Unsteady vehicle aerodynamics in gusty crosswind

Unsteady vehicle aerodynamics in gusty crosswind are investigated using both numerical simulation and wind tunnel experiments with the aim of providing an understanding of the unsteady aerodynamic behavior. For a realistic but generic sinusoidal gust event, significant unsteady effects are identified for the aerodynamic side force, roll and yaw moment. Unsteady side force and roll moment exhibit smaller amplitudes compared to steady-state measurements. The unsteady yaw moment, however, shows a significant increase in amplitude compared to the corresponding steady-state values. Furthermore, positive time delays are identified for the side force and the roll moment, while the yaw moment exhibits a negative time delay compared to the quasi-steady approximation. With the help of numerical simulations, an unsteady mechanism is derived where a delayed reaction of the wake flow causes a time delay and an increase of the load contributions of the rear end which eventually lead to the differences between the unsteady and quasi-steady aerodynamic loads. In this context, a characteristic wake topology is identified consisting of fluid entering into the wake region from the leeward side flanked by two counter-rotating vortices above and below, which is at the core of the proposed unsteady mechanism. Following the identification and characterization of the above described unsteady phenomena, their main features are verified experimentally. For this purpose, a complex experimental setup is applied that provides time-dependent flow conditions at realistic Reynolds and Strouhal numbers by rotating a 50% scale model around its vertical axis. Afterwards, the sensitivity of the identified unsteady effects to a variation of the gust parameters number of oscillations, gust frequency, vehicle speed, gust amplitude and initial yaw angle is investigated numerically using multiple-peak gust simulations. Finally, the influence of different vehicle rear end types and of geometrical variations is investigated.