Dr. William Anderson – May 10

Numerical Simulation of Inertia-Dominated Turbulent Flows

Dr. William Anderson

May 10, 2018

2:00 pm/NWC 560

Abstract: This talk presents a summary of three active research themes: (1) canonical wall-bounded turbulence and roughness effects; (2) aeolian processes on Earth and Mars; and (3) small-scale mixing in the ocean-mixed layer. On the first: turbulent flows respond to bounding walls with a predominant spanwise heterogeneity – that is, a heterogeneity parallel to the prevailing transport direction – with formation of Reynolds-averaged turbulent secondary flows. Results from large-eddy simulations and complementary experimental measurements of flow over spanwise-heterogeneous surfaces are shown: the resultant secondary cell location is clearly correlated with the surface characteristics, which ultimately dictates the Reynolds-averaged flow patterns. However, results also show the potential for instantaneous sign reversals in the rotational sense of the secondary cells. This is accomplished with probability density functions and conditional sampling. Second, ongoing results
to characterize the evolution of arid landscape features through aeolian processes – the wind-driven mobilization of sediment and dust – are shown. Models of sediment migration via saltation, the bouncing of sediment grains across arid surfaces, typically indicate that mass flux via saltation scales linearly with imposed aerodynamic stress, Q ~ τ; since stress scales to the square of velocity, it follows that Q ~ τ ~ |u|2. These rudimentary scaling arguments demonstrate the importance of turbulence in aeolian processes. Simulations have been used to study the evolution of vortical flow patterns (hairpins, streamwise rolls) in flow over aeolian barchans dunes, and to isolate driving aerodynamic mechanisms responsible for setting dune geometry. We are also working to establish the salient dynamical characteristics of atmospheric turbulence responsible for dust entrainment on Earth and Mars. Finally, results from ongoing efforts to model turbulent mixing in the ocean mixed layer
– the first ~100 m of the ocean, and the zone most closely modulated by atmospheric forcing – are shown. This zone regulates sequestration of anthropogenic carbon and heat from the atmosphere, accelerates the dispersion of surface-laden oil, and mixes agricultural nitrogen released at coastal inlets. Large-eddy simulation has been used to model Langmuir turbulence in coastal zones. Prognostic description of large-scale flow organization is confounded by the cumulative influence of imposed aerodynamic wind stress, bottom-bed (bathymetric) hydrodynamic stress, and orbital wave motion. Most prior studies have considered the aforementioned forces in a co-aligned arrangement, which is expected to be the exception in natural settings, not the norm. Our work focuses precisely on oblique forcing arrangements: we have used simulation results and mathematical deductions to explain the relative influence of different forces in setting the large-scale Langmuir cell organization.

Biographical Sketch: William Anderson is the Eugene McDermott Professor and Associate Professor of Mechanical Engineering at the University of Texas at Dallas. He received his doctoral degree in Mechanical Engineering from The Johns Hopkins University in 2011. His research interests are primarily numerical simulation of small-scale geophysical turbulence, and fundamental processes in rough-wall turbulence. He is a 2014 recipient of the Air Force Office of Scientific Research Young Investigator Program award. His research has been supported by the Army Research Office, National Science Foundation, Air Force Office of Scientific Research, and Texas General Land Office.