Active Control of Low Frequency Modes in an Aircraft Fuselage Using Spatially Weighted Arrays

2000 ◽  
Vol 122 (3) ◽  
pp. 227-234 ◽  
Author(s):  
Steven A. Lane ◽  
Robert L. Clark ◽  
Steve C. Southward
Keyword(s):  
Three Dimensional ◽  
Pressure Sensors ◽  
Low Frequency ◽  
Single Mode ◽  
Open Loop ◽  
H2 Control ◽  
Low Frequencies ◽  
Control Approach ◽  
Aircraft Fuselage ◽  
Transducer Arrays

Acoustic enclosures such as aircraft cabins often display lightly damped modal behavior at low frequencies where passive treatments are impractical due to mass and volume constraints. This work presents a feedback control approach using dynamic H2 controllers implemented with spatially weighted arrays of collocated pressure sensors and constant volume-velocity actuators. The open-loop system is shaped using spatially weighted transducer arrays to yield increased pole-zero separation, which results in better closed-loop performance. The transducer arrays are weighted to emphasize coupling to a particular acoustic mode or modes, which facilitates global control of the targeted dynamics. This work presents a method to determine the spatial weighting vectors for the transducer arrays from frequency response measurements. The development and implementation of low-order, dynamic H2 control laws is also discussed. Experimental results are presented for a single-mode and a multiple-mode controller implemented on an aircraft fuselage section, and demonstrate significant reduction of the targeted acoustic modes. To the best of the authors’ knowledge, this is the first experimental implementation of a feedback controller of this type capable of achieving such levels of global reduction in a three-dimensional acoustic system. [S0739-3717(00)02303-5]

The Use of Optimally Shaped Piezo-electric Film Sensors in the Active Control of Free Field Structural Radiation, Part 2: Feedback Control

1996 ◽  
Vol 118 (1) ◽  
pp. 112-121 ◽  
Author(s):  
S. D. Snyder ◽  
N. Tanaka ◽  
Y. Kikushima
Keyword(s):  
Feedback Control ◽  
Control Strategies ◽  
Free Field ◽  
Low Frequency ◽  
Acoustic Power ◽  
State Equations ◽  
Subsequent Increase ◽  
Low Frequencies ◽  
Control Approach ◽  
Iir Filters

Feedback control of free field structural radiation is considered. State equations are formulated with a transformation which decouples the acoustic power error criterion. Using the resultant equations, expressed in terms of “transformed mode” states, the order of the state equations can be significantly reduced at low frequencies. Two experimental implementations of feedback control strategies using shaped piezoelectric polymer film sensors to measure the transformed system states are described. The first of these is a simple analog implementation. The second implementation is in discrete time, where an adaptive algorithm for optimizing the weights of IIR filters for practical use is described. It is shown that by using the outlined control approach significant levels of low frequency acoustic power attenuation can be obtained with no control spillover and subsequent increase in higher frequency acoustic power output.

Receptivity to free-stream vorticity of flow past a flat plate with elliptic leading edge

2010 ◽  
Vol 653 ◽  
pp. 245-271 ◽  
Author(s):  
L.-U. SCHRADER ◽  
L. BRANDT ◽  
C. MAVRIPLIS ◽  
D. S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Three Dimensional ◽  
Low Frequency ◽  
Leading Edge ◽  
Sound Waves ◽  
Streamwise Vorticity ◽  
Low Frequencies ◽  
Free Stream Turbulence

Receptivity of the two-dimensional boundary layer on a flat plate with elliptic leading edge is studied by numerical simulation. Vortical perturbations in the oncoming free stream are considered, impinging on two leading edges with different aspect ratio to identify the effect of bluntness. The relevance of the three vorticity components of natural free-stream turbulence is illuminated by considering axial, vertical and spanwise vorticity separately at different angular frequencies. The boundary layer is most receptive to zero-frequency axial vorticity, triggering a streaky pattern of alternating positive and negative streamwise disturbance velocity. This is in line with earlier numerical studies on non-modal growth of elongated structures in the Blasius boundary layer. We find that the effect of leading-edge bluntness is insignificant for axial free-stream vortices alone. On the other hand, vertical free-stream vorticity is also able to excite non-modal instability in particular at zero and low frequencies. This mechanism relies on the generation of streamwise vorticity through stretching and tilting of the vertical vortex columns at the leading edge and is significantly stronger when the leading edge is blunt. It can thus be concluded that the non-modal boundary-layer response to a free-stream turbulence field with three-dimensional vorticity is enhanced in the presence of a blunt leading edge. At high frequencies of the disturbances the boundary layer becomes receptive to spanwise free-stream vorticity, triggering Tollmien–Schlichting (T-S) modes and receptivity increases with leading-edge bluntness. The receptivity coefficients to free-stream vortices are found to be about 15% of those to sound waves reported in the literature. For the boundary layers and free-stream perturbations considered, the amplitude of the T-S waves remains small compared with the low-frequency streak amplitudes.

