Publications

Numerical analysis of surface pressure coefficients for a building with balconies

Knowledge of the pressure distribution on building walls is essential to evaluate wind-induced natural ventilation and to assess wind loads on building walls and building components. Computational Fluid Dynamics (CFD) can be a valuable tool for determining mean wind pressure coefficients on building facades. However, while many CFD studies of mean wind pressure on buildings have been performed in the past, the vast majority of these studies focused on simple building geometries without facade details. This paper presents a systematic evaluation of 3D steady RANS CFD for predicting mean wind pressure distributions on windward and leeward surfaces of a medium-rise building with and without balconies. The evaluation is based on a grid-sensitivity analysis and on validation with windtunnel measurements. The impact of several computational parameters is also investigated, including the resolution of the computational grid, the turbulence model and building balconies. The results show that steady RANS can accurately reproduce the mean wind pressure distribution across the windward facade. 3D steady RANS CFD has also been shown to provide accurate predictions of the mean wind pressure at the leeward wall in case of a perpendicular approach flow wind direction. This however is not the case for oblique flow.

Performance of LES on non-uniform grids in a ventilated generic enclosure

Impact of wheel rotation on the aerodynamic drag of a time trial cyclist

Abstract Aerodynamic drag is the main resistive force in cycling at high speeds and on flat terrain. In wind tunnel tests or computational fluid dynamics simulations, the aerodynamic drag of cycling wheels is often investigated isolated from the rest of the bicycle, and sometimes in static rather than rotating conditions. It is not yet clear how these testing and simulating conditions influence the wheel aerodynamic performance and how the inclusion of wheel rotation influences the overall measured or computed cyclist drag. This study presents computational fluid dynamics simulations, validated with wind tunnel tests, that indicate that an isolated static spoked front wheel has a 2.2% larger drag area than the same wheel when rotating, and that a non-isolated static spoked front wheel has a 7.1% larger drag area than its rotating counterpart. However, rotating wheels are also subjected to the rotational moment, which increases the total power required to rotate and translate the wheel compared to static conditions where only translation is considered. The interaction with the bicycle frame and forks lowers the drag area of the front wheel by 8.8% for static and by 12.9% for the rotating condition, compared to the drag area of the isolated wheels. A different flow behavior is also found for static versus rotating wheels: large low-pressure regions develop from the hub for rotating wheels, together with a lower streamwise velocity region inside the circumference of the wheel compared to static wheels. The results are intended to help in the selection of testing/simulating methodologies for cycling spoked wheels.

CFD analysis of drag and convective heat transfer of individual body segments for different cyclist positions

This study aims at investigating drag and convective heat transfer for cyclists at a high spatial resolution. Such an increased spatial resolution, when combined with flow-field data, can increase insight in drag reduction mechanisms and in the thermo-physiological response of cyclists, related to heat stress and hygrothermal performance of clothing. Computational fluid dynamics (steady Reynolds-averaged Navier-Stokes) is used to evaluate the drag and convective heat transfer of 19 body segments of a cyclist for three different cyclist positions. The influence of wind speed on the drag is analysed, indicating a pronounced Reynolds number dependency of the drag, where more streamlined positions show a dependency up to higher Reynolds numbers. The drag and convective heat transfer coefficient (CHTC) of the body segments and the entire cyclist are compared for all positions at racing speeds, showing high drag values for the head, the legs and the arms and high CHTCs for the legs, the arms, the hands and the feet. The drag areas of individual body segments differ markedly for different cyclist positions whereas the convective heat losses of the body segments are found to be less sensitive to the position. CHTC-wind speed correlations are derived, in which the power-law exponent does not differ significantly for the individual body segments for all positions, where an average value of 0.84 is found. Similar CFD studies can be performed to assess drag and CHTCs at a higher spatial resolution for applications in other sport disciplines, bicycle equipment design or to assess convective moisture transfer.

3D RANS CFD simulations of gaseous pollutant transport in generic enclosures: two case studies

On the suitability of steady RANS CFD for forced mixing ventilation at transitional slot Reynolds numbers

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%.

The relation between the acoustics of open-plan study environments, well-being and learning performance of students

13th International Conference on Wind Engineering Foreword

Comparison of calculation models for wind-driven rain deposition on building facades

Towards The Integration Of The Urban Heat Island In Building Energy Simulations

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.

Numerical-physical modelling of the long jump flight of female athletes: Impact of jump style, hairstyle and clothing

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.

Characterization of aerodynamic performance of vertical axis wind turbines: Impact of operational parameters

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.

Numerical analysis of forced convective heat transfer coefficients at the facades of a low-rise building: influence of wind direction

PIV measurements of a plane wall jet in a confined space at transitional slot Reynolds numbers

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.

CFD simulation of train aerodynamics: Train-induced wind conditions at an underground railroad passenger platform

Evaporative cooling by water spray systems: CFD simulation, experimental validation and sensitivity analysis

A short review of recent research activities for characterization of aerodynamic optimization of vertical axis wind turbines

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.

Efficiency of a heated air curtain under cross-jet temperature gradients

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.