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Publication Metadata only “O/F shift” in hybrid rockets(American Institute of Aeronautics and Astronautics, 2014) Toson, Elena; Evans, Brian; Department of Mechanical Engineering; Karabeyoğlu, Mustafa Arif; Faculty Member; Department of Mechanical Engineering; College of Engineering; 114595For most hybrid rocket systems, oxidizer to fuel ratio (O/F) changes over time due to 1) natural growth of the fuel port diameter and 2) oxidizer flow rate variations, if throttling is employed. This phenomenon, which is referred to as “O/F shift”, leads to a reduction in motor performance. Note that liquid or solid rocket motors are not subject to temporal O/F variations, which is wrongfully considered as one of the most critical disadvantages of hybrid rockets. In this paper, the effect of “O/F shift” is quantified for hybrid rocket motors. Analytical formulas for the temporal O/F variation and the overall c* efficiency drop associated with the variation has been derived for single circular port motors. It has been shown that for a typical motor, c* efficiency drop due to O/F variation is well below 0.2%, a value which is too small to be measured in an actual motor test. It is also shown that for a wagon wheel type multiport configuration (with triangular ports), efficiency drop is significantly worse than the single circular port case. Even for the multiport systems, the shift does not have a controlling effect on the overall efficiency of the motor. A number of strategies have been outlined to control the adverse effects of O/F variation in a hybrid rocket. For a single circular port design with limited throttling, no mitigation is required. For systems with deep throttling requirements, aft oxidizer injection seems like a viable strategy to retain a high level of overall efficiency.Publication Metadata only 3D surface topography analysis in 5-axis ball-end milling(Elsevier, 2017) N/A; Department of Mechanical Engineering; Khavidaki, Sayed Ehsan Layegh; Lazoğlu, İsmail; PHD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179391This article presents a new analytical model to predict the topography and roughness of the machined surface in 5-axis ball-end milling operation for the first time. The model is able to predict the surface topography and profile roughness parameters such as 3D average roughness (Sa) and 3D root mean square roughness (Sq) by considering the process parameters such as the feedrate, number of flutes, step over and depth of cut as well as the effects of eccentricity and tool runout in 5-axis ball-end milling. This model allows to simulate the effects of the lead and tilt angles on the machined surface quality in the virtual environment prior to the costly 5-axis machining operations. The effectiveness of the introduced surface topography prediction model is validated experimentally by conducting 5-axis ball-end milling tests in various cutting conditions. (C) 2017 Published by Elsevier Ltd on behalf of CIRP.Publication Metadata only A CAM-based path generation method for rapid prototyping applications(Springer London Ltd, 2011) N/A; Department of Mechanical Engineering; Lazoğlu, İsmail; N/A; Faculty Member; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); N/A; College of Engineering; N/A; 179391A wide range of rapid prototyping (RP) methods are available commercially. Even though the hardware and production materials of these RP methods differ, their production techniques are built on the same idea: layer-by-layer material additive manufacturing. Whatever the material is used, it is deposited, vulcanized, or melted by following a pre-determined path, and each layer is stowed on the previous one to create the 3D model which is designed by using a computer-aided design program. The path which is followed while creating the model is very crucial. In this paper, a novel idea for path generation for RP processes is introduced. This new method is based on computer numerical controlled milling operation. Although the RP process and the milling process are completely opposite of each other since one of them is an additive and the other one is a subtractive method, the paths which are followed for these operations are very similar and based on the same idea: The progress goes on layer by layer. In this novel method, cutter location source files are used to create paths for RP processes. Examples of the prototypes produced by using this new method are also presented in the paper.Publication Metadata only A challenge for peptide coarse graining: transferability of fragment-based models(Wiley-V C H Verlag Gmbh, 2011) Villa, Alessandra; Peter, Christine; N/A; Department of Mechanical Engineering; Engin, Özge; Sayar, Mehmet; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 109820Peptides are highly promising building blocks for design