Steven J. Beresh

Penetration of a Transverse Supersonic Jet into a Subsonic Compressible Crossflow

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∞ 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∞ 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∞ ,b utat two lower M∞ 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.

Crossplane Velocimetry of a Transverse Supersonic Jet in a Transonic Crossflow

Stereoscopic particle image velocimetry has been employed to study the interaction created by a supersonic axisymmetric jet exhausting transversely from a flat plate into a transonic crossflow. Data have been acquired in the crossplane of the interaction at a single station in the far field, from which the velocity field identifies 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 ofthejet-to-freestream dynamic pressure ratio reveal the resulting change in the vortex characteristics, where the vortex strength, size, and position are established from the derived vorticity field. Sufficient data were acquired at one condition to determine all six unique components of the turbulent stress tensor, providing the mean spatial character of the anisotropic turbulence. A measureable degree of asymmetry is observed in the size and lateral position of the counter-rotating vortex pair. Self-similarity is established laterally when dimensions are scaled by either the vortex diameter or the horizontal vortex spacing; in the wall-normal direction, scaling is found with the vortex diameter once the vortex penetration is accounted.

Fluctuating wall pressures measured beneath a supersonic turbulent boundary layer

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 data 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 ω−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 ω2 dependence. When normalized by outer flow variables, a slight dependence upon the Reynolds number is detected, but Mach number is the dominant parameter. Normalization by inner flow variables is largely successful for the ω−1 region but does not apply for lower frequencies. A comparison of the pressure fluctuation intensities with 50 years of historic...

Turbulent Characteristics of a Transverse Supersonic Jet in a Subsonic Compressible Crossflow

Fluctuating velocity fields have been determined from particle image velocimetry data acquired in the farfield of the interaction generated by an overexpanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. Peak magnitudes of the turbulent stresses were found to be larger and located further from the wall for greater values of the jet-to-freestream dynamic pressure ratio J while the crossflow Mach number M∞ remained constant. These stress magnitudes diminish with downstream distance as their peak location moves further from the wall. The vertical positions of the peak normal stresses and shear stress inflection point coincide with the maximum mean streamwise velocity deficit induced by the jet. Instantaneous realizations of the velocity fluctuation fields reveal large-scale structures whose mean diameter is greater for larger J and decreases with downstream distance. The integral length scale calculated from profiles of the correlation coefficient instead shows an increase downstream; the discrepancy between the two length scales results from a low-pass filter effect of the correlation coefficient. Similarity was demonstrated for constant J at two transonic M∞, but not at two lower M∞ where the flowfield does not distinguish between the jet and its wake.

Interaction of a planar shock wave with a dense particle curtain: Modeling and experiments

The interaction of a planar shock wave with a dense particle curtain is investigated through modeling and experiments. The physics in the interaction between a shock wave with a dense gas-particle mixture is markedly differently from that with a dilute mixture. Following the passage of the shock wave, the dense particle curtain expands rapidly as it propagates downstream and pressures equilibrate throughout the flow field. In the simulations, the particles are viewed as point-particles and are traced in a Lagrangian framework. A physics-based model is then developed to account for interphase coupling. Compared to the standard drag law, four major improvements are made in the present interphase coupling model to take into account: (1) unsteady force contributions to particle force; (2) effect of compressibility on hydrodynamic forces; (3) effect of particle volume fraction on hydrodynamic forces; (4) effect of inter-particle collision. The complex behavior of the dense particle curtain is due to the interp...

Comparison of Experimentally Measured and Computed Second-Mode Disturbances in Hypersonic Boundary-Layers

Abstract : Experiments were carried out in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University and the Sandia National Laboratories Hypersonic Wind Tunnel at Mach 5 and 8. The purpose was to measure second-mode boundary-layer instabilities on a 7 deg half-angle cone at zero angle of attack using surface pressure sensors. Second-mode waves were successfully measured at all three Mach numbers, including under both noisy and quiet conditions. The most amplified second-mode disturbance frequencies compared well to the linear Parabolized Stability Equations computed by the STABL software suite. The eigenfunctions are also reported, to aid in the development of new instrumentation methods. The e(expn N) method is used in an attempt to determine the N factor for transition onset prediction in both tunnels.

Temperature imaging in nonpremixed flames by joint filtered Rayleigh and Raman scattering

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.

Pulse-Burst PIV in a High-Speed Wind Tunnel.

Time-resolved particle image velocimetry (TR-PIV) has been achieved in a high-speed wind tunnel, providing velocity field movies of compressible turbulence events. The requirements of high-speed flows demand greater energy at faster pulse rates than possible with the TR-PIV systems developed for low-speed flows. This has been realized using a pulse-burst laser to obtain movies at up to 50 kHz, with higher speeds possible at the cost of spatial resolution. The constraints imposed by use of a pulse-burst laser are limited burst duration of 10.2 ms and a low duty cycle for data acquisition. Pulse-burst PIV has been demonstrated in a supersonic jet exhausting into a transonic crossflow and in transonic flow over a rectangular cavity. The velocity field sequences reveal the passage of turbulent structures and can be used to find velocity power spectra at every point in the field, providing spatial distributions of acoustic modes. The present work represents the first use of TR-PIV in a high-speed ground-test facility.

Shock Tube Investigation of Unsteady Drag in Shock- Particle Interactions

A reassessment of historical drag coefficient data for spherical particles accelerated in shock-induced flows has motivated new shock tube experiments of particle response to the passage of a normal shock wave. Particle drag coefficients were measured by tracking the trajectories of 1-mm spheres in the wake of incident shocks of Mach numbers 1.68, 1.93, and 2.05. Data clearly show that as the Mach number increases, the drag coefficient increases substantially, consistent with past experiments. This increase significantly exceeds the drag predicted by incompressible standard drag models, but recently developed compressible drag models return values quite close to the current measurements. Low values for the acceleration parameter indicate that unsteadiness should not be expected to contribute to the drag increase. These observations suggest that elevated particle drag coefficients can be attributed to increased compressibility rather than flow unsteadiness.

Fluid-structure interactions in compressible cavity flows

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.