LOAD_UNDEX

Loads
*LOAD_UNDEX
"Optional title"
coid
entype, enid, $x_c$, $y_c$, $z_c$, gdir, $d$
$m_{HE}$, $\rho_{HE}$, $p_c$, $v_c$, $A$, $B$, $\gamma$, $K_c$
$C_d$, $E_d$, $\dot{u}_0$
Parameter definition
VariableDescription
coid Command ID (optional)
entype Entity type
options: P, PS, G, GS, FS, ALL
enid Entity identification number
$x_c$, $y_c$, $z_c$ Explosive charge location
gdir Gravity loading direction
options: X, Y, Z
$d$ Water depth
$m_{HE}$ Mass of explosive charge
$\rho_{HE}$ Density of explosive charge
$p_c$ Charge pressure constant
$v_c$ Charge velocity constant
$A$ Charge constant (dimensionless)
$B$ Charge constant (dimensionless)
$\gamma$ Heat capacity ratio
$K_c$ Adabatic charge constant
$C_d$ Bubble migration drag coefficient
$E_d$ Bubble migration exponent
default: 2
$\dot{u}_0$ Reference migration velocity
default: 1
Description

This command is used to define pressure loads from underwater explosions. The command is based on semi-empirical equations as suggested by Geers and Hunter (An integrated wave-effects model for an underwater explosion bubble, Journal of the Acoustical Society of America, 2002).

Pressure loads from both the initial shock wave and from the following bubble osciallations are generated.

Shock phase

During the shock phase the incident pressure at a distance $L$ from the bubble center is defined as:

$\displaystyle{p(L,t) = \frac{\rho_w}{4 \pi L} \left( \frac{R_c}{L} \right)^A \ddot{V}( \left[ R_c/L \right]^B t)}$

where $\rho_w$ is the water density, $R_c$ is the initial charge radius, $R$ is the current bubble radius and $V$ is the bubble volume:

$\displaystyle{ R_c = \left( \frac{3 m_{HE}}{4 \pi \rho_{HE}} \right)^{1/3} }$
$\displaystyle{V(t) = \frac{4}{3} \pi R_c^3 + \frac{4 \pi R_c}{\rho_w} \left[ 1.5857 \frac{t}{t_c} - 5.8293 + 0.4609 \mathrm{e}^{-1.338 t/t_c} + 5.3684 \mathrm{e}^{-0.1805 t/t_c} \right] p_c t_c^2}$
$\displaystyle{ t_c = \frac{R_c}{v_c} }$

Oscillation phase

The oscillation phase is assumed to begin at time $t_i = 7 t_c$. During the oscillation phase the bubble radius $R$ and its vertical migration $u$ are assumed to evolve according to:

$\displaystyle{\dot{R} = -\frac{\phi_{l0}}{R} -\frac{1}{c_w} \left( \dot{\phi}_{l0} - \dot{R}^2 - \frac{1}{3}\dot{u}^2 - \frac{2}{3}\dot{u}\frac{\phi_{l1}}{R} \right) }$
$\displaystyle{\dot{u} = -2 \frac{\phi_{l1}}{R} -\frac{1}{c_w} \left( \dot{\phi}_{l1} - 2 \dot{R} \dot{u} \right) }$

where:

$\displaystyle{\dot{\phi}_{l0} = \frac{1}{1 + \zeta} \left[ \left( \frac{1}{2} + \frac{\rho_g}{2\rho_w} + \zeta \right) \left( \dot{R}^2 + \frac{\dot{u}^2}{3} \right) - \frac{\rho_g}{\rho_w} c_g \frac{\phi_{l0}}{R} + \frac{2}{3} \left( 1 + \zeta \right) \dot{u} \frac{\phi_{l1}}{R} - Z \right] }$
$\displaystyle{\dot{\phi}_{l1} = \frac{1}{1 + \zeta} \left[ \left( 1 + \frac{\rho_g}{\rho_w} + 2 \zeta \right) \dot{R} \dot{u} -\left( 1 - \frac{\rho_g}{\rho_w} \right) g R - \frac{\rho_g}{\rho_w} c_g \frac{2 \phi_{l1} + \phi_{g1}}{R} + \frac{3}{8} C_d \vert \dot{u} \vert^{E_d} \dot{u}_0^{2-E_d} \right] }$
$\displaystyle{\dot{\phi}_{g1} = \frac{1}{1 + \zeta} \left[ \left( 2 + \frac{c_g}{c_w} + \zeta \right) \dot{R} \dot{u} - \frac{c_g}{c_w} \left( 1 - \frac{\rho_g}{\rho_w} \right) g R - c_g \frac{2 \phi_{l1} + \phi_{g1}}{R} - \frac{3 c_g}{8 c_w} C_d \vert \dot{u} \vert^{E_d} \dot{u}_0^{2-E_d} \right] }$
$\displaystyle{ Z = \frac{1}{\rho_w} \left( p_g - p_i + \rho_w g u \right) + \frac{1}{3} \left[ \left( \frac{\phi_{l1}}{R} \right)^2 - \frac{\rho_g}{\rho_w} \left( \frac{\phi_{g1}}{R} \right)^2 \right] }$
$\displaystyle{ \zeta = \frac{\rho_g c_g}{\rho_w c_w} }$

