SAE International Unsteady Aerodynamic Response of a Vehicle by Natural Wind Generator of a Full-Scale Wind Tunnel 2017-01-1549

In recent years, the automotive manufacturers have been working to reduce fuel consumption in order to cut down on CO 2 emissions, promoting weight reduction as one of the fuel saving countermeasures. On the other hand, this trend of weight reduction is well known to reduce vehicle stability in response to disturbances. Thus, automotive aerodynamic development is required not only to reduce aerodynamic drag, which contributes directly to lower fuel consumption, but also to develop technology for controlling unstable vehicle behavior caused by natural wind. In order to control the unstable vehicle motion changed by external contour modification, it is necessary to understand unsteady aerodynamic forces that fluctuating natural wind in real-world environments exerts on vehicles. In the past, some studies have reported the characteristics of unsteady aerodynamic forces induced by natural winds, comparing to steady aerodynamic forces obtained from conventional wind tunnel tests. Although there are few studies using full-scale vehicle, the characteristics of unsteady aerodynamic forces are not understood enough use for the vehicle development. In this study, a wind tunnel nozzle with oscillating plates and airfoils was newly developed for a full-scale wind tunnel. This device is called Natural Wind Generator (NWG) and can simulate natural wind with good repeatability. The authors attempted to clarify how unsteady aerodynamic forces induced by natural wind are different from steady aerodynamic forces. For experiments, a full-scale hatchback vehicle model was used. The unsteady aerodynamic forces acting on the model induced by the natural wind simulated using NWG were measured using a newly designed plate-type load cell. The comparisons between steady and unsteady aerodynamic forces show that the fluctuating aerodynamic forces induced by the natural wind have strong unsteady nature. In addition, the unsteady aerodynamic response characteristics evaluated by frequency response functions with respect to yaw angle fluctuations were changed to a great extent by modifying diffuser upsweep angles of the model. The evaluation method as mentioned in this study is effective to control unstable vehicle behavior caused by natural wind.