Studies Of Vortical Structures And Transition Mechanisms In Transitional Boundary Layers

Transition from laminar flow to turbulent flow is one of the most important and complex problems in the field of fluid mechanics research. Vortical structures and transition mechanisms in the transition process of compressible boundary layer flow over a flat plate are investigated and explored using the most accurate method, direct numerical simulation of Navier-Stokes equations. The details of late stages of boundary layer transition based on the spatial model are revealed. Previous numerical simulation and experimental results are verified. Based on our accurate simulation results, some new ideas are put forward and some new mechanisms are studied and revealed.In this paper, direct numerical simulation has been carried out. The time-dependent Navier-Stokes equations are solved directly by a third-order TVD Runge-Kutta method. A sixth order central compact scheme that facilitates high resolution of the flow field is used for spatial discretization together with a sixth order implicit compact filter. To avoid possible non-physical wave reflection from the boundaries, the non-reflecting boundary conditions including transverse and viscous effects are specified at the far field and the outflow boundaries. The inflow is specified by laminar flow profile with imposed eigenmodes of two-dimensional and three-dimensional Tollmien-Schlichting (T-S) waves. The parallel computation is accomplished through the Message Passing Interface (MPI) together with a domain decomposition approach. The DNS results show the mean flow properties, such as the skin friction coefficients and the mean velocity profile, wall shear linear law, log law in the turbulent region, as well as the spatial evolution of disturbance modes, which agree very well with the theoretic and experimental results.This paper is devoted to thorough and considerate researches of transition process in a boundary layer, especially the vortical structures at the late stages of nonlinear transition process, including the formation of typical vortical structures, important flow transition events and their relationship, voritcal structures at very late stages of transition and the flow randomization process. The mechanisms of boundary layer transition are investigated, and the major research works include:1. Typical vortical structures in transition process are discussed, focused on vortical structures and some new mechanism of the formation and evolution of streamwise vortex and ring-like vortex. Our numerical results shown that the mechanisms of disturbance development predominant at late stages of boundary-layer transition have universal typical vortex structure, despite the two regimes of transition (K- and H-regimes) have different peculiar properties and relative spatial positions at weakly nonlinear stages. In this paper, a new study for the mechanism of the ring-like vortex formation is discovered. According to our recent DNS results, the ring-like vortices generation is caused by the interaction between the prime streamwise vortices and secondary streamwise vortices. The ring-like vortex formation is a key issue of flow transition. The rotation movements of ring-like vortices induce huge momentum and energy transfer form high energy inviscid outer part of boundary layer to low energy inner part of boundary layer. There is no turbulence without ring-like vortices.2. Some important events in transition process and the corresponding effects on transitional flow field are studied by fine gird direct numerical simulation results. Complex ejection and sweep movements have close relationship with vorical structures. The details of flow structure around?vortex and ring-like vortex are studied. The relationship between ejection/sweep movements and vorical structures has been verified, which is consistent with experimental work. It is revealed that the mechanism of the second sweep generation, the positive spike formation, high shear layer and Reynolds stress distribution, and their relationship with ring-like vortices. In particular, numerical results show that positive spikes in the near-wall region induced by the ring-like vortices, propagate downstream with the same speed as the ring-like vortices, and the speed is several time faster than surrounding flow.3. At the very late stages of transition, complex U-shaped vortex, barrel vortex, turbulent spot and small scale vortex are investigated. In particular, It is found that while new ring-like vortex are generated, vorticity of the leading ring-like vortex became weaker and dissipated gradually; on the other hand, small scale vortex is generated under the interaction between secondary vortex and solid wall, and induced by the evolution of large vortex structure.In brief, a high-precision and high efficiency of numerical simulation method for compressible boundary layer transition is developed. Through the accurate numerical simulation of the vortical structures and analysis of the mechanism in transition process of compressible boundary layer flow, the whole procedure at the late stage of boundary layer transition has been clearly presented. Several new mechanisms are analyzed and revealed, such as the formation and evolution of ring-like vortices, the production of second sweep and positive spikes, and the formation theory of small-scale vortex structure. The research has important theoretical significance and application prospects.