A self-sustained sound, more usually known as a whistle, refers to a distinct tonal noise created due to the interaction between the sound and flow field. When a positive feedback loop is formed between the two fields, the energy in the mean flow will be transferred into the sound wave, thus giving rise to a whistle. In engineering practice, whistles are destructive as they can produce high sound and vibration levels and may result in risk for mechanical failures. In this work, a flow-related high level tonal noise was found during a measurement on a particle agglomeration pipe, which is a quasi-periodic corrugated structure designed for the exhaust system of heavy-duty trucks. The purpose of the pipe is to enhance particle agglomeration to increase the size of exhaust gas particles. To investigate the origin of the detected tonal noise additional measurements were carried out. Based on the measurement result, the aero-acoustic coupling in the agglomeration pipe was analyzed, revealing that the pipe has a large potentiality to amplify the incident sound power in the presence of a mean flow. Furthermore, the Nyquist stability criterion was applied to confirm the existence of exponentially growing modes in the system at certain conditions.

Real-time hybrid substructuring (RTHS) is a relatively new method of vibration testing for characterizing the system-level performance of physical hardware components. With RTHS, a dynamic system is partitioned into physical and numerical substructures and interfaced together in real-time similar to hardware-in-the-loop testing. This paper presents an overview of RTHS including the challenges posed by its real-time constraints and the application to system-level testing of physical vibration control devices and mechanical equipment. Two laboratory RTHS experiments performed at the University of Connecticut Structures Research Laboratory are used to demonstrate the benefit of RTHS. The first test examines the connected control method using viscous damper hardware as the physical substructure coupled to adjacent base isolation systems as the numerical substructure. The second test involves a multi-stage isolation system comprised of an operating mechanical component on isolators as the physical substructure coupled to an intermediate mass on similar isolators as the numerical substructure. In these RTHS tests, feedforward inverse compensation combined with feedback is used to compensate the frequency-dependent dynamics of the multi-actuator system. Experimental results demonstrate that RTHS accurately captures the system-level behavior of the coupled system and allows for repeatable tests of various conditions and potential system improvements to be efficiently examined.

Locations of sensors and actuators have major impact on the characteristics of control systems. In this paper a procedure for sensor and actuator placement for vibration control systems is presented. Two different performance criteria are used in order to find optimal positions for the system under consideration. One is considering observability and controllability only, the other one includes knowledge about a disturbance and the control objective. Both criteria are applied to a clamped plate resulting in different optimal sensor and actuator positions. The resulting configurations are investigated by comparison of optimal feed forward and H ∞ feedback control of the system with identical disturbances and control objectives but different sensor and actuator positions. The required control effort and achieved amplitude reduction are employed to rate the different performance criteria that were used to determine the sensor and actuator positions. It is shown that, by placing sensors and actuators on the basis of an adequate performance criterion, increased control performance in terms of amplitude reduction per control effort is achieved.

Numerical simulations are presented on a feedback active control strategy for flow-induced off-track vibration of the head gimbals assembly (HGA) supporting the slider in hard disk drives, through suppressing pressure fluctuations around the HGA. A virtual sensing method is employed to enable the feedback signal changeable from pressure fluctuations at the physical sensor position to those at single “virtual sensor” positions closely around the HGA or a spatial average of pressure fluctuations along an HGA surface. Based on a linear control methodology, performance of the proposed active control strategy with different feedback signals has been investigated in two-dimensional simulations, where a physical pressure sensor and a pressure actuator are assumed on the inner-surface of the HDD cover to detect the pressure fluctuations and to actuate active pressure oscillations into HDD space respectively. The results show effective control on the HGA off-track vibration when the feedback signal is configured to minimize pressure fluctuations at specific positions closely around the HGA, such as the wake region. It is also shown that satisfying control effect can be achieved on the HGA off-track vibration in the global spectrum when the feedback signal is configured to minimize the spatial average of pressure fluctuations along the upper surface of the HGA.

This paper addresses the construction, measurement, and analysis of a double-panel active partition (DPAP) and its accompanying analog feedback controllers. The DPAP was constructed by attaching an aluminum cone loudspeaker at each end of a short segment of a circular duct. Two analog feedback controllers were designed and built using the measured frequency response function of each panel. Two independent (decoupled) feedback controllers were then used to minimize the vibration amplitude of each panel in the presence of an acoustic disturbance. A normal-incidence transmission loss measurement system was used to assess the performance of the DPAP and of a single panel passive partition. Error signal attenuations show that it is both feasible and effective to simultaneously control both panels with decoupled feedback controllers, and that simultaneously controlling both panels of the DPAP has a distinct advantage over controlling a single panel. The reduction in vibration amplitude across the surface of the transmitting panel was confirmed with scanning laser vibrometer measurements. Transmission loss results were obtained for two passive and three active configurations. The average normal incidence transmission loss over the active measurement bandwidth (50–1,000 Hz) for the active double-panel was 60 dB. This is an average of 39 dB more transmission loss than a passive single panel partition.