1.1. Types of Coupled-Field Analysis

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The procedure for a coupled-field analysis depends on which fields are being coupled, but two distinct methods can be identified: load transfer and direct. These methods are described briefly below, and in the following chapters in detail:

ANSYS also offers the following additional coupled-field methods:

1.1.1. Direct Method

The direct method usually involves just one analysis that uses a coupled-field element type containing all necessary degrees of freedom. Coupling is handled by calculating element matrices or element load vectors that contain all necessary terms. An example of a direct method coupled-field analysis is a piezoelectric analysis using the PLANE223, SOLID226, or SOLID227 elements. Another example is MEMS analysis with the TRANS126 element.

A FLOTRAN analysis using the FLOTRAN elements is another direct method. Refer to the Fluids Analysis Guide for detailed procedures for a FLOTRAN analysis.

1.1.2. Load Transfer Methods

The load transfer methods involve two or more analyses, each belonging to a different field. You couple the two fields by applying results from one analysis as loads in another analysis. There are different types of load transfer analyses, explained in the following sections.

1.1.2.1. Load Transfer Coupled Analysis - ANSYS Multi-field solver

The ANSYS Multi-field solver, available for a large class of coupled analysis problems, is an automated tool for solving load transfer coupled field problems. It supersedes the physics file-based procedure and provides a robust, accurate, and easy to use tool for solving load transfer coupled physics problems. Each physics is created as a field with an independent solid model and mesh. Surfaces or volumes are identified for coupled load transfer. A multi-field solver command set configures the problem and defines the solution sequencing. Coupled loads are automatically transferred across dissimilar meshes by the solver. The solver is applicable to static, harmonic, and transient analysis, depending on the physics requirements. Any number of fields may be solved in a sequential (or mixed sequential-simultaneous) manner.

Two versions of the ANSYS Multi-field solver, designed for different applications, offer their own benefits and different procedures:

  • MFS - Single code: The basic ANSYS Multi-field solver used if the simulation involves small models with all physics field contained within a single product executable (e.g., ANSYS Multiphysics). The MFS - Single code solver uses iterative coupling where each physics is solved sequentially, and each matrix equation is solved separately. The solver iterates between each physics field until loads transferred across the physics interfaces converge.

  • MFX - Multiple code: The enhanced ANSYS Multi-field solver used for simulations with physics fields distributed between more than one product executable (e.g., between ANSYS Multiphysics and ANSYS CFX). The MFX solver can accommodate much larger models than the MFS version. The MFX - Multiple code solver uses iterative coupling where each physics is solved either simultaneously or sequentially, and each matrix equation is solved separately. The solver iterates between each physics field until loads transferred across the physics interfaces converge.

See "Multi-field Analysis Using Code Coupling" for detailed procedures.

1.1.2.2. Load Transfer Coupled Analysis - Physics File

With a physics file-based load transfer, you must explicitly transfer loads using the physics environment. An example of this type of analysis is a sequential thermal-stress analysis where nodal temperatures from the thermal analysis are applied as "body force" loads in the subsequent stress analysis. The physics analysis is based on a single finite element mesh across physics. You create physics files that define the physics environment; these files configure the database and prepare the single mesh for a given physics simulation. The general process is to read in the first physics file and solve. Then read in the next physics field, specify the loads to be transferred, and solve the second physics. Use the LDREAD command to link the different physics environments and apply the specified results data from the first physics environment as loads for the next environment's solution across a node-node similar mesh interface. You can also use LDREAD to read results from one analysis as loads in a subsequent analysis, without the use of physics files. See "Load Transfer Coupled Physics Analysis" for detailed procedures.

1.1.2.3. Load Transfer Coupled Analysis - Unidirectional Load Transfer

You can also couple a fluid-solid interaction analysis by unidirectional load transfer. This method requires that you know that the fluid analysis results do not affect the solid loads significantly, or vice-versa. Loads from an ANSYS Multiphysics analysis can be unidirectionally transferred to a CFX fluid analysis, or loads from a CFX fluid analysis can be transferred to an ANSYS Multiphysics analysis. The load transfer occurs external to the analyses. See "Unidirectional Load Transfer" for detailed procedures on both ANSYS-to-CFX and CFX-to-ANSYS unidirectional methods.

1.1.3. When to Use Direct vs. Load Transfer

Direct coupling is advantageous when the coupled-field interaction involves strongly-coupled physics or is highly nonlinear and is best solved in a single solution using a coupled formulation. Examples of direct coupling include piezoelectric analysis, conjugate heat transfer with fluid flow, and circuit-electromagnetic analysis. Elements are specifically formulated to solve these coupled-field interactions directly.

For coupling situations which do not exhibit a high degree of nonlinear interaction, the load transfer method is more efficient and flexible because you can perform the two analyses independently of each other. Coupling may be recursive, where iterations between the different physics are performed until the desired level of convergence is achieved. In a load transfer thermal-stress analysis, for example, you can perform a nonlinear transient thermal analysis followed by a linear static stress analysis. You can then use nodal temperatures from any load step or time-point in the thermal analysis as loads for the stress analysis. In a load transfer coupling analysis, you can perform a nonlinear transient fluid-solid interaction analysis, using the FLOTRAN fluid elements and ANSYS structural, thermal or coupled field elements.

Direct coupling typically requires less user-intervention because the coupled-field elements handle the load transfer. Some analyses must be done using direct coupling (such as piezoelectric analyses). The load transfer method requires that you define more details and manually specify the loads to be transferred, but offers more flexibility in that you can transfer loads between dissimilar meshes and between different analyses.

The following tables provides some general guidelines on using each method.

