You can define the initial fluid flow velocity in elements; however, if such conditions are omitted, a default value of zero is assumed. Initial velocities must be defined in global directions, regardless of the use of local transformations (see “Transformed coordinate systems,” Section 2.1.5).

For incompressible flow Abaqus/CFD automatically uses the user-defined boundary conditions and tests the specified initial velocity to be sure that the initial velocity field is divergence-free and that the velocity boundary conditions are compatible with the initial velocity field. If they are not, the initial velocity is projected onto a divergence-free subspace, yielding initial conditions that define a well-posed incompressible Navier-Stokes problem. Therefore, in some circumstances, the user-specified initial velocity may be overridden with a velocity that is divergence-free and matches the velocity boundary conditions.

*INITIAL CONDITIONS, TYPE=VELOCITY, ELEMENT AVERAGE

You can define the initial fluid density in elements. However, if the initial condition is omitted, the material density definition is assumed as default (see “Density,” Section 21.2.1). Similarly, if the initial density is specified on an element set that does not include all fluid elements, the material density is assumed as the default for those elements not contained in the element set.

*INITIAL CONDITIONS, TYPE=DENSITY, ELEMENT AVERAGE

For incompressible flows it is not necessary to prescribe the initial pressure condition since the initial pressure field is computed automatically from the initial velocity field and boundary conditions. This is done to ensure proper starting conditions for incompressible flows.

If the energy equation is solved, the initial fluid temperature in elements must be defined.

*INITIAL CONDITIONS, TYPE=TEMPERATURE, ELEMENT AVERAGE

If the Spalart-Allmaras turbulence model is active, you must prescribe an initial value for the Spalart-Allmaras turbulent eddy viscosity that is greater than zero and roughly three to five times the kinematic viscosity. The kinematic viscosity is the ratio of the fluid viscosity and density (). For more information, see “Viscosity,” Section 26.1.4.

*INITIAL CONDITIONS, TYPE=TURBNU, ELEMENT AVERAGE

If the RNG k– turbulence model is active, initial conditions need to be specified for both k and . The k and values must be greater than zero. A simple procedure to approximate the initial conditions can be obtained from values of the turbulence intensity and an approximate initial turbulent eddy viscosity as described below.

The turbulent kinetic energy is defined as

Therefore, an estimation for the initial conditions for the turbulent kinetic energy, k, can be expressed in terms of the characteristic velocity and turbulence intensity as

The initial value for the turbulent kinetic energy dissipation, , can be obtained from a known/proposed level of the turbulent eddy viscosity, , as

*INITIAL CONDITIONS, TYPE=TURBKE, ELEMENT AVERAGE

*INITIAL CONDITIONS, TYPE=TURBEPS, ELEMENT AVERAGE