In this work, a large eddy simulation (LES) model, which includes momentum and heat source (or sink) inside the tree planting layer, is proposed for the simulation of flow in a street canyon with tree planting. Vegetation canopy layer simulation shows that this model can be used to simulate the velocity distribution and temperature variation inside the canopy layer. Effects of atmospheric instability on flow and pollutant distribution in a street canyon with tree planting of an aspect ratio of 0.5 are studied. Results show that compared with the canyon with no tree planting (or the exposed street canyon), the canyon with tree planting shows a reduced wind circulation and pollutant exchange rate (PER) at the top layer of the street canyon, which induces the increase in the pollutant concentrations near road surface, leeward wall and windward wall. When street canyon atmosphere is under a strongly unstable condition, wind velocity decreases while pollutant concentration is increased in the areas near the street canyon top, road surface, leeward and windward walls, compared with the wind velocity in the street canyon with the neutral stratification. When street canyon atmosphere is under a weakly unstable condition, wind velocity weakens near the street canyon top and windward wall, but strengthens near the road surface and leeward wall, and pollutant concentration is decreased near the leeward and windward walls and is increased between the two rows of trees. When the street canyon atmosphere is under an unstable condition, PER is lower than that under the neutral stratification.
GU ZhaoLin1, ZHANG YunWei2 & LEI KangBin3 1 School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China
A new method for the solution of population balance equations(PBE) describing the micro-processes such as nucleation,growth,aggregation of particle swarms in a multiphase system is proposed.The method is based on the fixed pivot moment and allows arbitrary number of moments to be tracked si-multaneously.By expressing PBEs for both batch and continuous operations in a general form,and using weighted residual method to derive the moment equations,different moments can be tracked directly.The numerical density function is assumed to be a summation of several weighted Dirac Delta functions,and the integral and derivative terms in PBEs are transformed to a summation in order to reduce computational cost.Simulations of a batch nucleation-growth process and a continuous ag-gregation-growth process have demonstrated good agreement with the corresponding analytical solu-tions,with relative errors less than 108%.Simulation of a combined nucleation-growth-aggregation process,which does not have an analytical solution,is also included,so as to reproduce the mi-cro-behaviors of such a complex system,demonstrating the feasibility and reliability of this method.
