Publications

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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.

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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.

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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.

The breakup of liquids due to aerodynamic forces has been widely studied. However, the literature contains limited quantified data on secondary droplet sizes, particularly as a function of time. Here, a column of liquid water is subjected to a step change in relative gas velocity using a shock tube. A unique digital in-line holography (DIH) configuration is proposed which quantifies the secondary droplets sizes, three-dimensional position, and three-component velocities at 100 kHz. Results quantify the detailed evolution of the characteristic mean diameters and droplet size-velocity correlations as a function of distance downstream from the initial location of the water column. Accuracy of the measurements is confirmed through mass balance. These data give unprecedented detail on the breakup process which will be useful for improved model development and validation.

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. Mode matching in the spanwise direction did little to increase vibrations.

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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.

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