Laminar-turbulent transition detection on airfoils by high-frequency microphone measurements

In the present work, various data processing methods for laminar to turbulent transition detection on airfoils are assessed based on experimental data. For this purpose, NACA 63-418 airfoil profile with surface microphones flush mounted both on the suction and the pressure side is used in the wind tunnel experiments.

Reynolds numbers are changed in the range from 1.6·106 to 6·106 for various angles of attack. In this way, the transition behaviour of the airfoil is characterized. Time signals and spectrogram of the data, chordwise pressure level changes, and first moments are investigated. Standard deviations of pressure are observed to exhibit a peak at transition and slightly decreases for the fully turbulent flow staying always higher than the laminar flow.

The most robust method to detect the transition location is found to be the characteristic frequency approach by spectral moments provided that the inflow turbulence is low. The numerical results are in agreement with experimental results on the pressure side where natural transition occurs. However, the transition due to surface irregularity and microphone placement occurs on the suction side.

This cannot be predicted by the numerical tools. The Tollmien-Schlichting wave frequencies and neutral curve points are determined from the experiments. The analysis shows that the most common curve length of the transition process is around 15% to 20%, and it can vary between 0% to 30 % of the chord.

Increasing the Reynolds number leads to an earlier transition position closer to leading edge at both upper and lower surfaces.