High-Lift Configuration

Figure: Sketch of wind tunnel and two-element high-lift configuration

The present numerical study is related to experimental investigations by Tinapp [1] and Spang [7]. The test model is a generic two element high-lift configuration, consisting of a NACA 4412 main airfoil and a NACA 4415 flap with $c_k/c = 0.4$ relative chord length. Both profiles have bluff trailing edges. A previous study [8] of a similar test case showed that due to strong blocking the effect of the tunnel walls is very important and needs to be considered. The main airfoil is mounted at $52\%$ of the tunnel height ($h = 7.8 \ c$), whereas the flap is situated at a fixed position underneath the trailing edge of the main airfoil, thus forming a gap of $FG=0.078c$ with an overlap of $FO=0.027c$ (Fig. ). In the numerical study, the angle of attack is fixed at $\alpha=3^\circ$ for the main airfoil and $\beta = \alpha + \delta_f = 40^\circ$ for the flap.

The freestream velocity is $u_{\infty}=14 m/s$ corresponding to a Reynolds-Number of $Re=1.6 \cdot 10^5$ based on the main-airfoil chord. Transition is fixed at the positions of turbulator strips at 4.5% chord on the main airfoil and 2.8% chord on the flap according to the experimental setup.

In the experiments [7] periodic oscillating pressure pulses are generated externally by an electrodynamic shaker driving a small piston. It results in an oscillating jet emanating perpendicular to the chord from the narrow slot $4\%$ chord behind the flap leading edge. This excitation is presumed to be completely two-dimensional and it does not introduce extra mass-flux (zero-net-mass). To model the excitation apparatus, a suction/blowing type boundary condition is used. The perturbation to the flowfield is introduced through the inlet-velocity $u_e$ to a small chamber representing the excitation slot:

$$u_{e}(t) = u_{a} \ \sqrt{2} \cdot cos(2\pi t F)$$ (1)

where $F$ is the pertubation frequency and $u_a$ is the RMS velocity of the excitation oscillation. The excitation intensity is given by the non-dimensional steady momentum blowing coefficient $C_{\mu}=2 \frac{H}{c} \left( \frac{u_a}{u_\infty} \right)^2$ with is the slot-width $H=0.004 \ c_k$.