# Outlet Regions

How to create Outlet regions in SimLab.

From the menu bar, click nFX Analysis > Boundary Conditions > Outlet.

## Introduction

Outlet regions mimic the behavior of outlet and simple outlet using a combination of removal and extended regions. They allow particles of any phase to be removed from anywhere within the computational domain. Every particle that enters an Outlet region will be removed from the simulation permanently.

## Outlet Region Types

Currently, there are three types of outlet regions:

• OUTREG_SPHERE
• OUTREG_CYLINDER
• OUTREG_CUBOID

Cylindrical and rectangular shaped Outlet regions can optionally have an extension region, similar to Inlet region extensions, which can define either velocity or acceleration within its volume. An extension is an extended region in the opposite direction of the Outlet region's normal where a constant particle velocity or acceleration in the normal direction is enforced.

Spherical Outlet regions cannot have an extension and acts as a simple particle sink.

## Description

When used without extensions, Outlet regions act as pseudo zero pressure and zero stress boundaries with no backflow. Since the particles are missing neighbors (incomplete support), these conditions are not strictly met, hence pseudo. In a way, this is the same behavior observed for a free surface (void interface).

When used with an extension that is well populated, the behavior depends on the type of extension:
Velocity
When using the velocity extension type, the extension sets the normal component of particle velocity to the specified value. The particle velocity is free to evolve in directions perpendicular to the Outlet regions normal directions. The particles immediately outside of the extension will see this as some form of constant velocity outlet boundary. This is a loose form of boundary condition and the velocity felt by the particles immediately outside of the extension is not exactly equal to the Outlet region velocity. The pressure immediately outside the extension will adjust itself to support the flow through the extension. Applying zero velocity in velocity type extension will result in a clogged Outlet region.
Acceleration
When using acceleration extension type, the extension sets the normal component of particle acceleration to the specified value. The particle acceleration is free to evolve in directions perpendicular to the Outlet regions normal direction. For particles immediately outside of the extension, the particles in the extension represent a form of outlet boundary with constant acceleration. The pressure immediately outside the extension will adjust itself to support the flow through the extension.
Start Time / End Time
Specifies the start and finish times which define the window where the Outlet region is active.
Ramp Up / Down Time
This parameter specifies a time window for the acceleration and deceleration of the fluid. At the start time, the flow will ramp up from zero to the specified value over this time. If ramp down is specified, the velocity will ramp back down to zero prior to the end time. This minimizes pressure fluctuations and instabilities caused by instantaneous changes in velocity.
Material
Outlet regions can be set to apply only to specific phases. For example, removing one fluid only. It is also possible to specify wall phases. In this situation, an opening can be created in the wall by the Outlet region.

## Other Options

A free text box where additional parameters can be specified.