Malte Misol

Experiments on Active Control of Counter-Rotating Open Rotor Interior Noise

The efficiency of future aircraft has to be increased because of the CO2 restrictions layed down by nthe European Union. Two key technologies to reach this ambitious goal are a consequent light-weight ndesign of future aircraft and new engine concepts like the Counter-Rotating open rotors (CROR). nHowever, the combination of lightweight materials like carbon-fibre-reinforced plastics (CFRP) with nCROR is acoustically demanding because of the very high sound pressures emitted by this type of nengine and the poor transmission loss of CFRP structures in the lower frequency range. nTherefore, this work conducts a preliminary study to improve the transmission loss of a CFRP panel nexcited by a synthesised CROR pressure field in lower frequency range. As a first step, a typical naircraft fuselage panel mounted in a sound transmission loss facility is equipped with actuators nand sensors to implement multiple-input multiple-output (MIMO) feedforward control of flexural nvibration. The CFRP panel is excited via a CROR pressure field synthesised by a 112-channel loud nspeaker array. The active vibration control (AVC) system is realised by accelerometers and inertial nexciters. A considerable vibration reduction of the flexural vibration on the accelerometers is achieved. nThe local attenuation around the accelerometers leads to a new controlled vibration pattern that nradiates sound in a different way than the uncontrolled one. The difficulties in reducing the radiated nsound power through the AVC system are due to low observability, the “pinning” effect, and the nrestructured vibration patterns. All of these effects are studied in detail through surface vibration nscans and sound intensity measurements. Additionally, the radiation resistance matrix is used to nanalyse the controlled vibration patterns.

Performance of active feedforward control systems in non-ideal, synthesized diffuse sound fields

The acoustic performance of passive or active panel structures is usually tested in sound transmission loss facilities. A reverberant sending room, equipped with one or a number of independent sound sources, is used to generate a diffuse sound field excitation which acts as a disturbance source on the structure under investigation. The spatial correlation and coherence of such a synthesized non-ideal diffuse-sound-field excitation, however, might deviate significantly from the ideal case. This has consequences for the operation of an active feedforward control system which heavily relies on the acquisition of coherent disturbance source information. This work, therefore, evaluates the spatial correlation and coherence of ideal and non-ideal diffuse sound fields and considers the implications on the performance of a feedforward control system. The system under consideration is an aircraft-typical double panel system, equipped with an active sidewall panel (lining), which is realized in a transmission loss facility. Experimental results for different numbers of sound sources in the reverberation room are compared to simulation results of a comparable generic double panel system excited by an ideal diffuse sound field. It is shown that the number of statistically independent noise sources acting on the primary structure of the double panel system depends not only on the type of diffuse sound field but also on the sample lengths of the processed signals. The experimental results show that the number of reference sensors required for a defined control performance exhibits an inverse relationship to control filter length.

Causal feedforward control of a stochastically excited fuselage structure with active sidewall panel.

This paper provides experimental results of an aircraft-relevant double panel structure mounted in a sound transmission loss facility. The primary structure of the double panel system is excited either by a stochastic point force or by a diffuse sound field synthesized in the reverberation room of the transmission loss facility. The secondary structure, which is connected to the frames of the primary structure, is augmented by actuators and sensors implementing an active feedforward control system. Special emphasis is placed on the causality of the active feedforward control system and its implications on the disturbance rejection at the error sensors. The coherence of the sensor signals is analyzed for the two different disturbance excitations. Experimental results are presented regarding the causality, coherence, and disturbance rejection of the active feedforward control system. Furthermore, the sound transmission loss of the double panel system is evaluated for different configurations of the active system. A principal result of this work is the evidence that it is possible to strongly influence the transmission of stochastic disturbance sources through double panel configurations by means of an active feedforward control system.

