Abstract
Dynamic stall is a phenomenon on the retreating blade of a helicopter which can lead to excessive control loads. In order to understand dynamic stall and fill the gap between the investigations on pitching wings and full helicopter rotor blades, a numerical investigation of a single rotating and pitching blade is carried out. The flow phenomena thereupon including the Ω-shaped dynamic stall vortex, the interaction of the leading edge vortex with the tip vortex, and a newly noticed vortex structure originating inboard are examined; they show similarities to pitching wings, while also possessing their unique features of a rotating system. The leading edge/tip vortex interaction dominates the post-stall stage. A newly noticed swell structure is observed to have a great impact on the load in the post-stall stage. With such a high Reynolds number, the Coriolis force exerted on the leading edge vortex is negligible compared to the pressure force. The force history/vortex structure of the slice r/R = 0.898 is compared with a 2D pitching airfoil with the same harmonic pitch motion, and the current simulation shows the important role played by the swell structure in the recovery stage.
Highlights
Aerospace 2021, 8, 90. https://Dynamic stall on the retreating blade of a helicopter is a factor that inhibits the progress of fast forward flight or manoeuvre flight; the subsequent aero-elastic response of the blade serves as a main source of vibrations and high structure loads
Ref. [7] summarised early progress made in understanding dynamic stall, including the contribution of the dynamic stall vortex shedding to the surge of the normal force, the chord-wise force and the negative pitch moment; the role that reduced frequency or the pitching rate plays in the process; and the onset of vortex shedding as a result of the reverse flow upstream propagation from the trailing edge
A numerical investigation with URANS using SA and k − ω SST turbulence models was carried out on a single rotating flat rectangular blade with a NACA 0012 airfoil, and the flow feature of the SA result was researched in detail
Summary
Aerospace 2021, 8, 90. https://Dynamic stall on the retreating blade of a helicopter is a factor that inhibits the progress of fast forward flight or manoeuvre flight; the subsequent aero-elastic response of the blade serves as a main source of vibrations and high structure loads. Numerous experimental investigations [1,2,3,4,5] on oscillating airfoils; especially, the NACA 0012 airfoil was carried out in order to understand the mechanism of dynamic stall, as well as how the airfoil type, pitching amplitude, pitching rate, reduced frequency and the compressible effect impact aerodynamic coefficients. [7] summarised early progress made in understanding dynamic stall, including the contribution of the dynamic stall vortex shedding to the surge of the normal force, the chord-wise force and the negative pitch moment; the role that reduced frequency or the pitching rate plays in the process; and the onset of vortex shedding as a result of the reverse flow upstream propagation from the trailing edge Ref. [7] summarised early progress made in understanding dynamic stall, including the contribution of the dynamic stall vortex shedding to the surge of the normal force, the chord-wise force and the negative pitch moment; the role that reduced frequency or the pitching rate plays in the process; and the onset of vortex shedding as a result of the reverse flow upstream propagation from the trailing edge
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