SCHOOL OF MECHANICAL ENGINEERING SEMINAR
Monday, December 22, 2014 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Direct Numerical Simulations (DNS) Investigation of Turbulent Coal Combustion

Dr. Tamir Brosh

Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, UK

Coal combustion is responsible for about 40% of the world’s power requirements. This process accounts for about 24% of the world greenhouse gases generation. While many resources are invested in the improvement of cleaner, alternative, energy sources, the increasing demand for power generation means that the coal power plant will continue to play a major role in the world’s power production for the foreseeable future. Thus, improved fundamental understanding of coal particle-laden turbulent reacting flows is important for optimizing the coal combustion process and designing new generation energy-efficient and environment-friendly boilers and furnaces.

Recent advances in high performance computing have made it possible to obtain fundamental physical information regarding coal combustion based on numerical simulations, which is either difficult or impossible to obtain experimentally in the hostile furnace environment. Up until now most computational simulations of turbulent coal combustion have been carried out using Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES). However, the fidelity of RANS and LES simulations strongly depends on the accuracy of the turbulence and combustion models and are often subject to strong assumptions involved in the development of these models.

The current research focuses on the generation and analysis of data from Direct Numerical Simulations (DNS) of the carrier gas phase coupled with Lagrangian coal particles without making too restrictive physical approximations regarding the underlying reacting turbulent flow of the gas phase in order to obtain fundamental understanding of the coal combustion process.

Short Bio

Dr. Tamir Brosh obtained B.Sc. (2007) in Mechanical Engineering from Ben-Gurion University of the Negev where he also completed is M.Sc. (2009) and Ph.D. (2012). During his M.Sc. and Ph.D. Dr Brosh worked on Discrete Element Modelling of particle comminution in jet-mills. After completing his Ph.D., Dr Brosh took a postdoctoral research associate position in the School of Mechanical and Systems Engineering of Newcastle University UK where he worked on turbulent coal combustion using Direct Numerical Simulations. Dr Brosh is currently a visiting researcher at Newcastle University where he continues to work on coal combustion, and also contributing to the work on droplet combustion and flame wall interactions

School of Mechanical Engineering Seminar
Wednesday, January 21, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Arrays of Parametrically Excited Micro Cantilevers

Interacting through Fringing Electrostatic Fields

Inbar Schneider

School of Mechanical Engineering, Faculty of Engineering

Tel Aviv University, Israel

Dynamics of large arrays of micro- and nanoelectromechanical (MEMS/NEMS) oscillators in most cases assembled from cantilever-type individual elements interacting through nonlinear elastic, electromagnetic, or dissipative forces have received a great deal of attention over recent years. Apart from their rich dynamic behavior, these devices can be potentially used in ultrasensitive chemical or biological sensors or for light processing. Among the architectures reported so far the arrays interacting trough mechanical coupling support traveling waves (has real eigenfequencies in the case of a bounded system) and exhibit many interesting phenomena but lack tunability. On the other hand, the arrays interacting through electrostatic forces provided by close-gap electrodes can be easily tuned by voltage but does not support traveling waves. In the present work, we introduce a design of an array incorporating mechanically and electrostatically coupled micro cantilevers interacting through fringing electrostatic fields. This simple robust device architecture supports traveling waves and is distinguished by an easily tunable coupling stiffness and an efficient parametric excitation using a time-dependent voltage.  Reduced order model of the cantilevers was built based on the Galerkin decomposition and was used for the investigation of the interplay between the elastic and electrostatic coupling forces and their influence on the array’s dynamics. The non-local mechanical coupling matrix was extracted using the full scale finite elements analysis of the structure combined with sub-structuring procedure. The electrostatic interaction forces were approximated by a fit based on the the-dimensional numerical analysis.  The resonant responses of the arrays consisting of 500 mm long and 5 mm thick single crystal Si cantilevers were visualized by time-averaged temporally aliased video imaging and measured by laser Doppler vibrometry. Collective behavior, synchronization and abrupt transitions between standing wave patterns in arrays of micromechanical oscillators were observed in the experiments. Our experimental and model results collectively demonstrate that under a slowly varying drive frequency the standing wave patterns remain unchanged in certain frequencies intervals, followed by an abrupt change in the pattern.

School of Mechanical Engineering Seminar
Wednesday, January 21, 2014 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Minimal invasive medical micro-robot for brain parenchyma burrowing

Abadi Avi

MSc Student of Dr. Kosa Gabor

Microsystem locomotion is a key component for minimally invasive neurosurgical procedures. A self-propelling micro-robot facilitates targeted drug delivery, biopsy and neuro-stimulator positioning in brain parenchyma. In high frequency small deformation burrowing conditions, the soft brain tissue behaves as a viscoelastic fluidic environment and therefore Stokes swimmer techniques can be applied to move in the tissue.

We use a piezoelectric vibrating bimorph bender in order to propel an electrode in the brain parenchyma. The vibrational motion of the beam is stimulated by a top piezoelectric layer, divided into three separately actuated segments. Flexural vibration is created by each segment by sinusoidal excitation. The frequency, amplitudes and phases combinations govern the total shape of the beam’s vibration.

In order to discard the need for actuation modeling, we utilize the bottom piezoelectric layer of the bimorph for sensing. Three separated sensing segments convert the bending strain of the beam to electrical displacements and measured as voltages. By this measurement we are able to identify the frequency response (FR) of the beam vibration. We investigate the FR of a fully clamped commercially available piezoelectric bimorph in silicon oil. The implementation of the sensing abilities obtains maximal flow of the silicon oil, which indicates maximal propulsion forces. The suggested open loop control enables the system’s identification of a swimming micro-robot in highly viscous fluids. Results were confirmed using particle image velocimetry (PIV) methods under a microscope camera.

School of Mechanical Engineering Seminar
Wednesday, November 26, 2014 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Crack-arresting voided periodic materials

Or Hadar

MSc Student of Prof. Shmuel Ryvkin

Evaluation of the stress field around the tip of a semi-infinite crack embedded in a material with periodic microstructure is very difficult because it is necessary to carry out a large volume of calculations. This work presents and implements a novel method to perform this calculation. The conditions at the boundaries of a rectangular domain around the tip are formulated by the use of K-field for the homogeneous material possessing effective elastic properties and then the finite discrete Fourier transform is applied. This allows to replace standard analysis of a large periodic domain with many cells by the analysis of a single repetitive cell in the transform space which can be carried out by any numerical method. Consequently, the volume of calculations in comparison with the standard approach is reduced and the problem of a macrocrack embedded in a material with fine microstructure can be addressed without simplifying assumptions. The accuracy of the proposed approach is verified by a comparison with the analytical solution for a crack embedded in a homogeneous plane and with the known results for low-density voided material.

Application of the suggested method is given for a crack in a two-dimensional periodically voided material with triangular isotropic layout. The representative cell problem is resolved by the finite element method. It was assumed that the parent material is brittle and fracture toughness of the voided material is determined by the stress criterion for crack propagation, i.e. the crack will propagate when the maximal tensile stress around the tip will reach the tensile strength of the parent material. The dependence of the fracture toughness upon the material relative density is investigated for circular and hexagon voids, as well as its dependence on the shape of the voids for high density material. A comparison of the fracture toughnesses of the solid and voided materials has shown for which parameter combinations voided ones will provide better crack resistance. Results are given for materials commonly used: alumina 99.9%, silicon nitride, sapphire, boron nitride and mullite.