People

Supervisor 
Çetin YILMAZ
cetin.yilmaz@boun.edu.tr

BS - Mechanical Engineering, Boğaziçi University '00
MS - Mechanical Engineering, University of Michigan '01
MS - Mathematics, University of Michigan '02
PhD - Mechanical Engineering, University of Michigan '05

Current Students 
PhD Students 
Osman YÜKSEL
osman.yuksel@boun.edu.tr

BS - Bogazici University, Chemical Engineering '08
MS - Bogazici University, Mechanical Engineering '12
PhD - Bogazici University, Mechanical Engineering - present

Thesis topic: Shape and topology optimization of phononic band gap structures

Mustafa Umut ÖZCAN
umutozcan84@gmail.com

BS - Marmara University, Mechanical Engineering '07
MS - Koc University, Mechanical Engineering '09
PhD - Bogazici University, Mechanical Engineering - present

Thesis topic: Dynamic modeling of rubber mounts (Co-supervised with Prof. Fazıl Ö. Sönmez)

MS Students 
Adil Han ORTA
adil.orta@gmail.com

BS - Bogazici University, Mechanical Engineering '15
MS - Bogazici University, Mechanical Engineering - present

Thesis topic: Axial to rotary motion conversion mechanisms

Alumni 
MS Students 

Efe ÖZKAYA 

BS - Bogazici University, Mechanical Engineering '13
MS - Bogazici University, Mechanical Engineering '16
 

Title: Eddy current damping (ECD) applications for vibration isolation purposes

Abstract: In this thesis, passive eddy current damping (ECD) is used in two different novel designs that show vibration stop bands. One includes an inertial amplification mechanism and the other includes local resonators. In both designs, copper tube-ring magnet couple is used as a passive eddy current damper. The reason for such usage arises from the fact that this damping method does not require an external power source and its damping constant can be easily changed. The change in the damping constant is obtained by adjusting the distance between the conducting material which is the copper tube and the magnetic field generator which is the magnet. In the first design, which includes an inertial amplification mechanism, vertical motion is dampened by copper tube-ring magnet couple, which serves as a viscous damping component. For this design, a prototype is produced and experimental measurements are taken to show the effect of damping on vibration amplitudes. In the second design, which includes local resonators, it is aimed to generate a vibration stop band at low frequencies. The effect of ECD on stop band width, resonance peaks and anti-resonance depth on the frequency response results are observed. For comparison purposes, frequency response of this design is obtained analytically, experimentally and numerically. By both of these designs, it is shown that ECD can be used in different structures to attenuate vibrations.

Currently, PhD Student in the Department of Mechanical Engineering at Stevens Institute of Technology

Gizem ACAR 

BS - Bogazici University, Mechanical Engineering '10
MS - Bogazici University, Mechanical Engineering '12
 

Title: Phononic band gaps in two-dimensional periodic structures with inertial amplification mechanisms

Abstract: In this thesis, a 2D periodic structure equipped with inertial amplification mechanisms is designed. The structure is optimized to obtain  a wide and deep phononic band gap in low frequency ranges. The aim is to prevent wave propagation, hence suppress mechanical vibrations. In the literature, there are two common ways to generate band gaps, Bragg scattering and resonance scattering. Alternative to these methods, inertial amplification method is used in this study. Different types of inertial amplification mechanisms are discussed. Then, a 1D distributed parameter model, which is equivalent to the proposed inertial amplification mechanism, is used to construct the 2D periodic structure. First two natural frequencies of the 1D model are found analytically. The model is designed to have a band gap between these two natural frequencies. Yet, in order to calculate the frequencies more accurately and easily optimize the model, Finite Element Analysis is conducted on the model. The 2D periodic structure is composed of two different 1D unit models. These models are optimized so that the 2D structure has a wide and deep band gap at low frequencies. Prototypes of the two 1D unit models and the 2D structure are produced, and frequency responses of them are obtained by experimental modal analysis. The experimental and numerical frequency response results match quite well, which validate that the 2D structure has a wide and deep band gap.

Currently, PhD Student in the Department of Mechanical Engineering at Michigan State University

Osman YÜKSEL

BS - Bogazici University, Chemical Engineering '08
MS - Bogazici University, Mechanical Engineering '12

Title: Active noise control in a duct with flow (Co-supervised with Prof. Eşref Eşkinat)

Abstract: In this thesis, active noise control in a duct with flow is investigated. A one dimensional acoustic duct model, in which fluid    medium  inside the duct has a mean flow velocity, is presented. The acoustic duct model is solved in Laplace domain and infinite dimensional system transfer functions are obtained. For controller designs, appropriate microphone and noise canceling source locations inside the duct are determined. In numerical studies, ideal boundary condition case (open end) and general boundary condition case (frequency dependent impedance end) are investigated. For these boundary conditions, low order finite dimensional transfer function approximations of actual system transfer functions are obtained. It is found that, in a selected frequency range, approximations represent actual system in a satisfactory way. By using approximated system transfer functions, low order optimal H2 and H? controllers are synthesized via linear matrix inequalities method found in linear time invariant finite dimensional control theory. Closed loop frequency response and time domain simulations show that the controllers successfully suppress unwanted sound which propagates along the duct.

