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Design of innovative CROR blade and pylon

Final Report Summary - DINNO-CROR (Design of innovative CROR blade and pylon)

Executive summary:

The global objective for NUMECA and VKI in this project is to develop innovative contra-rotating open rotor (CROR) low-noise concepts. The activities related to porous treatment in work package (WP)2 aim at assessing whether porous liners located on the downstream blades of the CROR have a potential for significant noise reduction. A scaled-down mock-up has been designed, manufactured and assembled, with dimensions and operating parameters yielding a fair similarity with the full scale application (e.g. with regard to the reduced frequency of the incoming disturbances). However, not all parameters (e.g. Reynolds number and Mach number) could be preserved. The results showed that a strong correlation could be established between the wall surface pressure and far-acoustic fields. The aerodynamic balance measurements, primarily intended to measure static forces, proved to respond fast enough in order to provide integrated force spectra as well. These spectra correlate well with the unsteady pressure spectra obtained by means of electrets microphones. This opened interesting perspectives for assessing the effect of porous liners by looking directly on the wall pressure spectra and correlating them with the measured acoustic fields that should be consistent with the integrated force spectra. The results indicate that in order to observe a reduction of the noise above the measurement uncertainty, the porosity of the liners must reach quite high values (of the order of 25 %). Such perforations are likely to cause unacceptable performance loss for the present model. This has been later confirmed by measurements obtained with the aerodynamic balance.

A state-of-the-art report on dielectric barrier discharge (DBD) actuators and their potential use for CROR has been written. Preliminary computational fluid dynamic (CFD) computations have been performed to assess the effects of a DBD plasma actuator placed at the trailing edge (blunt) of the front blade in a CROR configuration. The effect of the plasma actuator is modelled as a new type of boundary condition on the blunt side of the airfoil where a 5 m / s velocity is imposed. (Maximum velocities of this order have been experimentally measured above DBD actuators.) The modeled plasma actuation causes no significant change in the wake behind the airfoil. This confirms that the DBD technology has a very limited potential for tone noise attenuation in realistic CROR configuration. Therefore, it was decided to stop the activities related to this concept and replace them with additional CFD / computational aeroacoustic (CAA) simulations of CROR configurations.

Aerodynamic and aeroacoustic simulations were carried out by NUMECA for the Airbus AI-PX7 CROR configuration at take-off conditions with the inclusion of an upstream pylon. The presence of the pylon marginally affects the global aerodynamic performance characteristics of the front and aft blades in our computations. The NLH method showed its limits in capturing the thin wake of the pylon through five or eight harmonics. The directivity of the emitted noise for the rotor-alone tones from the front rotor was found to be significantly affected by the presence of the pylon, with a significant increase in SPL in the upstream direction.

Project context and objectives:

To fulfil the SFWA objectives of reduced engine noise, a major effort is required towards innovative noise control methodologies and improved predictive capability of the CFD / CAA software systems. The present project combines an experimental investigation of two, low technology readiness level (TRL) flow / noise control options, associated with an innovative and highly efficient numerical CFD / CAA approach.

On the experimental side, the first noise control method, based on porous treatment of the blades, will be tested in an anechoic facility. The second concept relies on active blade surface to control the front rotor wake by actuators on the front rotor blade, possibly DBD plasma actuators.

On the numerical side, the DINNO-CROR proposal is based on an advanced new approach for the CFD determination of the noise sources and on the acoustic analogy for far-field noise propagation. While the CAA approach relies on a time domain formulation of the FW-H equations, the critical issue remains to deliver fast and accurate unsteady CFD solutions for prediction of the noise sources. The DINNO-CROR project will apply the nonlinear harmonic method (NLH) which allows a gain in CPU compared to current CFD methodologies, of close to three orders of magnitude. This method has been largely validated and applied on multistage turbines and compressors, and its extension to CRORs has recently been initiated. In the present project it will be further extended to include the physics of the investigated noise control systems.

