Inherent Mechanism Of Breakdown In Transition & The Eddy Viscosity Model Of Periodic Turbulent

Laminar-turbulent transition is an old yet unsolved problem. During the past hundred years,People has made great progress in resolving the problems in the engineering. Notwithstanding the great effort made, there is an important question that has not been addressed, that is, what is the inherent mechanism of breakdown that eventually leads to transition? The conventional idea is that the transition starts from the amplification of disturbances, and when the disturbances become larger, higher harmonics will be generated due to non-linear effect, which makes the flow more and more complicated, and finally the flow becomes turbulent. Though the scenario seems clear, yet there is a missing link, that is, what happens in the breakdown process. Here we show by analyzing the results from direct numerical simulations that the change of stability characteristics of the mean flow profile plays a key role in the breakdown process.At present, only the turbulent model can be used to dealing with the engineering turbulent problems. Most of those methods accepted to enclosing the equations are using some kind of eddy viscosities to joint Reynolds-stress with Mean Rate-of-Strain. The deference among deferent models or sub-grad models is only the deferent methods of the calculating of eddy viscosities. Despite of what those methods are, the eddy viscosities which have been used are all real numbers, that is the phase between Reynolds-stress and Mean Rate-of-Strain did not been considered in all those models. The study of the form of eddy viscosity of turbulent modeling recently shows that it should not always be a real one which were used in all the exist turbulent eddy viscosity models, and thus a new challenge is posed for the eddy viscosities of turbulence model. In this paper, for the purpose of get the exact form of eddy viscosity , the phase of Reynolds-stress and Mean Rate-of-Strain in a channel flow with periodic suck-blow boundary condition was studied by the means of DNS, and at the same time, Reynolds Number, suck-blow amplitude and the moving of suck-blow, which may affect on the phases, have been discussed also. More attention was paid on the phase difference of the two stresses. The result shows that phase difference shouldbe considered in concerned eddy viscosities. At the end, the form of eddy viscosity in this flow was discussed.