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 efficiency is limited in terms of the wall shear stress reduction, but is high as long as the turbulent wall activity is concerned. The wall shear stress is reduced due both to the reduction of the shear production in the viscous sublayer and to the contribution of the turbulent body force. The dissipation involving in the streamwise vorticity fluctuations transport equation increases significantly and overcomes the production in a thin layer near the wall leading to a drastic diminution of the turbulent wall shear stress fluctuations.
讲座题目三：近壁湍流的随机同步方法
Imperfect
Stochastic
Synchronization
of
the
Near
Wall
Turbulence
讲座时间：2009年7月2日上午10：00
讲座地点：北京航空航天大学流体所会议室
Introduction：Instantaneous amplitude and phase concept emerging from analytical signal formulation is applied to the wavelet coefficients of streamwise velocity fluctuations in the buffer layer of a near wall turbulent flow. Experiments and direct numerical simulations show both the existence of long periods of inert zones wherein the local phase is constant. These regions are separated by random phase jumps. The local amplitude is globally highly intermittent, but not in the phase locked regions wherein it varies smoothly. These behaviours are reminiscent of phase synchronization phenomena observed in stochastic chaotic systems. The lengths of the constant phase inert (laminar) zones reveal a type-I intermittency behaviour, in concordance with saddle-node bifurcation, and the periodic orbits of saddle nature recently identified in Couette turbulence. The observed phenomena are related to the footprint of coherent structures convecting in the low buffer layer and synchronizing the wall turbulence.
The results emerging from the reaction of the wall turbulence to a localized unsteady blowing are reinterpreted in the light of the phase synchronization phenomena. The wall shear indeed synchronizes with the localized blowing when the suboptimal control is applied downstream of the localized intervention that increases the efficiency of the control strategy by a factor of two. This is related to the concept of chaos synchronization and control.