Seung-Bu Park

Urban Flow and Dispersion Simulation Using a CFD Model Coupled to a Mesoscale Model

Abstract Flow and pollutant dispersion in a densely built-up area of Seoul, Korea, are numerically examined using a computational fluid dynamics (CFD) model coupled to a mesoscale model [fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)]. The CFD model used is a Reynolds-averaged Navier–Stokes equations model with the renormalization group k − e turbulence model. A one-way nesting method is employed in this study. MM5-simulated data are linearly interpolated in time and space to provide time-dependent boundary conditions for the CFD model integration. In the MM5 simulation, four one-way nested computational domains are considered, and the innermost domain with a horizontal grid size of 1 km covers the Seoul metropolitan area and its adjacent areas, including a part of the Yellow Sea. The NCEP final analysis data are used as initial and boundary conditions for MM5. MM5 is integrated for 48 h starting from 0300 LST 1 June 2004 and the coupled CFD–M...

A large-eddy simulation study of thermal effects on turbulent flow and dispersion in and above a street canyon

Thermal effects on turbulent flow and dispersion in and above an idealized street canyon with a street aspect ratio of 1 are numerically investigated using the parallelized large-eddy simulation model (“PALM”). Each of upwind building wall, street bottom, and downwind building wall is heated, and passive scalars are emitted from the street bottom. When compared with the neutral (no heating) case, the heating of the upwind building wall or street bottom strengthens a primary vortex in the street canyon and the heating of the downwind building wall induces a shrunken primary vortex and a winding flow between the vortex and the downwind building wall. Heating also induces higher turbulent kinetic energy and stronger turbulent fluxes at the rooftop height. In the neutral case, turbulent eddies generated by shear instability dominate mixing at the rooftop height and appear as band-shaped perturbations in the time–space plots of turbulent momentum and scalar fluxes. In all of the heating cases, buoyancy-generated turbulent eddies as well as shear-generated turbulent eddies contribute to turbulent momentum and scalar fluxes and band-shaped or lump-shaped perturbations appear at the rooftop height. A quadrant analysis shows that at the rooftop height, in the neutral case and in the case with upwind building-wall heating, sweep events are less frequent but contribute more to turbulent momentum flux than do ejection events. By contrast, in the case with street-bottom and downwind building-wall heating, the frequency of sweep events is similar to that of ejection events and the contribution of ejection events to turbulent momentum flux is comparable to that of sweep events.

A Large-Eddy Simulation Study of Thermal Effects on Turbulence Coherent Structures in and above a Building Array

AbstractThermal effects on turbulent flow in and above a cubical building array are numerically investigated using the parallelized large-eddy simulation model (PALM). Two cases (no heating and bottom heating) are simulated and are compared with each other, focusing on thermal effects on turbulence coherent structures. In the no-heating case, the streaky or streamwise-elongated structures of low-speed regions appear above the building array and ejections in the low-speed regions play an important role in transporting momentum downward. In the bottom-heating case, plume-shaped structures appear with streamwise-elongated structures and the magnitude of vertical turbulent momentum flux averaged over the low-speed regions increases. Elliptical structures of negative streamwise velocity perturbation and vortical structures similar to hairpin vortices appear above the building array in the conditionally averaged fields in both cases, and the coherent structures expand more vertically when the bottom is heated. ...

Large-eddy simulation of turbulent flow in a densely built-up urban area

Turbulent flow in a densely built-up area of Seoul, South Korea, is numerically investigated using the parallelized large-eddy simulation model. Based on the analysis of streamwise velocity and column-averaged vertical turbulent momentum flux, three areas of interest are selected: a downstream area of an apartment complex, an area behind high-rise buildings, and a park area. In the downstream area of the apartment complex, a large wake develops and a region of strong vertical turbulent momentum flux appears above the wake. At the height of maximum vertical turbulent momentum flux magnitude, all the four quadrant events occur in larger magnitude and contribute more to the vertical turbulent momentum transport than the averages in the main domain. In the area behind the high-rise buildings, fluctuating wakes and vortices are distinct flow structures around the top of the tallest building and updrafts induced by the flow structures appear as strong ejections just behind the high-rise buildings or farther downstream. While strong ejections are dominant at building-top heights, downdrafts along the windward walls of high-rise buildings are distinct below building-top heights and they induce high turbulent kinetic energy and winding flow around the high-rise buildings near the ground surface, transporting momentum downward and intermittently into nearby streets. In the park area located downstream in the main domain, turbulent eddies exist well above the ground surface, and the thickness of the interfacial region between low-speed air and high-speed air increases and complex turbulent flow appears in the interfacial region.

