CFD Module Updates
For users of the CFD Module, COMSOL Multiphysics® version 6.0 brings interfaces for modeling flow in rotating machinery, new functionality for large eddy simulations, and several new tutorial models. Read about these updates and more below.
Rotating Machinery, High Mach Number Flow
With the new release, the CFD Module allows for the formulation of high Mach number flow problems in rotating machinery. A new Rotating Machinery, High Mach Number Flow branch contains physics interfaces that define the momentum, continuity, and energy equations for laminar and turbulent flows. Typical uses include modeling turbomachinery, propellers, and helicopter rotors.
Automatic Wall Treatment for Large Eddy Simulation (LES)
For the LES interfaces, automatic wall treatment has been added that allows for a slightly coarser mesh near walls, when the flow close to walls is less interesting. This substantially reduces the computational cost but does not necessarily decrease accuracy in the solution of the flow field away from the walls using LES. Automatic wall treatment may be used to model separation from sharp edges, since the transition of the boundary layer from laminar to turbulent is then instantaneous.
Thermal Wall Functions for LES
In cases where the flow is not resolved all the way to the wall, wall functions for heat transfer must also be used. In COMSOL Multiphysics® version 6.0, wall functions for heat transfer are available for LES when combined with automatic wall treatment for the flow. The thermal wall functions are automatically added when Wall treatment is set to Automatic in the LES interface. This approach may be used for separation from sharp edges and buoyant separation perpendicular to a smooth surface.
Introducing Flow-Induced Noise
A hybrid computational aeroacoustic (CAA) method is introduced for modeling flow-induced noise. It is based on a one-way coupling between the turbulent flow sources and the acoustic equations. The method assumes that no back-coupling exists from the acoustic field to the flow field. The computational method is based on the FEM discretization of Lighthill’s acoustic analogy (wave equation). This formulation of the equations ensures that any solid, which can be fixed or vibrating, boundaries are implicitly taken into account. Two flow-induced noise options are available: the Lighthill analogy and the simpler aeroacoustic wave equation (AWE) analogy.
The new functionality relies on coupling a large eddy simulation (LES) fluid flow model, solved using the CFD Module, to the Aeroacoustic Flow Source domain feature in Pressure Acoustics, Frequency Domain. The coupling is achieved by using the Aeroacoustic Flow Source Coupling multiphysics coupling and the dedicated Transient Mapping study. Note that this feature requires the Acoustics Module.
Rotating Machinery, Phase Transport Mixture Model
For users that also have the Mixer Module, you can now simulate phase separation in rotating machinery with multiple dispersed phases where the centrifugal force can be used to fractionate particles by density, size, and shape. A new Rotating Machinery, Phase Transport Mixture Model branch contains predefined multiphysics interfaces that make it easier to set up these models. These interfaces can also be used to simulate the mixing of several phases that would otherwise separate due to sedimentation or flotation processes.
Density of a mixture of heavy and light particles in a rotating vessel with a conically shaped bottom. The heavy particles form a sediment on the outer boundary of the vessel, whereas a mixture of water and light particles is continuously compacted in the center of the vessel.
A mixture of water and heavy and light particles in a mixing tank. Without stirring, the light particles (slice with red gradient) float to the top and the heavy particles (slice with blue gradient) sink to the bottom. Stirring mixes the phases again.
Porous Slip for the Brinkman Equations Interface
The boundary layer in flow in porous media may be very thin and impractical to resolve in a Brinkman equations model. The new Porous slip wall treatment option allows you to account for walls without resolving the full flow profile in the boundary layer. Instead, a stress condition is applied at the surfaces, yielding a decent accuracy in the bulk flow by utilizing an asymptotic solution of the boundary layer velocity profile. The functionality is activated in the Brinkman Equations interface Settings window and is then used for the default wall condition. This new feature may be used in most models involving subsurface flow described by the Brinkman equations and where the model domain is large.
Heat Transfer in Porous Media
The heat transfer in porous media functionality has been revamped to make it more user friendly. A new Porous Media physics area is now available under the Heat Transfer branch and includes the Heat Transfer in Porous Media, Local Thermal Nonequilibrium, and Heat Transfer in Packed Bed interfaces. All of these interfaces are similar in function, the difference being that the default Porous Medium node within all these interfaces has one of three options selected: Local thermal equilibrium, Local thermal nonequilibrium, or Packed bed. The latter option has been described above and the Local Thermal Nonequilibrium interface has replaced the multiphysics coupling and corresponds to a two-temperature model, one for the fluid phase and one for the solid phase. Typical applications can involve rapid heating or cooling of a porous medium due to strong convection in the liquid phase and high conduction in the solid phase like in metal foams. When the Local Thermal Equilibrium interface is selected, new averaging options are available to define the effective thermal conductivity depending on the porous medium configuration.
In addition, postprocessing variables are available in a unified way for homogenized quantities for the three types of porous media. View the new porous media additions in these existing tutorial models:
Nonisothermal Flow in Porous Media
The new Nonisothermal Flow, Brinkman Equations multiphysics interface automatically adds the coupling between heat transfer and fluid flow in porous media. It combines the Heat Transfer in Porous Media and Brinkman Equations interfaces. You can see this new feature in the existing Free Convection in a Porous Medium tutorial model.
Two-Phase Flow in Porous Media
A new multiphysics interface combines the Brinkman Equations and the Level Set interfaces, and automatically adds a Two-Phase Flow, Level Set coupling node. It solves the conservation of momentum and a continuity of mass with the Brinkman equations. The interface between two immiscible fluids in porous media is tracked with the level-set function.
Greatly Improved Handling of Porous Materials
Porous materials are now defined in the Phase-Specific Properties table in the Porous Material node. In addition, subnodes may be added for the solid and fluid features where several subnodes may be defined for each phase. This allows for the use of one and the same porous material for fluid flow, chemical species transport, and heat transfer without having to duplicate material properties and settings.
Source Terms for the Shallow Water Equations Interface
The shallow water equations give a 1D or 2D approximation of shallow flows by averaging along the depth. Rain, local upwellings, pumping devices, or boundary stresses have to be introduced as source terms in the model equations. This was previously possible through the equation view, but the ability to add momentum and mass sources is now available as predefined settings in the flow interface.
New Approximate Schur Complement Method for the Vanka Solver
The Vanka solver is extended with a new approximate factorization method for its matrix blocks. When using the Block solver method Direct, stored factorization, there is now an option to Use approximate factorization that is using a Schur complement approximation for larger blocks. This method can save significant memory and CPU time for large blocks, as encountered in, for example, large 3D fluid flow models with the fully developed inflow boundary conditions. This method is available both from the Vanka solver as well as from the SCGS solver with the Vanka option enabled.
New Tutorial Models
COMSOL Multiphysics® version 6.0 brings several new tutorial models to the CFD Module.
FSI Analysis of a Sports Car Side Door
Dam Breaking on a Column, Level Set
This transient model uses the Two-Phase Flow, Level Set, Laminar Flow interface to model the impact of a water wave on a column. The pressure force on the column is calculated and can be compared with experimental results. This model is also available in the Application Library in a different version based on the shallow water equations.
Application Library Title:
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Free Surface Mixer, Level Set (requires Mixer Module)
Vortex generated by the rotating fluid in a free surface mixer calculated using a Rotating Machinery, Turbulent Flow interface from the Mixer Module together with the Level Set method.
Application Library Title:
Download from the Application Gallery