A Computational Investigation of Side-by-Side Rotors in Ground Effect

This study investigates the interactional aerodynamics of hovering side-by-side rotors in ground effect. The 5.5 ft diameter, 3-bladed fixed-pitched rotors are simulated using CFD at a targeted 5 lb= f t2 disk loading. Simulations are performed using the commercial Navier Stokes solver, AcuSolve, with a detached eddy simulation (DES) model.
Side-by-side rotors are simulated at two heights above the ground (H/D = 0.5 and H/D = 1), and with two hub-hub separation distances (3R and 2.5R). The performance of side-by-side rotors in ground effect are compared to isolated rotors out of ground effect. Between the side-by-side rotors in ground effect, a highly turbulent mixing region is
identified where the wakes of each rotor collide. The flow fountains upwards, as well as exits outwards (along a direction normal to a plane connecting the two rotor hubs). The fountaining between rotors reaches up to 1.5R above the ground, and as blades at H/D = 0.5 traverse the highly turbulent flow, strong vibratory loading is induced, and
a larger thrust loss is observed outboard between the rotors. Side-by-side rotors at H/D = 0.5 with 2.5R hub-hub spacing produce peak-to-peak thrust oscillations up to 16% the steady thrust. Rotors positioned higher, at H/D = 1 are above the turbulent mixing flow, and produce significantly lower vibratory loads. The spacing between rotors
at H/D = 0.5 and 3R hub-hub separation allows strong vortical structures to develop between the rotors which move from side-to-side over multiple revolutions. When the vorticity is positioned closer to one of the rotors, it produces a greater lift deficit over the outboard region and higher vibratory loading. For rotors closer together, at H/D = 0.5
and 2.5R separation, the vortical structures between rotors are constrained to a more concentrated area, and show less side-to-side drift.