Publications

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The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 0.8. Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex. Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding. Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations. Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius. Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude while greater turbulent kinetic energy penetrates the vortex core. An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core.

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A sub-scale experiment has been constructed using fins mounted on one wall of a transonic wind tunnel to investigate the influence of fin trailing vortices upon downstream control surfaces. Data are collected using a fin balance instrumenting the downstream fin to measure the aerodynamic forces of the interaction, combined with stereoscopic Particle Image Velocimetry to determine vortex properties. The fin balance data show that the response of the downstream fin essentially is shifted from the baseline single-fin data dependent upon the angle of attack of the upstream fin. Freestream Mach number and the spacing between fins have secondary effects. The velocimetry shows that the vortex strength increases markedly with upstream fin angle of attack, though even an uncanted fin generates a noticeable wake. No variation with Mach number can be discerned in the normalized velocity data. Correlations between the force data and the velocimetry suggest that the interaction is fundamentally a result of an angle of attack superposed upon the downstream fin by the vortex shed from the upstream fin tip. The Mach number influence arises from differing vortex lift on the leading edge of the downstream fin even when the impinging vortex is Mach invariant.

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Particle image velocimetry (PIV) data have been acquired using three different configurations in the far-field of the interaction of a transverse supersonic jet with a transonic crossflow. The configurations included two-dimensional PIV in the centerline streamwise plane at two overlapping stations, as well as stereoscopic PIV in both the same streamwise plane and the crossplane. The streamwise data show the downstream evolution of the interaction whereas the crossplane data directly reveal its vortex structure. The measurement planes intersect at a common line, allowing a comparison of those mean velocity components and turbulent stresses common to all configurations. All data from the streamwise plane agree to within their estimated uncertainties, but data from the crossplane exhibit reduced velocity and turbulent stress magnitudes by a small but significant degree. Additionally, the vertical positions of the peak velocities are slightly nearer the wall for the crossplane configuration. This comparison suggests that routine methods of uncertainty quantification for data used in the validation of computational models may not fully capture the error sources of an experiment.

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Joint fuel Raman and filtered Rayleigh-scattering (FRS) imaging is demonstrated in a laminar methane-air diffusion flame. These experiments are, to our knowledge, the first reported extension of the FRS technique to nonpremixed combustion. This joint imaging approach allows for correction of the FRS images for the large variations in Rayleigh cross section that occur in diffusion flames and for a secondary measurement of fuel mole fraction. The temperature-dependent filtered Rayleigh cross sections are computed with a six-moment kinetic model for calculation of major-species Rayleigh-Brillouin line shapes and a flamelet-based model for physically judicious estimates of gas-phase chemical composition. Shot-averaged temperatures, fuel mole fractions, and fuel number densities from steady and vortex-strained diffusion flames stabilized on a Wolfhard-Parker slot burner are presented, and a detailed uncertainty analysis reveals that the FRS-measured temperatures are accurate to within ±4.5 to 6% of the local absolute temperature. © 2005 Optical Society of America.

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A stereoscopic particle image velocimetry (PIV) instrument has been constructed for a transonic wind tunnel to study the interaction created by a supersonic axisymmetric jet exhausting from a flat plate into a subsonic compressible crossflow. Data have been acquired in the crossplane of the interaction at a single station in the farfield, in which the bulk particle motion is aligned with the out-of-plane velocity component. The resulting vector fields distinctly show the strength and location of the induced counter-rotating vortex pair as well as the remnant of the horseshoe vortex that wraps around the jet plume as it first exhausts from the nozzle. Data taken for four different values of the jet-to-freestream dynamic pressure ratio reveal the resulting change in vortex strength, size, and position. Vorticity fields were derived from the in-plane velocity data, but limited convergence of the present small data sets prevented any conclusions about the symmetry of the flowfield. Comparison of the present data is made with two-dimensional PIV data previously acquired in the streamwise plane.

