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Acoustic excitation of Tollmien-Schlichting waves due to localised surface roughness

Placidi, Marco, Gaster, Michael and Atkin, Chris J. (2020) Acoustic excitation of Tollmien-Schlichting waves due to localised surface roughness Journal of Fluid Mechanics.

JFM-19-RP-1198.R3.pdf - Accepted version Manuscript

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Experiments on the receptivity of two-dimensional boundary layers to acoustic disturbances from two-dimensional roughness strips were performed in a low-turbulence wind tunnel on a at plate model. The freestream was subjected to a plane acoustic wave so that a Stokes Layer (SL) was created on the plate, thus generating a Tollmien-Schlichting (T-S) wave through the receptivity process. An improved technique to measure the T-S component is described based on a retracting two-dimensional roughness, which allowed for phase-locked measurements at the acoustic wave frequency to be made. This improved technique enables both protuberances and cavities to be explored in the range 30�m < jhj < 750�m (equivalent to 0:025 < jhj=��B < 0:630 in relative roughness height to the local unperturbed Blasius boundary layer displacement thickness). These depths are designed to cover both the predicted linear and non-linear response of the T-S excitation. Experimentally, cavities had not previously been explored. Results show that a linear regime is identifiable for both positive and negative roughness heights up to � 150 �m (jhj=��B � 0:126). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of signi�cant depth, the non-linear behaviour is found to be milder than in the case of protuberances - this is attributed to the flow physics in the near field of the surface features. Nonetheless, results for positive heights agree well with previous theoretical work which predicted a linear disturbance response for small-height perturbations.

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Mechanical Engineering Sciences
Authors :
Gaster, Michael
Atkin, Chris J.
Date : 21 May 2020
Funders : EPSRC, Airbus Central R & T
DOI : 10.1017/jfm.2020.349
Grant Title : EPSRC Grant
Copyright Disclaimer : © The Author(s), 2020. Published by Cambridge University Press
Depositing User : James Marshall
Date Deposited : 29 May 2020 15:45
Last Modified : 30 Oct 2020 02:08

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