Pressure Directed Inlet-Outlet Velocity - Boundary Condition Description
Pressure Directed Inlet-Outlet Velocity is a velocity boundary condition for open inlets–outlets applied for patches on which the pressure is prescribed and the flow may reverse. It applies Zero Gradient when the flux leaves the domain, and when the flux enters, it imposes a velocity parallel to a direction vector chosen by the user, instead of strictly normal to the face (in contrast to Pressure Normal Inlet-Outlet Velocity). The condition is a derivative of Pressure Inlet-Outlet Velocity.
This 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 User defines the direction of the inflow through the directional cosine.
Pressure Directed Inlet-Outlet Velocity - Boundary Condition Understanding Pressure Directed Inlet-Outlet Velocity
The Pressure Directed 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:
\(\vec U_{p} = \vec d \frac{\phi}{\vec d \cdot A}\)
where:
\(\vec U_{p}\) - velocity vector at the patch
\(\vec d\) - unit inlet direction
\(\phi\) - flux through the face
\(A\) - face area vector
Pressure Directed 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 determined by the shape of the geometry. For instance, consider a ventilation channel between room and environment: if the pressure outside the room is higher, air will flow into the room through the channel.
Pressure Directed Inlet-Outlet Velocity is similar to Pressure Inlet-Outlet Velocity and Pressure Normal Inlet-Outlet Velocity boundary conditions. To avoid misunderstanding, all three boundary conditions are summarized in the table below.
| 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 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 |
Pressure Directed Inlet-Outlet Velocity - Boundary Condition Application & Physical Interpretation
Pressure Directed Inlet-Outlet Velocity is tailored for boundaries where the flow speed is uncertain and are regulated by the pressure difference across the boundary but the flow direction is predefined.
Pressure Directed Inlet-Outlet Velocity in Heat Transfer
Example applications: HVAC, room temperature distribution
This problem can be addressed using the buoyantSimpleFoam 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). In such cases, Pressure Directed Inlet-Outlet Velocity is applied at the outlet to adjust the flow according to the main driving factor and to stabilize the calculations.
| Physics | Pressure | Velocity | Temperature |
|---|---|---|---|
Inlet | Fixed Flux Pressure | Fixed Value | Fixed Value |
Outlet (wind tunnel) | Fixed Value | Pressure Directed Inlet-Outlet Velocity | Zero Gradient |
Pressure Directed Inlet-Outlet Velocity in channel flow applications
Example applications: channel flows, T-junction flows
This problem can be tackled using the pimpleFoam solver. In channel flows, the flow is generally driven by a pressure difference. The Total 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.
| Physics | Pressure | Velocity |
|---|---|---|
Inlet | Total Pressure | Pressure Directed Inlet-Outlet Velocity |
Outlet | Fixed Value | Inlet-Outlet |
Pressure Directed Inlet-Outlet Velocity in open-boundary flows (multiphase) applications
Example applications: dam break, river flows
Pressure Directed 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 or Zero Gradient.
| Physics | Pressure | Velocity |
|---|---|---|
Atmosphere (open boundary) | Calculated | Pressure Directed Inlet-Outlet Velocity |
Pressure Directed Inlet-Outlet Velocity - Boundary Condition Pressure Directed Inlet-Outlet Velocity in SimFlow
To define Pressure Directed Inlet-Outlet Velocity condition on the patch, the proper option must be selected from the drop-down menu in Boundary Conditions tab - Figure 1. The required input will define the flow direction.
Pressure Directed Inlet-Outlet Velocity - Boundary Condition Pressure Directed Inlet-Outlet Velocity - Alternatives
In this section, we propose boundary conditions that are alternative to Pressure Directed 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 |
|---|---|
similar definition to Pressure Directed Inlet-Outlet Velocity, but the flow occurs in the direction normal to the boundary face | |
works similarily to Pressure Directed Inlet-Outlet Velocity, but allows a direction to be adjusted by the pressure condition |