fluent中多孔介质模型的设置

7.19.6 User Inputs for Porous Media

When you are modeling a porous region, the only additional inputs for the

problem setup are as follows. Optional inputs are indicated as such.

1.

Define the porous zone.

2.

Define the porous velocity formulation. (optional)

3.

Identify the fluid material flowing through the porous medium.

4.

Enable reactions for the porous zone, if appropriate, and select the

reaction mechanism.

5.

Enable the

Relative Velocity Resistance Formulation

. By default, this

option is already enabled and takes the moving porous media into

consideration (as described in Section

7.19.6

).

6.

Set the viscous resistance coefficients (

or

in Equation

7.19-1

,

in

in Equation

7.19-2

) and the inertial resistance coefficients (

Equation

7.19-1

, or

in Equation

7.19-2

), and define the direction

vectors for which they apply. Alternatively, specify the coefficients for the

power-law model.

7.

Specify the porosity of the porous medium.

8.

Select the material contained in the porous medium (required only for

models that include heat transfer). Note that the specific heat capacity,

,

for the selected material in the porous zone can only be entered as a constant

value.

9.

Set the volumetric heat generation rate in the solid portion of the porous

medium (or any other sources, such as mass or momentum). (optional)

10.

Set any fixed values for solution variables in the fluid region

(optional).

11.

Suppress the turbulent viscosity in the porous region, if appropriate.

12.

Specify the rotation axis and/or zone motion, if relevant.

Methods for determining the resistance coefficients and/or permeability are

presented below. If you choose to use the power-law approximation of the

porous-media momentum source term, you will enter the

coefficients

and

in Equation

7.19-3

instead of the resistance

coefficients and flow direction.

You will set all parameters for the porous medium in

the

Fluid

panel

(Figure

7.19.1

), which is opened from the

Boundary

Conditions

panel

(as described in Section

7.1.4

).

Figure 7.19.1:

The

Fluid

Panel for a Porous Zone

Defining the Porous Zone

As mentioned in Section

7.1

, a porous zone is modeled as a special type of

fluid zone. To indicate that the fluid zone is a porous region, enable

the

Porous Zone

option in the

Fluid

panel. The panel will expand to show

the porous media inputs (as shown in Figure

7.19.1

).

Defining the Porous Velocity Formulation

The

Solver

panel contains a

Porous Formulation

region where you can

instruct

FLUENT

to use either a superficial or physical velocity in the

porous medium simulation. By default, the velocity is set to

Superficial

Velocity

. For details about using the

Physical Velocity

formulation, see

Section

7.19.7

.

Defining the Fluid Passing Through the Porous Medium

To define the fluid that passes through the porous medium, select the

appropriate fluid in the

Material Name

drop-down list in the

Fluid

panel

. If

you want to check or modify the properties of the selected material, you can

click

Edit...

to open the

Material

panel; this panel contains just the

properties of the selected material, not the full contents of the

standard

Materials

panel.

If you are modeling species transport or multiphase flow,

the

Material Name

list will not appear in the

Fluid

panel. For

species calculations, the mixture material for all fluid/porous

zones will be the material you specified in the

Species

Model

panel

.

For

multiphase

flows,

the

materials

are

specified

when

you define the phases, as described in Section

23.10.3

.

Enabling Reactions in a Porous Zone

If you are modeling species transport with reactions, you can enable

reactions in a porous zone by turning on the

Reaction

option in

the

Fluid

panel and selecting a mechanism in the

Reaction

Mechanism

drop-down list.

If your mechanism contains wall surface reactions, you will also need to

specify a value for the

Surface-to-Volume Ratio

. This value is the surface

area of the pore walls per unit volume (

), and can be thought of as a

measure of catalyst loading. With this value,

FLUENT

can calculate the

total surface area on which the reaction takes place in each cell by

multiplying

by the volume of the cell. See Section

14.1.4

for details

about defining reaction mechanisms. See Section

14.2

for details about wall

surface reactions.

