The rotor hub is one of the primary sources of high-speed rotorcraft parasite drag. To better understand the flow physics of the rotor hubs, counter-rotating coaxial rotor hub flows were simulated, followed by the experiment at the Penn State University. For the simulations, the Mercury computational fluid dynamics (CFD) framework employs an unstructured/Cartesian multi-mesh paradigm and Spalart-Allmaras delayed detached eddy simulation (SA-DDES) turbulent modeling. The coaxial hub flow physics was studied at two advance ratios by building up the hub components. The interference between the hub and the fairing components induced higher mean hub drag and unsteady harmonics in both hub and fairing drags. Different advance ratios affected hub wake structures and velocity fields at near- and mid-wake distances. Finally, the complete hub model in the water tunnel was simulated at the advance ratio of 0.25. The mean and unsteady drag predictions were compared with the experiment as well as the mean wake velocity fields. Additionally, the effect of hub shafts on the hub drags and wake velocity fields was studied. These efforts were based on the fourth Rotor Hub Flow Prediction Workshop in 2022.
Experimental near-wake flow field measurements from two Penn State University water tunnel tests of a defeatured helicopter hub are compared with two unsteady computational fluid dynamics (CFD) analyses, CREATE(TM)-AV Helios and University of Maryland Mercury. Both CFD frameworks employ an unstructured/Cartesian multi-mesh paradigm and turbulent Spalart-Allmaras Detached Eddy Simulation (SA-DES) modeling. Experimental velocimetry measurements of mean wake velocities, harmonic content, and Reynolds stresses provide valuable validation data for CFD. Overall, the two CFD solvers were in good agreement with each other and qualitatively captured the mean and harmonic content of the wake structures with accuracy. Flow feature dissimilarities between the advancing and retreating sides were differentiated and indicated dominant regions of harmonic flow disturbances biased toward the retreating side, in good agreement with experimental observations. Quantitatively, some variation in velocity deficits and downwash were noted, either in profile character, magnitude, and/or location. Encouragingly, there was little tendency of excessive dissipation in the CFD near wake, and the harmonic content actually tended towards a common overprediction. Reynolds number effects were minimal, and grid density effects were studied but inconclusive. These efforts were part of the 3rd PSU Rotor Hub Flow Workshop in 2020.
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