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“From Laminar to Turbulence: The Origin of Enhanced Skin Friction in Transitional Boundary Layer”
Presented by MENGZE WANG
(Advisers: Profs. Tamer Zaki & Gregory Eyink)
Transition to turbulence in wall-bounded flows is accompanied by a significant increase in skin friction, whose origin has been speculated but never rigorously demonstrated. A number of candidate physical mechanisms seem relevant including a mean flow effect, vorticity flux from the wall, and the impinging flow associated with turbulent spots. None of these conjectures, however, provide a definitive and quantitative explanation. In this work, we express the skin friction as the expectation of the stochastic Cauchy invariant, which is evaluated using the interior vorticity and wall vorticity flux (Lighthill source) along stochastic Lagrangian trajectories in backward time. The contributions to wall shear stress from nonlinear advection, viscous diffusion, vortex stretching and tilting can be quantified. Our analysis is performed using the transitional boundary layer dataset of the Johns Hopkins Turbulence Databases. Our results demonstrate that the contributions of the Lighthill source and vertical advection of outer vorticity are not negligible. However, they are much less important than the stretching of near-wall vorticity which is the dominant source of skin-friction increase during laminar-to-turbulent transition.
“A Data-Driven Framework for the Isolation, Tracking and Aerodynamic Load Estimation of Distinct Vortex Structures”
Presented by KARTHIK MENON
(Advisers: Profs. Rajat Mittal & Tamer Zaki)
This work presents a physics-based and data-driven computational framework for the analysis of vortex-dominated fluid-structure interaction problems. The dynamics of such problems are typically dictated by multiple distinct, force-producing vortical structures. However, accurately estimating the aerodynamic loads induced by each of these vortex structures in complex viscous flows remains an open question. In the analysis framework presented here, a rigorous force and moment partitioning method is used in conjunction with clustering techniques to simultaneously isolate, track, and quantify the force-production due to several distinct vortex structures in complex, unsteady flow-fields. This flexible, automated framework allows us to precisely compute the force and moment induced by each vortical structure on an immersed body, and also correlate the spatio-temporal evolution of each structure to its dynamical influence on the problem.