Thomas Haase

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.

Validation of spatially resolved all sky imager derived DNI nowcasts

Mainly due to clouds, Direct Normal Irradiance (DNI) displays short-term local variabilities affecting the efficiency of concentrating solar power (CSP) plants. To enable efficient plant operation, DNI nowcasts in high spatial and temporal resolutions for 15 to 30 minutes ahead are required. Ground-based All Sky Imagers (ASI) can be used to detect, track and predict 3D positions of clouds possibly shading the plant. The accuracy and reliability of these ASI-derived DNI nowcasts must be known to allow its application in solar power plants. Within the framework of the European project DNICast, an ASI-based nowcasting system was developed and implemented at the Plataforma Solar de Almeria (PSA). Its validation methodology and validation results are presented in this work. The nowcasting system outperforms persistence forecasts for volatile irradiance situations.

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.

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.

Experimental Realization of a Sound Radiation Filter for Feedforward Control to Improve Active Structural Acoustic Control Systems

Due to the strengthened CO2 and NOx regulations, future vehicles have to be lightweight and efficient. But, lightweight structures are prone to vibrations and radiate sound efficiently. Therefore, many active control approaches are studied to lower noise radiation besides the passive methods. One active approach for reducing Sound radiation from structures is the active structural acoustic control (ASAC). Since the early 90’s, several theoretical studies regarding ASAC systems were presented, but only very little experimental investigations can be found for this alternative to passive damping solutions. The theoretical simulations show promising results of ASAC systems compared to active vibration control approaches. So, for that reason in this paper an experiment is conducted to investigate the performance of an ASAC system in the frequency range up to 600 Hz. A regular sensor grid of 24 accelerometers that are interconnected to establish six radiation signals is applied to an aluminum plate. The plate is excited by a point force and the feedforward control system counteracts these vibrations with 2 inertial actuators. The radiated sound power can be reduced by 4 dB integrated over the targeted frequency band. Compared to an active vibration control system which results only in a 0.2 dB decreased sound radiation, the ASAC system is beneficial. Furthermore, the filter length and the causality aspect of the experiments are analyzed in order to investigate the digital signal processing requirements. It can be shown that short filters are sufficient for an ASAC System compared to the filter length needed for an AVC system. In the last part of this paper some technical simplifications are investigated in order to reduce the complexity of the sound Radiation filters. It is shown that the high pass filters needed to model the radiation efficiency can be neglected with a very small loss of performance.

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.

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.