Influence of geometric parameters of façade protruding ribs on turbulent flow statistics in street canyons: A large-eddy simulation study

How gable roofs change the mechanisms of turbulent vertical momentum transfer: A LES study on two-dimensional urban canyons

Large eddy simulations (LES) are widely used to study the effects of surface morphology on turbulence statistics, exchange processes and turbulence topology in urban canopies. However, as LES are only approximations of reality, special attention is needed for the computational model set-up to ensure an accurate representation of the physical processes of interest. This paper shows that the choice of the numerical domain can significantly affect the accuracy of turbulent flow statistics, potentially causing a mismatch between numerical studies and experimental data. The study examines the influence of cross-stream aspect ratio (YAR), streamwise aspect ratio (XAR) and scale separation (SS) on first- and second-order flow statistics and turbulence topology. It is found that domains with a low YAR underestimate the velocity variance, while those with a low XAR overestimate the variance value. The study proposes a new approach based on the Buckingham Pi theorem to evaluate the effect of SS, as the existing method has major limitations for canopy flows. The results suggest that domains with small SS underpredict the variance value. To minimise the artificial impact of the numerical domain on turbulent flow statistics, the study recommends guidelines for future research, including a YAR of 3 or more, an XAR of 6 or more and an SS of 12 or more. Error tables are presented to allow researchers to select smaller domains than recommended, depending on their research interests in specific parts of the flow.

Shaping buildings to promote street ventilation: A large-eddy simulation study

High resolution urban large-eddy simulation studies from street canyon to neighbourhood scale

The exchange of dissolved matter between a straight open channel and a series of shallow embayments present at one of its sides is investigated using large eddy simulation (LES). The direct link between the mechanism of mass exchange and the dynamics of coherent structures is demonstrated. It is shown that for the geometrical configuration considered in the present study, the mass exchange process is very non-uniform over the depth in the vicinity of the channel–embayment interface. Most of the contaminant is ejected from the embayments close to the free surface. The amount of contaminant re-entrained into the embayments situated downstream of the one in which contaminant was introduced is quantified. The mass exchange coefficient predicted by LES does not vary significantly with the embayment rank and is in very good agreement with the one predicted by the model proposed by Weitbrecht et al. (J Hydraul Eng 134(2):173–183, 2008) based on the value of a dimensionless morphometric groyne-field parameter. Field experiments were purposely performed in a natural stream with embayments whose length over width ratios were close to the ratio in the LES study. The concentration fields predicted by LES are compared with video-records of colored dye used to visualize the mass exchange in the field experiment. It is shown that, for both LES and the field experiment, the dominant passage frequency of the eddies inside the interfacial mixing layer is well predicted by the analytical model of Sukhodolov and Sukhodolova (in: Cowen et al (eds) Hydraulic measurements & experimental methods. Proceedings of international conference, Lake Placid, USA, pp 172–177, 2007). The model is then used to scale the time in the LES animations and field video-records showing the temporal evolution of the concentration field. The results of the comparison indicate several similarities in the mixing process, despite the differences in the bathymetry and the large difference in the Reynolds number between LES and the field experiment. This proves the usefulness of performing detailed LES and laboratory studies in well-controlled environments to understand mass-exchange processes around river groyne fields.

Large Eddy Simulation study on the structure of turbulent flow in a complex city

Urban street canyons are critical areas for air quality concerns, where interactions among wind flow, traffic, thermodynamics, and pollutant dispersion pose significant challenges for microscale climate modeling. In this study, we use high-resolution large-eddy simulation (LES) to investigate the effects of traffic-induced turbulence and exhaust emissions on pollutant transport within street canyons. The Imposed Velocity Method (IVM) for incorporating vehicle-induced effects was developed, implemented, and validated by extending the PALM model system.Through a comprehensive parameter study, we examine how traffic, thermodynamics, and varying wind conditions interact to influence local flow dynamics and pollutant dispersion. We systematically vary vehicle speed, wind speed, and thermal boundary conditions to represent a range of realistic traffic and diurnal heating scenarios. This parametric approach allows us to assess how changes in the wind-to-vehicle speed ratio and different heating configurations influence the interplay between vehicle-induced turbulence, thermal effects, and overall flow dynamics in the street canyon. We also investigate the impact of emission source representation by comparing a point and line source approach, revealing how these modeling choices influence the pollutant transport. This comparison underscores the importance of accurately modeling vehicular emissions to avoid under- or over-estimation of local concentrations.In this presentation, we will share our latest results and discuss key insights into the role of vehicle-induced turbulence (VIT) in influencing urban air quality.

Progressive optimization for the inverse design of 2D cascades

Effects of differential wall heating in street canyons on dispersion and ventilation characteristics of a passive scalar

Urban areas, home to about half of the world’s population, are increasingly adopting cool roofs to mitigate heat. These roofs reflect more sunlight and absorb less solar energy compared to conventional roofs, thereby reducing sensible heat fluxes from building rooftops. Many previous studies on the effects of cool roofs on near-surface air temperature were performed with traditional weather and climate models (e.g., the Weather Research and Forecasting or WRF model), where the near-surface air temperature is parameterized based on schemes designed for non-urban environments (such as Monin-Obukhov similarity theory) and does not represent the outdoor air temperature felt by urban residents. To quantify the effects of cool roofs on air temperature within the urban canyon, we perform building-resolving large-eddy simulations across different but idealized urban canyons that mimic different local climate zones. Our findings reveal that the cooling sensitivity, which characterizes the air temperature change (K) per unit amount of forcing (W/m2), increases with the canyon aspect ratio (AR) up to AR = 1, after which it decreases. We further compare the LES results to the simulated results by the single-layer urban canopy model in WRF. This study contributes to the understanding of how cool roofs impact within-canyon air temperatures in different urban settings.

