Publications

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The flow over aircraft bays exhibits many characteristics of cavity flows, namely resonant pressures that can create high structural loading. Most studies have represented these bays as rectangular cavities; however, this simplification neglects many features of the actual flight geometry which could affect the unsteady pressure field and resulting loading in the bay. To address this shortcoming, a complex cavity geometry was developed to incorporate more realistic aircraft-bay features including shaped inlets and internal cavity variations. A parametric study of these features at Mach 1.5, 2.0, and 2.5 was conducted to identify key differences from simple rectangular cavity flows. The frequency of the basic rectangular cavity modes could be predicted by theory; however, most complex geometries shifted these frequencies. Geometric changes that constricted the flow tended to enhance cavity modes and create higher pressure fluctuations. Other features, such as a leading edge ramp, lifted the shear layer higher with respect to the aft cavity wall and led to cavity tone suppression. Complex features that introduced spanwise non-uniformity into the shear layer also led to a reduction of cavity tones, especially at the aft end of the cavity.

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Experiments were conducted at freestream Mach numbers of 0.55, 0.80, and 0.90 in open cavity flows having a length-to-depth ratio L/D of 5 and an incoming turbulent boundary having a thickness of about 0.5D. To ascertain aspect ratio effects, the length-to-width ratio L/W was varied between 1.00, 1.67, and 5.00. Two stereoscopic PIV systems were used simultaneously to characterize the flow in the plane at the spanwise center of the cavity. For each aspect ratio, trends in the mean and turbulence fields were identified, regardless of Mach number. The recirculation region had the weakest reverse velocities in the L/W = 1.67 cavity, a trend previously observed at supersonic Mach numbers. Also, like the previous supersonic experiments, the L/W = 1.00 and L/W = 5.00 mean streamwise velocities were similar. The L/W = 1.00 cavity flows had the highest turbulence intensities, whereas the two narrower cavities exhibited lower turbulence intensities of a comparable level. This is in contrast to previous supersonic experiments, which showed the lowest turbulence levels in the L/W = 1.67 cavity.

Data have been acquired from a spanwise array of fluctuating wall pressure sensors beneath a wind tunnel wall boundary layer at Mach 2, then invoking Taylor's hypothesis allows the temporal signals to be converted into a spatial map of the wall pressure field. Different frequency ranges of pressure fluctuations may be accessed by bandpass filtering the signals. In all frequency ranges, this reveals signatures of coherent structures where negative pressure events are interspersed amongst positive events, with some degree of alternation in the streamwise direction. Within lower frequency ranges, streaks of instantaneously correlated pressure fluctuations elongated in the streamwise direction exhibit a spanwise meander and show apparent merging of pressure events. Coherent length scales based on single-sensor correlations are artificially shortened by neglecting this meander and merging, but are captured correctly using the sensor array. These measurements are consistent with similar observations by other researchers in the velocity field above the wall, and explain the presence of the flat portion of the wall pressure spectrum at frequencies well below those associated with the boundary layer thickness. However, the pressure data lack the common spanwise alternation of positive and negative events found in velocity data, and conversely demonstrate a weak positive correlation in the spanwise direction at low frequencies. © 2013 AIP Publishing LLC.

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.

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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 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.

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 flow induced by incident shocks at Mach numbers 1.68, 1.93, and 2.04. The necessary data accuracy is obtained by accounting for the shock tube wall boundary layer growth and avoiding interactions between multiple particles. Similar to past experiments, the current data clearly show that as the Mach number increases, the drag coefficient increases substantially. This increase significantly exceeds the drag predicted by incompressible standard drag models, but a recently developed compressible drag correlation returns values quite close to the current measurements. Recent theoretical work and low particle accelerations indicate that unsteadiness should not be expected to contribute to the drag increase over the relatively long time scales of the experiments. These observations suggest that elevated particle drag coefficients are a quasi-steady phenomenon attributed to increased compressibility rather than true flow unsteadiness. © 2012 American Institute of Physics.

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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.

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