Chaoqun Liu

By Ping Lu, Manoj Thapa and Chaoqun Liu

Part of the book: Advances in Modeling of Fluid Dynamics

By Chaoqun Liu and Shuhyi Chen

Turbulence is one of the most important but unresolved problems in modern fluid dynamics. Mathematically, one counterexample can overthrow a theory and we really do not need a second example. However, classical and current turbulence theories are filled with not only one but many self-contradictions. Therefore, we really do not have the right turbulence theory. The transition community believes that turbulence is generated by “vortex breakdown,” but the turbulence community believes there are coherent structures in fully developed turbulence after vortex breakdown. It is equivalent to say we study the structure of a house after the house collapses and breaks down. This is really ridiculous. There are many others. According to Liu, (1) flow transition is not a process of vortex breakdown but turbulence vortex structure buildup; (2) the nature of turbulence generation is that fluids cannot tolerate high shear and shear must transfer to rotation and form a very fast rotation core; (3) “shear layer instability” is the “mother of turbulence”; (4) turbulence small scales are generated by multiple-level shear layers that are generated by multiple level sweeps, ejections, negative and positive spikes; (5) large vortex provides energy to smaller vortices through fast rotation, which causes strong sweeps. According to Liu, the nature of the flow transition is mainly caused by vorticity rollup from the wall. Flow transition is vorticity redistribution and increment from near wall to whole boundary layer. In addition, flow transition is a process of non-rotational vorticity transferring to rotational vorticity.

Part of the book: New Perspectives in Fluid Dynamics

By Yifei Yu and Chaoqun Liu

Vortex definition and identification are extremely important for the study of fluid dynamics research. Liutex is a newly proposed concept that correctly represents vortex. Liutex is a vector whose direction is the local rotation axis and whose magnitude is twice the angular speed. To identify the unique structure of a vortex, a method known as the Liutex Core Line method has been developed, which displays the rotational core axis of a vortex. However, the original method is a manual method, which is not practical for real application, and an automatic algorithm is required for practical usage. Xu et al. proposed an algorithm by selecting the best line from a group of candidate lines, which is an important progress. In this chapter, from another perspective to solve this problem, a new algorithm is introduced based on forward Liutex magnitude gradient lines. Since gradient lines have the feature that they advance to the local maximums, the route will still result in a unique line, which avoids the process to find the best line. This algorithm has achieved some success for the Lambda vortex in early boundary layer transition.

Part of the book: Vortex Simulation and Identification