Previous efforts determined a set of calibrated, optimal model parameter values for Reynolds-averaged Navier–Stokes (RANS) simulations of a compressible jet in crossflow (JIC) using a $k–ε$ turbulence model. These parameters were derived by comparing simulation results to particle image velocimetry (PIV) data of a complementary JIC experiment under a limited set of flow conditions. Here, a $k–ε$ model using both nominal and calibrated parameters is validated against PIV data acquired from a much wider variety of JIC cases, including a realistic flight vehicle. The results from the simulations using the calibrated model parameters showed considerable improvements over those using the nominal values, even for cases that were not used in the calibration procedure that defined the optimal parameters. This improvement is demonstrated using a number of quality metrics that test the spatial alignment of the jet core, the magnitudes of multiple flow variables, and the location and strengths of vortices in the counter-rotating vortex cores on the PIV planes. These results suggest that the calibrated parameters have applicability well outside the specific flow case used in defining them and that with the right model parameters, RANS solutions for the JIC can be improved significantly over those obtained from the nominal model.
Femtosecond laser electronic excitation tagging (FLEET) is a powerful unseeded velocimetry technique typically used to measure one component of velocity along a line, or two or three components from a dot. In this Letter, we demonstrate a dotted-line FLEET technique which combines the dense profile capability of a line with the ability to perform two-component velocimetry with a single camera on a dot. Our set-up uses a single beam path to create multiple simultaneous spots, more than previously achieved in other FLEET spot configurations. We perform dotted-line FLEET measurements downstream of a highly turbulent, supersonic nitrogen free jet. Dotted-line FLEET is created by focusing light transmitted by a periodic mask with rectangular slits of 1.6 × 40 mm2 and an edge-to-edge spacing of 0.5 mm, then focusing the imaged light at the measurement region. Up to seven symmetric dots spaced approximately 0.9 mm apart, with mean full-width at half maximum diameters between 150 and 350 µm, are simultaneously imaged. Both streamwise and radial velocities are computed and presented in this Letter.
Time-resolved particle image velocimetry (TR-PIV) has become widespread in fluid dynamics. Essentially a velocity field movie, the dynamic content provides temporal as well as spatial information, in contrast to conventional PIV offering only statistical ensembles of flow quantities. From these time series arise further analyses such as accelerometry, space-time correlations, frequency spectra of turbulence including spatial variability, and derivation of pressure fields and forces. The historical development of TR-PIV is chronicled, culminating in an assessment of the current state of technology in high-repetition-rate lasers and high-speed cameras. Commercialization of pulse-burst lasers has expanded TR-PIV into more flows, including the compressible regime, and has achieved MHz rates. Particle response times and peak locking during image interrogation require attention but generally are not impediments to success. Accuracy considerations are discussed, including the risks of noise and aliasing in spectral content. Oversampled TR-PIV measurements allow use of multi-frame image interrogation methods, which improve the precision of the correlation and raise the velocity dynamic range of PIV. In combination with volumetric methods and data assimilation, a full four-dimensional description of a flow is not only achievable but becoming standardized. A survey of exemplary applications is followed by a few predictions concerning the future of TR-PIV.
The development of new hypersonic flight vehicles is limited by the physical understanding that may be obtained from ground test facilities. This has motivated the present development of a temporally and spatially resolved velocimetry measurement for Sandia National Laboratories (SNL) Hypersonic Wind Tunnel (HWT) using Femtosecond Laser Electronic Excitation Tagging (FLEET). First, a multi-line FLEET technique has been created for the first time and tested in a supersonic jet, allowing simultaneous measurements of velocities along multiple profiles in a flow. Secondly, two different approaches have been demonstrated for generating dotted FLEET lines. One employs a slit mask pattern focused into points to yield a dotted line, allowing for two- or three-component velocity measurements free of contamination between components. The other dotted-line approach is based upon an optical wedge array and yields a grid of points rather than a dotted line. Two successful FLEET measurement campaigns have been conducted in SNL’s HWT. The first effort established optimal diagnostic configurations in the hypersonic environment based on earlier benchtop reproductions, including validation of the use of a 267 nm beam to boost the measurement signal-to-noise ratio (SNR) with minimal risk of perturbing the flow and greater simplicity than a comparable resonant technique at 202 nm. The same FLEET system subsequently was reconstituted to demonstrate the ability to make velocimetry measurements of hypersonic turbulence in a realistic flow field. Mean velocity profiles and turbulence intensity profiles of the shear layer in the wake of a hypersonic cone model were measured at several different downstream stations, proving the viability of FLEET as a hypersonic diagnostic.
This experimental study explores the fluid-structure interactions occurring between a control surface and the hypersonic flow deflected by it. The control surface is simplified for this work as a spanwise finite wedge placed on a longitudinally sliced part of the cone. The front surface of the wedge is a thin panel which is designed to respond to the unsteady fluid loading arising from the shock-wave/boundary layer interactions. Experiments have been conducted in the Sandia Hypersonic Wind Tunnel at Mach 5 and Mach 8 at wedge angles of 10◦, 20◦ and 30◦ . High-speed schlieren and backside panel accelerometer measurements capture the unsteady flow dynamics and structural response of the thin panel, respectively. For attached or small separation interactions, the transitional regime has the strongest panel fluctuations with convective shock undulations induced by the boundary layer disturbance shown to be associated with dominant panel vibrations. For large separated interactions, shear layer flapping can excite select panel modes. Heating of the panel causes a downward shift in natural mode frequencies.
This study seeks to simplify the optical requirements for multi-line FLEET (Femtosecond Laser Electronic Excitation Tagging) generation by focusing the image of a periodic slit-mask with a cylindrical and spherical lens. Geometry effects on the signal were analyzed over fifteen mask iterations. The signal for each mask was found to vary with mask standoff from the focusing optics, which was optimized based on maximizing the Signal-to-Noise Ratio (SNR) for each mask. The number of generated lines was found to decrease with slit spacing while the separation of the lines increased. FLEET line spacing was determined by a constant magnification value of the imaged masks’ slit spacing. From the geometry study, two masks that produced three to five lines spaced at 0.8–1 mm apart with SNR > 4 were chosen to demonstrate the multi-line technique in a supersonic free-jet. Velocity calculations from this data showed good agreement with schlieren imaging of compressible flow structures.
Multi-frame correlation algorithms for time-resolved PIV have been shown in previous studies to reduce noise and error levels in comparison with conventional two-frame correlations. However, none of these prior efforts tested the accuracy of the algorithms in spectral space. Even should a multi-frame algorithm reduce the error of vector computations summed over an entire data set, this does not imply that these improvements are observed at all frequencies. The present study examines the accuracy of velocity spectra in comparison with simultaneous hot-wire data. Results indicate that the high-frequency content of the spectrum is very sensitive to choice of the interrogation algorithm and may not return an accurate response. A top-hat-weighted sliding sum-of-correlation is contaminated by high-frequency ringing whereas Gaussian weighting is indistinguishable from a low-pass filtering effect. Some evidence suggests the pyramid correlation modestly increases bandwidth of the measurement at high frequencies. The apparent benefits of multi-frame interrogation algorithms may be limited in their ability to reveal additional spectral content of the flow.
