Outlet-Inlet - Boundary Condition Description
Outlet-Inlet is a mixed boundary condition that alternates between Zero Gradient and Fixed Value, depending on the local flow direction at the boundary.
If the flow is leaving the domain (outflow, positive flux), the Fixed Value boundary condition is applied. If the flow is entering the domain (inflow, negative flux), the Zero Gradient boundary condition is used instead.
Outlet-Inlet is typically applied at the inlet of a domain or at boundaries expected to function as inlets but may, under certain conditions, allow fluid to exit. Such conditions can occur in buoyancy-driven flows, atmospheric simulations, or free-surface scenarios where the flow direction may reverse.
This boundary condition is applicable to all variables, including both scalars and vectors.
Outlet-Inlet - Boundary Condition Understanding Outlet-Inlet
Mixed-type boundary conditions, such as Outlet-Inlet, are often required in CFD simulations at boundaries where the flow direction can change during the simulation. These boundary conditions dynamically respond to whether the flow on a given patch is directed into or out of the computational domain.
It is important not to confuse Outlet-Inlet with Inlet-Outlet. While both are mixed-type boundary conditions that switch between Zero Gradient and Fixed Value based on the local flow direction, their applications are different:
- Inlet-Outlet is typically used at boundaries that primarily act as outlets but may occasionally admit reverse flow into the domain.
- When the flux is directed into the domain (reverse flow), Inlet-Outlet applies a Fixed Value condition.
- When the flux is directed out of the domain (positive flux), it applies Zero Gradient, allowing variables (e.g., velocity, temperature) to "float" and follow the internal field gradient.
- In contrast, Outlet-Inlet is applied to inlets and operates in the opposite way.
- When the flux is directed out of the domain (reverse flow at the inlet), it applies Fixed Value, as specified by the user.
- When the flux is directed into the domain (inflow), it applies Zero Gradient, allowing the internal field to determine the inflow condition without imposing a fixed value at the boundary.
Outlet-Inlet - Graphical Representation
Outlet-Inlet graphically can be presented as in Figure 1.
The upper row shows that when the flux is directed into the domain, Outlet-Inlet applies a zero-gradient condition, which does not alter the inlet profile. However, if part of the inlet patch experiences a reversal in flow direction, the user-specified value is applied to the affected portion of the boundary, while the rest remains unmodified, as shown in the lower row.
Outlet-Inlet - Boundary Condition Application & Physical Interpretation
Outlet-Inlet is used in simulations where a boundary primarily serves as an inflow but may occasionally act as an outflow. It is a mixed-type condition that dynamically adapts based on the direction of flow at the boundary. When the flow enters the domain, the condition applies a zero-gradient boundary, allowing the field values at the boundary to be determined by the internal solution. This ensures smooth and natural inflow behavior. If the flow reverses and exits the domain, the user-specified fixed value is imposed on the boundary.
Outlet-Inlet in Buoyancy-Driven (Heat Transfer) applications
Example applications: natural ventilation in a building
The problem can be solved using buoyantPimpleFoam (solver). We can consider the fresh air supply as a large reservoir at a uniform temperature. Most of the time, flow enters the domain from this reservoir. Rather than prescribing a strict uniform temperature by Fixed Value, it is more natural to allow the internal solution to stabilize. So for inflow, Zero Gradient. can be used.
But if, at some time, the flow reverses (due to a complicated flow pattern for example) and the domain pushes fluid out, we want to stabilize the flow by applying the prescribed, known value.
| Physics | Velocity | Pressure p-\(\rho\)gh | Temperature |
|---|---|---|---|
Inlet | Fixed Value | Fixed Flux Pressure | Outlet-Inlet |
Outlet-Inlet - Boundary Condition Outlet-Inlet in SimFlow
The definition of boundary conditions in SimFlow is both simple and intuitive. To specify the Outlet-Inlet boundary condition, the user must navigate to the Boundary Conditions panel, select the appropriate boundary, and choose the correct option from the drop-down menu.
Outlet Value needs to be specified by the user. This value is used for reverse flows at the boundary.
Outlet-Inlet - Boundary Condition Outlet-Inlet - Alternatives
In this section, we propose boundary conditions that are alternative to Outlet-Inlet. 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 |
|---|---|
belongs to the Neumann boundary conditions, sets the normal gradient of any variable to zero | |
fixed value on the patch | |
similar to Zero Gradient but with an additional feature, which prevents so-called backflows |