Simulation-based life-cycle-engineering and the ASCI program have resulted in models of unprecedented size and fidelity. The validation of these models requires high-resolution, multi-parameter diagnostics. Within the thermal-fluids disciplines, the need for detailed, high-fidelity measurements exceeds the limits of current engineering sciences capabilities and severely tests the state of the art. The focus of this LDRD is the development and application of filtered Rayleigh scattering (FRS) for high-resolution, nonintrusive measurement of gas-phase velocity and temperature. With FRS, the flow is laser-illuminated and Rayleigh scattering from naturally occurring sources is detected through a molecular filter. The filtered transmission may be interpreted to yield point or planar measurements of three-component velocities and/or thermodynamic state. Different experimental configurations may be employed to obtain compromises between spatial resolution, time resolution, and the quantity of simultaneously measured flow variables. In this report, we present the results of a three-year LDRD-funded effort to develop FRS combustion thermometry and Aerosciences velocity measurement systems. The working principles and details of our FRS opto-electronic system are presented in detail. For combustion thermometry we present 2-D, spatially correlated FRS results from nonsooting premixed and diffusion flames and from a sooting premixed flame. The FRS-measured temperatures are accurate to within {+-}50 K (3%) in a premixed CH4-air flame and within {+-}100 K for a vortex-strained diluted CH4-air diffusion flame where the FRS technique is severely tested by large variation in scattering cross section. In the diffusion flame work, FRS has been combined with Raman imaging of the CH4 fuel molecule to correct for the local light scattering properties of the combustion gases. To our knowledge, this is the first extension of FRS to nonpremixed combustion and the first use of joint FRS-Raman imaging. FRS has been applied to a sooting C2H4-air flame and combined with LII to assess the upper sooting limit where FRS may be utilized. The results from this sooting flame show FRS temperatures has potential for quantitative temperature imaging for soot volume fractions of order 0.1 ppm. FRS velocity measurements have been performed in a Mach 3.7 overexpanded nitrogen jet. The FRS results are in good agreement with expected velocities as predicted by inviscid analysis of the jet flowfield. We have constructed a second FRS opto-electronic system for measurements at Sandia's hypersonic wind tunnel. The details of this second FRS system are provided here. This facility is currently being used for velocity characterization of these production hypersonic facilities.

A particle image velocimetry instrument has been constructed for a transonic wind tunnel and applied to study the interaction created by a supersonic axisymmetric jet exhausting from a flat plate into a subsonic compressible crossflow. Data have been acquired in two configurations; one is a two-dimensional measurement on the streamwise plane along the wind tunnel centerline, and the other is a stereoscopic measurement in the crossplane of the interaction. The presence of the induced counter-rotating vortex pair is clearly visible in both data sets. The streamwise-plane data determined the strength and location of the vortices using the vertical velocity component while the crossplane data directly provided a measurement of the vortical motion. A comparison of the vertical velocity component measured using each configuration showed reasonable agreement.

Particle image velocimetry data have been acquired in the far field of the interaction generated by an overexpanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. Mean velocity fields were found in the streamwise plane along the flowfield centerline for different values of the crossflow Mach number M{sub {infinity}} and the jet-to-freestream dynamic pressure ratio J. The magnitude of the streamwise velocity deficit and the vertical velocity component both decay with downstream distance and were observed to be greater for larger J while M{sub {infinity}} remained constant. Jet trajectories derived independently using the maxima of each of these two velocity components are not identical, but show increasing jet penetration for larger J. Similarity in the normalized velocity field was found for constant J at two different transonic M{sub {infinity}}, but at two lower M{sub {infinity}} the jet appeared to interact with the wall boundary layer and data did not collapse. The magnitude and width of the peak in the vertical velocity component both increase with J, suggesting that the strength and size of the counter-rotating vortex pair increase and, thus, may have a stronger influence on aerodynamic surfaces despite further jet penetration from the wall.

