Turbulent characteristics and energy transfer in the far field of active-grid turbulence

Abstract

Turbulent characteristics in the far field of active-grid turbulence have been investigated through wind tunnel experiments using hot-wire anemometry. Two forcing protocols are employed following previous studies: one is the double-random mode and the other is the open mode with the grid remaining static with minimum blockage. The integral length scale L for the double-random modes slightly decreases with streamwise distance in the far field as observed in the near field of the active-grid turbulence. The nondimensional dissipation rate C ϵ for the double-random modes is around 0.5. This asymptotic value is different from those reported in previous active-grid turbulence experiments and could be nonuniversal. The equilibrium scaling L/λ = Cϵ Reλ/15 (λ is the Taylor microscale and Reλ is the turbulent Reynolds number) with a constant Cϵ is established in the far field of the double-random modes regardless of active-grid motions. The sum of production and destruction terms in the enstrophy budget equation for homogeneous and isotropic turbulence S + 2G/Reλ (S is the skewness of the longitudinal velocity derivative and G is the destruction coefficient) is proportional to Reλ-1 and close to zero in the present active-grid turbulence, suggesting that the equilibrium scaling is possibly related to the balance between the production and destruction of the enstrophy.