Dept. of Mechanical Engineering - Master's degree

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https://hdl.handle.net/11693/13901

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Open Access
Modelling a janus particle activated by an optical potential
(Bilkent University, 2023-09) Rauf, Umar
The flow around a Janus particle under an optical potential is investigated. The thermal effects are majorly overlooked while studying the Janus particle although fluid heating caused by optical potential might negligible but the fluid heating caused due to Janus particle acting as a heating source is not negligible. The stream function approach is used to model the behavior of an activated Janus particle. The explicit finite difference method (FDM) is used to numerically obtain the solution. BIL-FLOW, an in-house FDM code is developed and validated for flow around a Janus particle under an optical potential. Additionally, The BIL-OP, an in-house optical module based on geometric ray approximation is developed and validated. BIL-OP utilizes linear algebra to calculate three dimensional (3D) optical force.
Embargo
Microwave resonant sensor integration with impedance cytometry in microfluidic platform for probing micro-scale dielectric permittivity
(Bilkent University, 2023-09) Tefek, Uzay
This thesis presents a novel multiphysical sensor that integrates low-frequency impedance cytometry with high-frequency microwave capacitance sensing. The characterization of microscale objects, including microparticles and cells, is essential in various scientific disciplines, such as biology, materials science, and environmental science. Accurate identification and classification of these microscale entities are critical for applications ranging from drug delivery optimization to environmental impact assessment, however, the current techniques fall short in terms of the rapidity and cost-effectiveness necessary for analyzing extensive populations. To address this challenge, our hybrid sensor combines low-frequency impedance cytometry and high-frequency microwave capacitance sensing for material characterization based on dielectric permittivity. This integration offers a rapid, cost-effective, and highly accurate method for identifying and characterizing microscale particles and cells. Experimental studies demonstrate the sensor’s efficacy, achieving remarkable signal-to-noise ratios. The sensor’s versatility ex-tends monitoring permittivity changes in single cells exposed to fixing agents offering valuable insights into cellular properties. In summary, this thesis introduces an innovative multiphysical sensor that advances microscale entity analysis, enabling rapid and precise identification and characterization.
Open Access
Development of a fault-tolerant model predictive controller for vehicle lateral stability
(Bilkent University, 2023-08-01) Köysüren, Muhammed Kemal
Recently, there has been an increased interest in the automotive industry using scaled test vehicles to test the real-time performance of modeling and control algorithms. A scaled prototype, specifically developed and instrumented for enhancing vehicle lateral stability, offers distinct advantages in terms of cost reduction and the ability to repeat tests under various vehicle maneuvering scenarios rapidly. First, the mechatronic design of a 1:8 scaled electric vehicle with 4-wheel-drive and 4-wheel-independent-steering was done and the prototype vehicle was built. Plant model parameters such as the cornering coefficients of the tires are estimated using various methods such as traditional neural network training, a Physics Informed Deep Learning (PIDL) algorithm, and Pacejka’s tire modeling procedure. Secondly, a fault-tolerant reconfigurable model predictive controller (MPC) is proposed to enhance reference tracking for four-wheel-drive and four-wheel-steering vehicles under concurrent steering actuator faults. The method detects, isolates, and estimates fault magnitudes, which inform adjustments to the MPC formula. Performance validation is conducted through obstacle avoidance maneuvers with a control-oriented vehicle model and real-time applicability tests with a Processor-in-the-Loop system using a high-fidelity vehicle model. The test results confirm the proposed algorithm’s superior performance over the conventional MPC. Lastly, a computationally efficient two-path optimal control allocation method is proposed to reduce controller block execution time in vehicle ECU. High-fidelity results prove the computational cost reduction of the proposed algorithm over the conventional allocation method.
