STUDY OF THE NONLINEARITY IN THE AEROELASTIC RESPONSE AT HIGH ANGLE OF ATTACK BY WIND TUNNEL TESTS ON THE ACTIVE AEROELASTIC TEST BENCH

摘要

The dynamic response of a cantilever wing at small and large angles of attack was tested using the Active Aeroelastic Test Bench (AATB) in the wind tunnel at low wind velocity (8 m/s, Re ≈ 120,000). The compliant wing was excited at the root of the wing with a small 2° pitch angle. Advantage was taken from the fact that the AATB allows for arbitrary excitation of the wing in pitch and plunge at its constrained root. This property is exploited in order to excite the wing with an odd random phase multisine. The response of the instrumented wing was recorded by using both fibre Bragg grating (FBG) optical strain gauges and piezoelectric accelerometers. Using the spectral properties of the applied odd random phase multisine, the level of stochastic nonlinear distortions and the Best Linear Approximation (BLA) of the FRF was estimated for different angles of attack. A clear increase of the level of stochastic nonlinearities was observed when increasing the angle of attack up to 17.5°. These distortions were mainly focussed around the resonance frequencies and in the very low frequency range. This latter could only be observed in the measurements of the FBG strain gauges due to the high pass filter effect of the piezoelectric accelerometers. Comparison of the identified BLA with gust response measurements showed that in general good predictions from the BLA model were possible when the Signal to Distortion Ratio (SDR) was above 10 dB. For the tested wing this was for angles of up to 12°. Above this angle all resonance frequencies were still well approximated both in frequency and amplitude, however, a very important low frequency contribution was missing from the predicted data.