Active Control of Boundary Layer Transition by Mutual Interaction of Turbulent Spots

为了实验研究湍流斑内部涡在合并过程中的相互作用现象，在层流边界层中人工发展了一对湍流斑。然后，通过诱导多个湍流点形成一个合成的过渡边界层，以阐明它们的相互作用过程对边界层结构的影响。通过对30通道X热线测量的湍流点的瞬时和总体平均视图的分析，我们得出了以下结论。(1)湍流斑的箭头状头部由许多沿流方向拉长的小尺度条纹状结构组成，交错排列。(2)相邻的条纹结构除箭头翼尖处外均为反向旋转的纵向涡对。(3)当两个点开始合并时，每个内翼尖处的纵向涡开始接触。它们之间的相互作用产生强烈的上冲，并在上部合并区产生强烈的速度缺陷区。(4)不稳定的速度剖面增强了合并点顶部的展向涡。(5)跨向涡构成了连接其下方纵向涡的大尺度马蹄形涡头，并可能在下游湍流边界层中成长为湍流凸起。(6)当湍流斑阵列成长为湍流边界层时，其周期性结构仍然在边界层外缘周围存在。(7)形成的湍流边界层在两个相邻点之间的区域比在每个点的中心线上发展得更快。

In order to experimentally study interaction phenomena between the internal vortices of turbulent spots in their merging process, a pair of turbulent spots was artificially development in a laminar boundary layer. Then, a synthetic transitional boundary layer was generated by inducing many turbulent spots to clarify the effects of their interaction process on the structure of the boundary layer. Analysis on the instantaneous and ensemble-averaged views of the turbulent spots measured by a 30 channel X hot-wire led us to the following conclusions.(1) The arrow-shaped head of a turbulent spot is composed of many small-scale streaky structures elongated in the streamwise direction and arranged in the staggered style.(2) Each adjacent streaky structure conforms counter-rotating longitudinal vortex pairs except for the ones at both wingtips of the arrowhead.(3) The longitudinal vortex at each inside wingtip comes into contact, as the two spots begin to merge. Mutual interaction between them generates a strong upwash and give birth to a strong velocity-defect region in the upper merged region.(4) The unstable velocity profile there enhances spanwise vortices at the top of the merged spot.(5) The spanwise vortices constitute the heads of large-scale horseshoe vortices connecting the longitudinal vortices beneath and may grow into turbulent bulges in the downstream turbulent boundary layer.(6) As the array of turbulent spots grows up into a turbulent boundary layer, its periodical structure survives still around the boundary layer's outer edge.(7) The resultant turbulent boundary layer develops more rapidly in the region between two adjacent spots than along the center line of each spot.