COMSOL Multiphysics version 4 introduces many new ideas and concepts making it easier for you to create models. The following lists the most important updates to the graphical user interface.

#### New layout:

In version 4 it is easier to organize and design models with the
intuitive structure of the *COMSOL Desktop*. The *Model Builder *displays
all the features of your model in one place.

#### Personalized desktop:

Control how you organize the *COMSOL Desktop *layout—your
preferences are saved for the next time you open COMSOL Multiphysics.

#### Sequences of operations:

You can now build custom model sequences to generate your geometry, physics, mesh, studies, and results. The sequences can be edited and changes are automatically updated across the model. Operations in the sequences can be modified by the parametric solver.

#### Geometric parameter sweeps:

With geometry sequences COMSOL can perform geometric parameter sweeps with full associativity from the user interface.

#### Material settings:

Materials are now administrated in one node in the Model Builder. You can select a material and its properties for each domain and for all physics in that domain.

#### Multiple solutions and meshes:

With the new layout, you can save and view multiple solutions and
meshes, and compare and contrast the results in the *Results*
branch.

#### Probes:

Use probes to visualize scalar quantities during computations. The quantities can be defined as integrals, max/min, the average of a field quantity, or the value at a point. This now works for time stepping and parameter sweeps.

#### Dynamic help:

The new context-dependent help enables easy browsing with extended
search functionality. The *Help Menu *provides access to the
complete documentation set in PDF and HTML formats.

#### Improved graphics:

Faster, better looking graphics.

#### Model settings:

Access to the model settings is easier and more intuitive. When you select a node in the Model Builder, a docked window containing associated settings displays at the same time.

#### Predefined selections:

Define selections of domains, boundaries, edges, and points. These predefined selections are available in the Settings windows for the physics interfaces, meshing, and when studying the results.

#### LiveLink™ family of products for integrating with CAD and MATLAB®

The new LiveLink™ for SolidWorks®, LiveLink™ for Inventor®, and LiveLink™ for Pro/ENGINEER® products connect COMSOL Multiphysics directly with these CAD programs for interactively linking parameters specified in a CAD system with simulation geometry. In addition, LiveLink for MATLAB is available for incorporating COMSOL Multiphysics models in the MATLAB technical computing and programming environment.

#### Cluster support

A floating network license for COMSOL Multiphysics can be extended at no additional cost to computational nodes for clusters on the Windows Cluster Server 2003, Windows HPC Server 2008, and Linux platforms.

Version 4 also introduces:

- New and enhanced physics interfaces and predefined multiphysics interfaces

- New parallel solvers and higher solver performance:

- A modal solver for frequency response and time domain (for structural and acoustics simulations, for example)

- The MUMPS and SPOOLES direct solvers for parallel and cluster computing

- A time discrete solver

- A fast-frequency sweep solver using AWE (asymptotic waveform evaluation) for electromagnetic waves simulations, for example

## New Modules in Version 4.0a

COMSOL Multiphysics 4.0a includes four new modules:

*CFD Module*: for modeling laminar and turbulent single-phase and multiphase fluid flow, conjugate heat transfer, porous media flow, thin-film flow and lubrication, and other applications within fluid dynamics.

*Batteries and Fuel Cells Module*: for modeling batteries, fuel cells, and other electrochemical applications.

*Plasma Module*: for modeling plasma physics such as inductively-coupled and capacitively-coupled plasma, microwave plasma, drift-diffusion processes, and heavy species transport.

*Chemical Reaction Engineering Module*: For modeling chemical reactions and chemical species transport using both space-dependent and space-independent models. The chemical reactions can be coupled to heat transfer and fluid flow. This module replaces the Chemical Engineering Module and the Reaction Engineering Module in 4.0 (see “Backward Compatibility vs. Version 4.0” below).

## Backward Compatibility vs. Version 4.0

### Chemical Engineering and Reaction Engineering Modules

COMSOL Multiphysics 4.0a introduces the new Chemical Reaction Engineering Module, which merges the Chemical Engineering Module and the Reaction Engineering Module. All the reaction engineering tools are now integrated with the chemical species transport tools in the same module. The Chemical Reaction Engineering Module is focused on reaction engineering and chemical species transport and for that reason excludes many of the fluid flow capabilities of the former Chemical Engineering Module. Those fluid flow capabilities are instead included in the new CFD Module.

### Changed default Element Order For some Physics Interfaces

Many of the transport interfaces for fluid flow, heat transfer, and chemical species transport now use first-order elements (shape functions) by default instead of second-order elements which was the default in version 4.0. The reason for this is that higher-order elements are computationally more costly than linear elements, and also more prone to introduce spurious oscillations, which reduces the numerical robustness.

For the following physics interfaces, the default element order has changed:

- The Laminar Flow and Turbulent Flow single-phase flow interfaces uses linear elements for both the velocity field and the pressure (P1 + P1 elements).

- The Phase-Field Two-Phase Flow interface uses linear elements for both the velocity field and the pressure (P1 + P1 elements).

