High-Schmidt-number scalar transfer in regular and fractal grid turbulence

Turbulent mixing of high-Schmidt-number passive scalars in regular and fractal grid turbulence is experimentally investigated using a water channel. A turbulence-generating grid is installed at the entrance of the test section, which is 1.5 m in length and 0.1 m×0.1 m in cross section. Two types of grids are used: one is a regular grid consisting of square-mesh and biplane constructions, and the other is a square-type fractal grid, which was first investigated by Hurst and Vassilicos (2007 Phys. Fluids19 035103) and Seoud and Vassilicos (2007 Phys. Fluids19 105108). The two grids have the same solidity of 0.36. The Reynolds number based on the mesh size, Re_M=U ₀M_eff/ν, is 2500 in both flows, where U₀ is the cross-sectionally averaged mean velocity, M_eff is the effective mesh size and ν is the kinematic viscosity. A fluorescent dye (rhodamine B) is homogeneously premixed only in the lower stream and therefore the scalar mixing layers with an initial step profile develop downstream of the grids. The Schmidt number of the dye is O(10³). The time-resolved particle image velocimetry and the planar laser-induced fluorescence techniques are used to measure the velocity and concentration fields. The results show that the turbulent mixing in fractal grid turbulence is more strongly enhanced than that in regular grid turbulence for the same mesh Reynolds number Re_M. The profile of instantaneous scalar dissipation shows that scalar dissipation takes place locally even in the far downstream region at x/M_eff=120 in fractal grid turbulence.