Turbulent spot formation in stably stratified three-dimensional Yukawa liquids

A plane-Couette flow (PCF) is considered in a stably stratified three-dimensional (3D) Yukawa liquid, perturbed initially with a finite amplitude 3D perturbation. Stable stratification in density is achieved by subjecting the medium to external gravity. We demonstrate turbulent spot formation in a stably stratified PCF. The dynamics of the system is shown to depend upon the value of kappa, which is associated with the range of interaction. We have performed "first principles" classical 3D molecular dynamics (MD) simulations for "hypergravity" (1.3g0, g0 is Earth's gravitational force for unit mass in our normalized unit) and "milligravity" (9x10-3g0) cases for kappa, 1.0 and 4.0, respectively. We extract relevant fluid quantities from MD data. For the hypergravity case, when the system is evolved in time under stable stratification, the kinetic energy is observed to deposit in the lower wave vectors (Kx, Kz), leading to "inverse cascade" in the plane perpendicular to the direction of stratification. As a result, we observe large-scale structure formation in velocity and streamwise vorticity fields. Nucleation of velocity streak is observed for the first time in the stably stratified case in our simulation. The coherent structures in the velocity and streamwise vorticity fields are found to sustain for longer period of time for the stably stratified cases as compared to the unstratified case. For the milligravity case, the large-scale dynamics is observed to enhance. Unlike unstratified PCF, a background flow in Y -averaged streamwise fluid velocity field is observed for the stably stratified case. We believe that our results using "first principles" classical MD simulations on subcritical turbulence in stably stratified Yukawa liquid, may have ramifications to wider class turbulence problems.