A Computational Fluid Dynamics (CFD) study has been conducted to analyze the efficiency of the newly designed drag-driven Torque Wind Turbine (TWT). Specific objectives of this study are the analysis of the flow field around one blade of the turbine fixed in a static position, and the generation of the power and torque coefficient curves for a single rotating blade. The commercial software Ansys Fluent was used to perform the simulations adopting the SST k-ω turbulence model. A single blade of the turbine offers a maximum power coeffi- cient around 0.065 at a tip speed ratio ranging from 0.45 to 0.55 depending on the wind speed.
Accurate CFD simulation of coupled outdoor wind flow and indoor air flow is essential for the design and evaluation of natural cross-ventilation strategies for buildings. It is widely recognized that CFD simulations can be very sensitive to the large number of computational parameters that have to be set by the user. Therefore, detailed and generic sensitivity analyses of the impact of these parameters on the simulation results are important to provide guidance for the execution and evaluation of future CFD studies. A detailed review of the literature indicates that there is a lack of extensive generic sensitivity studies for CFD simulation of natural cross-ventilation. In order to provide such a study, this paper presents a series of coupled 3D steady RANS simulations for a generic isolated building. The CFD simulations are validated based on detailed wind tunnel experiments with Particle Image Velocimetry. The impact of a wide range of computational parameters is investigated, including the size of the computational domain, the resolution of the computational grid, the inlet turbulent kinetic energy profile of the atmospheric boundary layer, the turbulence model, the order of the discretization schemes and the iterative convergence criteria. Specific attention is given to the problem of oscillatory convergence that was observed during some of these coupled CFD simulations. Based on this analysis, the paper identifies the most important parameters. The intention is to contribute to improved accuracy, reliability and evaluation of coupled CFD simulations for cross-ventilation assessment.
Idealized sources are commonly used to reproduce the traffic emission in street canyons in experimental and numerical investigations. However, it remains unclear whether idealized sources can accurately reproduce the pollutant dispersion compared to more realistic sources. The goal of this paper is to investigate the impact of idealized and realistic sources on traffic-induced pollutant concentration in a street canyon by numerical simulation with Computational Fluid Dynamics (CFD). First, the scale-adaptive simulation (SAS) results of mean velocity and concentration are compared with wind-tunnel (WT) data for idealized line sources (ILS) and a satisfactory agreement is found between the SAS and WT results. Next, SAS are performed to investigate the impact on mean velocity and mean pollutant concentration of ILS versus multiple realistic car sources (MRCS) cases, designed to mimic a configuration of traffic jam-like conditions (i.e. rows of idling cars) in a street canyon. Different levels of geometrical simplifications of sources and total emission rates are considered. Although this research study focuses on these aforementioned specific set of conditions and conclusions relate solely to these circumstances, the SAS results show that the overall-averaged concentration associated with MRCS and the simplified source-geometry cases is about 1.18-6.22 times larger than that by ILS. Thus, ILS may be inadequate to reproduce the pollutant concentration in real scenarios. This study can contribute to enhancing the pollution prediction accuracy and reduce the potential risk of human exposure.
The forced convective heat transfer at the surfaces of a cubic building in an atmospheric boundary layer is investigated with CFD. Heat transfer in the boundary layer is modelled using low-Reynolds number modelling (LRNM) and standard wall functions (SWF). SWF are found to overestimate the convective heat transfer coefficient (CHTC) significantly. Using customised wall functions (CWF) for temperature, derived from LRNM data, a good agreement with LRNM is found for the CHTC, with relatively coarse grids in the near-wall region, compared to LRNM. Furthermore, correlations of the CHTC with the wind speed are reported for the windward surface.
Forced mixing ventilation is a commonly used ventilation principle in which air is forced into the upper part of the room at relatively high speed. Attachment of the wall jet to the ceiling, also known as the ‘Coanda effect’, is used to ensure that the air does not enter the occupant zone too early, thus preventing discomfort for the room occupants. Most mixing ventilation studies in the past have been conducted for wall jets with slot Reynolds numbers that are considered to be in the turbulent regime, while transitional flows can be present for lower slot Reynolds numbers. Previous studies have indicated possible deficiencies of the commonly used Reynolds-Averaged Navier-Stokes (RANS) equations in combination with a turbulence model to provide closure, when applied for transitional flows. This paper presents the first results of an experimental and numerical study of forced mixing ventilation at transitional slot Reynolds numbers, with focus on assessing the accuracy of steady RANS in combination with four frequently used turbulence models in Computational Fluid Dynamics (CFD) for indoor airflow. Visualisations with fluorescent dye seem to indicate that for a slot height (h/L) of 0.1 the flow is transitional for slot Reynolds numbers from about 800 to 2,900. 3D steady-state RANS simulations in combination with four turbulence models have been compared with time-averaged PIV measurements, for h/L = 0.1 and a slot Reynolds number of 1,500. In general, the SST k-? model shows the best agreement with the PIV measurements, whereas the standard, realizable and RNG k-e models show rather large deviations, at least in this particular case. Further validation studies should be made to analyse the accuracy of the CFD simulations for other slot heights and other slot Reynolds numbers.
