Inlet mode

What is imposed on an inlet: a uniform velocity normal to the inlet, a volume flow rate, a mass flow rate, or a velocity vector in global components. The choice decides which OpenFOAM boundary condition the program writes into 0/U.

Last updated 2026-11-03

Theory

The inlet patch is the set of CAD faces assigned to it in the Geometry step. The program reads the exported surface of the patch and computes its area $A$ and its area-weighted mean outward normal $\mathbf{n}$ ; every mode is turned into a velocity on the faces of the meshed patch:

mode	boundary condition in `0/U`	what is exact
Velocity	`fixedValue`, $\mathbf{U} = -U\,\mathbf{n}$ (uniform, along the inward mean normal)	the velocity; the flow rate is $U$ times the meshed area
Volume flow	`flowRateInletVelocity`, `volumetricFlowRate` $Q$	the flow rate, whatever the meshed area
Mass flow	`flowRateInletVelocity`, `massFlowRate` $\dot m$ (incompressible: with `rhoInlet`)	the mass flow
Velocity vector	`fixedValue`, $\mathbf{U} = (U_x, U_y, U_z)$ as typed	the vector; the flow rate is its inward normal component times the meshed area

A flow-rate inlet spreads the flow uniformly over the meshed faces, normal to each face. Because the mesher does not reproduce a port's area exactly, a velocity inlet delivers its flow through the meshed area, not the CAD area: with the default implicit feature snapping the meshed port areas are 99.5-99.97 % of the CAD on straight pipes (Edge snapping); a flow-rate inlet is exact either way. Volume or mass flow is therefore the better choice when the flow rate is the known quantity, as it usually is for an engine.

For a compressible run the mass flow is divided by the solved density on the inlet face; no inlet density is entered (Inlet density).

The same velocity also starts the potential-flow initialisation: a flow-rate inlet writes the vector $-\mathbf{n}\,Q/A$ as the starting value, because the potential-flow solver builds its inlet flux from that entry.

Equations

Q = U A \ \ (\text{velocity}), \qquad Q = \frac{\dot m}{\rho_{in}} \ \ (\text{mass flow}), \qquad Q = (-\mathbf{U}\cdot\mathbf{n})\,A \ \ (\text{vector})

(1)

where	meaning	unit
$Q$	Volume flow rate into the domain	m³/s
$U$	Inlet velocity (velocity mode)	m/s
$A$	Area of the inlet patch	m²
$\dot m$	Mass flow rate	kg/s
$\rho_{in}$	Inlet density	kg/m³
$\mathbf{n}$	Area-weighted mean outward unit normal of the patch	-

Default and rule

Default: Velocity (uniform_velocity)
Rule: Velocity, 10 m/s (Inlet velocity / flow rate).
Source: Boundary conditions as documented for OpenFOAM (OpenFOAM user guide, 2026). The port-area comparison (meshed against CAD area) comes from the program's validation runs on straight pipes and on case V2 (a two-inlet single-chamber muffler with internal inlet and outlet tubes; explicit snapping: 95-96 % of the CAD area; implicit: 99.5-99.97 % on pipes, 99.83-99.88 % on V2).

Valid range Velocity | Volume flow | Mass flow | Velocity vector

Which mode

Use volume or mass flow when the flow rate is known; use velocity when the velocity itself is the specification (for example a reference case defined by a bulk velocity).

High Mach number

Saving asks for confirmation when an inlet's estimated Mach number exceeds 0.3 for an incompressible case or 0.6 for a compressible one.

Velocity vector

The vector mode has not yet been validated on the solver; see Inlet velocity vector, x component.

References 1

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

Theory

Equations

Default and rule

Valid range Velocity | Volume flow | Mass flow | Velocity vector

Related 7

References 1