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Analysis of a drag reduced flat plate turbulent boundary layer via uniform momentum zones

机译：均匀动量区对减阻平板湍流边界层的分析

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High spatial resolution particle-image velocimetry (Ply) and micro-particle tracking velocimetry PTV) measurements have been performed to analyze the flow structures in a turbulent flat plate boundary layer undergoing spanwise traveling transversal surface waves. The scope of this investigation is to examine the connection between friction drag reduction and the structural properties of the instantaneous flow fields manipulated by the surface wave. The experiments were conducted for a momentum based Reynolds number of Re-theta = 2250 at dimensionless wave amplitudes of A(+) = 6, 7, 8, and 11, a wave period of T+ = 94, and a wave length of lambda(+) = 3738. Local drag reduction of up to 4.9% at 0.087 boundary layer thicknesses downstream of the moving surface was measured. The Reynolds shear stress in the logarithmic region (50 <= y(+) <= 250) is suppressed due to the surface waves, whereas the wall-normal velocity fluctuations are enhanced in the outer layer. The turbulent contribution term in the Fukagata, Iwamoto, and Kasagi (FIK) friction drag decomposition [18] is reduced by 2.7% due to the dampened Reynolds shear stress above the wave crest. The analysis of the large-scale uniform momentum zones (UMZs) indicates that the population of these zones above the wave trough is nearly identical compared to the non-actuated turbulent boundary layer, while above the wave crest the number of UMZs is decreased by 3.5%. The conditional statistics of the modal velocity based on the near-wall velocity fluctuation evidence that there are strong links between the near-wall flow events and the UMZs. The near-wall flow structures of high fluctuating intensity are less correlated to the large-scale UMZs due to the spanwise surface wave motion, which diminishes the momentum transfer from the outer large-scale energetic structures to the near-wall flow, and thus, leads to the friction drag reduction. (C) 2019 Elsevier Masson SAS. All rights reserved.
机译：已经进行了高空间分辨率粒子图像测速（Ply）和微粒追踪测速（PTV）测量，以分析湍流平板边界层中沿展向传播的横向表面波的流动结构。这项研究的范围是检查摩擦阻力减小与表面波操纵的瞬时流场的结构特性之间的联系。在A（+）= 6、7、8和11的无量纲波幅，T + = 94的波周期和lambda（）的无量纲波振幅下，基于动量的Re-theta = 2250雷诺数进行了实验+）=3738。在移动表面下游的0.087边界层厚度处，局部阻力降低了4.9％。对数区域（50 <= y（+）<= 250）中的雷诺剪切应力由于表面波而受到抑制，而外层的壁法向速度波动则增加了。由于波峰上方雷诺兹剪切应力的衰减，深冈，岩本和ata木（FIK）摩擦阻力分解[18]中的湍流贡献项减少了2.7％。对大型均匀动量带（UMZs）的分析表明，与未驱动的湍流边界层相比，波谷上方的这些区域的人口几乎相同，而在波峰上方，UMZ的数量减少了3.5 ％。基于近壁速度波动的模态速度的条件统计表明，近壁流动事件与UMZ之间存在很强的联系。由于翼展方向的表面波运动，具有高波动强度的近壁流动结构与大型UMZ的相关性较小，从而减少了从外部大型高能结构向近壁流动的动量传递，因此，导致摩擦阻力减小。 （C）2019 Elsevier Masson SAS。版权所有。

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