Linear and nonlinear effects in stably stratified turbulent channel flow

Even small levels of stable stratification significantly alter the dynamics of wall-bounded turbulent flows. The observed changes are due to (i) modifications to the linear dynamics and (ii) spectral and wall-normal redistribution of energy in the nonlinear terms. This work examines the two from an input-output perspective, at moderate Reynolds number. The linear dynamics are described by the resolvent operator where the effect of stratification is represented by changes in the forcing-response relation, due to the modified mean flow profiles and bulk Richardson number. As for the nonlinear terms, their statistics are computed from direct numerical simulations, and contrasted for the isothermal and weakly-stratified cases. Numerical experiments where we modify the resolvent operator and the nonlinear forcing in isolation differentiate the contributions of the linear and nonlinear effects. The analysis draws attention to the channel center where the influence of stratification is most evident.