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John R. Taylor
Department of Mechanical and Aerospace Engineering, University of California, San Diego La Jolla, CA

Sutanu Sarkar
Mechanical and Aerospace Engineering, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093

Vincenzo Armenio
Dipartimento di Ingegneria Civile Universita degli Studi di Trieste 34127 Trieste, Italy; Dipartimento di Ingegneria Civile e Ambientale, Universitá di Trieste, Piazzale Europa 1, 34127 Trieste, Italy


Large Eddy Simulation (LES) has been used to study flow driven by a constant pressure gradient in an open channel with stable stratification imposed by a constant heat flux at the free surface and an adiabatic bottom wall. Under these conditions a turbulent mixed layer develops underneath a strongly stratified pycnocline. Turbulent properties in the pycnocline and at the free surface are examined as a function of the imposed stratification. It is found that increasing the friction Richardson number, a measure of the relative importance of stratification with respect to boundary layer turbulence, leads to a stronger, thicker pycnocline which eventually limits the impact of wall-generated turbulence on the free surface. Increasing stratification also leads to an increase in the pressure-driven mean streamwise velocity near the free surface and a corresponding increase in mean shear, a decrease in the turbulent Reynolds stress, and a substantial decrease in the eddy viscosity. PDF's and visualizations are employed to understand how stratification affects the interaction of bottom turbulence with the free surface.