Time-resolved particle image velocimetry was conducted at 40 kHz using a pulse-burst laser in the supersonic wake of a wall-mounted hemisphere. Velocity fields suggest a recirculation region with two lobes, in which flow moves away from the wall near the centerline and recirculates back toward the hemisphere off the centerline, contrary to transonic configurations. Spatio-temporal cross-correlations and conditional ensemble averages relate the characteristic behavior of the unsteady shock motion to the flapping of the shear layer. At Mach 1.5, oblique shocks develop, associated with vortical structures in the shear layer and convect downstream in tandem; a weak periodicity is observed. Shock motion at Mach 2.0 appears somewhat different, wherein multiple weak disturbances propagate from shear-layer turbulent structures to form an oblique shock that ripples as these vortices pass by. Bifurcated shock feet coalesce and break apart without evident periodicity. Power spectra show a preferred frequency of shear-layer flapping and shock motion for Mach 1.5, but at Mach 2.0, a weak preferred frequency at the same Strouhal number of 0.32 is found only for oblique shock motion and not shear-layer unsteadiness.
The mechanism by which aerodynamic effects of jet/fin interaction arise from the flow structure of a jet in crossflow is explored using particle image velocimetry measurements of the crossplane velocity field as it impinges on a downstream fin instrumented with high-frequency pressure sensors. A Mach 3.7 jet issues into a Mach 0.8 crossflow from either a normal or inclined nozzle, and three lateral fin locations are tested. Conditional ensemble-averaged velocity fields are generated based upon the simultaneous pressure condition. Additional analysis relates instantaneous velocity vectors to pressure fluctuations. The pressure differential across the fin is driven by variations in the spanwise velocity component, which substitutes for the induced angle of attack on the fin. Pressure changes at the fin tip are strongly related to fluctuations in the streamwise velocity deficit, wherein lower pressure is associated with higher velocity and vice versa. The normal nozzle produces a counter-rotating vortex pair that passes above the fin, and pressure fluctuations are principally driven by the wall horseshoe vortex and the jet wake deficit. In conclusion, the inclined nozzle produces a vortex pair that impinges the fin and yields stronger pressure fluctuations driven more directly by turbulence originating from the jet mixing.
Fluid-structure interactions were studies on a 7° half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 5 and 8 and in the Purdue Boeing/AFOSR Mach 6 Quiet Tunnel. A thin composite panel was integrated into the cone and the response to boundary-layer disturbances was characterized by accelerometers on the backside of the panel. Here, under quiet-flow conditions at Mach 6, the cone boundary layer remained laminar. Artificially generated turbulent spots excited a directionally dependent panel response which would last much longer than the spot duration.
The mechanism by which aerodynamic effects of jet/fin interaction arise from the flow structure of a jet in crossflow is explored using particle image velocimetry measurements of the crossplane velocity field as it impinges on a downstream fin instrumented with high-frequency pressure sensors. A Mach 3.7 jet issues into a Mach 0.8 crossflow from either a normal or inclined nozzle, and three lateral fin locations are tested. Conditional ensemble-averaged velocity fields are generated based upon the simultaneous pressure condition. Additional analysis relates instantaneous velocity vectors to pressure fluctuations. The pressure differential across the fin is driven by variations in the spanwise velocity component, which substitutes for the induced angle of attack on the fin. Pressure changes at the fin tip are strongly related to fluctuations in the streamwise velocity deficit, wherein lower pressure is associated with higher velocity and vice versa. The normal nozzle produces a counter-rotating vortex pair that passes above the fin, and pressure fluctuations are principally driven by the wall horseshoe vortex and the jet wake deficit. The inclined nozzle produces a vortex pair that impinges the fin and yields stronger pressure fluctuations driven more directly by turbulence originating from the jet mixing.
The spanwise variation of resonance dynamics in the Mach 0.94 flow over a finite-span cavity was explored using stereoscopic time-resolved particle image velocimetry (TR-PIV) and time-resolved pressure sensitive paint (TR-PSP). The TR-PSP data were obtained along the cavity floor, whereas the TR-PIV measurements were made in a planform plane just above the cavity lip line. The pressure data showed relatively coherent distributions across the span. In contrast, the PIV showed a significant variation in resonance dynamics to occur across the span in the plane above the cavity. A substantial influence of the sidewalls appears to stem from spillage vortices. At the first cavity mode frequency, streamwise velocity fluctuations were several times higher near the sidewalls in comparison to the centerline values. Importantly, PSDs of streamwise velocity in the region of the spillage vortices showed a large peak to occur at mode one, indicating velocity fluctuations in these regions can have a preferred frequency. The resonance fluctuations in the velocity fields at modes two and three demonstrated a complex spatial dependence that varied with spanwise location.
