Sports stadia are increasingly used to host a wide variety of activities that attract large attendances, ranging from sports matches to concerts, festivities and conferences. One of the crucial aspects of spectator comfort in open stadia is protection from wind and rain. However, in many stadia this part of spectator comfort is insufficiently taken care of. The main reason is that stadia and stadium roofs are often designed with only vertical rainfall in mind, neglecting the influence of wind that can sweep the rain onto the stands. This wind-driven rain (WDR) can reach a large area of the stand underneath the roof, resulting in discomfort for the spectators in this area. This paper presents 3D Computational Fluid Dynamics (CFD) simulations of WDR shelter for 12 different generic stadia configurations that are representative for a wide range of existing stadia. The wind-flow patterns are determined by steady-state Reynolds-averaged Navi-er-Stokes (RANS) simulations, after which the WDR trajectories are calculated using Lagrangian particle tracking. This study demonstrates the influence of both stand arrangement and roof slope on the area of the stand that is wetted by WDR. It shows the importance of taking into account WDR in the stadium design process, and it provides some design guidelines to avoid this type of spectator discomfort.
Information on pedestrian-level wind (PLW) speed for wind comfort assessment can be obtained by wind-tunnel measurements or Computational Fluid Dynamics (CFD) simulations. Wind-tunnel measurements for PLW are routinely performed with low-cost techniques such as hot-wire or hot-film anemometers, Irwin probes or sand erosion, while Laser-Doppler Anemometry (LDA) and Particle-Image Velocimetry (PIV) are less often used because they are more expensive. CFD simulations are routinely performed by the relatively low-cost steady Reynolds-Averaged Navier–Stokes (RANS) approach. Large-Eddy Simulation (LES) is less often used because of its larger complexity and cost. This paper reviews wind-tunnel and CFD techniques to determine PLW speeds expressed generally in terms of amplification factors defined as the ratio of local mean wind speed to mean wind speed at the same position without buildings present. Some comparative studies systematically indicate that the low-cost wind-tunnel techniques and steady RANS simulations can provide accurate results (∼10%) at high amplification factors (>1) while their accuracy can deteriorate at lower amplification factors (<1). This does not necessarily compromise the accuracy of PLW comfort assessment, because the higher amplification factors provide the largest contribution to the discomfort exceedance probability in the comfort criterion. Although LDA, PIV and LES are inherently more accurate techniques, this paper supports the continued use of faster and less expensive techniques for PLW studies. Extrapolating a previous saying, we argue that pedestrian-level wind comfort is one of the few topics in wind engineering where nature is kind to us concerning turbulent flows.
Forced mixing ventilation is a commonly used ventilation principle in which air is forced into the upper part of the room at relatively high speeds. Attachment of the wall jet to the ceiling, also known as the 'Coanda effect', is used to ensure the air does not enter the occupant zone too early, thus preventing discomfort of the room occupants. Most mixing ventilation studies in the past have been conducted for wall jets with slot Reynolds numbers (Reslot) that are considered to be in the turbulent regime, while a transitional flow regime can be present for low Reslot values. However, previous Computational Fluid Dynamics (CFD) 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 a numerical analysis of forced mixing ventilation at a transitional slot Reynolds number (Reslot ≈ 1,000). CFD simulations of transitional wall jets in a confined space are conducted, using steady-state RANS modelling with difference turbulence models to provide closure. The results of the RANS simulations are compared with Particle Image Velocimetry (PIV) measurements in a reduced-scale model to assess the capability of the models to predict the transitional flow pattern. In addition, the ventilation efficiency is determined, demonstrating differences up to 36% in the calculated air exchange efficiency using different turbulence models.
This paper presents a practical numerical method to determine both the spatial and temporal distribution of driving rain on buildings. It is based on an existing numerical simulation technique and uses the building geometry and climatic data at the building site as input. The method is applied to determine the 3D spatial and temporal distribution of wind-driven rain on the facade a low-rise building of complex geometry. Distinct wetting patterns are found. The important causes giving rise to these particular patterns are identified : (1) sweeping of raindrops towards vertical building edges, (2) sweeping of raindrops towards top edges, (3) shelter effect by various roof overhang configurations. The comparison of the numerical results with full-scale measurements in both space and time for a number of on site recorded rain events shows the numerical method to yield accurate results.
