Theory

With the vector mode the program writes fixedValue with $(U_x, U_y, U_z)$ into 0/U for the inlet patch (OpenFOAM user guide, 2026): every face of the inlet gets the same vector. Only the component along the inward normal of the inlet carries flow into the domain, equation (1); the inward normal is minus the area-weighted mean outward normal of the exported inlet surface. The card shows that normal, the normal component, the angle between the vector and the normal, and the resulting volume flow.

Where the program needs a single inlet velocity, it uses:

the normal component for the flow rate (transient flow-through time, flow-rate bookkeeping);
the magnitude $|\mathbf{U}|$ for the Mach number, the turbulence estimate and the first-layer plan of the mesh.

Where no normal is known, $|\mathbf{U}|A$ is used as an upper bound of the flow. A flow-rate sweep keeps the direction and scales the vector to the inlet's share of the total flow.

When the vector mode is first selected, the three boxes start from the current setting converted into a vector: the current velocity (or flow rate divided by the area and density) along the inward normal.

Equations

u_n = -\mathbf{U}\cdot\mathbf{n} = -(U_x n_x + U_y n_y + U_z n_z), \qquad Q = u_n A, \qquad \theta = \arccos\frac{u_n}{\lvert\mathbf{U}\rvert}

(1)

where	meaning	unit
$\mathbf{U}$	Inlet velocity vector $(U_x, U_y, U_z)$	m/s
$\mathbf{n}$	Area-weighted mean OUTWARD unit normal of the inlet patch	-
$u_n$	Velocity component into the domain	m/s
$Q$	Volume flow rate into the domain	m³/s
$A$	Inlet patch area	m²
$\theta$	Angle between the vector and the inward normal	°

Default and rule

Default: the z component of the current inlet velocity along the inward normal
Rule: The current inlet velocity along the inward normal, split into its components.
Source: The boundary condition is OpenFOAM's fixedValue (OpenFOAM user guide, 2026); the conversion to a normal component and a flow rate is the program's own.

Valid range any; the vector must point into the domain (inward normal component > 0)

Pointing outwards

A vector whose inward normal component is zero or negative is refused when the case is built (it would be an outlet); the card warns while typing.

Uniform over the face

The same vector is imposed on every face of the inlet; a swirl or a profile cannot be entered.

Verified

Solved on a straight pipe (2026-11-11): the inlet mass flow equals that of the normal inlet with the same normal component; the cross-flow adds its kinetic energy to the inlet total pressure.

References 1

OpenCFD Ltd. (2026). OpenFOAM user guide. openfoam.com. Web page

Theory

Equations

Default and rule

Valid range any; the vector must point into the domain (inward normal component > 0)

Related 5

References 1