Study On CFD Simulation Of Turbulence Wind Field Near Ground

The turbulence wind field near ground is the main subject of wind engineering. CFD simulation approach is economic both in time and expenses, able to provide comprehensive information or results, and convenient for parameter analysis. It also is a growing approach for wind engineering. The existing empirical models of downbursts is low accuracy and only applicable for limited region, furthermore, the non-stationary wind loads generated in moving downbursts are difficult to be reproduced. Studies on wind field in hilly terrain are meaningful for determining wind loads on structures, micro-siting wind turbines and evaluating wind resources. The gust effects on pedestrian comfort cannot be ignored, and researches on evaluation of pedestrian wind environment using CFD is scared. Peak local fluctuating pressure is several times larger than mean pressure and able to magnify responses of structures, however, it is hard to simulate fluctuating pressure in CFD for present. For the issues addressed above, the main contents of this dissertation including 4 parts:A simple and novel vertical shaping function of radial velocities and a revision to radial shaping function of radial velocities are proposed to improve the simplicity and accuracy of the existing model for 3-D axisymmetric steady-state flows of downbursts and impinging jets. From the mass conservation equation of fluid mechanics, a general method is refined to derive vertical velocities from radial velocities. By incorporating varying characteristic lengths related functions: z_m(r)(δ(r)), r_m (z), u_m (r)和u_m (z), the revised model is able to compute the nonlinear developments of downburst boundary layer. Through comparing with CFD simulation results, the applicable region and precision of revised model are validated and verified. By imposing an atmospheric boundary layer wind, the famous Andrews AFB moving downburst was reproduced in CFD simulation and the results is consistent in both wind velocities and directions with field data. The computational results also revealed that the length scale Rc which influences the horizontal profiles of radial velocity should take different values for the initial and mature stage, and for the location of FFD (Front Flank Downburst) and RFD (Rear Flank Downburst). The wind characteristics of 6 typical hills (15°, 30°, 45°escarpments and ridges) covered by grass and forest vegetation were investigated by wind tunnel test. The velocity ratio distribution on the crest and downwind slope were obtained, and the effects of slopes, hill shapes and ground roughness on the wind velocity ratios were summarized. Through comparison with Codes from several countries, 5 recommended revisions were proposed for the current Chinese loadings Codes. Two strong wind events over a real complex terrain were simulated using CFD. Based on the comparison with field data from 9 observation towers and error analysis, the best turbulence model and grid size for complex terrain simulation is obtained. Using wind velocity ratios from CFD simulation and longtime meteorology statistical data from ambient flat terrain, a composite full-direction wind resources evaluation method aim for complex hilly terrain was proposed.A criterion that takes into account both mean and gust wind effects on people was proposed. Combined with the wind environment problems emerged from a semi-open type large-span building, 3-years long hour by hour wind records from 44 meteorological stations around the site were collected, and the CDFs parameters of mean wind speeds in 8 directions were estimated. The distribution of wind velocity ratios at pedestrian level was obtained using CFD simulation. Using the approach of threshold exceedance probability, the map of wind discomfort threshold exceedance probabilities for full directions was present, and the pedestrian wind environment around this building is evaluated.The discretizing and synthesizing of random flow generation technique (DSRFG) incorporated with large eddy simulation was adopted to produce a spatially correlated turbulent inflow for the simulation study. With regard to the 486m-long roof of Shenzhen Citizens Centre, its mean, fluctuating and peak pressure coefficient distributions, and spectra, spatial correlation coefficients and probability characteristics of pressure fluctuations were simulated in CFD and wind tunnel. The comparative study demonstrated that the LES integrated with the DSRFG technique could provide satisfactory prediction of wind effects on the long-span roof with complex shape, especially on separation zones along leading eaves where the worst negative wind-induced pressures commonly occur. The probabilities and spectra of pressure at roof eave, middle and rear sections were well generated. The non-gaussian characteristics and intermittence phenomenon in flow separation region were also captured. The space correlation of fluctuating pressure for longitudinal and lateral directions is agreed well with experimental data.