A Study On The Inflow Turbulence In Simulation Of Microscale Atmospheric Environment By Multi-Scale Coupling Method

The study of microscale atmospheric environment in urban areas is significant for developing urban-canopy models,fast-response models and evaluation of urban environment,etc.It is promising to simulate microscale flow and dispersion by coupling microscale models with mesoscale models,of which one of the challenges is to add smallscale turbulence at the coupling interface.To solve this problem,the principal work and main findings in the present work are summarized as follows:(1)The characteristics of inner-and outer-layer turbulence in boundary layers with large roughness are investigated.Results show that turbulence in the outer layer follows the outer-layer-similarity law while turbulence in the inner layer is strongly influenced by the sizes and configurations of the roughness.(2)Theoretical analysis and numerical simulations are conducted to investigate the influence of inflow turbulence on the turbulent field downstream in boundary layers with large roughness.It is found that as the distance from the ground increases,the streamwise distance influenced by the inflow turbulence increases as well.The influence of the inflow inner-and outer-layer turbulence is also investigated,with results showing that turbulence in the inner layer develops rapidly and thus the inflow inner-layer turbulence dominates in the near field,while turbulence in the outer layer develops slowly and then the inflow outer-layer turbulence can influence the inner-layer turbulence in the far field by dispersion or advection.(3)An inner-and outer-layer combination based turbulence-generation method is proposed.In the outer layer,turbulence is generated by transforming the pre-computed database according to outer-layer-similarity law.In the inner-layer,the geometrical parameters of underlying surface,free velocity of the pre-computed turbulence are consistent with the target problem.The turbulence generated in the inner and outer layers are then combined in a weighted sum.The method is tested in a flow over a cluster of model buildings.Results show that the transition height should be located in the roughness sublayer,e.g.1.5h,where h is the averaged building height.By comparing the present method with previous turbulence-generation methods,it proves that the distance needed for the inflow turbulence to redevelop is reduced in the present method.(4)The turbulence-generation method is applied in multi-scale coupling method to generate small-scale turbulence at the coupling interface.The WRF(Weather Research & Forecasting)model is used as the mesoscale model and Large-Eddy Simulation model is used as the microscale model.The coupling simulation is applied in a flow with a pointsource dispersion over the central business district in an actual urban area.Results show that the new coupling method can improve the predictions of wind direction in the urban canopy,the TKE field,the scalar plume,the maximum and r.m.s.of concentration.These results indicate that the new method is an effective way to simulate the complex flow and dispersion in urban canopies(or flow among large roughness).