Publications

CFD simulation of stratified indoor environment in displacement ventilation: Validation and sensitivity analysis

Towards accurate CFD simulations of vertical axis wind turbines at different tip speed ratios and solidities: Guidelines for azimuthal increment, domain size and convergence

The accuracy of CFD simulations of vertical axis wind turbines (VAWTs) is known to be significantly associated with the computational parameters, such as azimuthal increment, domain size and number of turbine revolutions before reaching a statistically steady state condition (convergence). A detailed review of the literature, however, indicates that there is a lack of extensive parametric studies investigating the impact of the computational parameters. The current study, therefore, intends to systematically investigate the impact of these parameters, on the simulation results to guide the execution of accurate CFD simulations of VAWTs at different tip speed ratios (λ) and solidities (σ). The evaluation is based on 110 CFD simulations validated with wind-tunnel measurements for two VAWTs. Instantaneous moment coefficient, Cm, and power coefficient, CP, are studied for each case using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with the 4-equation transition SST turbulence model. The results show that the azimuthal increment dθ is largely dependent on tip speed ratio. For moderate to high λ, the minimum requirement for dθ is 0.5° while this decreases to 0.1° at low to moderate λ. The need for finer time steps is associated to the flow complexities related to dynamic stall on turbine blades and blade-wake interactions at low λ. In addition, the minimum distance from the turbine center to the domain inlet and outlet is 15 and 10 times the turbine diameter, respectively. It is also shown that 20–30 turbine revolutions are required to ensure statistically converged solutions. The current findings can serve as guidelines towards accurate and reliable CFD simulations of VAWTs at different tip speed ratios and solidities.

Wind interference between two high-rise building models: On the influence of shielding, channeling and buffeting on peak pressures

The influence of interference between two high-rise building models on the minimum peak pressures was investigated in an atmospheric boundary layer wind tunnel. Pressure measur€ments were performed on a square model with an aspect ratio of 1 to 4. The influence of an interfering model with the same height as well as half the height was investigated. A square full-height model resulted in minimum peak pressures on the facades 50% larger than observed in the isolated configuration. This increase was not observed for a circular full-height model. Both a square and circular half-height model resulted in increases of 30% on the facades at the same height as the roof of the interfering model. This paper gives a detailed discussion of these increases in terms of the wind behaviour and types ofinterference (i.e. channelling, buffeting and shielding)

The effect of an urban park on the microclimate in its vicinity: a case study for Antwerp, Belgium

ABSTRACT An urban park can exhibit a cooling effect not only in the park itself, but also in its wake with respect to the prevailing wind. This study investigates the cooling potential of an urban park in Antwerp, Belgium focusing on two parameters: (1) intensity of the cooling effect, indicating the maximum reduction in air temperature and (2) range of the cooling effect, indicating the maximum horizontal distance where a minimum of 0.1 °C cooling effect is present. Computational fluid dynamics (CFD) simulations are performed using the three‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations. Coupled simulations of wind flow and heat transfer consider all the conductive, convective and radiative heat transfer processes but not the long‐wave radiation heat exchange between buildings, trees and ground. The vegetation model used in this study is evaluated with experimental data from an earlier study. Moreover, the simulated air temperatures inside the Antwerp city center are compared with available measurement data. Following the fairly‐good predictions from CFD simulations, results from the following cases are compared: (1) base case with the park, (2) a case with an open square instead of the park, and (3) a case with representative buildings instead of the park. The results indicate a maximum daytime intensity and range of the cooling effect of 3.4 °C and 498 m at 1500 LST (UTC + 2, Central European Summer Time). Further analysis shows that the cooling effect is more profound closer to the ground. Apart from the daytime cooling effect, the park investigated has the potential to lower wind velocities, resulting in increased air temperatures during the night‐time (at 0000 LST) by 0.9 °C, although this number might be slightly different given the neglect of long‐wave radiation heat exchange between buildings, trees and ground.

Efficient and high-resolution simulation of pollutant dispersion in complex urban environments by island-based recurrence CFD

We applied data-based recurrence CFD (rCFD) to model pollutant dispersion in near-field flow configurations. In case of complex topologies, the global-domain version of rCFD fails to account for local recurrent flow features. We therefore developed a novel island-based version of rCFD, which partitions the computational domain to isolate islands of high recurrence prominence, and subsequently defines a distinct recurrence path for each of these islands. We applied island-based rCFD to pollutant dispersion for two side-by-side cubical buildings with three different gap widths in between them and a real urban environment. We showed that numerical predictions of pollutant dispersion by island-based rCFD were in excellent agreement with full CFD simulations, thus outperforming the global-domain version of rCFD. In both applications, island-based rCFD simulations ran three orders of magnitude faster than corresponding full CFD simulations. In the second application, this speed-up enabled real-time simulations on a computational grid of 10 million cells.

Effect of the shaft on the aerodynamic performance of urban vertical axis wind turbines

The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are performed on a high-resolution computational grid. The evaluation is based on validation with wind-tunnel measurements. The results show that the power loss increases asymptotically with increasing δ due to the higher width and length of the shaft wake as the blades pass through a larger region with lower velocity in the downwind area. A maximum power loss of 5.5% compared to the hypothetical case without shaft is observed for δ = 16%. The addition of surface roughness is shown to be an effective approach to shift the flow over the shaft into the critical regime, reducing the shaft drag and wake width as a result of a delay in separation. For an optimal dimensionless equivalent sand-grain roughness height of 0.08, the turbine power coefficient at δ = 4% improves by 1.7%, which is equivalent to a 69% recovery of the corresponding turbine power loss. The results are found to be virtually independent of the shaft-to-turbine rotational speed ratio.

