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Reduced-order analysis of the acoustic near field of a ducted axial fan
International Journal of Heat and Fluid Flow ( IF 2.6 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.ijheatfluidflow.2020.108657
Seyed Mohsen Alavi Moghadam , Simon Loosen , Matthias Meinke , Wolfgang Schröder

Abstract Spatio-temporal and modal analyses of the tip-vortex system of a ducted axial fan are performed based on highly resolved large-eddy simulations by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The Reynolds number based on the tip speed of the blade and the diameter of the fan D o is Re = 9.36 × 10 5 , the Mach number is M = 0.136 , and the rotational speed Ω = 3000 rpm. The tip clearances s / D o = 0.001 , 0.005 , and 0.01 are investigated for the design and off-design operating conditions. Only at the off-design condition, a blade-tip-vortex interaction is observed for s / D o = 0.01 and s / D o = 0.005 while further reduction of the tip clearance to s / D o = 0.001 prevents the cyclic interaction of the neighboring blades tip vortex with the current blade and a fully transitional region develops on the suction side of the blade. The POD results capture the narrowband cyclic interaction of the blade wake at the off-design condition. The first two energetic mode pairs can be linked to the interaction of the blades with the incoming wakes of the preceding blades. Compared to the POD results, the DMD modes corroborate the findings of blade-wake interactions. Furthermore, they highlight the connection to boundary layer transition on the suction side of the blades. The spatio-temporal DMD analyses at the characteristic frequencies of the blade-wake interaction reveal the generation of pressure waves which evolve in the azimuthal direction like isolated bubbles and propagate upstream of the blade. It causes two peaks in the spectrum of the acoustic pressure in the near field acoustics in agreement with the experimental sound spectra for s / D o = 0.01 and s / D o = 0.005 at the off-design condition.

中文翻译:

管道轴流风机声近场的降阶分析

摘要 基于高分辨率的大涡模拟,通过适当的正交分解 (POD) 和动态模式分解 (DMD),对管道轴流风机的叶尖涡系统进行时空和模态分析。基于叶片叶尖速度和风扇直径D o 的雷诺数为Re = 9.36 × 10 5 ,马赫数为M = 0.136,转速Ω = 3000 rpm。针对设计和非设计操作条件研究了尖端间隙 s / D o = 0.001、0.005 和 0.01。仅在非设计条件下,在 s / D o = 0.01 和 s / D o = 0.005 时观察到叶片 - 尖端 - 涡流相互作用,同时将尖端间隙进一步减小到 s / D o = 0。001 可防止相邻叶片尖端涡流与当前叶片的循环相互作用,并且在叶片的吸力侧形成完全过渡区域。POD 结果捕获了非设计条件下叶片尾流的窄带循环相互作用。前两个能量模式对可以与叶片与前面叶片的传入尾流的相互作用相关联。与 POD 结果相比,DMD 模式证实了叶片尾流相互作用的发现。此外,它们强调了与叶片吸力侧边界层过渡的连接。在叶片尾流相互作用的特征频率下的时空 DMD 分析揭示了压力波的产生,这些压力波像孤立的气泡一样在方位角方向发展并在叶片上游传播。
更新日期:2020-10-01
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