Schumacher J, Eckhardt B
Fachbereich Physik, Philipps-Universität Marburg, D-35032 Marburg, Germany.
Phys Rev E Stat Nonlin Soft Matter Phys. 2001 Apr;63(4 Pt 2):046307. doi: 10.1103/PhysRevE.63.046307. Epub 2001 Mar 27.
The evolution of turbulent spots in a parallel shear flow is studied by means of full three-dimensional numerical simulations. The flow is bounded by free surfaces and driven by a volume force. Three regions in the spanwise spot cross section can be identified: a turbulent interior, an interface layer with prominent streamwise streaks and vortices, and a laminar exterior region with a large scale flow induced by the presence of the spot. The lift-up of streamwise streaks that is caused by non-normal amplification is clearly detected in the region adjacent to the spot interface. The spot can be characterized by an exponentially decaying front that moves with a speed different from that of the cross-stream outflow or the spanwise phase velocity of the streamwise roll pattern. Growth of the spots seems to be intimately connected to the large scale outside flow, for a turbulent ribbon extending across the box in the downstream direction does not show the large scale flow and does not grow. Quantitatively, the large scale flow induces a linear instability in the neighborhood of the spot, but the associated front velocity is too small to explain the spot spreading.
通过全三维数值模拟研究了平行剪切流中湍流斑的演化。流动由自由表面界定，并由体积力驱动。在展向斑的横截面上可识别出三个区域：一个湍流内部区域、一个具有明显流向条纹和涡旋的界面层，以及一个由斑的存在引起大尺度流动的层流外部区域。在与斑界面相邻的区域中，清楚地检测到由非正规放大引起的流向条纹的提升。斑的特征可以是一个指数衰减的前沿，其移动速度不同于横向流出速度或流向滚动模式的展向相速度。斑的增长似乎与外部大尺度流动密切相关，因为在下游方向延伸穿过盒子的湍流带没有显示出大尺度流动且不会增长。定量地说，大尺度流动在斑的附近诱发了线性不稳定性，但相关的前沿速度太小，无法解释斑的扩展。
