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Dissipation in turbulent flows

[ jeudi 22-05-2014 13:00 | anglais ]Dissipation in turbulent flows John Christos Vassilicos Imperial College London
Résumé :
A region exists in the turbulence decay region in the lee of various fractal and regular grids which lasts for a number of turnover times and where a new non-equilibrium law holds: Cε ≈ ReI=ReI where Cε is the dimensionless dissipation coefficient, ReI is a global inlet Reynolds number and Rel is a local Reynolds number based on local turbulence intensity and integral scale. There are well-defined -5=3 energy spectra over more than a decade in this region even though the dissipation scalings are not in accord with usual Richardson-Kolmogorov equilibrium arguments. Following this region in the downstream direction is another decay region where Cε ≈ const (i.e. ε ∼ uI3=L).
The nonequilibrium turbulence dissipation law implies the existence of axisymmetric turbulent wake regions where the mean ow velocity decit decays as the inverse of the distance from the wake-generating body and the wake width grows as the square root of that distance. This behavior is dierent from any documented boundary-free turbulent shear ow to date. Its existence is conrmed in wind tunnel experiments of wakes generated by plates with irregular edges placed normal to an incoming free stream. The downstream distance over which these new non-equilibrium wake laws are observed would be of the order of the kilometer for an irregular object such as a tree of about 10 meters size.