A. Akay | Bilkent University (original) (raw)

Papers by A. Akay

Research paper thumbnail of Finite Element Model for Hysteretic Friction Damping of Traveling Wave Vibration in Axisymmetric Structures

Volume 9: Mechanical Systems and Control, Parts A, B, and C, 2007

Research paper thumbnail of Optimizing Distributed-Contact Friction in Ring Dampers

Volume 1: 20th Biennial Conference on Mechanical Vibration and Noise, Parts A, B, and C, 2005

Research paper thumbnail of Distributed friction damping of travelling wave vibration in rods

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2008

A ring damper can be affixed to a rotating base structure such as a gear, an automotive brake rot... more A ring damper can be affixed to a rotating base structure such as a gear, an automotive brake rotor or a gas turbine's labyrinth air seal. Depending on the frequency range, wavenumber and level of preload, vibration of the base structure can be effectively and passively attenuated by friction that develops along the interface between it and the damper. The assembly is modelled as two rods that couple in longitudinal vibration through spatially distributed hysteretic friction, with each rod having periodic boundary conditions in a manner analogous to an unwrapped ring and disc. As is representative of rotating machinery applications, the system is driven by a travelling wave disturbance, and for that form of excitation, the base structure's and the damper's responses are determined without the need for computationally intensive simulation. The damper's performance can be optimized with respect to normal preload, and its effectiveness is insensitive to variations in preload or the excitation's magnitude when its natural frequency is substantially lower than the base structure's in the absence of contact.

Research paper thumbnail of Radiation from Bending Waves in the Presence of a Flow Boundary Layer

Journal of Vibration and Acoustics, 1993

The effect of refraction by a flow boundary layer on the radiation of sound from a bending wave o... more The effect of refraction by a flow boundary layer on the radiation of sound from a bending wave on a plate is investigated. Based on the relative velocities of the bending wave and the flow, four qualitatively different regimes are identified and results of numerical studies of radiation impedance and power emission are presented and discussed. For comparison, the role

Research paper thumbnail of S123 a New Recommendation at the Following of Uro-Oncological Tumors: Reminder Short Message Service (SMS)

European Urology Supplements, 2011

eur urol Suppl 2011;10(9):604 and fluoroscopy technology. The harmful effects of ionizing radiati... more eur urol Suppl 2011;10(9):604 and fluoroscopy technology. The harmful effects of ionizing radiation (such as hair loss, erythema, and dermatitis) were recognized shortly after the discovery of the X ray by Wilhelm Conrad Roentgen in 1895. Because of these effects, the hospital personnel needed to avoid of overexposure. Notwithstanding these observations, protection of staff exposed to X-rays and gamma rays from radium was poorly coordinated. With these facts in mind, we undertook a survey of urologists working in Turkey to evaluate their attitude and behavior of the protection of the radiation exposure during some of the more common endourological procedures requested. material & methods: The study included urology residents, specialists and all academic degree of urologists from university hospitals, education and research hospitals, state hospitals and private hospitals in Turkey. The questionnaire that was administered to the study participants was composed of demographic questions concerning age, gender, institution, current status of duty, as well as questions about uses of dosimeters and flexible protective clothing such as aprons, thyroid shields, eyeglasses, and gloves during fluoroscopy guided endourological procedures. The questionnaire was sent to 1796 urologists by e-mail between May and June 2011. Results: Of the 1796 questionnaires sent, 394 questionnaires were answered and 363 of these had completed answers. Mean age of the participants was 40,10±8,44 years. The numbers of doctors who are exposed the ionizing radiation was 307 (84,58%). 192 (62,54 %) participants reported that they expose the radiation from 1 to 5 endourological procedures per week. The numbers of doctors exposed the radiation above 5 times per week and less than 1 time per week were 30(9,77 %) and 85(27,69 %) respectively. behavior of urologists about uses of dosimeters and flexible protective clothes.

Research paper thumbnail of Effects of Viscous and Thermal Losses on Radiation from an Infinite Plate

Journal of Vibration and Acoustics, 1992

Research paper thumbnail of Frequency intermittency and energy pumping by linear attachments

Journal of Sound and Vibration, 2014

The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.