- Heat Transfer in Fluids and Heat Transfer in Porous Media use linear elements for the temperature.

- All interfaces in the Chemical Species Transport branch (Transport of Diluted Species, Transport of Concentrated Species, Nernst-Planck Equations, Solute Transport, and Species Transport in Porous Media) use linear elements for the species concentrations.

- In the Fluid-Structure Interaction interface you can control the element order for the structural mechanics and the fluid flow separately. The default uses second-order elements for the displacements in the structure and linear elements for the velocity field and pressure in the fluid.

- The Non-Isothermal Flow and Conjugate Heat Transfer interfaces use the same element order for the temperature as for the velocity field. The default setting uses linear elements for the temperature and the velocity field.

- In the Thermal Stress interface the element order for the temperature is one order lower than for the displacements. The default setting uses second-order elements for the displacements and linear elements for the temperature.

With the new default element orders, it can still be beneficial to use second-order elements rather than the default linear elements in the following cases:

- Laminar Flow — for low flow velocities.

- Phase-Field Two-Phase Flow — for microfluidics applications where convection is small.

- Heat Transfer in Participating Media — for cases when the media is approximated as stationary.

- Heat Transfer in Porous Media — when convection is negligible.

- Chemical Transport interfaces — when simulating only diffusion.

In these cases, increasing the element order is an effective alternative to refining the mesh.

## Backward Compatibility vs. Version 3.5a

### Deformed Geometry Interface

The *Parameterized Geometry* application mode in versions 3.5a,
which is limited to 2D, is replaced with the *Deformed Geometry*
interface in version 4.0a. This interface is available in 2D and 3D. The
*Deformed Geometry* interface deforms the mesh using an arbitrary
Lagrangian-Eulerian (ALE) method and is not the parameterized geometry
using geometric parameter sweeps (see above), which is new functionality
in version 4.0a.

In the version 4.0a interface, the *Linear Displacement* and *Similarity
Transform* boundary conditions are not yet available as preset
conditions. Those boundary conditions are planned for version 4.1.

In version 4.0a, you can create the corresponding conditions by manually entering variables.

### Pair Boundary Conditions

*Pairs* are used to connect boundaries between domains that are
separated by an *assembly boundary* (boundary between different
parts in an assembly). To create an assembly, you have to either import
it from a CAD package or actively form an assembly as a final step in
the COMSOL geometry sequence.

*Pair boundary conditions* are available as general conditions
for all application modes in versions 3.5a. Most pair boundary
conditions that were available in COMSOL 3.5a can be found in the
physics interfaces in version 4.0a. Exceptions from this are listed
under backward compatibility for each product.

### Periodic PAIR Boundary Conditions

*Periodic boundary conditions *are used to model repetitive
structures, where one boundary in a domain is identical to another
boundary in the same domain.

Periodic boundary conditions are available as general conditions for all application modes (now called physics interfaces) in version 3.5a. In version 4.0, this functionality is a tailored condition for each physics interface.

However, some physics interfaces may lack periodic boundary
conditions in version 4.0; see the *Backwards Compatibility*
section for the modules below.

Note that, in the future, all physics interfaces will include periodic boundary conditions.

### PDE Modes

#### Basis Functions or Elements

The PDE application mode in version 3.5a includes a number of
possible *basis **functions* or *elements* to be used in
the finite element formulation of a set of equations.

Only *Lagrange* basis elements are available in the *PDE
interfaces* in version 4.0a using the *General form* or *Coefficient
form*. *Discontinuous elements* are available by entering
expressions as *weak contributions* using auxiliary *dependent
variables*.

Other elements are not yet available in the PDE interfaces.

### Weak Constraints

There are some physics interfaces that are not able to define
constraints as weak constraints; see the *Backwards Compatibility*
sections for the modules below.

### Axisymmetric Models

In version 3.5a equations in axisymmetric application modes use the *independent
**variable* for the radius, r,
to account for axisymmetry. In version 4.0a, the equations are
compensated by using the factor 2πr.

### Display of Equations

The equations for the physics interfaces are not displayed graphically in version 4.0a. However, using dynamic help, you can get an overview of the equations formulated by a physics interface by just one click. Full support for display of equations is planned for version 4.1.

### Report Generator

The *report generator* is not yet implemented in version 4.0a. A
report generator is planned for 4.1.

### Backward Compatibility for Pre-3.5a Models

COMSOL 4.0a can load models saved from version 3.5a. For loading
models from earlier COMSOL versions than 3.5a you need to load them in
COMSOL 3.5a and then save them. For simplifying this task a utility is
available where you can convert all files in a directory from versions
3.0-3.5 to version 3.5a. See the section “COMSOL Convertpre35a Command”
on page 43 for Windows, section “COMSOL Convertpre35a Command” on page
76 for Linux, section “COMSOL Convertpre35a Command” on page 101 for the
Mac in the *COMSOL Installation and Operations **Guide* for
more information.