and development of novel materials with potential application areas ranging from drug design to biotechnology. The necessity to understand the structural and thermodynamic properties of these complex materials has led to a dramatic increase in the development of computational techniques geared specifically towards peptide-based systems. Both all-atom (AA) and coarse-grained (CG) simulations of such materials have become extremely important, where the latter is an indispensable tool for reaching the time and length scales relevant to the experiments. Here, we review different approaches and discuss the challenges in the development of CG models for peptides. In particular, we concentrate on the transferability of fragment-based CG models. We analyze the transferability of a solvent-free CG model developed to model hydrophobic phenylalanine dipeptides (FF) in water. Here, we employ the same CG strategy-with non-bonded potentials based on peptide fragments-to two other hydrophobic dipeptides, valine-phenylalanine (VF) and isoleucine-phenylalanine (IF). In line with the previously developed model, the dipeptides are described by seven beads and the potentials developed for FF (bonded and non-bonded) are directly applied to describe the phenylalanine and backbone atoms, while new potentials are developed to account for the valine and isoleucine sidechains. By comparing AA and CG intra and intermolecular samplings, we show the ability of the CG model to reproduce the conformational behavior and thermodynamic association properties of the corresponding atomistic systems.Publication Open Access A comparative analysis of austenite-to-martensite and austenite-to-bainite phase transformation kinetics in steels(Taylor _ Francis, 2013) Holzweissig, M.J.; Lambers, H.-G.; Maier, H.J.; Department of Mechanical Engineering; Uslu, Mehmet Can; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 23433This paper presents a comparison of the macroscopic transformation strain evolution as a function of the bainite and martensite phase fractions in steels. Specifically, the evolution of anisotropic strain with phase fraction follows a linear trend for the martensitic transformation due to continuous stress-induced variant selection. In the case of bainitic transformation, the anisotropic strain evolves non-linearly owing to diffusion, minimizing the distortion around the bainitic sheaves and further promoting stress-induced variant selection at the early stages of the bainitic phase transformation. However, this effectiveness is reduced when the bainitic sheaves start constricting the growth of each other.Publication Metadata only A comparison of solid model and three-orthogonal dexelfield methods for cutter-workpiece engagement calculations in three- and five-axis virtual milling(Springer London Ltd, 2015) Erdim, H.; N/A; Department of Mechanical Engineering; Boz, Yaman; Lazoğlu, İsmail; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179391Virtual simulation of three- and five-axis milling processes has started to become more important in recent years in various industries such as aerospace, die-mold, and biomedical industries in order to improve productivity. In order to obtain desired surface quality and productivity, process parameters such as feedrate, spindle speed, and axial and radial depths of cut have to be selected appropriately by using an accurate process model of milling. Accurate process modeling requires instantaneous calculation of cutter-workpiece engagement (CWE) geometry. Cutter-workpiece engagement basically maps the cutting flute entry/exit locations as a function of height, and it is one of the most important requirements for prediction of cutting forces. The CWE calculation is a challenging and hard problem when the geometry of the workpiece is changing arbitrarily in the case of five-axis milling. In this study, two different methods of obtaining CWE maps for three- and five-axis flat and ball-end milling are developed. The first method is a discrete model which uses three-orthogonal dexelfield, and the second method is a solid modeler-based model using Parasolid boundary representation kernel. Both CWE calculation methods are compared in terms of speed, accuracy, and performance for three- and five-axis milling of ball-end and flat-end mill tools. It is shown that the solid modeling-based method is faster and more accurate. The proposed methods are experimentally and computationally verified in simulating milling of complex three-axis and five-axis examples as well as predicting cutting forces.Publication Metadata only A comprehensive evaluation of parameters governing the cyclic stability of ultrafine-grained FCC alloys(Elsevier Science Sa, 2011) Niendorf, T.; Maier, H. J.; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433The current paper presents results of a thorough experimental