Abstract This paper presents a numerical study on the required momentum flux ratio to prevent air curtain breakthrough in case of cross-curtain (i.e. cross-jet) pressure gradients. 2D steady Reynolds-averaged Navier-Stokes (RANS) CFD simulations with the RNG k - ε turbulence model are employed for jet Reynolds numbers ranging from 5,000 to 30,000. First, the computational model is validated based on particle image velocimetry (PIV) measurements. Second, the influence of several jet parameters on the separation efficiency is evaluated for a moderate cross-jet pressure difference of 10 Pa. These are the ratio of the jet discharge momentum flux to the jet cross-flow momentum flux (momentum flux ratio), the jet height-to-width ratio and the jet discharge angle. Finally, the minimum deflection modulus to prevent jet breakthrough and the corresponding momentum flux ratio by an analytical equation and by CFD are compared. The results show that, for the configuration under study: (1) jets with the smallest height-to-width ratios ( β = 18) provide the highest separation efficiency; (2) inclined jets with discharge angles α 0 = 5° and 10° provide slightly higher separation efficiency than straight jets ( α 0 = 0°) and jets with α 0 = 20°; (3) the maximum modified separation efficiency is reached at lower momentum flux ratios for jets with smaller height-to-width ratios and for inclined jets; (4) the analytical and CFD values of the optimal momentum flux ratio differ with up to 31.2%. This study shows how the separation efficiency of air curtains can be improved by adjusting certain jet parameters.
In recent years, many accidents have occurred between cyclists and in-race motorcycles, even yielding fatal injuries. The accidents and the potential aerodynamics issues have impelled the present authors to perform dedicated wind-tunnel measurements and Computational Fluid Dynamics (CFD) simulations to assess cyclist drag reduction when followed by one, two or three motorcycles. The 3D steady-state Reynolds-Averaged Navier–Stokes simulations with the standard k–ε model are validated by the wind-tunnel tests. The cyclist drag reduction goes up to 8.7% for a single trailing motorcycle and to 13.9% for three trailing motorcycles at a distance of 0.25 m behind the cyclist. This distance is not uncommon in elite races, as evidenced by the many recent accidents. The effect by a single following motorcycle at realistic short distances d=0.25 m (8.7%), d=0.5 m (6.4%) and d=1 m (3.8%) is larger than that by a following car at realistic short distance d=5 m (1.4%). Therefore it could be argued that in-race motorcycles are not only more dangerous but also aerodynamically more influential. This study reinforces the necessity for the International Cycling Union to change the rules concerning in-race motorcycles, not only to avoid accidents but also to avoid unwanted aerodynamic benefits.
Tandem cycling enables visually impaired athletes to compete in cycling in the Paralympics. Tandem aerodynamics can be analysed by track measurements, wind-tunnel experiments and numerical simulations with computational fluid dynamics (CFD). However, the proximity of the pilot (front) and the stoker (rear) and the associated strong aerodynamic interactions between both athletes present substantial challenges for CFD simulations, the results of which can be very sensitive to computational parameters such as grid topology and turbulence model. To the best of our knowledge, this paper presents the first CFD and wind-tunnel investigation on tandem cycling aerodynamics. The study analyses the influence of the CFD grid topology and the turbulence model on the aerodynamic forces on pilot and stoker and compares the results with wind-tunnel measurements. It is shown that certain combinations of grid topology and turbulence model give trends that are opposite to those shown by other combinations. Indeed, some combinations provide counter-intuitive drag outcomes with the stoker experiencing a drag force up to 28% greater than the pilot. Furthermore, the application of a blockage correction for two athlete bodies in close proximity is investigated. Based on a large number of CFD simulations and validation with wind-tunnel measurements, this paper provides guidelines for the accurate CFD simulation of tandem aerodynamics.
This article aims at formulating a necessary level of detail in models for the atmospheric moisture load for attaining an accurate assessment of building components’ moisture responses. Simulated moisture responses of a ceramic brick outer leaf and rendering are compared for different models for the two key parts of the atmospheric moisture load: wind-driven rain and evaporative drying. It is shown that a constant wind-driven-rain coefficient yields only slight loss of accuracy, but the determination of the constant value has to take the influences of variability with rain intensity and wind speed and of temporal averaging into account. For the evaporative drying it is shown that the variations of the surface moisture and heat transfer coefficients with wind speed, and their overall order of magnitude, heavily influence the moisture responses. While most of the analysis is illustrated with the ceramic brick results only, it is noted that similar conclusions result for the slightly less sensitive rendering.
Generating economical, high-resolution and high-quality computational grids for Large Eddy Simulation (LES) of wind flow and convective heat transfer (CHT) around surface-mounted obstacles is not straightforward. When the grid size is used as filter, LES grids should ideally consist of cubic cells, while CHT requires a very high near-wall resolution to resolve the thin viscous sublayer and buffer layer that represent the largest resistance to CHT. To avoid very high cell numbers and the need for excessive computational resources, non-conformal grids can be considered. This paper provides a detailed evaluation of the performance of non-conformal grids with cubic cells, for wind flow and CHT around a wall-mounted cubic obstacle. LES results on non-conformal versus conformal grids are compared with each other and with wind-tunnel measurements of wind speed and surface temperature. Moreover, sensitivity analysis is performed concerning the impact of overall grid resolution, subdomain size and grid refinement ratio. Average absolute deviations between LES on non-conformal versus conformal grids are about 0.9% (0.5 °C) for surface temperature on all cube surfaces. Comparison with experiments shows for the non-conformal grid an average and maximum absolute deviation for surface temperature of 2.0% (1.1 °C) and 7.6% (3.6 °C), respectively. The sensitivity analysis shows minor impact of subdomain size on convective heat transfer coefficients (CHTC) where, on average, absolute deviations of less than 2.2% are observed. This study shows that non-conformal grids can strongly reduce the total cell count (here by a factor up to 30.2) without significantly compromising the accuracy of results.
