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Published Articles

The Volume 8, No 4, December 2003

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Displacement Sensitivity Control for Beams and Plates using Electro-Optic Holography

Jonathan D. Blotter, Tony J. Anderson


Electro-Optic Holography (EOH) is a whole-field, laser-based, displacement measurement technique. In conventional EOH, the displacement sensitivity is fixed by the wavelength of the laser source. When infrared diode lasers are used, the fringe patterns can become highly dense and unresolvable, limiting the measurable range of displacements. This paper presents and extends a technique for controlling the displacement sensitivity of an EOH system using diode laser current modulation. This technique is known as Frequency Translated Electro- Optic Holography (FTEOH). Using diode laser current modulation, the fringe patterns are based on higher order Bessel functions and the sensitivity of the EOH system can either be increased or decreased. The amount of sensitivity reduction is limited only by the frequency limit of the signal generator that modulates the current supplied to the diode laser. EOH and FTEOH experimental results for a cantilever beam and a simply supported plate are presented and shown to agree well.

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Selective Noise Cancellation using Independent Component Analysis

Jun-Il Sohn, Sang-Woo Ban and Minho Lee


A conventional active noise control (ANC) system adjusts coefficients of an adaptive filter to minimise energy of the input signal at the error microphone. When a desired signal exists with a noise signal around the error microphone, the ANC system removes even the desired signal. In this paper, we propose new ANC systems that selectively cancel only the noise signal in the mixture of signals at a specific location. Blind source separation (BSS), which is based on independent component analysis (ICA) and which separates the desired sound signal from the unwanted noise signal, is used as a preprocessor of the proposed ANC system. In order to enhance the performance of noise separation, we developed a teacher-forced BSS learning algorithm. The teacher signal is obtained from a loudspeaker of the ANC system. Computer simulation results show that the proposed ANC system, in conjunction with the BSS algorithm, effectively cancels only the ship engine noise signal from the linear and convolved signal mixtures, which include the human voice. Additionally, experimental results for practical applications in a real situation show good performance cancelling only the cooling fan noise from the signal mixtures, which include the voice signal.

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Computation of Broadband Noise Radiated by a Ducted Fan in a Uniform Flow

Serge Lewy


Ducted-fan broadband noise often dominates overall sound levels radiated by a modern aircraft high-bypassratio turbofan at subsonic rotational tip speeds. As a result, its prediction has become a challenge for additional noise reduction. Equations were established in a previous work for a uniform flow inside the duct, the free medium being at rest (case of static tests). However, induct and free-field acoustic powers are not balanced if there is a flow mismatch at the duct exit. Free-field computation is thus extended to a medium in translation which better simulates flight conditions (the same velocity being taken throughout the space). The radiation model of Tyler and Sofrin (Rayleighs integral) is no longer valid, and a routine based on the Kirchhoff integral equation has been implemented. It is checked that all the previous results are verified, both in the no-flow case and with flow only inside the duct. Computations assuming a uniform flow throughout the space now lead to the same acoustic power inside the duct and in the free field. This is a basic validation of the prediction model. It is mainly found that flow velocity tends to increase rotor forward radiation and to decrease aft radiation.

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Effect of Root Flexibility on Bladed-Disk Dynamics

A. S. Mohamad and A. A. N. Al-Jawi


Mode localisation in nearly periodic engineering structures has become one of the most important parameters in the dynamics of such structures. The bladed-disk assembly is one such structure with many practical applica- tions. In this paper, the free mode localisation in a bladed-disk assembly is considered, where the blades are modelled with root flexibility using Green's functions. The roots consist of idealised rectangular blocks mounted on linear and rotational springs. The beams are then mono-coupled by linear springs to form the cyclic chain. It is found that two propagation bands appear in addition to those for beams with fixed roots. The frequencies at which these flexibility modes occur can be obtained by solving an eighth-order frequency polynomial whose co- efficients depend only on the root flexibility parameters. Natural frequencies are calculated for tuned and mis- tuned cases using the Greens function formulation and the finite element method, and these are found to be in good agreement. The mode shapes corresponding to selected frequencies are computed using the finite element method. The effects of the root parameters on the mode localisation are also studied.

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Tension in Fluttering Flags

Peter M. Moretti


When a flag flutters, tension is dynamically induced by the two-dimensional vibratory motion. The dynamic process, involving centrifugal forces due to the curved path of the trailing edge, is similar to the whipping of an oscillating rope, and accounts for most of the dragforce observed at the flagpole attachment (luff). Conversely, the induced tension, combined with the curvature of the fabric, opposes the pressure forces from the flow field and extracts momentum from it. In order to estimate post-critical flag and panel flutter amplitudes, it is neces- sary to compute the structural stiffening due to dynamically induced tension. Tension in typical flag flutter mo- tion, consisting of a travelling wave growing in amplitude as it progresses towards the trailing edge (leech), is obtained by approximate analysis, using a computer algebra system. The time-averaged tension depends on the square of the velocity amplitude of the oscillating fabric; the distribution of time-averaged tension is shown for a typical flag flutter motion. An estimate of the tension fluctuations is developed: the fluctuations are small (rela- tive to the average tension) at locations several wavelengths from the leech, but are important near the leech. The general partial differential equation (PDE) of motion is obtained from Hamilton's principle. The induced- tension term in the governing PDE derives from the in-plane kinetic energy of the flag motion. Dynamically in- duced tension is shown to be important if the stiffness of the fabric is low: an order-of-magnitude criterion is presented.

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Comparative Study of Active Control of a Large Composite I-Beam

Gangbing Song and Vineet Sethi


This paper presents comparative experimental results of various active vibration control methods of a pultruded fibre-reinforced polymer (FRP) composites thin-walled I-beam using smart sensors and actuators. The FRP I-beams are made of E-glass fibers and polyester resins. The FRP I-beam is in a cantilevered configuration. PZT (Lead zirconate titanate) type of piezoelectric ceramic patches are used as smart sensors and actuators. These patches are surface-bonded near the cantilevered end of the I-beam. Utilising results from open-loop experimental testing, several active vibration control methods, such as positive position feedback (PPF), strain rate feedback (SRF), lead compensator and pole placement control are investigated. A comparative study and analysis of the control results obtained from the experiment was done. The study showed that the PPF control provides better vibration suppression than the SRF, lead compensator or pole placement control. Pole placement control results were superior to lead control, and SRF provided intermediate results. Experimental results demonstrate that the proposed methods achieve effective vibration control of the FRP I-beam.

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Circumferential-Wave Dispersion Curves for Water-Immersed, Water-Filled Spherical Steel Shells

Michael F. Werby and Herbert Ueberall


Frequency resonances in the scattering amplitude of acoustic waves from submerged elastic objects can be iso- lated by subtracting an underlying nonresonant background amplitude that is due to backscattered specular re- flection. The resonances are caused by the phase matching of circumferential (surface) waves, and thus deter- mine the phase-velocity dispersion curves of such waves. These are obtained here for the case of a spherical steel shell in water. If the shell is evacuated, Lamb-type waves A0 and S0 (analogous to those on a plate with one-sided water loading) are found, as well as the Scholte-Stoneley (A) wave that propagates in the loading fluid. If the shell is filled with water, new circumferential waves propagating in the filler fluid appear and carry out a motion coupled with each other, causing drastic repulsion phenomena among their dispersion curves.

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