-
[show abstract]
[hide abstract]
ABSTRACT: A turbofan engine acoustic liner is effective at reducing propagated noise. However, there is room for improving performance or making the liner more compact. Plasma actuators have been shown to be useful flow control devices in a number of fluid flow situations. Combining the plasma actuator (PSJA) and passive liner technologies offers the promise that the attenuation of the acoustic liner can be improved with the addition of the active actuators. The research was divided into two realms: oscillatory flow and bias flow induced by the plasma actuator. A waveguide was constructed with a disturbance speaker at one end, an anechoic termination at the other end, and a mounting for Helmholtz resonators (HRs) in the middle. The PSJA technology was integrated into the neck of the a single HR. A single frequency, feed forward test was conducted to evaluate the oscillatory flow. This test showed that the implementation of the PSJA with a feed forward system could produce an additional 3 dB of attenuation. For this test arrangement the bias flow did not show any additional measurable attenuation. Interpretation of the results are discussed.
The Journal of the Acoustical Society of America 10/2010; 128(4):2286. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A decentralized model-based control strategy is designed to reduce low-frequency sound radiation from periodically stiffened panels. While decentralized control systems tend to be scalable, performance can be limited due to modeling error introduced by the unmodeled interaction between neighboring control units. Since bounds on modeling error are not known in advance, it is difficult to ensure the decentralized control system will be robust without making the controller overly conservative. Therefore an iterative approach is suggested, which utilizes frequency-shaped loop recovery. The approach accounts for modeling error introduced by neighboring control loops, requires no communication between subsystems, and is relatively simple. The control strategy is evaluated numerically using a model of a stiffened aluminum panel that is representative of the sidewall of an aircraft. Simulations demonstrate that the iterative approach can achieve significant reductions in radiated sound power from the stiffened panel without destabilizing neighboring control units.
The Journal of the Acoustical Society of America 10/2010; 128(4):1729-37. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: When combined with attached motors and rotating machinery, the lightly damped, thick plating required in maritime applications becomes a broadband noise and vibration control problem. A typical solution is to adhere heavy and dense damping materials for dissipation of the plate vibrational energy. In order to attenuate low frequencies, significant mass must be added to the structure. This paper will review the development of two, new passive treatments intended to resolve this issue. HG blankets are constructed using small masses embedded into poroelastic material. Together with the inherent stiffness of the poroelastic material, the masses become embedded mass-spring dampers and their presence is found to notably increase the low-frequency transmission loss of the host material. DVAs are compact vibration absorbers that distribute continuous mass and spring elements over the surface while generating ample reactive damping at low frequencies. This paper will overview the concepts and development of adapting DVAs and HG blankets for use on heavy plate structures, their testing for broadband control performance, as well as their versatility for thinner panels. A comparison with a conventional, marine noise control treatment will be considered. [This work was supported by Northrop Grumman Shipbuilding-Newport News.].
The Journal of the Acoustical Society of America 03/2010; 127(3):1992. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Low frequency noise is a significant annoyance at very low amplitudes, particularly when outdoor community noise from mechanical equipment penetrates commercial and residential building structures. Mid and high frequencies are selectively attenuated by the exterior walls. However, mitigation of the low frequency noise has been very challenging due to the longer wavelength and higher energy. This work presents case studies of an advanced acoustical material in typical metal panels to mitigate community noise from outdoor mechanical equipment. These advanced materials were developed with a unique combination of patented acoustical and vibration absorbing technology, based of distributed absorbers, layered with standard acoustic materials. The materials provide enhanced low frequency noise (below 300 Hz) over state of the art 10 cm metal panels filled with the fiberglass absorber. The case studies demonstrate superior acoustic performance in low frequency without substantial increase in weight.
