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 PLANE222, PLANE223, SOLID225, SOLID226, or SOLID227 elements. Another example is MEMS analysis with the TRANS126 element. See Direct Coupled-Field Analysis for more details and examples.

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.

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.

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 table provides some general guidelines on using each method for different analyses and applications.

In addition to the analysis methods discussed above, 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. Mechanical APDL 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 Mechanical APDL circuit capability enables 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.