Experimental Study of an Active Window for Silent and Comfortable Vehicle Cabins

The poor sound insulation of windows especially at low frequencies constitutes a severe problem, both in transportation and in the building sector. Due to additional constraints on vehicles or aircrafts regarding energy efficiency and lightweight construction, the demand of light-weight-compliant noise-reduction solutions is amplified in the transportation industry. Simultaneously, in order to satisfy the customer demands on visual comfort and modern design, the relative size of glazed surfaces increases in all sectors. The experimental study presented below considers the feasibility of actively controlled windows for noise reduction in passenger compartments by using the example of an automobile windshield. The active windshield consists of the passive windshield, augmented with piezoceramic actuators and sensors. The main focus of the subsequent work was the development and evaluation of feedforward and feedback control strategies with regard to interior noise reduction. The structural excitation of the windshield was realized by an electrodynamic exciter (shaker) applied at the roof brace between the A-pillars. By this choice it was possible to emulate the structural excitation of the windshield through the car body, induced by coasting and motor-force harmonics. The laboratory setup does not permit the consideration of hydrodynamic and acoustic loads, which might be important as well. However, the experimental results indicate the high noise reduction potential of active structural acoustic control of structure-borne sound that radiates into a cavity.

Reduction of Turbulent Boundary Layer Noise with Actively Controlled Carbon Fiber Reinforced Plastic Panels

The turbulent boundary layer (TBL) is one of the dominant external noise sources in high subsonic aircrafts. Especially in modern aircrafts where common materials for fuselages are currently substituted by carbon-fiber-reinforced-plastics (CFRP), it is essential to avoid a decrease of passenger comfort as a result of an inferior transmission loss of the new materials. To increase the transmission loss of CFRP panels they are equipped with active noise reduction systems. In this paper the results of an experimental study in the aeroacoustic wind tunnel of the German Aerospace Center (DLR) are presented. An active panel excited by a TBL is tested at flow speeds up to Mach 0.16. The CFRP panel (500 × 800 × 2.7 mm3) is equipped with five piezo-ceramic patch actuators and ten accelerometers. Active structural acoustic control (ASAC) and active vibration control (AVC) are used to reduce the broadband TBL noise transmission in the bandwidth from 1 to 500 Hz. Feedforward (FF) and feedback (FB) control algorithms are applied in the experiments and show high performance even in presence of plant uncertainties. To improve control results the generalized plant framework of robust control is utilized for global feedback control. Finally, an overview of the achieved results is given.

Feedforward and feedback control of double panel partition with active trim panel

Feedback Control for the reduction of a stochastic broadband disturbance is widely used in experimtal investigations, where as feedforward control is often used in one-dimensional cases only. However, if the spatial and temporal correlations of the reference signals and the disturbance signals are high a feedforward controller is able to achieve a broadband disturbance reduction. In this research the placement of actuators and sensors is investigated to meet the special requirements of a feedforward control system (coherence, causality) to improve the sound transmission loss (<500Hz). Furthermore a double panel partition is realized in a sound transmission loss facility to investigate feedback and feedforward control systems. The experimental results prove the ability of the feedforward controlled smart trim panel to considerably reduce radiated sound power in third octave bands.

Active CFRP-Panels for Reduction of Low-Frequency Turbulent Boundary Layer Noise

The increasing use of modern lightweight materials in the aircraft industry raises the ndemand for innovative and lightweight-compliant noise abatement techniques. Smartstructures ntechnology is able to overcome the passive constraints in the low-frequency range n(<500Hz) by augmenting the light-weight structure with structurally integrated transducers nand a suitable control scheme. In these investigations, an active carbon fiber reinforced nplastic (CFRP) panel was designed, manufactured and experimentally investigated in an nacoustic wind tunnel. Measurement data of turbulent wall pressure fluctuations, structural nvibration and active sound power provide a database for the verification and validation of nthe developed simulation models. Active control of the smart CFRP-panel showed a nbroadband reduction of third-octave band sound power level up to 6dB(A).