Omnidirectional reductions in drag and fluctuating forces can be achieved for a circular cylinder subjected to cross-flow by attaching cylindrical rings along its span at an interval of several diameters. In this work, the effects of ring configuration, the diameter D, spanwise width W, and spanwise pitch P, on the vortex shedding suppression was investigated. As a result, it was found that the periodicity in the pressure fluctuation on the sides of the cylinder disappeared for Red ≥ 20000 at ring configurations of D/d = 1.3, W/d = 1 and P/d≈3. At this configuration, the fluctuating lift force reduced markedly to about 1/30 of a 2D cylinder due to the suppression of the periodic shedding together with the weakening of the spanwise correlation. The mechanism of this was explored through flow visualizations and PIV measurements, which was considered as follows: A spanwise pressure gradient originated from a stepwise change in the diameter induces a spanwise flow, which brings the corner vortex to the side of the ring. This promotes the turbulent transition in the shear layer separated from the ring for Red ≥ 20000. As a result, the wake behind the ring markedly shrinks, which induces a pair of large transverse circulations just behind the ring edges. Consequently, two-dimensional spanwise vortices are obstructed to form, resulting in the suppression of the periodicity in the vortex shedding.
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机译:对于承受横流的圆柱体,通过沿其跨径以几个直径的间隔安装圆柱环,可以实现全方位减小阻力和波动力。在这项工作中,研究了环结构,直径D,翼展方向宽度W和翼展方向间距P对涡流脱落抑制的影响。结果发现,在D / d = 1.3,W / d = 1和P /d≈3的环形配置下,对于Red≥20000,气缸侧面的压力波动的周期消失了。在这种构造下,由于抑制了周期性脱落以及翼展方向相关性的减弱,波动的升力显着减小至2D圆柱的约1/30。通过流动可视化和PIV测量探索了这种机制,其考虑如下:由直径的逐步变化产生的翼展方向压力梯度会引起翼展方向流动,从而将角涡旋带到环的侧面。对于Red≥20000,这会在与圆环分离的剪切层中促进湍流过渡。结果,圆环后面的尾流明显收缩,从而在圆环边缘后面引起了一对较大的横向循环。因此,阻碍了二维展向涡旋的形成,从而抑制了涡旋脱落的周期性。
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