Elementary Boundary Conditions

Boundary elements allow prescription of element values at domain boundaries. They can be specified by assigning Material LAW11 (or LAW18 in Purely Thermal Material cases) to boundary elements (quads in 2D analysis and solids in 3D analysis) or EBCS to boundary faces of elements. For each variable P, rho, T, k, epsilon, internal energy, the following can be prescribed:
  • Imposed varying conditions according to user function
  • Continuity
  • Smoothly varying predefined function

Non-reflective frontiers (NRF) (Material 11, option 3) ensure free field impedance to pressure and velocity fields.

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.

Generally the following elementary boundary conditions are used:
  • Inlet, flux is imposed using imposed velocities; 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 Vin)^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.

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.