Motorcycle Aerodynamics

Abstract

The work of this thesis is a synergistic interaction of laboratory experimentation and numerical simulation. The focus of the work is to establish how the aerodynamics of the fluid flow around a motorcycle is responsible for head-buffeting forces on a rider. The experimentation was primarily carried out utilizing several full-scale motorcycles in a specially designed wind tunnel, supplemented by additional experiments on an outdoor test track. Primary among the measurements were pressure distributions on the surface of a specially designed rider’s helmet and accelerometer outputs. These data were post-processed to give forces and frequency responses as a function of the position of the windshield of the motorcycle. With regard to numerical simulation, a model was formulated which included both the fluid mechanics of the air passing over the motorcycle and the structure of the machine. Several levels of description were employed for the simulation, including two-dimensional, partial three-dimensional, and full three-dimensional. The numerical calculations were carried out for both steady state and transient operation. The turbulence model used in the simulations had been validated earlier. Numerous comparisons and correlations were made between the predictions of the simulations and the measured data. They included: a) Correlated wind tunnel drag measurements to CFD predictions b) Created a computational model that will accurately predict buffeting levels for a given motorcycle and rider c) Identified contributing factors to head buffeting d) Correlated quantitative pressure and accelerometer results with riders’ subjective feedback e) Proved validity of computational model through multiple correlation activities involving experimental data With regard to head buffeting, it was found that the tallest position of the windshield gave rise to minimal vortex impingement on the rider’s helmet. In contrast, maximum impingement occurred when the windshield was set at its lowest position. When there was no windshield present, impingement was a minimum. The subjective opinions of the riders with regard to the impacts of head buffeting closely matched inferences abstracted from the quantitative pressure and accelerometer data. It was found that combining the pressure forces and the accelerometer readings gave rise to excellent verification of Newton’s Second Law.