$p_i$ is the initial hydrostatic pressure prior to detonation, $\rho_w$ is the water density, $\rho_g$ is the bubble gas density, $p_g$ is the bubble pressure, $c_w$ is the speed of sound in the water and $c_g$ is the current speed of sound in the gas.

$\displaystyle{ p_i = p_{atm} + \rho g d }$
$\displaystyle{ p_g = K_c \left( \frac{R_c}{R} \right)^{3\gamma} }$
$\displaystyle{ \rho_g = \rho_{HE} \left( \frac{R_c}{R} \right)^3 }$
$\displaystyle{ c_g = \sqrt{\frac{\gamma K_c}{\rho_{HE}}} \left( \frac{R_c}{R} \right)^{\frac{3}{2}(\gamma-1)} }$

$p_{atm}$ is the atmospheric pressure and $g$ is the gravity constant. Initial conditions are:

$\displaystyle{ \phi_{l0}(t=t_i) = -R \dot{R} \left[ 1 - \frac{1}{2} \left( 1 - \frac{\rho_g}{\rho_w} \right) \frac{\dot{R}}{c_w} + \zeta \right] + \frac{R}{c_w} Z }$
$\displaystyle{ \phi_{l1}(t=t_i) = 0 }$
$\displaystyle{ \phi_{g1}(t=t_i) = -\left( 1 - \frac{\rho_g}{\rho_w} \right) \frac{g R^2}{\zeta c_w} }$

In the oscillation phase the incident pressure at distance $L$ from the center of the bubble is defined as:

$\displaystyle{ p(L,t) = \rho_w \frac{R^2 \ddot{R} + 2 R \dot{R}^2}{L} }$

Reflected pressure

The reflected pressure against a surface is:

$\displaystyle{ p_R(L,t) = p(L,t) \cdot \left( 1 + \mathrm{cos}(\alpha) \right) }$

$\alpha$ is the angle between the surface normal and the vector from the surface to the center of the bubble.

Example
Underwater explosion (TNT)

A simple example of an underwater explosion using TNT parameters from Geers and Hunter (2002).

*UNIT_SYSTEM
SI
#
# --- PARAMETERS ---
#
*PARAMETER
x_c =-10, "HE charge x-coordinate"
y_c = 0, "HE charge y-coordinate"
z_c = 0, "HE charge z-coordinate"
m_HE = 0.3, "Explosive mass"
rho_HE = 1630.0, "Explosive density"
p_c = 1.67e9, "Explosive pressure constant"
v_c = 1010.0, "Explosive velocity constant"
A = 0.18, "Charge constant"
B = 0.185, "Charge constant"
K_c = 1045.0e6, "Adiabatic charge constant"
gamma = 1.3, "Ratio of heat capacitites"
depth = 100.0, "Water depth"
C_d = 0.5, "Drag coefficient"
E_d = 2.0, "Migration exponent"
#
# --- TIME and SENSORS ---
#
*TIME
0.1, 0, 0, 1.0e-6
*OUTPUT_SENSOR
"Front"
1, 1, 0, 0, 0
"Top"
2, 1, 1, 0, 1
#
# --- MESH ---
#
*COMPONENT_BOX
"Box"
1, 1, 4, 4, 4
0, -1, -1, 2, 1, 1
#
# --- MATERIAL ---
#
*MAT_RIGID
1, 1000.0
#
# --- PART ---
#
*PART
"Box"
1, 1
#
# --- LOAD ---
#
*LOAD_UNDEX
"Simple test"
1
P, 1, [%x_c], [%y_c], [%z_c], Z, [%depth]
[%m_HE], [%rho_HE], [%p_c], [%v_c], [%A], [%B], [%K_c], [%gamma]
[%C_d], [%E_d]
*END

Rigid box exposed to underwater blast load