Spectral Characteristics of the Near-Wall Turbulence in an Unsteady Channel Flow

[+] Author and Article Information
Sedat F. Tardu

 Laboratoire des Ecoulements Géophysiques et Industriels, B.P. 53 X, 38041 Grenoble, Francesedat.tardu@hmg.inpg.fr

J. Appl. Mech 74(1), 172-175 (Nov 29, 2005) (4 pages) doi:10.1115/1.2166650 History: Received October 04, 2004; Revised November 29, 2005


The modulation characteristics of the turbulent wall shear stress and longitudinal intensities in the inner layer are experimentally investigated in an unsteady channel flow wherein the centerline velocity varies in time in a sinusoidal manner. The fluctuating wall shear stress and velocity signals are temporally filtered and subsequently phase averaged. It is shown that the outer structures corresponding to the low spectrum range have a constant time lag with respect to the centerline velocity modulation. The inner active structures, in particular those with a frequency band containing the mean ejection frequency of the corresponding steady flow dominate the dynamics of the near-wall unsteady turbulence. The structures respond to the imposed shear oscillations in a complex way, depending both on their characteristic scales and the thickness of the oscillating shear zone in which they are embedded.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Phase shift and corresponding time lag of the modulation of the longitudinal turbulent intensity depicted by Filter 0 at several wall normal positions in the inner layer

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Figure 2

Modulation characteristics of the inner structures depicted by Filter 1. (a) Time lag, (b) critical nondimensional frequency and wall normal distance beyond which the phase shift is constant (left) together with the critical Stokes length (right) related to the inner structures 2 (see the text).

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Figure 3

Phase shift of the inner structures depicted by Filter 2 versus the frequency at different wall normal locations in the inner layer

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Figure 4

Relative amplitude of the filtered and unfiltered streamwise fluctuating velocity at (a)y+=20, (b)y+=100

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Figure 5

Schematic representation of the frequency response of structures 2




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