575 publications from this institution
Wall jets are important for a wide variety of engineering applications, including ventilation of confined spaces and cooling and drying processes. Although a lot of experimental studies have been devoted to wall jets, many of these have focused on laminar or turbulent wall jets. There is a lack of experimental data on transitional wall jets, especially transitional wall jets released into a confined space or enclosure. This paper presents flow visualizations and high-resolution Particle Image Velocimetry measurements of isothermal transitional plane wall jets injected through a rectangular slot in a confined space. As opposed to many previous studies, not only the wall jet region but also the recirculation region in the remainder of the enclosure is analyzed. The data and analysis in this paper provide new insights into the behavior of transitional plane wall jets in a confined space and will be useful for the validation of numerical simulations of this type of jets.
Traffic is one of the main sources of particulate matter (PM) inside urban areas. This paper provides a preliminary assessment of the potential to reduce outdoor PM concentrations by local removal inside semi-enclosed parking garages. The assessment is performed by computational fluid dynamics (CFD) based on the 3D steady RANS equations and an Eulerian advection-diffusion equation. First, an extensive CFD validation study is performed with gas dispersion wind-tunnel measurements. Next, the case study for Eindhoven city center is conducted on a high-resolution grid including 16 semi-enclosed garages. Traffic intensities on the streets and in the garages are converted to PM10 source terms. The garages are ventilated with outdoor air. Simulations are performed with and without removal units in the garages. The case study is not intended to reproduce a particular pollution episode but to provide a preliminary indication of the potential reduction in PM10 for representative meteorological and traffic conditions. The results show that 594 removal units allow reductions in local outdoor PM10 by up to 50% close to the garages while reductions up to 10% are achieved further downstream. It is concluded that local removal in semi-enclosed parking garages can be an effective strategy towards improved outdoor air quality.
Air curtains (ACs) are of interest in building applications to support energy efficiency and air quality by restricting heat and mass transport across continuously open entrances. Environmental conditions to which ACs are generally subjected, such as differences in temperature and pressure, induce loads on ACs that alter their flow pattern, hence can considerably influence their separation efficiency. Therefore, proper design and implementation of ACs demands detailed knowledge of the impact of these parameters both individually and combined. This is addressed here by a systematic evaluation of the AC separation efficiency under moderate environmental temperature (5 °C ≤ ΔT ≤ 25 °C) and pressure (1 Pa ≤ ΔP ≤ 8 Pa) difference conditions. RANS CFD simulations of an AC are conducted on verified computational grids, validated with experimental data and calibrated with field measurements. Results show a strong yet non-linear dependency of AC performance on environmental parameters and indicate there is an optimal separation efficiency (based on total mass transfer) in which competing effects between a given jet momentum flux on the one hand and cross-jet loads caused by temperature or pressure differences on the other hand are counterbalanced. Findings suggest to dynamically control the supplied AC jet momentum as a function of variable environmental conditions in order to continually deliver an optimal performance.
Accurate and reliable computational fluid dynamics (CFD) simulations are essential for the assessment of cross-ventilation of buildings. To determine which CFD models are most suitable, validation studies are required. A detailed review of the literature indicates that most CFD validation studies only employed the 3D steady Reynolds-averaged Navier-Stokes (RANS) approach and/or focused on a limited set of flow parameters. Therefore, the objective of this paper is the validation of both 3D steady RANS simulations and large eddy simulation (LES) of cross-ventilation in a generic isolated enclosure with wind-tunnel measurements. The evaluation is based on five parameters: mean velocity, turbulent kinetic energy, ventilation flow rate, incoming jet angle and incoming jet spreading width. The RANS simulations are conducted with the standard k-ε (SKE), RNG k-ε, realizable k-ε (RLZ), SST k-ω and RSM turbulence models, whereas the LES is performed with the dynamic Smagorinsky subgrid-scale model. SST/RNG/RSM reproduce the experimentally observed direction of the incoming jet, but all RANS models fail in reproducing the turbulent kinetic energy, which is too low especially above and below the jet, because steady RANS does not capture the vertical flapping of the jet. This transient feature is reproduced by LES, resulting in a better reproduction of all three measured parameters (velocity, turbulent kinetic energy, volume flow rate). It is concluded that choice of the model (RANS vs. LES) actually depends on which parameter is the target parameter, noting that the use of LES entails an increase in computational demand with a factor of ≈80–100.
