Porous Baffle Interface

Modeling

Simcenter STAR-CCM+ provides two forms of porous baffle.

• Resistance Based

  This baffle treatment is suitable for modeling open baffles where the flow is predominantly normal to the surface, such as that found in grills and screens.

  For turbulent flows, this option is appropriate if the holes in the baffle are such that their characteristic length is similar to the length scale of the turbulence.
• Shear Stress Based

  This baffle treatment, sometimes referred to as a wall-like or a wall-based porous baffle, is best suited to applications where the flow is mostly tangential to the baffle. This option is also appropriate if the velocities at the two sides of the porous baffle interface are different.

  For turbulent flows, this option is appropriate if the turbulence has a scale that reduces significantly at the baffle.

Simcenter STAR-CCM+ models a porous baffle as follows:

• The baffle has infinitesimal thickness.
• If the porous baffle is thought of as a perforated surface, then the ratio of the area of the holes to the area of the surface, is called the porosity, $\chi$. Wire screens would have a large value of $\chi$, while something like a perforated plate would have a smaller value.
• Each interface has a direction from side 0 to side 1. When you create the interface, select side 0 first, then side 1. For a porous-baffle interface, the upstream is side 0. Therefore if you select the sides in the wrong order, the sign of the pressure jump is reversed.
• Turbulence generation at the baffle depends on the ratio of the pore size to the characteristic turbulent scales. If the baffle pores are much smaller than the turbulent scales, the baffle behaves like a wall, so the Shear Stress Based baffle is appropriate. If the baffle pores are larger, the turbulent eddies can penetrate through the baffle (which has a structure similar to a grill, rather than a porous plate), so the Resistance Based baffle is appropriate.

• For a shear-based porous baffle, the viscous shear stress is approximated to be the same as the shear stress at the interface assuming it to be a solid wall. The porosity, $\chi$, is used only for heat and mass transfer calculations at the porous baffle surface and is not used for flow calculation.

  For a resistance-based porous baffle, the viscous stresses depend on the tangential resistance coefficients.

• For heat transfer analysis, the porous baffle offers a finite thermal resistance. Conduction is modeled as a one-dimensional process and thermal inertia is neglected. A thermal resistance per unit area, $R$, for heat transfer can be specified. The relation between the heat flux across the contact interface, $\dot{q}"$, and the temperature difference, $\mathrm{\Delta }T$, is given by Eqn. (79). Estimate the thermal resistance using Eqn. (82).
• The total heat transfer across the porous baffle is computed as a linear combination of the heat transfer due to convection and diffusion in the fluid and the conduction through the baffle. The porosity, $\chi$ , provides the linear weighting factor.
• It is assumed that the direction of the flow is unchanged as it passes through the baffle.

Setting up a Porous Baffle

To set up a porous baffle:

1. Set the interface Type property to Porous Baffle Interface.
2. In the Physics Conditions of the interface, specify the Porous Baffle Treatment method:
   • Resistance Based
   • Shear Stress Based
   See Porous Baffle Physics Conditions.
3. Specify the appropriate Physics Values.

   For a Resistance Based porous baffle, both normal resistance and tangential resistance is applied to the baffle. Set the following:

   • Porous Inertial Resistance
   • Porous Inertial Tangential Resistance
   • Porous Viscous Resistance
   • Porous Viscous Tangential Resistance

   For a Shear Stress Based porous baffle, normal resistance only is applied to the baffle.

   • Porous Inertial Resistance
   • Porous Viscous Resistance

   See Porous Baffle Physics Values.

4. For a Resistance Based porous baffle, if a turbulence model is activated in the physics continuum, you also specify the following:

   • Turbulence Intensity
   • Turbulent Length Scale

     The length scale of the turbulence is related to the characteristic geometry scale of the baffle itself, for example, the perforation diameter.

   The turbulence specification in a porous baffle is not a boundary condition, but is a source, in the same way as for Particle-Induced Turbulence. It can be considered as particle-induced turbulence, where the particles are the fixed components of the baffle.

   See Porous Baffle Physics Values.

