Journal of Fluid Mechanics



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Linear energy amplification in turbulent channels


JUAN C. del ÁLAMO a1p1 and JAVIER JIMÉNEZ a1a2
a1 School of Aeronautics, Universidad Politécnica de Madrid, 28040 Madrid, Spain
a2 Center for Turbulence Research, Stanford University, Stanford, CA 94305, USA

Article author query
del alamo jc   [Google Scholar] 
jimenez j   [Google Scholar] 
 

Abstract

We study the temporal stability of the Orr–Sommerfeld and Squire equations in channels with turbulent mean velocity profiles and turbulent eddy viscosities. Friction Reynolds numbers up to $Re_\tau \,{=}\, 2\,{\times}\, 10^4$ are considered. All the eigensolutions of the problem are damped, but initial perturbations with wavelengths $\lambda_x \,{>}\, \lambda_z$ can grow temporarily before decaying. The most amplified solutions reproduce the organization of turbulent structures in actual channels, including their self-similar spreading in the logarithmic region. The typical widths of the near-wall streaks and of the large-scale structures of the outer layer, $\lambda_z^+ \,{=}\, 100$ and $\lambda_z/h \,{=}\, 3$, are predicted well. The dynamics of the most amplified solutions is roughly the same regardless of the wavelength of the perturbations and of the Reynolds number. They start with a wall-normal $v$ event which does not grow but which forces streamwise velocity fluctuations by stirring the mean shear ($uv\,{<}\,0$). The resulting $u$ fluctuations grow significantly and last longer than the $v$ ones, and contain nearly all the kinetic energy at the instant of maximum amplification.

(Published Online July 19 2006)
(Received February 7 2006)
(Revised April 26 2006)


Correspondence:
p1 Present address: Department of Mechanical and Aerospace Engineering, UCSD, La Jolla, CA 92093, USA.


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