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Boundary-layer receptivity due to distributed surface imperfections of a deterministic or random natureAcoustic receptivity of a Blasius boundary layer in the presence of distributed surface irregularities is investigated analytically. It is shown that, out of the entire spatial spectrum of the surface irregularities, only a small band of Fourier components can lead to an efficient conversion of the acoustic input at any given frequency to an unstable eigenmode of the boundary layer flow. The location, and width, of this most receptive band of wavenumbers corresponds to a relative detuning of O(R sub l.b.(exp -3/8)) with respect to the lower-neutral instability wavenumber at the frequency under consideration, R sub l.b. being the Reynolds number based on a typical boundary-layer thickness at the lower branch of the neutral stability curve. Surface imperfections in the form of discrete mode waviness in this range of wavenumbers lead to initial instability amplitudes which are O(R sub l.b.(exp 3/8)) larger than those caused by a single, isolated roughness element. In contrast, irregularities with a continuous spatial spectrum produce much smaller instability amplitudes, even compared to the isolated case, since the increase due to the resonant nature of the response is more than that compensated for by the asymptotically small band-width of the receptivity process. Analytical expressions for the maximum possible instability amplitudes, as well as their expectation for an ensemble of statistically irregular surfaces with random phase distributions, are also presented.
Document ID
19920017438
Acquisition Source
Legacy CDMS
Document Type
Contractor Report (CR)
Authors
Choudhari, Meelan (High Technology Corp. Hampton, VA, United States)
Date Acquired
September 6, 2013
Publication Date
May 1, 1992
Publication Information
Publisher: NASA
Subject Category
Fluid Mechanics And Heat Transfer
Report/Patent Number
NASA-CR-4439
NAS 1.26:4439
Accession Number
92N26681
Funding Number(s)
CONTRACT_GRANT: NAS1-18240
PROJECT: RTOP 537-03-23-03
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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