Unsteady Aerodynamics of Co-Axial Rotor Blade Crossings

Paper #:
  • 2017-01-2024

  • 2017-09-19
The crossing event of the blades of a coaxial counter-rotating rotor is a potential source of noise and severe blade loads. Blade crossings occur several times each rotor revolution. Most of the radial variation in the flow field can be efficiently captured by stacking the results of a 2-D analysis using two airfoils approaching each other at the appropriate relative Mach number and separation distance. Previously, this phenomenon was analyzed by simulating two airfoils passing each other at specified speeds and vertical separation distances, using the compressible Navier-Stokes solver OVERFLOW. The simulations explored thickness, circulation, and compressible effects. Results revealed the complex nature of the aerodynamic and fluid dynamic impulses generated by blade-blade interactions, with implications for aeroelastic loads and aeroacoustic sources. In this paper, the unsteady aerodynamic effects of wake vortices and associated blade pressure field persistence are studied for a coaxial rotor system. These effects are expected to significantly influence the bending-torsion coupling of a rotor blade response and acoustic noise sources. The coaxial rotor system is now simulated using two trains of airfoils, vertically offset, traveling in opposite directions. The current simulation represents a more realistic excitation field compared to a simulation of just two airfoils traveling in opposite directions since the flow field persistence and shed vorticity from prior crossing events will affect each pair of upper/lower airfoils. The velocity and pressure distributions on the airfoils, and in the space between the airfoils are computed before, at, and after each airfoil crossing. Results from the multiple-airfoil simulation show noticeable changes in the aerodynamics and fluid mechanics compared to the two-airfoil results.
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