CFD Module Updates

For users of the CFD Module, COMSOL Multiphysics® version 5.6 brings two new fluid flow interfaces, new options for turbulent intensity and turbulence length scale, and automatic handling of disjoint selections. Learn more about these and other new CFD features below.

Performance Improvements for Multicore and Cluster Computing

COMSOL Multiphysics® version 5.6 includes several performance improvements for the solution process. In particular, the memory requirements are reduced for Jacobian matrix assembly as well as for the algebraic multigrid preconditioner. The reduction is significant for both multicore and cluster computing. Furthermore, the most important smoothers used in the multigrid method are now more efficient, in particular for cluster computing.

As an illustration of the improvement, consider the following CFD benchmark of an Ahmed Body featuring turbulent flow. The model used in the benchmark has a refined mesh, as compared to the Application Gallery version, with 6.3 million degrees of freedom on a 16-node cluster. In this comparison, COMSOL Multiphysics® version 5.5 update 3 and version 5.6 are installed on a cluster where each node has 48 cores (2x Intel® Xeon® Platinum 8260 24 cores). The solvers used in the comparison are the algebraic multigrid solver (SA-AMG) as a preconditioner to GMRES with the Symmetrically Coupled Gauss-Seidel smoother (denoted MG in the comparison graphs). In addition, the overlapping Domain Decomposition (Schwarz) method is used as a preconditioner to GMRES with the multigrid method as a domain solver (denoted DD). The graphs below show the performance as computation time vs. number of nodes and memory usage vs. number of nodes.

A 3D model of an Ahmed body, visualized with a metallic sheen, and yellow, green, turquoise, and blue streamlines, and a white mesh around it.
Streamlines around the Ahmed Body and mesh in a vertical cross section.

A chart of time from 0 to 1800 seconds vs. number of nodes from 1 to 16 and blue lines for COMSOL Multiphysics version 5.5 update 3 and yellow lines for version 5.6.
Wall-clock computation time for a refined version of the Ahmed Body benchmark model.

A chart of memory from 0 to 5000 MB vs. number of nodes from 1 to 16 and blue lines for COMSOL Multiphysics version 5.5 update 3 and yellow lines for version 5.6.
Physical memory use for a refined version of the Ahmed Body benchmark model.

Compressible Dispersed Multiphase Flow

Many natural phenomena, manufacturing processes, and separation processes involve dispersed multiphase flows with particles, bubbles, or droplets transported by a continuous phase. The Phase Transport, Mixture Model interfaces, which allow for an arbitrary number of dispersed phases, have been revised to support compressible mixture flows. These flow interfaces also have improved support for deforming and rotating domains. Additionally, the Phase Transport, Mixture Model interface can now be used together with the level set method in simulations that combine a separated and dispersed modeling approach. You can view this updated formulation feature in the new Droplet Rising Through a Suspension tutorial model.

An oil droplet rising through a suspension. The suspension is initially stratified, with a dense layer between two clear layers, and the droplet is initially located in the bottom clear layer. This model demonstrates the combination of a dispersed multiphase flow model with a level set model.

Nonisothermal Mixture Model

Simulating processes in which bubbles are formed in a liquid, such as nucleate boiling or cavitation processes, requires the coupling of multiphase flow with heat transfer. The new Nonisothermal Mixture Model interfaces provide exactly this functionality: They couple a Laminar Flow or Turbulent Flow (RANS) interface, a Phase Transport interface, and a Heat Transfer in Fluids interface using the new Nonisothermal Flow, Mixture Model and Nonisothermal Mixture Model multiphysics coupling nodes. All RANS turbulence models are supported by the Nonisothermal Mixture Model interfaces.

A cylindrical annulus model with the inside exposed and a heat camera color table visualizing the temperature, streamlines denoting bubble flux, and a color gradient from dark blue to white showing the bubble volume fraction.
Nucleate boiling in a cylindrical annulus. The temperature on the inner cylinder, the bubble volume fraction, and the streamlines of the bubble flux are shown in the figure.

Shallow Water Equations

The shallow water approximation is frequently applied in oceanographic and atmospheric applications to predict effects of tsunami impacts, areas affected by pollution, coastal erosion, and polar ice-cap melting, to mention a few. The new Shallow Water Equations, Time Explicit interface uses a depth-averaged formulation to solve free-surface flows in 1D and 2D domains. The bottom topography in a model can conveniently be defined from a digital elevation model (DEM). You can see this feature in the new Tsunami Runup onto a Complex 3D Beach, Monai Valley tutorial model.

Tsunami impact simulated with the Shallow Water Equations, Time Explicit interface.

Total Pressure Condition

For certain applications, such as in pump simulations, it is more convenient to specify the total (or stagnation) pressure on inlet and outlet boundaries. COMSOL Multiphysics® version 5.6 includes options for imposing either pointwise values or average values of the total pressure on inlets and outlets for incompressible flow.

A model of a fan in a brown tunnel with airflow around the blades and behind the fan.
Circulation generated from fan blades. Averaged total pressure conditions are imposed on the inlet and outlet boundaries.

New Averaging Options for Fluid Properties Across Phase Interfaces

When the density and/or viscosity ratio in a two-phase flow simulation is large, the use of volume-averaged fluid properties across the phase interface may lead to excessive smearing. Sharpening the transition zone for the fluid properties, or displacing it into one of the two phases, may improve accuracy and/or convergence in some cases. In version 5.6, smoothed Heaviside functions and harmonic volume averages can be used for both density and viscosity. For the viscosity, mass averaging and harmonic mass averaging are also available as options.

