High-fidelity Numerical Modeling and Analysis of Low Reynolds Number Airfoils and Synthetic Jet

The shear layer development over a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and with large-eddy simulation. Two angles-of-attack (AOA) were considered: 5deg and 12deg. Experiments and numerics confirm two flow regimes. The first regime (AOA=5deg) exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. The stability equations exhibit significant sensitivity to base flow variations, making experimental-numerical comparisons challenging. Linear stability analysis suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency, and the importance of nonlinear interactions with harmonics during transition.;The sensitivity of the Orr-Sommerfeld equation due to base flow deviations was investigated with a Monte Carlo-type perturbation strategy and a Chebyshev collocation method. A separated boundary layer with a nominal shape factor of H=5.9 was perturbed for both velocity and wall-normal position deviations. Wide bands of eigenvalue spectra were obtained due to both perturbations. The standard deviation of the peak growth rate and frequency was found to be independent of Reynolds number. To broaden the results, six boundary layers were investigated with shape factors H=5.9-22. Perturbations resulting in a standard deviation of 1 % of the nominal shape factor were applied. Sensitivities of the peak growth rate and frequency are more pronounced at lower shape factors.;The effect of synthetic jet cavity shape was investigated numerically and experimentally. In the examination of three cavities it was found that the cavity with the sharpest nozzle-to-cavity transition transmits the most momentum at the exit. The sharp transition within the cavity results in a stronger vortex, higher self-induced inward velocity, and more room for flow to exit the cavity during expulsion. It is shown that the shape of the internal cavity plays an important role in the flow behaviour at the nozzle exit. From a computational perspective, the flow field within the cavity must be computed to obtain accurate exit conditions for the synthetic jet.