Table 1.1  Choosing a Method

MethodApplication
Load-Transfer
Thermal-structuralVaried
Electromagnetic-thermal, electromagnetic-thermal-structuralInduction heating, RF heating, Peltier coolers
Electrostatic-structural, electrostatic-structural-fluidicMEMS
Magnetic-structuralSolenoids, electromagnetic machines
FSI, CFX- and FLOTRAN-basedAerospace, automative fuel, hydraulic systems, MEMS fluid damping, drug delivery pumps, heart valves
Electromagnetic-solid-fluidFluid handling systems, EFI, hydraulic systems
Thermal-CFDElectronics cooling
Direct
Thermal-structuralVaried, such as gas turbines, MEMS resonators
Acoustic-structuralAcoustics, sonar, SAW
PiezoelectricMicrophones, sensors, actuators, transducers, resonators
ElectroelasticMEMS
PiezoresistivePressure sensors, strain gauges, accelerometers
Thermal-electricTemperature sensors, thermal management, Peltiere cooler, thermoelectric generators
Electrostatic-structuralMEMS
Circuit coupled electromagneticsMotors, MEMS
Electro-thermal-structural-magneticIC, PCB electro-thermal stress, MEMS actuators
Fluid-thermalPiping networks, manifolds

Table 1.2  Methods Available

Coupled PhysicsLoad TransferDirectComments
Thermal-structuralANSYS Multi-field solverPLANE13, SOLID5, SOLID98, PLANE223, SOLID226, SOLID227. See Structural-Thermal Analysis.Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Thermal-electricANSYS Multi-field solver PLANE223, SOLID226, SOLID227 (Joule, Seebeck, Peltier, Thompson). See Thermal-Electric Analysis for a complete list of elements.Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Thermal-electric- structuralANSYS Multi-field solverPLANE223, SOLID226, SOLID227 . See Structural-Thermal-Electric Analyses for a complete list of elements.Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method. Joule heating is supported by both the direct and load-transfer methods. Seebeck, Peltier, and Thompson effects are available only with the direct method.
Piezoelectric--- PLANE13, SOLID5, SOLID98, PLANE223, SOLID226, SOLID227. See Piezoelectric Analysis. 
Electroelastic--- PLANE223, SOLID226, SOLID227. See Electroelastic Analysis. 
Piezoresistive--- PLANE223, SOLID226, SOLID227. See Piezoresistive Analysis. 
Electromagnetic-thermalANSYS Multi-field solver PLANE13, SOLID5, SOLID98Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Electromagnetic-thermal- structuralANSYS Multi-field solverPLANE13, SOLID5, SOLID98Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Acoustic-Structural (Inviscid FSI)--- FLUID29, FLUID30 
Circuit-coupled electromagnetic--- CIRCU124 + PLANE53, SHELL99, or SOLID117. CIRCU94. See "Coupled Physics Circuit Simulation". 
Electrostatic-structuralANSYS Multi-field solverTRANS109, TRANS126. See Electromechanical Analysis.Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Electromagnetic-structural- fluidic (FLOTRAN-based)ANSYS Multi-field solver--- Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Magnetic-structuralANSYS Multi-field solverPLANE13, SOLID62, SOLID5, SOLID98. See Magneto-Structural Analysis.Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.
Fluid-thermal (FLOTRAN - based)ANSYS Multi-field solver MFSFLOTRAN Conjugate heat transfer 
Fluid-thermal (CFX - based)ANSYS Multi-field solver MFXCFX Conjugate heat transfer 
FSI (FLOTRAN - based)ANSYS Multi-field solver MFS---  
FSI (CFX - based)ANSYS Multi-field solver MFX, Unidirectional ANSYS to CFX Load Transfer (EXPROFILE), Unidirectional CFX to ANSYS Load Transfer (MFIMPORT)--- Use the MFX solver if you need to iterate between the separate codes. Otherwise, use the appropriate unidirectional option.
Magnetic-fluidANSYS Multi-field solver ---

Can also use LDREAD, but we recommend using the ANSYS Multi-field solver if using the load transfer method.

LDREAD can read Lorentz forces into CFD mesh. LDREAD can also account for conventional velocity effect (PLANE53, SOLID97, SOLID117) by importing CFD calculated velocity distribution to an electromagnetic model to simulate electric power generation.

1.1.4. Additional Analysis Methods

In addition to the methods discussed above, ANSYS also offers the following methods:

1.1.4.1. Reduced Order Modeling

Reduced Order Modeling describes a solution method for efficiently solving coupled-field problems involving flexible structures. The reduced order modeling (ROM) method is based on a modal representation of the structural response. The deformed structural domain is described by a factored sum of the mode shapes (eigenvectors). The resulting ROM is essentially an analytical expression for the response of a system to any arbitrary excitation. This methodology has been implemented for coupled electrostatic-structural analysis and is applicable to micro-electromechanical systems (MEMS). See "Reduced Order Modeling" for detailed procedures.

1.1.4.2. Coupled Physics Circuit Simulation

You can often perform coupled physics simulations using a circuit analogy. Components such as "lumped" resistors, sources, capacitors, and inductors can represent electrical devices. Equivalent inductances and resistances can represent magnetic devices, and springs, masses, and dampers can represent mechanical devices. ANSYS offers a set of tools to perform coupled simulations through circuits. A Circuit Builder is available to conveniently create circuit elements for electrical, magnetic, piezoelectric, and mechanical devices. The ANSYS circuit capability allows you to combine both lumped elements, where appropriate, with a "distributed" finite element model in regions where characterization requires a full finite element solution. A common degree-of-freedom set allows the combination of lumped and distributed models. See "Coupled Physics Circuit Simulation" for detailed procedures.