The primary parameter of a standard k-ϵ model, Cμ, was calculated from stereoscopic particle image velocimetry (PIV) data for a supersonic jet exhausting into a transonic crossflow. This required the determination of turbulent kinetic energy, turbulent eddy viscosity, and turbulent energy dissipation rate. Image interrogation was optimized, with different procedures used for mean strain rates and Reynolds stresses, to produce useful turbulent eddy viscosity fields. The eddy viscosity was calculated by a least-squares fit to all components of the three-dimensional strain-rate tensor that were available from the PIV data. This eliminated artifacts and noise observed when using a single strain component. Local dissipation rates were determined via Kolmogorov’s similarity hypotheses and the second-order structure function. The eddy viscosity and dissipation rates were then combined to determine Cμ. Considerable spatial variation was observed in Cμ, with the highest values found in regions where turbulent kinetic energy was relatively ow but where turbulent mixing was important, e.g., along the high-strain jet edges and in the wake region. This suggests that use of a constant Cμ in modeling may lead to poor Reynolds stress predictions at mixing interfaces. A data-driven modeling approach that can predict this spatial variation of Cμ based on known state variables may lead to improved simulation results without the need for calibration.
A simple linear configuration for multi-line femtosecond laser electronic excitation tagging (FLEET) velocimetry is used for the first time, to the best of our knowledge, to image an overexpanded unsteady supersonic jet. The FLEET lines are spaced 0.5-1.0mmapart, and up to six lines can be used simultaneously to visualize the flowfield. These lines are created using periodic masks, despite the mask blocking 25%-30%of the 10 mJ incident beam.Maps of mean singlecomponent velocity in the direction along the principal flow axis, and turbulence intensity in that same direction, are created using multi-line FLEET, and computed velocities agree well with those obtained from single-line (traditional) FLEET. Compared to traditional FLEET, multi-line FLEET offers increased simultaneous spatial coverage and the ability to produce spatial correlations in the streamwise direction. This FLEET permutation is especially well suited for short-duration test facilities.
Two techniques have extended the effective frequency limits of postage-stamp PIV, in which a pulse-burst laser and very small fields of view combine to achieve high repetition rates. An interpolation scheme reduced measurement noise, raising the effective frequency response of previous 400-kHz measurements from about 120 kHz to 200 kHz. The other technique increased the PIV acquisition rate to very nearly MHz rates (990 kHz) by using a faster camera. Charge leaked through the camera shift register at these framing rates but this was shown not to bias the measurements. The increased framing rate provided oversampled data and enabled use of multi-frame correlation algorithms for a lower noise floor, increasing the effective frequency response to 240 kHz where the interrogation window size begins to spatially filter the data. Good agreement between the interpolation technique and the MHz-rate PIV measurements was established. The velocity spectra suggest turbulence power-law scaling in the inertial subrange steeper than the theoretical-5/3 scaling, attributed to an absence of isotropy.
A blind CFD validation challenge is being organized for the unsteady transonic shock motion induced by the Sandia Axisymmetric Transonic Hump, which echoes the Bachalo-Johnson configuration. The wind tunnel and model geometry will be released at the start of the validation challenge along with flow boundary conditions. Primary data concerning the unsteady separation region will be released at the conclusion of the challenge after computational entrants have been submitted. This paper details the organization of the challenge, its schedule, and the metrics of comparison by which the models will be assessed.
Bench-top tests are conducted to characterize Femtosecond Laser Electronic Excitation Tagging (FLEET) in static low pressure (35 mTorr-760 Torr) conditions, and to measure the acoustic disturbance caused by the resulting filament as a function of tagging wavelength and energy. The FLEET line thickness as a function of pressure and delay is described by a simple diffusion model. Initial FLEET measurements in a Mach 8 flow show that gate times of ≥ 1µs can produce visible smearing of the FLEET emission and challenge the traditional Gaussian fitting methods used to find the line center. To minimize flow perturbations and uncertainty of the final line position, several recommendations are offered: using third harmonic FLEET at 267 nm for superior signal levels with lower energy deposition than both 800 nm and 400 nm FLEET, and short camera delays and exposure times to reduce fitting uncertainty. This guidance is implemented in a Mach 8 test condition and results are presented.
An experimental characterization of the flow environment for the Sandia Axisymmetric Transonic Hump is presented. This is an axisymmetric model with a circular hump tested at a transonic Mach number, similar to the classic Bachalo-Johnson configuration. The flow is turbulent approaching the hump and becomes locally supersonic at the apex. This leads to a shock-wave/boundary-layer interaction, an unsteady separation bubble, and flow reattachment downstream. The characterization focuses on the quantities required to set proper boundary conditions for computational efforts described in the companion paper, including: 1) stagnation and test section pressure and temperature; 2) turbulence intensity; and 3) tunnel wall boundary layer profiles. Model characterization upstream of the hump includes: 1) surface shear stress; and 2) boundary layer profiles. Note: Numerical values characterizing the experiment have been redacted from this version of the paper. Model geometry and boundary conditions will be withheld until the official start of the Validation Challenge, at which time a revised version of this paper will become available. Data surrounding the hump are considered final results and will be withheld until completion of the Validation Challenge.
Here, experiments were performed within Sandia National Labs’ Multiphase Shock Tube to measure and quantify the shock-induced dispersal of a shock/dense particle curtain interaction. Following interaction with a planar travelling shock wave, schlieren imaging at 75 kHz was used to track the upstream and downstream edges of the curtain. Data were obtained for two particle diameter ranges ($d_{p}=106{-}125$,$300{-}355~\unicode[STIX]{x03BC}\text{m}$) across Mach numbers ranging from 1.24 to 2.02. Using these data, along with data compiled from the literature, the dispersion of a dense curtain was studied for multiple Mach numbers (1.2–2.6), particle sizes ($100{-}1000~\unicode[STIX]{x03BC}\text{m}$) and volume fractions (9–32 %). Data were non-dimensionalized according to two different scaling methods found within the literature, with time scales defined based on either particle propagation time or pressure ratio across a reflected shock. The data refelct that spreading of the particle curtain is a function of the volume fraction, with the effectiveness of each time scale based on the proximity of a given curtain’s volume fraction to the dilute mixture regime. It is observed that volume fraction corrections applied to a traditional particle propagation time scale result in the best collapse of the data between the two time scales tested here. In addition, a constant-thickness regime has been identified, which has not been noted within previous literature.
Time-resolved particle image velocimetry was conducted at 40 kHz using a pulse-burst laser in the supersonic wake of a wall-mounted hemisphere. Velocity fields suggest a recirculation region with two lobes, in which flow moves away from the wall near the centerline and recirculates back toward the hemisphere off the centerline, contrary to transonic configurations. Spatio-temporal cross-correlations and conditional ensemble averages relate the characteristic behavior of the unsteady shock motion to the flapping of the shear layer. At Mach 1.5, oblique shocks develop, associated with vortical structures in the shear layer and convect downstream in tandem; a weak periodicity is observed. Shock motion at Mach 2.0 appears somewhat different, wherein multiple weak disturbances propagate from shear-layer turbulent structures to form an oblique shock that ripples as these vortices pass by. Bifurcated shock feet coalesce and break apart without evident periodicity. Power spectra show a preferred frequency of shear-layer flapping and shock motion for Mach 1.5, but at Mach 2.0, a weak preferred frequency at the same Strouhal number of 0.32 is found only for oblique shock motion and not shear-layer unsteadiness.
