表面活性剂减阻水溶液在紊流边界层中的流动结构

Turbulent Structure of Drag-reducing Surfactant Solutions Flow in Boundary Layers on Flat Plates

为揭示表面活性剂减阻水溶液在边界层中的减阻力学特性,使用二维激光多普勒测速仪(LDV)对十六烷基三甲基溴化铵和伴随盐水杨酸钠的减阻水溶液在零压梯度紊流平板边界层中的流动结构进行实验研究。结果发现,与水流相比,减阻水溶液的主流方向脉动强度峰值出现位置远离壁面,在靠近边界层中部,出现第二峰值,并处于非“剪切诱导状态(SIS)”;雷诺应力的象限构成中,上喷流(Ejection)和下扫流(Sweep)的贡献明显被抑制以致使雷诺应力沿整个边界层内部基本为零;在主流速度脉动能谱中,低频运动强度大于水流,而高频运动强度低于水流。结果说明表面活性剂减阻水溶液具有降低紊流脉动各个成分相关度、提高流动结构尺度的作用。

To clarify the drag-reducing behaviors of a surfactant solution in turbulent boundary layers, the turbulence structures of cetyltrimethylammonium bromide （CTAB） surfactant solution mixed with NaSal which provide counter-ions in a zero-pressure gradient boundary layer on fiat plates were investigated by a two dimensional laser doppler velocimetry（LDV）. The results were found as follows： The peak of the streamwise velocity turbulence intensity for surfactant solution appeared away farther from the wall than that for water, and the second peak occured near the center of the boundary layer where the flow was not in SIS. The drag-reducing surfactant solution depressed the motions associated with ejections of low momentum fluid away from the wall and sweeps of high-momentum fluid toward the wall so that Reynolds shear stress was almost zero throughout the thickness of the boundary layer. In the power spectrum of the streamwise velocity fluctuation, the power intensity at low frequencies was larger and the power intensity at high frequencies was smaller than that for water. These showed that the surfactant additives can weaken the correlation between the two components of velocity fluctuation, and enlarge the turbulent structure scales in the boundary layers.