Impacts of Mesoscale Wind on Turbulent Flow and Ventilation in a Densely Built-up Urban Area

AbstractTurbulent flow in a densely built-up area of Seoul, South Korea, for 0900–1500 LST 31 May 2008 is simulated using the parallelized large-eddy simulation model (PALM) coupled to a mesoscale model (Weather Research and Forecasting Model). Time-varying inflow that is composed of mesoscale wind and turbulent signals induces different mean flows and turbulence structures depending on time. Sweeps induced by upper flow are distinct for 0900–0910 LST, and strong ejections and weaker sweeps are dominant for 1450–1500 LST at height z = 200 m. To investigate pedestrian wind environment and ventilation, mean wind velocity and turbulent kinetic energy at 2.5 m above streets are analyzed. The reference mean wind speed at z = 600 m continuously increases after 1010 LST. The pedestrian mean streamwise velocity tends to decrease after 1100 LST, although the pedestrian mean wind speed tends to slowly increase. Whereas the temporal velocity variations related to mesoscale wind are distinct in a street canyon and an...

Sensitivity of Ozone to NO x and VOCs in a Street Canyon

The sensitivity of ozone to and volatile organic compounds (VOCs) emission rates under different ventilation rates and emission ratios in a street canyon is investigated using a chemistry box model. The carbon bond mechanism IV (CBM-IV) with 36 gaseous species and 93 chemical reactions is incorporated. and VOCs emission rates considered range from 0.01 to with intervals of . Three different ventilation rates and three different emission ratios are considered. The simulation results show that the ozone concentration decreases with increasing emission rate but increases with increasing VOCs emission rate. When the emission ratio of VOCs to is smaller than about 4, the ozone concentration is lower in the street canyon than in the background. On average, the magnitude of the sensitivity of ozone to emission rate is significantly larger than that to VOCs emission rate. As the emission rate increases, the magnitude of the sensitivity of ozone to and VOCs emission rates decreases. Because the ozone concentration is lower in the street canyon than in the background, the increased ventilation rate enhances ozone inflow from the background. Therefore, the increase in ventilation rate results in the increase in ozone concentration and the decrease in the magnitude of the sensitivity of ozone to and VOCs emission rates when the emission ratio of VOCs to is smaller than about 4. On the other hand, the increase in emission ratio results in the increase in ozone concentration because the chemical ozone production due to the photolysis is enhanced. In the present experimental setup, the contribution of the change in emission ratio to the change in the sensitivity of ozone to emission rate is larger than that of the change in ventilation rate.

A Large-Eddy Simulation Study of Bottom-Heating Effects on Scalar Dispersion in and above a Cubical Building Array

AbstractThermal effects on scalar dispersion in and above a cubical building array are numerically investigated using the parallelized large-eddy simulation model (PALM). Two cases (no heating and bottom heating) are simulated, and scalar dispersion patterns in the two cases are compared. In the no-heating case, scalar ejections in the low-speed flow structures play an important role in transporting scalar upward above the building array. In the bottom-heating case, streamwise elongated and isolated scalar ejections appear below upper low-speed and upper high-speed regions above the building array. In both cases, bottom-emitted scalar flux is balanced by streamwise scalar advection and vertical turbulent scalar flux at the rooftop height. The vertical turbulent scalar flux at the rooftop height is mainly composed of scalar ejections and scalar sweeps that are related to low- and high-speed flow structures, respectively. Furthermore, the low- and high-speed flow structures at the rooftop height induce span...

An Investigation of Flow and Pollutant Dispersion in Three-Dimensional Asymmetric Street Canyons Using a CFD Model

A three-dimensional computational fluid dynamics (CFD) model with the renormalization group (RNG) turbulence model is used to examine the effects of difference in building height on flow and pollutant dispersion in asymmetric street canyons. Three numerical experiments with different street canyons formed by two isolated buildings are performed. In the experiment with equal building height, a portal vortex is formed in the street canyon and a typical recirculation zone is formed behind the downwind building. In the experiment with the downwind building being higher than the upwind building, the ambient flow comes into the street canyon at the front of the downwind building and incoming flow diverges strongly in the street canyon. Hence, pollutants released therein are strongly dispersed through the lateral sides of the street canyon. In the experiment with the upwind building being higher than the downwind building, a large recirculation zone is formed behind the upwind building, which is disturbed by the downwind building. Pollutants are weakly dispersed from the street canyon and the residue concentration ratio is largest among the three experiments. This study shows that the difference in upwind and downwind building height significantly influences flow and pollutant dispersion in and around the street canyon.