The development of the unsteady pressure field on the floor of a rectangular cavity was studied at Mach 0.9 using high-frequency pressure-sensitive paint. Power spectral amplitudes at each cavity resonance exhibit a spatial distribution with an oscillatory pattern; additional maxima and minima appear as the mode number is increased. This spatial distribution also appears in the propagation velocity of modal pressure disturbances. This behavior was tied to the superposition of a downstream-propagating shear-layer disturbance and an upstream-propagating acoustic wave of different amplitudes and convection velocities, consistent with the classical Rossiter model. The summation of these waves generates an interference pattern in the spatial pressure amplitudes and resulting phase velocity of the resonant pressure fluctuations.
Pulse-burst particle image velocimetry has been used to acquire time-resolved data at 37.5 kHz of the flow over a finite-width rectangular cavity at Mach 0.8. Power spectra of the particle image velocimetry data reveal four resonance modes that match the frequencies detected simultaneously using high-frequency wall pressure sensors, but whose magnitudes exhibit spatial dependence throughout the cavity. Spatiotemporal cross correlations of velocity to pressure were calculated after bandpass filtering for specific resonance frequencies. Cross-correlation magnitudes express the distribution of resonance energy, revealing local maxima and minima at the edges of the shear layer attributable to wave interference between downstream-and upstream-propagating disturbances. Turbulence intensities were calculated using a triple decomposition and are greatest in the core of the shear layer for higher modes, where resonant energies ordinarily are lower. Most of the energy for the lowest mode lies in the recirculation region and results principally from turbulence rather than resonance. Together, the velocity-pressure cross correlations and the triple-decomposition turbulence intensities explain the sources of energy identified in the spatial distributions of power spectra amplitudes.
Time-resolved particle image velocimetry recently has been demonstrated in high-speed flows using a pulse-burst laser at repetition rates reaching 50 kHz. Turbulent behavior can be measured at still higher frequencies if the field of view is greatly reduced and lower laser pulse energy is accepted. Current technology allows image acquisition at 400 kHz for sequences exceeding 4,000 frames, but for an array of only 128 × 120 pixels, giving the moniker of “postage-stamp PIV.” The technique has been tested far downstream of a supersonic jet exhausting into a transonic crossflow. Two-component measurements appear valid until 100 kHz at which point a noise floor emerges dependent upon the reduction of peak locking. Stereoscopic measurement offers three-component data for turbulent kinetic energy spectra, but exhibits a reduced signal bandwidth and higher noise in the out-of-plane component due to the oblique camera images. The resulting spectra 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 a decade of mid-range frequencies corresponding to the energetic eddies measured by PIV, which appears to have been previously unrecognized for high-speed free shear flows.
Boundary-layer transition was measured on a pitched, 7° half-angle cone in a Mach 8 conventional wind tunnel. On a smooth cone, transition via second-mode waves was ob- served at all angles of attack. In addition, naturally-excited stationary crossow waves were apparent in temperature sensitive paint images, but did not appear to lead to transition. Two patterns of roughness elements were used to generate higher-amplitude stationary crossow waves. Breakdown of the stationary waves was observed. The roughness resulted in instability amplitudes nearly an order of magnitude larger than the smooth cone at the same Reynolds numbers and higher instability growth rates. Transition occurred 30% - 40% sooner using the roughness elements with peak amplitudes near 15 - 20%, for α ≥ 4°. A low-frequency, coherent wave was measured at all angles of attack. The calculated phase velocity shows a strong dependence on angle of attack, but the propagation angle is similar for all non-zero α. The measured wave properties are curiously similar to measurements of a suspected tunnel-noise-driven instability made on an elliptic cone at Mach 6.
Fluid-structure interactions were studied on a 7 * half-angle cone in the Sandia Hypersonic Wind Tunnel at Mach 8 over a range of freestream Reynolds numbers b etween 3 . 3 and 14 . 5 x 10 6 / m . A thin panel with tunable structural natural frequencies was integrated into the cone and exposed to naturally developing boundary layers. An elevated panel re sponse was measured during boundary- layer transition at frequencies corresponding to the turbu lent burst rate, and lower vibrations were measured under a turbulent boundary layer. Controlled pert urbations from an electrical discharge were then introduced into the boundary layer at varying freq uencies corresponding to the struc- tural natural frequencies of the panel. The perturbations w ere not strong enough to drive a panel response exceeding that due to natural transition. Instead at high repetition rates, the perturber modified the turbulent burst rate and intermittency on the co ne and therefore changed the condi- tions for when an elevated transitional panel vibration res ponse occurred. Studies were also conducted in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. Under quiet flow, natural transition does not occur, and the c ontrolled perturbations are the only disturbance source. A clear panel response to turbulent spo ts created by the controlled pertur- bations was observed at varying frequencies of spot generat ion. The quiet-flow measurements confirm the clear relationship between turbulent spot passa ge and panel vibration.