Effect of Unsteady Lift Force on Vehicle Dynamics in Heave and Pitch Motion 2014-01-0576
The change in the aerodynamic lift force (henceforth CL) by heave motion is discussed in this paper in order to clarify the effect of aerodynamic characteristics on the vehicle dynamic performance. We considered that phenomenon in actual car running at 160km/h and 1Hz heave frequency. Using a towing tank to change its water from the air to the working fluid to more easily observe this phenomenon. That makes possible to observe the same phenomenon with reduced velocity and small models under same Strouhal number condition. This method can be reducing vehicle speed to 3m/s (1/15 actual) and frequency to 0.2Hz (1/5 actual) in case using 40% scaled model. The results of these tests showed that unsteady CL is proportional to heave motion. These results showed the proportional relationship between unsteady CL and heave motion. The formularization of unsteady CL made it possible to introduce shape coefficients to vehicle dynamics simulations as functions of heave velocity. This makes it possible to consider the effect of unsteady CL on dynamic performance at the initial stages of the development process. In addition, unsteady CFD was conducted by means of URANS with dynamic mesh in order to study the mechanism of unsteady CL, and the predictive accuracy of the method was verified. Predictive accuracy was within 10%, making the method adequate for an understanding of qualitative phenomena. The flow visualization by this unsteady CFD method clarified that unsteady CL during heave motion was produced by the pressure change in the under-floor area. Next, the unsteady front and rear CL during vehicle vibration was obtained accurately in slow motion by towing test. These values were analyzed with a two-degree-of-freedom mass-spring-damper model to quantify the effect of unsteady CL on flat ride. Moreover, we investigate about pitch motion. Numerical simulations were conducted with the change in the initial vehicle ride height and frequency of pitch motion in order to investigate the sensitivity of unsteady CL to these parameters. These simulation results indicated that the shape coefficient αl changed significantly from Strouhal number around 1.0. In initial ride height, αl increases as the closer to the ground by ground effect. It was possible to perform introduction CL change into the vehicle motion by formulating the test results with a simple model in heave motion. it is also analyzed by CFD. This will enable us to show the possibility to study the dynamics of the early stages of vehicle development.