Coupled Field Analysis (with Emphasis on Thermal-Stress) Chapter 10.

Coupled Field Analysis (with Emphasis on Thermal-Stress)

Chapter 10

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Training ManualWhat is a Coupled-Field Analysis?

• A Coupled-Field analysis is one in which the interaction (coupling) between two or more types of phenomena (fields) is considered. Such analyses may involve direct or indirect coupling of fields.

This is a list of ANSYS elements that can be used in direct coupled field analyses. Not all elements have thermal DOF.

This is a list of ANSYS elements that can be used in direct coupled field analyses. Not all elements have thermal DOF.

• When performing a directly coupled analysis, DOF from multiple fields (e.g., thermal-electric) are computed simultaneously. This is called the Direct Method and is only necessary when the individual field responses of the model are dependent upon each other. Directly coupled analyses are usually nonlinear since equilibrium must satisfied based on multiple criteria. With more DOF active per node, matrix equations are larger and more costly to analyze than for single-field models of comparable size.

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What is a Coupled-Field Analysis? (continued)

• An indirectly coupled analysis involves the solution of single-field models in a particular sequence. The results of one analysis are used as loads for the following analysis. This is also known as the Sequential Method of coupled analysis.

• This method of analysis is applicable when there is one-way interaction between fields. For example, the field response of a system (e.g., thermal) may significantly influence the response of the system to another field (e.g., structural) but not vice-versa. This method is usually more efficient than the Direct Method, and it does not require use of special elements.

• In this chapter we will only discuss coupling that involves thermal phenomena. Be aware that some ANSYS products do not support all kinds of coupled-field element types and analysis options. For example, the ANSYS Thermal product only supports Thermal-Electric direct coupling. Refer to the on-line Coupled-Field Analysis Guide for more information.

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Training ManualDirect Method - Examples

In the context of convection options presented in Chapter 7, the FLUID66 and FLUID116 thermal-fluid elements were introduced. These elements featured temperature and pressure DOF, qualifying them as directly coupled-field elements.

ANSYS has other coupled-field elements which feature combinations of structural, thermal, electrical, and magnetic DOF. Most practical problems involve interaction of only a few fields. Here are some common examples of direct coupled field analysis involving thermal phenomena:

Thermal-Structural: Hot Rolling of Aluminum Sheet

Temperature of sheet effects elastic-plastic material properties and thermal strains.

Mechanical and thermal loads produce large strains in sheet. New thermal analysis must be performed to account for shape change.

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Training ManualDirect Method - Examples (continued)

Thermal-Electromagnetic: Induction Heating of a Steel Billet

An induction coil generates a magnetic field around the billet. This field induces an alternating current within the billet resulting in Joule heating.

The billet experiences a significant rise in temperature due to the heating. Because of the extreme variation in temperature encountered, the temperature-dependence of the resistively and relative permeability of the billet must be accounted for. Consequently, the strength and direction of the magnetic field changes.

For a harmonic magnetic analysis such as this, once the magnetic vector potential {A} has been solved for, the current density vector {J} can be computed. This is used with resistivity to compute Joule heating in the matrix equivalent of this simple A-C circuit relationship:

For a harmonic magnetic analysis such as this, once the magnetic vector potential {A} has been solved for, the current density vector {J} can be computed. This is used with resistivity to compute Joule heating in the matrix equivalent of this simple A-C circuit relationship:

2

2maxRI

PowerRMSQ j

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Training ManualDirect Method - Preprocessing

• When preprocessing for a direct coupled-field analysis, remember the following:

• Use coupled-field elements with the appropriate DOF set for the fields involved. Single-field elements may be used in parts of the model where coupling is not needed.

• Closely study element options, material properties and real constants for each element type. Coupled-field elements typically have more limitations than comparable single-field elements (e.g., the PLANE13 does not permit mass transport of heat as does the PLANE55, the SOLID5 does not permit plasticity or creep as does the SOLID45).

• Use consistent units across all fields. For example, when performing a thermal-electric analysis do not use Btu/s when electrical power unit is expressed in Watts (Joules/s).

