The Direct Numerical Simulation Of The Thermally Stratified Turbulent Channel Flow

In this paper,the heat transport and the three-dimensional structure of temperature fluctuations in the thermally stratified turbulent channel flow is investigated by direct numerical simulations with the Boussinesq approximation,with the Reynolds number Re_τ=393.7,Prandtl number Pr=0.71,Richardson numbers Ri_τ=-10,0 and 10.The results show that:1.Due to the coupling of the thermal buoyancy and the shearing of turbulence,turbulence lead the heat transport.In the inner region,the viscous diffusion increases under the unstable stratification condition,which makes the temperature pulsation in the near-wall region larger than that under the neutral condition,so the viscous dissipation term increases in order to balance the budget.In the outer region,under the unstable stratification condition,thermal buoyancy enhances the vertical heat transport and decreases the average temperature gradient,resulting in a smaller generation term under the unstable stratification than that under the neutral condition,thus reducing the root mean square of temperature and heat flux in the flow direction.The production term under the stable stratification condition is larger than that under the neutral condition,resulting in a larger averaged temperature gradient,thus suppressing the vertical temperature transport,which means an increase in temperature pulsation.Thus the root mean square temperature and the heat flux in the flow direction increase,and the vertical heat flux decrease.2.Under stable and neutral conditions,the three-dimensional structure of temperature pulsation is similar to the morphological characteristics of flow velocity pulsation in the near-wall region.The temperature structure scale under stable stratification(Ri_τ=10)decreases with height and is less than that under neutral condition,while the temperature structure scale under unstable stratification(Ri_τ=-10)increases with height and is larger than that under neutral condition.Near the wall,the inclination angle of the temperature structure is about 30 degrees under neutral conditions,and the inclination angle increases with the increase of height.In neutral and stable conditions,the volume of temperature structure increases linearly with height,while this law is now satisfied under the condition of unstable stratification.