Free wake analysis of compressible rotor flow fields in hover

Free Wake Analysis of Compressible Rotor Flow Fields in Hover* John Steinhofft* K. Ramachandran:j:* The University of Tennessee Space Institute Tullahoma, Tennessee In the computation of helicopter rotor flow fields, it is well known that wake effects can be very important. In addition, compressibility can be very important since, at the speeds of modern rotors, shocks can start to appear near the blade tip. Extensive work has been done on incompressible rotor flows using vortex tracking methods to follow the wake for free wake analyses. These, of course, cannot treat compressibility effects. On the other hand, extensive work has also been done for fixed wings in transonic flow, using compressible potential flow and Euler equation methods. This work may have problems if used for free wake analysis of rotor flows due to the inability of potential flow solvers as currently used to allow a free wake, and Euler solvers to propagate the wake over significant distances without numerical diffusion and spreading. In this paper we describe a unified method that is fully compressible, and which computes the wake without requiring external specification of the wake, or separate computations for the wake and blade region. This method has been developed into a computer program ("HELIX I") for the computation of compressible rotor flow fields in hover with free wakes. Results of this program are also described. The method utilizes a modification of a ~ompressible finite volume potential flow technique. This technique has been extensively used in the fixed-wing industry for research and wing design, and has been shown to be effective for resolving shocks and to be in good agreement with experiment for subsonic and transonic flows. We obtain similar agreement for rotor flow field solutions, as shown by computed surface pressure plots. In addition, with our new wake treatment, we are able to compute wake positions that are in substantial agreement with experiment. * Supported by ARO Contract DAAG29-K0019 * t Associate Professor * :j:Graduate Research Associate 20-1