Assessment of Blast Loading
Courtesy Karen Riddette, WorleyParsons (Sydney, Australia)
Like the U.S., Australia has recently found itself subject to increased threat of terrorism, resulting in greater importance being placed on the security of public buildings and facilities. Also the proper storage of explosive materials requires an understanding of the likely effects of an accidental blast.
WorleyParsons has used FLOW-3D to examine how blast loadings are affected by complex building geometries, and determine the effectiveness of blast mitigation solutions.
Based on a large bank of historical test data, the U.S. Army has generated a series of empirical curves that can be used to calculate blast overpressures at a particular distance from the ground zero. See Figures 1 and 2. A blast wave consists of an initial pressure peak, which then decays back to atmospheric pressure, and into the sub-atmospheric ‘negative phase’. These values are for a 'side-on' pressure – for reflected waves from buildings, the peak pressure can be up to six times higher.
However, these values do not make allowance for shielding or complex blast reflections that can occur in enclosed areas or city blocks, which is where a 3D blast model becomes useful.
Balloon Model: Initiation of Blast
A CFD model was initiated using a "balloon model," where a spherical high pressure zone is created, which initiates a radiating shock wave, followed by a zone of rarefaction behind it. See Figure 3. This can be seen in the animation. Also note the high pressures as the blast wave is reflected from an obstacle.
By adjusting the initial pressure and temperature in the balloon, the shock wave can be calibrated to closely match the empirical data. It can be seen that the peak of the shockwave is flattened slightly due to the smearing of the wave as it passes through the computational grid. See Figure 4.
However it is generally the impulse which is of most interest to structural engineers, as this controls the loading and dynamic effects on structural elements. It can be seen that the calculated loading is in reasonable agreement with the empirical data – well within the margin of uncertainty associated with the potential size of a terrorist blast. See Figure 5.
Instead of starting with the balloon model, it is also possible to apply the pressure wave directly to the boundary of a cylindrical mesh. This animation is an example of recent work we have been carrying out looking at the blast loading on a large storage tank.