John F. Henfling

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.

Supersonic Flow over a Finite-Width Rectangular Cavity

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. T...

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.

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.

Width effects in transonic flow over a rectangular cavity

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.

Vortex Structure Produced by a Laterally Inclined Supersonic Jet in Transonic Crossflow

Stereoscopic particle image velocimetry data have been collected for a supersonic jet exhausting into a transonic crossflow from a laterally inclined scarfed nozzle installed flush in a flat plate. Mean velocity fields and their derived vorticity were measured at a single crossplane in the far-field for varied jet-to-freestream dynamic pressure ratio J and nozzle inclination angles of 0,15,30, and 45 deg. The data reveal the induced counterrotating vortex pair and the horseshoe vortex formed as the jet exits the nozzle, both of which appear symmetric for the uncanted nozzle. As the nozzle inclination is increased, the vortex pair becomes tilted opposite the nozzle cant and shifts closer to the wall, where the lower vortex attains a larger strength than its counterpart until it eventually becomes so dominant that the opposite vortex is undetectable. Regardless of nozzle cant, vortex strengths increase with larger J. Vortex distances from the wall increase with J at lower nozzle cants, but at larger inclination, the position of the vortex nearer the wall varies less with J than does its complement. Thus, at a sufficiently large nozzle cant, the jet interaction effectively produces a single vortex whose position does not shift greatly with jet strength.

Flow Separation Inside a Supersonic Nozzle Exhausting into a Subsonic Compressible Crossflow

Surface pressure data have been acquired along the nozzle wall and surrounding the exit plane for an axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible crossflow. These measurements have shown that the backpressure is sufficient to instigate nozzle flow separation under flowfield conditions that may be found in flight. The separation line has been found to be axially asymmetric, which results from the angular variation in the backpressure on the nozzle generated by the jets interaction with the freestream. As either the jet-to-freestream momentum ratio or the crossflow freestream Mach number is independently reduced, the size of the separated flow region becomes larger because the backpressure on the nozzle is increased relative to the jet stagnation pressure. Schlieren imaging is consistent with these observations and provides further elucidation of the resulting jet shock wave structure. Comparison of the data to correlations derived from freejet separation data is possible by employment of these predictions in a piecewise fashion around the perimeter of the nozzle.