As an interdisciplinary study area, multiphysics spans over many science and engineering disciplines. Multiphysics is a practice built on mathematics, physics, application, and numerical analysis. The mathematics involved usually contains partial differential equations and tensor analysis.
The physics refers to common types of physical processes, e.g., heat transfer (thermo-), pore water movement (hydro-), concentration field (concentro or diffuso/convecto/advecto), stress and strain (mechano-), dynamics (dyno-), chemical reactions (chemo- or chemico-), electrostatics (electro-), neutronics (neutro-), and magnetostatics (magneto-).
There are multiple definitions for multiphysics. In a broad sense, multiphysics refers to simulations that involve multiple physical models or multiple simultaneous physical phenomena. The inclusion of “multiple physical models” makes this definition a very broad and general concept, but this definition is a little bit self-contradictory as the implication of physical models may include that of physical phenomena.
COMSOL defines multiphysics in a relatively narrow sense: multiphysics includes 1. coupled physical phenomena in computer simulation and 2. the study of multiple interacting physical properties. In another definition, a multiphysics system consists of more than one component governed by its own principle(s) for evolution or equilibrium, typically conservation or constitutive laws. This definition is very close to the previous one except for that it does not emphasize physical properties.
In a more strict way, multiphysics can be defined as those processes including closely coupled interactions among separate continuum physics phenomena. In this definition, two-way exchange of information between physical fields, which could involve implicit convergence within a time step is the essential feature.
Based on the above definitions, multiphysics is defined as the coupled processes or systems involving more than one simultaneously occurring physical fields and also the studies of and knowledge about these processes and systems.
History and future
Multiphysics is neither a research concept far from daily life nor a recently developed theory or technique. In fact, we live in a multiphysics world. Natural and artificial systems are running with various types of physical phenomena at different spatial and temporal scales: from atoms to galaxies and from pico-seconds to centuries. A few representative examples in fundamental and applied sciences are loads and deformations on solids, complex flows, fluid-structure interactions, plasma and chemical processes, thermo-mechanical and electromagnetic systems.
Multiphysics has rapidly developed into a research and application area across many science and engineering disciplines. There is a clear trend that more and more challenging problems we are faced with involve physical processes that cannot be covered by a single traditional discipline. This trend requires us to extend our analysis capacity to solve more complicated and more multidisciplinary problems.
Modern academic communities are confronted with problems of rapidly increasing complexity, which straddle across the traditional disciplinary boundaries between physics, chemistry, material science and biology. Multiphysics has also become a frontier in industrial practice. Simulation programs have been evolving into a tool in design, product development, and quality control. During these creation processes, engineers are now required to think in areas outside of their training, even with the assistance of the simulation tools. It is more and more necessary for the modern engineers to know and grasp the concept of what is known deep inside the engineering world as “multiphysics.” 
The auto industry gives out a good example. Traditionally, different groups of people focus on the structure, fluids, electromagnets and the other individual aspect separately. By contrast, the intersection of aspects, which may represent two physics topics and once was a gray area, can be the essential link in the life cycle of the product. As commented by, “Design engineers are running more and more multiphysics simulations every day because they need to add reality into their models.”
Types of multiphysics
The part “physics” in “multiphysics” denotes “physical field”. There, multiphysics means the coexistence of multiple physical fields in a process or a system. In physics, a field is a physical quantity that has a value for each point in space and time. For example, on a weather map, a vector at each point of the map can used to represent the surface wind velocity with both speed and direction for the movement of air at that point.
How to do multiphysics
The implementation of multiphysics usually follows the following procedure: identifying a multiphysical process/system, developing a mathematical description of this process/system, discretizing this mathematical model into an algebraic system, solving this algebraic equation system, and postprocessing the data.
The abstraction of a multiphysical problem from a complex phenomenon and the description of such a problem are usually not emphasized but very critical to the success of the multiphysics analysis. This requires to identify the system to be analyzed, including geometry, materials and dominant mechanisms. The identified system will be interpreted using mathematics languages (function, tensor, differential equation) as computational domain, boundary conditions, auxiliary equations and governing equations. Discretization, solution and postprocessing are carried out using computers. Therefore, the above procedure is not much different from those in general numerical simulation based on the discretization of partial differential equations.
A mathematical model is essentially a set of equations. The equations can be divided into three categories according to the nature and intended role: governing equations, auxiliary equations and boundary/initial conditions. A governing equation describes the major physical mechanisms and process without further revealing the change and nonlinearity of the material properties. For example, in a heat transfer problem, the governing equation could describe a process in which the thermal energy (represented using temperature or enthalpy) at an infinitesimal point or a representative element volume is changed due to energy transferred from surrounding points via conduction, advection, radiation, and internal heat sources or any combinations of these four heat transfer mechanisms as the following equation:
Couplings between fields can be achieved in each category.
Multiphysics is usually numerical implemented with discretization methods such finite element method, finite difference method, and finite volume method. Many software packages mainly rely on the finite element method or similar commonplace numerical methods for simulating coupled physics: thermal stress, electro- and acousto- magnetomechanical interaction.
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