Forcing Boundary-Layer Transition on a Single-Element Wing in Ground Effect

Abstract

The transition from a laminar to turbulent boundary layer on a wing operating at low Reynolds numbers can have a large effect on its aerodynamic performance. For a wing operating in ground effect, where very low pressures and large pressure gradients are common, the effect is even greater. A study was conducted into the effect of forcing boundary-layer transition on the suction surface of an inverted GA(W)-1 section single-element wing in ground effect, which is representative of a racing-car front wing. Transition to a turbulent boundary layer was forced at varying chordwise locations and compared to the free-transition case using experimental and computational methods. Forcing transition caused the laminar-separation bubble, which was the unforced transition mechanism, to be eliminated in all cases and trailing-edge separation to occur instead. The aerodynamic forces produced by the wing with trailing-edge separation were shown to be dependent on trip location. As the trip was moved upstream the separation point also moved upstream, this led to an increase in drag and reduction in downforce. In addition to significant changes to the pressure field around the wing, turbulent energy in the wake was considerably reduced by forcing transition. The differences between free- and forced-transition wings were shown to be significant, highlighting the importance of modeling transition for ground-effect wings. Additionally, it has been shown that while it is possible to reproduce the force coefficient of a higher Reynolds-number case by forcing the boundary layer to a turbulent state, the flow features, both on-surface and off-surface, are not recreated.

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

“…For the latter, the fully turbulent boundary layer which forms from the leading edge of the model means that no laminar separation occurs, and thus the bubble does not form, and instead turbulent separation occurs close to the trailing edge. 11 The important difference, therefore, is that the k-k L -ω model allows a laminar separation to occur, while the k-ω SST model only contains turbulent separations. The difference between such separation phenomena is that a turbulent boundary layer is more resilient to the adverse pressure gradient, due to fluctuating velocity components providing momentum transfer normal to the surface, and thus separation occurs at a more downstream location.…”

“…Previous studies have shown the separation bubble to be a force-enhancement mechanism as it alters the effective shape of the wing to give it more effective camber; this helps to maintain suction levels and increase circulation. 10,11 It is the increase in circulation that causes suction across the entire lower surface to be increased. In addition to altering the aerodynamic forces that the wing generates, changes to the pressure field on a three-dimensional wing can also affect the formation of wing-tip vortices.…”

“…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.…”

“…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 and 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.…”

“…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.…”