5. Specify any tangential velocity on the porous baffle surface.

   This step is necessary if the baffle is rotating, for example.

   For a Resistance Based porous baffle, the flow velocity is the same on both sides of the baffle. You specify the flow velocity on the upstream side of the baffle. For a Shear Stress Based porous baffle, you can specify different flow velocities on either side of the porous baffle.

   Set the appropriate values in the Regions > [region] > Boundaries > [interface boundary] > Physics Conditions > Tangential Velocity Specification node for each side of the interface.

Porous Baffle Properties

Use the properties that are listed below to adjust the specifications of a Porous Baffle interface node.

Geometry

Specifies the geometry source option:

• Boundaries: The interface is a boundary-mode interface, which is created by selecting two boundaries.
• Contacts: The interface is a contact-mode interface. which is defined directly from part contacts.

Boundary-0

Indicates the "fixed" side of the interface (Read Only).

Boundary-1

Indicates the "adapted" side of the interface (Read Only).

Vertices from boundary-1 are projected onto boundary-0. The orientation can be swapped through the right-click action Reverse Orientation.

Contacts

Displays the part surface contacts. When the Geometry property is Contacts, it shows the part surface contacts. Geometry property is Boundaries, it shows no value.

Type

Defines the type of interface. Must be set to Porous Baffle Interface.

Topology

Defines the connection type between the interfaces:

• In-place—uses the in-place topology.
• Periodic—uses the periodic topology.
• Repeating—uses the repeating topology.

Connectivity

Defines how the boundaries on the two sides of the interface are connected. Porous baffle interfaces have Imprinted connectivity type. The imprint connectivity type indicates the interface is within regions that are discretized using the finite volume method. The imprint procedure creates intersected faces.

Allow Per-Contact Values

Allows you to define the interface tolerance individually for each contact. Activates the child property Specify by Part Subgroup.

注	This property is only valid for contact-mode boundary interfaces. See also: Contact-Mode Boundary Interfaces.

Close Adjacent Cells

Creates a watertight intersection by fixing the cell connectivity between the interface faces and side faces along common edges.

With this option, the topology-based intersector adds extra edges to eliminate the gap between the sides, which reduces spurious oscillations in physical quantities.

This property is only available when the Direct Intersector of the Interface Manager node is set to Topology-Based and the Connectivity of the interface is Imprinted.

注	Close Adjacent Cells is not compatible with DFBI Motion.

Reset on Relative Motion

• When activated, the interface is reset when there is any relative motion between the two sides.
• When deactivated, the interface is reset when the relative motion between the two sides exceeds a relative tolerance based upon minimum edge length.

注	This property is only available when the Connectivity of the interface is Imprinted.

Porous Baffle Physics Conditions

Baffle Thermal Option

Available only when an energy model is activated in the physics continuum.

• Non-Conducting
• Conducting

See Baffle Interface.

Energy Source Option

Available only when the Baffle Thermal Option is set to Conducting.

• None
• Heat Flux

  Defined only over the solid part of the baffle and so is proportional to $1-\chi$.

• Heat Source

  Defined over the entire baffle and so is not dependent on porosity $\chi$.

See Baffle Interface.

Porous Baffle Treatment

• Resistance Based

  This baffle treatment is suitable for modeling open baffles where the flow is predominantly normal to the surface, such as that found in grills and screens.

  Both normal resistance and tangential resistance is applied to the baffle: you specify the Porous Inertial Resistance, Porous Viscous Resistance, Porous Inertial Tangential Resistance, and Porous Viscous Tangential Resistance.

  Resistance in the tangential direction is considered homogeneous. The resistance is applied to the multiphase flow, rather than applied individually for each phase, so a porous baffle is not suitable for filtration-type applications.

  If a turbulence model is activated in the physics continuum, you also specify the Turbulence Intensity and the Turbulent Length Scale.

  The flow velocity is the same on both sides of the porous baffle. You specify the flow velocity on the upstream side of the porous baffle (in the Regions > [region] > Boundaries > [interface boundary] > Physics Conditions > Tangential Velocity Specification node).

  See Asymmetric Values on Interface Boundaries.

