Oliver Unruh

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

Numerical and Experimental Study of Sound Power Reduction Performance of Acoustic Black Holes in Rectangular Plates

Global attenuation of structural velocities is one of the most effective approaches in order to reduce noise emitted by shell structures such as a car roof or aircraft fuselage panels. This global reduction can be achieved by the application of passive damping treatments like constraint layer damping on large fractions of the vibrating surface. The main disadvantage of this approach arises from the fact that it leads to increasing total cost and weight of the structure. To overcome this problem, acoustic black holes can be used to create locations with high vibration amplitudes and low bending waves velocity in order to dissipate the energy of structure borne sound by very limited application of damping treatments. Acoustic black holes are funnel shaped thickness reductions that attract sound radiating bending waves and allow a global vibration reduction by an acceptable use of additional damping. This paper presents the results of a numerical and experimental study of acoustic black holes located on a rectangular plate. The presented work is focused on the influence of size, position and number of acoustic black holes on the acoustic performance. A large number of different configurations of these parameters is studied by finite-element-analysis and evaluated in terms of vibration amplitude and sound power level. In order to confirm simulation results the most efficient configuration is implemented in laboratory setup and characterized in terms of vibrational and acoustic performance.

Active Control of Counter-Rotating Open Rotor Interior Noise in a Dornier 728 Experimental Aircraft: Optimised Sensor Placement

Future aircrafts have to meet the restrictive fuel consumption guidelines of the European Committee. Therefore lightweight materials like carbon fibre reinforced plastics (CFRP) and new engine concepts are investigated. Counter Rotating Open Rotors (CROR) are a very promising approach for a more efficient aircraft. But in combination with a CFRP fuselage the interior noise levels will be increased dramatically because of the high pressures on the fuselage and the low transmission loss of CFRP. In this work active concepts as a lightweight compliant solution will experimentally investigated to improve the acoustic comfort of future aircrafts. A typical aircraft sidewall panel is therefore realized in a sound transmission loss facility and will be controlled actively by a feedforward control system. The panel is excited by a typical multi tonal pressure field which is generated by a speaker array. This preliminary study shows that an active structural acoustic control system can effectively improve the acoustic comfort inside the cabin.

Sound Radiation Properties of Complex Modes in Rectangular Plates: A Numerical Study

The sound radiation properties of real normal modes are often used to characterise acoustic sources such as vibrating plates. The assumption of homogeneously distributed structural damping is usually made and in consequence the eigenmodes are real. With the increasing use of high-performance composite structures and the demanding requirements for acoustic comfort, damping treatments such as constrained layer damping or embedded elastomer layers may be distributed locally at some critical locations of the vibrating structure. This violates the assumption of homogeneously distributed structural damping and results in complex mode shapes. In this case, dynamics of the vibrating systems within the resonances is no longer dominated by pure standing waves, but also by a superposition with travelling waves. This paper presents the results of a numerical study on sound radiation properties of this phenom-enon, generally known as complex modes of vibration. The finite element model of an inhomoge-neously damped plate is used to generate and investigate complex vibration patterns. The level of modal complexity of the generated complex modes is characterized by the modal collinearity index and the spatial distribution of the standing wave ratio. The fluid structure interaction is implemented by a elemental radiator approach and used for the acoustic characterisation of vibration patterns in terms of far-field radiation efficiencies and spatial distributions of sound intensity. It is shown that the eigenvector complexity can seriously affect the sound radiation properties of structural modes.

Application of a load-bearing passive and active vibration isolation system in hydraulic drives

Hydraulic drives are widely used in many engineering applications due to their high power to weight ratio. The high power output of the hydraulic drives produces high static and dynamic reaction forces and moments which must be carried by the mounts and the surrounding structure. A drawback of hydraulic drives based on rotating pistons consists in multi-tonal disturbances which propagate through the mounts and the load bearing structure and produce structure borne sound at the surrounding structures and cavities. One possible approach to overcome this drawback is to use an optimised mounting, which combines vibration isolation in the main disturbance direction with the capability to carry the reaction forces and moments. This paper presents an experimental study, which addresses the vibration isolation performance of an optimised mounting. A dummy hydraulic drive is attached to a generic surrounding structure with optimised mounting and excited by multiple shakers. In order to improve the performance of the passive vibration isolation system, piezoelectric transducers are applied on the mounting and integrated into a feed-forward control loop. It is shown that the optimised mounting of the hydraulic drive decreases the vibration transmission to the surrounding structure by 8 dB. The presented study also reveals that the use of the active control system leads to a further decrease of vibration transmission of up to 14 dB and also allows an improvement of the vibration isolation in an additional degree of freedom and higher harmonic frequencies.

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.

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 demand for innovative and lightweight-compliant noise abatement techniques. Smartstructures technology is able to overcome the passive constraints in the low-frequency range (<500Hz) by augmenting the light-weight structure with structurally integrated transducers and a suitable control scheme. In these investigations, an active carbon fiber reinforced plastic (CFRP) panel was designed, manufactured and experimentally investigated in an acoustic wind tunnel. Measurement data of turbulent wall pressure fluctuations, structural vibration and active sound power provide a database for the verification and validation of the developed simulation models. Active control of the smart CFRP-panel showed a broadband reduction of third-octave band sound power level up to 6dB(A).