Three-dimensional simulation of a flapping flag in a uniform flow
Three-dimensional simulation of a flapping flag in a uniform flow
复制标题
DOI：
10.1017/s0022112010000248
复制
发表时间：
2010-06-25
影响因子：
3.7
通讯作者：
Sung, Hyung Jin
中科院分区：
文献类型：
--
作者：
Huang, Wei-Xi;Sung, Hyung Jin
本文建立了一个三维计算模型，模拟均匀流场中旗的运动。通过一系列的数值试验，研究了摆振后流体-旗子耦合系统的非线性动力学特性。在低雷诺数下，当重力被排除时，旗形襟翼关于其中心线对称，并且在后缘上的拐角附近观察到展向方向上的弯曲。随着雷诺数的增加，展向弯曲由于侧边附近的正压力以及流体的粘性力的减小而变得平坦。在一定的临界雷诺数，旗失去了对称的中心线，这是示出相关的耦合流体旗不稳定性。从旗帜脱落的三维涡结构显示出显着的差异，从二维模拟的结果。通过连接在旗侧边缘和后缘处产生的那些，在旗后方形成发夹或O形旋涡结构。这样的旋涡结构通过减小旗上的压力差而对旗具有稳定效果。此外，侧缘附近的正压力与中心区域相比显著降低，导致展向弯曲。基于旗帜长度定义的斯特劳哈尔数稍微依赖于雷诺数和旗帜宽度，但与密度比成比例，如St类似于rho（-1/2）。另一方面，基于拍打幅度的斯特劳哈尔数保持接近0.2，与飞行或游泳动物的报告值一致。然后模拟了一面旗帜在重力作用下的挥舞，该旗帜沿着负展向定向。观察到旗帜下垂和上角的滚动运动。重力的双重作用，即不稳定的效果，如旗惯性和稳定效果，通过增加纵向张力。
A three-dimensional computational model is developed for simulating the flag motion in a uniform flow. The nonlinear dynamics of the coupled fluid-flag system after setting up of flapping is investigated by a series of numerical tests. At low Reynolds numbers, the flag flaps symmetrically about its centreline when gravity is excluded, and the bending in the spanwise direction is observed near the corners on the trailing edge. As the Reynolds number increases, the spanwise bending is flattened due to the decrease of the positive pressure near the side edges as well as the viscous force of the fluid. At a certain critical Reynolds number, the flag loses its symmetry about the centreline, which is shown to be related to the coupled fluid-flag instability. The three-dimensional vortical structures shed from the flag show a significant difference from the results of two-dimensional simulations. Hairpin or O-shaped vortical structures are formed behind the flag by connecting those generated at the flag side edges and the trailing edge. Such vortical structures have a stabilization effect on the flag by reducing the pressure difference across the flag. Moreover, the positive pressure near the side edges is significantly reduced as compared with that in the center region, causing the spanwise bending. The Strouhal number defined based on the flag length is slightly dependent on the Reynolds number and the flag width, but scales with the density ratio as St similar to rho(-1/2) On the other hand, the flapping-amplitude-based Strouhal number remains close to 0.2, consistent with the values reported for flying or swimming animals. A flag flapping under gravity is then simulated, which is directed along the negative spanwise direction. The sagging down of the flag and the rolling motion of the upper corner are observed. The dual effects of gravity are demonstrated, i.e. the destabilization effect like the flag inertia and the stabilization effect by increasing the longitudinal tension force.