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This paper describes the application of a filtered-Rayleigh-scattering (FRS) instrument for nonintrusive temperature imaging in a vortex-driven diffusion flame. The FRS technique provides quantitative, spatially correlated temperature data without the flow intrusion or time lag associated with physical probes. Use of a molecular iodine filter relaxes the requirement for clean, particulate-free flowfields and offers the potential for imaging near walls, test section windows and in sooty flames, all of which are precluded in conventional Rayleigh imaging, where background interference from these sources typically overwhelms the weak molecular scattering signal. For combustion applications, FRS allows for full-field temperature imaging without chemical seeding of the flowfield, which makes FRS an attractive alternative to other laser-based imaging methods such as planar laser-induced fluorescence (PLIF). In this work, the details of our FRS imaging system are presented and temperature measurements from an acoustically forced diffusion flame are provided. The local Rayleigh cross-section is corrected using Raman imaging measurements of the methane fuel molecule, which are then correlated to other major species using a laminar flamelet approach. To our knowledge, this is the first report of joint Raman/FRS imaging for nonpremixed combustion. Measurements are presented from flames driven at 7.5 Hz, where a single vortex stretches the flame, and at 90 Hz, where two consecutive vortices interact to cause a repeatable strain-induced flame-quenching event.

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Despite many decades of jet-in-crossflow experimentation, a distinct lack of data remains for a supersonic jet exhausting into a subsonic compressible crossflow. The present investigation seeks to address this deficiency by examining the flowfield structure of a Mach 3.73 jet injected transversely from a flat plate into a subsonic compressible freestream. The experimental results described herein include the mean surface pressure field as mapped using static pressure taps on the flat plate and an identification of flow features by employing an oil-based surface flow tracer. The possibility of flow separation within the nozzle itself also is addressed using pressure taps along the nozzle interior wall, as is the asymmetry of the separation line due to the variation of the local backpressure around the perimeter of the nozzle orifice resulting from the jet-in-crossflow interaction. Pressure data both on the flat plate and within the nozzle are presented at numerous angles with respect to the crossflow freestream direction to provide a breadth of measurements throughout the interaction region. Since the data are intended for use in validating computational models, attention is paid to providing details regarding the experimental geometry, boundary conditions, flowfield nonuniformities, and uncertainty analyses. Eight different sets of data are provided, covering a range of values of the jet-to-freestream dynamic pressure ratio from 2.8 to 16.9 and a freestream Mach number range of 0.5 to 0.8.

The present document summarizes the experimental efforts of a three-year study funded under the Laboratory Directed Research and Development program of Sandia National Laboratories. The Innovative Diagnostics LDRD project was designed to develop new measurement capabilities to examine the interaction of a propulsive spin jet in a transonic freestream for a model in a wind tunnel. The project motivation was the type of jet/fin interactions commonly occurring during deployment of weapon systems. In particular, the two phenomena of interest were the interaction of the propulsive spin jet with the freestream in the vicinity of the nozzle and the impact of the spin rocket plume and its vortices on the downstream fins. The main thrust of the technical developments was to incorporate small-size, Lagrangian sensors for pressure and roll-rate on a scale model and include data acquisition, transmission, and power circuitry onboard. FY01 was the final year of the three-year LDRD project and the team accomplished much of the project goals including use of micron-scale pressure sensors, an onboard telemetry system for data acquisition and transfer, onboard jet exhaust, and roll-rate measurements. A new wind tunnel model was designed, fabricated, and tested for the program which incorporated the ability to house multiple MEMS-based pressure sensors, interchangeable vehicle fins with pressure instrumentation, an onboard multiple-channel telemetry data package, and a high-pressure jet exhaust simulating a spin rocket motor plume. Experiments were conducted for a variety of MEMS-based pressure sensors to determine performance and sensitivity in order to select pressure transducers for use. The data acquisition and analysis path was most successful by using multiple, 16-channel data processors with telemetry capability to a receiver outside the wind tunnel. The development of the various instrumentation paths led to the fabrication and installation of a new wind tunnel model for baseline non-rotating experiments to validate the durability of the technologies and techniques. The program successfully investigated a wide variety of instrumentation and experimental techniques and ended with basic experiments for a non-rotating model with jet-on with the onboard jets operating and both rotating and non-rotating model conditions.