Periodic excitation with different intensity

To assess the effectiveness of oscillatory blowing and suction, various blowing coefficients $C_{\mu}$ are tested at fixed excitation frequency $F^+=1.03$.

For $C_{\mu}=40$, the pressure distribution obtained from the experiments and numerical results can be compared. Flow separation on the flap cannot completely be avoided, but the reverse flow region is located at about $35\%-40\%$ flap chord in both studies (Fig. ). The unsteady pressure distribution is exposed to strong oscillation due to the convected vortices which cannot be captured by the ''slow'' pressure tubes in the experimental investigations. This might explain the different pressure levels in the suction peaks on main airfoil and flap.

Figure: Mean pressure distribution for $F^+=1.03$ excitation frequency and $C_{\mu}=40 \cdot 10^{-5}$ intensity

Compared to the separated flow in the baseline simulations at $C_{\mu}=0$, excitation with low intensity does not increase the lift or delay separation (Fig.  and ). If $C_{\mu}$ becomes larger than $C_{\mu}>25 \cdot 10^{-5}$, however, lift continously climbes. This effect is perfectly in line with the experiments, but the maximum achievable intensity was limited to $C_{\mu}=65 \cdot 10^{-5}$, there. The present study shows that further increasing intensity might bring another gain in lift. In former investigations [4] flow control by tangential suction and blowing with stronger excitation also results in further increase of lift. In the present case of vertical excitation, separation position and drag coefficient, however, do not improve for extremely strong suction and blowing.

Figure: Mean separation position for $F^+=1.03$ excitation frequency and different intensities

Figure: Mean lift coefficient for $F^+=1.03$ excitation frequency and different intensities

Figure: Mean drag coefficient for $F^+=1.03$ excitation frequency and different intensities