Large Eddy Simulation Of Coherent Structure In Boundary Layer Turbulence Over Flat Wall And Riblets Surface

There are large eddy coherent structures in turbulent flow. The coherentstructures is the domination of the turbulent momentum, energy movement and isthe leading function of the generation of turbulent energy and the manipulation ofthe turbulence, it is the essence feature of turbulent flow. People can understand theprocess of turbulent generation, development and evolvement morecomprehensively through studying coherent structures, and offer likelihood methodof manipulating turbulence by changing coherent structures. There are lots offactors influencing turbulence, such as making use of riblets surface to changecoherent structures of turbulence, it is used widely in the investigation about dragreduction in spaceflight and navigation.By analyzing the flow visualization results of the low speed streaks in the nearwall region for the turbulent boundary layer flow over flat wall and reblets surface,it is found that low speed streaks appear to be flat and linear. The average of thenon-dimensional low speed streaks is influenced by the scale of riblets surfaces,and its value is less than the flat wall results. For the same riblets plate at a fixedposition, the average of the non-dimensional low speed streaks decreases as thenon-dimensional distance decreases. This implies that the riblets plate stronglyinfluences flow in the near wall region.This paper takes the coherent structure in boundary layer over flat wall andriblets surface as the research objective, three-dimensional flow over flat wall andriblets surface is simulated by virtue of large eddy simulation given by the sameinlet condition, boundary condition, turbulence intensity and subgrid model. Thesubgrid model is Germano dynamical model and the numerical difference schemeis center difference scheme of two-order precision. The grids at flow direction areassigned equably, the nodes are arranged with index mode at normal direction, andthe nodes are assigned encrypt gradually from the bottom to top, so as to ensurethere are 70% of nodes near the boundary. Using CFD software, the full developedturbulent flow in channels at Reynolds number 6 × 106 is simulated respectively.The paper analyzed the generation, evolution and instant vorticity field of thecoherent structure over flat wall and riblets surface, including little scale turbulentstructure and instantaneous state course, such as the spurt and sweeping course ofstrips, and near wall turbulent structure, and etc. The outcomes can be validated byits experimental counterpart. The results show the moment developing process offlow over flat wall and riblets surface and the evolvement of coherent structure,and also completely simulate all kinds of detail process in development ofboundary layer over flat wall and riblets surface. Through comparing, it is foundthat the low speed streaks appear to be flat and linear over riblets surface. Thismeans that the stream-wise grooves limit the flow motion in the spanwise direction,and the flow stability is enhanced.This paper also calculated the resistance of the boundary layer over flat walland riblets surface by virtue of large eddy simulation. Obtained the curve ofresistance of the two boundary in the same inlet, boundary and other externalconditions. Through comparison, the friction resistance of boundary layer overriblets surface is smaller than that of flat wall. These results offer theoreticalfoundation for the mechanism of drag reduction. At the same time, these alsoprovide the theoretical support for changing the coherent of turbulent flow tocontrol turbulence.