Mechanical Engineering

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Author: search.filters.author.Akl, Wael

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    Vibration of Periodic Drill-Strings with Local Sources of Resonance
    (MDPI, 2021-07-17) Akl, Wael; Alsupie, Hajid; Sassi, Sadok; Baz, Amr M.
    A new class of drill-strings is proposed for attenuating undesirable vibrations to ensure effective operation. The drill-string is provided with passive periodic inserts, which are integrated with sources of local resonance (LR). The inserts make the drill-string act as a low frequency pass mechanical filter for the transmission of vibration along the drill-string. Proper design of the periodic inserts with sources of LR tend to shift these stop bands towards zones of lower frequencies to enable confining the dominant modes of vibration of the drill-string within these bands. In this manner, propagation of the vibration along the drill-string can be completely blocked. A finite element model (FEM) is developed using ANSYS to investigate the bandgap characteristics of the proposed drill-string with sources of LR. The developed FEM accounts for bending, torsional, and axial vibrations of the drill-string in order to demonstrate the effectiveness of the periodic inserts with LR in simultaneous control of these combined modes as compared to conventional solid periodic inserts, which are only limited to controlling bending vibrations. The effect of the design parameters of the periodic inserts with LR on the bandgap characteristics of the drill-string is investigated to establish guidelines of this class of drill-strings.
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    SMART FOAM FOR ACTIVE VIBRATION AND NOISE CONTROL
    (2004-03-16) Akl, Wael; Baz, Amr M; Mechanical Engineering
    A new class of smart foams is introduced to simultaneously control the vibration and noise radiation from flexible plates coupled with acoustic cavities. The proposed smart foam consists of a passive foam layer bonded to one surface of an active piezoelectric composite whose other surface is bonded to the surface of the vibrating plate. In this manner, the active piezoelectric composite can control from one side the porosity and the acoustic absorption characteristics of the foam and from the other side can suppress the vibration of the flexible plate. With such capabilities, the proposed smart foam can simultaneously control structural and acoustic cavity modes over a broad frequency range. A comprehensive theoretical study of the smart foam elements is introduced, in order to optimize the design and performance of this hybrid actuator. Feedback control of the reflected sound field was numerically and experimentally investigated, using an impedance tube, and showed a great improvement in the sound absorption coefficient. A finite element model is developed to study the interactions among the foam, the active piezoelectric composite, the flexible plate and an acoustic cavity. The developed finite element model is a reduced 2-dimensional model based on the 1st order shear deformation theory, which was compared with the original 3-dimensional model and it managed to capture all the dynamic characteristics of the foam provided a proper thickness to width ratio is maintained. The model enables the prediction of the plate vibration and the sound pressure level inside the acoustic cavity for a simple PD feedback control strategy of the active piezoelectric composite. It enables also the computation of the acoustic absorption characteristics of the foam. The predictions of the model are also validated experimentally. The developed theoretical and experimental techniques will provide invaluable tools for the design and application of the proposed smart foam to a wide variety of systems such as passenger cars, helicopter, aircraft cabins and other flexible enclosures, where their operation as quiet platforms is critical to the success of their mission.