Abstract Motorcycles are present in cycling races for reasons including television broadcasting. During parts of the race, these motorcycles ride in front of individual or groups of cyclists. Concerns have been expressed in the professional cycling community that these motorcycles can provide aerodynamic benefits in terms of drag reduction for the cyclists drafting behind them. However, to the best of our knowledge, no information about the extent of these benefits is present in the scientific literature. Therefore, this paper analyses the potential drag reduction for a cyclist by drafting behind a motorcycle. Wind tunnel measurements and numerical simulations with computational fluid dynamics were performed. It was shown that drafting at separation distances d = 2.64, 10, 30 and 50 m can reduce the drag of the cyclist down to 52, 77, 88 and 93% of that of an isolated cyclist, respectively. A cyclist power model is used to convert these drag reductions into potential time gains. For a non-drafting cyclist at a speed of 54 km/h on level road in calm weather, the time gains by drafting at d = 2.64, 10, 30 and 50 m are 12.7, 5.4, 2.7 and 1.6 s per km, respectively. These time differences can influence the outcome of cycling races. The current rules of the International Cycling Union do not prevent these aerodynamic benefits from occurring in races.
In large eddy simulations (LES) of the atmospheric boundary layer (ABL), the choice of inflow method and the specification of surface roughness are crucial for obtaining accurate results. The consistency between inflow conditions and wall boundary condition has been investigated in depth for the RANS approach, whereas it needs further analysis for LES. This paper investigates the combined effect of inflow method and aerodynamic roughness length for LES of the neutral ABL. Three basic inflow generators in combination with three terrain types are evaluated in terms of streamwise homogeneity of the vertical profiles of mean velocity and turbulence kinetic energy. The results show that the precursor method best preserves the homogeneity of the profiles for all roughness lengths considered, with mean absolute deviations up to 0.7% for mean velocity and 3.5% for turbulence kinetic energy at the outlet of the computational domain. Among the synthetic methods, the Vortex Method performs satisfactorily for mean velocity and, to a lesser extent, turbulence kinetic energy, whereas the random flow generation (RFG) method leads to the largest deviations from the target profiles. Finally, the value of the aerodynamic roughness length is found to only weakly influence the performance of the inflow methods considered.
Large-Eddy Simulation of pollutant dispersion from a stack on the roof of a low-rise building in downtown Montreal is performed. Two wind directions are considered, with different wind flow patterns and plume behaviours. The resulting mean concentration field is observed and analysed with the computed (mean) convective and turbulent mass fluxes. These two concepts allow gaining some insight into the dispersion process and analysing the deficiencies of less sophisticated turbulence models. When the emitting building is located downstream of a high-rise building (case SW), the turbulent mass flux is directed from the high to low levels of mean concentration and the gradient diffusion hypothesis often used with steady models is verified. However, when the influence of the surrounding buildings is smaller (case W), a counter-gradient mechanism is observed in the streamwise direction, confirming the results obtained on isolated buildings. The present study supports the use of generic, simplified cases to investigate environmental processes; the conclusions can subsequently be applied to real and more complex cases.
A practical numerical method for driving rain estimation was presented in “Driving Rain on Building Envelopes—I” (Blocken and Carmeliet, 2000). An important prerequisite in employing this method is that the climatic data used as input are representative. In this paper, the attainment of representative experimental data for driving rain estimation is analysed. The importance of a sufficiently small time step to obtain representative climatic data measurements is indicated. It is shown that representative averaged values for wind speed and rainfall intensity for longer time steps can be obtained by averaging the measured data with the rainfall amounts as weighting factors. The effects of using different averaging techniques on the accuracy of the calculated driving rain results are investigated. It is found that the presented weighted averaging technique can provide accurate representative averaged data, whereas commonly used averaging techniques can give rise to large errors.
At present no clear procedure exists for the development of optimal remedial measures in pedestrian wind comfort studies. Instead, an appeal is made to the experience and intuition of the investigator to assess a priori which possible measures might be effective and could therefore be considered for further analysis. In this paper the use of adjoint Computational Fluid Dynamics (CFD) methods is investigated as a first step towards a more rigorous wind comfort optimization framework. These methods can be used to determine the sensitivity of mean wind velocity in a target area to grid deformations, which near building surfaces is indicative of the effect a geometric change in a certain location can have. It is shown that this method can be used to generate sensitivity maps in which the most opportune locations for remedial action are graphically displayed.
Adaptation to climate change on an urban scale is important, as increases in temperature will be inevitable according to future climate scenarios. In this study, Computational Fluid Dynamics (CFD) simulations are performed for the Bergpolder Zuid district in Rotterdam, the Netherlands. The simulations take into account wind flow, solar radiation, heat transfer and evaporative cooling. Two validation studies are performed; one for the surface temperatures and one for the evaporative cooling model. In the final stage, three water ponds are added to the domain and a comparative study is performed. It is concluded that evaporative cooling can reduce the air temperature at pedestrian height (1.8 meter) by 0.9°C to 2.0°C.
