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Kovalev
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К
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А.Сухоруков
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23.06.2002 00:43:00
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Рубрики
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Прочее;
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Nemnogo o detonacii
Perevodit mne len' - esli kto English ne uchil - to vkratce detonaciua - ochen chrenovaya shtuka dlia tech kto blizko stoit.
Detonation - An explosion in which the speed of the reaction front exceeds the speed of sound in a material and hence the reaction front precedes the shock wave
IN the study of weapons a low explosive burns but a high explosive detonates-a very different phenomenon. An initial shock compresses a high-explosive material heating it and causing chemical decomposition. The formation of chemical products releases enormous amounts of energy in just billionths of a second. This process sustains the shock wave which travels at supersonic velocity. All of this happens almost instantaneously to produce a blast of rapidly expanding hot gases.
In the brief instant of a high-explosive detonation some remarkable events take place: the shock wave produces pressure up to 500 000 times that of Earth's atmosphere the detonation wave travels as fast as 10 kilometers per second temperatures can soar to 5 500 kelvins and power approaches 20 billion watts per square centimeter.
There are many problems of interest to the Army that involve high-velocity impact or explosive detonation. Included are gun-launched penetrators impacting armored targets air-delivered penetrators impacting buried concrete targets detonation of warheads and blast effects on structures such as buildings and ships. These problems involve large distortions of the materials at high loading rates and they are a challenge to accurately model. Because testing of these events is expensive and time-consuming it is very helpful to the engineer/scientist to accurately simulate them on the computer. This capability leads to better designs and shorter design cycles. This article will focus on the state-of-the-art technique of combining finite elements and meshless particles to model high-velocity impact and explosive detonation.
This class of problems involves both severe deformations of solids or large fluid flows and small-deformation structural responses. Recent advances in particle methods have included the robust simulation of severe deformations and flows. Since finite elements model structural response with significantly greater efficiency it is desirable to combine these two numerical techniques for this class of problems and use the particles only for the material undergoing large deformations or flows.
What is a Detonation Wave?
Detonation waves are perhaps the most extensively studied and severe of all the combustion processes and were discovered over a century ago when Mallard and LeChatelier and Berthelot and Vielle discovered that low velocity flames propagating in a reactive gaseous mixture could suddenly acquire very high velocities accompanied by substantial increases in temperature and pressure near the flame front. It was then realised that this supersonic combustion consisted of a shock front closely followed by a reaction - the two components of a detonation wave.
Characteristics of a detonation wave
In a simple one dimensional theory a detonation wave can be regarded as consisting of an extremely strong shock wave closely followed by an exothermic reaction capable of providing enough energy to sustain the wave. It is far more violent and destructive than a shock wave due to the presence of a greater amount of energy produced from the reaction and the chemical combustion is continuously initiated by the adiabatic compression and heating of the gaseous medium behind the shock front. Two of the main characteristics of a detonation are that it propagates with a steady constant velocity and a sharp peak in pressure called the Von Neumann spike is observed at the detonation front. This is associated with the finite rate chemistry which takes place after an extremely short induction time. High instantaneous over-pressures are therefore associated with detonations particularly in the transition to detonation process.
Initiation of a detonaton
Detonations may be initiated by a variety of techniques in all of which the degree of confinement or the geometry of the container play a dominant role. These include initiation from an accelerating flame (deflagration to detonation transition) direct initiation by a high energy spark or by shock wave initiation. In the latter method if the induced chemical reactions are extremely exothermic then the shock wave may accelerate and use the chemical energy released from the reactions to undergo some form of explosive transition. Equilibrium is then reached the shock wave is self supporting and a detonation wave travelling at a characteristic velocity is established.
See also detonation computer simulation pictures at:
http://www.math.ntnu.no/~andreas/fronttrack/gas/box/