• Due to the iterative solution required, thermal coupled-field elements are not suitable for use within substructures.

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Direct Method - Loading, Solution, and Postprocessing • When loading, solving, and postprocessing a direct coupled-field model remember the following:

• If a Transient analysis type is selected for coupled-field elements with temperature DOF:

– Transient thermal effects can be modeled by all coupled-field element types.

– Transient electrical effects (capacitance, inductance) are not accounted for in thermal-electric analyses (i.e., only TEMP and VOLT DOF active).

– Coupled-field elements with magnetic vector potential DOF can model transient magnetic events (e.g., SOLID62). Elements with scalar potential DOF can only simulate static events (SOLID5).

• Study the DOF set and allowable types of loads for each element. Coupled-field elements allow multiple types (D, F, SF, BF) of loads to be applied at the same location (node, element face, etc.).

• Coupled-field analyses can be highly nonlinear. Consider using the Predictor and Line Search features to enhance convergence.

• Consider using the Multi-Plots feature to display results from different fields in different windows at the same time.

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Training ManualSequential Method

• The Sequential Method is used to solve indirect coupled-field analysis. Recall that it involves performing two single-field analyses sequentially (rather than simultaneously) with the results from the first analysis used as loads for the second analysis. For example:

Thermal Structural

Thermal Structural

Many problems involve significant thermal-to-structural coupling (temperatures cause thermal expansion)

But not Vice-Versa

the structural-to-thermal coupling is negligible (strains are small enough that original thermal solution is unaffected)

• When applicable, this approach is usually more desirable than a direct coupled-field analysis since single-field elements can be used and no multi-field iteration is required.

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Training ManualSequential Method - Examples

Temperatures in blade and disk produce thermal expansion strains. This has significant impact on stresses.

Since strains are small and contact regions have flat-on-flat geometry, temperature solution does not have to be updated.

Disk Sector

Airfoil

Platform

Root

Here are some common examples of indirect coupled field analysis involving thermal phenomena which can be solved using the sequential method:

Thermal-Structural:Turbine Blade Attachment Analysis

This kind of analysis is known as a Thermal-Stress analysis. This very common form of analysis is discussed in more detail later in the Chapter.This kind of analysis is known as a Thermal-Stress analysis. This very common form of analysis is discussed in more detail later in the Chapter.

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Training ManualSequential Method - Examples (continued)

Thermal-Electric:

Electrical Heater Embedded in a Glass Plate

An electrical heater embedded in a thin glass plate has current flowing through it. This causes Joule heating within the wire.

The wire and plate experience a rise in temperature due to the heating. Because the temperature variation in the system is not large, the temperature-dependence of the resistively can be ignored. As a result, the current does not change.

Once the voltage {V} has been solved for, it is used with resistivity to compute Joule heating using the matrix form of this simple equation:

RV

PowerQ j

2

+ V -

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Training ManualSequential Method - Procedures

• There are two basic ways to perform sequential coupled-field analysis in ANSYS. They are primarily differentiated with regard to how characteristics from each field are represented:

– Physics Environment Approach - One central database is built for use with all fields. Multiple physics environment files are written describing characteristics of each field individually.

– Manual Method - Multiple databases are created and stored, one for each field to be studied. All characteristics of an individual field are stored in database.

– We will discuss each of these approaches and their respective advantages in the following viewgraphs.

• Note: The sequential method can use the final analysis results to loop back to the initial analysis. This is normally accomplished manually by the analyst.

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Training ManualPhysics Environments

• To help automate sequential coupled-field analyses, ANSYS allows the user to define several Physics Environments for a single model. A Physics Environment represents the model characteristics necessary to completely describe the behavior of single field. A Physics Environment File is ASCII-formatted and contains the following information:

• Element Types and Options

• Nodal and Element Coordinate Systems

• Coupled Sets and Constraint Equations

• Analysis and Load Step Options

• Loads and Boundary Conditions

• GUI Preferences and Title

When building a database for use with physics environments, you must select element types with characteristics that are compatible across physics environments. For example 8-noded thermal brick elements are compatible with 8-noded structural brick elements, not 10-noded structural tets:

yes

no

Be careful when using degenerate element shapes. Element types with similar base configurations do not always support the same degenerate shapes.

