Trimmed Actuator Disk Modeling for Helicopter Rotor

The goal of this work has been to develop a method to compute the trim commands for an helicopter rotor using an Actuator Disc (AD) model where the source distribution and the orientation of the disc with respect to the shaft axis are adapted during the simulation, in order to meet the prescribed trim state. In a standard AD simulation the momentum source is assigned a priori, being usually provided by a CFD or BEM simulation with imposed kinematics. In this work the source distribution is provided by a multibody trimmed rotor simulation. The trimmed AD model has been embedded in ROSITA [1], a compressible RANS solver developed at Politecnico di Milano. ROSITA uses a system of overset grids (Chimera), which allow to give the actuator disc grid the same orientation as the rotor tip path plane without the need of remeshing. Thanks to this particular feature, both propulsive and moment trim can be performed. The source terms on the disc are obtained from a loose coupling between a CSD code (MBDyn [2]) and ROSITA. In particular when MBDyn has reached a periodic trimmed state, it provides a rotor map (a radial and azimuthal force distribution) and the disc orientation to the CFD solver. ROSITA is then run till a steady inflow condition is reached at the disc surface, thus providing an updated inflow map to the CSD solver. By iterating this procedure it is possible to reach the convergence at the desired rotor state. The coupling method has demonstrated to have a good convergence rate, in fact the solution becomes stable after about 5-10 cycles. As it can be seen in figures 1 and 2, both the shape of the pressure distribution on the rotor disc and the Fourier components of the hinge angles have reached a converged state. Results achieved with this simplified rotor model compare positively with both experimental results (fig. 3) and trimmed numerical simulations of the full rotor, thus representing a good compromise between the quality of results and the computational effort.