Analysis of Turbulent Flow Behavior in Helicopter Rotor Hub Wakes

The rotor hub is one of the most important features of all helicopters, as it provides the pilot a means for controlling the vehicle by changing the characteristics of the main and tail rotors. The hub also provides a structural foundation for the rotors and allows for the rotor blades to respond to aerodynamic forces while maintaining controllability and stability. Due to the inherent geometry and high rate of rotation, the rotor hub in its current form acts a large bluff body and is the primary source of parasite drag on the helicopter, despite its relatively small size. The rotor hub also produces a highly turbulent wake which can affect the performance of the vehicle's empennage and tail rotor. Much of the characteristics and behaviors of this wake are still difficult to predict and analyze, but the application of numerical simulations makes this task easier and more efficient. The turbulent and frequency content characteristics were examined in the wakes of four helicopter rotor hub geometries in forward flight. Computational fluid dynamics (CFD) simulations were performed using NASA's OVERFLOW 2.2n Reynolds-averaged Navier Stokes solver, and the simulations imposed flow conditions based on previous and current experimental and numerical studies. Surface force and velocity harmonics for several frequencies were computed and qualitatively compared against available experimental results. Components of the Reynolds stress tensor were computed and examined. Production and transport of the turbulent kinetic energy are examined through the rotor hub wakes at six stream-wise coordinates. Frequency content was found to be concentrated towards the retreating side of all hubs in most of the frequencies examined, and certain geometrical features of the hubs were found to contribute significantly greater portions of this frequency content than others. Reynolds stresses showed similar concentrations as the mean velocity contours, which displayed a general bias towards the advancing side due to the increased relative velocity. Modal analysis of the instantaneous Reynolds stresses showed that perturbations directly behind the advancing side could only be captured with a large set of modes. Integrations of a turbulent kinetic energy flux and the stream-wise third-order moment showed a nearly-linear relation between the frontal area of the hubs and the magnitudes of these quantities.