Publications

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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.

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 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.

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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.

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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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