光滑圆盘上小半球对边界层发展影响的实验研究

应用氢气泡流动显示技术对圆盘上小半球对边界层转捩的影响进行了观测.实验结果表明当Rer＞302时,将会从小半球脱落周期性的发卡涡.发卡涡在自诱导速度的作用下产生倾斜向上的运动,发卡涡头部顶端率先进入高速流区,因而比其根部运动更快,使发卡涡受到流向的拉伸,增加其流向的涡强,增加了流向涡强将使发卡涡的头部有更大的向上速度,由此而形成了不断加强的拉伸、自诱导过程,这就使三维扰动快速增长,导致边界层速度剖面出现局部的暂时变形,从而产生一个强剪切层,强剪切层很不稳定,导致发卡涡破裂而形成湍斑,在下游发展成完全湍流,而在边界层转捩过程中则观察到了水充速度有很强的负脉动.在小半球前缘附近会形成稳定的standing涡,并对standing涡及发卡涡对周围流体的诱导作用进行了细致的分析.小半球对边界层的扰动以锲形向下游传播,锲形的半顶角称为扰动扩散角,一个半球与三个半球的尾迹区没有明显的区别,每个半球扰动的扩散角均为5.7°.

Investigation of semi-spheres on the boundary layer development over a smooth circular plate

The hydrogen bubble flow visualization technique was used to study the effects of small semisphere on the boundary layer transition of flow over a smooth circular plate. The experimental results indicated that the periodic hairpin vortex will shed from the semi-sphere when Rer( Reynolds number based on the radius of the semi-sphere r) was greater than 302. Due to the effect of self-induced velocity, the hairpin vortex lifted with an inclined angle. The head of the hairpin vortex moved into the higher velocity region first, so the head possessed a higher velocity than the leg part of the hairpin vortex. The hairpin vortex was strentched at the streamwise direction, which increased the streamwise vortex strength and led to much higher upward velocity of the vortex head. The continuously created strentched and self-induced processes led to the three dimensional disturbance growing quickly, so the velocity profile of the boundary layer was suffered from the temporal local strain. The strong unstable shear layer was created, which led the hairpin vortex to be broken down into turbulent spots. With the further development of the turbulent spot, the fully developed turbulent boundary layer was achieved. In addition, the strong negative fluctuation of flow velocity was observed in the transition process. The stable standing vortex was formed in the near front region of the semi-sphere, the standing vortex and the induced effect of the hairpin vortex were analyzed in detail. The disturbance created by the small semisphere was spreaded in the form of wedged shape, and the spreading angle was 5.7° for both one and three semi-spheres cases.