Small-scale Turbulence Characteristics Of Cylinder And Screen Strip Wakes

Turbulence is a kind of complex flow phenomenon in nature.Turbulence can be roughly divided into large-scale turbulence and small-scale turbulence.Many researchers believe that small-scale is most universal.Therefore,small-scale turbulence has always been the focus of domestic and foreign scholars.Local isotropy is an important hypothesis in the theory of small-scale turbulence.In many cases,small-scale turbulence is anisotropy.The reason may be that small-scale turbulence is affected by some large-scale structures at finite Reynolds number.Coherent structure is a kind of large-scale structure.It is also worth exploring whether the higher order turbulence statistics satisfy local isotropy when the lower order turbulence statistics satisfy local isotropy.Firstly,some isotropic tensors of the fourth-order,fifth-order and sixth-order in the temperature field are derived,and the relations among the statistics are given.In the range 40?x/h?120 downstream of a screen strip wake,the degree of deviation from local isotropy of second-order statistics,third-order statistics and fourth-order statistics(the statistics here are all composed of (?)ui/(?)xj)are compared.Overall,the higher the order of the statistic is,the more the statistic deviates from local isotropy.It shows that when the low-order turbulence statistics satisfy local isotropy,the higher-order turbulence statistics may deviate from local isotropy.Next,the distribution of the second-order phase average structure function at phase and scale(r-?)plane is calculated in a screen strip wake.It is found that the energy of the total fluctuation is mainly derived from the random motion at small-scale.Along the y-direction as the position deviates from the center line of the wake the scale effected by coherent motion is smaller.Finally,non-dimensional transport equation for the mean turbulent energy dissipation rate tensor in homogeneous and local isotropic turbulence is derived.It is found that if turbulence Reynolds number is sufficiently high,the effect of non-dimensional mean shear parameter on small-scale and local isotropy is negligible.In other words,local isotropy is possible in the presence of mean shear.The terms of the transport equation for the components ?((?)?/(?)x1)2,?((?)?/(?)x2)2 and ?((?)?/(?)x3)2 of temperature dissipation rate are calculated at x/d=240 in a cylinder wake.It is found that the production due to the stretching of the temperature field by the turbulent strain field and dissipation of temperature dissipation components are dominant in the process of the three temperature dissipation components.Large-scale term also contributes to this process.This reflects the influence of large-scale term on small-scale term.In general,the contribution of large-scale term is the largest in the transport process of ?((?)?/(?)x2)2,while the contribution of large-scale term is the least in the transport process of ?((?)?/(?)x1)2.