A new wind tunnel experiment is underway to provide a comprehensive CFD validation dataset of an unsteady, transonic flow. The experiment is based on the work of Bachalo and Johnson; an axisymmetric model with a spherical hump is tested at a transonic Mach number. The flow is turbulent approaching the hump and becomes locally supersonic at the apex. This leads to a shock-wave/boundary-layer interaction, an unsteady separation bubble, and flow reattachment downstream. A suite of diagnostics characterizes the flow: oil-flow surface visualization for shock and reattachment locations, particle image velocimetry for mean flow and turbulence properties, fast pressure-sensitive paint for model pressure distributions and unsteadiness, high-speed Schlieren for shock position and motion, and oil-film interferometry for surface shear stress. This will provide a new level of detail for validation studies; therefore, a blind comparison, or ‘CFD Challenge’ is proposed to the community. Participants are to be provided the geometry, incoming boundary layer, and boundary conditions, and are free to simulate with their method of choice and submit their results. A blind comparison will be made to the new experimental data, with the goal of evaluating the state of various CFD methods for use in unsteady, transonic flows.
Femtosecond Laser Electronic Excitation Tagging (FLEET) is used to measure velocity flowfields in the wake of a sharp 7◦ half-angle cone in nitrogen at Mach 8, over freestream Reynolds numbers from 4.3∗106 /m to 13.8∗106 /m. Flow tagging reveals expected wake features such as the separation shear layer and two-dimensional velocity components. Frequency-tripled FLEET has a longer lifetime and is more energy efficient by tenfold compared to 800 nm FLEET. Additionally, FLEET lines written with 267 nm are three times longer and 25% thinner than that written with 800 nm at a 1 µs delay. Two gated detection systems are compared. While the PIMAX 3 ICCD offers variable gating and fewer imaging artifacts than a LaVision IRO coupled to a Photron SA-Z, its slow readout speed renders it ineffective for capturing hypersonic velocity fluctuations. FLEET can be detected to 25 µs following excitation within 10 mm downstream of the model base, but delays greater than 4 µs have deteriorated signal-to-noise and line fit uncertainties greater than 10%. In a hypersonic nitrogen flow, exposures of just several hundred nanoseconds are long enough to produce saturated signals and/or increase the line thickness, thereby adding to measurement uncertainty. Velocity calculated between the first two delays offer the lowest uncertainty (less than 3% of the mean velocity).
The mechanism by which aerodynamic effects of jet/fin interaction arise from the flow structure of a jet in crossflow is explored using particle image velocimetry measurements of the crossplane velocity field as it impinges on a downstream fin instrumented with high-frequency pressure sensors. A Mach 3.7 jet issues into a Mach 0.8 crossflow from either a normal or inclined nozzle, and three lateral fin locations are tested. Conditional ensemble-averaged velocity fields are generated based upon the simultaneous pressure condition. Additional analysis relates instantaneous velocity vectors to pressure fluctuations. The pressure differential across the fin is driven by variations in the spanwise velocity component, which substitutes for the induced angle of attack on the fin. Pressure changes at the fin tip are strongly related to fluctuations in the streamwise velocity deficit, wherein lower pressure is associated with higher velocity and vice versa. The normal nozzle produces a counter-rotating vortex pair that passes above the fin, and pressure fluctuations are principally driven by the wall horseshoe vortex and the jet wake deficit. In conclusion, the inclined nozzle produces a vortex pair that impinges the fin and yields stronger pressure fluctuations driven more directly by turbulence originating from the jet mixing.
Fluid-structure interactions were studies on a 7° half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 5 and 8 and in the Purdue Boeing/AFOSR Mach 6 Quiet Tunnel. A thin composite panel was integrated into the cone and the response to boundary-layer disturbances was characterized by accelerometers on the backside of the panel. Here, under quiet-flow conditions at Mach 6, the cone boundary layer remained laminar. Artificially generated turbulent spots excited a directionally dependent panel response which would last much longer than the spot duration.
The mechanism by which aerodynamic effects of jet/fin interaction arise from the flow structure of a jet in crossflow is explored using particle image velocimetry measurements of the crossplane velocity field as it impinges on a downstream fin instrumented with high-frequency pressure sensors. A Mach 3.7 jet issues into a Mach 0.8 crossflow from either a normal or inclined nozzle, and three lateral fin locations are tested. Conditional ensemble-averaged velocity fields are generated based upon the simultaneous pressure condition. Additional analysis relates instantaneous velocity vectors to pressure fluctuations. The pressure differential across the fin is driven by variations in the spanwise velocity component, which substitutes for the induced angle of attack on the fin. Pressure changes at the fin tip are strongly related to fluctuations in the streamwise velocity deficit, wherein lower pressure is associated with higher velocity and vice versa. The normal nozzle produces a counter-rotating vortex pair that passes above the fin, and pressure fluctuations are principally driven by the wall horseshoe vortex and the jet wake deficit. The inclined nozzle produces a vortex pair that impinges the fin and yields stronger pressure fluctuations driven more directly by turbulence originating from the jet mixing.
High-speed, time-resolved particle image velocimetry with a pulse-burst laser was used to measure the gas-phase velocity upstream and downstream of a shock wave-particle curtain interaction at three shock Mach numbers (1.22, 1.40, and 1.45) at a repetition rate of 37.5 kHz. The particle curtain was formed from free-falling soda-lime particles resulting in volume fractions of 9% or 23% at mid-height, depending on particle diameter (106-125 and 300-355 μm, respectively). Following impingement by a shock wave, a pressure difference was created between the upstream and downstream sides of the curtain, which accelerated flow through the curtain. Jetting of flow through the curtain was observed downstream once deformation of the curtain began, demonstrating a long-term unsteady effect. Using a control volume approach, the unsteady drag on the curtain was estimated from velocity and pressure data. The drag imposed on the curtain has a strong volume fraction dependence with a prolonged unsteadiness following initial shock impingement. In addition, the data suggest that the resulting pressure difference following the propagation of the reflected and transmitted shock waves is the primary component to curtain drag.
The spanwise variation of resonance dynamics in the Mach 0.94 flow over a finite-span cavity was explored using stereoscopic time-resolved particle image velocimetry (TR-PIV) and time-resolved pressure sensitive paint (TR-PSP). The TR-PSP data were obtained along the cavity floor, whereas the TR-PIV measurements were made in a planform plane just above the cavity lip line. The pressure data showed relatively coherent distributions across the span. In contrast, the PIV showed a significant variation in resonance dynamics to occur across the span in the plane above the cavity. A substantial influence of the sidewalls appears to stem from spillage vortices. At the first cavity mode frequency, streamwise velocity fluctuations were several times higher near the sidewalls in comparison to the centerline values. Importantly, PSDs of streamwise velocity in the region of the spillage vortices showed a large peak to occur at mode one, indicating velocity fluctuations in these regions can have a preferred frequency. The resonance fluctuations in the velocity fields at modes two and three demonstrated a complex spatial dependence that varied with spanwise location.
