Katya Marie Casper

Fluid-structure interactions in compressible cavity flows

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.

High-Speed Schlieren Imaging of Disturbances in a Transitional Hypersonic Boundary Layer.

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.

Toward Transition Statistics Measured on a 7-Degree Hypersonic Cone for Turbulent Spot Modeling.

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 and 8. To relate the intermittent disturbances to the average characteristics of transition on the cone, the statistical distribution of these disturbances must be known. These include the boundarylayer intermittency, burst rate, and average disturbance length. Traditional low-speed methods to characterize intermittency identify only turbulent/nonturbulent regions. However at high M , instability waves become an important part of the transitional region. Algorithms to distinguish instability waves from turbulence in both the pressure and schlieren measurements are being developed and the corresponding intermittency, burst rate, and average burst length of both regions have been provisionally computed for several cases at Mach 5 and 8. Distinguishing instability waves from turbulence gives a better description of the intermittent boundary layer at high M and will allow the fluctuations associated with boundary-layer instabilities to be incorporated into transitional models.

Simultaneous Pressure Measurements and High-Speed Schlieren Imaging of Disturbances in a Transitional Hypersonic Boundary Layer.

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.

Complex Geometry Effects on Subsonic Cavity Flows.

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.

Characterization of freestream disturbances in conventional hypersonic wind tunnels

Although low-disturbance (“quiet”) hypersonic wind tunnels are believed to provide more reliable extrapolation of boundary-layer transition behavior from ground to flight, the presently available q...

Compilation and Analysis of Second Mode Amplitudes on Sharp Cones in Hypersonic Wind Tunnels

This research effort coordinated by the NATO AVT-240 specialists’ group compiles and analyzes second-mode amplitudes on sharp slender cones at 0 degrees angle of attack. The analysis focuses on pressure fluctuations measured with piezoelectric sensors in 11 hypersonic wind tunnels operated by 9 organizations located in 3 NATO countries (Belgium, Germany, and USA) and Japan. The measurements are at freestream Mach numbers between 5 and 14, unit Reynolds numbers Re/m between 1.5 and 16 million per meter, and wall-to total temperature ratios between 0.1 and 0.8. The study shows that second-mode growth rates can be predicted with Parabolized Stability Equations (PSE) over the wide range of conditions. The maximum second-mode amplitudes vary weakly at edge Mach number Me greater than ~5.8, but significantly decrease at lower Me. The maximum N factor envelope from PSE and the measured amplitudes are used to estimate the initial amplitudes A0. At each Mach number, A0 varies approximately as Re/m^(-1). This leads to transition N factors that increase with Re/m. This behavior is consistent with the results from Marineau (AIAA Journal, Vol. 55, No. 2, 2017).