An examination of ground effect phenomena and the development of ground effect induced flow separation on a downforce generating wing

摘要

'Ground effect' is a phenomenon that can be characterised as a change in the aerodynamic behaviour of a body when it operates in close proximity to the ground. For many years, it has been exploited in the motor racing industry where inverted wings that produce beneficial downforce (negative lift) are used. Doing so allows these inverted wings to produce yet more downforce.Despite this application being utilised extensively, several aspects of how ground effect manifests about wings is still not well understood. Chief among them is how flow separation develops about inverted wings in ground effect. It has been shown previously that flow separation develops much more rapidly near the ground, however, a full explanation for how this transpires has been lacking. Additionally, an understanding of how ground effect works about differently shaped wings, and the underlying causes which generate these different behaviours, has not been elucidated.The influence of ground effect on several aerofoil and wing arrangements has been studied through various Computational Fluid Dynamics (CFD) analyses. The CFD modelling work throughout has been validated by way of moving-ground wind tunnel experimentation, using non-intrusive smoke flow visualisation and Laser Doppler Anemometry (LDA) measurement.A two-dimensional CFD investigation was undertaken to observe the ground effect behaviour about both an upright and inverted Tyrrell aerofoil, in order to clarify and differentiate the workings of ground effect for both cases. Two general mechanisms were identified in both cases: a reduction in effective angle of attack near the ground and a significant diversion of flow over the wing, when near the ground. Additional analyses on various inverted aerofoil geometries revealed that top surface camber is important in the generation of underwing suction and that a rapid upward curve on the bottom side of the aerofoil increases the effectiveness of the diffuser effect at the rear of the aerofoil.A project was undertaken to develop a CFD model which would be able to effectively model the unsteady turbulent behaviour about a quasi-two-dimensional inverted Tyrrell wing in extreme ground effect. A model which utilises span-normal periodic boundary walls was developed which offered a significant computational efficiency. Various CFD methodologies were also assessed resulting in an LES model which uses the Dynamic Smagorinsky-Lilly sub-grid turbulence model.Extensive LES simulations were conducted to model the ground effect induced flow separation that occurs on an inverted Tyrrell wing in extreme ground effect. Featuring a separation bubble near the leading edge, both a progressive and a sudden form of separation were identified. The progressive form (worsening as the ground is approached) initiated at the trailing edge and was found to be caused by the strengthening diffuser effect whose growing outlet-to-inlet ratio eventually creates a jet flow near the ground which cannot supply kinetic energy to the boundary layer. This form of separation was found not to be due to an excessive adverse pressure gradient, as often thought. The sudden form of separation is associated with the eventual inability of the separated shear layer to reattach. This was caused by both the diversion of flow over the wing, starving the boundary layer of kinetic energy, and also due to movement of the turbulent transition point further away from the boundary layer which ultimately prevents the separated shear layer from reattaching.