Research paper thumbnail of A Theoretical and Experimental Investigation of Contact Loss in the Clearances of a Four-Bar Mechanism

Journal of Mechanisms Transmissions and Automation in Design, 1986

Research paper thumbnail of Measurement of transient friction hysteresis

Research paper thumbnail of Sources of noise and vibration in a mechanical system with clearances

Inherent in the design of any mechanism with moving parts is the requirement for clearances. In t... more Inherent in the design of any mechanism with moving parts is the requirement for clearances. In the case of an engine, this requirement is enhanced by the extremes of both load and temperature under which some parts are expected to function. Collision of the parts in these connections where clearances exist are influenced by transient combustion forces as well as inertial forces. Impacts in the joints of a system are a major source of sound, vibration and wear. The mechanism of sound generation follows the dynamic response of the system components. The resulting transient acoustic field is comprised of radiation from the forced and free vibrations of the system. Radiation due to inertial forces are generally of lower frequency whereas sounds produced by the impacts exhibit higher levels with higher spectral content. This study investigates the sound and vibration response of a four-bar mechanism in the absence of external forces.

Research paper thumbnail of Fracture of polymeric materials under tensile loads

Research paper thumbnail of Dissipation in a finite-size bath

Physical Review E, 2011

We investigate the interaction of a particle with a finite-size bath represented by a set of inde... more We investigate the interaction of a particle with a finite-size bath represented by a set of independent linear oscillators with frequencies that fall within a finite bandwidth. We discover that when the oscillators have particular frequency distributions, the finite-size bath behaves much as an infinite-size bath exhibiting dissipation properties and thus allowing irreversible energy absorption from a particle immersed in it. We also present a reinterpretation of the Langevin equation using a perturbation approach in which the small parameter represents the inverse of the number of oscillators in the bath, elucidating the relationship between finite-size and infinite-size bath responses.

Research paper thumbnail of Acoustical observation of bubble oscillations induced by bubble popping

Physical Review E, 2007

Acoustic measurements of aqueous foams show three distinct radiation mechanisms that contribute t... more Acoustic measurements of aqueous foams show three distinct radiation mechanisms that contribute to the sound pressure field: oscillations of a bubble surface that precede popping due to the instability of thin liquid film, impulsive radiation due to bursts of bubbles, and oscillations from neighboring bubbles excited by a burst bubble. The movies captured by a fast camera confirm that the bubbles adjacent to a breaking bubble oscillate under the influence of the pressure generated by the burst bubble. The spectra of resulting transient sounds fall in the range of 2-8 kHz and those from bubble oscillations correlate well with the bubble size.

Research paper thumbnail of Relation of Dry-Friction to Surface Roughness

Journal of Tribology, 1997

Research paper thumbnail of Discussion: “Normal and Angular Motions at Rough Planar Contacts During Sliding With Friction” (Hess, D. P., and Soom, A., 1992, ASME J. Tribol., 114, pp. 567–578)

Journal of Tribology, 1992

Research paper thumbnail of Vibration absorption using non-dissipative complex attachments with impacts and parametric stiffness

The Journal of the Acoustical Society of America, 2009

The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.

Research paper thumbnail of Energy equipartition and frequency distribution in complex attachments

The Journal of the Acoustical Society of America, 2009

As reported in several recent publications, an undamped simple oscillator with a complex attachme... more As reported in several recent publications, an undamped simple oscillator with a complex attachment that consists of a set of undamped parallel resonators can exhibit unusual energy sharing properties. The conservative set of oscillators of the attachment can absorb nearly all the impulsive energy applied to the primary oscillator to which it is connected. The key factor in the ability of the attachment to absorb energy with near irreversibility correlates with the natural frequency distribution of the resonators within it. The reported results also show that a family of optimal frequency distributions can be determined on the basis of a variational approach, minimizing a certain functional related to the system response. The present paper establishes a link between these optimal frequency distributions and the energy equipartition principle: optimal frequency distributions are those that spread the injected energy as uniformly as possible over the degrees of freedom or over the modes of the system. Theoretical as well as numerical results presented support this point of view.

Research paper thumbnail of Theoretical foundations of apparent-damping phenomena and nearly irreversible energy exchange in linear conservative systems

The Journal of the Acoustical Society of America, 2007

This paper discusses a class of unexpected irreversible phenomena that can develop in linear cons... more This paper discusses a class of unexpected irreversible phenomena that can develop in linear conservative systems and provides a theoretical foundation that explains the underlying principles. Recent studies have shown that energy can be introduced to a linear system with near irreversibility, or energy within a system can migrate to a subsystem nearly irreversibly, even in the absence of dissipation, provided that the system has a particular natural frequency distribution. The present work introduces a general theory that provides a mathematical foundation and a physical explanation for the near irreversibility phenomena observed and reported in previous publications. Inspired by the properties of probability distribution functions, the general formulation developed here is based on particular properties of harmonic series, which form the common basis of linear dynamic system models. The results demonstrate the existence of a special class of linear nondissipative dynamic systems that exhibit nearly irreversible energy exchange and possess a decaying impulse response. In addition to uncovering a new class of dynamic system properties, the results have far-reaching implications in engineering applications where classical vibration damping or absorption techniques may not be effective. Furthermore, the results also support the notion of nearly irreversible energy transfer in conservative linear systems, which until now has been a concept associated exclusively with nonlinear systems.

