Boundary conditions
Introduction
This page details all the boundary conditions a user has access to as well as what the required input is. A boundary condition makes it so that a problem only has one solution. For example, defining an inlet temperature for the coolant will (for a given heatsink, pump, and heat load) fix the temperatures for the rest of the entities. A boundary condition allows a user to match the model as close as possible to reality; is a pump driving the fluid motion and if so, is this pump positioned upstream or downstream, etc.?
General overview
The following list shows all the available boundary conditions, applicability, and input parameters. Parameters denoted with * are required, the others are optional entries. Click on the name of the boundary condition for more information.
Boundary type | Boundary condition | Applicable to | Input Parameters | ||
---|---|---|---|---|---|
Fluid | Solid | Fluid | Solid | ||
Inlet | FixedFlowRateInlet | , , or , | |||
PressureInlet | , | ||||
PumpInlet | , Performance curve* | ||||
FanInlet | , Performance curve* | ||||
Outlet | PressureOutlet | | |||
FanOutlet | Performance curve* | ||||
PumpOutlet | Performance curve* | ||||
MinFlowrateOutlet | , or | ||||
Wall | FixedTemperatureWall | | | ||
HeatedWall | , , | | |||
ExternalWall | , , , , | , , , , | |||
InsulatedWall | | ||||
Slip | |||||
Cyclic | Another boundary of type 'Cyclic' | Another boundary of type 'Cyclic' |
Important
Any boundary that isn't an interface or has any special boundary condition imposed is by default adiabatic. This means that it will be treated as if a wall with perfect insulation is placed next to it.
For fluids will also the no-slip condition be imposed.
Boundary Types
FixedFlowRateInlet
Name | fixedFlowRateInlet |
Type | Inlet |
Applicable to | Fluids |
Input parameters | The temperature in at the patch should be specified, as well as either the velocity in , the mass flow rate in or the volumetric flow rate in , depending on the chosen option. Also a rotational velocity in can be set. |
With this boundary condition, a flow entering the domain at a certain velocity is specified. The flow can be described by:
- Fixed value velocity: the magnitude of the inlet velocity at the patch is specified.
- Mass flow rate: an average mass flow rate is specified, which is translated to a velocity at the patch with the fluid density by the software.
- Volumetric flow rate: the average volumetric flow rate at the patch is specified. The inlet velocity is then automatically calculated by the software.
A fixedFlowRateInlet boundary condition is typically used in combination with a pressureOutlet boundary condition.
PressureInlet
Name | pressureInlet |
Type | Inlet |
Applicable to | Fluids |
Input parameters | The temperature in and pressure in at the patch should be specified. |
This boundary condition defines an inflow condition based on the inputted total pressure at the boundary. A pressureInlet condition is typically used in combination with a pressureOulet boundary condition.
The total pressure in relates to the static pressure in through the following relation:
Where:
- represents the density of the fluid in
- represents the velocity of the fluid in
- represents the volumetric flow rate of the fluid in
- represents the area of the boundary patch in
PumpInlet
Name | pumpInlet |
Type | Inlet |
Applicable to | Fluids |
Input parameters | The temperature in at the patch should be specified, as well as the performance curve of the pump. |
This boundary condition sets the total pressure at the patch based on the pressure drop as a function of the volumetric flow rate as specified in the performance curve of the pump. This boundary condition is typically used in combination with a pressureOutlet boundary condition at the outlet.
The total pressure in relates to the static pressure in through the following relation:
Where:
- represents the density of the fluid in
- represents the velocity of the fluid in
- represents the volumetric flow rate of the fluid in
- represents the area of the boundary patch in
FanInlet
Name | fanInlet |
Type | Inlet |
Applicable to | Fluids |
Input parameters | The temperature in at the patch should be specified, as well as the performance curve of the fan. |
This boundary condition can be used to set a total pressure inlet condition for a fan. The total pressure at the patch is set based on the pressure drop which is specified in the performance curve of the fan as a function of the volumetric flow rate. This boundary condition is typically used in combination with a pressureOutlet boundary condition at the outlet.
The total pressure in relates to the static pressure in through the following relation:
Where:
- represents the density of the fluid in
- represents the velocity of the fluid in
- represents the volumetric flow rate of the fluid in
- represents the area of the boundary patch in
PressureOutlet
Name | pressureOutlet |
Type | Outlet |
Applicable to | Fluids |
Input parameters | Optionally, a pressure at the patch can be specified in . If no pressure is specified, it is set to 0 . This boundary patch fixes the static pressure at that boundary. |
This boundary condition defines an outflow condition based on the specified static pressure at the boundary. A pressureOutlet boundary condition is typically used in combination with a fixedFlowRateInlet, a fanInlet, or a pumpInlet.