The development of the unsteady pressure field on the floor of a rectangular cavity was studied at Mach 0.9 using high-frequency pressure-sensitive paint. Power spectral amplitudes at each cavity resonance exhibit a spatial distribution with an oscillatory pattern; additional maxima and minima appear as the mode number is increased. This spatial distribution also appears in the propagation velocity of modal pressure disturbances. This behavior was tied to the superposition of a downstream-propagating shear-layer disturbance and an upstream-propagating acoustic wave of different amplitudes and convection velocities, consistent with the classical Rossiter model. The summation of these waves generates an interference pattern in the spatial pressure amplitudes and resulting phase velocity of the resonant pressure fluctuations.
Pulse-burst particle image velocimetry has been used to acquire time-resolved data at 37.5 kHz of the flow over a finite-width rectangular cavity at Mach 0.8. Power spectra of the particle image velocimetry data reveal four resonance modes that match the frequencies detected simultaneously using high-frequency wall pressure sensors, but whose magnitudes exhibit spatial dependence throughout the cavity. Spatiotemporal cross correlations of velocity to pressure were calculated after bandpass filtering for specific resonance frequencies. Cross-correlation magnitudes express the distribution of resonance energy, revealing local maxima and minima at the edges of the shear layer attributable to wave interference between downstream-and upstream-propagating disturbances. Turbulence intensities were calculated using a triple decomposition and are greatest in the core of the shear layer for higher modes, where resonant energies ordinarily are lower. Most of the energy for the lowest mode lies in the recirculation region and results principally from turbulence rather than resonance. Together, the velocity-pressure cross correlations and the triple-decomposition turbulence intensities explain the sources of energy identified in the spatial distributions of power spectra amplitudes.
Time-resolved particle image velocimetry recently has been demonstrated in high-speed flows using a pulse-burst laser at repetition rates reaching 50 kHz. Turbulent behavior can be measured at still higher frequencies if the field of view is greatly reduced and lower laser pulse energy is accepted. Current technology allows image acquisition at 400 kHz for sequences exceeding 4,000 frames, but for an array of only 128 × 120 pixels, giving the moniker of “postage-stamp PIV.” The technique has been tested far downstream of a supersonic jet exhausting into a transonic crossflow. Two-component measurements appear valid until 100 kHz at which point a noise floor emerges dependent upon the reduction of peak locking. Stereoscopic measurement offers three-component data for turbulent kinetic energy spectra, but exhibits a reduced signal bandwidth and higher noise in the out-of-plane component due to the oblique camera images. The resulting spectra reveal two regions exhibiting power-law dependence describing the turbulent decay. One is the well-known inertial subrange with a slope of -5/3 at high frequencies. The other displays a -1 power-law dependence for a decade of mid-range frequencies corresponding to the energetic eddies measured by PIV, which appears to have been previously unrecognized for high-speed free shear flows.
Experiments were performed within Sandia National Labs’ Multiphase Shock Tube to measure and quantify the transient behavior of a dense particle curtain, following interaction with a planar shock wave. The data obtained are in the form of two particle diameter ranges (dp= 106-125, 300-355 µm) across Mach numbers ranging from 1.24-2.02. Using these data, along with data compiled from literature, the dispersion of a dense curtain was studied for multiple Mach numbers, particle sizes, and volume fractions. High-speed Schlieren imaging at 75 kHz was used to track the upstream and downstream edges of the curtains over time. Non-dimensionalization of the data was then carried out according to two different scaling methods found within the literature, with time scales defined based on either particle time of flight or pressure ratio across a reflected shock. The data show that spreading of the particle curtain is a function of the volume fraction, with the effectiveness of each timescale based on the proximity of a given curtain’s volume fraction to the dilute mixture regime. A new scaling argument is defined here, based on a simplified force balance, which shows improved collapse of the curtain spreading data across the volume fractions presented. It is seen that volume fraction corrections applied to a traditional time of flight timescale result in the best collapse of the data between the two timescales tested here.
The resonance modes in Mach 0.94 turbulent flow over a cavity having a length-to-depth ratio of five were explored using time-resolved particle image velocimetry and time-resolved pressure sensitive paint. Mode-switching occurred in the velocity field simultaneous with the pressure field. The first cavity mode corresponded to large-scale motions in shear layer and in the vicinity of the recirculation region, whereas the second and third modes contained organized structures associated with shear layer vortices. Modal surface pressures exhibited streamwise periodicity generated by the interference of downstream-traveling disturbances in shear layer with upstream-traveling acoustical waves. Because of this interference, the modal velocity fields also exhibited local maxima at locations containing pressure minima and vice-versa. Modal convective (phase) velocities, based on cross-correlations of bandpass-filtered velocity fields, decreased with decreasing mode number as the modal activity resided in lower portions of the cavity. These phase velocities also exhibited streamwise periodicity caused by wave interference. The measurements demonstrate that despite the complexities inherent in compressible cavity flows, many of the most prevalent resonance dynamics can be described with simple acoustical analogies.
we studied the influence of compressibility on the shear layer over a rectangular cavity of variable width in a free stream Mach number range of 0.6–2.5 using particle image velocimetry data in the streamwise centre plane. As the Mach number increases, the vertical component of the turbulence intensity diminishes modestly in the widest cavity, but the two narrower cavities show a more substantial drop in all three components as well as the turbulent shear stress. Furthermore, this contrasts with canonical free shear layers, which show significant reductions in only the vertical component and the turbulent shear stress due to compressibility. The vorticity thickness of the cavity shear layer grows rapidly as it initially develops, then transitions to a slower growth rate once its instability saturates. When normalized by their estimated incompressible values, the growth rates prior to saturation display the classic compressibility effect of suppression as the convective Mach number rises, in excellent agreement with comparable free shear layer data. The specific trend of the reduction in growth rate due to compressibility is modified by the cavity width.
Fluid-structure interactions were studied on a 7 * half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 8 over a range of freestream Reynolds numbers b etween 3 . 3 and 14 . 5 x 10 6 / m . A thin panel with tunable structural natural frequencies was integrated into the cone and exposed to naturally developing boundary layers. An elevated panel re sponse was measured during boundary- layer transition at frequencies corresponding to the turbu lent burst rate, and lower vibrations were measured under a turbulent boundary layer. Controlled pert urbations from an electrical discharge were then introduced into the boundary layer at varying freq uencies corresponding to the struc- tural natural frequencies of the panel. The perturbations w ere not strong enough to drive a panel response exceeding that due to natural transition. Instead at high repetition rates, the perturber modified the turbulent burst rate and intermittency on the co ne and therefore changed the condi- tions for when an elevated transitional panel vibration res ponse occurred. Studies were also conducted in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. Under quiet flow, natural transition does not occur, and the c ontrolled perturbations are the only disturbance source. A clear panel response to turbulent spo ts created by the controlled pertur- bations was observed at varying frequencies of spot generat ion. The quiet-flow measurements confirm the clear relationship between turbulent spot passa ge and panel vibration.
