School of Mechanical Engineering Seminar
Wednesday, June 12^th, 2019 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

ENERGY ABSORBING LOAD LIMITER OPTIMIZATION

Nadav Zimron Politi

M.Sc Student of Prof. Shmuel Ryvkin

The landing dynamics of a legged landers in planetary missions, such as the “Beresheet” lunar lander, is a key point in the success of such missions. In order to fulfil a successful landing, the Landing Gear must have energy absorbing capabilities in order to prevent high decelerations passing through to the spacecraft structure. The energy absorbing elements must have both tension and compression capabilities, withstand extreme outer space conditions and have a maximum energy absorbing capability to mass ratio.

This work presents a method for the solution of elastic-plastic finite deformation of curved bars with rectangular cross-section. An incremental procedure is implemented for the analysis of an initially curved bar, with a varying curvature, varying cross section and elastic-plastic linear hardening material properties. The simulation is verified against Elastica solutions for the constant cross-section elastic case and with test result for a curved bar with varying cross section undergoing elastic-plastic finite deformations.

Nonlinear Dynamics and Stability of
Optically Torqued Nanoparticles

Tomer Berghaus

   Professor Touvia Miloh-Tel Aviv University

Professor Oded Gottlieb-Technion Israel Institute of Technology

In this study we investigate the nonlinear dynamics and stability of optically torqued nano-ellipsoids, immersed in a liquid, subjected to a linear polarized electromagnetic excitation in the Rayleigh regime, i.e., when the particle size is considerably smaller than the exciting wave length. The orientation of nonspherical particles can be controlled through the torque exerted by the radiation carrying spin angular momentum. In recent years, this subject has received a growing amount of attention, but no comprehensive three-dimensional model of the problem has been found in the literature. The general equations of motion for the nanoparticle, in the Rayleigh regime under the assumption of low Reynolds number, are formulated using Euler angles, and as a result yield a set of coupled nonlinear differential equations, which are analyzed analytically and numerically. The analytical investigation for single-frequency excitation is performed on an autonomous formulation which reveals multiple coexisting equilibrium solutions that can exhibit loss of asymptotic stability culminating with periodic and nonstationary chaotic like solutions.  

Significant parts The first part of the study were was devoted to comparing the resulting numerical and analytic solutions we received for the planar configuration to experimental and analytical results published in previous studies. We distinguished between a number of limiting cases, that give a general view of the problem, and discuss their stability. It was found that the planer conic dynamics of an ellipsoid is obtained from degeneration of the three-dimensional equations is governed by a saddle-node bifurcation. In addition a Hopf bifurcation was discovered in the Eulerian frame for 'prolate' and 'oblate' ellipsoids. Three-dimensional rotational dynamics of ellipsoid subject to single-frequency excitation produced, for different values of the bifurcation variables, periodic, quasiperiodic and chaotic like behavior in the Cartesian space.

Such a model is critical to the understanding of more complicated dynamics that arise from additional kinds of excitation of the nanoparticle and to hydrodynamics of nanofluidic motion

Once the analysis model has been verified, a parametric study is conducted in order to optimize the load limiter mass in the framework of keeping the same force-stroke relation and without reaching failure. The suggested method is implemented for improvement of a basic circular shape load limiter. It was shown that at least 4% improvement by mass can be achieved.

The presented thesis provides a simple and convenient tool for the design process of energy absorbing load limiters. The load limiter force selection and required stroke are linked with the resulting load limiter mass.

Speaker: Elad Domanovitz

Ph.D. student under the supervision of Prof. Uri Erez

Wednesday, May 1^st, 2019 at 15:00
Room 011, Kitot Bldg., Faculty of Engineering

Diversity Combining via Universal Dimension-Reducing Space-Time Transformations

Abstract

Receiver diversity combining methods play a key role in combating the detrimental effects of fading in wireless communication and other applications. A novel diversity combining method is proposed for multiple-antenna reception, where a universal, i.e., channel independent, orthogonal dimension-reducing space-time transformation is applied prior to quantization of the signals. The scheme may be considered as the counterpart of Alamouti modulation, and more generally of orthogonal space–time block codes.  For the case of two antennas, the scheme provides the same worst-case performance as that afforded by selection diversity.

An important application of the scheme is to a scenario involving a single-antenna source node communicating with a destination node that is equipped with two antennas via multiple single-antenna relay nodes, where each relay is subject to an individual power constraint, where ultra-reliable and low-latency communication are desired. The latter requirement translates to considering only schemes that make use of local channel state information. Whereas for a receiver equipped with a single antenna, distributed beamforming is a well-known and adequate solution, no straightforward extension is known to the case of multiple receive antennas. We demonstrate how the novel receive diversity scheme enables to extend the distributed beamforming method to such scenarios. In comparison to a single-antenna destination node, the method doubles the diversity order without requiring any channel state information at the receiver while at the same time retaining the array gain offered by the relays.

Speaker: Jordan Yaniv

M.Sc. student under the supervision of Prof. Ariel Shamir and Prof. Shmuel Avidan

Wednesday, April 17th, 2019 at 15:00

Room 011, Kitot Bldg., Faculty of Engineering

The Face of Art: Landmark Detection and Geometric Style in Portraits

Abstract

Facial Landmark detection in natural images is a very active research domain. Impressive progress has been made in recent years, with the rise of neural-network based methods and large-scale datasets. However, it is still a challenging and largely unexplored problem in the artistic portraits domain. Compared to natural face images, artistic portraits are much more diverse. They contain a much wider style variation in both geometry and texture and are more complex to analyze. Moreover, datasets that are necessary to train neural networks are unavailable.

We propose a method for artistic augmentation of natural face images that enables training deep neural networks for landmark detection in artistic portraits.

We utilize conventional facial landmarks datasets, and transform their content from natural images into ``artistic face'' images. In addition, we use a feature-based landmark correction step, to reduce the dependency between the different facial features, which is necessary due to position and shape variations of facial landmarks in artworks.

To evaluate our landmark detection framework, we created an ``Artistic-Faces'' dataset, containing 160 artworks of various art genres, artists and styles, with a large variation in both geometry and texture.

Using our method, we can detect facial features in artistic portraits and analyze their geometric style. This allows the definition of signatures for artistic styles of artworks and artists, that encode both the geometry and the texture style. It also allows us to present a geometric-aware style transfer method for portraits.