Steady and unsteady turbulent flows and pollutant dispersion in 2D street canyons with novel boundary conditions and various Re numbers

Abstract

The unsteady approaching flow (UAF) and its influence upon the flow pattern within street canyon are investigated from a perspective of mechanism in this study. Based on wind tunnel test (WTT) measurements, the boundary conditions for the approaching flow over 2D street canyons (H/W = 2.4) are described in continuous functions of reference velocity and height. The turbulence kinetic energy (TKE) profiles present a normal-log distribution in WTT; the turbulence eddy dissipation rate (TED) profiles are modified according to WTTs and basic governing equations. With a sinusoidally varying reference velocity, these novel boundary conditions are then applied to study the influence of UAF (with periodic sinusoidal fluctuations) upon the flow pattern within 2D street canyons. CFD simulations with proposed scenario agree well with WTT for steady and unsteady approaching flows regarding velocity magnitude, vortex position and TKE. UAF can also enhance street canyon's ventilation as pollutant decays faster than that under steady approaching flow. The proposed scenario is finally extended to the investigation of Re independence and shows the larger normalized TKE at the periphery prolongs the procedure of reaching Re independence. Once it reaches Re independence, the flow pattern under a larger TKE has a higher eddy centre and velocity magnitude.