Investigation Of Predicted Helicopter Rotorhub Drag and Wake Flow with Reduced Order Modeling

The rotor hub is one of the most important components of the modern helicopter. This complex collection of linkages and plates has numerous responsibilities, including the translation of pilot input to system response, anchoring the blades to the rotor mast, and sustaining the various forces transmitted by the blades. Due its intricate design and relatively small sized components the rotor hub interacts with the incoming flow to create a highly chaotic, turbulent wake which impinges on the fuselage and empennage. This assembly has also been found to be one of the primary contributors to the total vehicle parasite drag. Unfortunately studying the rotor hub and its wake more closely is made difficult by the limitation of both modern experimental and computational methods. From an experimental standpoint tests are expensive to run, difficult to gather large amounts of data from, and can require full or high scale Reynolds numbers. Computational Fluid Dynamics (CFD) predictions of hub flows are limited by high grid resolution requirements, and lengthy grid generation and simulation times. Modal decompositions provide robust options for reduced order modeling of fluid flows. Several modal decomposition methods are tested for the validity of their application to the complex flow fields that form around rotor hubs. Four variations of two rotor hub designs, a baseline and low drag, are simulated in forward flight. This selection of hubs was chose to examine the effects of both hub geometry and aerodynamic optimization on the rotor hub surface forces and wake. Flow solutions were found using the OVERFLOW2.2n overset, structured, RANS solver created and maintained by NASA. Simulations were conducted using a fully turbulent model and the grid generation and computational equations specifics are discussed in further detail. Each of the four hub variants was subjected to the same flow conditions. Several variants of modal decomposition and other post processing techniques were used on the resultant surface force and wake data in order to Characterize the hub flow field.