Open Access
Towards improved adaptive control: human pilot models & memory-augmented architectures
(Bilkent University, 2023-07) Habboush, Abdullah
To facilitate the implementation of adaptive control methods, this dissertation introduces novel solutions to key problems that hinder the employment of adaptive controllers in industrial applications. We present techniques that are inspired by humans’ versatility in the control loop, where we focus on understanding how humans adapt in the face of anomalies, and how they use their memory to better recover from them. Towards that end, we propose adaptive human pilot models suited for the prediction of human behavior in the loop with an adaptive controller. These models serve as valuable tools to test the interactions between human pilots and adaptive control systems in the simulation environment in order to ensure safe operation in the presence of an anomaly. Furthermore, the development of the models is carried out based on rigorous Lyapunov stability analyses, which can provide analytical insights into how to better design adaptive con-trollers for manned applications. Apart from their unfavorable interactions with human pilots, another issue that accounts for the scarce employment of adaptive controllers in piloted applications lies in their transient characteristics. While numerous works are devoted to improving the transients of adaptive control systems, in this dissertation, we focus on taking advantage of it ﬁrst by providing adaptive controllers with human-like memory capabilities. We propose a memory architecture that can make use of stored data about the transients of previously experienced anomalies to aid in obtaining a resilient system against uncertainties. Thus, the proposed memory architecture enables adaptive controllers to rely on memory rather than exploration to better recover from familiar anomalies. The eﬀectiveness of the architecture is validated through numerical simulations, and a rigorous Lyapunov stability analysis is provided.
Open Access
Isogeometric boundary element formulation for deformable particles in microchannel confinement
(Bilkent University, 2023-08) Gümüş, Özgür Can
Numerical simulations of deformable particles are essential to understand quantities that are not possible with experimental techniques. The boundary element method is an advantageous technique to analyze deformable particles in viscous flow conditions since it reduces the dimensionality of the problem by one for linear partial differential equations. Isogeometric boundary element formulation is proposed to model the motion of deformable particles which provides unique ad-vantages in terms of the higher-order continuity of elements and exact geometry representation. Moreover, it enables the calculation of surface normal and curvature analytically. Deformable particles, more specifically droplets, may undergo high deformation which deteriorates the mesh. Moreover, numerical inaccuracies result in a nonphysical change in the volume of the particle. Hence, volume correction and mesh relaxation algorithms are implemented in the isogeometric boundary element method to alleviate the aforementioned numerical artifacts. Without loss of generality, droplets in free and bounded flow cases are formulated and several benchmark problems are solved to assess the accuracy of the proposed formulation. Isogeometric boundary element method supported by stabilization methods explained in the study allows for obtaining stable and accurate results with low-resolution simulations.
Open Access
Active lubrication interfaces with tunable micro-textures
(Bilkent University, 2023-07) Pekol, Sena
This thesis investigates a homogenization-based space-time optimization framework in the context of hydrodynamic lubrication in order to design micro-textures which can be actively controlled through external stimuli. The response at the interface is established via the Reynolds equation to describe the physics of the lubrication for a small film thickness. Subsequently, the interface is subjected to multiscale analysis and effective macroscopic parameters are derived via homogenization method. In order to calculate the macroscopic parameters, Finite Element (FE) formulation is employed and the implementation of the parameters in the in-house FE code is demonstrated. For the suboptimality problem due to typically employed fixed unit cell in FE analysis, a geometry optimization scheme is developed. Thereafter, a sensitivity based topology optimization framework is introduced with the aim of identifying the spatial distribution and temporal variation of the micro-texture, and the shape of the unit cell which together help achieve the targeted lubrication response. The performance of the employed framework is assessed through objectives which ultimately determine the macroscopic flux at the interface as well as the frictional traction that is associated with the macroscopic dissipation at the interface. Finally, three-dimensional realizations are constructed for active micro-textures by adopting a readily deployable experimental architecture.
Open Access
Magnetic microfluidic platform for bacteria isolation and detection
(Bilkent University, 2023-07) Babaie, Zahra
S. pneumoniae is widely recognized as a leading cause of respiratory infections worldwide, often resulting in high mortality rates. However, the advent of microfluidic technologies has brought significant advancements, including the simplified, sensitive, cost-effective, and rapid approach to pneumococcal bacteremia detection. In this study, we have revised the microfluidic magnetic platform patented by our research group to isolate the bacteria using synthesized micro-magnetic beads bonded with aptamer. In the initial phase of the experiments, the inlet flow rate was adjusted to match the rotational speed of the magnetic unit to retain the micro-magnetic beads within the fabricated microfluidic channel effectively. Subsequently, a series of experiments were conducted to assess microfluidics-based bacteria isolation at various flow rates, involving seven different concentrations of bacteria, and the outcomes were compared with those obtained through batch-type isolation method. The isolation selectivity of target bacteria from complex samples was also assessed with control bacteria at two different concentrations. Furthermore, the isolated micro-magnetic beads-bacteria complexes were also transferred to interdigitated microelectrodes unit for electro-chemical impedance spectroscopy analysis. The ultimate objective is to integrate the isolation and detection components on the same microfluidic platform which leads to a rapid and precise diagnosis of bacterial infections.
