Turbulence and Mixing in Stratified Flows

The flow of a dense fluid along a sloping bottom is a basic geophysical process in scenarios ranging from deep ocean outflows to downslope mountain winds.  Gravity currents also provide an important mechanism for the transfer of sediments.  The establishment of these flows in many cases derives from a region of hydraulic control due to geometric constriction, which generates selective acceleration of the stratified fluid.  This acceleration, in turn, destabilizes the flow to shear instabilities that result in turbulent mixing and transfer of mass and momentum.

Laboratory experiments carried out  with Larry Armi at Scripps Institution of Oceanography examined the mixing and entrainment mechanisms at the shear interface of a developing downslope current.  These revealed an initial, unstable region characterized by rapid development, low shear Richardson numbers and high entrainment.  A subsequent high Richardson number region is characterized by weak entrainment and collapsing turbulence. 

Density gradient field with density and velocity 

profiles for downslope current vs. alongslope distance

Realtime and vorticity field images of instability in accelerating shear flow.

PUBLICATIONS

G. Pawlak and L. Armi, "Mixing and Entrainment in Developing Stratified Currents", Journal of Fluid Mechanics, Vol. 424, 2000, p. 45-73.

G. Pawlak and L. Armi, "Vortex Dynamics in a Spatially Accelerating Shear Layer", Journal of Fluid Mechanics,  Vol. 376, Dec. 1998, p. 1-35.

G. Pawlak and L. Armi, "Hydraulics of a Two Layer Arrested Wedge Flow", Journal of Hydraulic Research, Vol. 35, No. 5, 1997, p. 603-618.

G. Pawlak and L. Armi,  "Stability and Mixing of a Two Layer Exchange Flow", Dynamics of Atmospheres and Oceans, Vol. 24, Jan. 1996, p. 139-151.