Extraction of Dynamic Inflow Models for Coaxial and Tandem Rotors from CFD Simulations

The performance and inflow characteristics of representative coaxial rotor and tandem configurations have been studied. Two different solvers were used. The first approach is the well-established RotCFD solver that uses a wake capturing approach on a Cartesian coordinate system, with radially distributed rotating body forces representing the forces exerted by the rotor blades on the fluid. The second approach, called GT-Hybrid, uses Navier-Stokes analyses over small regions surrounding the rotor blades, and a free wake model outside the Navier-Stokes domains. There are significant differences in the way the flow was modeled with these two solvers. Nevertheless, the two solvers gave consistent, physically realistic results for representative coaxial and tandem rotor configurations. The inflow velocity field was examined, and the first three terms in the dynamic inflow (uniform component, and the lateral and longitudinal variations) were extracted. Comparisons with a viscous vortex particle method and the dynamic inflow theory was done for the coaxial rotor in forward flight and good agreement was observed. Significant interference of the upper rotor downwash over the lower rotor was captured and quantified for this coaxial rotor. For the tandem rotor configuration, the effects of overlap on the inflow was captured. In addition, the longitudinal variation of the inflow was properly captured with both solvers.