利用轴对称行波壁减阻

将已有的无限长行波壁无粘周期分离流的二维解析理论推广到轴对称流,作为研究用柔壁行波主动控制实施减阻的基础．物理上看,在一个特定的临界行波速下会形成自发的周期性分离-再附流,每个波谷的分离区捕获一个稳定的涡环．此涡环列形成“流体滚动轴承”,把近壁流和主流隔开,使前者只有周期性弱剪切．这时,柱体的压阻和摩阻均将趋于零．本文建立了双列交错涡环的理论模型来模拟这个物理图景,证实上述临界波速惟一地存在,并用理论选择行波壁的波形并决定临界波速．发现：在同样波幅下,随柱体半径趋向无限大,临界波速的渐进值并不等于相应的二维值．

Drag Reduction by Axisymmetric Traveling Wavy Wall

This paper extends the previous two-dimensional analytical theory of the inviscid periodic separated flow over an infinitely long traveling wavy wall to axisymmetric flow, which serves as a theoretical guidance to the study on active drag-reduction flow control by flexible wall with traveling waves. Physically, at a special critical wave speed the flow may have a naturally periodical separated-reattached pattern, with each wave trough capturing a stable vortex ring. This row of vortex rings forms a "fluid roller bearing" over the wall, which separates the near-wall layer from the external main stream such that the former only has a weak periodic shearing. In this state the total drag of the columnar body due to both pressure and skin friction will tend to vanish. This paper establishes an inviscid theoretical model to mimic this flow physics by using two rows of staggered vortex rings. The theory confirms the unique existence of the critical wave speed and can be used to select the wave pattern of the flexible wall as well as the value of the critical wave speed. It is found that, as the radius of the columnar body approaches infinity, the asymptotic critical wave speed does not equal the two-dimensional value for the same amplitude.