blastFoam | Comparison of explosive airblast calculators
In this short article we compare blastFoam simulation results with other software and semi-empirical methods to provide confidence in the code's ability to accurately calculate airblast parameters.
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Comparison with Software
In 2013 Browning [1] used the programs Ansys LS- DYNA and CTH to simulate an explosion of 3,629kg (8,000lb) of TNT (spherical free-air burst), with a charge radius of 0.81m. The simulations were also conducted by Shin [2] while exploring the simulation capabilities of the explicit code Ansys Autodyn, and by Karlos [3] with the EUROPLEXUS (EPX) code. In 2021, simulations were also conducted by the Synthetik team using the opensource blastFoam code.
Incident peak overpressure and incident impulse results were recorded at scaled distances of 1.19 (18.3m), 1.59 (24.4m) and 1.98 m/kg^(1/3) (30.5m) for a range of mesh resolutions: cell sizes of 25.4, 50.8, and 76.2 mm.
All simulations employed the Jones-Wilkins-Lee (JWL) equation of state for the TNT charge.
The results show that all computer codes calculate similar peak incident overpressure values for the examined scaled distances. As anticipated, the codes give slightly higher peak overpressure values if a finer resolution mesh is used. Similar conclusions can be drawn for the incident impulse values, however, both Ansys LS-DYNA and EPX do see higher impulse values with coarser meshes.
The variations in the results from these programs can likely be attributed to the different techniques and solvers used when calculating the activation of the JWL material and the subsequent propagation of the blast wave. Although, perhaps an area deserving of additional focus is the relatively wide disparity in impulse predictions between each of the semi-empirical methods and the software outputs.
Comparison with Semi-Empirical References
In addition to high-fidelity physics-based methods such as Computational Fluid Dynamics (CFD) software, blast wave parameters can be calculated through various informative sources that are available in the open literature. Among the most reliable and widely used references are: 1) the work of W.E. Baker’s 'Explosions in Air' [4] and a handbook of a similar title by the U.S. Army Materiel Command, and 2) a technical report by Kingery and Bulmash [5] that has been incorporated into U.S. military publications and software.
These references produce results that are similar for pressure but there are some significant differences in the values of specific impulse [6]. As both methods are based on experimental data, the relative accuracy of each is difficult to determine, and this raises the question of why there are differences in two seemingly accepted methods.
Both 'Explosions in Air' and 'Kingery and Bulmash' make use of research data from others, and Kingery notes that various parameters have been measured in different ways [5]. Thus, the differing impulse values could be attributed to blast duration measurements as well as a change in the way that the decay values were determined.
Goodman [7], Baker [4], and Bulson [8] make a point of the fact that due to conditions, differing materials, differing instruments, and other things, it is hard to get two explosions by differing researchers to agree. Furthermore, all authors noted that their data were not internally consistent, because the properties at the front were based on experimental data alone and the time-histories were based primarily on adjusted theoretical calculations.
As an example, the modified Friedlander equation is used ubiquitously as a general or idealized form of a pressure-time history after a detonation event, and the equation can generate a curve that matches several positive-phase parameters (e.g., time of arrival, peak pressure, positive phase duration, and specific impulse) without matching the empirical wave form [9].
These issues are exacerbated for the negative-phase, as only positive-phase parameters govern the Friedlander profile, and hence the wave form below ambient pressure is very unlikely to match empirical predictions.
The time-shifted data facilitates better direct comparison, and one can observe how the modified Friedlander ‘fit’ to the positive-phase parameters result in curves that are asymptotic to ambient pressure (i.e., an overpressure of 0 kPa). By contrast, blastFoam delivers a wave form and specific impulse that fall between the semi-empirical predictions, and also reduces to ambient pressure in a manner that is much more likely to give realistic negative-phase values, and that is more representative of what is likely to be observed by instrumentation during an explosive test.
References
Browning S., Sherburn A., Schwer E., (2013) “Predicting blast loads using LSDYNA and CTH”, Proceedings, ASCE Structures Congress, Pittsburgh.
Shin J., Whittaker A., Amjad A., (2014) “Air-blast effects on civil structures”, Technical Report MCEER-14-0006, University of Buffalo, NY.
Karlos V., Solomos G., Larcher M., (2016) “Analysis of blast parameters in the near-field for spherical free-air explosions”, JRC Technical Report.
Baker, W.E., Explosions in Air, University of Texas Press, Austin, 1973.
Kingery, C.N. and Bulmash, G., “Technical Report ARBRL-TR-02555: Air Blast Parameters from TNT Spherical Air Burst and Hemispherical Surface Burst.” AD-B082713, U.S. Army Ballistic Research Laboratory, Aberdeen Proving Ground, MD, April 1984.
J. M. K. Chock, “Review of Methods for Calculating Pressure Profiles of Explosive Air Blast and its Sample Application,” p. 143.
H. J. Goodman, Compiled Free-Air Blast Data on Bare Spherical Pentolite, Ballistic Research Laboratories Report No. 1092, February 1960.
Bulson, P.S., Explosive Loading of Engineering Structures, E & FN Spon, London, 1997.
S. E. Rigby, A. Tyas, T. Bennett, S. D. Clarke, and S. D. Fay, “The Negative Phase of the Blast Load,” International Journal of Protective Structures, vol. 5, no. 1, pp. 1–19, Mar. 2014, doi: 10.1260/2041-4196.5.1.1.
Software Code Availability
blastFoam: https://github.com/synthetik-technologies/blastfoam
Ansys Autodyn: https://www.ansys.com/products/structures/ansys-autodyn
Ansys LS-DYNA: https://www.ansys.com/products/structures/ansys-ls-dyna
EUROPLEXUS: https://ec.europa.eu/jrc/en/scientific-tool/europlexus-simulation-software
CTH: https://www.sandia.gov/cth/
About blastFoam:
blastFoam is an opensource solver for multi-component compressible flow with application to high-explosive detonation, explosive safety and airblast.
The solver is based on the OpenFOAM® framework and provides solutions to highly compressible systems including single and multi-phase compressible flow, and single- and multi-velocity systems.
blastFoam provides implementations of the essential numerical methods (e.g. 2nd and 3rd order schemes), equations of state (e.g. ideal gas, stiffened gas, Jones-Wilkins-Lee, etc.), run-time selectable flux schemes (e.g. HLL, HLLC, AUSM+, Kurganov/Tadmor) and high-order explicit time integration (e.g. 2nd, 3rd, and 4th order).
blastFoam provides activation and explosive burn models to simulate the initiation and expansion of energetic materials, as well as afterburn models to simulate under-oxygenated explosives that exhibit delayed energy release.
Synthetik's opensource CFD airblast code based on the OpenFOAM framework is available on GitHub: https://github.com/synthetik-technologies/blastfoam
blastFoam is developed by Synthetik Applied Technologies:
https://www.synthetik-technologies.com/
Disclaimer:
This offering is not approved or endorsed by OpenCFD Limited, producer and distributor of the OpenFOAM software via www.openfoam.com, and owner of the OPENFOAM and OpenCFD trade marks.