Currently, PhD Student in the Department of Mechanical Engineering at Boğaziçi University

Kamil KOÇAK

BS - Bogazici University, Mechanical Engineering '08
MS - Bogazici University, Mechanical Engineering '11
  

Title: Design of a compliant lever-type passive vibration isolator using quasi-zero-stiffness mechanism

Abstract: In this thesis, design and analysis of a new passive vibration isolator usingcompliant lever-type and quasi-zero-stiffness (QZS) mechanisms is provided. At first, various systems are designed without QZS mechanism. It is aimed to create antiresonance frequency by using lever-type mechanism. Anti-resonance occurs when the inertial force generated by the levered mass cancels the spring force. Since the lever type mechanism increases the effect of the isolator mass, it is capable of isolating a relatively heavy body with little mass in itself. These designs are examined analytically, numerically and experimentally. First, the systems are modeled with lumped and distributed parameters. Then, the system is modeled by finite element method (FEM). Optimization of the design is made to get highest isolation in the widest frequency range. As a result, the optimized design with desired properties is found. At last, the optimized design is manufactured and tested. In the second part, QZS mechanism is attached to the design. This design consists of multiple parts which renders the system adjustable. The equations of negative stiffness mechanism are provided. FEM results are compared with the analytical results. The working principle of adjustable mechanisms are explained. As a result, design and analysis of a new compliant lever type passive vibration isolator using QZS mechanisms is provided. Finally, the design is manufactured and tested. This design can also be adjusted for the designed vibration isolation frequency range and good isolation levels can be acquired especially at low frequencies.

Currently, PhD Student in the Department of Mechanical Engineering at Georgia Institute of Technology

Mustafa Ali ACAR

BS - Bogazici University, Mechanical Engineering '08
MS - Bogazici University, Mechanical Engineering '11 

Title: Design of an adaptive-passive dynamic vibration absorber composed of a string-mass system equipped with negative stiffness tension  adjusting mechanism 

Abstract: In this study, a new adaptive-passive dynamic vibration absorber design is discussed. This proposed design is composed of a stiff  string under tension with a central mass attachment as a dynamic vibration absorber (DVA), a negative stiffness mechanism as a string tension adjustment aid and a tuning controller to make it adaptive. Dynamic properties of adaptive-passive DVA systems are adjusted in real-time by generally varying their stiffness. The adaptive-passive DVA design subject to this thesis uses the string tension as a tuning parameter. The dependency of the natural frequencies of this system on the string tension is analyzed using finite element method and verified analytically. Additionally, a method for adjusting the string tension with almost zero effort is proposed. To achieve this goal, the design incorporated a negative stiffness element to create a quasi-zero stiffness and constant zero-force mechanism when combined with the string. Force-displacement analysis of a system composed of a pre-loaded spring and a rigid link is examined analytically. It was shown that the system can have constant negative stiffness behavior. A string tension adjustment algorithm is created which tunes the DVA system depending on the magnitude and frequency of the most dominant component of the vibration signal. Finally, a prototype of the complete design is built. A series of experimental procedures are conducted on the prototype with the intention of verifying the theoretical calculations. Results obtained from these experiments are also given in the thesis.

Currently, PhD Student in the Department of Mechanical Engineering at Michigan State University

Veysel DOĞAN

BS - Middle East Technical University, Mechanical Engineering '84
MS - Bogazici University, Mechanical Engineering '11
  

Title: Tuned mass damper applications on slender structures to improve earthquake and wind response 

Abstract: The subject of the study is to improve earthquake and wind response of slender structures by using Tuned Mass Damper (TMD)  applications. TMD application on main structure reduces the structural response amplitude by creating additional damping. A vibration analysis employing transfer matrices was applied for a slender structure with varying cross sections. The analysis is applied to an existing reinforced concrete minaret structure as a case study due to its poor dynamic response and lack of sufficient studies. Best applicable and efficient TMD type was investigated to improve the dynamic response of selected minaret structure. Structure response was analyzed with and without TMD installation by using MATLAB for discrete mass model. Harmonic excitation was considered to simulate the ground motion and improvements in the response were discussed. SAP2000 software was also used to analyze the same structure through Finite Element Method technique. 1999 Kocaeli and 1999 Düzce earthquakes ground motion records were used to verify the effectiveness of the developed TMD. Wind response is also considered. Detailed fabrication drawings were prepared by considering the challenging installation constraints. Feasibility study of the developed TMD was discussed for applications either in new structures or retrofits.

