Application of an efficient flow control system on helicopter rotor blades may lead to improved aerodynamic performance. Recently, the own invention of a passive vortex generator (Rod Vortex Generator-RVG) has been analysed for channel and wing flows proving its capability to reduce flow separation. The application of this passive flow control device on helicopter rotor blades is described in the present paper. The basic flow mechanism is based on the intensification of exchange of momentum in the direction normal to the wall by a streamwise vortex. High momentum air is transferred to the low momentum region close to the surface and therefore the separation bubble is reduced. The present CFD investigation was carried out with the FLOWer code from DLR which solves the Favre-averaged Navier-Stokes equations using the chimera overlapping grids technique and LEA (Linear Explicit Algebraic Stress) k-ω turbulence model. The validation of the numerical setup for high-speed transonic hover conditions is based on a comparison with experimental data obtained by Caradonna and Tung (1981). For forward flight regime, the validation is based on a comparison with flight test data gathered by Cross and Watts for the AH-1G helicopter (1988). It has been proven that the application of the proposed flow control system reduces the size of the separation bubble increasing the aerodynamic performance in both states of flight. Nomenclature Symbols c blade chord (m) d rod diameter (m) h rod height (m) L spacing between vortex generators (m) M Mach number (-) M T tip Mach number (-) M ∞ forward flight Mach number (-) Re T tip Reynolds number (-) r rod radius (m) V ∞ forward flight velocity (km/h) C f skin friction coefficient (-) C P blade pressure coefficient (-) C Po rotor power coefficient (-) C Q rotor torque coefficient (-) C T rotor thrust coefficient (-) α rod skew angle (˚) β c backward disk tilt (˚) β s sideways disk tilt (˚) δ boundary layer thickness (m) µ rotor advance ratio (-) θ 0 blade collective angle (˚) θ c lateral cyclic coefficient (˚) θ s longitudinal cyclic coefficient (˚) ϴ rod pitch angle (˚) ψ blade azimuthal position (˚) Abbreviations C-T Caradonna and Tung FC Flow Control RVGs Rod Vortex Generators VGs Vortex Generators 40th European Rotorcraft Forum 2014, Proceedings of a meeting held 2

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Strong, normal shock wave, terminating a local supersonic area on an airfoil, not only limits aerodynamic performance but also becomes a source of a high-speed impulsive helicopter noise. The application of a passive control system (a cavity covered by a perforated plate) on a rotor blade should reduce the noise created by a moving shock. This article covers the numerical implementation of the Bohning/Doerffer transpiration law into the SPARC code and includes an extended validation against the experimental data for relatively simple geometries of transonic nozzles. It is a first step towards a full simulation of a helicopter rotor equipped with a noise reducing passive control device in hover and in forward flight conditions.

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