Meteorological measurements are conducted in Antwerp, Belgium in July 2013, followed by CFD urban microclimate simulations considering the same city and time period. The simulations are found to be able to reproduce measured air temperatures inside central Antwerp with an average absolute difference of 0.88 °C. The simulation results supplemented with measurements are used to generate location-specific Microclimatic Conditions (MCs) in three locations: (1) a rural location outside Antwerp; (2) an urban location inside Antwerp, away from an urban park; and (3) another urban location, close to the same park. Building Energy Simulations (BES) are performed for 36 cases based on three different MCs, two building use types and six sets of construction characteristics, ranging from pre-1946 buildings to new, low-energy buildings. Monthly Cooling Demands (CDs) are extracted for each case and compared with each other. The results demonstrate that compared to the air temperatures in the rural area, on average, air temperatures at the urban sites away and close to the park are 3.3 °C and 2.4 °C higher, respectively. This leads to an additional monthly CD of up to 90%. CDs of buildings with better thermal insulation and lower infiltration rates can increase by 48% once moved from the rural location to an urban location, which may lead to the reconsideration of design guidelines of low-energy buildings exposed to an urban MC. Although the proximity of an urban park cannot fully compensate the increased CD by an urban MC, residential buildings close to the park are found to have on average 13.9% less CD during July 2013, compared with buildings away from the same park. The influence of the urban park on the CDs of buildings in its vicinity is strongly linked to the meteorological wind direction. Professionals focusing on energy-efficient buildings in cities are advised to conduct energy predictions with location-specific MC data, instead of only using city-averaged meteorological data.
There is a growing interest in wind energy harvesting in the built environment. Vertical axis wind turbines (VAWT) seem to represent an ideal candidate for this purpose due to their omni-directional operation. However, as a result of a comparatively small amount of research on VAWTs during the last decades they fall short of horizontal axis wind turbines (HAWT) with respect to aerodynamic efficiency. Therefore, more research is required in order to optimize VAWT performance. Several research methods including numerical modeling of varying complexity and large-scale wind tunnel measurements have aided the optimization of VAWTs. Designing new airfoils, finding the optimum solidity and tip speed ratio, blade fixed pitch angle, incoming flow Reynolds number and skewness angle and flow control are some of the adopted approaches to improve VAWT performance. The current study intends to present a short review of conducted research and propose new directions for future research.
Accurate prediction of ventilation flow is of primary importance for designing a healthy, comfortable, and energy-efficient indoor environment. Since the 1970s, the use of computational fluid dynamics (CFD) has increased tremendously, and nowadays, it is one of the primary methods to assess ventilation flow in buildings. The most commonly used numerical approach consists of solving the steady Reynolds-averaged Navier-Stokes (RANS) equations with a turbulence model to provide closure. This article presents a detailed validation study of steady RANS for isothermal forced mixing ventilation of a cubical enclosure driven by a transitional wall jet. The validation is performed using particle image velocimetry (PIV) measurements for slot Reynolds numbers of 1000 and 2500. Results obtained with the renormalization group (RNG) k-ε model, a low-Reynolds k-ε model, the shear stress transport (SST) k-ω model, and a Reynolds stress model (RSM) are compared with detailed experimental data. In general, the RNG k-ε model shows the weakest performance, whereas the low-Re k-ε model shows the best agreement with the measurements. In addition, the influence of the turbulence model on the predicted air exchange efficiency in the cubical enclosure is analyzed, indicating differences up to 44% for this particular case.This article presents a detailed numerical study of isothermal forced mixing ventilation driven by a low-velocity (transitional) wall jet using steady computational fluid dynamics (CFD) simulations. It is shown that the numerically obtained room airflow patterns are highly dependent on the chosen turbulence model and large differences with experimentally obtained velocity fields can be present. The renormalization group (RNG) k-ε model, which is commonly used for room airflow modeling, shows the largest deviations from the measured velocities, indicating the care that must be taken when selecting a turbulence model for room airflow prediction. As a result of the different predictions of the flow pattern in the room, large differences are present between the predicted air exchange efficiency obtained with the four tested turbulence models, which can be as high as 44%.
Weather conditions in an urban environment differ from the conditions in a rural environment. This phenomenon is known as the urban heat island (UHI) effect. In this study the urban climate was monitored at five locations in Rotterdam (the Netherlands) for a period of 1.5 years. The urban heat island intensity was subsequently calculated by the difference between these results and measurements of the Royal Dutch meteorological institute (KNMI) at a rural area 5 km from the centre of Rotterdam. A data-driven method based on a neural network was used for the prediction of the UHI intensity. In this method the UHI intensity at a specific time is calculated as a function of eight weather parameters at that specific time as well as the preceding three hours. The results show a mean squared error on the test data of 0.18 °C. The results therefore indicate that the model reproduces the transient behaviour of the UHI intensity in an accurate manner. This approach can therefore be used to convert weather data of a rural area in weather data for an urban area and subsequently be used in building performance simulations.
