This section described new functionality and backward compatibility with respect to the implementations in the Chemical Engineering Module in COMSOL 3.5a.
New Functionality in Version 4
- A new Low-Reynolds Number k-ε Turbulence Model interface is introduced, which yields high accuracy in the description of the flow close to walls. Mass and heat transport are also accurately described by this formulation.
- The improved k-ε Turbulence Model interface implementation gives greater robustness when the value of the turbulence intensity is small. The wall functions are also more accurate and require less input compared to previous versions.
- The new turbulence model formulations allow for transient simulations of turbulent flow. You can run time-dependent simulations without having to start with a steady flow solution as initial condition.
- The option to use psuedo time stepping when computing stationary solutions has been added to the Single-Phase Flow physics interfaces (including Brinkman Equations and Free and Porous Media Flow). Pseudo-time stepping increases the robustness when solving highly non-linear problems and reduces the need for solving in several steps.
- The multiphysics interface Rotating Machinery has been substantially improved. Using the Rotating Domain feature one or several rotating domains can be included in a model. The axis of rotation can easily be altered as well as the rotation direction and frequency. The Rotating Wall feature automatically adds the proper velocity constraints on boundaries of the Rotating Domain feature which do not correspond to the interface between the rotating and stationary domain.
- The possibility to include a Forchheimer Drag extension to the fluid flow resistance in porous media flow has been added.
- It is now possible to include a convective term in the Brinkman Equations. This means that porous media flow including higher fluid velocity can be studied. The inclusion of the term is controlled using the Neglect inertial term (Stokes-Brinkman) switch available in the Brinkman Equations and the Free and Porous Media physics interfaces.
- Stabilization has been included for the Brinkman equations. The stabilization provides increased robustness and less computational cost for a given accuracy compared to previous versions. It is also imperative when solving problems including the convective term.
- You can now add and remove species in the Chemical Species Transport interfaces. This means that you do not have to start with all species in your transport model. Instead you can add species one by one, thus reducing the risks for introducing errors.
- Two new diffusion models, a mixture-averaged diffusion model and a model based on Fick’s law, are introduced in the Transport of Concentrated Species interface. These diffusion models are less computationally demanding than the Maxwell-Stefan diffusion model, and the latter model require less input data for the interaction between species in the mixture. The new models can be used in convectively dominated problems, where high accuracy in the diffusion interaction is not required, or when interaction data is not available.
- The improved stabilization for Chemical Species Transport in version 4.0 yields higher accuracy with a relatively coarse mesh compared to version 3.5a. This also results in increased robustness and less computational cost for a given accuracy compared to previous versions.
- A new Open Boundary condition is introduced in the physics interfaces for turbulent Single Phase Flow, Heat Transfer, and Chemical Species Transport. The condition is designed to be used on model boundaries including both in and outflow sections. On sections with inflow the solution is not affected, while at occurrences of inflow a user defined exterior condition is enforced.
- A new physics interface for Species Transport in Porous Media accounts for the effect of the tortuous path in porous media. This path results in the additional dispersion perpendicular to the main flow of a transported species caused by the convective flux. The dispersion of species in porous media is thus more accurately described compared to previous implementations.
- A new physics interface for Heat Transfer in Porous Media can be used to accurately study heat transfer in porous catalysts, filters, and other unit operations involving porous media.
Backward Compatibility vs. Version 3.5a
k-ε Turbulence Model
The new wall functions have the potential to deliver higher accuracy than the formulation used in 3.5a. They may however require finer wall resolution. Hence, a 3.5a turbulence model can often benefit from an additional boundary layer mesh or refined boundary layer mesh when imported into 4.0a.
k-ω Turbulence Model
The k-ω turbulence model physics interface is not yet implemented in version 4.0a. It is planned for the CFD Module in version 4.1.
Version 4.0a includes automatic translation of models built with the previous k-ω turbulence model. When opened, the full model, including initial values and boundary conditions, is converted to the k-ε turbulence model. Once opened, the model can also be also be changed to the Low-Reynolds k-e Turbulence Model interface. The latter physics interface present an excellent alternative for higher accuracy in models including confined flows.
Pseudo Application Modes
The Pseudo application modes for species transport in version 3.5a allow for the use of the dependent variable for time as a space coordinate in the direction of the flow.
The corresponding physics interfaces are not yet implemented in version 4.0a. They are planned for a later version.
Meanwhile, you can either create this alternative description manually, by relating time to a position along the length of the reactor using the axial velocity, or you can use a full 2D or 3D model.
Special Basis Functions or Elements
None of the special basis functions or elements for the finite element formulation of flow problems included in version 3.5a are available in version 4.0a. However, the new stabilization functionality in version 4.0a for fluid flow is identical to using the bubble elements in 3.5a.
Other special elements that were available in 3.5a will not be re-implemented in version 4. The reason for this is that the stabilized formulation in version 4.0a gives high accuracy to a relatively small computational cost compared to the special elements.
Thin Boundary Layer Pair Boundary Conditions
The thin boundary layer approximation approximates the mass flux perpendicular to an interface according to:
where ni denotes the flux of species i, n the normal vector, cs the surface concentration, and cb the bulk concentration of species i.
In the case where cs actually is a concentration in a separate domain, so that the interface between two domains requires a discontinuous concentration but a continuous flux, this condition could be defined in 3.5a using pair boundary conditions.
Version 3.5a models using this functionality are not automatically converted to version 4.0a.
However, you can covert these models manually in version 4.0a by using separate fields for the surface and bulk concentrations. The analogy is also valid for heat flux.
