“Instability characteristics of cooled hypersonic boundary layers”
Publication Type:
Conference
Authors:
Co-Authors:
D. Gaitonde
Year Published:
2020
Abstract:
Mack mode or second mode instability plays a critical role in hypersonic boundary layer (HBL) transition. The sensitivity of Mack modes to wall thermal conditions, specifically cooling, is of great interest, since it can destabilize this mode. In this work, we utilize linear stability theory (LST) and direct numerical simulations (DNS), to characterize the behavior of Mack mode instability in a Mach 6 flat plate boundary layer, with varying degrees of wall cooling. LST highlights the shift of second mode instability from mode~S to mode~F, as the wall is progressively cooled. This is followed by the mode~F merging with the slow continuous acoustic spectra, resulting in supersonic phase speeds in the instability waves, inducing it to radiate into the freestream. A physical analysis of the eigenmodes of this radiating instability wave shows that the oscillatory component outside the HBL contains comparable amounts of vorticity and acoustic fluctuations. Corresponding DNSs capture the radiating waves, and the net outward acoustic flux of energy is quantified. Although the radiated energy-flux increases monotonically with wall cooling, the perturbation amplitudes over the wall are not abated, due to the trapped component of second mode oscillations.
Conference Name:
AIAA Science and Technology Forum and Exposition 2020
Conference Location:
Orlando, FL
Other Numbers:
Refereed Designation:
Date Published:
1/5/2020