湍流边界层拟序结构的实验研究

EXPERIMENTAL STUDIES ON COHERENT STRUCTURES IN TURBULENT BOUNDARY LAYERS

20世纪60年代后,先后从流动显示发现了快慢斑、猝发、上升流、下扫流和多种涡结构等湍流边界层的拟序结构．它们对湍流边界层的摩阻、传热传质和湍动能的产生等特性有重要影响．涡结构是上述拟序结构的核心,它影响其它拟序结构的发展和演变．发卡涡通常被认为是基本涡结构．发卡涡等涡结构的再生,是湍流边界层拟序结构能够自持续的必要的因素．壁面低速流上升产生猝发,是湍流边界层湍能的主要来源;条件采样是测量猝发频率和其它拟序结构出现频率的重要手段．流动显示对湍流边界层拟序结构作了大量定性观察,有许多减阻和增加传热率等应用性研究在此基础上发展起来．80年代后,出现了测量湍流边界层的瞬时流速矢量场的多热线法和PIV技术,三维PIV技术可望将来为湍流边界层的实验研究带来重大进展．本文评述了流动显示法、多热线法和PIV技术的优点和不足之处,以及它们在对湍流边界层拟序结构的研究中的贡献．

Since the 60s of the last century, the low-speed streaks, high-speed streaks, burst, ejections, sweeps and various vortical structures in turbulent boundary layers were discovered using flow visualization techniques. These coherent structures have significant influences on the skin friction and heat transfer of turbulent boundary layers, and also on the generation of turbulence kinetic energy. The induced velocities of vortical structures influence the evolution of other coherent structures, thus they are the kernels of the coherent structures in a turbulent boundary layers. Hairpin vortex is generally accepted as the basic structure in a turbulent boundary layer. There were many papers on hairpin vortex after the 80s. The generation and re-generation of hairpins and other vortices are the essential factor for the self-sustaining of coherent structures in turbulent boundary layers. It becomes one of the hot topics in studies of turbulent boundary layers in the 90s. Burst created by the low speed fluid ejected from wall region is the most important source for the generation of turbulence energy. Conditional sampling is an important technique for measuring the frequency of burst and the frequency of occurrence of other coherent structures. Observations using flow visualizations revealed the basic qualitative characteristics of coherent structures, on which many ideas were developed for drag reduction and enhancing heat transfer. After the 80s, multi-hot wire and PIV techniques were developed, which were applied for measuring the instantaneous flow velocity vector field. The development of three-dimensional PIV technique may be a break-through in the experimental research on coherent structures. In the present paper, discussions are focused on flow visualization, multi-hot wire and PIV techniques, their advantages and defects, also their contributions to their investigations of coherent structures.