Open Access
Design, characterization, and applications of soft 3D printed strain gauges
(Bilkent University, 2023-07) Özbek, Doğa
The development of soft sensors for integration into untethered miniature robots is significant for improving their environmental perception in physically challenging scenarios, such as collapsed buildings after an earthquake. The primary objective is to design and manufacture reliable soft sensors that serve as structural and sensing elements within the robots, eliminating the need for post-processing methods like data-driven learning and optimization. The soft sensors employ resistive sensing, similar to strain gauges, and are implemented on a Wheatstone bridge to convert resistive changes into voltage changes under me-chanical actuation or deformation. The study explores two categories of soft sensor designs: sheet-type and 3D shaped sensors. Sheet-type sensors are embedded in the C-legs of a soft quadruped robot (SQuad), enabling gait control, while 3D shaped sensors are structurally integrated into the robots to enhance environmental perception. Manufacturing of the soft sensors is made accessible and efficient through 3D printing technology, using conductive Thermoplastic Polyurethane (cTPU) as the printing material. Challenges arise in integrating the soft sensors into the robots while preserving their soft nature, locomotion, and agility. The thesis addresses these challenges by implementing the soft sensor concept in various robots and their parts, including the C-leg of SQuad, Modular Soft Quadruped (M-SQuad), Suspensionized Soft Quadruped (S-SQuad), Sensorized Collision Resilient Robot (SCoReR), and a tail for Reconfigurable Miniature Modular Robot (ReMBot). The soft sensors enable different functionalities to these robots, such as gait control feedback, obstacle detection, inclination detection, and collision detection, enhancing the adaptability of the robots in physically challenging environments. The thesis highlights the potential of soft 3D printed strain gauges. The ease of manufacturing and cost-efficiency of these sensors make them promising for applications in wearable robots and human-computer interfaces. Future directions are highlighted, emphasizing the need for detailed sensor characterization experiments and the development of detection algorithms to improve reliability. Additionally, a dynamic model of the coil-shaped sensors is proposed to simulate resistance changes, streamlining the design process without repetitive manufacturing iterations. As a result, this thesis presents a reliable soft sensor design, manufacturing, and integration into untethered miniature robots. The outcome of this work demonstrates the effectiveness of soft sensors in enhancing environmental perception, paving the way for innovative solutions in force measurement applications and human-computer interactions.
Open Access
ReMBot: A reconfigurable, miniature, modular robot with soft connection mechanisms
(Bilkent University, 2023-07) Uğur, Mustafa
Nature has been a valuable source of inspiration for engineers, leading to the development of diverse materials, mechanisms, and algorithms that have enhanced human life. One fascinating idea borrowed from nature is the collaborative work of ant colonies. Ants work together to accomplish tasks that are impossible to achieve individually, such as constructing bridges by connecting to one another. Researchers have been motivated by such examples to create reconfigurable robots that can perform various exciting tasks, such as climbing stairs, crossing gaps, moving objects, and assisting in furniture building, by moving as separate modules and docking to each other using different connection mechanisms. However, the connection mechanism remains a challenge. Many of the existing designs re-quire an actuator or a driving circuit which makes the control harder and limits the robot’s motion. This thesis presents ReMBot: A self-reconfigurable, miniature, modular robot with a soft connection mechanism. The robot comprises multiple modules, each equipped with backbones featuring permanent magnets. Using permanent magnets offers reconfigurability without requiring additional power, actuation, or a driving circuit while enhancing the robot’s compliance. The modules, including the body, electronics, actuators, c-shaped soft legs, and backbones with magnets, weigh 29.43 grams and have 82 mm x 60 mm x 14.7 mm dimensions. These module specifications, combined with the whole system design, allow ReMBot modules to execute path-tracking tasks, dock and undock, and sense the connection between modules. Their ability to connect and maintain a longer structure enables the ReMBot to climb obstacles higher than itself. Soft c-shaped legs enable modules to dock successfully by ensuring successful path-tracking tasks while they help them to move in different terrains like gravel, sand, or grass. The modules’ miniature structure, ease of manufacture, and affordability make them a suitable option for multiple use cases. The robot’s wireless communication capability makes it a strong contender for surveillance in confined spaces like collapsed buildings and nuclear sites, large areas like farmlands, and even planetary exploration missions.
