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

Abstract

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 · 10 6 to 6 · 10 6 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. KEYWORDSboundary layer, laminar-turbulent transition, transition detection, wind tunnel experiments, wind turbine airfoil INTRODUCTIONAs the size of modern wind turbines is steadily growing, high Reynolds number experimental data for the wind turbine airfoils and blades are needed for verification and improvement of the aerodynamic prediction tools in the design process. In order to contribute to the development of the airfoil design and to accurately predict airfoil characteristics, the determination of the laminar-turbulent transition point is very important. 1Power curves, which are the indication of the electrical power output at different incoming wind speeds, are highly affected by the lift to drag ratio of the aerodynamic profiles of wind turbines. However, the maximum lift coefficient must be limited due to structural load constraints, which makes drag force one of the major parameters to be studied. 2 Skin friction drag of a laminar boundary layer is significantly lower than that of a turbulent boundary layer. Therefore, by the detection of the transition location from laminar to turbulent flow, it is possible to have an improved frictional drag estimation. Moreover, transition location control mechanisms can be applied to obtain higher lift to drag ratio airfoils.It was hypothesized by Reynolds that the transition is a result of instabilities in the laminar flow, which was further developed by Rayleigh. 3On a given body, starting f...

Transition characteristics measured on a 2MW 80m diameter wind turbine rotor in comparison with transition data from wind tunnel measurements

“…The x tr p is shown on the upper graph of Fig. 8 in comparison with the results from wind tunnel testing at Re 5 mill on the similar 2D airfoil section model as presented in [12]. A considerable shift of x tr p towards higher AoA is seen which amounts to 8 to 10 deg when x tr p is close to the LE.…”

“…8 shows the comparison of rotor vs. wind tunnel transition results for the suction side. Again the wind tunnel results are from [12] where it was found that the transition was shifted towards lower AoA than e.g predictions by Xfoil indicated. The causes were also discussed and a combination of interaction from neighbouring/upstream microphones and the surface irregularities were mentioned as possible effects.…”

“…The wind tunnel experiment, the transition detection procedure and the results have been thoroughly described in [12] and will only be summarized here. The blade section was made in an aluminium shell that was sanded as the final step in the manufacturing proces, Fig.…”

“…below 500 Hz) is much lower. It means that a method based on the first moment of spectra as discussed in [12] seems to be slightly more precise than the above method.…”

“…See [12] for comparison of the two detection procedures. The wind tunnel transition results in the present paper are based on the moment method and it should also be noted that a frequency interval from 2 to 7 kHz was used in the integration of the spectra for the wind tunnel.…”