# Pressure Normal Inlet-Outlet Velocity

Source: https://help.sim-flow.com/boundary-conditions/pressure-normal-inlet-outlet-velocity
Last updated: 2026-10-01

## Description

Pressure Normal Inlet-Outlet Velocity is a velocity boundary condition for open inlets–outlets. It applies [Zero Gradient](https://help.sim-flow.com/boundary-conditions/zero-gradient) when the flux leaves the domain, and when the flux enters, it imposes a purely **normal** velocity derived from the flux. The condition is a derivative of [Pressure Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-inlet-outlet-velocity).

The boundary condition is specifically designed for boundaries where the flow speed is not known in advance and is controlled by the pressure difference across the boundary. The inlet direction is predefined and always normal to the surface of the boundary.

## Understanding Pressure Normal Inlet-Outlet Velocity

The Pressure Normal Inlet-Outlet Velocity boundary condition determines the velocity at a boundary patch based on the neighboring cell values and the specified patch pressure. The velocity is calculated based on the flux through the boundary according to the equation:

\[\vec U_{p} = \vec n \frac{\phi}{|A|}\]

where:

- \(\vec U_{p}\) - velocity vector on face **i**
- \(\vec n\) - face normal vector
- \(\phi\) - face flux
- \(A\) - face area magnitude

Pressure Normal Inlet-Outlet Velocity is designed to handle both inlet and outlet flow conditions based on pressure. In real-world scenarios, there are situations where you don’t have a fixed velocity inlet or a pressure outlet, but rather a boundary where the flow can either enter or exit based on the pressure conditions within the domain. Moreover, the direction of inflow is always associated with the normal to the patch surface.

Pressure Normal Inlet-Outlet Velocity is similar to [Pressure Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-inlet-outlet-velocity) and [Pressure Directed Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-directed-inlet-outlet-velocity) boundary conditions. To avoid misunderstanding, all three boundary conditions are summarized in the table below.

Comparison of Pressure Normal Inlet-Outlet Velocity with Pressure Inlet-Outlet Velocity and Pressure Directed Inlet-Outlet Velocity

| Feature | Pressure Inlet-Outlet Velocity | Pressure Normal Inlet-Outlet Velocity | Pressure Directed Inlet-Outlet Velocity |
| --- | --- | --- | --- |
| Application | Open inlet/outlet with known pressure | Open inlet/outlet with known pressure, but velocity forced in the normal direction to the patch | Open inlet/outlet with known pressure, but velocity forced in a user-defined direction |
| Outflow | Zero Gradient applied | Zero Gradient applied | Zero Gradient applied |
| Inflow - how \(U\) is set | Normal component based on the internal field | \(U\) determined based on flux \(\vec U_{p} = \vec n \frac{\phi}{A}\) in the normal direction | \(U\) with normal and tangential components \(\vec U_{p} = \vec d \frac{\phi}{\vec d \cdot A}\) |
| Typical uses | Ducts or plenums where the mass-flow is pressure-driven but swirl or sliding motion may exist at the inlet | Open surfaces where any inflow should be perpendicular | Skew nozzles, HVAC diffusers, jet fans – anywhere the pressure-driven jet must enter at a specific angle |

## Application & Physical Interpretation

Pressure Normal Inlet-Outlet Velocity is tailored for boundaries where the flow speed is uncertain and is regulated by the pressure difference across the boundary, but the flow direction is predefined and normal to the boundary. Tangential components are zeroed by the boundary condition.

### Pressure Normal Inlet-Outlet Velocity in Heat Transfer applications

**Example applications**: HVAC, room temperature distribution

This problem can be addressed using the [buoyantSimpleFoam](https://help.sim-flow.com/solvers/buoyant-simple-foam) solver. In ventilation-related problems, understanding the temperature distribution is often crucial. The flow is driven either by setting a constant velocity at the inlet (or specifying a mass/volumetric flow rate) or by the pressure difference ([Total Pressure](https://help.sim-flow.com/boundary-conditions#section-pressure)). In such cases, Pressure Normal Inlet-Outlet Velocity is applied at the outlet to adjust the flow according to the main driving factor and to stabilize the calculations.

Example Boundary Conditions set for Heat Transfer applications

| Physics | Pressure | Velocity | Temperature |
| --- | --- | --- | --- |
| Inlet | Fixed Flux Pressure | Fixed Value | Fixed Value |
| Outlet (wind tunnel) | Fixed Value | **Pressure Normal Inlet-Outlet Velocity** | Zero Gradient |

### Pressure Normal Inlet-Outlet Velocity in channel flow applications

**Example applications**: channel flows, T-junction flows

This problem can be tackled using the [pimpleFoam](https://help.sim-flow.com/solvers/pimple-foam) solver. In channel flows, the flow is generally driven by a pressure difference. The [Total Pressure](https://help.sim-flow.com/boundary-conditions#section-pressure) is specified at the inlet, requiring the velocity to adjust accordingly. However, the flow direction may be determined by the diffuser or deflector unit.

Example Boundary Conditions set for channel flow applications

| Physics | Pressure | Velocity |
| --- | --- | --- |
| Inlet | Total Pressure | **Pressure Normal Inlet-Outlet Velocity** |
| Outlet | Fixed Value | Inlet-Outlet |

### Pressure Normal Inlet-Outlet Velocity in open-boundary flows (multiphase) applications

**Example applications**: dam break, river flows

Pressure Normal Inlet-Outlet Velocity is often used as a so-called open boundary because of its unique properties to adjust and work both as the velocity inlet and [Zero Gradient](https://help.sim-flow.com/boundary-conditions/zero-gradient).

Example Boundary Conditions set for open boundary applications

| Physics | Pressure | Velocity |
| --- | --- | --- |
| Atmosphere (open boundary) | Calculated | **Pressure Normal Inlet-Outlet Velocity** |

## Pressure Normal Inlet-Outlet Velocity in SimFlow

To define the Pressure Normal Inlet-Outlet Velocity boundary condition on the patch, the proper option must be selected from the drop-down menu in the Boundary Conditions tab - [Figure 1](https://help.sim-flow.com/boundary-conditions/pressure-normal-inlet-outlet-velocity#fig-sketch-01).

![Pressure Normal Inlet Outlet Velocity boundary condition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/pressure-normal-inlet-outlet-velocity/pressure-normal-inlet-outlet-velocity-bc-01.png)

Figure 1. Pressure Normal Inlet Outlet Velocity boundary condition in SimFlow

## Pressure Normal Inlet-Outlet Velocity - Alternatives

In this section, we propose boundary conditions that are alternative to Pressure Normal Inlet-Outlet Velocity. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.

| Boundary Condition | Description |
| --- | --- |
| [Pressure Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-inlet-outlet-velocity) | works similarly to Pressure Normal Inlet-Outlet Velocity, but allows a direction to be adjusted by the pressure condition |
| [Pressure Directed Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-directed-inlet-outlet-velocity) | similar definition to Pressure Normal Inlet-Outlet Velocity, but the flow occurs in the direction specified by the User |