Open Access
Towards mode shape independent nanoelectromechanical mass spectrometry under atmospheric conditions
(Bilkent University, 2023-06) Kaynak, Batuhan Emre
Nanoelectromechanical systems are being utilized in mass sensing applications owing to their small footprints and high mass sensitivities, targeting masses that are unreachable by most other techniques (>10 MDa) while also being able to measure masses in the kDa range. Recently, these sensors were deployed under atmospheric conditions by integrating a polymeric focusing lens, which increased capture efficiencies and decreased the system cost, both of which have been significant challenges in the NEMS-MS field. However, when deployed under atmospheric conditions, the displacement profiles of these sensors become dependent on the environment due to dissipative effects, unlike symmetric displacement profiles that follow the mode shapes in vacuum. Since the frequency shift by particle landing depends on the landing position, the corrections to the governing equations make the analysis more accurate, especially when the quality factor of the sensor is low. Therefore, devices that enable single mode mass sensing under atmospheric conditions remove the need for corrections and enable mode shape independent mass sensing. Here, we propose devices with uniform regions in their mode shapes, which removes the position dependency when calculating the mass of a landing particle. The uniform mode shapes are analyzed and validated by simulations and experiments using 200 nm fluorescent polystyrene nanoparticles whilst considering the landing positions of the particles and frequency shifts. After the uniform mode shape validation, we conducted 40 nm gold nanoparticle mass sensing experiments. Therefore, the proposed devices allowed us to perform mode shape independent mass sensing under atmospheric conditions.
Open Access
A miniature, foldable, collision resilient quadcopter
(Bilkent University, 2023-06) Bakır, Alihan
In the fields of surveillance, mapping and security, the use of unmanned aerial vehicles (UAVs), is becoming inevitable day by day, especially with their au-tonomous movement capabilities. The main reason for the increasing use of un- manned aerial vehicles is their ability to map and survey unknown and dangerous places such as caves without risking human life. At the same time, the ability to conduct aerial surveillance in full autonomy for public safety and health is also an important factor. Today, UAVs are used for many purposes, such as cave mapping and surveying, detection and intervention of forest fires, and inspection of outdoor and crowded areas for security purposes, often accompanied by an operator. Quadcopters are also used for missions that do not require long duration flight, as they are able to both hold position and ensure a highly stable flight. In addition to their many advantages, these UAVs are very sensitive to even the slightest impact, so their fully autonomous flight is still limited to controlled areas. Various studies are being conducted to increase the crash resistance of the quadcopters. Among these researches, there are different ideas such as protective shells and bumpers that surround the UAV and absorb the impact in case of collisions. The spherical cases that surround the UAV are usually in a mesh structure to be lightweight and not obstruct the airflow. Bumpers designed to protect the most sensitive parts of the UAV, such as the motors and propellers, are insufficient to protect the body. For these reasons, making different parts of the UAV from more flexible materials will eliminate the vulnerability of the UAV and increase its resistance to collisions. In this thesis, in order to increase the impact resistance of quadcopters and to ensure that they do not break, robots with some flexible and some rigid parts were tested and the results of these tests were evaluated in detail. During these tests, the effects of the compliance of the robot’s arms and the compliance of the bumpers protecting the propellers upon the impact were analyzed. In order to make this comparison, flexible and rigid robot bodies with dimensions as close as possible to each other, as well as rigid and flexible bumpers of similar size and structure were designed. The flexible bumper and body were produced by cutting PET sheets via a laser cutter and folding them in an origami-inspired pattern. This production method adds flexibility thanks to the thinness of the PET sheets and structural rigidity thanks to the origami-inspired folding technique. To in- crease the flexibility of the robot in the event of a collision and the stability of the motors, inserts made of TPU are inserted into the body. In addition to impact resilience, this thesis also discusses a soft sensor that can be attached to collision-resiliant drones. This sensor, made of conductive TPU, allows the robot to sense its surroundings by giving it a sense of touch. Thanks to the flexible sensor, robots can detect when a collision occurs and react accordingly. This sensor works entirely based on the flexibility of the UAV’s bumpers and senses the bending of these bumpers. Therefore, such a sensor cannot be used on rigid hulls and bumpers as they do not bend.