The present study investigates the impact of building façade geometrical details on the pollutant transport mechanism in long street canyons. Large-eddy simulations (LES), extensively validated with experiments, are performed for four cases: (i) street canyon without façade balconies, (ii) street canyon with balconies at both windward and leeward façades, (ii) street canyon with balconies only at the windward façade and (iv) street canyon with balconies only at the leeward façade. The results show that the building balconies can strongly affect the wind flow field and pollutant dispersion in long street canyons. The most significant impact is observed for the two street canyon cases with balconies at the windward façade, which strongly obstruct the airflow from penetrating deep into the bottom of the canyon. The presence of balconies only at the windward façade and at both façades can increase the area-weighted mean pollutant concentration in the vertical center plane inside the canyon by 80% and 106%, respectively, and reduce the mean pollutant exchange velocity (Ue) by 46% and 54%, respectively. The analysis of the vertical mean convective and turbulent mass fluxes indicates that the presence of balconies mainly decreases the convective contribution to Ue, while the impact on the turbulent contribution is smaller.
Wheels account for about 10% of the total aerodynamic drag of a cyclist. While different types of wheels are commercially available, spoked wheels are commonly used in cycling races. This paper presents computational fluid dynamics (CFD) simulations of spoked wheels, and systematically evaluates different boundary conditions that are usually used for rotation modelling of wheels, namely (i) rotational moving wall, (ii) moving reference frame (MRF) and (iii) sliding mesh. Steady and unsteady Reynolds-Averaged Navier-Stokes (RANS) CFD simulations are thus performed for an isolated wheel. Moreover, the impact of the volume enclosing the wheel, where the MRF is applied, on the predicted wheel drag is evaluated. The evaluation is based on validation with wind-tunnel measurements of force coefficients. The results of this study can be used for accurate CFD simulations of cyclist aerodynamics. 1Introduction In the 2017 Tour de France, 10 different manufacturers provided the wheels for the 22 teams in the race (Arthur, 2017), while each manufacturer had several wheels in their catalogue. The wheels are selected among the large variety of commercially available options based on their performance in terms of aerodynamic drag, weight, inertia and stiffness (Kyle, 1995). The aerodynamic performance of wheels is of significant importance since the drag of both wheels is responsible for about 10% of the total cyclist resistance (Greenwell et al., 1995). It is usually evaluated using wind-tunnel tests (Greenwell et al., 1995; Kyle, 1995, 1991; Tew and Sayers, 1999) and more recently using CFD simulations (Godo et al., 2010). CFD is capable of computing forces and moments acting on each single wheel’s component (rim, tyre, spokes and hub) and providing fundamental information that can be used by designers to improve the aerodynamic performance of wheels. Nevertheless, one critical aspect in the CFD simulations of wheels is appropriate rotation modelling approaches. To the best of our knowledge, the impact of different rotation modelling approaches on the accuracy of CFD simulations of cycling wheel aerodynamics has not yet been investigated. 2Methodology In this study, CFD simulations are first validated with the wind-tunnel measurements by Tew and Sayers (1999). The Campagnolo Shamal wheel has a 19 mm rim width and a 61 mm depth, spanning from the tyre to the rim edges (Fig. 1a). The spoke’s cross section is approximated to a rectangle of 3 mm 1 mm (Fig. 1b). The computational domain has a cross-section of 8.6 m 7.8 m. The upstream and downstream length of the domain are 3.9 m and 7.4 m, respectively. The computational grid consists of about 13.2 million cells, while about 246,000 surface cells are used on the wheel (Fig. 1b). The mean velocity inlet boundary condition is a uniform profile (48 km/h), accordingly to the experiment (Tew and Sayers, 1999). The 3D RANS equations are solved in combination with the k-ω SST turbulence model. It should be noted that the good performance of the k-ω SST turbulence model has been already shown in previous studies on cycling aerodynamics, e.g. Defraeye et al. (2010). Three approaches are evaluated to model the rotation of the wheel: (i) the rotational wall approach (RW), (ii) the moving reference frame approach (MRF) and (iii) the sliding mesh approach (SM). The latter two methods are applied on a volume surrounding the wheel, as shown in Fig. 1a and Fig. 1c. Moreover, the impact of the volume enclosing the wheel, where the MRF is applied, on the predicted wheel drag is evaluated. (Fig. 1c). 3Results The CFD results show a good agreement with the wind-tunnel results in terms of the drag coefficient with a deviation of about 1.8 % at 0˚ yaw angle. Further and more detailed information about the different approaches will be provided in the full paper.
