Research On Turbulence Generation And Sustenance Based On DNS Of Transitional Boundary Layer

Not only does boundary layer transition play a critical role in many fluid dynamic applications,but it also becomes a very important subject in the study of turbulence generation,development and sustenance because of its rather ordered vortical strucutres.In pursuit of the mechanism behind boundary layer transition,especially the late stages of trantion,a high fidelity direct numerical simulation was conducted on a flat plate boundary layer flow.Based on the data obtained,some new findings are gained.?1?Multiple level vortical structures are found in boundary layer transition.It is confirmed that ejections and sweeps excited by vortical structures are responsible for bringing energy from inviscid area to the bottom of the boundary layer.Then the high shear layer induced by positive spikes leads to the formation of new small scale vortices.In addition,the vortices are quite stable and never have been observed to break down as stated in the classical turbulence theory.The development and formation of multiple level vortex packages are carefully investigated.As indicated,the transition from laminar flow to turbulence is a “buildup” process of vortical tructures.?2?Studies on some other issues concerning the boundary layer transition are also carried out,including turbulence bursting and intermittency,the increase of total vorticity during transition,whether vortices locate at areas with concentration of vorticity and so on.It is concluded that the intermittent bursting of turbulence results from the passage and development of vortical structures,and it is a manifestation of the regular “buildup” process of vortical structures from an Eulerian point of view.In addition,although vorticity flux must keep conserved during transition,negative and positive vorticity fluctuations are created due to tilting and stretching of vortex filaments according to vorticity transport equations,both of which can drive rotational movement of fluids and can not be simply assumed to balance each other.Therefore,the transition process can not be viewed as a vorticity redistribution process.It is also surprisingly discovered that the?-vortex does not represent the congregation of vortex filaments which is intuitively widely accepted as a key feature of vortices.?3?Vortex definition and identification have been a longstanding issue for turbulence research.A new vortex identification criterion called ?-method is proposed based on the idea that dissipation is minimized in a rigid rotation of fluids and the rotation is closedly reltated to the rate of dissipation.The comparison with other vortex identification methods like Q-criterion and ?₂-method is conducted and the advantages of the new method can be summarized as follows.1.The method is able to capture vortices well and very easy to perform;2.The physical meaning of ? is much clearer;3.The ?-criterion doesn't require a case related thresholds while Q and ?₂ methods do,and iso-surface of ?=0.52.can always capture vortices properly in all the cases we investigated.4.Both strong and weak vortices can be captured well simultaneously while improper Q and ?₂ thresholds may lead to strong vortices captured while weak ones omitted or weak vortices captured but strong ones smeared.