Luttwak G.,Rafael Laboratories |
Falcovitz J.,Hebrew University of Jerusalem
Computers and Fluids | Year: 2013
A recently formulated frame-invariant monotonicity criterion and slope limiter for vectors was applied to the Staggered Mesh Godunov-SMG/Q scheme for Lagrangian and ALE hydrodynamics. The VIP (vector-space polygon or polyhedron) was shown to be a natural extension of monotonicity constraints from scalar to vector variables. Taking notice of the fact that gradients of scalars are vectors, we now seek to use this new concept to devise better, and perhaps truly multidimensional, slope limiters for scalar variables. The proposed scheme constitutes a generalization of a 1D monotonic-averaging limiter for scalars, to a VIP type limiter for (vector) gradients of scalars in 2D or 3D. Test cases computed by the SMG/ALE scheme, using the new VIP limiter for gradients, are presented. As we can see from them, the new method, while being robust in strong shock computations, can better preserve gradients of density under advection. © 2012 Elsevier Ltd.
Rosenberg Z.,Rafael Laboratories |
Dekel E.,Rafael Laboratories
International Journal of Solids and Structures | Year: 2010
The process of ductile plate perforation by sharp-nosed rigid projectiles is further examined in this work through 2D numerical simulations. We highlight various features concerning the effective resisting stress (σr) which a finite thickness plate, with a flow stress of Yt, exerts on the projectile during perforation. In particular, we show that the normalized resisting stress (σr/Yt) can be represented as a unique function of the normalized thickness of the plate (H/D, where H is plate thickness and D is projectile diameter), for a large range of normalized thicknesses. Our simulations for very thin target plates show that the penetration process is achieved through the well-known dishing mechanism, where the target material is pushed forward by the projectile's nose. An important observation, which emerges from our simulations, is that the transition between the dishing and the hole enlargement mechanisms takes place at a normalized thickness of about H/D = 1/3. We also find that the normalized resistive stress for intermediate plate thicknesses, 1/3 < H/D < 1.0, is relatively constant at a value of σr/Yt = 2.0. This range of thicknesses conforms to a state of quasi plane stress in the plates. For thicker plates (H/D > 1) the σr/Yt ratio increases monotonically to values which represent the resistance to penetration of semi-infinite targets, where the stress state is characterized by plane strain conditions. Using a simple model, which is based on energy conservation, we can predict the values of the ballistic limit velocities for many projectile/target combinations, provided the perforation is done through the ductile hole enlargement mechanism. Good agreement is demonstrated between predictions from our model and experimental data from different sources, strongly enhancing the confidence in both the validity and usefulness of our model. © 2010 Elsevier Ltd. All rights reserved.
Rusnak I.,Rafael Laboratories
Journal of Guidance, Control, and Dynamics | Year: 2010
In preliminary analysis of guided systems, it is required to assess the miss distance performance from some small set of parameters. This paper presents analytical formulas of bounds on the achievable rms miss by a radar-guided missile against a variety of target maneuvers. These formulas use a set of core parameters that affect the miss distance; thus, they can be used for synthesis and analysis of the performance of radar-guided tactical missiles. The bound is derived, subject to the assumption that the missile guidance law and estimator are fully matched to the missile dynamics, the target maneuver, and the glint noise. The glint is the dominant noise source, the missile applies frequency agility, and there is no missile acceleration limit. No system can achieve smaller rms miss distance than the one presented, subject to the stated assumptions. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Djerassi S.,Rafael Laboratories
Multibody System Dynamics | Year: 2016
This work addresses the ‘hard collision’ approach to the solution of planar, simple non-holonomic systems undergoing a one-point collision-with-friction problem, showing that (i) there are no coherent types of collision whereby forward sliding follows sticking, unless the initial relative tangential velocity of the colliding points vanishes; and (ii) the type of collision can be determined directly, given the collision angle of incidence (Formula presented.) and Coulomb’s coefficient of friction (Formula presented.) between the colliding points. The classic hitting rod problem is used to illustrate the (Formula presented.)–(Formula presented.) collision-type dependence. Finally, the relation between collision with friction and tangential impact problems in multibody systems is briefly discussed. © 2016 Springer Science+Business Media Dordrecht
Marcus S.W.,Rafael Laboratories
IEEE Transactions on Antennas and Propagation | Year: 2013
The optical transformation is generally used to deform free space in order to obtain novel effects on waves. Such effects can be realized by replacing the deformed region by a 'metamaterial' characterized by ε and μ tensors that are determined by the transformation. A new type of transformation is proposed which not only deforms a free space region, but transforms a non-trivially shaped conducting surface that bounds this region as well. The deformations are designed to map the original region into a planar slab of metamaterial, and to map the conducting surface into a plate which bounds this slab. Since the fields outside the deformed region-which include the fields scattered by the original conducting surface-are unaffected by the transformation, they can, in effect, be produced by scattering from the planar metamaterial-coated plate created from the transformation. This substitution of a non-trivially shaped conducting surface by a planar metamaterial-coated plate is applied to (a) a face of a wedge to produce reflection from the planar metamaterial-coated plate that is not in the specular direction, and (b) a parabolic reflector antenna to design a planar system which produces the same radiation pattern as the reflector. Both devices are numerically validated. © 2013 IEEE.