Project results:

The goal of the DINNO-CROR project is to develop innovative CROR low noise concepts. The objective of the activities related to porous treatment were aimed at assessing experimentally whether porous liners located on the downstream blades of the CROR have a potential for significant noise reduction. A scaled-down mock-up has been designed, manufactured and assembled, with dimensions and operating parameters yielding a fair similarity with the full scale application (e.g. with regard to the reduced frequency of the incoming disturbances). However, not all parameters (e.g. Reynolds number and Mach number) could be preserved. The results showed that a strong correlation could be established between the wall surface pressure and far-acoustic fields. The aerodynamic balance measurements, primarily intended to measure static forces, proved to respond fast enough in order to provide integrated force spectra as well. These spectra correlate well with the unsteady pressure spectra obtained by means of electrets microphones. This opened interesting perspectives for assessing the effect of porous liners by looking directly on the wall pressure spectra and correlating them with the measured acoustic fields that should be consistent with the integrated force spectra. The results indicate that in order to observe a reduction of the noise above the measurement uncertainty, the porosity of the liners must reach quite high values (of the order of 25 %). Such perforations are likely to cause unacceptable performance loss for the present model. This has been later confirmed by measurements obtained with the aerodynamic balance.

A state-of-the-art report on DBD actuators and their potential use for CROR has been written. Preliminary CFD computations have been performed to assess the effects of a DBD plasma actuator placed at the trailing edge (blunt) of the front blade in a CROR configuration. The effect of the plasma actuator is modelled as a new type of boundary condition on the blunt side of the airfoil where a 5 m / s velocity is imposed. (Maximum velocities of this order have been experimentally measured above DBD actuators.) The modelled plasma actuation causes no significant change in the wake behind the airfoil. This confirms that the DBD technology has a very limited potential for tone noise attenuation in realistic CROR configuration. Therefore, it was decided to stop the activities related to this concept and replace them with additional CFD / CAA simulations of CROR configurations.

Aerodynamic and aeroacoustic simulations were carried out by NUMECA for the Airbus AI-PX7 CROR configuration at take-off conditions with the inclusion of an upstream pylon. The presence of the pylon marginally affects the global aerodynamic performance characteristics of the front and aft blades in our computations. The NLH method showed its limits in capturing the thin wake of the pylon through five or eight harmonics. The directivity of the emitted noise for the rotor-alone tones from the front rotor was found to be significantly affected by the presence of the pylon, with a significant increase in SPL in the upstream direction. Since studies from other partners within SFWA showed that the scooping concept is unpractical due to the large mass flow rate and large power needed to run the scooping system, it was decided to stop the activities related to the boundary layer absorption concept.

Potential impact:

During the course of the project, a close collaboration has been developed with Airbus industry and the rest of the SFWA-ITD partners. Meetings took place on a regular basis (Toulouse, Feb 2010; Toulouse, Sept. 2010; Braunschweig, Jun. 2011; Toulouse, Jan 2012). These meetings were open to the SFWA-ITD partners and guaranteed an efficient interface between the project and the SFWA-ITD team. It allowed Airbus industry to closely monitor the work performed during this project and to adapt the scope of the project according the latest information and results available.

Our studies led to a better fundamental understanding of the aerodynamics and aero-acoustics of CROR configurations for the end users of the DINNO-CROR project which are the SFWA-ITD partners. Clear guidelines have been formulated on how to simulate CROR configurations to produce accurate aerodynamic and aero-acoustic results. The innovative low noise concepts considered in this project did not appear promising for CROR configurations. This allowed nonetheless the SFWA-ITD partners to restrict the field of investigations for future concepts which should be further developed to higher TRL for limiting the aero-acoustic impact of the CROR.

There is no doubt that this project will support the global CLEANSKY objectives towards greener aircrafts. It should be noted that the promising results regarding the numerical simulations of CROR configurations led to a new Clean Sky project for NUMECA Int. (Clean Sky NAA-CROR, Call identifier: SP1-JTI-CS-2010-3). The collaboration with the SFWA-ITD partners therefore still continues in the framework of WP2.2.2 (Innovative power plant integration), task 6 (CROR assessment).

List of websites: Charles Hirsch
NUMECA int.
charles.hirsch@numeca.be
Fax: +32-264-79398

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