讲座人：Tardu F. Sedat，法国格勒诺布尔第一大学教授，英国帝国理工访问学者教授，曾在法国国家研究中心（CNRS）工作，研究领域涉及湍流机理、湍流控制、旋涡分离流、微流体和微电子机械系统等。在国际学术期刊Journal of Fluid Mechanics, Physics of Fluids等发表论文40多篇，撰写学术专著3部；在湍流、微流动和应用微电子机械系统（MEMS）进行流动控制方面居于国际领先地位。

讲座题目一：相互作用式旁路转捩机理

Interactive Bypass Transition Mechanism And Its Implications

讲座时间：2009年6月30日上午10：00

讲座地点：北京航空航天大学流体所会议室

Introduction：The interaction between two localized disturbances is analyzed in a subcritical channel flow through direct numerical simulations. The initial perturbations are in the form of two pairs of counter rotating vortices. One of them interacts with the wall normal vorticity layers set-up near the wall, by compressing or stretching locally part of them through the straining motion it induces. The breakdown of spanwise symmetry leads to the rapid development of a new wall normal vorticity patch that is tilted by the shear and rolls up into a new small-scale streamwise vortex. The process results in a localized turbulent spot at later stages of development. A detailed analysis is carried out to determine the role of different parameters entering in the physics of the mechanism. Several critical thresholds that trigger the interactive bypass transition process are found and analyzed. The similitude parameters resulting from the parametric investigation coincide well with those governing the self-sustaining Reynolds shear stress producing eddies in the buffer layer of a fully developed turbulent wall flow. It is suggested that the mechanism we propose may play some role in the regeneration cycle of the near wall turbulence generating structures by precisely bypassing the three-dimensional streak instability mechanism.

The second part of the lecture deals with the implications of the proposed mechanism in particular for mixing in low Reynolds number flows. The effect of interacting vortices on the mixing in a microchannel flow at a Reynolds number Re =10 and Prandtl number Pr =10 is investigated through direct numerical simulations. The configuration simulates two synthetic jets staggered in the lateral spanwise direction. It is shown that the strategy leads to the auto-regeneration of small-scale longitudinal vortical structures that increase significantly the mixing. Main characteristics of the perturbed flow field are in close similarity with a localized turbulent spot that is in principle inconceivable at such a small Reynolds number.

讲座题目二：主动式减阻控制：理论及其可行性

Active Control of Drag: Theory and Feasibility

讲座时间：2009年7月1日上午10：00

讲座地点：北京航空航天大学流体所会议室

Introduction：The activities of the Lab of Geophysical and Industrial Flows (LEGI) in flow control will be summarized in the first part of the talk. This includes, active and passive control of drag, electromagnetic flow control, and effects of a localized imposed unsteadiness on the wall turbulence response.

The second part wil be devoted to one-information suboptimal control strategies for wall drag reduction. The suboptimal control with the cost function directly connected to the wall shear and introduced for a while has been revisited through direct numerical simulations of high temporal and spatial resolution. Its effect on the fine structure of the wall turbulence has been analyzed in details, essentially through the spanwise vorticity transport mechanism.

It is shown that only half of the viscous sublayer is mainly affected by the control. The actuation ef