The Bubbly Flow, Low Re k-ε Interface
The Bubbly Flow, Low Re k-ε (bf) interface (), found under the Multiphase Flow>Bubbly Flow>Bubbly Flow, Turbulent Flow branch () when adding a physics interface, is used to model the flow of liquids with dispersed bubbles at high Reynolds numbers.
It is assumed that the bubbles only occupy a small volume fraction and that they always travel with their terminal velocity. It is thereby possible to solve only one set of Navier–Stokes equations for the liquid phase and to let the velocity of the bubbles be guided by a slip model. The pressure distribution is calculated from a mixture-averaged continuity equation. The volume fraction of bubbles is tracked by solving a transport equation for the effective gas density. Turbulence effects are modeled using the AKN two-equation k-ε with realizability constraints and bubble-induced turbulence production. The AKN model is a so-called low-Reynolds number model, which means that it can resolve the flow all the way down to the wall. The AKN model depends on the distance to the closest wall. The physics interface therefore includes a wall distance equation.
The physics interface can also model the distribution of the number density, which can be used to calculate the interfacial area, useful when simulating chemical reactions in the mixture.
The main physics node is the Fluid Properties feature, which adds the equations for turbulent bubbly flow and provides an interface for defining the fluid materials for the liquid and the gas and the slip velocity model to use.
When this physics interface is added, the following default physics nodes are also added in the Model Builder — Fluid Properties, Wall (the default boundary types are No slip for the liquid and No gas flux for the gas), and Initial Values.
Except where indicated below, the nodes settings for this physics interface are the same as for The Laminar Bubbly Flow Interface and The Bubbly Flow, k-ε Interface.
Turbulence
The default Turbulence model type is RANS. A different turbulence model can be selected under Turbulence model. The default turbulence model is Low Reynolds number k-ε.
Wall Treatment
The AKN model employs per default an Automatic wall treatment, which switches between a low-Reynolds-number formulation and a wall function formulation depending on how well resolved the flow is close to the wall. The automatic wall treatment gives a robust formulation that makes the most out of the available resolution.
Select the Low Re option in order to enforce resolution all the way down to the wall. This can be more accurate than the automatic wall treatment provided that the mesh is fine enough.
Edit Turbulence Model Parameters
Turbulence model parameters are optimized to fit as many flow types as possible, but for some special cases, better performance can be obtained by tuning the model parameters. For a description of the turbulence model and the included model parameters see Theory for the Turbulent Flow Interfaces.
Dependent Variables
The dependent variables (field variables) are the Velocity field, liquid phase u (SI unit: m/s), the Pressure p (SI unit: Pa), the Effective gas density rhogeff (SI unit: kg/m3), the Turbulent dissipation rate ep (SI unit: m2/s3), the Turbulent kinetic energy k (SI unit: m2/s2), the Reciprocal wall distance G (SI unit: 1/m), and the Number density, gas phase nd (SI unit: 1/m3).
The Bubbly Flow, Low Re k-ε interface requires a Wall Distance Initialization study step in the study previous to the Stationary or Time Dependent study step.
For study information, see Stationary with Initialization, Time Dependent with Initialization, and Wall Distance Initialization in the COMSOL Multiphysics Reference Manual.