SPECIAL CEAFM Seminar
Speaker: Tamer Zaki (Imperial College, London)
Title: "Receptivity, Instability and Breakdown to Turbulence in Boundary Layers"
Date: Tuesday, October 16, 2012 [Special Date]
Time: 12:00 p.m. [Special Time]
Location: Gilman 132 [Special Location]
The manner in which infinitesimal disturbances can cause organized fluid motion to become chaotic is an intriguing phenomenon. In addition to being of great theoretical interest, laminar-to-turbulence transition is of significant engineering importance due to its role in heat transfer, its influence on momentum mixing, and its effect on drag. In this work, we present complementary theoretical analysis and high-fidelity direct numerical simulations of transition to turbulence in boundary layers.
The proceedings of transition are not unique, and various pathways can ultimately lead to boundary- layer turbulence. These pathways have traditionally been grouped in two classes: the orderly and the bypass routes. Orderly transition has its origin in classical linear stability theory, which predicts a slow transition process starting from weak Tollmien-Schlichting instability waves. In engineering applications, the presence of free-stream disturbances promotes early breakdown to turbulence, and transition is said to “bypass” the classical Tollmien-Schlichting route.
Numerical simulations of bypass transition reveal that high-frequency disturbances from the free stream are expelled by the boundary-layer shear – a phenomenon known as shear sheltering. Using asymptotic analysis, we develop a physical understanding of the mechanics of shear sheltering, and explain how low-frequency free-stream perturbations can permeate the mean shear. These elongated disturbances force the boundary layer resonantly and lead to the amplification of streaks.
While the majority of the laminar streaks are innocuous, a small proportion undergoes a localized instability and breakdown to turbulence. Reports in the literature present conflicting views on the origin of streak breakdown – a matter that we address by performing secondary instability analyses of realistic streaks. The predicted streak instabilities are shown to cause breakdown to turbulence in complementary direct numerical simulations.
Figure 1: Snapshot of a boundary layer undergoing bypass transition beneath free-stream turbulence. Red and blue isosurfaces are high- and low-speed streaks. Localized turbulent spots are identified by isosurfaces of the λ2 criterion.
Tamer Zaki received his PhD in 2005 from Stanford University, where he was a member of the Flow Physics and Computational Engineering group. During his PhD, he was awarded the “Stanford Graduate Fellowship” and participated in the DoE-ASCI program both at Stanford and at Los Alamos National Lab where he was awarded the “Directors Fellowship”. After his PhD, he joined the faculty in the Department of Mechanical Engineering at Imperial College London in 2006. He has since established the Flow Science and Engineering group, with a research activity that spans transitional and turbulent shear flows, interfacial flows and, most recently, multi-scale modeling and complex fluids. He is also a member of the steering committee of the Network for Advanced Instability Methods, and a member of the management committee of the UK Turbulence Consortium.