A 1D plot with a blue line for volume average, green for Heaviside function, red for harmonic volume average, turquoise for mass average, and magenta for harmonic mass average.
Different options for averaging the viscosity across an air–water interface.

New Options for Turbulence Conditions

Three new options are available for the turbulent intensity on inlets and open boundaries: Low (0.01), Medium (0.05), and High (0.1). There is also a new Geometry based default option for the turbulence length scale on inlets. This option automatically calculates the hydraulic diameter of the inlet and sets the turbulence length scale to 0.07 times the hydraulic diameter, which is the recommended value for fully developed turbulent flow in pipes and channels.

The Inlet settings for Turbulent Flow in COMSOL Multiphysics version 5.6, with the Velocity and Turbulence Conditions sections expanded and a hydrocyclone model in the Graphics window.
New options for Turbulent Intensity and Turbulence Length Scale shown in the Settings window.

Automatic Handling of Disjoint Selections

For Fully Developed Flow on inlets and outlets, and Mass Flow on inlets, a selection consisting of disjoint boundaries can be handled within a single boundary feature. This new default option adds one equation for each connected component in a selection. The connected components in a disjoint selection are detected automatically.

A model of a multijet shown as partially transparent for an inside view.
The polymerization_multijet model has been updated with the new option for disjoint selections. Separate equations are added for the two types of inlets.

New Porous Medium Feature

A new feature for handling a porous medium is available for defining the different phases: solids, fluids, and immobile fluids. In the Heat Transfer in Porous Media interface, the Porous Medium feature is used to manage the material structure with a dedicated subfeature for each phase: Fluid, Porous Matrix, and optionally, Immobile Fluids. This new workflow provides added clarity and improves the user experience. It also facilitates multiphysics couplings in porous media in a more natural way. Combined with the Moisture Transport and Porous Media Flow interfaces, the heat transfer in porous media improvements enable the modeling of nonisothermal flow and latent heat storage in porous media.

You can see this new setup in the following models:

The Porous Medium feature settings in COMSOL Multiphysics version 5.6 with the Effective conductivity options shown and Reciprocal average highlighted.
A porous material with a fluid, a solid, and an immobile fluid combined with the Porous Medium feature in the Heat Transfer in Porous Media interface. The Settings window shows the selection of the model defining the effective thermal conductivity from the different phases in the porous medium.

Automatic Detection of Ideal Gas Material in Heat Transfer in Fluids

The Fluid feature, available within the various heat transfer interfaces, has been updated to take advantage of the ideal gas assumption to improve computational efficiency. The new From material option of the Fluid type list automatically detects whether the material applied on each domain selection is an ideal gas or not, and uses the relevant properties for either case. This may speed up computation when computing pressure work in compressible nonisothermal flows, for example. Since the gases available in COMSOL Multiphysics® and in the Material Library are modeled as ideal gases, many models with compressible nonisothermal flow are expected to benefit from this improvement.

A model of an LED bulb visualizing the velocity around the bulb in a color gradient from dark blue to white and the temperature in the bulb using the heat camera color table.
Temperature distribution (surface plot) and velocity (arrows and streamlines) in an LED bulb. By using the ideal gas formulation automatically, the computational time is 10% shorter in COMSOL Multiphysics® version 5.6.

Heat and Energy Balance

The postprocessing variables for energy and heat balance definition have been extended to cover new configurations. Specifically, the variables are for nonisothermal flow, to account for out-of-plane heat sources; for the work of volume forces, viscous dissipation, and pressure; for boundary stresses; and for enthalpy flux in cases of nonzero normal velocity on internal walls. The postprocessing variables or energy and heat balance definition have also been extended to layered materials. Energy and heat balance provide an alternative criterion to the solver error estimate to check the simulation accuracy. You can see this functionality demonstrated in the Electronic Chip Cooling model.

The COMSOL Multiphysics 5.6 UI with the Global Evaluation settings shown for a cross-flow heat exchanger model, which is shown in the Graphics window in a color gradient from dark red to white to indicate temperature in degrees Kelvin.
Verification on the energy balance in the cross-flow heat exchanger model. The total net energy rate entering at the hot inlet is compared with the energy balance for the entire model.

Easier Setup for Phase Field and Level Set Models

The Level Set and Phase Field interfaces have been restructured: Two Initial Values nodes are now added by default, and the previously used Initial Interface feature has been removed. Instead, the initial interface is automatically placed at the boundaries between the two Initial Values nodes with different initial phases.

Settings for the Initial Values, Fluid 2 feature. Note that the Initial Interface feature is no longer needed. The initial distribution of the level set or phase field function is solved for in the Phase Initialization study step.

New Tutorial Models

COMSOL Multiphysics® version 5.6 brings several new tutorial models to the CFD Module.

Droplet Rising Through a Suspension

An orange droplet rising through a gray suspension in a column.
An oil droplet rising through a dense suspension located between two clear-fluid layers.

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Polymer Electrolyte Membrane Electrolyzer

A model of an electrolyzer with the volume fraction of oxygen visualized in a dark blue to white color gradient.
Volume fraction of oxygen in a polymer electrolyte membrane electrolyzer.
Application Library Title:
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Dam Breaking on a Column, Shallow Water Equations

A model of a gray square column with blue water breaking on the column.
Shallow water equations simulation of the impact of a water wave on a column.

Application Library Title:
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Tsunami Runup onto a Complex 3D Beach, Monai Valley

A model of a gray beach and blue water representing tsunami runup.
Shallow water equations simulation of tsunami runup onto a beach.

Application Library Title:
Download from the Application Gallery