Field rotor measurements. Data sets prepared for analysis of stall hysteresis

As part of the JOULE-3 project 'STALLVIB' an analysis and synthesis of the data from the field rotor experiments at ECN, Delft University, Imperial College, NREL and Risø has been carried out. This has been done in order to see to what extent the data could be used for further development and validation of engineering dynamic stall models.

A detailed investigation of the influence of the post-processing of the different data sets has been performed. Further, important statistical functions such as PSD spectra, coherence and transfer functions have been derived for the data sets which can be used as basis for evaluation of the quality of the data seen relative to actual application of the data.

The importance of using an appropriate low-pass filtering to remove high frequency noise has been demonstrated when the relation between instantaneous values of e.g. #alpha# and C_N is considered. In general, the complicated measurement on a rotor of #alpha# and w and the interpretation of these parameters combined with the strongly three-dimensional, turbulent flow field around the rotating blade has the consequence that it seems difficult to derive systematic information from the different data sets about stall hysteresis.

In particular, the measurement of #alpha#, which determination of the stagnation point gives reasonable data below stall but fails in stall. On the other hand, measurements of #alpha# with a five hole pitot tube can be used also in the stall region. Another main problem is the non-dimensionalization of the coefficients C_N and C_r.

If the dynamic pressure used for the non-dimensionalization is not fully correlated with the aerodynamic pressure over the considered airfoil section due to e.g. influence of the gravity on the pressure pipes, the hysteresis loops will be distorted.