The effect of small-amplitude time-dependent changes to the surface morphology of a sphere
The effect of small-amplitude time-dependent changes to the surface morphology of a sphere
复制标题
小幅度随时间变化对球体表面形态的影响
DOI：
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发表时间：
2011
影响因子：
3.7
通讯作者：
B. McKeon
中科院分区：
文献类型：
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作者：
A. Norman;E. Kerrigan;B. McKeon
操纵流动分离的典型方法采用被动手段或主动技术，例如吹吸或等离子体加速。在这里，它表明，流动可以显着改变，通过对表面的形状进行微小的变化。使用一个非常简单的时间依赖性扰动的球体的表面进行的概念实验的证明：球直径的1%的粗糙度元素的方位角移动的不受控制的层流分离点上游的球体表面周围，与旋转频率一样大的旋涡脱落频率。一个关键的发现是，当粗糙度沿表面沿着移动时，由于球体边界层中产生的扰动，观察到对横向力的大影响的无量纲时间为= tU∞/D 4。这种缓慢的发展使得运动的部件在扩展区域上产生一个脱落的边界层。它示出，可以产生的横向力是一样大的阻力。同时，粒子图像测速和力测量结果表明，在尾流中产生了一对反向旋转的螺旋涡，对力有显著影响，并大大增加了尾流中的雷诺应力。由小表面扰动产生的流场的相对较大的扰动允许根据流向/径向平面中的测量值构造相位平均的三维（双速度分量）尾流结构。由于粗糙度元件引起的旋涡结构对具有名义上光滑表面或分布粗糙度的球体上的流动有影响。此外，它表明，振荡的粗糙度元素，或塑造其轨迹，可以产生一个平均横向力。
Typical approaches to manipulation of flow separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled laminar separation point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is = tU∞/D ≈ 4. This slow development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force.