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Boundary Conditions

The boundary condition parameters for the Flow module apply to Boundaries. They also apply to Interfaces for which the Flow module is Blanked on one side of the interface, creating in effect, a boundary.

The boundary conditions and associated Flow parameters are specified as follows:

  • Geometric Entities Panel > Boundaries > [Desired Boundary]
  • Properties Panel > Model Tab > Flow

The equations for the boundary conditions are explained in Modelling of Flow Boundaries. The boundary conditions available for Flow module are:

  

Figure 5.25 - Flow Boundary Condition

 

Wall

This corresponds to a solid boundary and means that there is shear (drag) and no normal component of velocity at the boundary (i.e. no through-flow). When the Turbulence module is active, the roughness of the wall can be accounted for using Wall Roughness Model. A Wall boundary can be assigned a shear wall velocity using Options.

Options

To introduce shear at the wall, the Wall boundary condition has the following Options:

  1. Stationary: Assumes that the wall is stationary.

  2. Cartesian: Introduces shear at the wall in terms of components of velocity. The velocity at a Wall boundary is input relative to the stationary reference frame.
  3. Tangential: Introduces shear at the wall in terms of the normal to the wall. The tangential velocity at a Wall boundary is input relative in terms of the Vector Normal to the Wall Velocity and a Value.

The velocity for a Wall boundary is intended to introduce a shear at the wall. In this case, only tangential velocities are used.

Numerically, the velocity for a Wall boundary introduces a momentum source into the momentum equation 5.13. It does not actually move the boundary, i.e. it does not change the shape of the domain.

 

Figure 5.26 - Flow Options

Wall Type

  1. Rigid: Produces a non-deforming wall.

  2. Flexible: Produces a deforming wall. It does not physically move the grid associated with the wall. The effect of the wall motion is included by changing the effective volume of the neighboring cell.

 

 

 

 

Figure 5.27 - Flow Wall Type

To model flexible walls, there are two models under Deformation Model:

The Displacement and Wall Velocity on the flexible wall can be displayed through the variable list under the Results Panel. These are not reflected in the X-Y Plots.

High Order Shear

This uses a parabolic function for the velocity profile near the wall instead of a linear function. This can be used for laminar flow near the wall. This can also be used if the near wall cell is within the laminar sub-layer for turbulent flows. This reduces the number of cells used to resolve the flow within thin gaps, which are heavily dominated by viscous shear forces.

 

 

 

 

Figure 5.28 - Flow High Order Shear

Specified Velocity

This sets the velocity on the boundary. The net mass flow out of the boundary is given by the fluid density and velocity (relative to the boundary area and orientation). Choose one of the following to specify the direction and magnitude of the velocity.

 

Cartesian: This defines the boundary velocity in terms of the velocity components relative to the model coordinate system.

  

Figure 5.29 - Specified Velocity - Cartesian

Boundary Normal: This defines the velocity normal to the boundary. The magnitude is controlled by the Normal Velocity Component. The Flow Direction can be Inflow, Outflow, or Both.

  • Inflow: Allows flow into the domain, such that a positive Value for the Volumetric Flux will result in a boundary velocity pointing into the domain.
  • Outflow: Allows flow out of the domain, such that a positive Value for the Volumetric Flux will result in a boundary velocity pointing out of the domain.
  • Both: Allows flow in or out of the domain. A positive Value for the Volumetric Flux will result in outflow. A negative Value for the Volumetric Flux will result in inflow.
  

Figure 5.30 - Specified Velocity - Boundary Normal
 

Note: For either Inflow or Outflow, a negative Normal Velocity Component will be reset to a positive value, such that the sign of the Value for the Volumetric Flux has no influence on the direction of flow.

Swirl: This defines a swirling flow at a boundary. The magnitude of the inflow is controlled by the Normal Velocity Component. The Flow Direction can be Inflow , Outflow or Both. The Swirl velocity is given through the: Rotational Direction, Rotational Speed, Rotational Center and Rotational Axis Vector

  

Figure 5.31 - Specified Velocity - Swirl
Specified Volumetric Flux

This sets the fluid’s volumetric flux (m3/s) at an opening. Thereby, it sets the velocity at the boundary. The corresponding mass flow is determined by the fluid density and velocity (relative to the boundary area and orientation). The velocity associated can be either Uniform or Fully Developed. The direction and magnitude of the velocity can be specified by the following options:

  • Flow Direction: Determined by selecting one of three options: Inflow, Outflow, or Both. The volumetric flux, is input under Value.
 

Figure 5.32 - Specified Volumetric Flux - Flow Direction

  • Velocity Profile: This may be set to the following:

    • Uniform: Constant velocity at the boundary based on the volumetric flux, , boundary area and orientation:

    • Fully Developed: Allows the velocity profile at the boundary to be similar (same shape) to the velocity profile at the cell centers immediately downstream.

  

Figure 5.33 - Specified Volumetric Flux - Velocity Profile
Specified Total Pressure

This defines the total pressure at an opening where flow is expected to enter or leave the domain. The velocity of the flow at the boundary is then computed as part of the solution.

The direction of the boundary velocity vector is given as Cartesian and Boundary Normal under the drop down list of Directional Option.

Velocity Profile: The following two options are available:

  1. Uniform: Velocity is uniform across the entire boundary.
  2. Zero Gradient: Local boundary cell velocity values are determined based on the zero gradient from the adjacent cell.
 

