Three-dimensional modifications

In the experiments three-dimensionally modified Gurney-flaps have been shown to reduce the drag [7]. As computational simulations of such configurations require a considerable numerical effort, only one selected version of slitted Gurney-flaps (fig. , right) is investigated here.

In fig. , the flow fields predicted by URANS and DES are plotted. Compared to the Gurney-flap without slits, the two-dimensional rolls are weakened and irregular vortices appear instead. Even though in both the URANS and the DES results, very complex structures dominate the near wake and in particular the region downstream of the slits, vortex shedding is still visible. Again the intensity of vortices predicted by DES is larger than in the case of URANS. Compared to the strong rolls behind the 2d Gurney, here the smaller structures with no common orientation tend to interact and dissipate faster. In the visualization of $\lambda_2$, they vanish without being convected far downstream. The main effect of the slits in drag reduction is based on this phenomenon.

The introduction of slits has a negative effect on the lift as the Gurney-flap no longer covers the complete span. In the present case, slits with $10\%$ of the span lead to a drop in lift of about $5\%$. In the experiments, the drag could be reduced by $10\%$ at the same time [7].

The Detached Eddy Simulation could reproduce this behavior predicting a reduction of mean drag by $12\%$ coupled with strong three-dimensionality and a significant reduction of lift fluctuation. In the URANS computations the fluctuations are also significantly damped. However, the drag is not reduced (see table ). The Strouhal numbers are almost unaffected by the slits in the DES, and are slightly increased in the URANS due to higher frequency structures.

The effect of Gurney slits on the frequency spectrum of lift is plotted in fig. . The DES as well as the URANS results show a reduction of the Strouhal peak for the slitted 3d Gurney coupled with a slight increase of low and high frequency noise in the DES.

Table: Integral quantities and Strouhal number for three-dimensional computations compared to experiments [7].

			$\overline{c_l}$		$c_l\,'$		$\overline{c_d}$		$St$
Exp. HQ17			$0.600$		$-$		$0.0112$		$-$
Exp. 2d Gurney			$0.851$		$-$		$0.0164$		$0.14$
URANS			$0.834$		$0.0090$		$0.0125$		$0.121$
DES			$0.823$		$0.0042$		$0.0122$		$0.126$
Exp. 3d Gurney			$0.813$		$-$		$0.0148$		$-$
URANS			$0.815$		$0.0069$		$0.0128$		$0.129$
DES			$0.810$		$0.0038$		$0.0118$		$0.124$

Figure: Frequency spectrum of lift for the airfoil with slitted Gurney-flap (3d Gurney).