Research paper thumbnail of Near-irreversibility in a conservative linear structure with singularity points in its modal density

The Journal of the Acoustical Society of America, 2006

Through two complementary approaches, using modal response and wave propagation, the analyses pre... more Through two complementary approaches, using modal response and wave propagation, the analyses presented here show the conditions under which a decaying impulse response, or a nearly irreversible energy trapping, takes place in a linear conservative continuous system. The results show that the basic foundation of near-irreversibility or apparent damping rests upon the presence of singularity points in the modal density of dynamic systems or, analogously, in the wave-stopping properties associated with these singularities. To illustrate the concept of apparent damping in detail, a simple undamped beam is modified to introduce a singularity point in its modal density distribution. Simulations show that a suitable application of a compressive axial force to an undamped beam placed on an elastic foundation attenuates its impulse response with time and develops the characteristics of a nearly irreversible energy trap.

Research paper thumbnail of Pseudo-damping in undamped plates and shells

The Journal of the Acoustical Society of America, 2006

Pseudo-damping is a counter-intuitive phenomenon observed in a special class of linear structures... more Pseudo-damping is a counter-intuitive phenomenon observed in a special class of linear structures that exhibit an impulse response characterized by a decaying amplitude, even in the absence of any dissipation mechanism. The conserved energy remains within but designated parts of the system. Pseudo-damping develops when the natural frequency distribution of the system includes condensation points. The recently formulated theoretical foundation of this phenomenon, based on mathematical properties of special trigonometric series, makes it possible to describe a class of mechanical systems capable of displaying pseudo-damping characteristics. They include systems with discrete oscillators and one-dimensional continuous beamlike structures already reported by the authors in recent studies. This paper examines development of pseudo-damping phenomenon in two-dimensional structures, using plates and shells as examples, and shows how a preloaded plate on an elastic foundation can lead to pseudo-damping. Moreover, in the case of curved shell elements examined here, pseudo-damping can result due to the curvature of the structure, which naturally introduces condensation points in the modal density.

Research paper thumbnail of Finite Element Model for Hysteretic Friction Damping of Traveling Wave Vibration in Axisymmetric Structures

Volume 9: Mechanical Systems and Control, Parts A, B, and C, 2007

Research paper thumbnail of Optimizing Distributed-Contact Friction in Ring Dampers

Volume 1: 20th Biennial Conference on Mechanical Vibration and Noise, Parts A, B, and C, 2005

Research paper thumbnail of Distributed friction damping of travelling wave vibration in rods

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2008

A ring damper can be affixed to a rotating base structure such as a gear, an automotive brake rot... more A ring damper can be affixed to a rotating base structure such as a gear, an automotive brake rotor or a gas turbine's labyrinth air seal. Depending on the frequency range, wavenumber and level of preload, vibration of the base structure can be effectively and passively attenuated by friction that develops along the interface between it and the damper. The assembly is modelled as two rods that couple in longitudinal vibration through spatially distributed hysteretic friction, with each rod having periodic boundary conditions in a manner analogous to an unwrapped ring and disc. As is representative of rotating machinery applications, the system is driven by a travelling wave disturbance, and for that form of excitation, the base structure's and the damper's responses are determined without the need for computationally intensive simulation. The damper's performance can be optimized with respect to normal preload, and its effectiveness is insensitive to variations in preload or the excitation's magnitude when its natural frequency is substantially lower than the base structure's in the absence of contact.

Research paper thumbnail of Radiation from Bending Waves in the Presence of a Flow Boundary Layer

Journal of Vibration and Acoustics, 1993

The effect of refraction by a flow boundary layer on the radiation of sound from a bending wave o... more The effect of refraction by a flow boundary layer on the radiation of sound from a bending wave on a plate is investigated. Based on the relative velocities of the bending wave and the flow, four qualitatively different regimes are identified and results of numerical studies of radiation impedance and power emission are presented and discussed. For comparison, the role

Research paper thumbnail of S123 a New Recommendation at the Following of Uro-Oncological Tumors: Reminder Short Message Service (SMS)

