Numerical investigations on small-scale rotor configurations with validation using acoustic wind tunnel data

Abstract

This paper addresses the acoustic and aerodynamic characteristics of small-scale rotor configurations, including the influence of the rotor-rotor interactions. For this purpose, a Rotor/Rotor/Pylon configuration is chosen for both the test and numerical simulations. The wind tunnel experiments on various rotor configuration were performed in DLR’s Acoustic Wind Tunnel Braunschweig (AWB). The experiments involve isolated rotors, and rotors in tandem and coaxial configuration in hover and forward flight. For numerical simulations an unsteady free wake 3-D panel method (UPM) is used to account for aerodynamic non-linear effects associated with the mutual interference among the Rotor/Rotor/Pylon configurations. The effect of the pylon is simulated using potential theory in form of a panelized body. Finally, the sound propagation into the far field is calculated with DLR’s FW-H code APSIM, using UPM blade surface pressure as input. The validation effort is supported by CFD TAU steady simulations on selected hover test cases. The experiments and numerical results indicate that the harmonic noise is the dominant source of the noise for the present rotor selection. Broadband noise is also observed in the experiment, but its contribution to the overall sound pressure is small. In the numerical simulations of both the coaxial and the tandem configuration, the interferences are particularly well visible and the noise directivity becomes more complex. There is no change in time averaged inflow by applying phase angles. In the coaxial condition, in hover, the phase delay between rotors doesn’t change the maximum noise level. In forward flight, the phase delay can influence the maximum level of the noise radiation. In both coaxial and tandem configuration, the position of the downstream rotor is key for the noise radiation and therefore avoiding the interaction with upstream wake can reduce the noise radiation.