Abstract The effect of aspect ratio on the flow over a hemispherical turret was investigated using Time Resolved Particle Image Velocimetry (TRPIV) in a water tunnel at a Reynolds number of 33,000, based on the turret diameter (d). The height of the pedestal (h) was varied to obtain four aspect ratios ( A R = h d ) of 0.5, 1, 1.5 and 2. For all cases, the boundary layer separated upstream of the models to form a classical horseshoe vortex that wrapped around the base of the turret models as it convected downstream. A bifurcation streamline resulted in the formation of a half saddle point near the forward attachment line that divided the flow into inward turning recirculating flow near the end wall, and the outer flow. The vertical displacement of the half saddle point continued to increase with increasing aspect ratio. Spectral analysis and Dynamic Mode Decomposition (DMD) was performed on the velocimetry data that showed existence of multiple peaks whose amplitudes increased with increasing aspect ratio and subharmonic distribution of energy. Results of power spectra were corroborated by DMD analysis which showed the gradual shift in the highest energy modes from high frequencies to lower ones and reduction of decay rates of lower energy modes with increasing aspect ratio. Surface flow visualization tests in the wind tunnel also showed the strong influence of aspect ratio on the near wall flow upstream and in the wake of the models vividly observed from the trajectories of the limiting streamlines.

中文翻译：

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纵横比对壁挂式半球形转塔上流动的影响

摘要 基于转塔直径 (d)，在雷诺数为 33,000 的水隧道中，使用时间分辨粒子图像测速法 (TRPIV) 研究了纵横比对半球转塔上流动的影响。基座的高度 (h) 发生变化以获得 0.5、1、1.5 和 2 的四个纵横比 (AR = hd)。对于所有情况，边界层在模型上游分离，形成一个经典的马蹄形涡旋环绕炮塔模型的底部，因为它在下游对流。分叉流线导致在前附着线附近形成半鞍点，将流动分为端壁附近向内转向的再循环流和外流。随着纵横比的增加，半鞍点的垂直位移继续增加。对测速数据进行频谱分析和动态模式分解 (DMD)，该数据显示存在多个峰值，其幅度随着纵横比和能量次谐波分布的增加而增加。功率谱的结果通过 DMD 分析得到证实，DMD 分析显示最高能量模式从高频逐渐转移到较低频率，并且随着纵横比的增加，较低能量模式的衰减率降低。风洞中的表面流可视化测试还表明，纵横比对上游近壁流的影响很大，并且从限制流线的轨迹生动地观察到模型之后。