Fluid–structure interactions that occur during aircraft internal store carriage were experimentally explored at Mach 0.58–1.47 using a generic, aerodynamic store installed in a rectangular cavity having a length-to-depth ratio of seven. The store vibrated in response to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance frequencies. Cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas the spanwise response to cavity tones was much more limited. Increased surface area associated with tail fins raised vibration levels. The store had interchangeable components to vary its natural frequencies by about 10–300 Hz. By tuning natural frequencies, mode-matched cases were explored where a prominent cavity tone frequency matched a structural natural frequency of the store. Mode matching in the streamwise and wall-normal directions produced substantial increases in peak store vibrations, though the response of the store remained linear with dynamic pressure. Near mode-matched frequencies, changes in cavity tone frequencies of only 1% altered store peak vibrations by as much as a factor of two. In conclusion, mode matching in the spanwise direction did little to increase vibrations.
Three stereoscopic PIV experiments have been examined to test the effectiveness of self-calibration under varied circumstances. Measurements taken in a streamwise plane yielded a robust self-calibration that returned common results regardless of the specific calibration procedure, but measurements in the crossplane exhibited substantial velocity bias errors whose nature was sensitive to the particulars of the self-calibration approach. Self-calibration is complicated by thick laser sheets and large stereoscopic camera angles and further exacerbated by small particle image diameters and high particle seeding density. Despite the different answers obtained by varied self-calibrations, each implementation locked onto an apparently valid solution with small residual disparity and converged adjustment of the calibration plane. Therefore, the convergence of self-calibration on a solution with small disparity is not sufficient to indicate negligible velocity error due to the stereo calibration.
Pulse-burst Particle Image Velocimetry (PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulent eddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing, both leading and trailing the reference eddy. This indicates the paired nature of the turbulent eddies and the tendency for these pairs to recur at repeatable spacing. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. The spatial scale of these vortices appears similar to previous observations of compressible jets in crossflow. Super-sampled velocity spectra to 150 kHz reveal a power-law dependency of -5/3 in the inertial subrange as well as a -1 dependency at lower frequencies attributed to the scales of the dominant shear-layer eddies.
Stereoscopic particle image velocimetry was used to experimentally measure the recirculating flow within finite-span cavities of varying complex geometry at a freestream Mach number of 0.8. Volumetric measurements were made to investigate the side wall influences by scanning a laser sheet across the cavity. Each of the geometries could be classied as an open-cavity, based on L/D. The addition of ramps altered the recirculation zone within the cavity, causing it to move along the streamwise direction. Within the simple rectangular cavity, a system of counter-rotating streamwise vortices formed due to spillage from along the side wall, which caused the mixing layer to develop a steady spanwise waviness. The ramped complex geometry, due to the presence of leading edge and side ramps, appeared to suppress the formation of streamwise vorticity associated with side wall spillage, resulting in a much more two-dimensional mixing layer.
Fluid-structure interactions were studied on a 7° half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 8 over a range of freestream Reynolds numbers between 3.3 and 14.5 × 106/m. A thin panel with tunable structural natural frequencies was integrated into the cone and exposed to naturally developing boundary layers. An elevated panel response was measured during boundary-layer transition at frequencies corresponding to the turbulent burst rate, and lower vibrations were measured under a turbulent boundary layer. Controlled perturbations from an electrical discharge were then introduced into the boundary layer at varying frequencies corresponding to the structural natural frequencies of the panel. The perturbations were not strong enough to drive a panel response exceeding that due to natural transition. Instead at high repetition rates, the perturber modified the turbulent burst rate and intermittency on the cone and therefore changed the conditions for when an elevated transitional panel vibration response occurred.
Time-resolved particle image velocimetry (TR-PIV) measurements were made in a shock tube using a pulse-burst laser. Two transient flowfields were investigated including the baseline flow in the empty shock tube and the wake growth downstream of a cylinder spanning the width of the test section. Boundary layer growth was observed following the passage of the incident shock in the baseline flow, while the core flow velocity increased with time. The measured core flow acceleration was compared to that predicted using a classical unsteady boundary layer growth model. The model typically provided good estimates of core flow acceleration at early times, but then typically underestimated the acceleration. As a result of wall boundary layers, a significant amount of spatial non-uniformity remained in the flow following the passage of the end-wall reflected shock, which could be an important factor in combustion chemistry experiments. In the transient wake growth measurements, the wake downstream of the cylinder was symmetric immediately following the passage of the incident shock. At later times (≈ 0.5 ms), the wake transitioned to a von Kármán vortex street. The TR-PIV data were bandpass filtered about the vortex shedding frequency to reveal additional details on the transient wake growth.
High-speed, time-resolved particle image velocimetry with a pulse-burst laser was used to measure the gas-phase velocity upstream and downstream of a shock wave-particle curtain interaction at three shock Mach numbers (1.19, 1.40, and 1.45), at a sampling rate of 37.5 kHz. The particle curtain, formed from free-falling soda-lime particles with diameters ranging from 300 - 355 μm, had a streamwise thickness of 3.5 mm and volume fraction of 9% at mid-height. Following impingement by a shock wave, a pressure difference was created between the upstream/downstream sides of the curtain, which accelerated flow through the curtain. Jetting of flow through the curtain was observed downstream once deformation of the curtain began, demonstrating a long-term unsteady effect. Using a control volume approach, the unsteady drag on the curtain was determined from velocity and pressure data. Initially, the pressure difference between the upstream and downstream sides of the curtain was the largest contributor to the total drag. The data suggests, however, that as time increases, the change in momentum flux could become the dominant component as the pressure difference decreases.
Time-resolved PIV has been accomplished in three high-speed flows using a pulse-burst laser: a supersonic jet exhausting into a transonic crossflow, a transonic flow over a rectangular cavity, and a shock-induced transient onset to cylinder vortex shedding. Temporal supersampling converts spatial information into temporal information by employing Taylor’s frozen turbulence hypothesis along local streamlines, providing frequency content until about 150 kHz where the noise floor is reached. The spectra consistently reveal two regions exhibiting power-law dependence describing the turbulent decay. One is the well-known inertial subrange with a slope of-5/3 at high frequencies. The other displays a-1 power-law dependence for as much as a decade of mid-range frequencies lying between the inertial subrange and the integral length scale. The evidence for the-1 power law is most convincing in the jet-in-crossflow experiment, which is dominated by in-plane convection and the vector spatial resolution does not impose an additional frequency constraint. Data from the transonic cavity flow that are least likely to be subject to attenuation due to limited spatial resolution or out-of-plane motion exhibit the strongest agreement with the-1 and-5/3 power laws. The cylinder wake data also appear to show the-1 regime and the inertial subrange in the near-wake, but farther downstream the frozen-turbulence assumption may deteriorate as large-scale vortices interact with one another in the von Kármán vortex street.
Pulse-burst particle image velocimetry (PIV) has been used to acquire time-resolved data at 37.5 kHz of the flow over a finite-width rectangular cavity at Mach 0.6, 0.8, and 0.94. Power spectra of the PIV data reveal four resonance modes that match the frequencies detected simultaneously using high-frequency wall pressure sensors. Velocity resonances exhibit spatial dependence in which the lowest-frequency acoustic mode is active within the recirculation region whereas the three higher modes are concentrated within the shear layer. Spatio-temporal cross-correlations were calculated from velocity data first bandpass filtered for specific resonance frequencies. The low-frequency acoustic mode shows properties of a standing wave without spatial correlation. Higher resonance modes are associated with alternating coherent structures whose size and spacing decrease for higher resonance modes and increase as structures convect downstream. The convection velocity appears identical for the high-frequency resonance modes, but it too increases with downstream distance. This is in contrast to the well-known Rossiter equation, which assumes a convection velocity constant in space.
