School of Mechanical Engineering Seminar
Monday, May 7, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Solitary waves on ferrofluids jet

Prof. Emilian Parau

 Univ. of East Anglia UK

'Travelling axisymmetric waves on the surface of a cylindrical ferrofluid jet are investigated. An azimuthal magnetic field is generated by an electric current flowing along a stationary metal rod which is mounted along the axis of the moving jet. Solitary waves are computed and their time evolution is discussed. Comparisons with weakly nonlinear theories such as KdV, and experiments are presented.'

School of Mechanical Engineering Seminar
Monday, June 18, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

A sublaminate meso-mechanics damage model for low-velocity impact analysis of multi-layered laminates

Yarden Zaki

MSc Student of Prof. Rami Haj-Ali

Low velocity impact (LVI) can induce hidden damage in laminated composite structures.  Thus this is a major risk source that may reduce strength and stiffness and limit serviceability. Wide spread delaminations are frequent outcome of LVI.   Hence, it is essential to develop predictive mechanical models for LVI damage.

 In this study, a through-thickness meso-mechanical sublaminate homogenization model is proposed for the damage analysis of composite plates subjected to low-velocity impact (LVI), see Figure-1. The sublaminate model performs through-thickness homogenization using the smallest repeated stack sequence of the multi-layered laminate. In-plane and out-of-plane damage models are introduced and examined for their ability to initiate and propagate progressive damage in the laminate. The sublaminate includes a thin cohesive layer that represents the intra-layer delamination damage mode.  Out-of-plane normal and shear failure criteria are combined to detect cohesive damage. The proposed sublaminate damage framework is implemented as an external user material of theABAQUS/explicit FEA program. LVI analyses of composite plates are performed and the resulted impact damage shapes are compared with test results taken from the literature.  Good prediction ability is shown for impact of multi-layered plates having several repeated stacks through their thickness.

Bio-Composites Based on Unique Coral Collagen Fibers towards

Tissue Engineered Blood Vessels

Shir Wertheimer

M.Sc. student of Prof. Rami Haj-Ali

Cardiovascular Diseases (CVDs), e.g. peripheral arterial diseases, cerebrovascular disease, and particularly coronary artery occlusion, constitute the leading cause of death worldwide. Approximately 31% of all global deaths are caused by CVDs, of which 42% are attributable to coronary artery disease. These are manifested by, inter alia, the mechanical degradation of the blood vessels.  The mechanical strength of the blood vessel is provided primarily by type I collagen fibers which are mainly found in the outer layer of blood vessels, known as the adventitia.

This study introduces a new bio-composite material system consisting of unique and long collagen fibers derived from corals – embedded within an alginate hydrogel matrix. The new bio-composite layers were used to fabricate a graft to be used towards tissue engineered blood vessels.  These constructs consisted of both circumferentially and longitudinally oriented collagen fibers. Mechanical properties of the grafts such as compliance and hoop stress-strain curve were investigated via a new experimental setup constructed in our lab for this purpose, allowing applied internal pressure levels (0-300 mmHg) with measured external deformations using an optical extensometer. Furthermore, numerical finite element simulations of the graft predicted close stiffness values to the measured, especially for relatively high pressure loads.  

Next, a biological biocompatibility study was conducted to examine cell growth within the manufactured composite construct.  Towards that goal, fibroblast cells were seeded within the collagen fibers alone and monitored in order to generate quantitative growth metrics. Subsequently, different bio-composite collagen-reinforced hydrogels were also fabricated and cell-seeded for similar bio-compatibility studies. The cells in all samples used for the biocompatibility tests were alive and well-functioning for more than 28 days.  They demonstrated higher growth rates during the first two weeks, followed by lower yet generally positive growth rates for the following two weeks. The cell orientation was shown to follow that of the collagen fibers for the duration of the experiment.

The novelty of this study is manifested in the use of naturally derived long collagen fibers for the development of a new class of tissue-engineered grafts. The proposed novel bio-composite and associated construct show a great potential for future tissue engineered replacements of blood vessels.

 (The talk will be given in English)

Speaker:     Dr. Amit Daniely
                     School of Computer Science and Engineering, Hebrew university 

Wednesday, May 2^nd, 2018
15:00 - 16:00

Room 011, Kitot Bldg., Faculty of Engineering

On Learning Theory and Neural Networks

Abstract

Can learning theory, as we know it today, form a theoretical basis for neural networks? I will try to discuss this question in light of classical and new results

Based on joint work with Roy Frostig, Vineet Gupta and Yoram Singer

School of Mechanical Engineering Seminar
Wednesday, June 13, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Scrutiny of Instabilities in Czochralski Crystal Growth Configuration Using Complementary Experimental Technologies

Mr. Evgeny Miroshnichenko

Mr. Evgeny Miroshnichenko

Ph.D Student of Prof. E. Kit

A parametric experimental study of cold plume instability that appears in the large-Prandtl-number Czochralski melt flows is reported. The critical temperature difference (the critical Grashof number) and the frequency of appearing oscillations were measured for varying Prandtl numbers, aspect ratios of the melt, and crystal/crucible radii ratio. The measurements were carried out by two independent and fully non-intrusive experimental techniques. The results are reported as dimensionless parametric dependencies, and then are joined into relatively simple empirical relations.

The parametric relations for the critical Grashof number and oscillations frequency are extended to include parameters of the capillary meniscus height for different Prandtl numbers, radii and aspect ratios. The results show that with increase of the meniscus height the critical temperature difference noticeably grows and sometimes doubles. The difference between results obtained for “short” and “tall” menisci is noticeable. This might explain the saturation of the dependence of the critical Grashof number on meniscus height. An additional qualitative experiment using PVC tube replaces the meniscus allows to observe even reverse the sense of the temperature dependence following the saturation.

Modeling of effects associated with the nonlinear crystal front shows different flow regimes distributed under the dummy. The results show that the presence of liquid filled cavity inside the crystal reduces dramatically the stability factor, even while axial symmetry preserved. Experiments preventing surface tension by covering of free surface estimated a contribution of Marangoni effect to instability. Therefore, a correction term considering these effects is required for the forgoing critical Grashof number and the frequency relations.

Two- and three dimensional flow patterns of supercritical regimes and different types of instability occurs in Czochralski model were visualized using Schlieren method.

Baroclinic instability caused by dummy rotation were also considered in purpose to examine Richardson/Reynolds numbers dependence over widened range of rotation rates.