Impacts of uneven surface heating of an ideal street canyon on airflows and indoor ventilation:Numerical study using OpenFOAM coupled with EnergyPlus

In the daytime,building facades and ground surfaces are heated by solar radiation,and the resulting buoyancy can change the flow field inside street canyons.In this study,the impacts of uneven and time-varying heating of surfaces inside an ideal urban street canyon(aspect ratio=1)on single-sided indoor natural ventilation are analyzed.This work introduces a methodology for a numerical approach based on the coupling between a computational fluid dynamics(CFD)model and an energy balance model.First,EnergyPlus is employed to calculate the time-varying and uneven surface heating.Simulations are performed at four typical study times(05:00,09:00,15:00,20:00)during a hot summer day(July 15)in Wuhan,China.Second,the surface temperature results are transferred to OpenFOAM for CFD simulation.Two inlet wind velocities(U_(ref))are investigated.The results show that for a relatively strong ambient wind(U_(ref)=3 m/s),the buoyancy caused by surface heating does not significantly change the airflow structures and indoor ventilation compared with those in the isothermal case.However,for a weak ambient wind(U_(ref)=0.5 m/s),the airflow structures inside the street canyon vary with the time of day.Moreover,the average air exchange rate(ACH)differs by-25.0% to 15.9%compared with that in the isothermal case,and the ACH of a single room increases by up to 2710%.The results indicate that the surface-heating-induced buoyancy is nonnegligible and should be carefully investigated,especially for weak ambient winds.

Increasing Ventilation by Passive Strategies: Analysis of Indoor Air Circulation Changes through the Utilization of Microclimate Elements

A demand for renewable alternatives that would be able to deal with the problems related to well-being is directly linked to the world’s growing needs to save energy and reduce environmental costs. For a project implementation addressing these issues, it is essential to know the climatic conditions of the target area. Taking natural ventilation, climatic factors, and renewable alternatives as important sources of comfort, in this work, passive strategies, through the utilization of microclimate elements as well as the location of outside obstacles, were imposed on an initial and specific project. The objective was to introduce obstacles which could interfere in the field of external wind and evaluate whether this outside intervention is able to make changes in indoor air circulation. The wind fields for the studied cases were obtained by computational simulations, and their consequences were analyzed to attain thermal comfort. The method adopted to obtain the wind fields was a Petrov-Galerkin type method, which is a stabilized mixed finite element method of the Navier-Stokes equations considering the incompressibility and formulated in primitive variables, velocity and pressure. The obtained results point to the solutions that promote the increase or decrease of the wind-field intensity.

Numerical studies on issues of Re-independence for indoor airflow and pollutant dispersion within an isolated building

This study conducted the numerical models validated by wind-tunnel experiments to investigate the issues of Re-independence of indoor airflow and pollutant dispersion within an isolated building.The window Reynolds number(Re_(w))was specified to characterize the indoor flow and dispersion.The indicators of RRC(ratio of relative change)or DR(K_DR)(difference ratio of dimensionless concentration)<5%were applied to quantitatively determine the critical Rew for indoor flow and turbulent diffusion.The results show that the critical Re(Re_(crit)) value is position-dependent,and Re_(crit) at the most unfavorable position should be suggested as the optimal value within the whole areas of interest.Thus Re(H,orit)=27,000 is recommended for the outdoor flows;while Re_(w,crit)=15,000 is determined for the indoor flows due to the lower part below the window showing the most unfavorable.The suggested Re_(w,crit)(=15,000)for indoor airflow and cross ventilation is independence of the window size.Moreover,taking K_DR≤5% as the indicator,the suggested Re_(w,crit) for ensuring indoor pollutant diffusion enter the independence regime should also be 15,000,indicating that indoor passive diffusion is completely determined by the flow structures.The contours of dimensionless velocity(U/U_(0))and concentration(K)against the increasing Re(w) further confirmed this critical value.This study further reveals the Re-independence issues for indoor flow and dispersion to ensure the reliability of the data obtained by reduced-scale numerical or wind-tunnel models.