Assessment and Optimization of the Aerodynamic and Acoustic Characteristics of a Counter Rotating Open Rotor

Abstract

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.

On the influence of trailing-edge serrations on open-rotor tonal noise

“…The fourth domain of the setup extends the mesh further in radial direction up to 10R, in order to remove a possible influence of the position of the far-field boundary on the performance of the CROR. 5 The part of the radial domain one and two with an axial extension of round about one chord length upstream (resp. downstream) of the blade rows is the high quality zone of the mesh, see Fig.…”

“…On one hand methods are developed to understand the noise generation mechanisms, 3, 4 on the other hand an automate design-to-noise procedure is being set up. [5][6][7] In the frame of the European Research Program "Joint Technology Initiative Clean-Sky", a task is dedicated to "Innovative CROR blade and pylon design". The implementation of serrations on the front rotors trailing edge is numerically investigated as a possible technology to reduce the tonal noise.…”

How to improve open rotor aerodynamics at cruise and take-off

“…13, as the incidence reduces by a factor of 2•5, the rotational speed increases by a factor of about 1•58. Note that 1•58 2 ≅ 2•5 and from Equation (6) it is expected that X ∝ U 2 i.…”

Performance prediction and progress towards multi-disciplinary design of contra-rotating open rotors

“…Pitch angles are adjusted to match the given shaft power at constant speed. The map is based on performance data published by Laban et al (9) and scaled to match the 3D-CFD data obtained by Schnell et al (10) . To quantify the potential fuel burn reduction of CROR engines in comparison to modern turbofan concepts, a survey study was carried out by Otten et al (11) .…”

“…7 for the front rotor: rather than comparing the absolute level of the static pressure distribution at a given operating point, here the difference of the normalised pressure for two different RPM is shown in order to highlight the ability of the numerical method to correctly model the RPM sensitivity on blade static pressure. domain on the CROR performance parameters (Top of Climb conditions) (10) . Figure 5.…”