Block Format Keyword Provides a fast way to simulate air blast pressure on a structure.

The Air Blast incident pressure is fitted from experimental data, then blast pressure is deduced from surface orientation to the detonation point. You must provide detonation point, detonation time and equivalent TNT mass.

This is a simplified loading method because the arrival time and incident pressure are not adjusted for obstacles. It also does not take into account confinement or ground effects.

## Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
surf_ID Exp_data I_tshift Ndt IZ Iform Node_ID
xdet Ydet Zdet Tdet WTNT
Pmin Tstop
Ground_ID

## Definition

Field Contents SI Unit Example

(Character, maximum 10 digits)

surf_ID Surface identifier.

(Integer, maximum 10 digits)

Exp_data Experiment data flag.
1 (Default)
UFC-03-340-02 Free Air, Spherical charge of TNT.
2
UFC-03-340-02 Ground Reflection, Hemispherical charge of TNT.
3
UFC-03-340-02 Air Burst, Spherical Charge over the ground.

(Integer, maximum 10 digits)

I_tshift Time shift flag.
1 (Default)
No shift.
2
Shift time to skip computation time from 0 to ${t}^{*}=\mathrm{inf}\left({T}_{arrival}\right)$ .

(Integer)

Ndt Number of intervals for minimum time step.

$\text{Δ}{t}_{blast}=\frac{\mathrm{inf}\left({T}_{0}\right)}{{N}_{dt}}$

Where, ${T}_{0}$ is the duration of positive phase.

Default = 100 (Integer)

IZ Scaled Distance update with time.
=1
Scaled Distance is computed at initial time and does not change with time.
=2 (Default)
Scaled Distance is updated at each time step.

(Integer)

Iform Modeling flag.
= 1
Friedlander model.
= 2 (Default)
Modified Friedlander model.

(Integer)

Node_ID Node identifier defining detonation point.

If defined, the flags Xdet, Ydet and Zdet are ignored.

Xdet Detonation Point X-coordinate.

Ignored if Node_ID ≠ 0.

Default = 0.0 (Real)

$\left[\text{m}\right]$
Ydet Detonation Point Y-coordinate.

Ignored if Node_ID ≠ 0.

Default = 0.0 (Real)

$\left[\text{m}\right]$
Zdet Detonation Point Z-coordinate.

Ignored if Node_ID ≠ 0.

Default = 0.0 (Real)

$\left[\text{m}\right]$
Tdet Detonation time.

Default = 0.0 (Real)

$\left[\text{s}\right]$
WTNT Equivalent TNT mass.

(Real)

$\left[\text{Kg}\right]$
Pmin Minimum pressure.

Default = -1020 (Real)

$\left[\text{Pa}\right]$
Tstop Stop time.

Default = 1020 (Real)

$\left[\text{s}\right]$
Ground_ID Surface identifier for ground definition.

Ignored if Exp_data=1.

Surface type is /SURF/PLANE

Default: Origin =(0,0,0), normal=(0,0,H)

1. Modeling situation is set with Exp_data flag. You provide explosion data (Xdet, Ydet, Zdet), explosion mass (WTNT) target surface (surf_ID), and detonation time (Tdet). All other parameters and flags have default values.
If Exp_data=3, explosive height must be defined.
At a given point over the user surface, the corresponding radius $R$ and the explosive mass WTNT is used to determine characteristic values of the blast wave (arrival time ${t}_{a}$ , maximum pressure Pmax, positive duration $\text{Δ}{t}_{+}$ , impulse ${I}_{+}$ , ...). Both incident wave and reflected wave are to follow Friedlander’s equation:
• If Imodel = 1 (Friedlander model)
${\mathrm{P}}_{Friedlander}\left(t\right)={P}_{}\cdot {e}^{\frac{-\left(t-{t}_{a}\right)}{\text{Δ}{t}_{+}}}\left(1-\frac{t-{t}_{a}}{\text{Δ}{t}_{+}}\right)$
• If Imodel = 2 (modified Friedlander model)
${\mathrm{P}}_{Friedlander}\left(t\right)={P}_{\mathrm{max}}\cdot {e}^{\frac{-b\left(t-{t}_{a}\right)}{\text{Δ}{t}_{+}}}\left(1-\frac{t-{t}_{a}}{\text{Δ}{t}_{+}}\right)$

Where, ${P}_{\mathrm{max}},\text{Δ}{t}_{+},{t}_{a}$ are experimentally known at a given scaled distance $\frac{R}{{W}^{1}{3}}}$ . 3

With the modified Friedlander model (Imodel=2), ‘b’ is a decay parameter introduced to fit the positive impulse.

'b’ is solved such as:
$\underset{{t}_{a}}{\overset{{t}_{a}+\text{Δ}{t}_{+}}{\int }}{\mathrm{P}}_{Friedlander}\left(t\right)dt={I}_{+}$
2. The fitted time history function ${\mathrm{P}}_{incident}\left(t\right)$ and ${\mathrm{P}}_{reflected}\left(t\right)$ are also used to compute blast loading ${\mathrm{P}}_{BLAST}\left(t\right)$ at a given face centroid Z’ (Figure 3). 2

Where, $\theta$ is the angle between the surface segment (centroid Z’) and the direction to detonation point.

This means that blast pressure is equal to reflected pressure if segment is directly facing the detonation point, and equal to incident pressure if segment is not facing the detonation point. This modeling is simple because arrival time and incident pressure are not adjusted with shadowing of the related structure. It also does not into account confinement and tunnel effect.

This also requires the surface to have outward normal vector.

3. If Iz =1, R is constant and computed during Starter at time=0.00. When Iz =2, $R=R\left(t\right)$ is updated for each cycle during Engine computation.
4. If WTNT is not set, the mass is zero and no pressure will be loaded on the related surface.
5. If modeled explosive is not TNT, an equivalent TNT mass must be provided.
6. The experimental data uses the unit system {cm, g, $\mu s$ }. The units defined in /BEGIN will be used to convert the experimental data units to the model units. Therefore, the units defined in /BEGIN must correctly match the units used in the model.
7. It is possible to skip computation time from $T=0$ to ${t}^{*}=\mathrm{inf}\left({T}_{arrival}\right)$ . The shift value is automatically computed during Starter execution. To disable a computation up to ${t}^{*}$ , the I_tshift value must be equal to 2.
8. The ${N}_{dt}$ parameter can impose a minimum time step, if structural one is not large enough. Imposing $\text{Δ}{t}_{blast}=\frac{\mathrm{inf}\left({T}_{0}\right)}{{N}_{dt}}$ ensures that there are sufficient time steps during positive phase, that is, during the exponential, decrease of the blast wave. By default, ${N}_{dt}=100$ .
9. Parameter ${P}_{\mathrm{min}}$ was introduced to keep positive part of the Friedlander blast model.
${\mathrm{P}}_{}\left(t\right)=\mathrm{max}\left({P}_{BLAST}\left(t\right),{P}_{\mathrm{min}}\right)$
1 Structures to resist the effects of accidental explosions. Departments of the Army, Navy, and Air Force, TM 5-1300/NAVFAC P-397/AFR 88-22, November 1990.
2 Randers-Pehrson, Glenn, and Kenneth A. Bannister. Airblast Loading Model for DYNA2D and DYNA3D. No. ARL-TR-1310. Army Research Lab Aberdeen Proving Ground MD, 1997.
3 Structures to resist the effects of accidental explosions, Unified Facilities Criteria (UFC), UFC 3-340-02, 5 December 2008.