Open Access
Aspects of constitutive modeling in continuum-kinematics-inspired peridynamics
(Bilkent University, 2022-10) Ekiz, Ekim
Continuum-kinematics-inspired Peridynamics (CPD) has been recently proposed as a geometrically exact formulation of peridynamics (PD) that is also thermo- dynamically and variationally consistent. Unlike the original formulation of PD, CPD can accurately capture the Poisson effect. CPD consists of one-, two- and three-neighbor interactions. The isotropic CPD formulation of non-local elasticity therefore involves three material constants associated with length, area and volume. This manuscript aims to establish the relationships between the material parameters of CPD and isotropic linear elasticity for two- and three-dimensional problems. Two alternatives for the CPD energy density are introduced. Analytical solutions of the energy densities for affine deformations are derived. It is shown that the three material parameters of CPD reduce to two independent pa- rameters in the linearized framework, and can be expressed in terms of any pairs of isotropic linear elasticity constants, such as Lame parameters. The analysis here provides a physical interpretation for the ﬁrst Lame constant. Finally, the admissible ranges for CPD material parameters are established.
Open Access
Numerical study on the dispersion and deposition of particles in evaporating sessile droplets
(Bilkent University, 2022-09) Erdem, Ali Kerem
Evaporating sessile droplets including dispersed particles are utilized in the coating, printing, and biomedical applications. Modeling this problem is a challenging process, therefore diﬀerent assumptions are used in the literature. It is important to have a model which covers both pinned and moving contact line regimes for the droplet, thus whole evaporation process and deposition proﬁle can be understood. Therefore, in this work, a numerical and mathematical model is derived to simulate two-dimensional symmetric thin evaporating sessile droplets whose contact line is ﬁrstly pinned and then moving. This model is derived by combining diﬀerent models in literature with the help of lubrication theory and rapid vertical diﬀusion assumption. This model includes a temporal change in the droplet’s surface height, contact line dynamics, particle dispersion, and deposition. The ﬁnite diﬀerence method is used in the numerical solution. Cases including pinned and moving contact lines in the literature are solved separately by diﬀerent numerical algorithms developed in this work and these algorithms were combined. This new algorithm ﬁrst solves a mathematical model in the pinned contact line regime. When the contact angle goes below the deﬁned limit, the second part of the algorithm solves the mathematical model in the moving contact line regime until 95 percent of the total particle mass is deposited. A parametric study has been done with the developed algorithm. A set of parameters is deﬁned and chosen parameters are changed to see their eﬀects. It is observed that increasing the Marangoni number and Capillary number, increased particle accumulation near the center. Decreasing evaporation number and increasing Damkohler number result in more uniform particle deposition.
Open Access
Design, fabrication, and soft impact modeling and simulation of a collision-resilient foldable micro quadcopter
(Bilkent University, 2022-09) Abazari, Amirali
Despite the appreciable advancements in mobile robot navigation and obstacle avoidance algorithms using an abundance of sensors and sensor fusion methods, the navigation of moving robots through confined and cluttered spaces is still a great challenge. The physical interaction and collision between mobile robots and surrounding obstacles in these environments are unavoidable. This becomes more concerning for the case of Vertical Take-Off and Landing (VTOL) Unmanned Aerial Vehicles (UAV) that the system is naturally unstable, and a minor fault or disturbance may result in severe crashes. In this thesis, a Collision-Resilient Quad-rotor Micro Aerial Vehicle (MAV) is designed using the Origami-inspired design fabrication techniques. The quadcopter is lightweight (220g max) and is designed for outdoor inspection and surveillance missions. This compliant drone provides an stable flight for a duration of 5 - 10 min, depends on the flight condition. A dynamic model is derived for the quadcopter to represent the realistic features designed for this specific UAV. In addition to that, the impact of the compliant body to surrounding obstacles is modeled as visco-elastic contact force and is added to the quadcopter’s dynamic model. The contact dynamic friction force between the protective soft bumpers and the surface is also modeled. The developed dynamic model is then used to simulate the impact of the collisionresilient quadcopter in two different simulation environments; MATLAB Simulink and ROS Gazebo. A cascaded PID control scheme is suggested for low-level (attitude) and high-level (global position) control of the drone in experiments and simulation. The result of these soft impact simulations closely imitate the collision-resilient properties of the actual quadcopter in experiments. Coefficient of Restitution (CoR) for the compliant drone impact, both in simulations and experiments, is in the interval [0.5 0.6]. This shows a great capacity for the drone to dampen the collisions.
