Investigation On The Entrainment Characteristics Of Sheared Convective Boundary Layer In The Atmosphere Environment

Stratified convective flow is a common flow phenomenon in the agrometeorological environment,and the atmospheric convective boundary layer(CBL)is a typical one.The turbulent characteristics of the atmospheric CBL with horizontal velocity shear are quite different from that of pure convective CBL without the shear.It is of great scientific significance to study the flow field characteristics of entrainment and mixing process of the sheared CBL and the relevant scaling relations for the evaluation of agricultural meteorology,the monitoring and analysis of drought in desert areas and the monitoring of air quality.In this thesis,by taking horizontal homogeneous and dry atmospheric CBL as the research object,the dynamic characteristics of turbulent entrainment in the sheared CBL are studied by using Large-Eddy simulations(LES)and stratified convective flume experiments.LES is carried out in a 6000 m×6000 m×2000 m atmospheric domain to study the effects of velocity difference,potential temperature gradient and buoyancy flux on the turbulent characteristics of CBL.(1)The results show that the formation of the entrainment layer is affected by the intersection of the coupled horizontal shear effect and the vertical buoyancy effect.(2)The thermal bubbles and velocity contours show the characteristics of turbulence intermittency and anisotropy,with the mixed layer dominated by stable small eddies,whereas stripped large eddies appearing in the entrainment layer under the shear effect in the flow direction,and the temperature peak surface of the bottom in the calculated domain presents a two-belt distribution along the spanwise direction,which indicates that the characteristic size of the entrainmented vortex is about 1/4?1/3 of the domain width.(3)By studying the vertical variation of buoyancy flux,it is found that the velocity difference of the entrainment layer and the surface buoyancy flux promote the entrainment process,and the height of the entrainment layer shifts to the layer with higher velocity.The potential temperature gradient in the free atmosphere inhibits the growth of the entrainment layer thickness,especially the upper limit of the entrainment layer.(4)It is further found that the phenomenon of shear restraining entrainment occurs in the cases with weak shear and strong potential temperature gradient.In other cases,with the enlargement of velocity difference,the turbulent kinetic energy generated by shear cannot be completely dissipated by viscosity,and the remaining part is balanced by the negative buoyancy of the entrainment layer,which makes the entrainment flux ratio increases,while the shear stress near the surface has no obvious effect on the turbulent kinetic energy of the entrainment layer.(5)The entrainment flux ratio is in the range of 0.176?0.385 for all the simulated cases,significantly higher than 0.2 of pure buoyancy CBL.The parameterization of characteristic quantity is deduced and verified:(1)Based on the turbulent kinetic energy equation in the framework of the CBL"first-order model",the generation and consumption contributions of shear and buoyancy turbulent kinetic energy are calculated respectively,and the formula of the proportion of shear generated turbulent kinetic energy consumed to promote entrainment is developed.The ratio is in the range of 0.3?0.38,and it seems to increase slightly with the enhancement of shear strength.(2)According to the data from simulated cases,the current available scaling relations of the entrainment layer thickness cannot reflect the overall influence of boundary characteristic parameters of the boundary layer on the entrainment process,leading to inaccurate or application limitation.This thesis replaces the potential temperature step value of the entrainment layer with the potential temperature gradient as the characteristic parameter,and incorporates the velocity difference into the characteristic velocity scale,thus the influence of shear effect on entrainment is fully considered.A new scaling relationship between the dimensionless entrainment layer thickness?(?)/(?) _iandRi_NC^-1/2is established,that is,?(?)/(?) _i=1.26Ri_NC^-1/2,whereRi_{N C} is the modified buoyancy Richardson number,and(35)z and z_iare the thickness and height of the entrainment layer.This new scaling relationship agrees well with the simulated data of Martin and Wyngaard and the available lidar results of Boers and Eloranta,lothon et al.Furthermore,a new experimental set-up is designed and constructed in this thesis to carry out the sheared stratified flow experiments with the Particle Image Velocimetry(PIV)technology.The experimental set-up simulates the stratified effect of CBL through the density difference between the saline water in the lower layer and the fresh water in the upper layer,imitates the shear effect through the different flow velocities of the upper and lower layers,and simulates the buoyancy flux from the surface of CBL through the freshwater plume from the micro holes in the bottom.The velocity vector data of the flow field are obtained by PIV.To understand the flow dyanmics characteristics of the sheared stratified boundary layer,a series of experiments are carried out under the conditions of flow velocity difference in the range of 0?0.25 m/s,bottom buoyant plume flow rate of20?40 L/h,and the density difference of 25?60 kg/m³.(1)The results show that when the velocity difference is small,the velocity shear is not strong enough to overcome viscous force to form entrained vortex,but then the velocity shear is over0.04 m/s,the formation,development and breaking process of the entrained vortex can be observed.After about 12z_iaway from the entrance of the flume,the entrainment layer in the middle is in a stable turbulent state.(2)It is also found that there are relatively large changes of the horizontal and vertical velocities in the middle layer.Taking the maximal velocity gradient as the height of the entrainment layer and the height difference between the velocity gradient becomes zero in the layer as the thickness of the entrainment layer,the effects of the experimental boundary conditions on the height and thickness of the entrainment layer are also studied.The results show that the influencing factors of entrainment are consistent with the simulated cases results.It is found that the thickness of the entrainment layer increases with the increased bottom plume flow rate which promotes the entrainment in the layer above,the increase of velocity shear,but on the contrary,the increase of the salinity in the layer below makes the density gradient in the middle interface,leading to reduced thickness of the entrainment layer.(3)With the experimental data,it is found that the scaling relationship between the entrainment layer thickness andRi_NC^-1/2is consistent with the simulation results,that is,?(?)/(?) _i=1.17Ri_NC^-12,but the coefficient is slightly smaller.The possible causes for this difference is also explored in the thesis.