Propagation of Low Frequency Elastic Disturbances in a Three-Dimensional Composite Material

1975 ◽  
Vol 42 (1) ◽  
pp. 159-164 ◽  
Author(s):  
W. Kohn
Keyword(s):  
Secular Equation ◽  
Displacement Vector ◽  
Three Dimensional ◽  
Low Frequency ◽  
Three Dimensions ◽  
Low Frequencies ◽  
One Dimensional ◽  
Coupled Partial Differential Equations ◽  
Constant Coefficients ◽  
The Mean

This paper is a generalization to three dimensions of an earlier study for one-dimensional composites. We show here that in the limit of low frequencies the displacement vector ui(r,t) can be written in the form ui (r,t) = (∂ij + vijl (r) ∂/∂xl + …) Uj (r,t). Here Uj (r,t) is a slowly varying vector function of r and t which describes the mean displacement of each cell of the composite. Its components satisfy a set of three coupled partial differential equations with constant coefficients. These coefficients are obtainable from the three-by-three secular equation which yields the low-lying normal mode frequencies, ω(k). Information about local strains is contained in the function vijl(r), which is characteristic of static deformations, and is discussed in detail. Among applications of this method is the structure of the head of a pulse propagating in an arbitrary direction.

Open Loop Control of Combustion Instability With a High-Momentum Air-Jet

2004 ◽  
Author(s):  
Jong Ho Uhm ◽  
Sumanta Acharya
Keyword(s):  
Low Frequency ◽  
Sine Wave ◽  
Open Loop ◽  
Effective Control ◽  
High Momentum ◽  
Open Loop Control ◽  
Low Frequencies ◽  
Loop Control ◽  
Wave Modulation ◽  
Air Jet

A new strategy for open-loop control of combustion oscillations using a high-momentum air-jet modulated at low frequencies is presented in this paper. The oscillations in the swirl-stabilized spray combustor of interest are dominated by an acoustic mode (235 Hz) with a low frequency (13 Hz) bulkmode (of the upstream cavity) oscillation superimposed. The most effective strategy for control is shown to be achieved through the use of a new concept which utilizes a high-momentum air-jet injected directly into the region of flame dynamics. It is shown that with a low frequency modulation (15 Hz) of the high momentum air-jet, the pressure oscillations can be reduced significantly (by a factor of nearly 10). Square wave modulation is shown to be considerably more effective than sine-wave modulation. These results are extremely promising since high bandwidth actuation is not required for effective control.

scholarly journals The Ultra-low Frequency and Broad Band Gaps Characteristics of Multilayer Composite Cylindrical Three-dimensional pentamode metamaterials

2021 ◽  
Author(s):  
Chengxin Cai ◽  
Xue Wang ◽  
Qifu Wang ◽  
Mingxing Li ◽  
Guangchen He ◽  
...  
Keyword(s):  
Acoustic Waves ◽  
Broad Band ◽  
Three Dimensional ◽  
Engineering Application ◽  
Low Frequency ◽  
Single Mode ◽  
Band Gaps ◽  
Multilayer Composite ◽  
Lattice Constants ◽  
Mode Area

Abstract For three-dimensional pentamode metamaterials, it is of great significance to realize underwater ultra-low frequency acoustic wave control. Therefore, two types multilayer composite cylindrical three-dimensional pentamode metamaterials with ultra-low frequency and broad band gaps are proposed in this paper. By using pentamode metamaterials with lattice constants on the order of centimeters, the phononic band gaps below 60 Hz and the single-mode area below 30Hz can be obtained. Compared with asymmetrical double-cone locally resonant pentamode metamaterials, the lower edge frequency, relative bandwidth and figure of merit of the first phononic band gap can be reduced by up to 61.4%, 10.3% and 40.6%, respectively. It will provide reference and guidance for the engineering application of pentamode metamaterials in controlling the ultra-low frequency broadband acoustic waves, vibration and noise reduction.