Open Access
Model-based optimization of microscale parts printed with projection-based continuous vat photopolymerization
(Bilkent University, 2022-08) Güven, Ege
Micro-scale additive manufacturing has seen significant growth over the past years, where improving the accuracy of complex micro-scale geometries is seen as an important challenge. Using grayscale images rather than black and white images during production is an effective method to improve the fabrication quality. This thesis presents a model-based optimization method for improving the dimensional accuracy of parts using voxel-based grayscale dynamic optimization during continuous 3D printing. A detailed solidification model has been developed and used to estimate the curing dynamics of the resin used in 3D printing. The irradiance of the light beam projected for each pixel influences a larger volume on the resin than the targeted voxel. The proposed model-based method optimizes the images considering the light distribution from all closely related pixels to maintain the accuracy of the micro part. The results of this method have been applied to the printing of complex 3D parts to show that optimized grayscale images improve the areas with overcuring significantly. It is shown that the number of overcured voxels was reduced by 24.7% compared to the original images. Actual printing results from the experimental setup confirm the improve-ments in the accuracy and precision of the printing method. The optimization method has been further improved by allowing variable printing speed during pro-duction and optimizing the speed profile of the print alongside grayscaling. This approach allows for printing of certain geometries that would otherwise be challenging to produce accurately. Computational limitations of performing speed and grayscale optimization simultaneously has been overcome by utilizing the symmetry of certain special cases to reduce optimization variables.
Open Access
Identification of internal process parameters of micro milling considering machined surface topography
(Bilkent University, 2022-07) Masrani, Abdulrzak
Micro-milling is a fast and versatile machining method that can be used to manufacture three-dimensional parts of a wide range of materials with high accuracy. Modeling of micro-milling processes is complex due to size effects, where the chip thickness becomes comparable to the cutting edge radius. Furthermore, tool runout and deflection effects on the process outputs are amplified and cannot be neglected. As the process is scaled down where micrometer accuracy is required; modeling and identifying the process parameters becomes essential to optimize or monitor the process. This study presents a systematic approach to force modeling and parameter identification of micro-milling processes. Finite element analysis of tool deflection is integrated into mechanistic modeling of micro-milling forces together with considering the trochoidal trajectory of the cutting teeth, tool runout, and chip thickness accumulation due to minimum uncut chip thickness. The internal process parameters are identified using the experimental cutting forces and machined surface topography with a novel methodology. The research results are experimentally validated by slot and side micro-milling tests on commercially pure titanium, using coated carbide micro-end-mills with diameters of 0.2 and 0.4 mm, and accurate predictions of model parameters and cutting forces are obtained. The proposed force models can be used in smart manufacturing and digital twin applications to reduce the time and costs associated with process optimization. The proposed parameter identification techniques can also help to reduce the need for advanced measurement systems.
Open Access
A unifying approach towards the geometrical instabilities of compressible, multilayer domains
(Bilkent University, 2022-07) Bakiler, Ayşe Derya
Instabilities that arise in layered systems have been a riveting course of study for the past few decades, having found utility in various fields, while also being frequently observed in biological systems. However, the large deformation bifurcation analysis of compressible domains remains vastly understudied compared to the incompressible case. In this work, we present a unifying approach for the instability analysis of multilayer compressible elastic domains under plane deformations, also extending the approach to include the general interface model and to capture growth-induced instabilities. First, a linear elastic, displacement-based approach to capture bilayer wrinkling is taken, outlining the basics of such an approach. Then, the large-deformation incremental analysis for a rectangular, compressible, hyperelastic domain under plane deformations is developed, which serves as a generic framework for other geometries. This framework is applied to beam, half-space, bilayer and trilayer structures. Next, the framework is extended to account for the general interface model, looking into coated half-spaces, coated beams, and bilayers with interfaces. Finally, the framework is derived to account for both compression and growth. Obtained analytical results for the onset of wrinkling are compared with computational simulations using the finite element method (FEM) enhanced with eigenvalue analysis, cultivating excellent agreements between analytical and numerical results all across the material and geometrical parameter spectrum, and portraying clearly the significant effect of compressibility on bifurcation behavior. The analytical framework presented here provides grounds for further works on other modes of instabilities and more complex geometries.
