Ityp = 3
This law enables to model a nonreflecting boundary (NRF).
Format
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

/MAT/LAW11/mat_ID/unit_ID or /MAT/BOUND/mat_ID/unit_ID  
mat_title  
${\rho}_{i}$  ${\rho}_{0}$  
Ityp  P_{sh} 
Ityp = 3  NonReflecting Boundary
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

c  ${l}_{c}$  
Blank Format  
Blank Format  
Blank Format  
Blank Format  
Blank Format  
Blank Format 
Definition
Field  Contents  SI Unit Example 

mat_ID  Material identifier. (Integer, maximum 10 digits) 

unit_ID  Unit Identifier. (Integer, maximum 10 digits) 

mat_title  Material title. (Character, maximum 100 characters) 

${\rho}_{i}$  Initial density. 3 (Real) 
$\left[\frac{\text{kg}}{{\text{m}}^{\text{3}}}\right]$ 
${\rho}_{0}$  Reference density used in E.O.S
(equation of state). Default ${\rho}_{0}={\rho}_{i}$ (Real) 
$\left[\frac{\text{kg}}{{\text{m}}^{\text{3}}}\right]$ 
Ityp  Boundary condition type. 1
(Integer) 

P_{sh}  Pressure shift. 2 (Real) 
$\left[\text{Pa}\right]$ 
c  Outlet sound speed. 1 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
${l}_{c}$  Characteristic length. 1 (Real) 
$\left[\text{m}\right]$ 
Comments
 NonReflecting Boundary formulation is
based on Bayliss & Turkel. ^{1} The objective is to impose a mean pressure which fluctuate with
rapid variations of pressure and velocity:$$\frac{\partial P}{\partial t}=\rho c\left(\frac{\partial}{\partial t}({V}_{n}){V}_{n}\cdot div(\overrightarrow{V}{V}_{n}\cdot \overrightarrow{n})\right)+c\frac{({P}_{\infty}P)}{2{l}_{c}}$$Pressure in the far field ${P}_{\infty}$ is imposed with a function of time. The transient pressure is derived from ${P}_{\infty}$ , the local velocity field V and the normal of the outlet facet:
 density, energy, temperature, turbulent energy and dissipation are imposed with a function of time as in Ityp = 2
 if the function number is 0, the neighbor element value is used to respect continuity
 acoustic impedance will be $\rho c$
 typical length
${l}_{c}$
is used to relax the effective pressure towards its
imposed value. It should be large compared to the highest wave length of interest in
the problem. The relaxation term acts as high pass filter whose frequency cutoff
is:$${f}_{c}=\frac{c}{4\cdot \pi \cdot {l}_{c}}$$
Where, sound speed c and characteristic length ${l}_{c}$ are two required parameters (non zero).
 The P_{sh} parameter enables shifting the output pressure which also becomes PP_{sh}. If using ${P}_{sh}=P\left(t=0\right)$ , the output pressure will be $\text{\Delta}P$ , with an initial value of 0.0.
 With thermal modeling, all thermal data ( ${T}_{0},\text{\hspace{0.17em}}{\rho}_{0}{C}_{P}$ , ...) can be defined with /HEAT/MAT.
 It is not possible to use this boundary material law with multimaterial ALE /MAT/LAW37 (BIPHAS) and /MAT/LAW51 (MULTIMAT).
^{1} A. Bayliss, E. Turkel,
"Outflow Boundary Condition for Fluid Dynamics", NASACR170367, Institute for Computer
Application in Science and Engineering, August 7, 1980