Approximate calculation of low‐frequency three‐dimensional magnetotelluric responses using a multiple plate model

Geophysics ◽  
1992 ◽  
Vol 57 (1) ◽  
pp. 106-115 ◽  
Author(s):  
Carlos Flores ◽  
R. Nigel Edwards
Keyword(s):  
Three Dimensional ◽  
Low Frequency ◽  
Test Site ◽  
Observation Point ◽  
Skin Depth ◽  
Approximate Algorithm ◽  
Published Data ◽  
Low Frequencies ◽  
Severe Problem ◽  
Layered Earth

The numerical methods now used to calculate the magnetotelluric (MT) response of three‐dimensional (3-D) conductive inhomogeneities are still expensive in terms of computing cost. This is a more severe problem in many actual field conditions where multiple conductors exist. There is then a clear need for the development of faster algorithms for data interpretation. We can rapidly compute the 3-D MT response of a multiple‐conductor structure embedded in a layered earth at low frequencies using an approximate algorithm where each conductor is simulated by a set of plates. There are two limitations to the technique. The first low‐frequency limitation is a requirement that the induction number of any plate in the model be small compared with unity. The second condition specifies that the range from an observation point on the surface of the earth to a typical plate must be small compared with the skin depth in the layered background medium. These conditions allow us to ignore local self‐induction within the plates and mutual induction between the plates and the host medium. The secondary anomalous fields generated by any plate are assumed to be due to a distribution of current dipoles in the plane of the plate. The strengths of the distributed dipoles are found by solving a Fredholm integral equation of the second kind with the layered earth MT electric field at depth as the driving function. MT responses obtained with our approximate algorithm for three prismatic models compare favorably with previously published data obtained with complete algorithms for the same models. The use of the technique to model multiple conductors is illustrated with controlled source MT data collected at the Cavendish Test Site.

Disparity-induced and blur-induced convergence eye movement and accommodation in the monkey

1986 ◽  
Vol 55 (5) ◽  
pp. 896-914 ◽  
Author(s):  
B. G. Cumming ◽  
S. J. Judge
Keyword(s):  
Eye Movement ◽  
Dimensional Space ◽  
Dynamic Control ◽  
Three Dimensional ◽  
Open Loop ◽  
Binocular Viewing ◽  
Monocular Viewing ◽  
Target Movement ◽  
Low Frequencies ◽  
First Case

The dynamics of vergence eye movement and of ocular accommodation were studied in two monkeys trained to track a haploscopically presented target that appeared to move in depth. The target was presented under four conditions: monocular viewing, normal binocular viewing, accommodation-open-loop binocular viewing, and "conflict" viewing, in which the accommodation and vergence stimuli did not correspond to those produced by any real target in three-dimensional space. The first and third conditions were chosen because in each case only one of the two primary cues that guide accommodation and vergence was operative: blur in the first case and disparity in the third. We usually studied responses to apparent target movement directly toward or away from the right eye, in which accommodation was measured. The latencies of the accommodation responses to steps toward the monkey were approximately 180 and 240 ms in the two monkeys, while in both monkeys the latencies of convergence were approximately 160 ms. Neither the vergence latencies nor the accommodation latencies were greatly different in monocular and binocular viewing. Responses to a sinusoidally moving target (frequencies 0.1-1.2 Hz; peak-to-peak amplitude 0.5-4 diopters or meter-angles) were studied in the first three of the above viewing conditions. In binocular viewing, even with accommodation open-loop, vergence and accommodation showed much smaller phase lags than in monocular viewing. Furthermore, in response to step changes, both vergence and accommodation velocities were higher in binocular viewing than in monocular viewing. Thus the dynamic control of both vergence and accommodation relies predominantly on disparity signals. At low frequencies (0.2 or 0.3 Hz) the monkeys showed only a modest ability to separate their accommodation and vergence responses when presented with conflicting blur and disparity cues. A simple linear calculation based on the data above was used to predict the responses in such situations. The predicted and observed responses were in reasonable agreement.

scholarly journals Into the dark: patterns of middle ear adaptations in subterranean eulipotyphlan mammals

