# FAQ/Physical

< FAQ

## 1 FAQ Section 3: Physical

Questions about the physics implemented (boundary conditions and similar)

### 1.1 General

#### 1.1.1 What is the meaning of the Field X

A table of the fields most commonly written by OpenFOAM-solvers can be found here.

#### 1.1.2 Where do I enter the fluid-density for icoFoam, turbFoam and other incompressible solvers?

You don't. Instead of the dynamic viscosity $\mu$ the kinematic viscosity $\nu=\frac{\mu}{\rho}$ is used by the OpenFOAM-solvers.

Note: the pressure has to be normalized with the density, too. One consequence of this is that the dimensions of pressure become pressure divided by density.

#### 1.1.3 What is the field phi that the solver is writing?

The mass flow through the cell faces ( $\rho \vec u \cdot \vec A$ with $\vec A$ the area of the face). Keep in mind that solvers for incompressible flow will unlikely use $\rho$.

### 1.2 Boundary Conditions

#### 1.2.1 What's the difference between the symmetryPlane and the zeroGradient boundary conditions?

The zeroGradient boundary condition sets the boundary value to the near-wall cell value.

A symmetryPlane boundary condition is a symmetry-plane which is equivalent to a zeroGradient for scalars, but not for vectors or tensors.

(Source: )

#### 1.2.2 What does the lInf parameter mean in pressureTransmissive boundary condition?

lInf is the relaxation length-scale (in m) for outgoing pressure waves to return to pInf. This stops the pressure in the domain from floating about if the inlet pressure is not specified. (source: )

### 1.3 Turbulence modeling

#### 1.3.1 How is wall-functions for RANS disabled and enabled?

All high-Re RANS turbulence models include wall-functions because it is inappropriate to use them without. Only the low-Re models operate without wall-functions as they include model-specific wall treatments.

(Source: )