/EBCS/PRES

Block Format Keyword Describes the elementary boundary condition to imposed density and pressure.

Format

Field	Contents	SI Unit Example
`ebcs_ID`	Elementary boundary condition identifier. (Integer, maximum 10 digits)
`unit_ID`	Unit Identifier (Integer, maximum 10 digits)
`ebcs_title`	Elementary boundary condition title. (Character, maximum 100 characters)
`surf_ID`	Surface identifier. (Integer)
`C`	Speed of sound. Default = 0 (Real)	$[\frac{m}{s}]$
`fct_ID`_pr	Function $f_{p r} (t)$ identifier for pressure. = 0 $P = F s c a l e_{p r}$ = n $P = F s c a l e_{p r} \cdot f_{p r} (t)$ (Integer)
`Fscale`_pr	Pressure scale factor. Default = 0 (Real)	$[Pa]$
`fct_ID`_rho	Function $f_{r h o} (t)$ identifier for density. = 0 $ρ = F s c a l e_{r h o}$ = n $ρ = F s c a l e_{r h o} \cdot f_{r h o} (t)$ (Integer)
`Fscale`_rho	Density scale factor. Default = 0 (Real)	$[\frac{kg}{m^{3}}]$
`fct_ID`_en	Function $f_{e n} (t)$ identifier for energy. = 0 $E = F s c a l e_{e n}$ = n $E = F s c a l e_{e n} \cdot f_{e n} (t)$ (Integer)
`Fscale`_en	Energy scale factor. Default = 0 (Real)	$[J]$
$l_{c}$	Characteristic length. Default = 0 (Real)	$[m]$
`r`₁	Linear resistance. 5 Default = 0 (Real)	$[\frac{kg}{m^{2} s}]$
`r`₂	Quadratic resistance. 5 Default = 0 (Real)	$[\frac{kg}{m^{3}}]$

Input is general, no prior assumptions are enforced! Verify that the elementary boundaries are consistent with general assumptions of ALE (equation closure).
It is not advised to use the Hydrodynamic Bi-material Liquid Gas Law (/MAT/LAW37 (BIPHAS)) with the elementary boundary conditions.
Density, pressure, and energy are imposed according to a scale factor and a time function. If the function number is 0, the imposed density, pressure and energy are used.
This keyword is less than four or equal to six (non-reflective frontiers (NRF)) using:
(1)
$\frac{\partial P}{\partial t} = ρ c \frac{\partial V_{n}}{\partial t} + c \frac{(P_{\infty} - P)}{l_{c}}$

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.

Where, $l_{c}$ is the characteristic length, to compute cutoff frequency $f_{c}$ as:(2)
$f_{c} = \frac{c}{2 π . l_{c}}$
A resistance pressure is computed and added to the current pressure.(3)
$P_{r e s} = r_{1} \cdot V_{n} + r_{2} \cdot V_{n} \cdot | V_{n} |$

It aims at modeling the friction loss due to the valves.