Forcing Boundary-Layer Transition on an Inverted Airfoil in Ground Effect

Abstract

The inuence of the laminar boundary-layer state on a wing operating in ground eect has been investigated using experiments with a model that provides twodimensional ow. The eect of a boundary-layer trip placed at varying distances from the leading edge was observed at various incidences in terms of on-surface characteristics, including pressure measurements, ow visualisation and hot-lm anemometry, and o-surface characteristics with velocity surveys below and behind the wing. The act of forcing transition led to downforce being reduced and drag increased, moreover it altered almost all aspects of the wing's aerodynamic characteristics, with the eect becoming greater as the trip was placed closer to the leading edge. These aspects include the replacement of a laminar separation bubble with trailing-edge separation, a thicker boundary layer, and a thicker wake with greater velocity decit. The importance of considering laminar phenomena for wings operating in ground eect has been shown. Nomenclature c Wing chord length, m C D Drag coecient C F Friction coecient

Recent advances and test processes in automotive and motorsports aerodynamic development

“…Common to both motorsports and road cars is the pursuit of increased aerodynamic efficiency. This has led to advancements in ground effect aerodynamics 1–8 and tyre aerodynamics. 9 Also, common to both motorsports and road cars is the need for increased aerodynamic development efficiency.…”

Modelling boundary-layer transition on wings operating in ground effect at low Reynolds numbers

“…Despite the low Reynolds number conditions of each of these studies, none has included natural transition phenomena as a laminar region was only included up to x/c = 0.1 such that comparison could be made to forced-transition experiments. [ Experimental studies that have used boundary-layer trips to cause premature transition to a turbulent boundary layer [9][10][11][12] have cited large differences in the aerodynamic forces and flow structures compared to the natural transition counterpart. Zerihan & Zhang [9] found increased trailing-edge separation when the turbulent boundary layer covered a larger portion of the wing, due to a thicker boundary layer encountering the adverse pressure gradient.…”

“…Roberts et al [11][12] showed that a wing with a longer turbulent portion gave a thicker wake and a significantly different pressure field around the wing. Additionally, the aerodynamic forces generated by the wing were proportional to the extent of the laminar boundary layer.…”

“…1–23 The computational studies 13–23 have, in general, used Reynolds-averaged Navier–Stokes (RANS) solvers with fully turbulent closure models; the low Reynolds number operating conditions for such simulations mean that in reality there are both laminar and turbulent effects to consider. Experimental studies, 9–13 however, have shown that both laminar and turbulent boundary layers exist on the wing, with transition occurring through a laminar separation bubble. In comparison with a turbulent boundary layer, laminar boundary layers are thinner, generate lower skin friction drag, and are less resilient to adverse pressure gradients; thus, if a significant portion of the wing contains a laminar boundary layer, then the assumption of fully turbulent flow can lead to erroneous results.…”

“…Correia et al 10 showed the laminar separation bubble to be a force-enhancement mechanism as it altered the effective shape of the wing akin to an increase in camber, thus increasing circulation. Moreover, Roberts et al 11,12 showed that a wing with a longer turbulent portion gave a thicker wake and a significantly different pressure field around the wing. In addition, the aerodynamic forces generated by the wing were proportional to the extent of the laminar boundary layer.…”