2017 ◽  
Vol 4 (9) ◽  
pp. 170608 ◽  
Author(s):  
Daisuke Koyabu ◽  
Misato Hosojima ◽  
Hideki Endo
Keyword(s):  
Middle Ear ◽  
Three Dimensional ◽  
Low Frequency ◽  
Frequency Noise ◽  
Terrestrial Environment ◽  
Low Frequencies ◽  
Geometric Morphometric ◽  
Key Innovation ◽  
Ear Ossicles ◽  
Stapes Footplate

Evolution of the middle ear ossicles was a key innovation for mammals, enhancing the transmission of airborne sound. Radiation into various habitats from a terrestrial environment resulted in diversification of the auditory mechanisms among mammals. However, due to the paucity of phylogenetically controlled investigations, how middle ear traits have diversified with functional specialization remains unclear. In order to identify the respective patterns for various lifestyles and to gain insights into fossil forms, we employed a high-resolution tomography technique and compared the middle ear morphology of eulipotyphlan species (moles, shrews and hedgehogs), a group that has radiated into various environments, such as terrestrial, aquatic and subterranean habitats. Three-dimensional geometric morphometric analysis was conducted within a phylogenetically controlled framework. Quantitative shapes were found to strongly reflect the degree of subterranean lifestyle and weakly involve phylogeny. Our analyses demonstrate that subterranean adaptation should include a relatively shorter anterior process of the malleus, an enlarged incus, an enlarged stapes footplate and a reduction of the orbicular apophysis. These traits arguably allow improving low-frequency sound transmission at low frequencies and inhibiting the low-frequency noise which disturbs the subterranean animals in hearing airborne sounds.

scholarly journals Investigation of Acoustic Properties on Wideband Sound-Absorber Composed of Hollow Perforated Spherical Structure with Extended Tubes and Porous Materials

2020 ◽  
Vol 10 (24) ◽  
pp. 8978
Author(s):  
Dengke Li ◽  
Zhongcheng Jiang ◽  
Lin Li ◽  
Xiaobo Liu ◽  
Xianfeng Wang ◽  
...  
Keyword(s):  
Sound Absorption ◽  
Three Dimensional ◽  
Low Frequency ◽  
Acoustic Properties ◽  
Low Frequencies ◽  
Frequency Sound ◽  
Sound Absorber ◽  
Spherical Structure ◽  
Absorption Performance ◽  
Melamine Foam

Traditional porous media such as melamine foam absorb sound due to their three-dimensional porous struts. However, the acoustic properties at low frequencies are greatly related to its thickness. In this paper, a novel type of thin and lightweight sound absorber composed of melamine foam and hollow perforated spherical structure with extended tubes (HPSET) is introduced to enhance the sound absorption performance at low frequencies. A theoretical model for the normal absorption coefficient of the HPSET with melamine foam is established. Good agreements are observed between the simulated and the experimental results. Compared with the virgin melamine foam, the proposed absorber can greatly improve the low-frequency sound absorption and retain the mid- to high-frequency sound absorption, while the thickness of the proposed absorber is less than 1/28 of the wavelength.

scholarly journals Stochastic BEM for the Vibroacoustic Analysis of Three-Dimensional Structures

2011 ◽  
Vol 2011 ◽  
pp. 1-12 ◽  
Author(s):  
R. D'Amico ◽  
A. Pratellesi ◽  
M. Pierini ◽  
N. Baldanzini
Keyword(s):  
Crucial Role ◽  
Three Dimensional ◽  
Low Frequency ◽  
Wide Frequency Range ◽  
Frequency Range ◽  
Small Perturbations ◽  
Low Frequencies ◽  
Response Sensitivity ◽  
Basic Assumptions ◽  
Prediction Reliability

Nowadays, extending the NVH prediction reliability to the whole frequency range is an attractive goal of vibroacoustics. Deterministic methodologies are well established for the low-frequency range, but, decreasing the wavelength, energy-based methods are necessary. In such a range, a crucial role is played by small perturbations which highly influence the response sensitivity. Moreover, taking into account these variations allows to make the product design more robust and even quicker. Introducing geometrical uncertainties within the classic BEM formulation allows to obtain the so-called stochastic BEM. As a result, the solution shows deterministic behaviour at low frequencies; decreasing the wavelength, the effect of the uncertainties smooths the response. Consequently, it is possible to obtain an averaged trend over the whole frequency range which asymptotically tends to the deterministic one. In this paper, we deal with three-dimensional acoustic SBEM. First, the formulation and its basic assumptions are presented. Secondly, they are applied to academic cases to show its potentialities in predicting vibroacoustic behaviour over a wide frequency range.

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