• Shear Stress Based

  This baffle treatment, sometimes referred to as a wall-like or a wall-based porous baffle, is best suited to applications where the flow is mostly tangential to the baffle.

  Normal resistance only is applied to the baffle: you specify the Porous Inertial Resistance and the Porous Viscous Resistance. Tangential drag is accounted for with a standard wall treatment.

  You can specify different flow velocities on either side of the porous baffle (in the Regions > [region] > Boundaries > [interface boundary] > Physics Conditions > Tangential Velocity Specification node for each side of the interface).

Porous Baffle Physics Values

Intersection

Available for all direct interfaces to control the intersection tolerance. The available properties for porous baffle interfaces are:

Specify by Part Subgroup

Specifies sub-grouping for contact-mode boundary interfaces to set up independent intersection properties. See also Contact-Mode Boundary Interfaces and 调整交界面相交容差（基于几何的方法）.

The remaining properties depend on the setting for the Direct Intersector property. This property is set on the Interfaces node (see Interfaces Properties):

Geometry-Based (Legacy)

This Direct Intersector option activates:

• Geometric Tolerance: Specifies the maximum projection distance of adapted vertices onto the fixed side in terms of a fraction of minimum local edge length around each vertex. For more details, refer to 调整交界面相交容差（基于几何的方法）.

Topology-Based with Connectivity Imprinted

• Match Outer Boundary: when activated, indicates that the interface boundaries are expected to fully overlap on large-scale topological features. Activate this option only when you expect a complete match of the two boundaries of an interface.

  By default, this property is deactivated.

• Projection Tolerance: maximum orthogonal projection distance in terms of a fraction of local element diameter. Places a limit on how the vertices of the adapted side get projected onto the fixed side. In cases where the two interface boundaries are separated by greater distances, you can increase the Projection Tolerance value to get fewer remainder faces.

  By default, the tolerance is set to 0.2.

• Angle Threshold: maximum angle (deg) by which the intersector identifies large-scale features that are mapped from the adapted side onto the fixed side. To preserve sharper features, specify a smaller Angle Threshold value. If the boundary meshes have spurious defects, you can specify a larger value.

  By default, the value is set to 45 deg.

Heat Flux

Available only when the Energy Source Option is set to Heat Flux. Proportional to $1-\chi$.

See Baffle Interface.

Heat Source

Available only when the Energy Source Option is set to Heat Source. Proportional to entire baffle area and not dependent on $\chi$.

See Baffle Interface.

Porosity

Available for single-phase flows only.

The factor $\chi$ in Eqn. (1859) that is used in calculating heat transfer and optionally viscous shear. The value ranges from 0 to 1. If the porous baffle is thought of as a perforated surface, then $\chi$ can be interpreted as the ratio of the area of the holes to the area of the surface.

Porous Inertial Resistance

For single-phase flows, the value of the coefficient $P_i$ in Eqn. (1858). For multiphase flows, the value of the same coefficient in the normal direction.

This value can be positive or 0. The default value is 0.

Porous Inertial Tangential Resistance

The value of the coefficient ${\alpha }_m$ in the tangential direction (see Eqn. (1870)).

Available only when the Porous Baffle Treatment is set to Resistance Based.

Porous Viscous Resistance

For single-phase flows, the value of the coefficient $P_v$ in Eqn. (1858). For multiphase flows, the value of the same coefficient in the normal direction.

This value can be positive or 0. The default value is 0.

Porous Viscous Tangential Resistance

The value of the coefficient ${\beta }_m$ in the tangential direction (see Eqn. (1870)).

Available only when the Porous Baffle Treatment is set to Resistance Based.

Thermal Resistance

Available only when the Baffle Thermal Option is set to Conducting.

See Baffle Interface.

Turbulence Intensity

Available only when the Porous Baffle Treatment is set to Resistance Based, and a turbulence model is activated in the physics continuum.

Turbulent Length Scale

The length scale of the turbulence is related to the characteristic geometry scale of the baffle itself, for example, the perforation diameter or tube spacing.

Available only when the Porous Baffle Treatment is set to Resistance Based, and a turbulence model is activated in the physics continuum.