- You can now add and remove species in the Chemical Species Transport physics 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 for chemical species transport in concentrated solutions. These diffusion models require less input data for the interaction between species in a solution and are less computationally demanding than the Maxwell-Stefan diffusion model. The new models can be used when interaction data is not available or when accuracy in the diffusion interaction is not required.
- A new Low-Reynolds Number k-ε Turbulence Model physics interface yields high accuracy in the description of the flow and transport of heat and mass transport close to walls.
- The improved k-ε Turbulence Model physics 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 version 3.5a.
- 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 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 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.
All backward compatibility issues are planned to be solved for version 4.0a unless explicitly stated.
K - ω Turbulence Model
The k-ω turbulence model physics interface is not yet implemented in version 4.0. It is planned for the CFD Module in version 4.1.
The Low-Reynolds k-ε turbulence model interface is an excellent alternative for higher accuracy in models including confined flows.
Periodic 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 in version 3.5a.
In version 4.0, the separate physics interfaces include this functionality as a tailored condition. These conditions are available for the physics interfaces for Single-Phase Flow, Two-Phase Flow (Level Sets and Phase Fields), Heat Transfer, and for Chemical Species Transport in free media.
They are not available in the physics interfaces for Species Transport in Porous Media, Bubbly Flow, and Mixture Models.
In addition, the periodic boundary conditions in version 4.0 use a new implementation, which is not yet backwards compatible with 3.5a. Models using periodic conditions have to be manually converted to version 4.0.
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 4.0. They are planned for 4.1.
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.
The application mode for Reacting Flow in version 3.5a automatically couples fluid flow to chemical species transport.
The physics interface for Reacting Flow is not yet implemented in version 4.0. It is planned for version 4.1.
However, you can easily create a coupled flow and chemical species transport model, including reactions, by selecting the flow field directly in the mass transport physics interface. The list of available flow fields is generated automatically.
The moving mesh interface for Rotating Machinery is not yet implemented in version 4.0.
It is not possible to combine two-phase flow with fluid-structure interaction. This is planned for the CFD Module in version 4.1.
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 4.0. However, the new stabilization functionality in version 4.0 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 4.0 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:
ni * n = –k(cb – cs)
where nidenotes 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.
Figure 1-2: Example of two domains with two separate dependent variables for chemical concentration.
Version 3.5a models using this functionality are not automatically converted to version 4.0.
However, you can covert these models manually in version 4.0 by using separate fields for the surface and bulk concentrations. The analogy is also valid for heat flux.