Aerodynamics of asymmetrical land speed record vehicles
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Although current market penetration of battery electric cars is low (0.1% worldwide), it is rapidly growing as the advantages of electric vehicles (EV) in reduced pollution, CO2 emissions and lower operating costs overcome their higher initial purchase price. As battery systems carry far less energy than traditional liquid fuels, EVs face challenges to maximize efficiency to achieve acceptable performance and range. One way to enhance EV efficiency is to reduce aerodynamic drag.
In racing, the goal is to achieve maximum performance for a given available energy, which provides a laboratory to study vehicle system optimization. This is especially true for land speed racing where the singular aim is to safely achieve the highest possible speed on a long, closed course, for example the Bonneville Salt Flats in Utah. The goal in this form of racing is to minimize aerodynamic drag while maintaining dynamic stability.
In this work, aerodynamic, rolling resistance, and tractive forces and moments were examined for an asymmetrical land speed record vehicle through computational fluid dynamics (CFD) studies and the analysis of the equations of motion. Validation of CFD technique was performed by comparison of numerical results to published drag and lift, velocity profile, and flow topology of a 25° and 35° Ahmed body using Reynolds Averaged Navier-Stokes (RANS) turbulence models (25° and 35° Ahmed body) and Large Eddy Simulation (LES) (25° Ahmed body). RANS simulations were found to predict Cd (-1.4% of published values) and Cl (+2.3% of published values), while LES was less successful for Cd (+8.4%), and Cl (-11.6%). Both methods predicted velocity profiles and wake structures well.
Studies were undertaken to characterize the dynamic and aerodynamic stability of a bluff-body four “wheel” (Ahmed Body) vehicle and a two-wheel streamlined electric land speed record motorcycle. The Ahmed body was found (from CFD) to have positive lift between 0° and 45° yaw angles, and then transition to negative lift (downforce) between 45° and 55° of yaw angle at a speed of 150 mph (67 m/s). The two-wheel streamlined motorcycle was found (from CFD) to create lift greater than the vehicle weight at yaw angles greater than 50° at 150 mph (67 m/s) and at yaw angles greater than 20° at speeds of 250 mph (112 m/s), the design speed of the vehicle. The addition of a longitudinal, dorsal “shark fin” was found to reduce this lift to below the vehicle weight even at a yaw angle of 90° at a speed of 150 mph (67 m/s).
Three-dimensional computational fluid dynamics (CFD) simulations were also used to characterize and enhance the aerodynamic performance of an electrically-powered racing sidecar. From the starting point of a Solidworks model from the laser-scan of an existing road- racing sidecar motorcycle, an extensive optimization program using ANSYS Fluent 17.0 (CFD), with 6-10 million-element, unstructured, tetrahedral meshes and a RANS turbulence model, was undertaken. Compared to the original starting point, the optimised sidecar CFD results indicated a 24.4% reduction in Cd, a change in Cl from +0.0026 (lift) to -0.255 (downforce). Lateral force coefficient (Cy) was reduced 11% compared to the original sidecar. From visualisations of the flow topology, large streamwise vortical structures originating from the shoulder regions of the rider were found to be the most significant sources of aerodynamic drag. Other parts of the sidecar body also produced streamwise vortices that contributed to pressure drag. Negative lift (downforce) was found to result primarily from the formation of a primary vortex along the leading edge of the underside of the splitter at the front of the vehicle.
Based upon these results, a new body for the sidecar was fabricated from composite materials. The modified sidecar was successful, setting four FIM world land speed records and one U.S. national land speed record in electric sidecar motorcycle classes at the Bonneville Salt Flats in Utah, U.S.A. in August 2016. Further validation of the new sidecar bodywork was undertaken with testing in a full-scale wind tunnel facility.
The asymmetrical aerodynamic forces generated by the sidecar, predicted from CFD, were found by the rider to not create significant dynamic instabilities at high speeds. Dynamic stability analyses predicted cross winds would require minor steering corrections by the rider and were found to have different effects depending upon their direction due to the aerodynamic asymmetry of the vehicle. Pitch and roll moments were found to show asymmetries but were judged by the rider to be negligible in their effect on vehicle stability. The stability predicted from CFD and dynamic modelling was thus confirmed by the rider’s experiences during successful land speed record attempts.