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首页> 外文期刊>Journal of Fluid Mechanics >Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer
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Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer

机译：三维分离湍流边界层相干结构的实验研究

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Coherent structures of a three-dimensional (3D) separation due to an adverse pressure gradient are investigated experimentally.The flow set-up consists of a flat plate to develop a turbulent boundary layer upstream of an asymmetric two-dimensional diffuser with one diverging surface.The diffuser surface has an initial mild curvature followed by a flat section where flow separation occurs.The top and the two sidewalls of the diffuser are not equipped with any flow control mechanism to form a 3D separation.Planar particle image velocimetry (PIV) using four side-by-side cameras is applied to characterize the flow with high spatial resolution over a large streamwise-wall-normal field of view (FOV).Tomographic PIV (tomo-PIV) is also applied for volumetric measurement in a domain flush with the flat surface of the diffuser.The mean flow obtained from averaging instantaneous velocity fields of this intermittent unsteady flow appears as a vortex with an elliptical cross-section.The major axis of the ellipse is tilted with respect to the streamwise direction.As a result, the average velocity in the mid-span of the diffuser has an upstream forward flow and a downstream backward flow, separated by a point of zero wall shear stress.Sweep motions mainly carry out transport of turbulent kinetic energy upstream of this point, while ejections dominate at the downstream region.In the instantaneous flow fields, forward and backward flows have equivalent strength, and the separation front is extended in the spanwise direction.The conditional average of the separation instants forms a saddle-point structure with streamlines converging in the spanwise direction.Proper orthogonal decomposition (POD) of the tomo-PIV data demonstrates that about 42% of the turbulent kinetic energy is present in the first pair of modes, with a strong spanwise component.The spatial modes of POD also show focus, node and saddle-point structures.The average of the coefficients of the dominant POD modes during
机译：通过实验研究了由于不利的压力梯度引起的三维（3D）分离的相干结构。流量设定由平板组成，以在不对称的二维扩散器上游的湍流边界层，其中一个发散表面。漫射器表面具有初始温和曲率，然后是流动分离发生的平坦部分。扩散器的顶部和两个侧壁不配备任何流量控制机构以形成3D分离。使用四个形成3D分离.Planar粒子图像Velocimetry（PIV）应用并排相机以在大型流动壁正常的视野（FOV）的情况下表征具有高空间分辨率的流量（FOV）.tomapace PIV（Tomo-PIV）也应用于域中的域中的体积测量漫射器的平坦表面。从平均这种间歇性不稳定流量的平均瞬时速度场获得的平均流量作为具有椭圆形横截面的涡流。主要轴线椭圆的椭圆相对于流动方向倾斜。结果，扩散器的中间跨度中的平均速度具有上游向前流动和下游向后流动，由零壁剪切应力的点分开。主要在该点上游进行湍流动能的运输，而喷射在下游区域主导。瞬时流场，向前和向后流具有等效强度，并且分离前沿沿南方方向延伸。条件平均值分离速度形成鞍点结构，其流线为流线型，在跨越致致扭曲的数据的突然方向上的分解（POD）表明，在第一对模式中存在约42％的湍流动能，其中有一个强制跨域组件。POD的空间模式也显示了焦点，节点和鞍点结构。主导荚段的系数的平均值

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