For several years, Safran has been involved in the design and optimization of contra rotating open rotors. This innovative architecture is known for allowing drastic reduction in fuel burn, but its development is facing complex technological challenges such as acoustics, aerodynamics, and weight penalty due to the mechanical complexity of an Open Rotor. Since 2010, Safran has been developing the experimental test bench HERA (1/5 mock-up scale) to improve the understanding of the complex aerodynamics and acoustics phenomena involved in the counter rotating propellers configuration. Isolated and installed low speed and high speed wind tunnel campaigns, including PIV measurements have been extremely helpful in defining design guidelines for full scale open rotor specification. These tests have been used as CFD feed-back among other purposes. An iterative process involving CFD optimization (in close collaboration with Cenaero) and wind tunnel test campaigns has been developed over the last 4 years and has led to the definition of an innovative design strategy, which has been successfully tested during the process of the full scale counter rotating propellers design for the SAGE2 ground test demonstrator engine. This phase has evidenced the absolute necessity of a multi-disciplinary design method when it comes to full scale and “rig-ready” design. Ensuring high propulsive efficiency and at the same time, minimizing the acoustic level, while maintaining severe mechanical constraints such as weight, inertia and proper dynamic positioning under control, requires a dedicated and integrated “all inclusive” design process. The aim of this paper is to present the design methodology and some of the wind tunnel tests results carried out over the last 4 years, which have led to the definition of a novel multidisciplinary design methodology that involves CFD, FEM and acoustics.
The current research focuses on the aerodynamic design of a centrifugal compressor and the effect of tip tailoring on the aerodynamic impeller efficiency. To this extent a high-fidelity multi-point design optimization process has been developed and exploited on a high pressure ratio transonic impeller. By manipulating the shape of the impeller blades and endwalls and by including advanced geometrical features such as winglets on the impeller blades, the behavior of the impeller flow has been investigated. Here, the results of three-dimensional RANS simulations with the Spalart-Allmaras turbulence model on a structured multi-block mesh is used for the evaluation of the flow characteristics. In the context of radial machines, the results of the aerodynamic design optimization show an important improvement of the impeller isentropic efficiency compared to the reference impeller, with a significant contribution from the presence of the impeller tip winglets. Furthermore, the integration of the impeller winglet has encouraged this study to provide a detailed analysis on the impeller flow structures in order to have a better understanding of the effects of tip tailoring on impeller performance.
This paper discusses the noise prediction of Contra Rotating Open Rotors (CROR) in the context of a multi-disciplinary rotor design. Propeller noise is believed to be one of the dominant sources of noise on CROR engines in all flight conditions that require high thrust. It is therefore important to consider acoustics as early as possible in the design process. At Snecma, blade design is tackled as a multi-disciplinary approach that involves among others; mechanics, dynamics, aerodynamics, acoustics and engine integration constraints. In this context, accurate prediction methods based on unsteady Computational Fluid Dynamics (CFD) cannot be used to predict tonal noise radiation because of the prohibitive time required for convergence. This paper focuses on the prediction of far field tonal interaction noise using the fast prediction platform Sandra, which complies with the timescale of a rotor design project. Sandra is composed of an aerodynamic module and an acoustic module. As a first approximation, the aerodynamic module uses either a lifting-line method or a lifting-surface method to compute the pressure fluctuations on the blades. A map of the flow velocity, which contains the velocity gradients of the wake and of the tip vortex behind the front rotor, is extracted from a Reynolds Averaged Navier-Stokes (RANS) CFD simulation downstream of the front rotor. A quasi-stationary approach, which uses the space and time symmetry of the isolated rotors CFD, allows the flow field to be decomposed along the azimuthal angle. The pressure fluctuations on the rear blade are finally computed for each position. The acoustic propagation is then performed in the time domain using Farassat’s formulation 1A of the Ffowcs Williams and Hawkings equation. It is shown that the physics of the interaction between the front and the rear rotor is well captured. Direct comparisons with the far field noise computed from a uRANS solution, and with experimental data, show very good agreement of the position of the angles of maximum noise radiation for harmonics of order 2 or lower. It is also shown that the relative noise radiation between various rotor geometries is reasonably well captured, which is a requirement to provide a fast and relevant method for multi-disciplinary propeller blade design.
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