Comparison to experiments

Two of the configurations, the twin bars (d) and wake body II (g), can be compared to experiments [7]. The lift is in good agreement whereas the effect of modifications to the drag is generally overpredicted (table ).

Table: Integral quantities and relative drag gain for modified Gurney-flaps (exp. Bechert et al. [7]).

			$\overline{c_l}$		$\overline{c_d}$		$\frac{\Delta c_d}{\Delta c_{d,Gurney}}$
a)			$0.6005$		$0.01115$
b)			$0.8512$		$0.01637$		$\pm 0$
d)			$0.8491$		$0.01560$		$-15\%$
g)			$0.8493$		$0.01428$		$-40\%$

Several reasons account for this misrepresentation. Beside the general problems coupled with accurate prediction of drag by CFD methods [22], the simulations in this particular case do not capture any three-dimensional effects which may have a strong influence on the drag prediction. Another reason is that no parasitic effects which may influence the experiments are considered. Finally URANS always concentrates almost all the energy in the dominant frequency. Therefore in the case of damping the corresponding flow structures, URANS results will always overpredict the gain.