Vehicle wakes in side wind
Doctoral thesis, 2021
Vehicle aerodynamics is typically assessed and developed in idealised conditions using low turbulence wind tunnels and numerical methods. Several aspects influencing vehicle aerodynamics are often neglected such as traffic and wind conditions. This thesis explores the effects of steady wind, or yawed flow, on the wake of vehicles. The goal is to increase the knowledge of the full wake behaviour at yaw and how it is related to the aerodynamic drag. For this, an optimisation method is used throughout this work to generate robust, low-drag, reference geometries. The optimisation is done at different yaw angles, allowing asymmetric geometries at yaw. The cycle averaged drag, which takes into account the driving cycle as well as the wind distribution, is also considered to create symmetric geometries which are insensitive to yaw. The optimisation is focused on base cavities and trailing edge modifications to these cavities.
Generally, the low-drag configurations have a more balanced wake, with and without side wind, where the recirculating flow in the wake is aligned with the vehicle. The improved balance allows the wake to move more freely which often increases the large scale coherent motions of the wake. These unsteady motions are linked to increases in drag in the literature, however, in this work, improving the wake balance was found to be the more important indicator of the overall drag. At yaw, the coherent unsteady motions are reduced as a result of the wake being locked in a more stable, but higher drag, upwash or downwash dominated state.
The wake becomes increasingly downwash or upwash dominated at yaw by a large rotating structure in the wake. The yaw insensitive designs have a wake that is slightly biased towards the top or bottom of the base at zero yaw to counteract the movement of the wake at yaw. Optimising the geometry without considering yaw can reduce the performance over the entire operating range. This highlights the importance of considering several operating conditions during vehicle development.
Chalmers, Mechanics and Maritime Sciences (M2), Vehicle Engineering and Autonomous Systems
Numerical analysis of a vehicle wake with tapered rear extensions under yaw conditions
Journal of Wind Engineering and Industrial Aerodynamics,; Vol. 179(2018)p. 308-318
Aerodynamic drag improvements on a square-back vehicle at yaw using a tapered cavity and asymmetric flaps
International Journal of Heat and Fluid Flow,; Vol. 86(2020)
Surrogate-based optimisation using adaptively scaled radial basis functions
Applied Soft Computing Journal,; Vol. 88(2020)
Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity
Experiments in Fluids,; Vol. 62(2021)
Magnus Urquhart, Simone Sebben. Optimisation of trailing edge flaps on the base cavity of a vehicle for improved performance at yaw
Vehicle aerodynamics is typically assessed and developed in perfect conditions using wind tunnels and computer simulations. There are several aspects that have an impact on the aerodynamic performance and the energy usage of a vehicle that are neglected such as traffic and wind conditions. This thesis explores the effects of steady wind on the aerodynamic drag.
The results show that the aerodynamic sensitivity to wind can be improved if the performance with wind is taken into consideration during vehicle design. This results in a trade-off where a slight reduction in performance in still conditions is balanced out against a larger gain when there is wind present. A robust design with low sensitivity to side wind can be achieved efficiently using optimisation techniques to automatically balance out the trade-off in performance.
Ökad energieffektivitet genom kontroll och reduktion av det inducerade motståndet
Swedish Energy Agency (2016-008677/43328-1), 2016-12-01 -- 2019-12-30.
Areas of Advance
C3SE (Chalmers Centre for Computational Science and Engineering)
Fluid Mechanics and Acoustics
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5031
Opponent: Prof. Jens Fransson, KTH, Sweden