Subsonic Compressible Crossflow Research Articles

Turbulent eddies in a compressible jet in crossflow measured using pulse-burst particle image velocimetry

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.

Turbulent Characteristics of a Transverse Supersonic Jetin 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.

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.

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 jet's 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.