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

Abstract

This study investigates the interactional aerodynamics of hovering side-by-side rotors in ground effect. The 5.5-ft diameter, three-bladed fixed-pitched rotors are simulated using computational fluid dynamics at a targeted 5 lb/ft2 disk loading. Simulations are performed using the commercial Navier–Stokes solver, AcuSolve®, with a delayed detached eddy simulation 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 (IGE) is compared to isolated rotors out of ground effect. Between the side-by-side rotors IGE, 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) with fountaining between the rotors reaching up to 0.75D above the ground. As blades at H/D = 0.5 traverse the highly turbulent flow, strong vibratory loading is induced and a thrust loss is observed over the outboard sections between the rotors that is large enough to negate any nominal ground effect benefits inboard. Side-by-side rotors at H/D = 0.5 with 2.5R hub–hub spacing produce peak-to-peak thrust oscillations up to 16% of the steady thrust. Rotors placed higher, at H/D = 1 are positioned 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 moves closer to one of the rotors, it produces a greater lift deficit over the outboard region and a stronger 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.