Influence of Schmidt number on the fluctuations of turbulent mass transfer to a wall

Measurements are presented on the influence of Schmidt number on the frequency of the mass transfer fluctuations at a solid boundary. The shape of the spectral function is similar at all Schmidt numbers. A relation between the mass transfer fluctuations and the fluctuating velocity field can be obtained only at high frequencies. A comparison of the scale and the frequency of the mass transfer fluctuations and the velocity fluctuations suggests that the rate of mass transfer is controlled by convective motions in the flow oriented eddies described by a number of previous investigators. However, the concentration fluctuations caused by these convective motions are greatly dampened close to the wall by molecular diffusion. Thus the mass transfer fluctuations reflect only the scale and not the frequency of the convective motions in the flow oriented eddies.