The street canyon phenomenon has a negative impact on air quality. Flow, temperature, and the concentration fields of pollutants affect street canyon. Many studies have focused on the concentration and flow fields; model experiments on the temperature field have been less examined. This experiment investigates the effect of model height and placement of flow and temperature fields. Temperature and flow fields are visualized using a smoke-wire method, one of the methods used to visualize a flow field. Visualization images were investigated in comparison to several other images. In turbulent flow conditions, temperature change of the street is verified when the turbulence parameter has changed. As a result of the experiment, data showing a high cooling effect was obtained. Furthermore, a trend was verified for cooling effects under turbulent flow conditions.

In the present paper, the results of a large eddy simulation (LES) study on hydrogen microjets (dj ∼ 0.5 mm) injected into a hot (1600 K) vitiated crossflow at different angles — namely, normal (90°) and inclined jet (30°), are presented. The goal is to explore the effects of injection angle on coherent turbulent structure formations, flame-vortex interactions, and wall heat flux contributions. The LES identifies the presence of the horseshoe vortex, the shear layer vortices (SLV), and the counter-rotating vortex pair (CVP), along with the shedding of spanwise-symmetry hairpin vortices in both the normal and inclined jets. The structures in the latter, however, appear more convoluted. In the near field, the SLV-induced flow is found to play a key role in the mixing and flame propagation in the windward side of the jet that is stabilized through the autoignition process along the front edge of both the normal and inclined jets close to their exits. The flame-shear layer offset phenomenon is also noticed on the windward side of the jets. In the far field, the CVP is found to be the dominating mechanism in the entrainment of the hot vitiated crossflow by the reacting jet and large-scale mixing. Its induced counter rotating flow field give rises to the flame propagation and the heat and species transfer from the windward to the leeward side of the jet near the injection wall. In the wake region, the combustion and its byproducts persist in closer proximity to the jet exit of the normal case because of the presence of a much stronger recirculation zone behind the jet. Accordingly, higher wall heat fluxes are obtained in this region for the normal jet. The mean wall heat flux values of both the normal and inclined jets decrease and approach each other with moving away from the jet exit in the streamwise direction. The findings indicate that the CVP-induced flow drastically increases heat transfer to the near wall region, resulting in a spanwise-symmetry heat flux profile with double peaks in downstream. The results of the present LES study are compared to the experimental data available in the literature by considering instantaneous hydroxide (OH) fields and mean wall heat fluxes.

In this paper, the results of a large eddy simulation (LES) study on hydrogen microjets (dj ∼ 0.5 mm) injected into a hot (1600 K) vitiated crossflow at different angles, namely, normal (90 deg) and inclined jet (30 deg), are presented. The goal is to explore the effects of injection angle on coherent turbulent structure formations, flame–vortex interactions, and wall heat flux contributions. The LES identifies the presence of the horseshoe vortex, the shear layer vortices (SLV), and the counter-rotating vortex pair (CVP), along with the shedding of spanwise-symmetry hairpin vortices in both the normal and inclined jets. The structures in the latter, however, appear more convoluted. In the near field, the SLV-induced flow is found to play a key role in the mixing and flame propagation in the windward side of the jet that is stabilized through the auto-ignition process along the front edge of both the normal and inclined jets close to their exits. The flame-shear layer offset phenomenon is also noticed on the windward side of the jets. In the far field, the CVP is found to be the dominating mechanism in the entrainment of the hot vitiated crossflow by the reacting jet and large-scale mixing. Its induced counter-rotating flow field give rises to the flame propagation and the heat and species transfer from the windward to the leeward side of the jet near the injection wall. In the wake region, the combustion and its byproducts persist in closer proximity to the jet exit of the normal case because of the presence of a much stronger recirculation zone behind the jet. Accordingly, higher wall heat fluxes are obtained in this region for the normal jet. The mean wall heat flux values of both the normal and inclined jets decrease and approach each other with moving away from the jet exit in the streamwise direction. The findings indicate that the CVP-induced flow drastically increases heat transfer to the near wall region, resulting in a spanwise-symmetry heat flux profile with double peaks in downstream. The results of the present LES study are compared to the experimental data available in the literature by considering instantaneous hydroxide (OH) fields and mean wall heat fluxes.

Large eddy simulation study on pedestrian-level wind environments around elevated walkways and influential factors in ideal urban street canyons

Large eddy simulations were performed to characterize the flow and mass transport mechanisms in the interior of two Pocillopora coral colonies with different geometries, one with a relatively loosely branched morphology (P. eydouxi), and the other with a relatively densely branched structure (P. meandrina). Detailed velocity vector and streamline fields were obtained inside both corals for the same unidirectional oncoming flow, and significant differences were found between their flow profiles and mass transport mechanisms. For the densely branched P. meandrina colony, a significant number of vortices were shed from individual branches, which passively stirred the water column and enhanced the mass transport rate inside the colony. In contrast, vortices were mostly absent within the more loosely branched P. eydouxi colony. To further understand the impact of the branch density on internal mass transport processes, the non-dimensional Stanton number for mass transfer, St, was calculated based on the local flow time scale and compared between the colonies. The results showed up to a 219% increase in St when the mean vortex diameter was used to calculate St, compared to calculations based on the mean branch diameter. Turbulent flow statistics, including the fluctuating velocity components, the mean Reynolds stress, and the variance of the velocity components were calculated and compared along the height of the flow domain. The comparison of turbulent flow statistics showed similar Reynolds stress profiles for both corals, but higher velocity variations, in the interior of the densely branched coral, P. meandrina.