Simulation of the Effects of an Air Blast Wave - ELSA - Europa

7 Implementation of an Air Blast Loading Function7.1 MotivationCalculations using the explosive with the JWL-equation may produce accurate results but need alsosmall elements. This results in small time step sizes and large computation times. There are severalpossibilities to reduce this cost, as shown in chapter 3. One of these is the use of a load-timefunction instead the fluid-structure interaction.This load-time function can be used if reflections of the air blast wave do not have an influence andif there is no need to consider the shadowing of the structure. The load-time function depends onthe size of the charge, the distance from the charge and some other conditions. The advantage ofthis method is the reduction of the computation time.Calculations with this method can be used to get an idea of the behaviour of the structure and to validateresults of a fluid-structure-interaction for special cases.7.2 Used FunctionThe modified Friedlander equation from Baker [2] (see equation(4)) can be used for the implementation.The parameters of this equation can be chosen from Kingery [14] or from Baker [2]. Theparameter b for the (decay) can be calculated with the knowledge of the maximum negativepressure or with the knowledge of the impulse. The value of b, as calculated in chapter 2.2.5, willbe used here.7.3 ImplementationThe load-time function is implemented as a new impedance. Impedances enable the input of boundaryconditions for special elements (CLxx) lying over the common elements. The command for theimpedance air blast wave (IMPE AIRB) allows the definition of a size of the charge, the origin ofthe detonation and the starting time of the detonation. Different conditions with different parametersare possible. Spherical and hemispherical explosion can be considered, also with reflectionconditions. The following conditions are possible by choosing the CONF parameter:1 = spherical detonation (full space), reflection conditions, equation form Kingery2 = spherical detonation (full space), no reflection conditions, equation form Kingery3 = spherical detonation (full space), no reflection conditions, equation form Baker4 = hemispherical detonation (half space), reflection conditions, equation form Kingery62