FLUCTUATION BACKGROUND DUE TO INCOMPRESSIBLE DISTURBANCES IN LAMINAR SHEAR FLOWS

The incompressible fluctuation background in laminar shear flows with a smooth velocity profile is investigated. Concrete calculations are p erformed for parallel Couette flow using nonmodal analysis of the line ar dynamics of the disturbances. Nonmodal analysis makes it possible t o grasp phenomena that could not be grasped in the early investigation s, and thereby makes it possible to represent the fluctuation backgrou nd in a completely new light: In incompressible shear flows the spatia l spectral energy density of the fluctuation background is anisotropic , and furthermore in certain regions of wave-number space it is higher than that of the thermal noise. It is also shown that in the stationa ry state of the nonequilibrium system studied there exists a new, indi rect channel for thermalization of the energy of the mean flow-energy is constantly transferred from the mean flow into the spatial Fourier harmonics of vortex pertubations and ultimately into heat. Possible ma nifestations of the fluctuation background described in this paper are listed. (C) 1997 American Institute of Physics.