In the attempt to reduce fuel consumption, a new generation of Ultra-High-By-Pass-Ratio (UHBR) turbofans have been introduced in the aeronautic industry which are structurally noisier especially at lower frequencies, because of their larger diameter, lower number of blades and rotational speed. Moreover, they present a shorter nacelle, leaving less available space for acoustic treatments. For this reason, innovation in the liner technology is highly demanded. In this contribution, we analyse the performances of an electroacoustic liner, made up of microphones (sensors) and small loudspeakers (actuators). Such array of electroacoustic resonators can feature an interesting boundary operator, called Advection Boundary Law. Such boundary law has been analysed in grazing-incident acoustic fields without air-flow and in case of plane waves. Here, we adapt such boundary condition to attenuate spinning modes. Numerical simulations in case of spinning-modes, shows the potentiality and the passivity issues of such innovative boundary law. Finally, a reproduction of a turbofan engine (scale 1:3) accomplishing real-life rotational speeds, allows to assess the performances of the Advection Boundary Law in presence of mean-flow and spinning-modes.
In the aim of attenuating noise transmission through air-ducts, research is prompted for overcoming the limitations of classical acoustic liners, especially in the aero-engines applications. The new generation of Ultra- High-By-Pass-Ratio (UHBR) turbofans while considerably reducing fuel consumption, increases noise pollution especially at lower frequencies because of their larger diameter, lower number of blades and rotational speed. Moreover, they present a shorter nacelle, leaving less available space for acoustic treatments. In case of simplified one-dimensional propagation, integral constraints exist which analytically define the limits of the scattering performances of reciprocal systems, such they are the local impedance liners, for a fixed length of the acoustic treatment along the duct. In this contribution, we analyse a special boundary condition breaking the reciprocity principle, and overcoming the limitations of locally reacting liners. We call it Advection Boundary Law as it introduces a convection on the boundary, responsible of non-reciprocal behaviour at grazing incidence, and of the enhancement of transmission loss with respect to pure locally-reacting resonators. Performances and passivity of such boundary law are numerically analysed first in grazing-incidence problems. The grazing-incidence problem is experimentally studied in a plane-wave acoustic waveguide lined by electroacoustic resonators which can be programmed to reproduce such advection boundary law.
This contribution focuses on innovative acoustic liner concepts for reducing noise transmission through acoustic waveguides. The technology employed to implement such innovative boundary treatments are electro-active acoustic absorbers making use of loudspeakers (as actuators) and microphones (as sensors). Its most ambitious application is in the nacelle of the new generation of Ultra-High-By-Pass-Ratio (UHBPR) turbofans. In particular, this work targets higher modal-order sound fields (such as the ones in the UHBPR in the frequencies of interest). A nonlocal boundary operator is presented to show the potentialities of programmable metasurfaces in overcoming the performances of classical liner technologies.
The SALUTE project aims at evaluating performance of electroacoustic metasurface, employing a surface array of controlled electroacoustic actuators, for smart acoustic lining under grazing turbulent flow to be used in UHBR Technologies Engines. Theoretical and numerical investigations have been carried out for designing innovative concepts for complex aero-acoustic characterization in an engine mock-up. A specific focus was placed in the realization of prototypes for evaluating the metacomposite liner performances in 3D liners close to real engine implementation, its process complexity and robustness. This project provides new tools for designing smart acoustic liners; while acoustical experimental tests demonstrate efficiency and robustness of such technology for controlling UHBR noise emission. This paper presents the concept development from theory to technological realization and characterization by produced numerical tools. The experimental results obtained with the liners in acoustic flow duct facilities (FDF) have been realized in the PHARE facilities of Ecole Centrale de Lyon. Different configurations of liners have been tested using similar flow conditions as in target engine: 1 passive liner used as reference and a 3D active liner based on an array of electroacoustic absorbers. The final tests campaign comprises acoustics and aerodynamics measurements to characterize the aeroacoustics flow conditions, the membrane behavior, the achieved synthetic acoustic impedance and the resulting insertion loss.
In this paper, experimental results of broadband noise reduction in an acoustic flow duct are presented. An active liner composed of an array of electroacoustic absorbers is used. The control law is based on the pressure-based, current driven digital architecture for impedance control with a local control strategy. A wind tunnel test rig named Caïman has been used for the experimental validation. The results confirm the adaptability and the stability of the whole system with the local control strategy. The air flow slightly reduces the efficiency while maintaining the adaptability and the stability.
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