Calculation of Multibladed Rotors in High-Speed Forward Flight with Weak Fluid-Structure-Coupling

Abstract

A weak coupling procedure for coupling the rotor simulation code S4 and the Euler/Navier-Stokes solver FLOWer was presented. The method allows to produce trimmed CFD solutions for rotors in high speed forward flight accounting for elastic blade deformations and viscous effects. The weak coupling procedure was applied to high-speed forward flight test cases of the 7A and the 7AD rotor. Isolated blade computations were carried out for both rotors while a chimera computation around the whole 4-bladed rotor was carried out only for the 7A rotor. The predicted pitching moment computed with the assumption of inviscid flow (Euler) showed large differences compared to the experimental data. Between 0° and 90° azimuth the Euler pitching moment even had the wrong sign. A clear improvement of the solution with respect to the existing experimental data because of the weak coupling was proven. The well known negative peak in the normal forces distribution close to the blade tip around 90°-120° for elastic blades in high speed forward flight was well reproduced with the coupled Navier-Stokes computation although the computational results show a phase shift of about 10-15° azimuth. While the predicted elastic torsion for the 7A test case showed a considerable content of a 5/rev motion this was not the case for the 7AD rotor. The reason for this unexpected difference has to be clarified. The chimera technique provided more accurate results at the cost of an increase of computational effort by more than one order of magnitude. Although the method was based on isolated blade computations it was able to predict the performance differences between the 7A and the /AD rotor in high-speed forward flight fairly well while the absolute power was overpredicted for both rotors by about 10%.