Single layer hydraulic model : a theoretical analysis of urban ventilation mechanism in atmospheric boundary layer stratification

Abstract

Numerical simulation with Computational Fluid Dynamics (CFD) is employed to study the dynamics over hypothetical urban areas in neutrally and stably stratified turbulent boundary layers. In neutral stratification, a total of 144 two-dimensional computational domains (18 different building-height-to-street-width (aspect) ratios with 8 distinct roof geometries) were used in the numerical simulations. In stable stratification, six three-dimensional computational domains with different stratifications (Froude number = 1.7, 2.4, 2.8, 3.1, 4.0 and 4.6) were used for the investigation of dynamics in interfacial flows with stationary hydraulic jump by the single layer hydraulic model. Reynolds-averaged-Navier-Strokes (RANS) and Large-Eddy-Simulation (LES) coupled with Volume-of-Fluid (VOF) method were used in the numerical simulation of neutrally and stably stratified flows respectively. Friction factor (f) was proposed as a parameter to quantify the aerodynamic resistance over idealized urban geometries. Air change rate (ACH) and pollutant change rate (PCH) were suggested to appraise ventilation and pollutant removal at the roof level of street canyons in neutrally stratified turbulent boundary layer respectively. The fluctuation component of ACH was found to be directly proportional to f^(1⁄2) and PCH was found to behave distinctively in different flow regimes. Both fluctuation components of ACH and PCH dominated the ventilation and pollutant removal mechanism that could serve as the minimum performance indices. The vertical dispersion coefficient σ_z, which is a parameter commonly used in Gaussian plume model, was used to describe the dispersion characteristics after a pollutant line source placed at the street level. It was found that σ_z was directly proportional to f^(1⁄4) and the frictional force exerted by rough surfaces was able to alter the near-field and far-field behaviors of pollutant dispersion. Three different flow regimes, namely submerged hydraulic jump, stationary hydraulic jump and supercritical hydraulic jump were identified in stably stratified turbulent boundary layer modeled by the single layer hydraulic model. The dynamics were found completely different in each regime that must be analyzed separately. Detailed comparisons of vertical profiles and ventilation performance were reported for the flows in stationary hydraulic jump and supercritical hydraulic jump regime. Flows in stationary hydraulic jump regime were selected for in-depth analysis with different stratifications. It was found that the total ACH remained constant regardless the variation in flow stratification. Furthermore, the fluctuation and mean components of ACH increased and decreased with increasing stability respectively. This finding is quite different from our conventional understanding of the suppressed turbulence in strong stratification. Finally, the coherent structures of flow in stably stratified turbulent boundary layer with stationary hydraulic jump were closely inspected. A secondary turbulence production mechanism was found in the more stable stratification flows. The turbulence generated in higher stability also dissipated slower and traveled further. These findings explained the stronger fluctuation component of ACH in more stable environment.