A Reduced-Order Model is a feature in a COMSOL Multiphysics model that encapsulates important aspects of the behavior of another model in a computationally efficient form. From the perspective of a COMSOL Multiphysics model that uses a reduced-order model, it is essentially a black box with a well-defined interface. Depending on the type of reduced-order model, it can, in addition to
inputs and
outputs, interact with the surrounding model via reduced-order equations, internal states, or matrix representations of the model.
A reduced-order model must be trained so as to reproduce the behavior of a full,
unreduced, COMSOL Multiphysics model and study step. The training of a reduced-order model is sometimes referred to as the
offline phase and its subsequent use as the
online phase. Reduced-order models are normally created and trained by a
Model Reduction study step (see
Model Reduction), but some types can also be imported from a COMSOL Reduced-Order Model file (
*.mphrom).
Outputs from a reduced-order model are made available to the model where it is used (the caller) in the form of variables or operators. These values or operators can be used as parts of expressions and equations when computing the calling model as well as in postprocessing.
The interface through which a calling model interacts with a reduced-order model in COMSOL Multiphysics belongs to one of two fundamental types: it is either stateless or
stateful. A reduced-order model with a stateless interface is a black box with a number of inputs and a number of outputs. The reduced-order model may have internal states that are solved by an internal solver, but these are not exposed to the solver used for solving the calling model. Because a reduced-order model with a stateless interface uses its own internal solver, it can be passed new input values and generate new results directly in postprocessing, without having to run a main solver.
When a model contains a reduced-order model, it is represented as a node under the Reduced-Order Modeling node (
) under
Global Definitions. See the following sections for information about the available reduced-order model types, concepts, and associated modeling tools: