Publications

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Fluid-structure interactions that occur during aircraft internal store carriage were experimentally explored at Mach 0.58-1.47 using a generic, aerodynamic store installed in a rectangular cavity having a length-To-depth ratio of seven. The store vibrated in response to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance frequencies. Cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas the spanwise response to cavity tones was much more limited. Increased surface area associated with tail fins raised vibration levels. The store had interchangeable components to vary its natural frequencies by about 10-300 Hz. By tuning natural frequencies, mode-matched cases were explored where a prominent cavity tone frequency matched a structural natural frequency of the store. Mode matching in the streamwise and wall-normal directions produced substantial increases in peak store vibrations, though the response of the store remained linear with dynamic pressure. Near mode-matched frequencies, changes in cavity tone frequencies of only 1% altered store peak vibrations by as much as a factor of two. Mode matching in the spanwise direction did little to increase vibrations.

The inherent relationship between boundary-layer stability, aerodynamic heating, and surface conditions makes the potential for interaction between the structural response and boundary-layer transition an important and challenging area of study in high-speed flows. This paper phenomenologically explores this interaction using a fundamental two-dimensional aerothermoelastic model under the assumption of an aluminum panel with simple supports. Specifically, an existing model is extended to examine the impact of transition onset location, transition length, and transitional overshoot in heat flux and fluctuating pressure on the structural response of surface panels. Transitional flow conditions are found to yield significantly increased thermal gradients, and they can result in higher maximumpanel temperatures compared to turbulent flow. Results indicate that overshoot in heat flux and fluctuating pressure reduces the flutter onset time and increases the strain energy accumulated in the panel. Furthermore, overshoot occurring near the midchord can yield average temperatures and peak displacements exceeding those experienced by the panel subject to turbulent flow. These results suggest that fully turbulent flow does not always conservatively predict the thermo-structural response of surface panels.

The influence of compressibility on the shear layer over a rectangular cavity of variable width has been studied at a freestream Mach number range of 0.6 to 2.5 using particle image velocimetry data in the streamwise center plane. As the Mach number increases, the vertical component of the turbulence intensity diminishes modestly in the widest cavity, but the two narrower cavities show a more substantial drop in all three components as well as the turbulent shear stress. This contrasts with canonical free shear layers, which show significant reductions in only the vertical component and the turbulent shear stress due to compressibility. The vorticity thickness of the cavity shear layer grows rapidly as it initially develops, then transitions to a slower growth rate once its instability saturates. When normalized by their estimated incompressible values, the growth rates prior to saturation display the classic compressibility effect of suppression as the convective Mach number rises, in excellent agreement with comparable free shear layer data. The specific trend of the reduction in growth rate due to compressibility is modified by the cavity width.

In previous studies, complex cavity geometries showed higher amplitude and more three- dimensional pressure fields than simple rectangular cavities. However, those studies relied on twenty point measurements within the cavity. To further understand the development of the pressure field within complex bays, high-frequency pressure-sensitive paint (PSP) was applied to the floor of an L/D = 7 complex cavity at Mach 0.9; unsteady pressure measurements were obtained at 10 kHz. Power spectra of the PSP measurements have a spatial distribution at each cavity resonance frequency with an oscillatory pattern; additional maxima and minima appear as the mode number is increased. This behavior was tied to the superposition of a downstream propagating shear-layer disturbance and an upstream propagating acoustic wave of different amplitudes, consistent with the classical Rossiter model. Complex geometries added spanwise asymmetries to the spatial pattern and amplified specific modes. These spatially dependent features of the pressure field might be missed by point measurements of the pressure field.

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 between 3.3 and 14.5 × 106/m. A thin panel with tunable structural natural frequencies was integrated into the cone and exposed to naturally developing boundary layers. An elevated panel response was measured during boundary-layer transition at frequencies corresponding to the turbulent burst rate, and lower vibrations were measured under a turbulent boundary layer. Controlled perturbations from an electrical discharge were then introduced into the boundary layer at varying frequencies corresponding to the structural natural frequencies of the panel. The perturbations were 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 cone and therefore changed the conditions for when an elevated transitional panel vibration response occurred.

The flow over an aircraft bay is often represented using a rectangular cavity; however, this simplification neglects many features of actual flight geometry that 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, internal cavity structure, and doors. A parametric study of these features was conducted based on fluctuating pressure measurements at subsonic and supersonic Mach numbers. Resonance frequencies and amplitudes increased in the complex geometry compared to a simple rectangular cavity that could produce severe loading conditions for store carriage. High-frequency content and dominant frequencies were generated by features that constricted the flow such as leading-edge overhangs, internal cavity variations, and the presence of closed doors. Broadband frequency components measured at the aft wall of the complex cavities were also significantly higher than in the rectangular geometry. These changes highlight the need to consider complex geometric effects when predicting the flight loading of aircraft bays.

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Experiments were performed to understand the complex fluid-structure interactions that occur during aircraft internal store carriage. A cylindrical store was installed in a rectangular 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 to 2.5 and the incoming boundary layer was turbulent. Fast-response pressure measurements provided aeroacoustic loading in the cavity, while triaxial accelerometers 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, and it exhibited a directionally dependent response to cavity resonance. Specifically, cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas a spanwise response was observed only occasionally. The streamwise and wall-normal responses were attributed to the longitudinal pressure waves and shear layer vortices known to occur during cavity resonance. Although the spanwise response to cavity tones was limited, broadband pressure fluctuations resulted in significant spanwise accelerations at store natural frequencies. The largest vibrations occurred when a cavity tone matched a structural natural frequency, although energy was transferred more efficiently to natural frequencies having predominantly streamwise and wall-normal motions.

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The flow over an open aircraft bay is often represented in a wind tunnel with a cavity. In flight, this flow is unconfined, though in experiments, the cavity is surrounded by wind tunnel walls. If untreated, wind tunnel wall effects can lead to significant distortions of cavity acoustics in subsonic flows. To understand and mitigate these cavity–tunnel interactions, a parametric approach was taken for flow over an L/D = 7 cavity at Mach numbers 0.6–0.8. With solid tunnel walls, a dominant cavity tone was observed, likely due to an interaction with a tunnel duct mode. An acoustic liner opposite the cavity decreased the amplitude of the dominant mode and its harmonics, a result observed by previous researchers. Acoustic dampeners were also placed in the tunnel sidewalls, which further decreased the dominant mode amplitudes and peak amplitudes associated with nonlinear interactions between cavity modes. This indicates that cavity resonance can be altered by tunnel sidewalls and that spanwise coupling should be addressed when conducting subsonic cavity experiments. Though mechanisms for dominant modes and nonlinear interactions likely exist in unconfined cavity flows, these effects can be amplified by the wind tunnel walls.

Sandia’s Hypersonic Wind Tunnel (HWT) became operational in 1962, providing a test capability for the nation’s nuclear weapons complex. The first modernization program was completed in 1977. A blowdown facility with a 0.46-m diameter test section, the HWT operates at Mach 5, 8, and 14 with stagnation pressures to 21 MPa and temperatures to 1400K. Minimal further alteration to the facility occurred until 2008, but in recent years the HWT has received considerable investment to ensure its viability for at least the next 25 years. This has included reconditioning of the vacuum spheres, replacement of the high-pressure air tanks for Mach 5, new compressors to provide the high-pressure air, upgrades to the cryogenic nitrogen source for Mach 8 and 14, an efficient high-pressure water cooling system for the nozzle throats, and refurbishment of the electric-resistance heaters. The HWT is now returning to operation following the largest of the modernization projects, in which the old variable transformer for the 3-MW electrical system powering the heaters was replaced with a silicon-controlled rectifier power system. The final planned upgrade is a complete redesign of the control console and much of the gas-handling equipment.

The flow over aircraft bays are often represented using 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, internal cavity variations, and doors. A parametric study of these features was conducted at subsonic Mach numbers. Increased higher frequency content and higher-amplitude fluctuations were found in the complex geometry that could produce severe loading conditions for stores carried within the bays. High-frequency content was generated by features that constricted the flow such as leading edge overhangs, internal cavity variations, and the presence of closed doors. Also, the Rossiter modes of the complex configurations were usually shifted in frequency from the simple rectangular cavity, and many modes had much higher amplitudes. Broadband frequency components measured at the aft wall of the complex cavities were also significantly higher than in the rectangular geometry. These changes highlight the need to consider complex geometric effects when predicting the flight loading of aircraft bays.