Currently, General Manager of Doka Endüstri

PhD Students 

Akın OKTAV 

BS - Istanbul Technical University, Mechanical Engineering '03
MS - Bogazici University, Automotive Engineering '05
PhD - Bogazici University, Mechanical Engineering '16

Title: Computational and experimental investigation of low frequency noise in passenger vehicles (Co-supervised with Prof. Günay Anlaş)

Abstract: Acoustic comfort of passenger vehicles has become a significant competition factor in the market, as much as the others, i.e. styling, power, fuel consumption and budget. Vibro-acoustic response studies play an important role in developing countermeasures to noise problems, before or after vehicle launch. In this study, low frequency noise characteristics of passenger vehicles are addressed. Vehicle noise variability and dominant paths that cause low frequency booms are investigated. To diagnose the cause of variability, a systematic approach is proposed, where all steps are explained briefly. It is deduced that predominant paths, which are found to be the main contributors of diagnosed booms, are also the root causes of variability observed. Current practice of experimental transfer path analysis is discussed in the context of trade-offs between accuracy and time cost. An overview of methods, which propose solutions for structure borne noise, is given, where assumptions, drawbacks and advantages of methods are stated theoretically. Applicability of methods is also investigated, where the engine induced structure borne noise of the sedan studied is taken as a reference problem. Sources of measurement errors, processing operations that affect results and physical obstacles faced in the application are analyzed. Effects of damping, reasons and methods to analyze them are discussed in detail. In this regard, a new procedure, which increases the accuracy of results, is also proposed. Coupled vibro-acoustic response of the sedan is analyzed, and the effect of folding rear seat aperture is studied. An analytical solution is proposed to calculate acoustic eigenfrequencies. Then, uncoupled acoustic eigenfrequencies of the actual cavity, where trunk and cabin cavities are connected through the aperture are computed. It is shown that planar acoustic eigenfrequencies of the sedan can approximately be calculated using the analytical solution proposed. To further clarify the impact of folding rear seat aperture, coupled vibroacoustic response of the sedan is analyzed through different case studies. It is observed that booms are highly correlated with the calculated uncoupled planar acoustic eigenfrequencies. It is concluded that proposed analytical solution can be effectively used in calculation of acoustic eigenfrequencies and identification of booms, rather performing a detailed computational work. Experimental modal analysis studies are carried out to update the computational model. The updated model is then used in modification prediction studies. Finally, it is shown that the procedures proposed works well with real problems.

Currently, Specialist in Vibration and Acoustics Laboratory at Boğaziçi University

Semih TANIKER

BS - Bogazici University, Mechanical Engineering '08
MS - Bogazici University, Mechanical Engineering '11
PhD - Bogazici University, Mechanical Engineering '15

Title: Design and analysis of three-dimensional phononic band gap structures with embedded inertial amplification mechanisms

Abstract: In this study, three-dimensional (3D) phononic band gap structures are investigated. First, infinite and finite periodic simple cubic, body centered cubic and face centered cubic lattices with and without inertial amplification mechanisms are considered. These 3D lattices are modeled with mass and spring elements that are parametrically varied to observe their effects on band gap (stop band) limits. When inertial amplification mechanisms are used in the infinite periodic lattices, wide low frequency band gaps are generated. Moreover, wide and deep phononic gaps are obtained by using moderate amount of unit cells in the case of finite periodic lattices. Then, 3D phononic band gap structures are formed using distributed parameter inertial amplification mechanisms. The resonance and antiresonance frequencies that characterize the first vibration stop band of the building block mechanism are obtained analytically and by finite element method. The mechanism is optimized to yield wide vibration stop bands in an octahedron and a 2x3 array of octahedrons. Furthermore, these structures are manufactured using a 3D polymer printer and their experimental frequency responses are obtained. Structural damping is added to the finite element model in order to match the resonant peak magnitudes of the numerical and experimental frequency response results. Moreover, a new inertial amplification mechanism is designed by adding constraining beams that reduce the degree of freedom of the initial mechanism. Consequently, ultra wide band gaps at low frequencies are attained. To sum up, it is demonstrated that the 3D structures built with inertial amplification mechanisms are capable of isolating excitations in longitudinal and two transverse directions in a very wide frequency range. Prototypes of the two 1D unit models and the 2D structure are produced, and frequency responses of them are obtained by experimental modal analysis. The experimental and numerical frequency response results match quite well, which validate that the 2D structure has a wide and deep band gap.

 

Currently, R&D Engineer at Roketsan