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Simulating 'realistic wind' for accurate fuel economy
Exa Corporation explains that with a 2% reduction in aerodynamic drag translating to a 1% improvement in real-world fuel economy, just how important realistic wind conditions in simulations
Submitted by: Exa Corporation
For a typical car driving at highway speeds, about 50% of the energy is spent to overcome the aerodynamic drag and the other 50% is required to overcome the mechanical losses (engine, transmission, tires, ancillary systems, etc.). This means that every 2% in aerodynamic drag reduction translates to a 1% improvement in fuel economy. As engines, transmissions and low rolling resistance tires become more efficient, especially in the case of electric vehicles, the reduction of vehicle aerodynamic drag under real world conditions on the road has increased impact on the fuel economy and driving range.
In the recent SAE paper (SAE 2015-01-1551) experimental on-road measurements were gathered by comparing the energy consumption of the vehicle with the characteristics of the on-road flow field including traffic turbulence and crosswind, as measured by a pitot-static probe mounted on the roof of the vehicle. The fluctuations in the measured wind are attributed mainly to turbulence caused by other vehicles on the road, while the measured yaw angle is attributed mainly to the prevailing crosswind and large-scale wind gusts. Figure 1 (below) shows the power consumption as a function of yaw angle. The data has been segmented by turbulence intensity, showing the influence of turbulence in addition to yaw angle as a source of drag. Lower turbulence intensities yielded a lower drag value. Conversely, data with high turbulence intensity levels indicated that added turbulence generally decreases yaw sensitivity to drag, but increases overall drag value.
The numerical simulations were carried out using Exa’s PowerFLOW. The realistic flow environment was modeled by including all the relevant traffic turbulence and wind structures that together make up the real on-road environment, including flow structures many times the size of the vehicle. These flow structures, specified at the inlet, evolved in time as they were transported downstream towards the vehicle. On top of these realistic wind fluctuations, a varying crosswind yaw angle was also added. The interaction between yaw angle, turbulence, and the vehicle flow field is illustrated in Figure 2 (below), where streamlines are colored by local yaw angle as they move through the domain. The realistic wind effects resulted in a drag increase of about ΔCD=+0.010, but also a slightly less steep yaw curve, indicating that the effect of realistic wind is more significant near zero yaw than at higher yaw angles.
Both the experimental and numerical data indicate a direct trend between environmental turbulence intensity and power consumption of the vehicle. Basically, this means that the real world conditions, including traffic turbulence and crosswind, will result in a reduction of the fuel economy of the vehicle. Therefore, improving the efficiency and performance of any vehicle on the open road requires that the main features of the real world conditions be included in the simulated environment.