Time-resolved particle image velocimetry (PIV) using a pulse-burst laser has been acquired of a supersonic jet issuing into a Mach 0.8 crossflow. Simultaneously, the final pulse pair in each burst has been imaged using conventional PIV cameras to produce an independent two-component measurement and two stereoscopic measurements. Each measurement depicts generally similar flowfield features with vorticity contours marking turbulent eddies at corresponding locations. Probability density functions of the velocity fluctuations are essentially indistinguishable but the precision uncertainty estimated using correlation statistics shows that the pulse-burst PIV data have notably greater uncertainty than the three conventional measurements. This occurs due to greater noise in the cameras and a smaller size for the final iteration of the interrogation window. A small degree of peak locking is observed in the aggregate of the pulse-burst PIV data set. However, some of the individual vector fields show peak locking to non-integer pixel values as a result of real physical effects in the flow. Even if peak locking results entirely from measurement bias, the effect occurs at too low a level to anticipate a significant effect on data analysis.
Mach 0.94 flow over a cavity having a length-to-depth ratio of five was explored using time-resolved particle image velocimetry (TR-PIV) with a burst-mode laser. The data were used to probe the resonance dynamics of the first three cavity (Rossiter) tones. Bandpass filtering was employed to reveal the coherent flow structure associated with each tone. The first Rossiter mode was associated with a propagation of large scale structures in the recirculation region, while the second and third modes contained organized structures consistent with convecting vortical disturbances. The wavelengths of the second and third modes were quite similar to those observed in a previous study by the current authors using phase-averaged PIV. Convective velocities computed using cross correlations in the unfiltered data showed the convective velocity increased with streamwise distance in a fashion similar to other studies. Convective velocities during cavity resonance were found to decrease with decreasing mode number, consistent with the modal activity residing in lower portions of the cavity in regions of lower local mean velocities. The convective velocity fields associated with resonance exhibited a streamwise periodicity consistent with wall-normal undulations in the resonant velocity fields; however, additional work is required to confirm this is not an analysis artifact.
The interaction of a Mach 1.67 shock wave with a dense particle curtain is quantified using flash radiography. These new data provide a view of particle transport inside a compressible, dense gas–solid flow of high optical opacity. The curtain, composed of 115-µm glass spheres, initially spans 87 % of the test section width and has a streamwise thickness of about 2 mm. Radiograph intensities are converted to particle volume fraction distributions using the Beer–Lambert law. The mass in the particle curtain, as determined from the X-ray data, is in reasonable agreement with that given from a simpler method using a load cell and particle imaging. Following shock impingement, the curtain propagates downstream and the peak volume fraction decreases from about 23 to about 4 % over a time of 340 µs. The propagation occurs asymmetrically, with the downstream side of the particle curtain experiencing a greater volume fraction gradient than the upstream side, attributable to the dependence of particle drag on volume fraction. Bulk particle transport is quantified from the time-dependent center of mass of the curtain. The bulk acceleration of the curtain is shown to be greater than that predicted for a single 115-µm particle in a Mach 1.67 shock-induced flow.
The flow over an aircraft bay is often represented using a rectangular cavity; however, this simplification neglects many features of actual flight geometry that could affect the unsteady pressure field and resulting loading in the bay. To address this shortcoming, a complex cavity geometry was developed to incorporate more realistic aircraft-bay features including shaped inlets, internal cavity structure, and doors. A parametric study of these features was conducted based on fluctuating pressure measurements at subsonic and supersonic Mach numbers. Resonance frequencies and amplitudes increased in the complex geometry compared to a simple rectangular cavity that could produce severe loading conditions for store carriage. High-frequency content and dominant frequencies were generated by features that constricted the flow such as leading-edge overhangs, internal cavity variations, and the presence of closed doors. Broadband frequency components measured at the aft wall of the complex cavities were also significantly higher than in the rectangular geometry. Furthermore, these changes highlight the need to consider complex geometric effects when predicting the flight loading of aircraft bays.
A previous experiment by the present authors studied the flow over a finite-width rectangular cavity at freestream Mach numbers 1.5–2.5. In addition, this investigation considered the influence of three-dimensional geometry that is not replicated by simplified cavities that extend across the entire wind-tunnel test section. The latter configurations have the attraction of easy optical access into the depths of the cavity, but they do not reproduce effects upon the turbulent structures and acoustic modes due to the length-to-width ratio, which is becoming recognized as an important parameter describing the nature of the flow within narrower cavities.
Experiments were performed to understand the complex fluid-structure interactions that occur during aircraft internal store carriage. A cylindrical store was installed in a rectangular cavity having a length-to-depth ratio of 3.33 and a length-to-width ratio of 1. The Mach number ranged from 0.6 to 2.5 and the incoming boundary layer was turbulent. Fast-response pressure measurements provided aeroacoustic loading in the cavity, while triaxial accelerometers provided simultaneous store response. Despite occupying only 6% of the cavity volume, the store significantly altered the cavity acoustics. The store responded to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance. Specifically, cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas a spanwise response was observed only occasionally. The streamwise and wall-normal responses were attributed to the longitudinal pressure waves and shear layer vortices known to occur during cavity resonance. Although the spanwise response to cavity tones was limited, broadband pressure fluctuations resulted in significant spanwise accelerations at store natural frequencies. The largest vibrations occurred when a cavity tone matched a structural natural frequency, although energy was transferred more efficiently to natural frequencies having predominantly streamwise and wall-normal motions.
The flow over an open aircraft bay is often represented in a wind tunnel with a cavity. In flight, this flow is unconfined, though in experiments, the cavity is surrounded by wind tunnel walls. If untreated, wind tunnel wall effects can lead to significant distortions of cavity acoustics in subsonic flows. To understand and mitigate these cavity–tunnel interactions, a parametric approach was taken for flow over an L/D = 7 cavity at Mach numbers 0.6–0.8. With solid tunnel walls, a dominant cavity tone was observed, likely due to an interaction with a tunnel duct mode. An acoustic liner opposite the cavity decreased the amplitude of the dominant mode and its harmonics, a result observed by previous researchers. Acoustic dampeners were also placed in the tunnel sidewalls, which further decreased the dominant mode amplitudes and peak amplitudes associated with nonlinear interactions between cavity modes. This indicates that cavity resonance can be altered by tunnel sidewalls and that spanwise coupling should be addressed when conducting subsonic cavity experiments. Though mechanisms for dominant modes and nonlinear interactions likely exist in unconfined cavity flows, these effects can be amplified by the wind tunnel walls.
