2009.04.02

222, Eigineering 2nd Bld, Hongo Campus

Scientific Manager Dr. Sylvain Lardeau

Department of Aeronautics, Imperial College London, UK

Title：Long and short-lived structures in turbulent and trasitional flows: modeling and simulation

Schedule：15:00 ~ 16:00 Thursday 2nd April, 2009

Place：222, Engineering 2nd Bld, Hongo Campus

Abstract： Coherent Structures (CS) have been studied experimentally and numerically for many years, although their formation and the role they play in the dynamic of turbulent flows are still unclear, while it remain crucial for several applications, such as flow control. Short-lived near-wall structures (e.g. hairpin vortices) are thought to be fundamental in the momentum exchange in the near-wall layer, and are usually of primary concern for effective flow control targeting, whilst their control has proved to be remarkably short-lived. On the other hand, long-lived, or large-scale fluctuations are usually difficult to detect (experiments) or to simulate. Present work at ICL is trying to address those issues by simulating and analysing highly-resolved, purpose-made, flow configurations. First, example of CS is given, at low-Reynolds number for several examples of increasing complexity: free-shear layers, wall-bounded flows at low Reynolds numbers, and attached and separated flows of practical configurations at high Reynolds numbers. Of particular interest are the fundamental implications for turbulence modelling.

Dr. Tamer Zaki

Department of Mechanical Engineering, Imperial College London, UK 

Title：Instances of Transition to turbulence in boundary layers

Schedule：16:00 ~ 17:00 Thursday 2nd April, 2009

Place：222, Engineering 2nd Bld, Hongo Campus 

Abstract：  Transition to turbulence in boundary layers is often classified as orderly or bypass. The early stages of orderly transition are characterized by the amplification of two-dimensional Tollmien-Schlichting instability waves. These waves develop a three-dimensional secondary instability, and subsequently breakdown to turbulence. When the transition deviates from this description, the breakdown mechanism is termed bypass transition.

Transition mechanisms are studied using direct numerical simulations (DNS). The view that bypass transition is the complement of the orderly route suggests that it can be studied starting from the complement of Tollmien-Schlichting waves in Orr-Sommerfeld theory.  "Continuous mode transition" refers to studies of the interaction of pairs of continuous modes. DNS of these bi-modal interactions can emulate breakdown of boundary layers in response to a fully turbulent free stream, but the two modes must be of low- and high-frequency. The former penetrates the boundary layer and generates the high-amplitude streaks, or Klebanoff distortions, which precede breakdown in bypass transition. The high-frequency mode only exists in the free stream, and is required in order to trigger the secondary instability of the streaks and breakdown into turbulent spots. A hybrid transition mechanism is also simulated, where both Tollmien-Schlichting and continuous modes are present in the flow and interact,. A Floquet analysis provides an explanation of the results observed in the DNS.