Wake bodies (f+g)

The following computations consider a combination of one vertical bar of height $0.6h$ and an airfoil-shaped wake body of length $3.0h$ and height $0.8h$, $1.2h$ downstream of the flap. The wake body is located vertically centered to the Gurney (wake body I) or shifted $0.2h$ downwards (wake body II).

The introduction of such bodies in the wake provides a significant reduction of drag ($73\%$ to $85\%$) which is associated with an almost complete suppression of the flow structures in the wake (fig. ). The drag reduction can be achieved even though separation occurs on the lower surface of wake body I indicating a suboptimal design. Due to an occurring recirculation bubble, the lift fluctuations are not damped by this configuration.

Table: Integral quantities, non-dimensional frequency and relative drag gain computed for the HQ17 airfoil with modified Gurney-flaps.

			$\overline{c_l}$		$c_l\,'$		$\overline{c_d}$		$\frac{\Delta c_d}{\Delta c_{d,Gurney}}$		$St$
a)			$0.636$		$0.0000$		$0.0080$				$0.117$
b)			$0.855$		$0.0044$		$0.0113$		$\pm 0$		$0.115$
c)			$0.848$		$0.0087$		$0.0101$		$-36\%$		$0.118$
d)			$0.847$		$0.0007$		$0.0101$		$-36\%$		$0.120$
e)			$0.835$		$0.0009$		$0.0117$		$+12\%$		$0.130$
f)			$0.829$		$0.0082$		$0.0089$		$-73\%$		$0.129$
g)			$0.840$		$0.0005$		$0.0085$		$-85\%$		$0.114$

Figure: Iso-contours of vorticity in the wake of Gurney-flaps with modifications.