European Urology Supplements, 2011

eur urol Suppl 2011;10(9):604 and fluoroscopy technology. The harmful effects of ionizing radiati... more eur urol Suppl 2011;10(9):604 and fluoroscopy technology. The harmful effects of ionizing radiation (such as hair loss, erythema, and dermatitis) were recognized shortly after the discovery of the X ray by Wilhelm Conrad Roentgen in 1895. Because of these effects, the hospital personnel needed to avoid of overexposure. Notwithstanding these observations, protection of staff exposed to X-rays and gamma rays from radium was poorly coordinated. With these facts in mind, we undertook a survey of urologists working in Turkey to evaluate their attitude and behavior of the protection of the radiation exposure during some of the more common endourological procedures requested. material & methods: The study included urology residents, specialists and all academic degree of urologists from university hospitals, education and research hospitals, state hospitals and private hospitals in Turkey. The questionnaire that was administered to the study participants was composed of demographic questions concerning age, gender, institution, current status of duty, as well as questions about uses of dosimeters and flexible protective clothing such as aprons, thyroid shields, eyeglasses, and gloves during fluoroscopy guided endourological procedures. The questionnaire was sent to 1796 urologists by e-mail between May and June 2011. Results: Of the 1796 questionnaires sent, 394 questionnaires were answered and 363 of these had completed answers. Mean age of the participants was 40,10±8,44 years. The numbers of doctors who are exposed the ionizing radiation was 307 (84,58%). 192 (62,54 %) participants reported that they expose the radiation from 1 to 5 endourological procedures per week. The numbers of doctors exposed the radiation above 5 times per week and less than 1 time per week were 30(9,77 %) and 85(27,69 %) respectively. behavior of urologists about uses of dosimeters and flexible protective clothes.

Research paper thumbnail of Effects of Viscous and Thermal Losses on Radiation from an Infinite Plate

Journal of Vibration and Acoustics, 1992

Research paper thumbnail of Frequency intermittency and energy pumping by linear attachments

Journal of Sound and Vibration, 2014

The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.

Research paper thumbnail of A Theoretical and Experimental Investigation of Contact Loss in the Clearances of a Four-Bar Mechanism

Journal of Mechanisms Transmissions and Automation in Design, 1986

Research paper thumbnail of Measurement of transient friction hysteresis

Research paper thumbnail of Sources of noise and vibration in a mechanical system with clearances

Inherent in the design of any mechanism with moving parts is the requirement for clearances. In t... more Inherent in the design of any mechanism with moving parts is the requirement for clearances. In the case of an engine, this requirement is enhanced by the extremes of both load and temperature under which some parts are expected to function. Collision of the parts in these connections where clearances exist are influenced by transient combustion forces as well as inertial forces. Impacts in the joints of a system are a major source of sound, vibration and wear. The mechanism of sound generation follows the dynamic response of the system components. The resulting transient acoustic field is comprised of radiation from the forced and free vibrations of the system. Radiation due to inertial forces are generally of lower frequency whereas sounds produced by the impacts exhibit higher levels with higher spectral content. This study investigates the sound and vibration response of a four-bar mechanism in the absence of external forces.

Research paper thumbnail of Fracture of polymeric materials under tensile loads

Research paper thumbnail of Dissipation in a finite-size bath

Physical Review E, 2011

We investigate the interaction of a particle with a finite-size bath represented by a set of inde... more We investigate the interaction of a particle with a finite-size bath represented by a set of independent linear oscillators with frequencies that fall within a finite bandwidth. We discover that when the oscillators have particular frequency distributions, the finite-size bath behaves much as an infinite-size bath exhibiting dissipation properties and thus allowing irreversible energy absorption from a particle immersed in it. We also present a reinterpretation of the Langevin equation using a perturbation approach in which the small parameter represents the inverse of the number of oscillators in the bath, elucidating the relationship between finite-size and infinite-size bath responses.

Research paper thumbnail of Acoustical observation of bubble oscillations induced by bubble popping

Physical Review E, 2007

Acoustic measurements of aqueous foams show three distinct radiation mechanisms that contribute t... more Acoustic measurements of aqueous foams show three distinct radiation mechanisms that contribute to the sound pressure field: oscillations of a bubble surface that precede popping due to the instability of thin liquid film, impulsive radiation due to bursts of bubbles, and oscillations from neighboring bubbles excited by a burst bubble. The movies captured by a fast camera confirm that the bubbles adjacent to a breaking bubble oscillate under the influence of the pressure generated by the burst bubble. The spectra of resulting transient sounds fall in the range of 2-8 kHz and those from bubble oscillations correlate well with the bubble size.

