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 side wall 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 suggest that zones of high turbulence were affixed to each side wall centered on the cavity lip, with a strip of turbulence stretched out across the cavity shear layer whose intensity was a function of the length-to-width ratio.
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.
Particle image velocimetry measurements have been conducted for supersonic flow over a three-dimensional cavity of variable width using two different experimental configurations. Two-component data were acquired of the entire streamwise extent of the cavity, peering partially into the cavity at an angle, which introduced a perspective bias error in the vertical velocity component. Stereoscopic data at the cavity's aft end were obtained using a more complex camera orientation to see much greater depth of the cavity without introducing perspective error. The data reveal the turbulent shear layer over the cavity and the recirculation region within it. Both the mean structure of the recirculation region and the shear layer turbulence intensity were found to be a function of the length-to-width ratio of the cavity. Large-scale turbulent eddies are prominent within the shear layer but not evident in the recirculation region.
Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes, which creates one of the largest sources of uncertainty in the simulation of energetic material detonation. To close this gap, a novel Multiphase Shock Tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 19%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 were achieved. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30-40% by the presence of the particle field. X-Ray diagnostics have been developed to penetrate the opacity of the flow, revealing the concentrations throughout the particle field as it expands and spreads downstream with time. Furthermore, an X-Ray particle tracking velocimetry diagnostic has been demonstrated to be feasible for this flow, which can be used to follow the trajectory of tracer particles seeded into the curtain. Additional experiments on single spherical particles accelerated behind an incident shock wave have shown that elevated particle drag coefficients can be attributed to increased compressibility rather than flow unsteadiness, clarifying confusing results from the historical database of shock tube experiments. The development of the Multiphase Shock Tube and associated diagnostic capabilities offers experimental capability to a previously inaccessible regime, which can provide unprecedented data concerning particle dynamics of dense gas-solid flows.