Summary

A two-element high-lift configuration with periodic blowing and suction through a narrow slot at the leading edge of the flap is simulated by an unsteady RANS simulation. The effect of vertical zero-net-mass excitation is studied. The computed results are compared with available wind tunnel test results to determine the prediction capability of the computational method. For better comparison, the wind tunnel walls are included in the numerical simulations.

For the baseline simulation without excitation, the influence of mesh resolution and time-stepping is studied and reasonable results can be obtained with the LLR $k$-$\omega$ turbulence model and correct transition fixing. Massive separation is predicted and the results agree fairly well with the experiments.

For the active control case, steady suction and blowing do not show satisfactory effects. To study flow control by periodic excitation various blowing coefficients are investigated at a given excitation frequency. In general, the computation as well as experiments show that the lift increases if the intensity exceeds a certain limit that is about $C_{\mu}>25 \cdot 10^{-5}$ in the present case. However, for very large intensities $C_{\mu}>50 \cdot 10^{-5}$ the positive effects become smaller and smaller.

By changing the excitation frequency at a constant intensity an optimum frequency can be identified. It corresponds to a sinusoidal lift and detaching vorticies of a suitable size. The mean lift coefficient increases by up to $30\%$ compared to the baseline simulation and separation can be delayed.

Numerical simulation of unsteady turbulent flow with a RANS approach is always questionable. In the present case, however, results are defensible as the existence of a spectral gap is demonstrated and qualitative agreement with the experiments is shown. In the future further work needs to be done in comparing RANS simulations to a large-eddy simulation (LES) or a detaches-eddy simulation (DES).