To fulfil the SFWA objectives of reduced engine noise, a major effort was required towards innovative noise control methodologies and improved predictive capability of the CFD/CAA software systems.
The present project combined an experimental investigation of two, low 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, was tested in an anechoic facility. The associated acoustic impedance was determined as input for the CFD/CAA approach. The second concept relied on active blade surface to control the front rotor wake by actuators on the front rotor blade, possibly DBD plasma actuators. The wake characteristics behind the actuated blades were tested in a cascade facility.
- On the numerical side, the DINNO-CROR proposal was 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 relied on a time domain formulation of the FW-H equations, the critical issue remained to deliver fast and accurate unsteady CFD-solutions for prediction of the noise sources.
The DINNO-CROR project applied 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 had been largely validated and applied on multistage turbines and compressors, and its extension to CROR’s had recently been initiated.
In the project it was further extended to include the physics of the investigated noise control systems. In TASK 2, the NLH methodology was extended to model the interaction of the rotor with the pylon. For Concept 1, the boundary layer absorption was modelled by introducing an impedance boundary conditions on the pylon; while for Concept 2, the non-radial pylon was modelled directly as a solid surface.
The global objective for NUMECA and VKI in this project was to develop innovative contra-rotating open rotor (CROR) low-noise concepts. The activities related to porous treatment in work package (WP)2 aimed 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 had 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 indicated 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 had 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 had been written. Preliminary computational fluid dynamic (CFD) computations had 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 was 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 confirmed 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.