This paper presents performance analysis and design of ducted propellers for lighter-than-air vehicles. Highfidelity CFD simulations were first performed on a detailed model of the propulsor, and the results were in very good agreement with available experimental data. Additional simulations were performed using a simplified geometry, to quantify the effect of the duct and of the blade twist on the propeller performance. It was shown that the duct is particularly effective at low flight speed, and that the blades with relatively high twist have better performance over the flight envelope. Design of the optimal twist distribution and of the duct shape was also attempted, by coupling the flow solver with a quasi-Newton optimisation method. Flow gradients were computed by solving the discrete adjoint of the Reynolds Averaged Navier-Stokes equations using a FixedPoint Iteration scheme or a nested Krylov method with deflated restarting. The results show that the ducted propeller propulsive efficiency can be increased by 2%.
This paper presents the development of a fully implicit, low-memory, discrete adjoint method by means of automatic source-code differentiation applied to the Helicopter Multi-Block computational fluid dynamics solver. The method is suitable for applications in flight mechanics as well as shape optimization, and is demonstrated in this paper for popular flow cases reported in the literature. In particular, adjoint CFD computations were undertaken for airfoils, wings and rotor blade cases, and the obtained results were found to agree well with published solutions and with finite differences of flow derivatives. The method has been demonstrated for inviscid and viscous cases and results suggest that the current implementation is robust and efficient. The cost of the adjoint computations is relatively low due to the employed source-code differentiation and most of the times it is no more than the cost of a steady-state flow solution.
The present study is one of the first attempts to exploit the GOAHEAD data base to perform a code-tocode evaluation on complete helicopter aerodynamics. The numerical results of two GOAHEAD partners, the German Aerospace Center (DLR) and Politecnico di Milano (PoliMi) are presented and compared to experimental measurements. The study also addresses an evaluation of two different approaches to predict helicopter flows. The first, applied by DLR, accounts for rotor trim and elastic effects by weak fluid-structure coupling. The PoliMi approach, on the other hand, enforces a prescribed kinematics, taken directly from the experiment, on a rigid blade. The simulations refer to a complete helicopter windtunnel model, featuring a scaled NH90 fuselage, the ONERA 7AD main rotor, a scaled BO105 tail rotor, a rotor hub and a pylon, all located inside the 8 m × 6 m test section of the DNW low-speed wind tunnel. The flight conditions correspond to cruise flight at Ma = 0.204 and fuselage attitude α = −2.5 •. The comparisons demonstrate the capability of present unsteady RANS solvers to predict flow fields around complete helicopters.
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