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The Israel Defense Forces (IDF; צְבָא הַהֲגָנָה לְיִשְׂרָאֵל , The Army of Defense for Israel), alternatively referred to by the Hebrew-language acronym Tzahal (צה״ל), is the national military of the State of Israel. It consists of three service branches: the Israeli Ground Forces, the Israeli Air Force, and the Israeli Navy. It is the sole military wing of the Israeli security apparatus. The IDF is headed by the Chief of the General Staff, who is subordinate to the Israeli Defense Minister. On the orders of David Ben-Gurion, the IDF was formed on 26 May 1948 and began to operate as a conscript military, drawing its initial recruits from the already-existing paramilitaries of the Yishuv—namely Haganah, the Irgun, and Lehi. Since its formation shortly after the Israeli Declaration of Independence, the IDF has participated in every armed conflict involving Israel. While it originally operated on three major fronts—against Lebanon and Syria in the north, against Jordan and Iraq in the east, and against Egypt in the south—the IDF has primarily shifted its focus to southern Lebanon and the Palestinian Territories since the 1979 Egypt–Israel peace treaty and 1994 Israel–Jordan peace treaty. However, notable Israeli–Syrian border incidents have occurred frequently since 2011 due to regional instability caused by the ongoing multi-sided Syrian Civil War. The IDF is unique among the militaries of the world due to its regulated conscription of women since its formation. It is one of the most prominent institutions in Israeli society due to its influence on the country's economy and political scene. It uses several technologies developed within Israel, with many of them made specifically to cater to its needs in its operational environment in the Levant. Prominent Israeli-developed military equipment includes: the Merkava, a main battle tank; the Achzarit and Namer - armored personnel carriers; Iron Dome, an air defense system; Trophy, an active protection system; IDF Caterpillar D9 armored bulldozer; Spike anti-tank guided missile family; MATADOR shoulder-launched rocket; the IMI Galil and IWI Tavor families of assault rifles; and the Uzi, a family of submachine guns.

Large Eddy Simulation of Particle Dynamics Inside a Differentially Heated Cavity

Christoph Bosshard

In nuclear safety, most severe accident scenarios lead to the presence of fission products in aerosol form in the closed containment atmosphere. It is important to understand the particle depletion process to estimate the risk of a release of radioactivity to the environment should a containment break occur. As a model for the containment, we use the three-dimensional differentially heated cavity (DHC) problem. DHC is a cubical box with a hot wall and a cold wall on vertical opposite sides. On the other walls of the cube we have adiabatic boundary conditions. For the velocity field the no-slip boundary condition is valid. The flow of the air in the cavity is described by the Boussinesq equations. Complex flow patterns develop and the flow characteristics depend on the non-dimensional Rayleigh and Prandtl numbers. The predominant flow type in the DHC is a turbulent natural convection flow. This work aims at reaching Rayleigh numbers and turbulent levels as high as possible given the available computational resources. The method used to simulate the turbulent flow is the large eddy simulation (LES) where the dynamics of the large eddies is resolved by the computational grid and the small eddies are modelled by the introduction of subgrid scale quantities using a filter function. Numerically, the LES equations are discretized by the spectral element method. Particle trajectories are computed using the Lagrangian particle tracking method, including the relevant forces (drag, gravity, thermophoresis). Four different particle sets with each set containing one million particles and diameters of 10 μm, 15 μm, 25 μm and 35 μm are simulated. The complexity and the size of the large three-dimensional problem requires the use of massively parallel supercomputers. Spectral element methods are naturally suitable for parallelisation by distributing the elements among the processors. For the Lagrangian particle tracking we use a method where equal numbers of particles are assigned to every processor. The flow field is broadcast and every particle processor tracks the assigned particles, a procedure which leads to a perfect load balancing. Simulation results for the flow field and particle sizes from 15 μm to 35 μm at a Rayleigh number of 109 are compared to previous results from a direct numerical simulation. First order statistics of the LES flow fields are in very good agreement with the direct numerical simulation while the agreement of second order moments is fair. Also the turbulent structures associated to the maximum of turbulent kinetic energy production are correctly reproduced. Particle statistics in the LES and the direct numerical simulation were similar and the settling rates practically identical. Contrary to previous particle simulations in LES, it was found that no model was necessary for the influence of the unresolved flow scales on the particle motions. This can be explained, because the important settling mechanism is through gravity and particle deposition at the walls by turbophoresis is negligible.

EPFL2012

Large Eddy Simulation of Particle Dynamics Inside a Differentially Heated Cavity

Christoph Bosshard

EPFL2012