# Uniform Inlet-Outlet

Source: https://help.sim-flow.com/boundary-conditions/uniform-inlet-outlet
Last updated: 2026-10-02

## Description

Uniform Inlet-Outlet is a derivative of [Inlet-Outlet](https://help.sim-flow.com/boundary-conditions/inlet-outlet). The boundary condition is used to specify a variable that acts as both an inlet and an outlet at a particular boundary of a computational domain. It is commonly employed for variables such as velocity or mass flow rate, where the value can vary depending on the flow conditions.

The Uniform Inlet-Outlet boundary condition allows for a smooth transition between an inflow and an outflow boundary. It is particularly useful when you have a boundary that can switch between behaving as an inlet or an outlet, depending on the local flow direction. The Uniform Inlet-Outlet boundary condition can be applied to all variable types (scalars, vectors, etc.).

Compared to [Inlet-Outlet](https://help.sim-flow.com/boundary-conditions/inlet-outlet), the Uniform Inlet-Outlet definition allows setting the value as a constant value or time-dependent.

## Understanding Uniform Inlet-Outlet

Uniform Inlet-Outlet can be represented graphically, as in the image below.

The illustration below depicts that if the flux is out of the domain, the Uniform Inlet-Outlet behaves as the [Zero Gradient](https://help.sim-flow.com/boundary-conditions/zero-gradient) boundary condition. However, if the flow conditions change and the flow is inward (towards the domain), the `Fixed Value` boundary condition is applied.

![Uniform Inlet-Outlet Boundary Condition](https://help.sim-flow.com/boundary-conditions/images/inlet-outlet/inlet-outlet-bc-3.jpg)

Figure 1. Uniform Inlet-Outlet Boundary Condition

## Application & Physical Interpretation

The Uniform Inlet-Outlet boundary condition is designed to efficiently handle flow at boundaries where the direction of flow can potentially reverse, making it particularly suitable for domains where inflow and outflow conditions are not strictly predefined. This boundary condition is adaptive, meaning it changes its behavior based on the local flow conditions at the boundary. Below are a few examples demonstrating how this boundary condition can be used and how to correctly interpret its meaning.

### Uniform Inlet-Outlet in Aerodynamics applications

**Example applications**: car, aircraft aerodynamics, wind tunnel experiment

This problem can be solved by using [simpleFoam (solver)](https://help.sim-flow.com/solvers/simple-foam). The velocity boundary condition at the outlet can be defined as Uniform Inlet-Outlet. This is critical because, in the wake of a vehicle (or any other object), flow regions might experience reverse flow due to vortices or turbulent eddies.

Example Boundary Conditions set for Aerodynamics applications

| Physics | Velocity | Pressure |
| --- | --- | --- |
| Outflow | **Uniform Inlet-Outlet** | Fixed Flux Pressure |

### Uniform Inlet-Outlet in VoF (Volume of Fluid) applications

**Example applications**: free surface flows, ship hull calculations, sloshing, ship hydrodynamics, wave impact on structures

For flows with a distinct interface between two fluids, like water and air (Volume of Fluid method used in [interFoam (solver)](https://help.sim-flow.com/solvers/inter-foam)), the Uniform Inlet-Outlet boundary condition for a volume fraction \(\alpha\) can be applied for so-called atmospheric boundaries of the domain, above the free surface. It will allow, in this situation, the air entrainment into the domain.

Example Boundary Conditions set for VoF applications

| Physics | Velocity | Pressure | Phases \(\alpha\) |
| --- | --- | --- | --- |
| Pressure Outlet | Pressure Inlet-Outlet Velocity | Fixed Value | **Uniform Inlet-Outlet** |

### Uniform Inlet-Outlet in Heat Transfer applications

**Example applications**: room ventilation, room heating, electronic cooling, atmospheric flows

Buoyancy-driven flows with heat exchange can be simulated in the [buoyantPimpleFoam (solver)](https://help.sim-flow.com/solvers/buoyant-pimple-foam). The Uniform Inlet-Outlet boundary condition for temperature T can be applied to the outlet out of the domain - again to prevent the backflows/reverse flows that might appear due to changing operating conditions.

Example Boundary Conditions set for Heat Transfer applications

| Physics | Velocity | Pressure | Thermal - T |
| --- | --- | --- | --- |
| Pressure Outlet | Pressure Inlet-Outlet Velocity | Fixed Value | **Uniform Inlet-Outlet** |

## Uniform Inlet-Outlet in SimFlow

### Constant Value in Uniform Inlet-Outlet

When the constant value of the variable is assigned using the Uniform Inlet-Outlet boundary condition, the definition is similar to [Inlet-Outlet](https://help.sim-flow.com/boundary-conditions/inlet-outlet).

![Constant Uniform Inlet-Outlet Boundary Condition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/uniform-inlet-outlet/uniform-inlet-outlet-bc-01.png)

Figure 2. Constant Uniform Inlet-Outlet Boundary Condition in SimFlow

### Time-dependent value in Uniform Inlet-Outlet

The variable can also be defined as time-dependent. In SimFlow, a convenient table with graphical representation is used to define the boundary condition.

![Time-dependent Uniform Inlet-Outlet Boundary Condition in SimFlow](https://help.sim-flow.com/boundary-conditions/images/uniform-inlet-outlet/uniform-inlet-outlet-bc-02.png)

Figure 3. Time-dependent Uniform Inlet-Outlet Boundary Condition in SimFlow

## Uniform Inlet-Outlet - Alternatives

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

Uniform Inlet-Outlet - Alternatives

| Boundary Condition | Description |
| --- | --- |
| [Inlet-Outlet](https://help.sim-flow.com/boundary-conditions/inlet-outlet) | similar to Zero Gradient but with an additional feature, which prevents so-called backflows |
| [Zero Gradient](https://help.sim-flow.com/boundary-conditions/zero-gradient) | prescribes normal gradient to zero |
| [Pressure Inlet-Outlet Velocity](https://help.sim-flow.com/boundary-conditions/pressure-inlet-outlet-velocity) | allows velocity to adjust at the outlet in order to satisfy the prescribed inlet velocity |