Fluid flow 3D elements are four- to eight-node isoparametric elements are formulated in 3D space. They have three velocity degrees of freedom defined, the X velocity, the Y velocity and the Z velocity per node (see Figures 1 to 4) and one pressure degree of freedom at the center of the element. The only constitutive model which is allowed for all 3D elements is the Newton-Stokes fluid model. These elements are useful for general flow situations where no apparent symmetry can be utilized to analyze the flow using 2D elements.

*Figure 1: 3D Quadrilateral Brick Element*

*Figure 2: 3D Six-Node (Wedge) Brick Element*

*Figure 3: 3D Five-Node (Pyramid) Brick Element*

*Figure 4: 3D Four-Node (Tetrahedron) Brick Element*

When applying loads to a surface number of a 3D brick part, be aware that some models may not have all the lines on the face to be loaded on the same surface number. What happens in this situation? If the model originated from a CAD solid model, all faces coincident with the surface of the CAD model will receive the load regardless of the surface number of the lines. In hand-built models and on CAD parts that are altered so that the part is no longer associated with the CAD part, the surface number that is common in any three of the four lines that define a face (four-node region) or two of the three lines (three-node region) determines the surface number of that face.

The element parameters for 3D elements will depend on the type of fluid flow analysis that is being performed.

*Steady Fluid Flow or Unsteady Fluid Flow Analysis Type*

If you are performing a steady or unsteady fluid flow analysis, first select the model that will be used for the viscosity of the fluid in the Viscosity Model drop-down box in the Element Definition dialog.

- The
*Newtonian*option should be used for fluids in which the shear stress is proportional to the rate of deformation. This is the most commonly used viscosity model. - The
*Power Law*option will enforce maximum and minimum limits for the viscosity of the fluid based on the shear rates. - The
*Carreau Model*option is recommended if the fluid will experience large differences in shear rate. This viscosity model will generate a curve that will characterize the viscosity over an infinite range of shear rates. - The
*Porous Media Model*option can be used to represent an area where the fluid is flowing through a porous media such as a filter. This is used to model a distributed resistance in the flow. (Porous media model is not available for open channel flow or multiphysics.)

Next, select the integration order that will be used for the 3D elements in this part in the *Integration Order* drop-down box. For rectangular shaped elements, select the *2nd Order* option. For moderately distorted elements, select the *3rd Order* option. For extremely distorted elements, select the *4th Order* option. The computation time for element stiffness formulation increases as the third power of the integration order. Consequently,
the lowest integration order which produces acceptable results should be used to reduce processing time.

*Flow through Porous Media Analysis Type*

If you are performing a Flow through Porous Media analysis, first select the type of material in the* Model* drop-down box in the *General* tab. If the part represents fluid that is not flowing through a porous media, select the* Fluid* option. If the part represents a porous media with uniform permeability in all directions, select the Isotropic option if
the Reynolds number for porous flow is less than 1. Select the *Isotropic power-law* option if the Reynolds number for porous flow is greater than 1. The Reynolds number for porous flow is calculated as . If the part represents a porous media with different permeabilities in three orthogonal directions, select the *Orthotropic* option if the Reynolds number for porous flow is less than 1. Select the* Orthotropic power-law* option if the Reynolds number for porous flow is greater than 1.

Control Orientation of 3D Elements

The controls used to orient the elements depend on the type of fluid flow analysis being performed.

*Steady Fluid Flow or Unsteady Fluid Flow Analysis Type*

If the part is using a *Viscosity Model* (set on the *General* tab) of* Porous Media Model*, use the *Orientation* tab to choose between a *Material model* of *Isotropic* or *Orthotropic*. The Isotropic material model has the same material properties in all directions; the Orthotropic material model has different
material properties in three orthogonal directions.

If using an orthotropic material model, you must define the orientation of material axes 1, 2 and 3. There are two basic methods to accomplish this.

The first method of orienting an orthotropic material is to select one of the global axes as material axis 1. If you select
the *Global X-direction* option in the *Material axis direction specified using *drop-down box, the orthogonal material axes will follow the X, Y and Z axes as follows:

- Material axis 1: X axis
- Material axis 2: Y axis
- Material axis 3: Z axis

If you select the *Global Y-direction* option in the *Material axis direction specified using* drop-down box, the orthogonal material axes follow the X, Y and Z axes as follows:

- Material axis 1: Y axis
- Material axis 2: Z axis
- Material axis 3: X axis

If you select the *Global Z-direction* option in the *Material axis direction specified using* drop-down box, the orthogonal material axes follow the X, Y and Z axes as follows:

- Material axis 1: Z axis
- Material axis 2: X axis
- Material axis 3: Y axis

With the first method, the axes can be rotated about the chosen global direction by entering an angle in the *Material Axis Rotation Angle* field. This angle follows the right-hand rule.

The second method of orienting an orthotropic material is to select the *Spatial Points* option in the *Material axis direction specified using *drop-down box. Next you must define the coordinates for three spatial points in the *Spatial point coordinates *table. Next, select the appropriate index for the spatial points in the *Orientation Node 1*, *Orientation Node 2* and *Orientation Node 3* drop-down boxes. Material axis 1 will be a vector from the spatial point in the Orientation Node 1 drop-down box to the spatial
point in the *Orientation Node 2* drop-down box. Material axis 2 will be perpendicular to local axis 1 and will be in the plane formed by orientation nodes
1 and 3. Material axis 3 will be calculated as the cross-product of material axis 1 and material axis 2.

ImportantThe spatial point coordinates are shared by all the parts in the model. Changing any of the coordinates in one part will affect
all other parts that use the same spatial point.

*Flow through Porous Media Analysis Type*

If the part is using an orthotropic material model, define the orientation of material axes 1, 2 and 3 in the *Orientation* tab of the *Element Definition* dialog.

The material axes for a 3D element are the r (sometimes known as n), s and t axes. These axes will be defined by specifying
three nodes in the *Orientation Node 1*, *Orientation Node 2* and* Orientation Node 3* fields. You must first check the model in the Results environment to determine the node numbers. The r (or n) axis will be
defined as the vector from *Orientation Node 1* to *Orientation Node 2*. The s axis will be perpendicular to the r axis and will pass through *Orientation Node 3*. The t axis will be the cross product of the r and s axes.

*Figure 5: Orientation of the Material Axes*

- Be sure that a units system is defined.
- Be sure that the model is using a fluid flow analysis type.
- Right-click the
*Element Type*heading for the part that you want to be 3D elements. - Select the 3D command.
- Right-click the
*Element Definition*heading. - Select the
*Edit Element Definition*command. - If you are performing a steady or unsteady fluid flow analysis, select the appropriate viscosity model in the
*Viscosity Model*drop-down box. - If you are performing a flow through porous media analysis, select the appropriate material model in the Material Model drop-down box.
- If you selected the
*Orthotropic*or*Orthotropic power-law*option in the*Material Model*drop-down box, define the orientation of the material axes in the*Orientation*tab. - Press the
*OK*button.