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The Polymer Flow Module Physics Interface Guide
The physics interfaces in this module are based on the laws for conservation of momentum, mass, and energy in fluids. The different flow models contain different combinations and formulations of the conservation laws that apply to the physics of the flow field. These laws of physics are translated into partial differential equations and are solved together with the specified initial and boundary conditions.
A physics interface defines a number of features. These features are used to specify the fluid properties, initial conditions, boundary conditions, and possible constraints. Each feature represents an operation describing a term or condition in the conservation equations. Such a term or condition can be defined on a geometric entity of the component, such as a domain, boundary, edge (for 2D components), or point.
Figure 4 shows the Model Builder, including a Laminar Flow interface, and the Settings window for the selected Fluid Properties 1 feature node. The Fluid Properties 1 node adds the marked terms to the component equations in a selected geometry domain. Furthermore, the Fluid Properties 1 feature may link to the Materials feature node to obtain physical properties such as density and constitutive parameters, in this case rubber modeled with a power-law fluid. The fluid properties, defined by the Rubber, Power law material, can be functions of the modeled physical quantities, such as pressure and temperature. In the same way, the Wall 1 node adds the boundary conditions at the walls of the fluid domain.
Figure 4: The Model Builder including a Laminar Flow interface (left), and the Settings window for Fluid Properties for the selected feature node (right). The Equation section in the Settings window shows the component equations and the terms added by the Fluid Properties 1 node. The added terms are underlined with a dotted line. The arrows also explain the link between the Materials node and the values for the fluid properties.
The Polymer Flow Module includes a number of Fluid Flow interfaces for different types of flow. It also includes Chemical Species Transport interfaces for reacting flows in multicomponent solutions, and physics interfaces for heat transfer in solids and in fluids found under the Heat Transfer branch.
Figure 5 shows the Polymer Flow Module interfaces as they are displayed when you add a physics interface (see also Physics Interface Guide by Space Dimension and Study Type for further information). A short description of the physics interfaces follows.
Figure 5: The physics interfaces for the Polymer Flow Module as shown in the Model Wizard
Single-Phase Flow
The Creeping Flow interface () approximates the Navier-Stokes equations for very low Reynolds numbers. This is often referred to as Stokes flow and is applicable when viscous effects are dominant, such as in very small channels or microfluidics devices.
The Laminar Flow interface () is primarily applied to flows at low to intermediate Reynolds numbers. This physics interface solves the Navier-Stokes equations for incompressible, weakly compressible, and compressible flows (up to Mach 0.3). The Laminar Flow interface also allows for simulation of non-Newtonian fluids.
The Rotating Machinery, Laminar Flow interface () combines the Laminar Flow interface and a Rotating Domain, and is applicable to fluid-flow problems where one or more of the boundaries rotate, for example in mixers and around propellers. The physics interface supports incompressible, weakly compressible and compressible (Mach < 0.3) laminar flows of Newtonian and non-Newtonian fluids.
The Viscoelastic Flow interface () is used to simulate incompressible isothermal flow of viscoelastic fluids. It solves the continuity equation, the momentum equation and a constitutive equation that defines the elastic stresses. There are four predefined models for the elastic stresses: Oldroyd-B, FENE-P, Giesekus and LPTT.
Multiphase Flow
The Two-Phase Flow, Level Set interface (), the Two-Phase Flow, Phase Field interface (), and the Two Phase Flow, Moving Mesh interface () are used to model two fluids separated by a fluid-fluid interface. The moving interface is tracked in detail using either the level set method, the phase field method, or by a moving mesh, respectively. The level set and phase field methods use a fixed mesh and solve additional equations to track the interface location. The moving mesh method solves the Navier Stokes equations on a moving mesh with boundary conditions to represent the interface. In this case equations must be solved for the mesh deformation. Since a surface in the geometry is used to represent the interface between the two fluids in the Moving Mesh interface, the interface itself cannot break up into multiple disconnected surfaces. This means that the Moving Mesh interface cannot be applied to problems such as droplet formation in inkjet devices (in these applications the level set or phase field interfaces are appropriate). These physics interfaces support incompressible flows, where one or both fluids can be non-Newtonian.
The Laminar Three-Phase Flow, Phase Field interface () models laminar flow of three incompressible phases which may be either Newtonian or non-Newtonian. The moving fluid-fluid interfaces between the three phases are tracked in detail using the phase-field method.
Nonisothermal Flow
The Nonisothermal Flow, Laminar Flow interface () is primarily applied to model flow at low to intermediate Reynolds numbers in situations where the temperature and flow fields have to be coupled. A typical example is natural convection, where thermal buoyancy forces drive the flow. This is a multiphysics interface for which the nonlocal couplings between fluid flow and heat transfer are set up automatically.
Reacting Flow
The Laminar Flow interface () under the Reacting Flow branch combines the functionality of the Single-Phase Flow and Transport of Diluted Species interfaces. The physics interface is primarily applied to model flow at low to intermediate Reynolds numbers in situations where the mass transport and flow fields have to be coupled.
Physics Interface Guide by Space Dimension and Study Type
 
Chemical Species Transport
Reacting Flow
Fluid Flow
Single-Phase Flow
Rotating Machinery, Fluid Flow
Multiphase Flow
Two-Phase Flow, Moving Mesh
Two-Phase Flow, Level Set
Two-Phase Flow, Phase Field
Three-Phase Flow, Phase Field
Nonisothermal Flow
Heat Transfer
Moving Interface
1 This physics interface is included with the core COMSOL package but has added functionality for this module.
2 This physics interface is a predefined multiphysics coupling that automatically adds all the physics interfaces and coupling features required.