CFD Pro 14.5 L12 Domain Interfaces

2013 ANSYS, Inc. December 12, 2013 1 Release 14.5

14. 5 Release

Introduction to ANSYS CFD Professional

Lecture 12 Domain Interfaces


Overview Domain Interfaces are used for:

Connection of unmatched meshes (hex to tet for example)

a single mesh file may contain non-matching mesh regions and require non-conformal interfaces

Connect different types of domains together (e.g. Fluid to Solid)

Create periodic regions within a domain


Overview Where two 3-d mesh regions meet there are two possible treatments Two elements share one face no interpolation necessary called a

conformal or one-to-one mesh interface

Every element has its own face interpolation of fluxes is necessary called non-conformal or unmatched and a Generalised Grid Interface (GGI) is required to connect the regions

conformal non-conformal


Inserting Domain Interfaces To create a domain interface right-

click on Flow Analysis or use the toolbar icon


Domain Interfaces and Boundary Objects After creating a domain interface 3 new objects

appear in the outline tree

The interface object is at the Flow Analysis level This is the object you should edit to make changes to the

domain interface

Within each domain a Side 1 or Side 2 boundary condition is automatically created

They will be automatically updated when changes are made to the interface object

In general do not edit these objects but you can:

specify sources

set wall boundary conditions for non-overlap regions

The Interface object

The Side 1 and Side 2

boundary conditions


Domain Interfaces Panel

Domain Interfaces connect two sets of surfaces together - Side 1 and Side 2

First select the combination of domains

Then select the Side 1 and Side 2 surface sets

The Domain (Filter) just limits the scope of the Region List to make selection easier

The Interface Models and Mesh Connection Method control how data is transferred across the interface

Wh

at?

H

ow

?


Interface Models

The available Interface Models are:

Translational Periodicity Simulates geometries that have translational periodicity

Allows for either the mass flow rate or the pressure change across the interface to be specified (Additional Interface Models see later)

Rotational Periodicity Simulates rotationally periodic geometries

General Connection For all other types of connections


Additional Interface Models

On the Additional Interface Models tab you can:

Represent source or sink of momentum, e.g. a fan or a porous wall, by specifying a pressure change between side 1 and side 2 or a mass flow rate

With a translational periodic interface this feature can be used to set up fully-developed flow in a short section of duct

Set up infinitely thin walls by selecting the appropriate wall boundary condition option for Mass and Momentum

- Can model heat conduction across wall (next slide)


Additional Interface Models (CHT) Domain interfaces can be used to model

conduction through a thin wall

Wall Thickness is not meshed Normal conduction only; neglects any in-plane

conduction

Set-up Create a Fluid-Fluid Domain Interface

On the Additional Interface Models tab set Mass and Momentum to No Slip Wall

Enable the Heat Transfer toggle and pick:

Thin Material and specify a Material & Thickness, or

Thermal Contact Resistance

Note : Other domain interface types (Fluid-Solid etc.) can use these options to represent coatings etc.


Additional Interface Models contd

On the Additional Interface Models tab you can:

Create a switch, using Conditional Connection Control, to determine whether the interface is open to flow or closed, i.e. a wall

Provide a logical expression, e.g. a function of time or pressure difference

Either define when the interface is open or when its state changes


Mesh Connection Method

1:1 Only use this option if you are sure that the nodes on Side 1 and Side 2 of the

interface match up exactly

Not used for Fluid Solid and Solid Solid interfaces

GGI Use this option when the nodes on the two sides are not aligned

For best results both sides should have fairly similar mesh length scales

Fluxes are conserved across the interface

If the size of the connection region for one side is different from the other, the connection will be automatically made between the mutually overlapping surfaces (for best results ensure both sides fully overlap)

Possible to perform a connection where there is a slight gap or interference between the two sides of the GGI connection

The gap should be small relative to the mesh length scale

When solving, GGI connections use more memory and CPU than 1:1 connections


Mesh Connection Method

Automatic

This is generally the recommended option when available

It will try to make a 1:1 connection if possible, otherwise GGI

The Mesh Match Tolerance under Edit > Options > Mesh determines how close nodes need to be before a 1:1 connection can be made

The default value of 0.005 (0.5%) is a fraction of the local mesh length scale

In some cases only the GGI option will be available

e.g. when there is a change of reference frame across the interface

In some situations a GGI connection will be used even when nodes match 1:1

e.g. Fluid Solid interfaces, since GGI connections are more accurate in these situations


GGI connections are used for Fluid-Solid and Solid-Solid interfaces

At a GGI interface, there is no discontinuity in values of temperature across the interface

the CFX Solver calculates a "surface temperature" based on a flux-conservation equation for the 'control surfaces' that lie between the fluid side and the solid side

At a solid-fluid 1:1 interface may result in a temperature discontinuity at the interface

this is because values are interpolated from the interface into the bulk of the solid domain using the value for the solid-side node at the interface, while values are interpolated from the interface into the bulk of the fluid domain using the value for the fluid-side node at the interface

GGI vs 1:1 for Conjugate Heat Transfer


Automatic Domain Interfaces

CFX-Pre automatically creates domain interfaces within a single mesh assembly

To connect multiple domains within the assembly

To connect non-matching meshes within the assembly

Right-click on Mesh > View by > Region Type to see a list of assemblies in the mesh

Always check the automatic interfaces to make sure they are correct!

Can disable automatic interface creation from Case Options > General in the Outline tree

You always need to create interfaces manually between mesh assemblies

CFD Pro 14.5 L12 Domain Interfaces

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