湍流模型和施密特数对街谷内污染扩散模拟的影响

Influence of turbulence models and Schmidt number on numerical simulation of pollutant dispersion in a street canyon

选取不同的湍动施密特数,分别采用标准κ-ε模型和realizableκ-ε模型对孤立街道峡谷内的气流运动和污染物扩散进行了数值模拟.计算得到的气流速度场和污染物浓度分布表明,标准κ-ε模型和realizableκ-ε模型预测出极为相似的气流旋涡结构,即在峡谷内生成一个中心大致位于峡谷中央的顺时针大旋涡,并在此大旋涡的作用下污染物往峡谷的背风侧积聚.通过对比分析峡谷迎风面和背风面上的无量纲污染物浓度计算结果与风洞试验数据表明,湍动施密特数的改变并不影响计算空气流场,但改变污染物扩散方程中的湍动扩散率,增大湍动施密特数将减弱污染物的湍动扩散作用,使得峡谷顶部污染物外溢减少,从而导致峡谷内的污染物浓度计算值增大;只要给定合适的计算参数,标准κ-ε模型和realizableκ-ε模型均能预测出峡谷内的污染物扩散分布,标准κ-ε模型合适的湍动施密特数为0.4～0.6,而realizableκ-ε模型则为0.3～0.5;当取湍动施密特数为0.7（Fluent软件系统中的默认值）时,标准κ-ε模型的计算精度高于realizableκ-ε模型的计算精度.

Numerical calculations were performed using the standard and realizable κ-ε turbulence models with various turbulent Schmidt numbers （Sct） to simulate the airflow and pollutant disper sion inside an isolated street canyon. The simulated wind fields and pollutant concentration contours show that the two studied κ-ε models produce a similar clockwise main vortex that controls pollutant transport inside the canyon. The calculated non-dimensional pollutant concentration profiles along the leeward and windward walls of the canyon were compared with the wind tunnel measured data. It is revealed that, Sct influences only the turbulent diffusion of pollutant and not the dynamic, fluid field, the pollutant discharge from the canyon is declined and therefore the computed pollu-tion level inside the canyon is increased by artificially increasing Set ; both the standard anti reanzable κ-ε models with the appropriate choice of coefficients are applicable to simulate the pollutant dispersion within an isolated canyon, and the optimum value of turbulent Schmidt number is in the range of 0. 4±0.6 for standard κ-ε model and 0. 3±0.5 for realizable κ-ε model; in the case of a Set = 0. 7（the default value of Set in Fluent）, the predictions of standard κ-ε model is in better agreement with the experimental data comparing to that of the realizable κ-ε model.