Open Access
A numerical algorithm to model wall slip and cavitation in two-dimensional hydrodynamically lubricated contacts
(Bilkent University, 2022-01) Çam, Mert Yusuf
Hydrodynamic lubrication takes a fundamental role in mechanical systems to reduce energy losses and prevent mechanical breakdown. In order to model hydrodynamic lubrication in thin films the solution of the Reynolds equation is required. However, the Reynolds equation cannot re ect all the lubricant characteristics in thin films. The effects of two critical factors, wall slip and cavitation, need to be considered. A new numerical solution of the Reynolds equation is presented to model two-dimensional hydrodynamic lubrication, including the linear complementary mass-conserving cavitation model and multi-linearity wall slip model. In addition, a new wall slip model has been proposed by modifying the multi-linearity wall slip model to make it more computationally affordable. The proposed mathematical model has been validated against available models in literature with the tests performed on journal bearings, slider bearings, squeeze dampers, and surface textured bearings. The proposed novel wall slip model is up to 9 times more computational affordable than the original multi-linearity wall slip model.
Open Access
Design of a droplet-based microfluidic system for hybrid polymer nanoparticle synthesis
(Bilkent University, 2021-12) Şahinoğlu, Osman Berkay
Droplet microfluidics is advantageous in synthesizing microparticles for both confining their size to the physical dimensions of the droplet and providing a monodisperse result due to rapid mixing inside the droplets. Thusly named microreactors became the focus of the microfluidics community in the recent decade due to their superior ability to control the reaction environment. In this study, for the application of microreactors, a hybrid organic-inorganic material that became prominent in last years named polyhedral oligomeric silsesquioxane (POSS) is chosen. POSS is a polymer that, beside its hybrid nature, shows heat resistance property which made its use in protective painting applications and high radical group affinity that can be utilized to further configure its material properties. This study proposes two microreaction systems for monomer POSS with ther-mal and photopolymerization methods that aim to increase monodispersity and solve the clogging problem encountered in previous studies by introducing the oil phase in the system. Feasibility of systems was investigated numerically using COMSOL Multiphysics and analyses showed adequate heating of and mixing in microreactors. A robust post-processing procedure is proposed to remove excess oil from the sample. Measurements showed microdroplet and sub-micron particle generation where the size distribution of these particles are quantified using MATLAB. Though the use of oil in the system proved to be another challange, hexane based substitute materials are proposed for future work.
Open Access
Modeling of droplet motion on textured surfaces
(Bilkent University, 2021-09) Naji, Mayssam
We describe the motion of a droplet on a textured ratchet track using a non-linear resonator model. A textured ratchet track is composed of semi-circular pillar array that induces a net surface tension gradient on a droplet placed on it. When a vertical vibration is applied, hysteresis is overcome, and the droplet moves towards the local lower energy barrier; however, due to the repetitive structure of texture, it keeps moving until the end of the track. The droplet motion depends on the amplitude and frequency of the vertical oscillation, and this dependence is nonlinear. Therefore, ﬁnding a fully analytical solution to represent this motion is not trivial. Consequently, the droplet motion still remains as a topic that needs further investigation. In this study we elaborate on the utility of double-pendulum as a basis for modeling the droplet motion on surfaces. Similar to the droplet motion, resonators, such as double pendulum, are simple, yet non-linear systems. Moreover, inverted double pendulum motion has key characteristics such as two phase motion and double peak motion, which are also observed in the droplet motion on textured ratchets. In this thesis, data processing models are developed to highlight the similarity between these two systems both qualitatively and quantitatively. After establishing this comparison, a model is proposed that utilizes an inverted double pendulum mounted on a moving cart to successfully simulate the motion of a droplet on a ratchet track. This methodology will lead to developing an accurate droplet-motion modeling approach which will be useful to understand droplet dynamics in more depth.