Two-component and stereoscopic particle image velocimetry measurements have been acquired in the streamwise plane for supersonic flow over a rectangular cavity of variable width, peering over the sidewall lip to view the depths of the cavity. The data reveal the turbulent shear layer over the cavity and the recirculation region within it. The mean position of the recirculation region was found to be a function of the length-to-width ratio of the cavity, as was the turbulence intensity within both the shear layer and the recirculation region. Compressibility effects were observed in which turbulence levels dropped, and the shear layer thickness decreased as the Mach number was raised from 1.5 to 2.0 and 2.5. Supplemental measurements in the crossplane and the planform view suggest that zones of high turbulence were affixed to each sidewall centered on the cavity lip, with a strip of turbulence stretched out across the cavity shear layer for which the intensity was a function of the length-to-width ratio. These sidewall features are attributed to spillage, which is greatly reduced for the narrowest cavity. Such effects cannot be found in experiments lacking finite spanwise extent.
Sandia’s Hypersonic Wind Tunnel (HWT) became operational in 1962, providing a test capability for the nation’s nuclear weapons complex. The first modernization program was completed in 1977. A blowdown facility with a 0.46-m diameter test section, the HWT operates at Mach 5, 8, and 14 with stagnation pressures to 21 MPa and temperatures to 1400K. Minimal further alteration to the facility occurred until 2008, but in recent years the HWT has received considerable investment to ensure its viability for at least the next 25 years. This has included reconditioning of the vacuum spheres, replacement of the high-pressure air tanks for Mach 5, new compressors to provide the high-pressure air, upgrades to the cryogenic nitrogen source for Mach 8 and 14, an efficient high-pressure water cooling system for the nozzle throats, and refurbishment of the electric-resistance heaters. The HWT is now returning to operation following the largest of the modernization projects, in which the old variable transformer for the 3-MW electrical system powering the heaters was replaced with a silicon-controlled rectifier power system. The final planned upgrade is a complete redesign of the control console and much of the gas-handling equipment.
Particle image velocimetry (PIV) measurements quantified the coherent structure of acoustic tones in a Mach 0.91 cavity flow. Stereoscopic PIV measurements were performed at 10-Hz and two-component, time-resolved data were obtained using a pulse-burst laser. The cavity had a square planform, a length-to-depth ratio of five, and an incoming turbulent boundary layer. Simultaneous fast-response pressure signals were bandpass filtered about each cavity tone frequency. The 10-Hz PIV data were then phase-averaged according to the bandpassed pressures to reveal the flow structure associated with the resonant tones. The first Rossiter mode was associated with large scale oscillations in the shear layer, while the second and third modes contained organized structures consistent with convecting vortical disturbances. The spatial wavelengths of the cavity tones, based on the vertical coherent velocity fields, were less than those predicted by the Rossiter relation. With increasing streamwise distance the spacing between structures increased and approached the predicted Rossiter value at the aft-end of the cavity. Moreover, the coherent structures appeared to rise vertically with downstream propagation. The time-resolved PIV data were bandpass filtered about the cavity tone frequencies to reveal flow structure. The resulting spacing between disturbances was similar to that in the phase-averaged flowfields.
To investigate the pressure-fluctuation field beneath turbulent spots in a hypersonic boundary layer, a study was conducted on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. Controlled disturbances were created by pulsed-glow perturbations based on the electrical breakdown of air. Under quiet-flow conditions, the nozzle-wall boundary layer remains laminar and grows very thick over the long nozzle length. This allows the development of large disturbances that can be well-resolved with high-frequency pressure transducers. A disturbance first grows into a second-mode instability wavepacket that is concentrated near its own centreline. Weaker disturbances are seen spreading from the centre. The waves grow and become nonlinear before breaking down to turbulence. The breakdown begins in the core of the packets where the wave amplitudes are largest. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction. The turbulent core grows downstream, resulting in a spot with a classical arrowhead shape. Behind the spot, a low-pressure calmed region develops. However, the spot is not merely a localized patch of turbulence; instability waves remain an integral part. Limited measurements of naturally occurring disturbances show many similar characteristics. From the controlled disturbance measurements, the convection velocity, spanwise spreading angle, and typical pressure-fluctuation field were obtained.
Simultaneous stereo PIV measurements of a round free jet were obtained from narrow and wide camera angles while a fifth camera viewed the laser sheet from 90 degrees to determine the two-component velocity field free of errors resulting from stereo calibration. Errors in mean velocities were small, but artificially reduced turbulent stresses were generated when self-calibration was not used, owing to a smearing effect that occurs when the two cameras are inadequately registered to each other. This difficulty worsened with increased laser sheet thickness. Spatial error in the stereo calibration process can artificially displace vector fields from the expected origin, which was detected through comparison to the simultaneous two-component measurement. Although this spatial offset typically is small with respect to statistical properties of a data set, it can be prominent when instantaneous snapshots of the velocity field are examined, particularly where the velocity gradient is momentarily large.
Experiments were conducted at freestream Mach numbers of 0.55, 0.80, and 0.90 in open cavity flows having a length-to-depth ratio L/D of 5 and an incoming turbulent boundary having a thickness of about 0.5D. To ascertain aspect ratio effects, the length-to-width ratio L/W was varied between 1.00, 1.67, and 5.00. Two stereoscopic PIV systems were used simultaneously to characterize the flow in the plane at the spanwise center of the cavity. For each aspect ratio, trends in the mean and turbulence fields were identified, regardless of Mach number. The recirculation region had the weakest reverse velocities in the L/W = 1.67 cavity, a trend previously observed at supersonic Mach numbers. Also, like the previous supersonic experiments, the L/W = 1.00 and L/W = 5.00 mean streamwise velocities were similar. The L/W = 1.00 cavity flows had the highest turbulence intensities, whereas the two narrower cavities exhibited lower turbulence intensities of a comparable level. This is in contrast to previous supersonic experiments, which showed the lowest turbulence levels in the L/W = 1.67 cavity.
High-frequency pressure sensors were used in conjunction with a high-speed schlieren system to study the growth and breakdown of boundary-layer disturbances into turbulent spots on a 7° cone in the Sandia Hypersonic Wind Tunnel at Mach 5 and 8. To relate the intermittent disturbances to the average characteristics of transition on the cone, the statistical distribution of these disturbances must be known. These include the boundarylayer intermittency, burst rate, and average disturbance length. Traditional low-speed methods to characterize intermittency identify only turbulent/nonturbulent regions. However at high M, instability waves become an important part of the transitional region. Algorithms to distinguish instability waves from turbulence in both the pressure and schlieren measurements are being developed and the corresponding intermittency, burst rate, and average burst length of both regions have been provisionally computed for several cases at Mach 5 and 8. Distinguishing instability waves from turbulence gives a better description of the intermittent boundary layer at high M and will allow the fluctuations associated with boundary-layer instabilities to be incorporated into transitional models.
Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes, which creates one of the largest sources of uncertainty in the simulation of energetic material detonation. To close this gap, a novel Multiphase Shock Tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 19%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 were achieved. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30-40% by the presence of the particle field. X-Ray diagnostics have been developed to penetrate the opacity of the flow, revealing the concentrations throughout the particle field as it expands and spreads downstream with time. Furthermore, an X-Ray particle tracking velocimetry diagnostic has been demonstrated to be feasible for this flow, which can be used to follow the trajectory of tracer particles seeded into the curtain. Additional experiments on single spherical particles accelerated behind an incident shock wave have shown that elevated particle drag coefficients can be attributed to increased compressibility rather than flow unsteadiness, clarifying confusing results from the historical database of shock tube experiments. The development of the Multiphase Shock Tube and associated diagnostic capabilities offers experimental capability to a previously inaccessible regime, which can provide unprecedented data concerning particle dynamics of dense gas-solid flows.
