Fixed Flux Pressure - Boundary Condition Description
Fixed Flux Pressure is a boundary condition specifically designed for pressure boundaries where the flux is predetermined or calculated from other variables in the system. Instead of setting fixed pressure at the boundary, it adjusts the pressure based on the specified flux, ensuring that the boundary’s net flux (flow rate) meets the desired or calculated value.
In other words, Fixed Flux Pressure chooses the pressure gradient such that the flux on the boundary matches that specified by the velocity boundary condition. This condition is typically applied for the domain inlet and walls.
Fixed Flux Pressure - Boundary Condition Understanding Fixed Flux Pressure
Fixed Flux Pressure adjusts the pressure gradient according to the formula:
\(\vec \nabla p_m = (\vec H/a_p \cdot \vec S_f - \vec U \cdot \vec S_f)\frac{(a_p)_f}{|| \vec S_f||}\)
where the subscript \(f\) represents values interpolated to the face and \(p_m = p - \rho \vec g h\) is the modified pressure.
The terms \(\vec H/a_p \cdot \vec S_f\) and \(\vec U \cdot \vec S_f\) represent the predicted and known flux, directly linked to the pressure equation. \(\vec S_f = ||S_f|| \cdot \vec n\) represents the face surface vector of the boundary surface, and \(a_p\) represents the matrix of coefficients associated with the cell center P. On a wall, the discrepancy in flux approaches zero, thereby transitioning the boundary condition to Zero Gradient.
Fixed Flux Pressure should be used for problems where body forces are considered, such as gravity or surface tension. Applying only Zero Gradient in these cases can result in a flux through the wall due to the influence of body forces. Fixed Flux Pressure boundary condition addresses this by compensating for the body forces to achieve zero flux. It adjusts the pressure gradient at the wall to balance all other flux contributions, ensuring accurate simulation results.
Fixed Flux Pressure - Boundary Condition Application & Physical Interpretation
The Fixed Flux Pressure boundary condition adjusts the pressure gradient such that the flux on the boundary is that specified by the velocity boundary condition.
Fixed Flux Pressure in Multiphase applications
Example applications: tank sloshing, ship motion, river flow, free-surface flows
Ship motion studies can be conducted using the interFoam (solver). The Fixed Flux Pressure boundary condition can be applied to the hull of a ship to ensure that the ship’s surface remains impermeable, even in the presence of gravity or surface tension forces.
| Physics | Modified Pressure \(p- \rho gh\) | Velocity |
|---|---|---|
Hull (wall) | Fixed Flux Pressure | Moving Wall Velocity |
Inlet | Fixed Flux Pressure | Fixed Value |
| Tutorial | Description |
|---|---|
The standard ship hull tutorial for multiphase flows typically involves simulating the interaction between the hull of a ship and the surrounding water and air. |
Fixed Flux Pressure in Heat Transfer applications
Example applications: heat exchangers, electronics cooling, room heating
A heat exchanger problem can be solved in CHT Multi Region SIMPLE (solver). Fixed Flux Pressure can be applied for the inlet boundary in order to guarantee the pressure gradient adjusts to the prescribed velocity flux.
| Physics | Modified Pressure \(p- \rho gh\) | Velocity | Temperature |
|---|---|---|---|
Inlet | Fixed Flux Pressure | Fixed Value | Fixed Value |
| Tutorial | Description |
|---|---|
The heat exchanger tutorial involves simulating the heat transfer between two fluids through a solid wall, which is a common scenario in various engineering applications such as HVAC systems, power plants, and chemical processing. |
Fixed Flux Pressure - Boundary Condition Fixed Flux Pressure in SimFlow
Fixed Flux Pressure definition in SimFlow is very easy and does not require any parameters. If one wants the pressure gradient to be calculated based on quantities from different patches, Fixed Flux Pressure must be selected from the drop-down menu on the given patch -Figure 1.
Fixed Flux Pressure - Boundary Condition Fixed Flux Pressure - Alternatives
In this section, we propose boundary conditions that are alternative to Fixed Flux Pressure. 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 |
|---|---|
the boundary condition specifies a constant gradient (rate of change) at the boundary |