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Training ManualPhysics Environments (continued)

• In addition to similar element order (e.g., shape function polynomial) and shape, most elements require similar element options (e.g., axisymmetric vs. planar 2-D Solids) for compatibility. However, some types of loads do not require complete compatibility between environments. For example, 8-noded thermal solids may be used to supply temperatures for 20-node structural brick elements. Some elements require special keyoption settings to be compatible with elements of dissimilar order.

• Element attribute numbering (MAT, REAL, TYPE) must be consistent from environment to environment.

• Use the Null Element Type (type # zero) for regions which do not participate in a particular physics environment (e.g., magnetic analysis requires modeling of air around objects whereas thermal and structural do not).

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• Also, make sure the mesh discretization is acceptable for capturing results you want from ALL physics environments. For example:

• The Physics Environment approach allows up to 9 environments be defined for a single model. This approach is usually best when more than two field interactions are to be considered or when the use of separate databases for each field is infeasible. For more information about the Physics Environment approach to indirect problems, refer to Chapter 2 of the on-line Coupled Field Analysis Guide.

This mesh density might be suitable for obtaining temperatures

in a thermal analysis, but . . .

. . . This mesh density is required to obtain stresses of comparable accuracy in a structural analysis

Physics Environments (continued)

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Training ManualThermal-Stress Analysis

• In the remainder of this Chapter, we will consider what is probably the most common form of indirect coupled-field analysis; Thermal-Stress analysis.

• Thermal-Stress analysis is considered to be an indirect problem because temperatures from the thermal analysis contribute significantly to the strains and stresses in the structural analysis, but the structural response of the model does not strongly impact the thermal solution.

• Since thermal-stress analysis involves the sequential interaction of only two fields, we will use the Manual Method (MM) of sequential coupling rather than the more elaborate Physics Environment Method (PEM) described previously. Here are some advantages and disadvantages to the MM:

Advantages to MM:

– There are fewer restrictions when building thermal and structural models. For example, the attribute numbering and mesh discretization may be different for thermal and structural models. PEM requires same model be used for all.

– MM utilizes simplified, robust approach which has been supported by ANSYS and used by customers for many years.

Disadvantages to MM:

– You must maintain both thermal and structural databases and results files. This increases the amount of disk space required as compared with the single model approach employed by the PEM.

– MM may be cumbersome if you wish to consider interactions with other fields later.

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Training ManualGeneral Procedure

• In a Thermal-Stress analysis, Nodal Temperatures from the thermal solution are used as Body Loads in the structural analysis.

• When using the Manual Method of sequential solution there are two options for applying thermal nodal temperatures to structural elements. The choice between them depends on whether or not the thermal and structural models utilize similar meshes:

– If meshes used for thermal and structural analyses have the SAME node numbering . . .

1

The thermal model is automatically converted to structural by means of the ETCHG command (see table for corresponding element types).

The thermal model is automatically converted to structural by means of the ETCHG command (see table for corresponding element types).

Thermal temperatures are read directly from the thermal results file and applied to the structural model using the LDREAD command.

Thermal temperatures are read directly from the thermal results file and applied to the structural model using the LDREAD command.

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Training ManualGeneral Procedure (continued)

– If meshes used for the thermal and structural analyses have DIFFERENT node numbering . . .

• Structural mesh is created apart from thermal model using mesh discretization that is optimized for capturing structural results.

• Structural body loads are mapped from the thermal analysis temperature distribution. This involves a more elaborate procedure which makes use of the BFINT command to interpolate thermal results (not available for use with Physics Environments).

– The next viewgraph provides a comparison of the procedures when using the same or different meshes.

2

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Training ManualThermal-Stress Analysis Flow Chart

No (Option 2)Same

Mesh?

