Particle image velocimetry measurements have been conducted for a Mach 0.8 flow over a wall-mounted hemisphere. The flow is strongly separated, with a mean recirculation length exceeding 5 δ and a mean reverse velocity of -0.2 U∞. The shock foot was found to typically sit just forward of the apex of the hemisphere and move within a range of about ±10 deg. Conditional averages based upon the shock foot location show that the separation shock is positioned upstream along the hemisphere surface when reverse velocities in the recirculation region are strong and is located downstream when they are weaker. The recirculation region appears smaller when the shock is located farther downstream. No correlation was detected of the incoming boundary layer with the shock position, nor with the wake recirculation velocities. These observations are consistent with recent studies concluding that for large strong separation regions, the dominant mechanism is the instability of the separated flow rather than a direct influence of the incoming boundary layer.
Experiments were performed to understand the complex fluid-structure interactions that occur during internal store carriage. A cylindrical store was installed in a 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 - 2.5 and the incoming turbulent boundary layer thickness was about 30-40% of the cavity depth. Fast-response pressure measurements provided aeroacoustic loading in the cavity, while triaxial accelerometers and laser Doppler vibrometry 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, as previously determined with modal hammer tests, and it exhibited a directional dependence to cavity resonance. Specifically, cavity tones excited the store in the streamwise and wall-normal directions consistently, while a spanwise response was observed only occasionally. The streamwise and wall-normal responses were attributed to the known pressure gradients in these directions. Furthermore, spanwise vibrations were greater at the downstream end of the cavity, attributable to decreased levels of flow coherence near the aftwall. Collectively, the data indicate the store response to be dependent on direction of vibration and position along the length of the store.
A high-speed schlieren system was developed for the Sandia Hypersonic Wind Tunnel. Schlieren images were captured at 290 kHz and used to study the growth and breakdown of second-mode instabilities into turbulent spots on a 7° cone. At Mach 5, wave packets would intermittently occur and break down into isolated turbulent spots surrounded by an otherwise smooth, laminar boundary layer. At Mach 8, the boundary layer was dominated by second-mode instabilities which would break down into larger regions of turbulence. Second-mode waves surrounded these turbulent patches as opposed to the smooth laminar flow seen at Mach 5. Detailed pressure and thermocouple measurements were also made along the cone at Mach 5, 8 and 14, in a separate tunnel entry. These measurements give an average picture of the transition behavior that complements the intermittent behavior captured by the schlieren system. At Mach 14, the boundary-layer remained laminar so the transition process could not be studied. However, the first measurements of second-mode waves were made in HWT-14.
High-frequency pressure sensors were used in conjunction with a high-speed schlieren system to study the growth and breakdown of boundary-layer disturbances into turbulent spots on a 7° cone in the Sandia Hypersonic Wind Tunnel. At Mach 5, intermittent low-frequency disturbances were observed in the schlieren videos. High-frequency secondmode wave packets would develop within these low-frequency disturbances and break down into isolated turbulent spots surrounded by an otherwise smooth, laminar boundary layer. Spanwise pressure measurements showed that these packets have a narrow spanwise extent before they break down. The resulting turbulent fluctuations still had a streaky structure reminiscent of the wave packets. At Mach 8, the boundary layer was dominated by secondmode instabilities that extended much further in the spanwise direction before breaking down into regions of turbulence. The amplitude of the turbulent pressure fluctuations was much lower than those within the second-mode waves. These turbulent patches were surrounded by waves as opposed to the smooth laminar flow seen at Mach 5. At Mach 14, second-mode instability wave packets were also observed. Theses waves had a much lower frequency and larger spanwise extent compared to lower Mach numbers. Only low freestream Reynolds numbers could be obtained, so these waves did not break down into turbulence.
Stereoscopic Particle Image Velocimetry data of a trailing vortex shed from a tapered fin installed on a wind-tunnel wall have been analyzed to provide turbulent statistics. After correcting for the effects of vortex meander, the radial and azimuthal turbulent normal stresses are smallest at the vortex center, reaching a maximum around its periphery to produce an annulus of turbulence. Conversely, the streamwise turbulent stress peaks at the vortex center. The ringed turbulent structure is consistent with rotation stabilizing the flow in the vortex core, whereas a fluctuating axial velocity contributes to vortex decay. All three turbulent normal stresses decay with downstream distance. Turbulent shear stresses also decay with downstream distance but possess a relatively small magnitude, suggesting minimal coupling between turbulent velocity components. The vortex turbulence is strongly anisotropic in a manner that varies greatly with spatial position. As the vortex strength is reduced, the axial turbulent normal stress diminishes more sharply than the two cross-plane turbulent normal stresses, possibly because the latter components are influenced by external turbulence spiraling towards the vortex core. The turbulent shear stresses do not show discernable reductions in magnitude with lower vortex strength.
The longitudinal merging of wave packets and turbulent spots in a hypersonic boundary layer was studied on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. Two pulsed glow perturbations were created in rapid succession to generate two closely spaced disturbances. The time between the perturbations was varied from run to run to simulate longitudinal merging. Preliminary results suggest that the growth of the trailing distur- bance seems to be suppressed by the presence of the leading disturbance. Conversely, the core of the leading disturbance appears unaffected by the presence of the trailing distur- bance and behaves as if isolated. This result is consistent with low-speed studies as well as DNS computations of longitudinal merging. However, the present results may be influ- enced by the perturber performance and therefore further studies of longitudinal merging are necessary to confirm the effect on the internal pressure structure of the interacting disturbances.
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.
