When: Mar 12 2021 @ 4:00 PM
Where: Join online via Zoom
Join online via Zoom

Department of Mechanical Engineering VIRTUAL GRADUATE SEMINAR IN FLUID MECHANICS
Join on-line via Zoom: https://wse.zoom.us/j/99813484575

“A Lagrangian Relaxation Wall Model for Large Eddy Simulations of Turbulent Flows”
Presented by MITCHELL FOWLER
(Advisers: Profs. Charles Meneveau & Tamer Zaki)
Near wall resolution requirements for large eddy simulations (LES) limit their applicability due to prohibitive computational costs. This necessitates the near wall region to modeled. The equilibrium wall model (EQWM) is the simplest and most widely adopted wall model in LES. The standard EQWM utilizes a known velocity profile, observed in a statistically stationary configuration, to relate the wall stress to the LES velocity. Even in non-equilibrium conditions the EQWM performs remarkably well since LES includes non-equilibrium effects outside the wall model layer. However, in rapidly changing conditions, near-wall changes are not captured by EQWM-LES. In this scenario, it appears conceptually important to be able to separate equilibrium and non-equilibrium effects to model each separately. In this work we develop and apply a composite wall model, comprised of quasi-equilibrium and non-equilibrium components, to a canonical non-equilibrium test case. The quasi-equilibrium component is derived from RANS-like integrated boundary layer equations and leads to an evolution equation for the wall stress which relaxes to its equilibrium value in a Lagrangian frame. While the derived equations are Lagrangian, some initial applications with advection neglected are presented. The non- equilibrium component is derived based on the observed Stokes-like layer response near the wall due to fast transients in the pressure gradient. The composite wall model thus responds properly under both quasi-equilibrium and non-equilibrium conditions providing greater range of applicability for wall modeled LES.
Funding Acknowledgement:
Research supported by the Office of Naval Research (grant N00014-17-1-2937).

“Restricted Nonlinear Large Eddy Simulations of Turbulent Flow over Spanwise Heterogeneous Strip Roughness”
Presented by BENJAMIN MINNICK
(Adviser: Prof. Dennice Gayme)
A number of engineering applications involve turbulent flow over heterogeneous rough surfaces, such as the atmospheric boundary layer over a forest or crop fields, flow over ship hulls with bio-fouling, or turbulent drag reduction over riblets. Despite the fundamental importance of such phenomenon, our understanding is limited due to the difficulty in simulating the vast range of configurations roughness topographies can take. In this work, we take advantage of the computational tractability of the restricted nonlinear large eddy simulation (RNL-LES) model to perform an extensive parametric study of flow over spanwise heterogeneous strip roughness at arbitrarily high Reynolds numbers. We first show that the RNL-LES model is capable of predicting the secondary flow induced by the roughness and the associated momentum pathways that have been observed in the literature. These results suggest that streamwise constant nonlinear interactions are sufficient to describe the turbulent mixing of momentum in these flows. We highlight on the importance of circulation in this flow and use the data from the parametric sweep to characterize the influence this intensified cross-plane mixing has on turbulent statistics and momentum transfer. We then use these results to inform a parametric model to predict secondary flow circulation for a given roughness topography.