Research paper thumbnail of Relation of Dry-Friction to Surface Roughness

Journal of Tribology, 1997

Research paper thumbnail of Discussion: “Normal and Angular Motions at Rough Planar Contacts During Sliding With Friction” (Hess, D. P., and Soom, A., 1992, ASME J. Tribol., 114, pp. 567–578)

Journal of Tribology, 1992

Research paper thumbnail of Vibration absorption using non-dissipative complex attachments with impacts and parametric stiffness

The Journal of the Acoustical Society of America, 2009

The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.

Research paper thumbnail of Energy equipartition and frequency distribution in complex attachments

The Journal of the Acoustical Society of America, 2009

As reported in several recent publications, an undamped simple oscillator with a complex attachme... more As reported in several recent publications, an undamped simple oscillator with a complex attachment that consists of a set of undamped parallel resonators can exhibit unusual energy sharing properties. The conservative set of oscillators of the attachment can absorb nearly all the impulsive energy applied to the primary oscillator to which it is connected. The key factor in the ability of the attachment to absorb energy with near irreversibility correlates with the natural frequency distribution of the resonators within it. The reported results also show that a family of optimal frequency distributions can be determined on the basis of a variational approach, minimizing a certain functional related to the system response. The present paper establishes a link between these optimal frequency distributions and the energy equipartition principle: optimal frequency distributions are those that spread the injected energy as uniformly as possible over the degrees of freedom or over the modes of the system. Theoretical as well as numerical results presented support this point of view.

Research paper thumbnail of Theoretical foundations of apparent-damping phenomena and nearly irreversible energy exchange in linear conservative systems

The Journal of the Acoustical Society of America, 2007

This paper discusses a class of unexpected irreversible phenomena that can develop in linear cons... more This paper discusses a class of unexpected irreversible phenomena that can develop in linear conservative systems and provides a theoretical foundation that explains the underlying principles. Recent studies have shown that energy can be introduced to a linear system with near irreversibility, or energy within a system can migrate to a subsystem nearly irreversibly, even in the absence of dissipation, provided that the system has a particular natural frequency distribution. The present work introduces a general theory that provides a mathematical foundation and a physical explanation for the near irreversibility phenomena observed and reported in previous publications. Inspired by the properties of probability distribution functions, the general formulation developed here is based on particular properties of harmonic series, which form the common basis of linear dynamic system models. The results demonstrate the existence of a special class of linear nondissipative dynamic systems that exhibit nearly irreversible energy exchange and possess a decaying impulse response. In addition to uncovering a new class of dynamic system properties, the results have far-reaching implications in engineering applications where classical vibration damping or absorption techniques may not be effective. Furthermore, the results also support the notion of nearly irreversible energy transfer in conservative linear systems, which until now has been a concept associated exclusively with nonlinear systems.

Research paper thumbnail of Near-irreversibility in a conservative linear structure with singularity points in its modal density

The Journal of the Acoustical Society of America, 2006

Through two complementary approaches, using modal response and wave propagation, the analyses pre... more Through two complementary approaches, using modal response and wave propagation, the analyses presented here show the conditions under which a decaying impulse response, or a nearly irreversible energy trapping, takes place in a linear conservative continuous system. The results show that the basic foundation of near-irreversibility or apparent damping rests upon the presence of singularity points in the modal density of dynamic systems or, analogously, in the wave-stopping properties associated with these singularities. To illustrate the concept of apparent damping in detail, a simple undamped beam is modified to introduce a singularity point in its modal density distribution. Simulations show that a suitable application of a compressive axial force to an undamped beam placed on an elastic foundation attenuates its impulse response with time and develops the characteristics of a nearly irreversible energy trap.

Research paper thumbnail of Pseudo-damping in undamped plates and shells

The Journal of the Acoustical Society of America, 2006

Pseudo-damping is a counter-intuitive phenomenon observed in a special class of linear structures... more Pseudo-damping is a counter-intuitive phenomenon observed in a special class of linear structures that exhibit an impulse response characterized by a decaying amplitude, even in the absence of any dissipation mechanism. The conserved energy remains within but designated parts of the system. Pseudo-damping develops when the natural frequency distribution of the system includes condensation points. The recently formulated theoretical foundation of this phenomenon, based on mathematical properties of special trigonometric series, makes it possible to describe a class of mechanical systems capable of displaying pseudo-damping characteristics. They include systems with discrete oscillators and one-dimensional continuous beamlike structures already reported by the authors in recent studies. This paper examines development of pseudo-damping phenomenon in two-dimensional structures, using plates and shells as examples, and shows how a preloaded plate on an elastic foundation can lead to pseudo-damping. Moreover, in the case of curved shell elements examined here, pseudo-damping can result due to the curvature of the structure, which naturally introduces condensation points in the modal density.