Air curtains (ACs) employ plane turbulent jets to separate two environments in terms of heat and mass transfer, while still allowing unrestricted access through the opening between these environments. Most previous studies focused on ACs discharged from nozzles located just above the opening. However, in some cases ACs have to be installed close to the ceiling at a substantial distance from the top of the opening. The AC blown downwards along the vertical wall then first resembles a wall jet and after reaching the top of the opening starts resembling a free jet. The present study analyzes the behavior and performance of an AC with upstream wall above the opening. 2D steady RANS CFD simulations are performed based on grid-sensitivity analyses and validation with experimental data for a wall jet and a free jet. The total opening height is 4 m and vertical walls of 0.5 m, 1 m and 2 m, partly closing this opening, are considered. AC performance is evaluated both with the separation efficiency η (based on infiltration) and the adapted separation efficiency η* (based on infiltration and exfiltration). It is shown that the presence of the wall reduces jet decay. The longer the wall, the larger the jet momentum over the opening height. This reduces infiltration and increases η, but it increases exfiltration and therefore decreases η*. In practice, the jet discharge velocity (jet momentum) will have to be adjusted to keep high η*.
The surface-averaged forced Convective Heat Transfer Coefficient (CHTCavg) at a windward building facade is influenced by the complex interaction between a wide range of parameters. Existing CHTC expressions, however, consider the impact of these parameters either incompletely or not at all. Earlier studies have shown that this shortcoming can lead to significant errors in Building Energy Simulations. In this paper, therefore, the combined impacts of wind speed (U10), building height (H) and width (W), and wind direction (θ) on the CHTCavg for the windward facade of buildings are systematically investigated. High-resolution CFD simulations of wind flow and forced convective heat transfer, validated with wind-tunnel measurements, are performed for 64 building geometries (10 m ≤ H and W ≤ 80 m), 8 wind directions (0° ≤ θ ≤ 78.75°) and 4 reference wind speeds (1 m/s ≤ U10 ≤ 4 m/s). The 3D steady RANS equations with the realizable k-ε turbulence model and the low-Re number Wolfshtein model are used. The results show that for a given building geometry and U10, the CHTCavg decreases as θ increases from 0° to 78.75°. The maximum reduction of about 42% occurs for the building with H = 8W = 80 m. In addition, for a given θ and U10, by increasing H, the CHTCavg increases, while increasing W has the opposite impact on the CHTCavg. Finally, a new generalized CHTC expression is presented as a function of U10, H, W and θ, and its accuracy is confirmed by detailed in-sample and out-of-sample evaluations.
Numerical simulations of room airflow using Computational Fluid Dynamics (CFD) are often based on the Reynolds-averaged Navier-Stokes (RANS) approach. Using this approach, only the averaged quantities are computed, whereas the effect of turbulence on the mean flow is modeled. Since the RANS approach does not provide information on velocity and concentration fluctuations, also the turbulent mass fluxes should be modeled. In the majority of the cases this is done by employing the standard gradient-diffusion hypothesis, which relates the turbulent mass flux to the mean concentration derivatives. In this paper a CFD analysis of pollutant dispersion in an enclosure ventilated by a transitional wall jet (Re˜ 2,500) is presented, using validated high-resolution RANS and Large Eddy Simulations (LES). The LES simulation shows that a counter-gradient turbulent mass flux is present, indicating that the standard gradient-diffusion hypothesis used in RANS is not valid in the entire flow domain. However, it is shown that for this particular case, the convective mass fluxes dominate over the turbulent mass fluxes, and that the predicted pollutant concentrations by RANS will therefore not differ significantly from those by LES. Paper_ID613
A few riders have adopted a rather exceptional and more aerodynamic sprint position where the torso is held low and nearly horizontal and close to the handle bar to reduce the frontal area. The question arises how much aerodynamic benefit can be gained by such a position. This paper presents an aerodynamic analysis of both the regular and the low sprint position in comparison to three more common cycling positions. Computational fluid dynamics simulations are performed with the 3D RANS simulations and the transition SST k–ω model, validated with wind-tunnel measurements. The results are analyzed in terms of frontal area, drag coefficient, drag area, air speed and static pressure distribution, and static pressure coefficient and skin friction coefficient on the cyclist surfaces. It is shown that the drag area for the low sprint position is 24% lower than for the regular position, which renders the former 15% faster than the latter. This 24% improvement is not only the result of the 19% reduction in frontal area, but also caused by a reduction of 7% in drag coefficient due to the changed body position and the related changes in pressure distribution. Evidently, specific training is required to exert large power in the low sprint position.
