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Department: search.filters.department.Department of Mechanical EngineeringSubject: search.filters.subject.Mathematics

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    A LES/PDF simulator on block-structured meshes
    (Taylor & Francis Ltd, 2019) Pope, Stephen B.; N/A; Department of Mechanical Engineering; Türkeri, Hasret; Muradoğlu, Metin; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 46561
    A block-structured mesh large-eddy simulation (LES)/probability density function (PDF) simulator is developed within the OpenFOAM framework for computational modelling of complex turbulent reacting flows. The LES/PDF solver is a hybrid solution methodology consisting of (i) a finite-volume (FV) method for solving the filtered mass and momentum equations (LES solver), and (ii) a Lagrangian particle-based Monte Carlo algorithm (PDF solver) for solving the modelled transport equation of the filtered joint PDF of compositions. Both the LES and the PDF methods are developed and combined to form a hybrid LES/PDF simulator entirely within the OpenFOAM framework. The in situ adaptive tabulation method [S.B. Pope, Computationally efficient implementation of combustion chemistry using in situ adaptive tabulation, Combust. Theory Model. 1 (1997), pp. 41-63; L. Lu, S.R. Lantz, Z. Ren, and B.S. Pope, Computationally efficient implementation of combustion chemistry in parallel PDF calculations, J. Comput. Phys. 228 (2009), pp. 5490-5525] is incorporated into the new LES/PDF solver for efficient computations of combustion chemistry with detailed reaction kinetics. The method is designed to utilise a block-structured mesh and can readily be extended to unstructured grids. The three-stage velocity interpolation method of Zhang and Haworth [A general mass consistency algorithm for hybrid particle/finite-volume PDF methods, J. Comput. Phys. 194 (2004), pp. 156-193] is adapted to interpolate the LES velocity field onto particle locations accurately and to enforce the consistency between LES and PDF fields at the numerical solution level. The hybrid algorithm is fully parallelised using the conventional domain decomposition approach. A detailed examination of the effects of each stage and the overall performance of the velocity interpolation algorithm is performed. Accurate coupling of the LES and PDF solvers is demonstrated using the one-way coupling methodology. Then the fully two-way coupled LES/PDF solver is successfully applied to simulate the Sandia Flame-D, and a turbulent non-swirling premixed flame and a turbulent swirling stratified flame from the Cambridge turbulent stratified flame series [M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames I: Non-swirling flows, Combust. Flame 159 (2012), pp. 2896-2911; M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames II: Swirling flows, Combust. Flame 159 (2012), pp. 2912-2929]. It is found that the LES/PDF method is very robust and the results are in good agreement with the experimental data for both flames.
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    Electromechanical modeling of silicon nanowire switches: size and boundary condition effects
    (Amer Inst Physics, 2020) Esfahani, Mohammad Nasr; Department of Mechanical Engineering; Department of Mechanical Engineering; Roudposhti, Speedeh Shahbeigi; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108
    Understanding the operational behavior of nanoelectromechanical systems (NEMS) is the preliminary step to design functional sensors and actuators. Miniaturization is considered for further improvement in sensitivity, while the extreme surface area in NEMS devices plays a leading role in the effective performance through size dependence physical properties. Nanowire (NW) switches are one such device with significant surface effects present on the pull-in voltage. This study introduces a new approach to implement the surface effect into electromechanical behavior of NW switches based on finite element analysis. The influence of size and boundary condition on pull-in voltage is studied for silicon NWs. Results demonstrate the importance of length-to-thickness ratio as a suitable parameter to express the surface effect rather than the surface-area-to-volume ratio.
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    Modeling cutting forces for five axis milling of sculptured surfaces
    (Trans Tech Publications Ltd, 2011) Erdim, H.; N/A; Department of Mechanical Engineering; Boz, Yaman; Lazoğlu, İsmail; Master Student; Faculty Member; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); Graduate School of Sciences and Engineering; College of Engineering; N/A; 179391
    5-axis ball-end milling processes are used in various industries such as aerospace, automotive, die-mold and biomedical industries. 5-axis machining provides reduced cycle times and more accurate machining via reduction in machining setups, use of shorter tools due to improved tool accessibility. However, desired machining productivity and precision can be obtained by physical modeling of machining processes via appropriate selection of process parameters. In response to this gap in the industry this paper presents a cutting force model for 5-axis ball-end milling cutting force prediction. Cutter-workpiece engagement is extracted via developed solid modeler based engagement model. Simultaneous 5-axis milling tests are conducted on A17075 workpiece material with a carbide cutting tool. Validation of the proposed model is performed for impeller hub roughing toolpaths. Validation test proves that presented model is computationally efficient and cutting forces can be predicted reasonably well. The result of validation test and detailed comparison with the simulation are also presented in the paper.
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    PublicationOpen Access
    Monolithic fabrication of silicon nanowires bridging thick silicon structures
    (Institute of Electrical and Electronics Engineers (IEEE), 2015) Peric, O.; Sacchetto, D.; Fantner, G.E.; Leblebici, Y.; Department of Mechanical Engineering; Taşdemir, Zuhal; Alaca, Burhanettin Erdem; Faculty Member; Department of Mechanical Engineering; College of Engineering; N/A; 115108
    A monolithic process is developed for the fabrication of Si nanowires within thick Si substrates. A combination of anisotropic etch and sidewall passivation is utilized to protect and release Si lines during the subsequent deep etch. An etch depth of 10 μm is demonstrated with a future prospect for 50 μm opening up new possibilities for the deterministic integration of nanowires with microsystems. Nanowires with in-plane dimensions as low as 20 nm and aspect ratios up to 150 are obtained. Nanomechanical characterization through bending tests further confirms structural integrity of the connection between nanowires and anchoring Si microstructures.