In-flight measurements of the of coherence length varying with altitude and speed using flight test data.

Abstract

Surface pressure fluctuations from several flight attitudes were evaluated in order to find the influence of altitude and speed on the coherence lengths of turbulent structures in the boundary layer. Flight tests were performed using an A320 airplane as a test carrier. Three consecutive window banks in the vicinity of the wing were used to install a pressure transducer array. The array consisted of three aluminum windows equipped with a total of 30 pinhole-mounted pressure transducers [29], [30] and [31]. Assuming a homogenous and ergodic flow the spatial differences between each transducer pairing were considered and the narrow-band coherence was calculated for each pairing. A two-dimensional exponential function was fitted through this coherence, taking the local frequency-dependent flow direction into account [32]. The decay coefficient representative for a remaining coherence of 36 percent was plotted over the frequency and compared with the Efimtsov prediction [33] in Fig. 10. The prediction was found to considerably underestimate the coherence lengths values below 1 kHz. The prediction parameters were adapted to fit the measurements and the resulting curves were compared between different flight altitudes at constant flow velocity and different flow velocities at constant altitude.