A novel multiphase shock tube has been constructed to test the interaction of a planar shock wave with a dense gas-solid field of particles. The particle field is generated by a gravity-fed method that results in a spanwise curtain of 100-micron particles producing a volume fraction of about 15%. Interactions with incident shock Mach numbers of 1.67 and 1.95 are reported. High-speed schlieren imaging is used to reveal the complex wave structure associated with the interaction. After the impingement of the incident shock, transmitted and reflected shocks are observed, which lead to differences in flow properties across the streamwise dimension of the curtain. Tens of microseconds after the onset of the interaction, the particle field begins to propagate downstream, and disperse. The spread of the particle field, as a function of its position, is seen to be nearly identical for both Mach numbers. Immediately downstream of the curtain, the peak pressures associated with the Mach 1.67 and 1.95 interactions are about 35% and 45% greater than tests without particles, respectively. For both Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced by about 30-40% by the presence of the particle field.
The development of turbulent spots in a hypersonic boundary layer was studied on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. Under quiet flow conditions, the nozzle wall boundary layer remains laminar and grows very thick over the long nozzle length. This allows the development of large turbulent spots that can be readily measured with pressure transducers. Measurements of naturally occurring wave packets and developing turbulent spots were made. The peak frequencies of these natural wave packets were in agreement with second-mode computations. For a controlled study, the breakdown of disturbances created by spark and glow perturbations were studied at similar freestream conditions. The spark perturbations were the most effective at creating large wave packets that broke down into turbulent spots. The flow disturbances created by the controlled perturbations were analyzed to obtain amplitude criteria for nonlinearity and breakdown as well as the convection velocities of the turbulent spots. Disturbances first grew into linear instability waves and then quickly became nonlinear. Throughout the nonlinear growth of the wave packets, large harmonics are visible in the power spectra. As breakdown begins, the peak amplitudes of the instability waves and harmonics decrease into the rising broad-band frequencies. Instability waves are still visible on either side of the growing turbulent spots during this breakdown process.
Previous wind tunnel experiments up to Mach 3 have provided fluctuating wall-pressure spectra beneath a supersonic turbulent boundary layer, which essentially are flat at low frequency and do not exhibit the theorized {psi}{sup 2} dependence. The flat portion of the spectrum extends over two orders of magnitude and represents structures reaching at least 100 {delta} in scale, raising questions about their physical origin. The spatial coherence required over these long lengths may arise from very-large-scale structures that have been detected in turbulent boundary layers due to groupings of hairpin vortices. To address this hypothesis, data have been acquired from a dense spanwise array of fluctuating wall pressure sensors, then invoking Taylor's Hypothesis and low-pass filtering the data allows the temporal signals to be converted into a spatial map of the wall pressure field. This reveals streaks of instantaneously correlated pressure fluctuations elongated in the streamwise direction and exhibiting spanwise alternation of positive and negative events that meander somewhat in tandem. As the low-pass filter cutoff is lowered, the fluctuating pressure magnitude of the coherent structures diminishes while their length increases.
A novel multiphase shock tube has recently been developed to study particle dynamics in gas-solid flows having particle volume fractions that reside between the dilute and granular regimes. The method for introducing particles into the tube involves the use of a gravity-fed contoured particle seeder, which is capable of producing dense fields of spatially isotropic particles. The facility is capable of producing planar shocks having a maximum shock Mach number of about 2.1 that propagate into air at initially ambient conditions. The primary purpose of this new facility is to provide high fidelity data of shock-particle interactions in flows having particle volume fractions of about 1 to 50%. To achieve this goal, the facility drives a planar shock into a spatially isotropic field, or curtain, of particles. Experiments are conducted for two configurations where the particle curtain is either parallel to the spanwise, or the streamwise direction. Arrays of high-frequency-response pressure transducers are placed near the particle curtain to measure the attenuation and shape change of the shock owing to its interaction with the dense gas particle field. In addition, simultaneous high-speed imaging is used to visualize the impact of the shock on the particle curtain and to measure the particle motion induced downstream of the shock.
High-frequency pressure-fluctuation measurements were made in AEDC Tunnel 9 at Mach 10 and the NASA Langley 15-Inch Mach 6 and 31-Inch Mach 10 tunnels. Measurements were made on a 7{sup o}-half-angle cone model. Pitot measurements of freestream pressure fluctuations were also made in Tunnel 9 and the Langley Mach-6 tunnel. For the first time, second-mode waves were measured in all of these tunnels, using 1-MHz-response pressure sensors. In Tunnel 9, second-mode waves could be seen in power spectra computed from records as short as 80 {micro}s. The second-mode wave amplitudes were observed to saturate and then begin to decrease in the Langley tunnels, indicating wave breakdown. Breakdown was estimated to occur near N {approx} 5 in the Langley Mach-10 tunnel. The unit-Reynolds-number variations in the data from Tunnel 9 were too large to see the same processes.
Wind tunnel experiments up to Mach 3 have provided fluctuating wall-pressure spectra beneath a supersonic turbulent boundary layer to frequencies reaching 400 kHz by combining signals from piezoresistive silicon pressure transducers effective at low- and mid-range frequencies and piezoelectric quartz sensors to detect high frequency events. Data were corrected for spatial attenuation at high frequencies and for wind-tunnel noise and vibration at low frequencies. The resulting power spectra revealed the {omega}{sup -1} dependence for fluctuations within the logarithmic region of the boundary layer, but are essentially flat at low frequency and do not exhibit the theorized {omega}{sup 2} dependence. Variations in the Reynolds number or streamwise measurement location collapse to a single curve for each Mach number when normalized by outer flow variables. Normalization by inner flow variables is successful for the {omega}{sup -1} region but less so for lower frequencies. A comparison of the pressure fluctuation intensities with fifty years of historical data shows their reported magnitude chiefly is a function of the frequency response of the sensors. The present corrected data yield results in excess of the bulk of the historical data, but uncorrected data are consistent with lower magnitudes. These trends suggest that much of the historical compressible database may be biased low, leading to the failure of several semi-empirical predictive models to accurately represent the power spectra acquired during the present experiments.
A novel multiphase shock tube to study particle dynamics in gas-solid flows has been constructed and tested. Currently, there is a gap in data for flows having particle volume fractions between the dusty and granular regimes. The primary purpose of this new facility is to fill that gap by providing high quality data of shock-particle interactions in flows having dense gas particle volume fractions. Towards this end, the facility aims to drive a shock into a spatially isotropic field, or curtain, of particles. Through bench-top experimentation, a method emerged for achieving this challenging task that involves the use of a gravity-fed contoured particle seeder. The seeding method is capable of producing fields of spatially isotropic particles having volume fractions of about 1 to 35%. The use of the seeder in combination with the shock tube allows for the testing of the impingement of a planar shock on a dense field of particles. The first experiments in the multiphase shock tube have been conducted and the facility is now operational.