The infiltration of unconditioned outdoor air to controlled indoor environments can have negative impacts on the energy performance and indoor environmental quality of buildings. Air curtains can be used to reduce infiltration through entrance doors where transit is frequent. The stability and performance of air curtains strongly depends on the combination of jet and environmental forces that act on them, including forces due to cross-jet density gradients and injection of buoyancy in the jet. This study addresses the performance of a heated air-curtain system by means of validated computational fluid dynamics (CFD) simulations. Two performance indicators are considered: (1) separation efficiency (related to mass transport), and (2) thermal efficiency (related to heat transport). The results indicate that based on the performance indicators, the use of a heated air curtain is not favorable over the use of an isothermal air curtain.
Vertical axis wind turbines (VAWTs) have received growing interest for off-shore application and in the urban environments mainly due to their omni-directional capability, scalability, robustness, low noise and costs. However, their aerodynamic performance is still not comparable with their horizontal axis counterparts. To enhance their performance, the impact of operational parameters such as tip speed ratio (λ), Reynolds number (Rec) and turbulence intensity (TI) on their power performance and aerodynamics needs to be deeply understood. The current study, therefore, intends to systematically investigate the effect of these parameters in order to provide a deeper insight into their impact on the aerodynamic performance of VAWTs. For this investigation, a Darrieus H-type VAWT has been employed. A wide range of the parameters is considered: λ = 1.2–6.0, Rec = 0.3 × 105–4.2 × 105 and TI = 0%–30% to analyze the turbine performance, turbine wake and dynamic loads on blades. High-fidelity computational fluid dynamics (CFD), extensively validated with experimental data, are employed. The results show that (i) variable-speed operation maintaining the optimal λ at different wind speeds improves the turbine power coefficient, e.g. up to 168% at 4 m/s, while keeping an almost constant thrust coefficient, (ii) the turbine performance and wake are Re-dependent up to the highest Rec studied, (iii) large TI (> 5%) improves the turbine performance in dynamic stall by promoting the laminar-to-turbulent transition and delaying stall on blades, however it deteriorates the optimal performance by introducing extra skin friction drag. The findings of the current study can support more accurate performance prediction of VAWTs for various operating conditions and can help the improvement of the aerodynamic performance of VAWTs.
The long jump is a track and field event in which the athlete sprints down a runway and tries to leap as far as possible from a take-off line. To the best of our knowledge, there are no published studies on the aerodynamic impact of jump style, hairstyle and clothing on the long jump distance. This paper presents a numerical-physical model of the long jump flight. It allows to predict flight distance and the impact of jump style, hairstyle and clothing. It consists of five submodels: an existing model of the sprint before take-off, a computational fluid dynamics (CFD) model of different body postures in flight, a set of physical wind tunnel models for CFD validation, a full-scale wind tunnel manikin with different hairstyles and clothing and a numerical model of the flight trajectory. Jump style only impacts flight distance by 1 cm or less. Hairstyle and clothing however can cause drag to vary by more than 25% and flight distance by more than 10 cm, mostly by impacting the take-off speed. In the long term, long jump events might see the introduction of hair caps and low-drag clothing to reduce aerodynamic resistance and level the playing field.
La dynamique de fluide computationnelle (CFD) est souvent utilisee pour etudier le transfert de chaleur entre un materiau et une lame d’air puisque la CFD peut calculer avec precision le profil de vitesse dans la couche limite. Cependant, les logiciels commerciaux de CFD comportent plusieurs limites pour modeliser l’humidite dans les materiaux poreux. Consequemment, un modele a ete developpe par les auteurs pour simuler le transfert d’humidite entre une lame d’air et un materiau poreux en utilisant la CFD integree a un modele de transfert de diffusion de l’humidite. Dans cet article, la CFD est utilisee pour modeliser la chaleur, le mouvement d’air et le transfert d’humidite dans l’air incluant l’echange radiatif et le transfert convectif de chaleur ainsi que la conduction de chaleur dans le materiau. Le transfert d’humidite dans le materiau est calcule en relais, a chaque pas de temps, avec un modele complementaire. Une etude de cas est realisee pour un ecoulement d’air laminaire au-dessus d’une eprouvette de bois.