Automobile designers and engineers are constantly focusing on aerodynamic optimization with the aim of reducing fuel consumption and particulate and gaseous emissions. In the last decade, a generic car body called DrivAer model has been frequently adopted in numerical and experimental investigations. However, the setup of the CFD domain, grid, turbulence model and numerical approach can significantly impact the accuracy of results and requires sensitivity studies. This is the goal of this study for which wind-tunnel (WT) tests and CFD simulations were carried out on the 1:4 reduced-scale DrivAer notchback model. The target parameters were the drag (CD) and lift (CL) coefficients. First, the CFD results were validated with the WT tests. Next, the impact of surface grid resolution, total prism layer height, first prism layer height, growth rate, cell type, upstream and downstream distances, turbulence models, and numerical approaches was systematically investigated. The sensitivity analysis provides a set of guidelines on optimal grid generation. The validation study indicates that a hybrid RANS-LES approach is a good trade-off for an accurate yet economical simulation of the aerodynamic forces. These guidelines are expected to help researchers and practitioners in CFD simulations of car aerodynamics.
Jachthuis St. Hubertus kent een lange historie van vochtproblemen. Slagregen Is de belangrijkste vochtbron voor de toren van het Jachthuis. Deze parameter is echter onvoldoende geimplementeerd als randvoorwaarde in de huidige generatie rekenmodellen voor hygrothermische analyse van bouwdelen. Daarom werden In dit onderzoek CFD-simulaties uitgevoerd om de slagregenbelasting op de toren te bepalen. Er werden ook slagregenmetingen uitgevoerd om de simulatles te valideren. De gevalideerde CFD-resultaten zullen in vervolgonderzoek gebrulkt worden als randvoorwaarde bij hygrothermische analyses om de vochtproblemen van de toren te analyseren en herstelmaatregelen te evalueren.
Abstract Some teams aiming for victory in a mountain stage in cycling take control in the uphill sections of the stage. While drafting, the team imposes a high speed at the front of the peloton defending their team leader from opponent’s attacks. Drafting is a well-known strategy on flat or descending sections and has been studied before in this context. However, there are no systematic and extensive studies in the scientific literature on the aerodynamic effect of uphill drafting. Some studies even suggested that for gradients above 7.2% the speeds drop to 17 km/h and the air resistance can be neglected. In this paper, uphill drafting is analyzed and quantified by means of drag reductions and power reductions obtained by computational fluid dynamics simulations validated with wind tunnel measurements. It is shown that even for gradients above 7.2%, drafting can yield substantial benefits. Drafting allows cyclists to save over 7% of power on a slope of 7.5% at a speed of 6 m/s. At a speed of 8 m/s, this reduction can exceed 16%. Sensitivity analyses indicate that significant power savings can be achieved, also with varying bicycle, cyclist, road and environmental characteristics.
The aim of this study is to assess wind flow modelling capabilities for a selected urban area -Quartiere la Venezia in Livorno city (Italy) - by means of wind tunnel tests and CFD simulations. Both wind tunnel tests and CFD simulations were performed on a reduced-scale model. In addition, in the CFD simulations two levels of detail were tested. A comparison in terms of mean wind velocity profiles was then made between wind tunnel test results and numerical simulations in order to understand how the different degrees of precision of the CFD models can affect the results. At a later stage these results will be complemented by full-scale experimental data, enabling a further assessment of modelling capabilities.
Courtyard and building geometry are crucial aspects to achieve efficient natural ventilation and indoor thermal comfort in a compact urban environment. In this paper, first, a comparison between different geometries of courtyards in terms of wind flow characteristics and indoor air speed is performed based on the validation of Computational Fluid Dynamics (CFD) simulations with 2D publis hed wind-tunnel experiments. Second, assessment of thermal comfort is made inside a number of selected dwelling's rooms facing different courtyard geometries. It is confirmed that rooms with cross ventilation have higher indoor air speed values and therefore a better thermal comfort than with single-side ventilation. The courtyard dimensions, the position of the room and the orientation are important aspects influencing the indoor air speed and thermal comfort.
