Study of Scale-Interactions in Strained and Destrained Turbulence

This paper examines the interactions among different length-scales of turbulence during straining and de-straining of the flow. Understanding scale-interactions is a crucial ingredient in formulating improved subgrid models for Large Eddy Simulations. In this experimental study, planar Particle Image Velocimetry (PIV) measurements are performed 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 are applied by pushing or pulling rectangular piston whose width is equal to that of the a rectangular tank towards and away from the bottom. The velocity of the piston is computer controlled and synchronized with the PIV system. The initial background turbulence, characterized by the distributions of rms values and energy spectra, confirms that that the turbulence is nearly isotropic and homogeneous. The applied straining is characterized using high-speed photography of the piston and by PIV measurements of the mean flow. The results consist of the time evolution of several turbulence parameters subjected to a sequence of straining and destraining motions, with particular emphasis on the Reynolds stresses, Sub-grid scale (SGS) stresses, SGS anisotropy and SGS dissipation. The paper also examines the scale dependence of the SGS stress and dissipation, and compares the energy flux between different scales during the straining and destraining parts of the deformation.