Pressure Normal Inlet-Outlet Velocity - Boundary Conditions

Pressure Normal Inlet-Outlet Velocity - Boundary Condition   Description

Pressure Normal Inlet-Outlet Velocity is a velocity boundary condition for open inlets–outlets. It applies 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.

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.

Pressure Normal Inlet-Outlet Velocity - Boundary Condition   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 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 and 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
FeaturePressure Inlet-Outlet VelocityPressure Normal Inlet-Outlet VelocityPressure 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 Normal Inlet-Outlet Velocity - Boundary Condition   Application & Physical Interpretation

Pressure Normal 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 and normal to 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 (solver) 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 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
PhysicsPressureVelocityTemperature

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 (solver) 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.

Example Boundary Conditions set for channel flow applications
PhysicsPressureVelocity

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 or Zero Gradient.

Example Boundary Conditions set for open boundary applications
PhysicsPressureVelocity

Atmosphere (open boundary)

Calculated

Pressure Normal Inlet-Outlet Velocity

Pressure Normal Inlet-Outlet Velocity - Boundary Condition   Pressure Normal Inlet-Outlet Velocity in SimFlow

To define Pressure Normal Inlet-Outlet Velocity boundary condition on the patch, the proper option must be selected from the drop-down menu in Boundary Conditions tab - Figure 1

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

Pressure Normal Inlet-Outlet Velocity - Boundary Condition   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 ConditionDescription

Pressure Inlet-Outlet Velocity

works similarily to Pressure Normal Inlet-Outlet Velocity, but allows a direction to be adjusted by the pressure condition

Pressure Directed Inlet-Outlet Velocity

similar definition to Pressure Normal Inlet-Outlet Velocity, but the flow occurs in the direction specified by the User