Figure 5.34 - Specified Total Pressure - Velocity Profile
Rotating Wall

This simulates the shear effect of a rotating wall. It requires specifying the following:

 

Figure 5.35 - Rotating Wall

Symmetry

This means that there is no shear (i.e. perfect slip) and no normal component of velocity at the boundary (i.e. no through-flow). Symmetry for flow also means there is no normal gradient of pressure at the boundary. This boundary condition is different from a wall boundary condition in that for a wall there is shear. This usually corresponds to a physical symmetry in the model, but it doesn’t have to as long as the effects of this boundary condition make sense. For example, it can be used in some cases to mimic a free surface.

Specified Pressure Outlet

This allows to set the static pressure at an opening where flow is expected to exit the domain. In case of back flow, a momentum source may also be added via the associated Back Flow Velocity (optional) input. Specified Pressure Outlet determines the mass flow across the boundary as part of the solution.

It has the following options:

Specified Pressure Inlet

This allows to set the static pressure at an opening where flow is expected to enter the domain. A momentum source may also be added at this type boundary via the associated velocity input. Specified Pressure Inlet determines the mass flow across the boundary as part of the solution.

It has following options:

 

Note: If the incoming fluid has a relatively high dynamic head, another option is to use the Specified Total Pressure boundary condition instead of Specified Pressure Inlet.
Resistor Capacitor

Resistor Capacitor: This enables the user to determine the flow () - pressure () relationship for a selected boundary. The following models are available under Model option.

 

  • DP-Q Curve: DP-Q Curve model has two input parameters: Volumetric Flux and Environment Pressure. Environment Pressure has the default value of 101325 Pa. Volumetric Flux must be entered through an expression editor in the form of .

  

Figure 5.38 - Resistor capacitor - DP-Q Curve

  • Orifice: Orifice model has three input parameters. They are Discharge Coefficient, Diameter and Environment Pressure. Environment Pressure has the default value of 101325 Pa.

  

Figure 5.39 - Resistor Capacitor - Orifice

  • Resistor: Resistor model has two input parameters. They are Resistor - r and Environment Pressure. Environment Pressure has the default value of 101325 Pa.

  

Figure 5.40 - Resistor Capacitor - Resistor

  • Capacitor: Capacitor model has two input parameters. They are Capacitor and Environment Pressure. Environment Pressure has the default value of 101325 Pa.

  

Figure 5.41 - Resistor Capacitor - Capacitor

  • 2 Elements: 2 Elements model has three input parameters. The system is modelled as a circuit with a Resistor and Capacitor. Specify the value of resistance of Resistor in the option Resistor - R and Capacitance in the option Capacitor. Environment Pressure has the default value of 101325 Pa.

  

Figure 5.42 - Resistor Capacitor - 2 Elements

  • 3 elements: 3 elements model has four input parameters. The system is modelled as a circuit with 2 resistors and a capacitor. Specify the value of resistance of Resistor in the option Resistor - R, 2nd resistance in the option Resistor - r and capacitance in the option Capacitor. Environment Pressure has the default value of 101325 Pa.

  

Figure 5.43 - Resistor Capacitor - Orifice
 

Note: This boundary condition is based on the 2-element and 3-element Windkessel model1 Daniel R. Kerner, Ph.D. . and Broemser, Ph., et. al., ``Uber die Messung des Schlagvolumens des Herzens auf unblutigem Weg'', Zeitung für Biologie 90 (1930) 467-507. often used for heart flow modelling.

 

Interface Condition

The following interface conditions and associated Flow parameters can be specified for a selected Interface.

Default Interface is the default option for an interface connecting fluid to fluid. The Interface Conditions for the Flow module are the same as for the boundary conditions, if and only if one side of the Interface has been Blanked for Flow. If, instead, the Flow module is Active on both sides of an Interface, then it can only be assigned as a Default Interface. The interfaces can be modelled as:

 

ClosedExample of adding Fan curve

In some applications, user need to add a fan curve when user do not want to model the actual fan geometry or if the geometry is complex. Simerics-MP provides an option to add the fan curves as a condition for an interface using the Expression Editor.

The user can add the fan curve by the following steps:

  1. Select MGI02_fan_mgi_roomaxial_room_mgi_fanaxial from the Interface list in the Geometric Entities Panel as shown in Figure 5.44.

  2. Select Fan or Pressure Jump under Model in the Properties Panel as shown in Figure 5.45.
  3. Enter -0.658,-0.6814,0.32 m for Flow Direction.
  4. Add the Pressure vs Flow data in Tables format and place the text file in the working folder. Here, text file is named as dpvsq.text.
  5. Click Edit Expressions icon next to Q-DP curve and enter DPvsQ = table("dpvsq.txt",Q) under Local Expressions.
  6. Click OK, to close the Expression Editor dialog box.
  7. Enter DPvsQ Pa for Q-DP curve under Model Tab of the Properties Panel.

 

Figure 5.44 - Fan curve interface

 

Figure 5.45 - Fan parameters

 

 

Note: For more information on how to add Pressure vs Flow curve in table format, refer Tables in the Expression Editor.

 

Output

The Flow module outputs available with the Default Interface include Area, Normal, Mass Flow Rate, Volumetric Flow Rate, Momentum, Pressure Force, Average Total Pressure, Pressure, and Average Static Pressure.

 

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