program undertaken to shed light onto the mechanisms dictating the cyclic stability in ultrafine-grained (UFG) alloys with a face-centered cubic structure. Cyclic deformation responses of several copper- and aluminum-based UFG alloys were investigated and the corresponding microstructural evolutions were analyzed with various microscopy techniques. The important finding is that a larger volume fraction of high-angle grain boundaries and solid solution hardening significantly improve the fatigue performance of these alloys at elevated temperatures and high strain rates, and under large applied strain amplitudes.Publication Metadata only A computational study of axial dispersion in segmented gas-liquid flow(American Institute of Physics (AIP) Publishing, 2007) Gunther, Axel; Stone, Howard A.; Department of Mechanical Engineering; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; 46561Axial dispersion of a tracer in a two-dimensional gas-liquid flow is studied computationally using a finite-volume/front-tracking method. The effects of Peclet number, capillary number, and segment size are examined. At low Peclet numbers, the axial dispersion is mainly controlled by the convection through the liquid films between the bubbles and channel walls. In this regime, the computational results are found to be in a very good agreement with the existing model due to Pedersen and Horvath [Ind. Eng. Chem. Fundam. 20, 181 (1981)]. At high Peclet numbers, the axial dispersion is mainly controlled by the molecular diffusion, with some convective enhancement. In this regime, a new model is proposed and found to agree well with the computational results. These Peclet number regimes are shown to persist for different slug lengths. The axial dispersion is found to depend weakly on the capillary number in the diffusion-controlled regime. Finally, computational simulations are performed for the cases of six bubbles to mimic bubble trains, and results are compared with the theoretical models.Publication Metadata only A computational study of drop formation in an axisymmetric flow-focusing device(Amer Soc Mechanical Engineers, 2006) Department of Mechanical Engineering; Department of Mechanical Engineering; Filiz, İsmail; Muradoğlu, Metin; N/A; Faculty Member; Department of Mechanical Engineering; College of Engineering; College of Engineering; N/A; 46561We investigate the formation and dynamics of drops computationally in an axisymetric geometry using a Front-Tracking/Finite-Difference (FT/FD) method. The effects of viscosity ratio between inner and outer liquids on the drop creation process and drop size distribution are examined. It is found that the viscosity ratio critically influences the drop formation process and the final drop distribution. We found that, for small viscosity ratios, i.e., 0.1 < lambda < 0.5 drop size is about the size of the orifice and drop distribution is highly monodisperse. When viscosity ratio is increased, i.e., 0.5 < lambda < I a smaller drop is created just after the main drop. For even higher viscosity ratios, the drop distribution is usually monodisperse but a satellite drop is created in some cases. The effect of the flow rates in the inner jet and the co flowing annulus are also studied. It is found that the drop size gets smaller as Q(in) / Q(out) is reduced while keeping the outer flow rate constant.Publication Open Access A computational study of droplet-based bioprinting: effects of viscoelasticity(American Institute of Physics (AIP) Publishing, 2019) Taşoğlu, Savaş; Department of Mechanical Engineering; Nooranidoost, Mohammad; Izbassarov, Daulet; Muradoğlu, Metin; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; N/A; N/A; 46561Despite significant progress, cell viability continues to be a central issue in droplet-based bioprinting applications. Common bioinks exhibit viscoelastic behavior owing to the presence of long-chain molecules in their mixture. We computationally study effects of viscoelasticity of bioinks on cell viability during deposition of cell-loaded droplets on a substrate using a compound droplet model. The inner droplet, which represents the cell, and the encapsulating droplet are modeled as viscoelastic liquids with different material properties, while the ambient fluid is Newtonian. The model proposed by Takamatsu and Rubinsky ["Viability of deformed cells," Cryobiology 39(3), 243-251 (1999)] is used to relate cell deformation to cell viability. We demonstrate that adding viscoelasticity to the encapsulating droplet fluid can significantly enhance the cell viability, suggesting that viscoelastic properties of bioinks can be tailored to achieve high cell viability in droplet-based bioprinting systems. The effects of the cell viscoelasticity are also examined, and it is shown that the Newtonian cell models may significantly overpredict the cell viability.