Effect of 3D Roughness Patch on Instability Amplification in a Supersonic Boundary Layer


Surface roughness is known to have a substantial impact on the aerothermodynamic loading of high-speed vehicles, particularly via its influence on the laminar-turbulent transition process within the boundary layer. Numerical simulations are performed to investigate the effects of a distributed region of densely packed, sinusoidal shape roughness elements on a Mach 3.5 flat plate boundary layer for flow conditions corresponding to the planned conditions of an upcoming experiment in the Mach 3.5 Supersonic Low Disturbance Tunnel at the NASA Langley Research Center. Analysis of convective instabilities in the wake of the roughness patch was reported in a previous paper and the current work extends that analysis to instability amplification across the length of the roughness patch. Quasiparallel stability analysis of the modified boundary layer flow over the patch indicates two dominant families of unstable disturbances, namely, a group of high frequency modes that amplify in localized regions along the roughness patch and another group of low frequency modes that have smaller peak amplification rates but amplify steadily both above the roughness patch and in the wake region behind it. The results suggest that the amplification factors associated with the high-frequency modes are sufficiently low, at least for the roughness patches considered in this paper, so that these modes are unlikely to have a major influence on the transition process. The amplification of the low-frequency modes within the region of the roughness patch is further quantified via direct numerical simulations. Results confirm the strongly destabilizing influence of the roughness patch on the first mode instabilities, yielding an N-factor increment of ΔN ≈ 3.6 for a roughness patch length of eight wavelengths.

Meeting Name

AIAA Scitech Forum, 2019 (2019: Jan. 7-11, San Diego, CA)


Mechanical and Aerospace Engineering

Keywords and Phrases

Amplification; Aviation; Boundary layer flow; NASA; Numerical models; Supersonic aircraft; Surface roughness; Thermodynamics; Wakes, American Institute of Aeronautics and Astronautics; Convective instabilities; Destabilizing influences; Flat plate boundary layers; German aerospace centers; Laminar turbulent transitions; NASA Langley Research Center; Supersonic boundary layer, Laminar boundary layer

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Article - Conference proceedings

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© 2019 German Aerospace Center (DLR), All rights reserved.

Publication Date

01 Jan 2019