The Design of Multi-Element Wings in Ground Effect

The racecar aerodynamicist is only too aware of the difficulties of optimising the performance of multiple element wings in ground effect. Experimental testing of a series of configurations usually answers the question "which of these is best?", but to determine why requires detailed and time consuming instrumentation to provide pressure distributions, wake surveys and surface flow visualisation studies. CFD techniques can be of some help at this stage by providing more detailed information on the flowfield than is usually available through wind tunnel tests alone.

Even with this level of capability, however, it is still difficult for the aerodynamicist to know how to change the geometry to eliminate diagnosed problems, and to make further improvements to the design. Usually a whole series of "cut and try" models need to be manufactured and tested in the hope of arriving at one demonstrating an improvement. Resorting to this approach for the design of multi-element wings is particularly hit and miss, since small changes to the profiles can have substantial effects on the characteristics of the wing, and such changes are usually non-intuitive.

Most CFD methods do not get around this problem. They simply mimic a wind tunnel in providing an analysis of the existing geometry, but do not provide any assistance as a design tool. The loadings on the wing assembly, and any areas of separation, are determined by the shape of the pressure distribution generated by the profiles. To achieve optimum performance from a wing assembly it is essential to be able to specify the pressure distribution and to compute the shape of the aerofoil sections required to achieve it. Careful control of the pressure distribution allows design for maximum downforce, for instance, with no flow separation over a given speed range.

Flow Solutions is now able to offer such a procedure for 3D wing design in ground effect. The design procedure is coupled to the NEWPAN panel method. Mainplane and flap elements can then be redesigned individually or simultaneously, at each section across the span, including the effects of interaction with other components.

Results for a typical front wing design procedure are shown in the following figure. At an inboard section the original design showed significant flow separation on the mainplane at low speed; after redesigning the pressure distribution an aerofoil is obtained generating more downforce and with no flow separation at low speed. Note that it would have been almost impossible to achieve this result by a trial and error process in the wind tunnel, not least because of scaling problems between the model and the full scale car.

There follows an example of the way in which we perform this process using NEWPAN2D in conjunction with NEWPAN and the post-processor VIEWPAN.