Study Of Dynamic And Thermal Characteristics In Turbulent Channel Flow Bounded By Rough Walls

Wall-bounded turbulence widely exists in nature and industrial applications,and turbulent shear flow with rough walls is one of the typical flow patterns in the environment and engineering technology.It is found that the roughness elements have influence on many aspects of turbulent flow,including the turbulent characteristics near the wall,resistance,heat and mass transfer,mixing and transport efficiency and so on.For example,the micro-rough structures on the surface of high-speed aircraft may increase the surface friction and temperature,and result in an increase of energy consumption and a hidden burning danger of skin material;On the other hand,the application of roughness elements in industrial components can effectively improve the heat exchange efficiency of the flow system.Therefore,the study on rough-wall turbulence has important scientific meaning and engineering application value.The study will help to understand the dynamics and thermodynamic mechanisms affecting the statistical characteristics of turbulence in the rough-wall boundary layer,optimize the design of the wall roughness element,reduce the flow resistance and improve heat transfer efficiency.Turbulent channel flows bounded by rough walls are numerically studies by direct numerical simulations(DNS)based on Navier-Stokes equation and the lattice Boltzmann equation.Three typical turbulent problems are studied,including turbulence and heat transfer modulations in channels roughened by cube-covered surface,turbulence modulations by spanwise oscillations of a channel wall with porous layer and turbulence modifications in open channel flow over an anisotropic porous wall.Friction Reynolds number Re?=180.In this study,it is found that rough surface has a significant influence on the mean and fluctuating quantities of the turbulent velocity and temperature fields,as well as the turbulent structure and heat transport process,and results in different behaviors of the flow resistance and heat transfer efficiency.The main results and conclusions are given as follows:(1)Turbulent channel flows roughed by cube-covered surface are studied numerically by lattice Boltzmann method.By changing the height of the cubes ?+ ?(6,18),it is found that temperature field and velocity field correlate strongly.The heat transfer augmentation is always accompanied by a drag increase,and both the velocity and thermal roughness function are related to the wall-normal velocity fluctuations vrms in the plane of the cube crests.Further analysis of the decomposition of the drag coefficient and Nusselt number found that the modulations of the turbulent structures by the cubes are the main factor for the enhancement of the drag coefficient and Nusselt number.In addition,although the heat transfer efficiency increases with the height of the cubes,considering the drag consumption,it is found that the net heat transfer efficiency is no longer monotonic to the height of the cubes,the maximum net heat transfer efficiency improvement(26%)is obtained at ?+=12.(2)The coupling effect of the porous media and the spanwise oscillation of wall surface on the modulation of the turbulent channel flow is numerically studied by the lattice Boltzmann method.The Darcy number and porosity are Da ?(5×1017,5x10-3)and? ?(0.4,0.9),respectively.The period and amplitude of the oscillation are T+=125 and Wm+=4.5.Firstly,we derive the theoretical solution to the Stokes second problem of two-layer fluids.It suggests that in the laminar state,the porous medium will amplify the effect of the wall oscillation,resulting in an increase of velocity near the wall,which may enhance the drag reduction rate of the wall oscillation.In the turbulent state,it is found that a significant net power saving%Pnet is obtained with lower permeability porous media.This lies in the modulation on the turbulent structure and Reynolds stress.For the first time,a shape parameter S is proposed to describe the correlation between the porous media and the drag coefficient,and the nonlinear relationship between the drag reduction rate and the shape parameter is obtained.The required power for oscillating wall is reduced,because in the specific decelerating phase,the fluid will promote the movement of the porous medium due to its inertia.The maximum drag reduction is obtained with porous parameter Da=5×10-7 and ?=0.8.At these parameters,a 13.02%net power saving%Pnet is obtained as compared to the fixed smooth wall case.(3)The finite difference method is used to numerically study the modulation of channel turbulence by anisotropic porous media.By changing the anisotropy ratio r ?(1,100)and thickness of the porous medium hp+ ?(0.9,54),it is found that the modulations of the turbulent structures near the wall have a significant influence on the turbulent drag.When the porous thickness hp+ is smaller than the critical thickness hc*or anisotropy ratio r is larger than the critical ratio r0,the sweeping and ejection movements are weakened by restriction of the formation of the hairpin vortices,which depresses the momentum exchange between the near wall region and outer layer.Consequently,the flow resistance is reduced.When the hp+ is larger than hc*or r is smaller than r0,due to the weakening effect of the wall-blocking,the turbulent vortex motions can penetrate deeply into the porous medium and cause a strong wall shear,subsequently the flow resistance increases.Dimensional analysis shows that the critical thickness of the porous medium hc*is linearly related to the Reynolds number of the spanwise permeability hc*?Rekz,which is consistent with the numerical result.In our study,when r=100 and hp+=9,the maximum drag reduction obtained is about 20.3%.