Hi everyone,
A brief introduction for those of you who do not know me...
I work in the Engineering Specialists group of SIMULIA and I focus on the acoustics and blast features of our product, Abaqus. During my support work and my interaction with the customers, I get the following question a lot...
Do I use the acoustic elements in Abaqus to model blasts? The answer to this question deserves a bit of explanation to lay things in perspective.
Two types of blast analysis are possible in Abaqus: underwater explosions and in-air explosions.
Underwater explosions:
1. Far-field underwater explosions can be modeled with the incident wave interaction approach, in which the water surrounding the submarine is modeled with acoustic elements. The reasoning behind this approach is that the acoustic pressures generated by the explosion (i.e., the pressure fluctuations about the mean), although capable of causing damage due to whipping phenomena, are small in comparison to the mean pressure in the water.
2. In near-field underwater explosions, there are two primary contributors to damage: 1. Very high over-pressures and 2. Direct contact with the explosive. These effects require special modeling techniques, including 1. Nonlinear pressure calculation, 2. Mesh motion, and 3. Contact. Acoustic elements in Abaqus possess inherent limitations that prevent using the incident wave interaction approach in this case. These limitations include an assumed linear response (which prevents accurate nonlinear pressure calculations), the inability to model changing contact conditions (due to the use of structural-acoustic tie constraints), and the lack of displacement degrees of freedom. To overcome these limitations, the Coupled Eulerian-Lagrangian (CEL) approach, together with the use of equation of state (EOS) material properties for the medium (to model nonlinear pressures), is a viable alternative for simulating near-field underwater explosions.
In-air explosions:
Since the mean pressure in air is much smaller than that in water, nonlinear effects become more pronounced at lower acoustic pressures. The implication is that acoustic elements cannot be used for either far-field or near-field in-air explosions.
To model far-field in-air explosions, consider that air applies a negligible mass loading on the structure---its only function is to serve as a medium to propagate the pressure from the source to the structure. If an empirical relation for the pressure due to the explosion is available, the loading can be applied directly to the structure. This eliminates the need to explicitly model the air, thus, reducing computational costs. A widely used empirical relation for the explosive pressure on a structure, CONWEP, is available in Abaqus as part of the incident wave interaction formulation.
Near-field in-air explosions can be modeled with the CEL approach; the modeling technique is similar to that for near-field underwater explosions.
Conclusions:
1. Far-field underwater explosions can be modeled with acoustic elements.
2. Near-field underwater explosions can be modeled using the CEL technique (acoustic elements cannot be used).
3. Far-field in-air explosions do not require modeling of the medium if an empirical relation for the pressure on the structure is known.
4. Near-field in-air explosions can be modeled using the CEL technique.