Including the Relative Velocity Resistance Formulation

Prior to

FLUENT

6.3, cases with moving reference frames used the

absolute velocities in the source calculations for inertial and viscous

resistance. This approach has been enhanced so that relative velocities are

used for the porous source calculations (Section

7.19.2

). Using the

Relative

Velocity Resistance Formulation

option (turned on by default) allows you

to better predict the source terms for cases involving moving meshes or

moving reference frames (MRF). This option works well in cases with

non- moving and moving porous media. Note that

FLUENT

will use the

appropriate velocities (relative or absolute), depending on your case setup.

Defining the Viscous and Inertial Resistance Coefficients

The viscous and inertial resistance coefficients are both defined in the same

manner. The basic approach for defining the coefficients using a Cartesian

coordinate system is to define one direction vector in 2D or two direction

vectors in 3D, and then specify the viscous and/or inertial resistance

coefficients in each direction. In 2D, the second direction, which is not

explicitly defined, is normal to the plane defined by the specified direction

vector and the

direction vector. In 3D, the third direction is normal to the

plane defined by the two specified direction vectors. For a 3D problem, the

second direction must be normal to the first. If you fail to specify two

normal directions, the solver will ensure that they are normal by ignoring

any component of the second direction that is in the first direction. You

should therefore be certain that the first direction is correctly specified.

You can also define the viscous and/or inertial resistance coefficients in

each direction using a user-defined function (UDF). The user-defined

options become available in the corresponding drop-down list when the

UDF has been created and loaded into

FLUENT

. Note that the coefficients

defined in the UDF must utilize the

DEFINE_PROFILE

macro. For more

information on creating and using user-defined function, see the separate

UDF Manual.

If you are modeling axisymmetric swirling flows, you can specify an

additional direction component for the viscous and/or inertial resistance

coefficients. This direction component is always tangential to the other two

specified directions. This option is available for both density-based and

pressure-based solvers.

In 3D, it is also possible to define the coefficients using a conical (or

cylindrical) coordinate system, as described below.

Note that the viscous and inertial resistance coefficients are

generally based on the superficial velocity of the fluid in the

porous media.

The procedure for defining resistance coefficients is as follows:

1.

Define the direction vectors.

?

To use a Cartesian coordinate system, simply specify the

Direction-1

Vector

and, for 3D, the

Direction-2 Vector

. The unspecified

direction will be determined as described above. These direction

vectors correspond to the principle axes of the porous media.

For some problems in which the principal axes of the porous medium

are not aligned with the coordinate axes of the domain, you may not

know a priori the direction vectors of the porous medium. In such

cases, the plane tool in 3D (or the line tool in 2D) can help you to

determine these direction vectors.

(a)

porous region. (Follow the instructions in

Section

27.6.1

or

27.5.1

for initializing the tool to a position on an

existing surface.)

(b)

Rotate the axes of the tool appropriately until they are aligned

with the porous medium.

(c)

Once the axes are aligned, click on the

Update From Plane

Tool

or

Update From Line Tool

button in

the

Fluid

panel.

FLUENT

will automatically set the

Direction-1

Vector

to the direction of the red arrow of the tool, and (in 3D)

the

Direction-2 Vector

to the direction of the green arrow.

?

To use a conical coordinate system (e.g., for an annular, conical filter

element), follow the steps below. This option is available only in 3D

cases.

(a)

Turn on the

Conical

option.

(b)

Specify the

Cone Axis Vector

and

Point on Cone Axis

. The

cone axis is specified as being in the direction of the

Cone Axis

Vector

(unit vector), and passing through the

Point on Cone Axis

.

The cone axis may or may not pass through the origin of the

coordinate system.

(c)

Set the

Cone Half Angle

(the angle between the cone's axis and

its surface, shown in Figure

7.19.2

). To use a cylindrical coordinate

system, set the

Cone Half Angle

to 0.

Figure 7.19.2:

Cone Half Angle

For some problems in which the axis of the conical filter element is

not aligned with the coordinate axes of the domain, you may not

know a priori the direction vector of the cone axis and coordinates of

a point on the cone axis. In such cases, the plane tool can help you to

determine the cone axis vector and point coordinates. One method is

as follows:

(a)

Select a boundary zone of the conical filter element that is

normal to the cone axis vector in the drop-down list next to the

Snap

to Zone

button.

(b)

Click on the

Snap to Zone

button.

FLUENT

will automatically

Cone Axis

Vector

and the

Point on Cone Axis

. (Note that you will still have to

set the

Cone Half Angle

yourself.)

