Pressure fluctuations beneath instability wave packets and turbulent spots in a hypersonic boundary layer

Katya M Casper, Purdue University

Abstract

During atmospheric reentry, hypersonic vehicles are subjected to high levels of boundary-layer pressure fluctuations that cause vibration of internal components. Current models are not adequate to predict these fluctuations. A more physics-based approach can be obtained by using a turbulent-spot model of transition. In order to gain a better understanding of the pressure-fluctuation field and the growth of turbulent spots in a hypersonic boundary layer, the development of disturbances was studied on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. 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. For a controlled study, disturbances were created by pulsed glow perturbations and studied at various freestream conditions. Both the centerline and the spanwise distribution of pressure fluctuations were measured as boundary-layer disturbances grew from linear instability wave packets into turbulent spots. A disturbance first grows into a linear instability wave packet and then quickly becomes nonlinear. At this point, the wave packet is still concentrated near the disturbance centerline, but weaker disturbances are seen spreading from the center. Throughout the nonlinear growth of the wave packets, large harmonics are visible in the power spectra. Breakdown to turbulence begins in the core of the wave packets where the wave amplitudes are largest. As breakdown begins, the peak amplitudes of the instability waves and harmonics decrease into the rising broadband frequencies. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction at a spreading angle. The turbulent core grows downstream, resulting in a turbulent spot with a typical arrowhead shape. However, the spot is not merely a localized patch of turbulence; instability waves are still an integral part of the disturbance. From these measurements, the convection velocity, spanwise spreading angle, and typical pressure-fluctuation field were obtained for the disturbances. These values as well as the transitional physics learned from this work can be included in engineering models for predicting a flight vehicle's environment.

Degree

Ph.D.

Advisors

Schneider, Purdue University.

Subject Area

Aerospace engineering

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