FanOutlet
Name | fanOutlet |
Type | Outlet |
Applicable to | Fluids |
Input parameters | The performance curve of the fan should be specified. |
This boundary condition sets the total pressure at the patch based on the pressure drop as a function of the volumetric flow rate as specified in the performance curve of the fan. A fanOutlet condition is typically used in combination with a pressureInlet condition at the inlet.
PumpOutlet
Name | pumpOutlet |
Type | Outlet |
Applicable to | Fluids |
Input parameters | The performance curve of the fan should be specified. |
This boundary condition sets the total pressure at the patch based on the pressure drop as a function of the volumetric flow rate as specified in the performance curve of the pump. A pumpOutlet condition is typically used in combination with a pressureInlet condition at the inlet.
MinFlowrateOutlet
Name | minFlowRateOutlet |
Type | Outlet |
Applicable to | Fluids |
Input parameters | A minFlowRateOutlet only requires one input parameter: either the velocity in the mass flow rate in or the volumetric flow rate in at the patch. |
This boundary condition ensures that a specified minimum flow rate is achieved through the outlet. This minimum flow rate can be described by:
- Fixed value velocity: the magnitude of the minimum outlet velocity at the patch is specified.
- Mass flow rate: the minimum average mass flow rate at the patch is specified. The minimum outlet velocity is then automatically calculated by the software.
- Volumetric flow rate: the minimum average volumetric flow rate at the patch is specified. The minimum outlet velocity is then automatically calculated by the software.
A minimumFlowRateOutlet boundary condition is typically used in combination with a pressureInlet boundary condition.
FixedTemperatureWall
Name | fixedTemperatureWall | |
Type | Wall | |
Applicable to | Fluids and solids | |
Input parameters | Fluid | The temperature in at the patch should be specified. |
Solid | The temperature in at the patch should be specified. |
With the fixedTemperatureWall, you can fix the temperature assigned to the patch. This is useful if you want to model a constant temperature heat source or sink. The following input options are available:
- A constant value over the entire patch, e.g.:
- A polynomial in function of spatial coordinates where 7 coefficients should be provided:
This array represents the coefficients of a polynomial:
- A table containing the coordinates and corresponding temperatures:
HeatedWall
Name | fixedTemperatureWall | |
Type | Wall | |
Applicable to | Fluids and solids | |
Input parameters | Fluid | The heating power in should be specified. Optionally, a thermal resistance can be applied by providing the thickness of the thermal resistance layer in and the conductivity of the resistance layer in . |
Solid | The heating power in should be specified. |
With the heatedWall, a constant heating power value can be assigned to the patch. The following input options are available:
- A constant power value over the entire patch, e.g.:
- A polynomial in function of spatial coordinates where 7 coefficients should be provided:
This array represents the coefficients of a polynomial:
- A table containing the coordinates and corresponding power:
ExternalWall
Name | fixedTemperatureWall | |
Type | Wall | |
Applicable to | Fluids and solids | |
Input parameters | Fluid | The heat transfer coefficient in and the temperature of the environment in should be specified. Optionally, a thermal resistance layer can be defined by setting the thickness of the layer in and the conductivity of the thermal resistance layer in . Optionally, an external emissivity in [−] can be set. |
Solid | The heat transfer coefficient in and the temperature of the environment in should be specified. Optionally, a thermal resistance layer can be defined by setting the thickness of the layer in and the conductivity of the thermal resistance layer in . Optionally, an external emissivity in [−] can be set. |
With this boundary condition, heat losses to or gains from the environment can be modeled. Optional thin thermal resistance layers can be specified through the thicknesses and the conductivity of the layers.
InsulatedWall
Name | fixedTemperatureWall | |
Type | Wall | |
Applicable to | Fluids and solids | |
Input parameters | Fluid | The roughness length in |
Solid | / |
As the name suggests, this boundary condition represents an adiabatic wall. These boundaries do not thermally interact with the rest of the case setup.
Slip
Name | slip |
Type | Wall |
Applicable to | Fluids |
Input parameters | / |
This boundary condition ensures that there can be no flow perpendicular to the boundary itself, but there can be flow (with a non-zero velocity) parallel to or along this boundary patch.
Cyclic
Name | Cyclic |
Type | Wall |
Applicable to | Fluids and solids |
Input parameters | Another boundary of type 'Cyclic' |
ColdStream allows you to assign periodicity. By using the Cyclic boundary patch option, the heat leaving through this boundary will enter again through the second, linked boundary.
Warning
To link 2 boundaries with one another, they must be assigned to the same parent region. The shape of the linked boundaries must also match with one another.
Updated about 4 hours ago