D I S C R E T I Z A T I O N

To display this section, click the Show button () and select Discretization. It controls the discretization (the element types used in the finite element formulation).

From the Discretization of fluids list select the element order for the velocity components and the pressure: P1+P1 (the default), P2+P1, or P3+P2.

•P1+P1 means linear elements for both the velocity components and the pressure field. This is the default element order for the Laminar Flow and Turbulent Flow flow interfaces. Linear elements are computationally cheaper than higher-order elements and are also less prone to introducing spurious oscillations, thereby improving the numerical robustness.

•P2+P1 means second-order elements for the velocity components and linear elements for the pressure field. This is the default for the Creeping Flow interface because second-order elements work well for low flow velocities.

•P3+P2 means third-order elements for the velocity components and second-order elements for the pressure field. This can add additional accuracy but it also adds additional degrees of freedom compared to P2+P1 elements.

The Creeping Flow Interface

The Creeping Flow interface (), found under the Single-Phase Flow branch () in the Model Wizard, has the equations, boundary conditions, and volume forces for modeling freely moving fluids using the Stokes equations, solving for the velocity field and the pressure. This interface can be used to study fluid flows at very low Reynolds numbers where the inertial term in the Navier-Stokes equations can be neglected. This single-phase flow type is also referred to as Stokes flow and occurs in systems with high viscosity or small flow length scales (for example in microfluidics and MEMS devices).

The main feature of the interface is the Fluid Properties feature, which adds the Stokes equations and provides an interface for defining the fluid material and its properties.

When this interface is added, these default nodes are also added to the Model Builder— Fluid Properties, Wall (the default boundary condition is No slip), and Initial Values.

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Right-click the Creeping Flow node to add other features that implement, for example, boundary conditions and volume forces.

Except where noted below, see The Laminar Flow Interface for all the

other settings.

Note

P H Y S I C A L M O D E L

By default in the Creeping Flow interface, the Neglect inertial term check box is selected, which produces the Stokes equations.

The first number is the element order for the fluid velocity, and the second number is the element order for the pressure. Contrary to general laminar and turbulent single-phase flow, P2+P1 elements are well suited for low flow velocities, so the default is to use P2+P1 elements rather than purely linear P1+P1 elements.

C O N S I S T E N T S T A B I L I Z A T I O N

To display this section, click the Show button () and select Stabilization. Only the Streamline diffusion consistent stabilization method is available for the Navier-Stokes equations. This check box is selected by default and should remain selected for optimal performance. This consistent stabilization method does not perturb the original transport equation. If required, click to clear the Streamline diffusion check box.

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