Ten questions concerning modeling of wind-driven rain in the built environment

A combined CFD–HAM approach for wind-driven rain on building facades

Numerical analysis of wind effects on mechanically induced depressurization for asbestos abatement

Asbestos abatement from buildings requires establishing depressurization through a mechanical ventilation system to prevent fiber leakage. However, atmospheric wind can cause momentary breaching of containment, increasing the probability of asbestos fiber leaking into the atmosphere. This necessitates the investigation of wind effects on mechanically-induced containment by experimental and numerical techniques. The paper focuses on the reliability of a ventilation network model (SYLVIA) to predict internal pressure field when coupled with external pressure predicted by wind-tunnel (WT) tests or large eddy simulations (LES). The internal pressure and the probability of containment breach predicted by WT-SYLVIA and LES-SYLVIA show reliable results.

CFD simulation of non-isothermal mixing ventilation in a generic enclosure: Impact of computational and physical parameters

Multiscale numerical simulations of climate change impact on urban microclimate and human health

As a result of climate change, projections show that the increasing urban population will be exposed to higher air temperatures (Hayes et al., 2014; Sheffield and Wood, 2008). In addition, due to climatic changes, the frequency and duration of heatwaves are expected to increase while the Urban Heat Island effect (UHI) is expected to become more severe (Emmanuel and Krüger, 2012; Kovats and Hajat, 2008; Mirzaei and Haghighat, 2010), causing increasing heat-related mortality and morbidity (Garssen et al., 2005; Xu et al., 2012). Meditteranean-type climate regions have been characterized as particularly vulnerable to climate change due to their high exposure to extreme weather phenomena (Diffenbaugh and Giorgi, 2012; Paz et al., 2016). The intensity of heat waves in these regions is expected to increase further, leading to other life-threatening consequences for urban populations in the Mediterranean region  (Legasa et al., 2020; Zittis et al., 2021). Under this context, the detailed investigation of the impact of climate change on the urban microclimate and human health is considered highly important. This study investigates the impact of climate change on urban microclimate and pedestrian thermal comfort through a series of numerical simulations. For this purpose, 3D Computational Fluid Dynamics (CFD) simulations are performed based on the Unsteady Raynolds-Average Navier-Stokes (URANS) equations for urban microclimate in a real compact heterogeneous urban area. The CFD validation is based on high-resolution field and laboratory measurements of the case study area in Nicosia, Cyprus (Antoniou et al., 2019, 2017). Simulations are performed for meteorological conditions based on field measurements performed in 2010, and for predicted meteorological conditions for 2050 derived from downscaled Regional Climate Models (RCMs). The RCM data are derived from the European Coordinate Regional Downscaling Experiment (EURO-CORDEX) database. CFD results are combined with radiation modeling results of the same urban area to calculate pedestrian thermal comfort using the Universal Thermal Comfort Index (UTCI). The results show that by 2050, a 2.3 °C  increase in the maximum air temperature is expected to occur, which can lead to a more than 240% increase in heat-related mortality. In addition, an increase of UTCI levels is also expected, which is more pronounced in the late afternoon hours, reaching up to 4.3 °C increase at 20:00. A significant change in the thermal stress categories is also identified between the 2010 and 2050 scenarios, where “very strong heat stress” conditions (UTCI: 38-46 °C) are expected to prevail for twice as long, increasing from 3.3 to 6.6 hours, and  “extreme heat stress” conditions (UTCI > 46 °C) appear at some locations of the area in 2050.    

CFD simulations of an isolated cycling spoked wheel: Impact of the ground and wheel/ground contact modeling

Spoked wheels are the most frequently used wheel type in road cycling competitions and their aerodynamic optimization is crucial for cyclist performance. The aerodynamic performance of wheels is generally analyzed by wind tunnel tests or CFD simulations for isolated wheels. There is a large number of options to model the wheel/ground contact in CFD simulations, including different clearances between tire and ground and different heights of solid contact patches (step). However, it is unclear to what extent these modeling options influence the CFD results. The present paper systematically analyzes the impact of these options on the computed forces and moments of an isolated cycling spoked wheel and elucidates the flow behavior around this wheel for zero yaw conditions. The wheel drag coefficient for the cases where the ground is included in the simulations using a clearance or a step is 1.0% and about 1.8% lower compared to the case without ground, respectively, whereas the rotational moment is about 2.0% lower for all the wheel/ground contact modeling approaches compared to the case without ground. The gap clearance (≤20 mm) and step height (≤10 mm) should be kept minimal to avoid a significant influence on the forces and moments. In addition, the presence of the ground influences the flow behavior in the lower section of the wheel including the pressure distribution on the exterior of the wheel. This study is intended to help researchers and manufacturers to perform accurate CFD simulations of cycling spoked wheels and to optimize their aerodynamics.

Near-field pollutant dispersion in an actual urban area: Analysis of the mass transport mechanism by high-resolution Large Eddy Simulations

Air Curtain Optimization

The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains.  This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’.

Stack gas dispersion measurements with Large Scale-PIV, Aspiration Probes and Light Scattering Techniques and comparison with CFD

Wind-driven rain on the facade of a monumental tower: Numerical simulation, full-scale validation and sensitivity analysis

On the predicted effectiveness of climate adaptation measures for residential buildings

Guidelines for the required time resolution of meteorological input data for wind-driven rain calculations on buildings