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The present study demonstrates the aerodynamic and acoustic optimization potential of a counter rotating open rotor. The objective was to maximize the propeller efficiency at top-of-climb conditions and to minimize the noise emission at takeoff while fulfilling the given thrust specifications at two operating conditions (Takeoff and Top of Climb) considered. Both objectives were successfully met by applying an efficient multi-objective optimization procedure in combination with a 3D RANS method. The acoustic evaluation was carried out with a coupled U-RANS and an analytic far field prediction method based on an integral Ffowcs-Williams Hawkings approach. This first part of the paper deals with the application of DLR’s CFD method TRACE to counter rotating open rotors. This study features the choice and placement of boundary conditions, resolution requirements and a corresponding meshing strategy. The aerodynamic performance in terms of thrust, torque and efficiency was evaluated based on steady state calculations with a mixing plane placed in between both rotors, which allowed for an efficient and reliable evaluation of the performance, in particular within the automatic optimization. The aerodynamic optimization was carried by the application of AutoOpti, a multi-objective optimization procedure based on an evolutionary algorithm which also was developed at the Institute of propulsion technology at DLR. The optimization presented in this paper features more than 1600 converged 3D steady-state CFD simulations at two operating conditions, Takeoff and Top of Climb respectively. In order to accelerate the optimization process a surrogate model based on a Kriging interpolation on the response surfaces was introduced. The main constrains and regions of interest during the optimization where a given power split between the rotors at takeoff, retaining an axial outflow at the aft rotor exit at top of climb and fulfilling the given thrust specifications at both operating conditions. Two objectives were defined: One was to maximize the (propeller) efficiency at top of climb conditions. The other objective was an acoustic criteria aiming at decreasing the rotor/rotor interaction noise at takeoff by smoothening the front rotor wakes. Approximately 100 geometric parameters were set free during the optimization to allow for a flexible definition of the 3D blade geometry in terms of rotor sweep, aft rotor clipping, hub contour as well as a flexible definition of different 2D profiles at different radial locations. The acoustic evaluation was carried out based on unsteady 3D-RANS computations with the same CFD method (TRACE) involving an efficient single-passage phase-lag approach. These unsteady results were coupled with the integral Ffowcs-Williams Hawkings method APSIM via a permeable control surface covering both rotors. The far field directivities and spectra for a linear microphone array were evaluated, here mainly at the takeoff certification point. This (still time consuming) acoustic evaluation was carried out after the automatic optimization for a few of the most promising individuals only and results will be presented in comparison with the baseline configuration. This detailed acoustic evaluation also allowed for an assessment of the effectiveness of the acoustic cost function as introduced within the automatic optimization.

Within the frame of the ongoing 7th Framework European Project DREAM, this article presents a synthesis of Cenaero, DLR, and Onera's joint effort to demonstrate the aerodynamic and acoustic optimization potential of a contra-rotating open rotor. Within WP 3.2 led by Snecma, the objective was to maximize the propellers efficiency at top-of-climb conditions and to minimize the noise emission at take-off focusing on interaction noise while fulfilling the thrust and torque split specifications at both operating points. These objectives were successfully met by the development and exploitation of efficient multi-objective three-dimensional Reynolds-averaged Navier–Stokes (RANS)-based surrogate-assisted optimization strategies. In order to assess the aerodynamic and acoustic characteristics of both baseline and optimized geometries, coupled unsteady RANS (URANS) simulation and farfield prediction based on an integral Ffowcs Williams–Hawkings approach were then carried out. The results demonstrate that although the acoustic criterion driving the optimization process did not lead to an improvement of the noise characteristics over the whole directivity range, it may be regarded as a cost-effective way to incorporate noise-related aspects into the design intent.

In order to solve the problem of inadequate penetration of low-density jet penetrating shell charge, the low-density PTFE are modified by adding a certain fraction of copper powder in the matrix. This treatment helps to increase the material density and jet energy, as well as to improve the static and dynamic mechanical properties of the material. AUTODYN-2D finite element software is used to simulate the process in which the low-density jet forms and penetrates shell charge. The simulation and experiment results show that in comparison to PTFE, the mechanical properties of PTFE/Cu have been significantly improved, and the jet has strong penetration capability. Under the same structural conditions, the penetration diameter of PTFE/Cu jet is about enhanced by 70 % for the panel and 30 % for the back plate compared to that of PTFE jet. As an experimental result, the penetration depth of the main armour by PTFE/Cu jet is enhanced by 5.38 % compared with that of PTFE jet.