Abstract
The coherent vortex simulation (CVS) decomposes each realization of a turbulent flow into two orthogonal components: An organized coherent flow and a random incoherent flow. They both contribute to all scales in the inertial range, but exhibit different statistical behaviors. The CVS decomposition is based on the nonlinear filtering of the vorticity field, projected onto an orthonormal wavelet basis made of compactly supported functions, and the computation of the induced velocity field using Biot–Savart’s relation. We apply it to a three-dimensional homogeneous isotropic turbulent flow with a Taylor microscale Reynolds number Rλ=168, computed by direct numerical simulation at resolution N=2563. Only 2.9%N wavelet modes correspond to the coherent flow made of vortex tubes, which contribute 99% of energy and 79% of enstrophy, and exhibit the same k−5/3 energy spectrum as the total flow. The remaining 97.1%N wavelet modes correspond to a incoherent random flow which is structureless, has an equipartition energy spectrum, and a Gaussian velocity probability distribution function (PDF). For the same flow and the same compression rate, the proper orthogonal decomposition (POD), which in this statistically homogeneous case degenerates into the Fourier basis, decomposes each flow realization into large scale and small scale flows, in a way similar to large eddy simulation (LES) filtering. It is shown that the large scale flow thus obtained does not extract the vortex tubes equally well as the coherent flow resulting from the CVS decomposition. Moreover, the small scale flow still contains coherent structures, and its velocity PDF is stretched exponential, while the incoherent flow is structureless, decorrelated, and its velocity PDF is Gaussian. Thus, modeling the effect of the incoherent flow discarded by CVS-wavelet shall be easier than modeling the effect of the small scale flow discarded by POD-Fourier or LES.
Highlights
Since the work presented in this paper has been performed at NASA-Ames during the CTRCenter for Turbulence Research Summer Program 2000,1 we recall the comments on turbulence research made in 1948 by Hugh L
Tollmien and Prandtl’s suggestion to split the turbulent fluctuations into non-diffusive and diffusive components is very similar to the concept behind coherent vortex simulationCVSwhich we introduced in Refs. 3–5
We have shown that the CVS decomposition, based on the nonlinear filtering of the wavelet coefficients of vorticity, is an efficient tool for extracting coherent vortices out of turbulent flows
Summary
Since the work presented in this paper has been performed at NASA-Ames during the CTRCenter for Turbulence Research Summer Program 2000,1 we recall the comments on turbulence research made in 1948 by Hugh L. Shall the flow be regarded as a mean flow that merely transports and distorts large eddies superposed on the flow, these eddies being of varying size and intensity?’’ This comment of Dryden, which assumes that turbulent flows are composed of coherent units of varying sizes and intensities which cannot be smoothed by averaging, supports our proposal of using the wavelet representation to study turbulent flows.. Since the flow studied here is statistically invariant by translation and rotation, in this case the POD degenerates into the Fourier basis, where the modes are sorted in increasing order of the wavenumber Such a homogeneous isotropic flow is the most difficult case to treat for both POD and CVS, but it is the most generic turbulent flow one can compute at large Reynolds numbers without any ad hoc turbulence model
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