October 25, 2019 - 2:00 pm
October 25, 2019 - 3:00 pm
Name: Dr. Inanc Senocak
Title: New insights into stratified atmospheric flows from studying idealized slope flows
Location: NWC 5600
Time: 2:00 PM
Series: Boundary Layer, Urban Meteorology, and Land-Surface Processes
Abstract: Stably stratified slope flows are commonly observed during nighttime in mountainous terrain or over large ice sheet and glaciers. Surface inclination alters the dynamics of turbulence due to oblique action of stratification relative to flow shear, leading to further departure from the classical aerodynamic boundary layer profile over a flat plate. Several mystifying fluid-physical processes that have been observed in strongly stratified flows, such as turbulent bursts, relaminarization and turbulence-wave interactions, make turbulence parameterization of stable conditions in weather prediction models a formidable task. The canonical fluid dynamics problem of slope flows with an exact laminar solution was originally formulated by Ludwig Prandtl in early 1940s1. Lykosov and Gutman introduced an extended version of the Prandtl model that takes into account the effect ambient winds2. Shapiro and Fedorovich refined the seminal work of Prandtl for different types of boundary cond
itions3. The exact solutions derived in those works provide excellent base profiles which we use to investigate the dynamical stability of viscous parallel slope flows with and without an ambient wind forcing. To this end, we apply the linear stability theory to the disturbance equations governing the slope flows and perform direct numerical simulations independently to uncover two distinct modes of instabilities in slope flows and explain the mechanisms underlying the observed flow instabilities. The transverse mode of instability manifests itself as stationary vortical rolls aligned in the along-slope direction, whereas the longitudinal mode emerges as waves propagating in the base-flow direction. We show that the progression of these instabilities toward turbulence depends on the slope angle, the Prandtl number and a newly introduced stratification perturbation parameter, which we interpret as a measure of the disturbance to the background stratification due to thermal forcing at
the surface. When ambient winds are active in the background, a fourth dimensionless number, which we call it the wind forcing parameter, enters into the picture. At constant Prandtl number, these two new dimensionless numbers along with the slope angle control the dynamical stability of slope flows, which constitute a strong counter evidence against relying solely on the gradient Richardson number as a criterion of stability for stratified atmospheric flow.
1. Prandtl, L. 1942 FÃ¼hrer durch die StrÃ¶mungslehre. Vieweg und Sohn.
2. Lykosov, VN & Gutman, LN 1972 Turbulent boundary-layer over a sloping underlying surface. Izv. Acad. Sci. USSR, Atmos. Ocean. Phys. 8 (8), 799.
3. Shapiro, A. & Fedorovich, E. 2004 Unsteady convectively driven flow along a vertical plate immersed in a stably stratified fluid. J. Fluid Mech. 498, 333-352.
Inanc Senocak is an associate professor of mechanical engineering at the University of Pittsburgh. He obtained his PhD degree in aerospace engineering from the University of Florida and his B.Sc. degree in mechanical engineering from the Middle East Technical University in Ankara, Turkey. He conducted postdoctoral studies at the Stanford University and the Los Alamos National Laboratory prior to starting his faculty career at the Boise State University in 2007. He is a fellow of the American Society of Mechanical Engineers (ASME), an associate fellow of the American Institute of Aeronautics and Astronautics (AIAA) and a past recipient of a CAREER Award from the National Science Foundation.