5A. Convert thermal model to structural (ETCHG)

5a. Clear thermal mesh and build structural model

Yes(Option 1)

5B. Read in temperature loads (LDREAD)

5b. Write node file (NWRITE) and save structural model

5c. Resume thermal model and perform temp interpolation (BFINT)

5d. Resume structural model and read in body load file (/INPUT)

6. Specify analysis type, analysis options, and load step options

7. Specify reference temperature and apply other structural loads.

8. Save and Solve

9. Postprocess results

END

1. Build, load, and solve thermal model

2. Postprocess to identify temps for structural

3. Set GUI preferences, change jobname and delete thermal loads, CEs, CPs

4. Define structural material properties

BEGIN Upper and lower case

letters are usedto differentiateOptions 1 and

2 in Step 5

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1. Build thermal model and perform a transient or steady-state thermal analysis to solve for temperatures at nodes.

2. Review thermal results and identify time-points (or load steps/substeps) with critical temperature gradients.

3a. Set GUI preferences to “Structural” and “Thermal”.

3b. Change the Jobname

Detailed Procedure

• The following is a step-by-step presentation of the thermal-stress analysis Flow Chart presented on the previous viewgraph.

2

1

3a

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Training ManualDetailed Procedure (continued)

3c. Delete all thermal Loads

3d. Delete Coupled Sets and Constraint Equations

3c

3d

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4. Define structural material properties including the mean (not instantaneous) coefficient of thermal expansion (ALPX).

Detailed Procedure (continued)

4

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• The next two viewgraphs (Steps 5A and 5B) assume that the thermal mesh will be used again in the structural model (Option 1).

5A. Change element types from thermal to structural (ETCHG command):

Check real constants and verify element options are correct for the new element types


5A

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5B. Apply temperature body loads from thermal analysis (LDREAD command):

9. Solve current load step

5B


Identify thermal results file

Identify result bytime or substep

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5a. Clear thermal mesh . . .

Delete thermal element types and define structural element types . . .

Change meshing controls and mesh structural model.


• The next six viewgraphs (Steps 5a-5d) assume that the thermal mesh will not be used again in the structural model (Option 2).

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5b. Select all nodes for which temperature body loads are to be defined and write a node file.

5b

Specify name ofnode file to write


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5c. Save structural model, change jobname back to the thermal jobname, resume thermal database . . .

enter the General Postprocessor . . .


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Read in desired results set, and . . .

Initiate body load interpolation:


Name of node file

Name of loadfile to write

Used for writingmultiple load files

Use solid-to-solid

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• There may be situations where the thermal mesh and structural mesh do not occupy the same exact regions of space. In such cases, ANSYS will extrapolate body loads to structural nodes falling outside the perimeter of the thermal model.

• The default proximity criterion used to determine if a structural node is close enough for extrapolation is 0.5 times the half-width of the nearest thermal element edge. An undocumented feature was introduced at Rev. 5.4 to allow user control of this tolerance:

• There is no GUI support for the undocumented extra field. Consequently, the command must be entered manually in the Input Window.

BFINT, Fname1, Ext1, Dir1, Fname2, Ext2, Dir2, KPOS, Clab, KSHS, EXTOL

Extrapolation with BFINT

EXAMPLE: When the structural mesh includes a fillet which is not present in the thermal model, some nodes may fall outside the perimeter of the thermal model. If the fillet is large enough and the thermal model fine enough, loads may not be written for some structural nodes in the fillet region.

Using the default tolerance, these two nodes would not be assigned a load

Thermal Mesh

Structural MeshPerimeter

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5d. Exit the General Postprocessor, change the jobname back to the structural jobname, resume the structural database . . .

enter Solution . . .

And read input from body load file to apply temperature loads:


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6a. Define structural analysis type(default is static)

6b. Specify analysis options (e.g., solver choice)

6c. Specify load step options(e.g., output controls)


6a

6b

6c

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7a. Set the strain free reference temperature for thermal expansion calculations (TREF):

7


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7b. Apply other structural loads.

8. Save model and Solve current load step.

99. Postprocess results:


7b

8

Coupled Field Analysis (with Emphasis on Thermal-Stress) Chapter 10.

Documents

Transcript of Coupled Field Analysis (with Emphasis on Thermal-Stress) Chapter 10.