School of Mechanical Engineering Seminar
Wednesday, June 12th, 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.
