# Elementary Boundary Conditions

Boundary elements allow prescription of element values at domain boundaries. They can be specified with two ways:

- /MAT/LAW11
- /MAT/LAW51
- /MAT/LAW18 (in Purely Thermal Material cases)

The other way is with ALE Elementary Boundary Conditions assign on surface of boundary elements.

- imposed varying conditions according to user function
- continuity
- smoothly varying predefined function

With Radioss ALE/CFD any combination of the above options can be specified; on the counterpart, the closure of the various convection and diffusion equations has to be verified carefully by you.

- Inlet:
Flux is imposed using imposed velocities /IMPVEL; density, energy, turbulent energy (that is: k) are imposed as constants. Continuity is imposed for pressure (display purposes only) and for epsilon. Turbulent energy, rho k is set to zero for external flows and to 1.5*rho*(0.06 V

_{in})^{2}for internal flows. - Outlet:
Continuity for all variables except pressure, which is imposed. When using the Non-reflective frontiers (NRF) option, you provide a value for sound speed and a typical relaxation length, which must be greater than the largest wave length of interest.

- Sides:
Continuity for all variables with the Non-reflective frontiers (NRF) option or slip conditions without boundary elements. It ensures free field impedance to pressure and velocity fields.

If an element does not exist at boundary, continuity is assumed; but kinematic conditions are necessary to disallow fluxes; otherwise the convection equation is not closed and the program can diverge.

- In Material
LAW11 below options used:
`Ityp`= 0 specifies stagnation conditions for perfect gas (Bernoulli inlet).`Ityp`= 1 specifies stagnation conditions for a linear compressible material (Bernoulli inlet).`Ityp`= 2 imposes values (inlet/outlet).`Ityp`= 3 is for non-reflective frontiers (outlet).

- In Material
LAW51 below options used:
`Iform`= 2 enables to impose sub-material states (density, energy, and volumetric fraction) which are also used to compute global material state. (inlet).`Iform`= 4 gas inlet conditions for multi-material ALE laws (Gas inlet stagnation).`Iform`= 5 liquid inlet conditions for multi-material ALE laws (Liquid inlet stagnation).`Iform`= 6 This boundary material enables you to simulate a non-reflecting outlet boundary for multi-material law /MAT/LAW51 (NRF outlet).

For example, in the input deck, density and energy are imposed constant at the inlet. Non-reflective frontiers are imposed at the outlet. The flux is then injected at inlet through imposed velocities at nodal points.

## Non-Reflective Frontiers (NRF)

`Ityp`=3 in LAW11 and `Iform`=6 in LAW51 are used to prevent
outgoing wave reflections on the boundaries of the domain.

- An average pressure is imposed via a function. A relaxation term is added to let the average pressure converge toward the imposed value. This is well suited for outlets.
- An average pressure is calculated from the neighboring element pressure and the pressure converges toward this always changing value.

The impedance of the boundary is exactly the wave impedance of a monopole radiating at distance
2`l`_{c} from the boundary, where `l`_{c} is specified in the input data for this law.

This non-reflective frontiers (NRF) is not effective when velocities are imposed or when nodes are fixed.