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.
L’analyse du comportement hygrothermique de l’enveloppe du bâtiment est requise pour evaluer les problemes relies a l’humidite et la chaleur. Les coefficients de transfert convectif sont particulierement importants pour ces calculs. Des guides, standards, et relations semi-empiriques existent pour la determination des coefficients de transfert convectif de chaleur, mais il y a peu d’information pour les coefficients de transfert convectif d’humidite. Les equations pour la chaleur, l’air, et l’humidite dans la couche limite sont analogues pour des conditions particulieres, et en consequence les coefficients de transfert convectif d’humidite sont souvent calcules avec des equations d’analogie, i.e. Lewis et Chilton-Colburn. Les analogies sont surtout applicables pour des ecoulements d’aire dans le regime laminaire, mais certaines etudes indiquent que les equations pourraient aussi etre valides pour des ecoulements d’aire turbulents. Par contre, les comparaisons entre les donnees experimentales et les coefficients calcules a l’aide des analogies montrent des differences pouvant atteindre 300%. Les parametres influencant les resultats sont, entre autres, l’echange radiatif, la chaleur latente, des materiaux au-dessous du point de saturation, etc. La dynamique de fluide computationnelle (CFD) est souvent utilisee pour etudier le transfert de chaleur entre un materiel et une lame d’air, puisque la CFD peut calculer avec precision le profil de vitesse dans la couche limite. Plusieurs limitations sont presentes dans les programmes commerciaux de CFD pour modeliser l’humidite dans des 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.
The high importance of indoor environment performance aspects such as surface condensation, mold growth, thermal comfort, etc., is widely recognized. High-resolution simulation of heat, air and moisture (HAM) transfer can be used to enhance the prediction and analysis of these aspects. For this purpose, a coupling mechanism has been developed in order to perform run-time external coupling between Building Energy simulation (BES) and Computational Fluid Dynamics (CFD). This paper presents the results of indoor humidity calculation using the new coupled tool for the BESTEST case600. The results are compared with stand-alone BES results and the need and importance of coupled simulations is discussed.
The Dutch railways provide transport for more than 1.2 million passengers a day, resulting in more than 5200 train rides. In order to avoid overcrowded trains during rush hours, it is planned to increase the amount of passenger and freight trains, and their running velocities. Pedestrian discomfort or danger on platforms can be caused by trains which are allowed to pass small railway stations at high speed of up to 140 km/h. A number of these railway stations lay underground and it is known that space confinement increases the slipstream forces and results in augmented velocities behind the running train (Gilbert et al., 2012). The purpose of this study is to evaluate the effect of a passing train on the wind flow induced inside a tunnel by means of CFD using LES simulation and to validate the simulations by wind-tunnel measurements.
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.
The current study systematically analyzes the impact of solidity (σ) and number of blades (n) on the aerodynamic performance of 2-, 3- and 4-bladed Darrieus H-type vertical axis wind turbines (VAWTs). Solidity varies within the wide range of 0.09–0.36. A large number of operational parameters, i.e., tip speed ratio (λ), Reynolds number (Re), turbulence intensity and reduced frequency (K) are investigated to provide a deeper insight into the impact of σ and n on the dynamic loads on blades, the turbine performance and the wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, extensively validated with experiments, are employed. The results show that the turbine optimal tip speed ratio (λopt) is invariant to a newly-introduced parameter 'σλ3', regardless of the turbine geometrical and operational characteristics. In addition, a new correlation is derived to estimate λopt as a function of σ, which can also be employed to predict the optimal σ for a turbine with a given λ. It is also found that: (i) for constant-speed urban VAWTs, which due to the low mean wind speed in the urban environment, frequently operate at moderate to high λ, a relatively-low σ is optimal; (ii) an optimal VAWT is a moderately-high-solidity variable-speed rotor maintaining a relatively-low λ, where due to the large blade chord length the resulting Re and K are favorably high; (iii) within the turbine optimal operational range, turbine power coefficient (CP) is almost independent of n. The present findings support the optimal aerodynamic design of small-to large-scale VAWTs.
