Numerical and experimental study of the aerodynamic characteristics around two-dimensional terrain with different slope angles

Complicated terrain was considered and simplified as two-dimensional(2D)terrain in a dynamical downscaling model and a parametric wind field model for typhoons developed by the Shanghai Typhoon Institute.The 2D terrain was further modeled as uphill and downhill segments with various slope angles relative to the incoming flow.The wind speed ratios and pressure characteristics around the 2D terrain were numerically and experimentally investigated in this study.Aerodynamic characteristics of the 2D terrain with a limitedlength upper surface were first investigated in the wind tunnel with sheared incoming flow.The corresponding numerical investigation was also conducted by using the commercial computational fluid dynamics code FLUENT with the realizable k-ε turbulence model.Special efforts were made to maintain the inflow boundary conditions throughout the computational domain.Aerodynamic characteristics were then investigated for the ideal 2D terrain with an unlimited-length upper surface by using a numerical method with uniform incoming flow.Comparisons of the different terrain models and incoming flows from the above studies show that the wind pressure coefficients and the wind speed ratios are both affected by the slope angle.A negative peak value of the wind pressure coefficients exists at the escarpment point,where flow separation occurs,for the uphill and downhill terrain models with slope angles of 40°and 30°,respectively.Correspondingly,the streamwise wind speed ratios at the points above the escarpment point for the uphill terrain model increase with increasing slope angle,reach their peak values at the slope angle of a=40°and decrease when the slope angle increases further.For the downhill terrain model,similar trends exist at the points above the escarpment point with the exception that the critical slope angle is a=30°.

Impact of physical representations in CALMET on the simulated wind field over land during Super Typhoon Meranti(2016)

A WRF(Weather Research and Forecasting Model)/CALMET(California Meteorological Model)coupled system is used to investigate the impact of physical representations in CALMET on simulations of the near-surface wind field of Super Typhoon Meranti(2016).The coupled system is configured with a horizontal grid spacing of 3 km in WRF and 500 m in CALMET,respectively.The model performance of the coupled WRF/CALMET system is evaluated by comparing the results of simulations with observational data from 981 automatic surface stations in Fujian Province.The root mean square error(RMSE)of the wind speed at 10 m in all CALMET simulations is significantly less than the WRF simulation by 20%^30%,suggesting that the coupled WRF/CALMET system is capable of representing more realistic simulated wind speed than the mesoscale model only.The impacts of three physical representations including blocking effects,kinematic effects of terrain and slope flows in CALMET are examined in a specified local region called Shishe Mountain.The results show that before the typhoon landfall in Xiamen,a net downslope flow that is tangent to the terrain is generated in the west of Shishe Mountain due to blocking effects with magnitude exceeding 10 m/s.However,the blocking effects seem to take no effect in the strong wind area after typhoon landfall.Whether being affected by the typhoon strong wind or not,the slope flows move downslope at night and upslope in the daytime due to the diurnal variability of the local heat flux with magnitude smaller than 3 m/s.The kinematic effects of terrain,which are speculated to play a significant role in the typhoon strong wind area,can only be applied to atmospheric flows in stable conditions when the wind field is quasinondivergent.

Numerical investigation of the wind environment around tall buildings in a central business district

The wind environment around tall buildings in a central business district(CBD)was numerically investigated.The district covers an area of-^4.0 km2 and features a high density of tall buildings.In this study,only buildings taller than 20 m were considered,resulting in 173 tall buildings in the analysis.The numerical investigation was realized using the commercial computational fluid dynamics code FLUENT with the realizable A-6:turbulence model.Special efforts were made to maintain inflow boundary conditions throughout the computational domain.The reliability of the numerical method was validated using results from an experimental investigation conducted in the core area of the CBD(〜1.5 km2).Experimental and numerical investigations of wind speed ratios at the center of the three tallest buildings in the CBD agree within an uncertainty factor of 2.0.Both the experimental and numerical results show that wind speed ratios in the wind field with exposure category D are higher than those from the wind field with exposure category B.Based on the above validation work,the wind environment around tall buildings in the whole CBD was then investigated by numerical simulation.Common flow phenomena and patterns,such as stagnation points,shielding effects,separation flow,and channeling flow,were identified around the tall buildings.The pedestrianlevel wind environment around tall buildings in the CBD was further evaluated using nearby meteorological wind data.The evaluation results show that some pedestrian activities,such as sitting at the center of the three tallest buildings,are unadvisable when the wind blows from the south-east.