The flow over aircraft bays exhibits many characteristics of cavity flows, namely resonant pressures that can create high structural loading. An extensive dataset of pressure measurements within both simple and complex cavities was previously obtained and analyzed using power-spectral densities, coherence levels, and cross correlations between sensor pairs within the cavity. More in-depth analysis of the flow structure is studied here using modal decomposition techniques. Both Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) were applied to the experimental and computational results within a simple rectangular cavity. POD was able to show that the cavity modes are coherent across the cavity width. Only higher modes that were associated with more turbulent fluctuations exhibited spanwise variations. These were concentrated at the aft end of the cavity. DMD was able to isolate structures associated with single frequencies in the flow. At the Rossiter frequencies, coherent structures across the front cavity width were found, while more complex shapes were observed at the cavity rear, consistent with the POD analysis. Additional DMD modes in between the dominant Rossiter frequencies also appeared. These additional modes were associated with a low-frequency modulation of the cavity tones.

Particle image velocimetry (PIV) measurements quantified the coherent structure of acoustic tones in a Mach 0.91 cavity flow. Stereoscopic PIV measurements were performed at 10-Hz and two-component, time-resolved data were obtained using a pulse-burst laser. The cavity had a square planform, a length-to-depth ratio of five, and an incoming turbulent boundary layer. Simultaneous fast-response pressure signals were bandpass filtered about each cavity tone frequency. The 10-Hz PIV data were then phase-averaged according to the bandpassed pressures to reveal the flow structure associated with the resonant tones. The first Rossiter mode was associated with large scale oscillations in the shear layer, while the second and third modes contained organized structures consistent with convecting vortical disturbances. The spatial wavelengths of the cavity tones, based on the vertical coherent velocity fields, were less than those predicted by the Rossiter relation. With increasing streamwise distance the spacing between structures increased and approached the predicted Rossiter value at the aft-end of the cavity. Moreover, the coherent structures appeared to rise vertically with downstream propagation. The time-resolved PIV data were bandpass filtered about the cavity tone frequencies to reveal flow structure. The resulting spacing between disturbances was similar to that in the phase-averaged flowfields.

Fluid-structure interactions that occur during aircraft internal store carriage were experimentally explored at Mach 0.94 and 1.47 using a generic, aerodynamic store installed in a rectangular cavity having a length-to-depth ratio of 7. Similar to previous studies using a cylindrical store, the aerodynamic store responded to the cavity flow at its natural structural frequencies, and it exhibited a directionally dependent response to cavity resonance. Cavity tones excited the store in the streamwise and wall-normal directions consistently, whereas the spanwise response was much more limited. The store had interchangeable components to vary its natural frequencies by about 10 - 300 Hz. By tuning natural frequencies, mode-matched cases were explored where a prominent cavity tone frequency matched a structural natural frequency of the store. Mode matching produced substantial increases in store vibrations, though the response of the store continued to scale linearly with the dynamic pressure or loading in the bay. Near mode matching frequencies, the response of the store was quite sensitive as changes in cavity tone frequencies of 1% altered store vibrations by as much as a factor of two.

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To investigate the pressure-fluctuation field beneath turbulent spots in a hypersonic boundary layer, a study was conducted on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. Controlled disturbances were created by pulsed-glow perturbations based on the electrical breakdown of air. Under quiet-flow conditions, the nozzle-wall boundary layer remains laminar and grows very thick over the long nozzle length. This allows the development of large disturbances that can be well-resolved with high-frequency pressure transducers. A disturbance first grows into a second-mode instability wavepacket that is concentrated near its own centreline. Weaker disturbances are seen spreading from the centre. The waves grow and become nonlinear before breaking down to turbulence. The breakdown begins in the core of the packets where the wave amplitudes are largest. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction. The turbulent core grows downstream, resulting in a spot with a classical arrowhead shape. Behind the spot, a low-pressure calmed region develops. However, the spot is not merely a localized patch of turbulence; instability waves remain an integral part. Limited measurements of naturally occurring disturbances show many similar characteristics. From the controlled disturbance measurements, the convection velocity, spanwise spreading angle, and typical pressure-fluctuation field were obtained.

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