Experiments on Differential Scalar Mixing in Turbulence in a Sheared, Stratified Flow

Laboratory experiments were performed to measure differential diffusion of temperature and salinity across a sheared density interface. The eddy diffusivity of temperature K-T exceeded the eddy diffusivity of salinity Ks by as much as 1.5 orders of magnitude at low s/vN(2), where a is the rate of dissipation of turbulent kinetic energy, v is the kinematic viscosity, and N is the buoyancy frequency in the pycnocline. The diffusivity ratio d = K-S/K-T increased from about 0.05 to 1 over the range 0.1 < epsilon/vN(2) <40. These differences made the eddy diffusivity of density depend on the density ratio. The trend of d with epsilon/vN(2) was consistent with trends found in other experiments, simulations, and theory, and the collapse of several datasets allowed the diffusivity ratio to be expressed as a function of epsilon/vN(2). However, shear decreased differential diffusion less in the experiments than predicted by theory for homogeneous turbulence subjected to constant shear and stratification. No strong effect of the density ratio on the diffusivity ratio was apparent. Because many flows in oceanography and limnology have values of epsilon/vN(2) low enough to exhibit significant differential diffusion, accounting for differential diffusion in interpreting measurements and modeling stratified water bodies is recommended.