Sason I.,Technion - Israel Institute of Technology |
Shuval B.,Rafael Laboratories
IEEE Transactions on Information Theory | Year: 2011
A design of robust error-correcting codes that achieve reliable communication over various channels is of great theoretical and practical interest. Such codes are termed universal. This paper considers the universality of low-density parity-check (LDPC) code ensembles over families of memoryless binary-input output-symmetric (MBIOS) channels. Universality is considered both under belief-propagation (BP) and maximum-likelihood (ML) decoding. For the BP decoding case, we derive a density-evolution-based analytical method for designing LDPC code ensembles that are universal over various families of MBIOS channels. We also derive a necessary condition for universality of LDPC code ensembles under BP decoding; this condition is used to provide bounds on the universally achievable fraction of capacity. These results enable us to provide conditions for reliable/unreliable communications under BP decoding that are based on the Bhattacharyya parameter of the channel. For the ML decoding case, we prove that properly selected regular LDPC code ensembles are universally capacity-achieving for the set of equi-capacity MBIOS channels and extend this result to punctured regular LDPC code ensembles. © 2011 IEEE.
Marcus S.W.,Rafael Laboratories
IEEE Transactions on Antennas and Propagation | Year: 2015
Previous determinations of bistatic radar cross section (RCS) of dipole clouds assumed incoherent scattering which does not hold near the forward scatter direction. In order to obtain unified expressions that are valid for scattering in all directions, the average bistatic RCS of a spherical chaff cloud is derived as the sum of incoherent and coherent contributions. Simplified forms for these expressions are provided for five representative directions/polarizations. In the forward scatter direction, the coherent contribution is dominant, but in the angular region about that direction, the bistatic RCS pattern is beam-like and is dependent on both the coherent and incoherent contributions. An expression is provided for the width of this 'beam.' The derived expressions are verified by statistical numerical RCS computations based on the method of moments. Since these numerical calculations include interparticle coupling, whereas the derived expressions assume that such coupling is not present, the numerical results also provide criteria for the interparticle distance that is required for this coupling to be ignored in bistatic RCS calculations. This distance is seen to depend on the direction of scatter. © 1963-2012 IEEE.
Partom Y.,Rafael Laboratories
Journal of Energetic Materials | Year: 2013
Shock initiation of homogeneous explosives has been a subject of research since the 1960s, with neat and sensitized nitromethane as the main materials for experiments. A shock initiation model of homogeneous explosives was established in the early 1960s. It involves a thermal explosion event at the shock entrance boundary, which develops into a superdetonation that overtakes the initial shock. In recent years, Sheffield and his group, using accurate experimental tools, were able to observe details of buildup of the superdetonation. There are many papers on modeling shock initiation of heterogeneous explosives, but there are only a few papers on modeling shock initiation of homogeneous explosives. In this article, bulk reaction reactive flow equations are used to model homogeneous shock initiation in an attempt to reproduce experimental data of Sheffield and his group. It was possible to reproduce the main features of the shock initiation process, including thermal explosion, superdetonation, input shock overtake, overdriven detonation after overtake, and the beginning of decay toward Chapman-Jouget (CJ) detonation. The time to overtake (TTO) as function of input pressure was also calculated and compared to the experimental TTO. © 2013 Copyright RAFAEL.
Djerassi S.,Rafael Laboratories
Multibody System Dynamics | Year: 2012
This paper deals with one-point collision with friction in three-dimensional, simple non-holonomic multibody systems. With Keller's idea regarding the normal impulse as an independent variable during collision, and with Coulomb's friction law, the system equations of motion reduce to five, coupled, nonlinear, first order differential equations. These equations have a singular point if sticking is reached, and their solution is 'navigated' through this singularity in a way leading to either sticking or sliding renewal in a uniquely defined direction. Here, two solutions are presented in connection with Newton's, Poisson's and Stronge's classical collision hypotheses. One is based on numerical integration of the five equations. The other, significantly faster, replaces the integration by a recursive summation. In connection with a two-sled collision problem, close agreement between the two solutions is obtained with a few summation steps. © Springer Science+Business Media B.V. 2011.
Bar D.E.,Rafael Laboratories |
Raboy S.,Rafael Laboratories
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2013
A novel approach to automatic detection of moving vehicles from a single satellite WorldView-2 imagery is presented. The technique is based on the time gap between three sensor band groups: panchromatic, multispectral, and four new additional multispectral bands. The entire process is automatic and includes movement estimation followed by moving object spectral restoration and construction of spatially built object profiles to estimate the movement direction and velocity. The approach neither relies on external information like road data or site models, nor is limited to vehicle type. The performance of the new approach is demonstrated via detection of several vehicle types on WorldView-2 satellite imagery of the San Francisco area. © 2008-2012 IEEE.