An alternate method is as follows:

(a)

(Follow the instructions in Section

27.6.1

for initializing the tool to a

position on an existing surface.)

(b)

Rotate and translate the axes of the tool appropriately until the

red arrow of the tool is pointing in the direction of the cone axis

vector and the origin of the tool is on the cone axis.

(c)

Once the axes and origin of the tool are aligned, click on

the

Update From Plane Tool

button in

the

Fluid

panel.

FLUENT

will automatically set the

Cone Axis

Vector

and the

Point on Cone Axis

. (Note that you will still have to

set the

Cone Half Angle

yourself.)

2.

Under

Viscous Resistance

, specify the viscous resistance

coefficient

in each direction.

Under

Inertial Resistance

, specify the inertial resistance coefficient

in

each direction. (You will need to scroll down with the scroll bar to view

these inputs.)

For porous media cases containing highly anisotropic inertial resistances,

enable

Alternative Formulation

under

Inertial Resistance

.

The

Alternative Formulation

option provides better stability to the

calculation when your porous medium is anisotropic. The pressure loss

through the medium depends on the magnitude of the velocity vector of

the

i

th component in the medium. Using the formulation of

Equation

7.19-6

yields the expression below:

(7.19-10)

Whether or not you use the

Alternative Formulation

option depends on

how well you can fit your experimentally determined pressure drop data to

the

FLUENT

model. For example, if the flow through the medium is

aligned with the grid in your

FLUENT

model, then it will not make a

difference whether or not you use the formulation.

For more infomation about simulations involving highly anisotropic porous

media, see Section

7.19.8

.

Note

that

the

alternative

formulation

is

compatible

only

with

the

pressure-based solver.

If you are using the

Conical

specification method,

Direction-1

is the cone

axis direction,

Direction-2

is the normal to the cone surface (radial (

)

direction for a cylinder), and

Direction-3

is the circumferential (

)

direction.

In 3D there are three possible categories of coefficients, and in 2D there are

two:

?

?

?

In the isotropic case, the resistance coefficients in all directions are

the same (e.g., a sponge). For an isotropic case, you must explicitly

set the resistance coefficients in each direction to the same value.

When (in 3D) the coefficients in two directions are the same and

those in the third direction are different or (in 2D) the coefficients in

the two directions are different, you must be careful to specify the

coefficients properly for each direction. For example, if you had a

porous region consisting of cylindrical straws with small holes in

them positioned parallel to the flow direction, the flow would pass

easily through the straws, but the flow in the other two directions

(through the small holes) would be very little. If you had a plane of

flat plates perpendicular to the flow direction, the flow would not

pass through them at all; it would instead move in the other two

directions.

In 3D the third possible case is one in which all three coefficients are

different. For example, if the porous region consisted of a plane of

irregularly-spaced objects (e.g., pins), the movement of flow between

the blockages would be different in each direction. You would

therefore need to specify different coefficients in each direction.

Methods for deriving viscous and inertial loss coefficients are described in

the sections that follow.

Deriving Porous Media Inputs Based on Superficial Velocity, Using a

Known Pressure Loss

When you use the porous media model, you must keep in mind that the

porous cells in

FLUENT

are

100% open

, and that the values that you

specify for

and/or

must be based on this assumption. Suppose,

however, that you know how the pressure drop varies with the velocity

through the actual device, which is only partially open to flow. The

following exercise is designed to show you how to compute a value

for

which is appropriate for the

FLUENT

model.

Consider a perforated plate which has 25% area open to flow. The pressure

drop through the plate is known to be 0.5 times the dynamic head in the

plate. The loss factor,

(7.19-11)

, defined as

is therefore 0.5, based on the actual fluid velocity in the plate, i.e., the

velocity through the 25% open area. To compute an appropriate value

for

, note that in the

FLUENT

model:

1.

The velocity through the perforated plate assumes that the plate is 100%

open.

2.

The loss coefficient must be converted into dynamic head loss per unit

length of the porous region.

Noting item 1, the first step is to compute an adjusted loss factor,

which would be based on the velocity of a 100% open area:

(7.19-12)

,

or, noting that for the same flow rate,

,