Scale Interactions of Turbulence Subjected to a Straining-Relaxation-Destraining Cycle

J. Chen, C. Meneveau & J. Katz
Department of Mechanical Engineering
Center for Environmental and Applied Fluid Mechanics The Johns Hopkins University, Baltimore MD 21218

ABSTRACT: The response of turbulence subjected to planar straining and de-straining is studied experimentally, and the impact of the applied distortions on the energy transfer across different length scales is quantified. The data are obtained using Planar Particle Image Velocimetry (PIV) in a water tank, in which high Reynolds number turbulence with very low mean velocity is generated by an array of spinning grids. Planar straining and de-straining mean flows are produced by pushing and pulling a rectangular piston towards, and away from, the bottom wall of the tank. The data are processed to yield the time evolution of Reynolds stresses, anisotropy tensors, turbulence kinetic energy production, and mean subgrid-scale (SGS) dissipation rate at various scales. During straining, the production rises rapidly. After the relaxation period the small-scale SGS stresses recover isotropy, but the Reynolds stresses at large scales still display significant anisotropy. Thus when destraining is applied, a strong negative production (mean backscatter) occurs, where turbulence fluctuations return kinetic energy to the mean flow. The SGS dissipation displays similar behavior at large filter scales, but the mean backscatter gradually disappears with decreasing filter-scales. Energy spectra are compared to predictions of Rapid Distortion Theory (RDT). Good agreement is found for the initial response but, as expected for the time-scale ratios of the experiment, turbulence relaxation causes discrepancies between measurements and RDT at later times.

J. Fluid Mech. 562, 123-150 (2006).

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