The Journal of the Acoustical Society of America 06/2008; 123(5):3811. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper describes a combined control strategy designed to reduce sound radiation from stiffened aircraft-style panels. In particular, the control approach uses robust active damping in addition to high-authority LQG control. Active damping is achieved using direct velocity feedback with triangularly shaped strain actuators and point velocity sensors [P. Gardonio and S.J. Elliott, JASA 117(4), 2046-2064 (2005)]. However unlike previous work, anisotropic actuators are used since they outperform traditional isotropic actuators in this application. While active damping is simple and robust, stability is guaranteed at the expense of performance. Therefore, this approach is often referred to as low-authority control. In contrast, LQG control strategies can achieve substantial reductions in sound radiation. Unfortunately, the unmodeled interaction between neighboring control units can destabilize decentralized control systems. Numerical simulations show that combining active damping and decentralized LQG control can be mutually beneficial. In particular, augmenting the in-bandwidth damping supplements the performance of the LQG control strategy and reduces the destabilizing interaction between neighboring control units. Therefore, the performance of the combined system can be better than the sum of each individual strategy.
The Journal of the Acoustical Society of America 06/2008; 123(5):3575. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A decentralized LQG-based control strategy is designed to reduce low-frequency sound transmission through periodically stiffened panels. While modern control strategies have been used to reduce sound radiation from relatively simple structural acoustic systems, significant implementation issues have to be addressed before these control strategies can be extended to large systems such as the fuselage of an aircraft. For instance, centralized approaches typically require a high level of connectivity and are computationally intensive, while decentralized strategies face stability problems caused by the unmodeled interaction between neighboring control units. Since accurate uncertainty bounds are not known a priori, it is difficult to ensure the decentralized control system will be robust without making the controller overly conservative. Therefore, an iterative approach is suggested, which utilizes frequency-shaped loop recovery. The approach accounts for modeling error introduced by neighboring control loops, requires no communication between subsystems, and is relatively simple. The control strategy is validated using real-time control experiments performed on a built-up aluminum test structure representative of the fuselage of an aircraft. Experiments demonstrate that the iterative approach is capable of achieving 12 dB peak reductions and a 3.6 dB integrated reduction in radiated sound power from the stiffened panel.
The Journal of the Acoustical Society of America 06/2008; 123(5):3870. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Mechanical equipment (fans, chillers, motors) produces airborne and structure-borne noise with a significant low frequency component. However, in many applications, mitigation of the low frequency noise requires implementation of significant mass barrier in combination with an air space. This work presents case studies on the use of a novel lightweight and thin acoustical material to mitigate low frequency noise from mechanical equipment. This advanced material, based on patented distributed absorber technology, was used to treat the metal housing of the mechanical equipment. The novel material effectively increased transmission loss of the metal housing below 300 Hz. The total noise reduction was 10-14 dB (A) with 5 dB improvement over typical flexible acoustical blankets below 300 Hz.
The Journal of the Acoustical Society of America 06/2008; 123(5):3259. · 1.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This study describes the development of a model that predicts the sound radiation from an aircraft style panel excited by a dynamic pressure field with arbitrary spatial correlation. Eventually, this model will be used to develop distributed feedback control strategies for systems with spatially correlated inputs. For this study, the input pressure field is simulated using an array of point forces on the panel. The excitation at each point is defined in the time domain using the spectral representation method [M. Shinozuka and C.-M. Jan, J. Sound Vibr. 25(1), 111-128 (1972)]. This method generates sample functions that match the spatial and temporal correlation characteristics of the desired pressure field. Three different types of excitations are considered: a turbulent boundary layer excitation, a diffuse field, and a spatially uncorrelated disturbance. This approach could be extended to a variety of other types of spatially correlated stochastic inputs as well. A convergence study is also presented to show how many point forces are required to approximate the spatial characteristics of each type of pressure field. [Work supported by NASA.]
10/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Experiments were carried out to demonstrate the ability of an artificial neural network (ANN) system to distinguish between the noise of two helicopters. The ANN is taught to identify helicopters by using two types of features: one that is associated with the ratio of the main-rotor to tail-rotor blade passage frequency (BPF), and the ohter that describes the distribution of peaks in the main-rotor spectrum, which is independent of the tail-rotor. It is shown that the ability of the ANN to identify helicopters is comparable to that of a conventional recognition system using the ratio of the main-rotor BPF to the tail-rotor BPF (when both the main- and the tail-rotor noise are present), but the performoance of ANN exceeds the conventional-method performance when the tail-rotor noise is absent. In addition, the results of ANN can be obtained as a function of propagation distance.
11/1990;
