CFD Module

The Euler-Euler Model, Turbulent Flow Interface

A new interface for turbulent dispersed two-phase flow is introduced with version 5.1. The Euler-Euler Model, Turbulent Flow interface can handle large ratios of the particle relaxation time to mean-flow time scales. This implies that the particles in the dispersed phase do not have to be in local force equilibrium with the continuous phase. An advantage with the Euler-Euler two-phase flow model is that it is also able to handle large differences in density between the dispersed and continuous phases, for example solid particles dispersed in air.

The turbulence in this interface is modeled using the standard k-ε turbulence model including realizability constraints. The interface also includes options to solve one set of k-ε equations for the mixture (Two-Phase Turbulence is set to Mixture) or to solve two sets of k-ε equations, one for each phase (Two-Phase Turbulence is set to Phase specific). By default, the former setting is applied.

Snapshot from the simulation of a bubble column using the Euler-Euler Model, Turbulent Flow interface. The dispersed phase (isocontour plot), velocity of the continuous phase (arrow plot), and turbulent kinetic energy of the mixture (slice color plot) are visualized. Snapshot from the simulation of a bubble column using the Euler-Euler Model, Turbulent Flow interface. The dispersed phase (isocontour plot), velocity of the continuous phase (arrow plot), and turbulent kinetic energy of the mixture (slice color plot) are visualized.

Snapshot from the simulation of a bubble column using the Euler-Euler Model, Turbulent Flow interface. The dispersed phase (isocontour plot), velocity of the continuous phase (arrow plot), and turbulent kinetic energy of the mixture (slice color plot) are visualized.

Coupled Porous Media Flow and Turbulent Flow

The Single-Phase Flow interfaces can now model turbulent flow in a free medium that is coupled to a porous medium. You can activate this functionality by adding a Fluid and Matrix Properties domain node for the Algebraic yPlus or L-VEL turbulence models. These turbulence models are only available in the CFD and Heat Transfer Modules, but you can still couple them to Porous media flow interfaces available in other modules.

You can either start with a porous media flow interface and add a free-flow domain or you can start with a free-flow interface and add a porous domain. The Enable porous media domains checkbox adds the Fluid and Matrix Properties feature. The Brinkman equations are solved in the porous domains and the Reynolds-averaged Navier-Stokes equations are solved in the free-flow domains.

Finally, your modeling capabilities have been extended by the fact that the Forchheimer term can be added to the equations for porous media flow. This allows for the description of high interstitial velocities (i.e., high velocities in the pores).

This figure shows a porous filter, furthest away from the viewer, supported by a perforated solid plate. A flow is pumped through the filter, where the effect of the porous filter and the perforations in the supporting plate on the turbulent flow are automatically accounted for in the flow interface. This figure shows a porous filter, furthest away from the viewer, supported by a perforated solid plate. A flow is pumped through the filter, where the effect of the porous filter and the perforations in the supporting plate on the turbulent flow are automatically accounted for in the flow interface.

This figure shows a porous filter, furthest away from the viewer, supported by a perforated solid plate. A flow is pumped through the filter, where the effect of the porous filter and the perforations in the supporting plate on the turbulent flow are automatically accounted for in the flow interface.

Capillary Pressure in the Two-Phase Darcy's Law Interface

One of the important terms in two-phase porous media flow is the capillary pressure, which signifies an averaged force necessary to move the interface, separating the two fluids, through the porous domain. The force works against the interfacial tension between the two phases. Capillary pressure is now available as an option for the Capillary Model feature in the Two-Phase Darcy's Law interface. The available capillary pressure models are: van Genuchten, Brooks and Corey, and the ability for you to define your own.

Two-phase flow in a porous filter placed between two perforated plates. The color plot shows the water saturation while the streamline represents the total flow for the mixture. Two-phase flow in a porous filter placed between two perforated plates. The color plot shows the water saturation while the streamline represents the total flow for the mixture.

Two-phase flow in a porous filter placed between two perforated plates. The color plot shows the water saturation while the streamline represents the total flow for the mixture.

New Inlet and Outlet Features for the Mixture Model and Bubbly Flow Interfaces

For the Inlet/Outlet feature in the Mixture Model interface, when the Mixture Boundary Condition is set to Velocity, there is now a Normal inflow/outflow velocity option in addition to the Velocity field option.

The new boundary conditions give improved stability in the solution of the benchmark model of an air-lift loop reactor. The new boundary conditions give improved stability in the solution of the benchmark model of an air-lift loop reactor.

The new boundary conditions give improved stability in the solution of the benchmark model of an air-lift loop reactor.

Pressure Condition

The Inlet features for the Bubbly Flow and Mixture Model interfaces have been updated with a new Pressure condition including a Suppress backflow option and the choice of Normal flow or User defined Flow direction. The new Pressure condition sets the normal stress at the boundary, which is more robust than the previous Pressure/No viscous stress condition.

Suppress Backflow Condition with Exterior Dispersed Phase/Gas Condition

The Mixture Model and Bubbly Flow interfaces have been updated with a new outlet Pressure condition, which includes options for Suppress backflow and Normal flow. In some cases (even when Suppress backflow is selected), it is not possible to avoid backflow on the entire boundary. For this reason, the Outlet features have been provided with an Exterior dispersed phase/gas conditions section including an input field for the Dispersed phase volume fraction/Effective gas density and options for specifying the number density when solved for. A Discontinuous Galerkin formulation is applied for the Dispersed Phase/Gas Boundary Condition in order to switch from a Dispersed phase/Gas outlet condition to a Dispersed phase/Gas concentration condition on the parts of the outlet where backflow occurs.

An example of a mixture stagnation flow for which the exterior dispersed phase volume fraction has been set to zero. An example of a mixture stagnation flow for which the exterior dispersed phase volume fraction has been set to zero.

An example of a mixture stagnation flow for which the exterior dispersed phase volume fraction has been set to zero.

Perforations for Thin-Film Flow

A new perforations feature is available for thin-film damping, enabling the modeling of thin-film flow in structures with perforations. The Perforations feature acts as a sink term for gases, which is proportional to both the ambient pressure and to the pressure difference with respect to the ambient pressure on the other side of the perforated surface. The constant of proportionality is known as the Perforation admittance (Y) and can either be defined directly or determined from the Bao model.

Out-of-Plane Motion Option for Border Flow Boundary Condition

A new option is available for the Border flow boundary condition for thin-film flow. Selecting Out-of-plane motion for the Border flow type calculates the pressure gradient at the boundary using the Gallis and Torczynski model. This model has been shown to agree well with detailed CFD and Monte Carlo simulations that model both the thin-film flow domain and the surrounding gas. The model applies for both rarefied and non-rarefied flows up to Knudsen numbers of approximately one.

Pseudo Time-Stepping for the Euler-Euler Model

The Euler-Euler Model interfaces now support pseudo time-stepping, which makes it easier to solve stationary models, especially for turbulent flow. The settings are found in the Advanced Settings section at the interface level.

Infinite Element Domains in Darcy's Law Interface

The Darcy's Law interface now supports infinite element domains and more advanced computations of boundary fluxes.