Experiments and Numerical Simulations of Flow Around a Bluff Body in Oscillatory Flow

淹没在振荡流中的钝体周围的流动由分离流包络组成，分离流包络具有起始流的流动特性和由于交替振荡流动而产生的有限长度的非定常尾迹。研究作用在钝体上的力和钝体周围的流态对于诸如桥墩、海底管道和其他海洋结构等流体工程结构是至关重要的。关于由振荡流（如波浪）引起的流体力，有许多方面尚不清楚。关于振荡流作用在圆柱上的流体力的实验研究已有报道。在这个流体动力学领域，Keulegan-Carpenter数和粘性参数beta是最重要的参数，其中KC=UmT/D （Um：最大流体速度，T：周期，D：体直径），beta=Re/KC-D^2IvT （v=运动粘度）。用u型管水箱进行了实验：(1)测量了圆柱体的更多ce系数，并与其他研究在u型管水箱中进行了比较，以验证实验设置。然后，在KC数从1到90的广泛范围内，测量了具有不同攻角的方形圆柱体和平板的直线和横向力。流动可视化还用于确认流型与力系数之间的对应关系。在测量的KC数范围内，出现了几种不同类型的流型，旋涡运动与内线力和横向力的变化密切相关。(2)采用u型管水箱，研究了横截面宽高比为0.2 ~ 3.0的矩形圆柱体浸没在振荡流中的受力及周围流态。特别是，我们发现，与其他情况下的d/h圆柱在振荡流动中相比，d/h=0.6圆柱的阻力系数CD只有在KC=120以上才最大。KC数与流型、CD与尾迹宽度之间存在良好的关系。有限差分法的数值模拟采用ALE（任意拉格朗日-欧拉）有限差分法模拟了淹没在静水中的振荡钝体（如圆柱体和方形体）的流动。在三维非定常流场、不可压缩流场和粘性流场的假设下进行了数值模拟。将预测结果与流体动力测量结果和流动可视化结果进行了比较。发现计算和测量的直线力之间有很好的一致性，即惯性和阻力系数的值，CM， CD由著名的莫里森方程表示，在KC值的范围内。数值模拟还成功地再现和识别了几种流型，如“纵向涡旋或本氏涡旋”、“非对称流型”、“横街流型”和圆柱“双对流型”的三维流型结构。结果表明，圆柱和方圆柱的流型计算结果与流态可视化结果吻合较好。少

The flow around a bluff body submerged in oscillatory flow consists of the envelopment of separating flow, which has flow-characteristics of a starting flow and an unsteady wake with a limited length because of flow of the oscillation in the alternate directions. The studies of forces acting upon and flow patterns around the bluff body are fundamental to such fluid engineering constructions as piers, submarine pipelines and other ocean structures. There are many aspects that remain indistinct yet about fluid forces induced by oscillating flow such as waves. Some experimental studies on fluid forces acting on a circular cylinder under oscillating flow were already reported. In this field of fluid dynamics, a Keulegan-Carpenter number and a viscous parameter beta, where KC=UmT/D (Um : a maximum fluid velocity, T : a period, D : a body diameter), beta=Re/KC-D^2IvT (v= kinematic viscosity) are the most important parameters.Experiments by using a U-tube water tank :(1) The fluid dynamic for … More ce coefficients of a circular cylinder were measured and compared with those of other studies in a U-tube water tank to verify the experimental set-up. Then, measurements of both in-line and transverse forces of a square cylinder and flat plates with various angles of attack were made in a wide range of KC numbers from 1 to 90. Flow visualization was also employed to confirm the correspondence between flow patterns and force coefficients. It is found that several classes of flow patterns appear within the measured KC number range and the vortex motions closely corresponds to variations of the in-line and transverse forces.(2) The forces acting upon and the flow patterns around a rectangular cylinder with a cross-section of width/height ratio of 0.2-3.0 submerged in oscillatory flow were studied by using a U-tube water tank. In particular, we found that a drag coefficient CD of the d/h=0.6 cylinder becomes the biggest only beyond KC=120, compared with other case of the d/h cylinder in oscillatory flow. The good relationships between KC number and the flow patterns, and CD and wake width were shown.Numerical Simulations by a finite difference methodAn ALE (Arbitrary Lagrangian-Eulerian) finite difference method has been employed to simulate the flow around an oscillating bluff body, e.g., a circular cylinder and a square one, submerged in a still water. Simulations have been carried out under the assumption of 3-dimensional, unsteady, incompressible and viscous flow. Predictions are compared with the results of measurements of fluid-dynamic forces and flow-visualization. There is found to be good agreement between the computed and measured in-line forces, i.e., the values of inertia and drag coefficients, CM, CD represented by the well-known Morison equation, across a range of KC values. Several types of flow patterns are successfully reproduced and identified by the numerical simulations also, such as 3-D flow-structure of "longitudinal vortices or Honji's Vortex", "asymmetric pattern", "transverse street" and "double pair" for a circular cylinder. It is confirmed that the computed flow patterns agree well with the flow-visualized ones for both the circular cylinder and the square cylinder. Less