An Estimation of the Effect of Turbulence from the Natural Wind and Traffic on the Cycle-Averaged-Drag Coefficient

A drag coefficient, which is representative of the drag of a car undergoing a particular drive cycle, known as the cycle-averaged-drag coefficient, has been previously developed. It was derived for different drive cycles using mean values for the natural wind. It assumed terrain dependent wind velocities based on the Weibull function, equi-probable wind direction and shear effects. It did not, however, include any effects of turbulence in the natural wind. Some recent research using active vanes in the wind tunnel to generate turbulence has suggested that the effect on drag can be evaluated from the quasi steady wind inputs. On this basis a simple quasi-steady theory for the effect of turbulence on car drag is developed and applied to predicting the cycle-averaged-drag coefficient for a range of cars of different types. The drag is always increased by the turbulence but in all cases is relatively small. Turbulence is also present when driving in traffic, but traffic also introduces a velocity deficit, which reduces drag. By making certain assumptions about traffic flow, a crude traffic model is developed and the impact of traffic on the cycle-averaged-drag has been derived. It is found that the combination of natural wind turbulence and the effects of traffic results in very small changes to the cycle-averaged-drag coefficient.