4.141 FLUID141 2-D Fluid-Thermal

4.141 FLUID141 2-D Fluid-Thermal (UP19980821 ) You can use FLUID141 to model transient or steady state fluid/thermal systems that involve fluid and/or non-fluid regions. The conservation equations for viscous fluid flow and energy are solved in the fluid region, while only the energy equation is solved in the non-fluid region. Use this FLOTRAN CFD element to solve for flow and temperature distributions within a region, as opposed to elements that model a network of one-dimensional regions hooked together (such as FLUID66).

For the FLOTRAN CFD elements, the velocities are obtained from the conservation of momentum principle, and the pressure is obtained from the conservation of mass principle. (The temperature, if required, is obtained from the law of conservation of energy.) A segregated sequential solver algorithm is used; that is, the matrix system derived from the finite element discretization of the governing equation for each degree of freedom is solved separately. The flow problem is nonlinear and the governing equations are coupled together. The sequential solution of all the governing equations, combined with the update of any temperature- or pressure-dependent properties, constitutes a global iteration. The number of global iterations required to achieve a converged solution may vary considerably, depending on the size and stability of the problem. Transport equations are solved for the mass fractions of up to six species.

You may solve the system of equations in a constant angular velocity rotating coordinate system. The degrees of freedom are velocities, pressure, and temperature. Two turbulence quantities, the turbulent kinetic energy and the turbulent kinetic energy dissipation rate, are calculated if you invoke an optional turbulence model. For axisymmetric models, you can calculate an optional swirl - velocity VZ normal to the plane. You also can specify swirl at the inlet or a boundary (moving wall).

Figure 4.141-1 FLUID141 2-D Fluid-Thermal Element



4.141.1 Input Data

Figure 4.141-1 shows the geometry, node locations, and the coordinate system for this element. The element is defined by three nodes (triangle) or four nodes (quadrilateral) and by isotropic material properties. The coordinate system is selected according to the value of KEYOPT(3), and may be either Cartesian, axisymmetric, or polar.

Section 2.7 describes element loads. The ANSYS CFD FLOTRAN Analysis Guide includes a discussion of which ANSYS commands are unavailable or inappropriate for FLUID141.

4.141.1.1 Fluid Elements

If the material number [MAT] of a FLUID141 element is 1, it is assumed to be a fluid element. Its properties - density, viscosity, thermal conductivity and specific heat - are defined with a series of FLDATA commands. You can analyze only one fluid, and it must be in a single phase. Thermal conductivity and specific heat are relevant (and necessary) only if the problem is thermal in nature. The properties can be a function of temperature through relationships specified by the FLDATA7,PROT command or through a property database (the file floprp.ans). In addition, the density may vary with pressure (per the ideal gas law) if the fluid is specified to be air or a gas.

Six turbulence models are available. You can activate turbulence modeling with the FLDATA24,SOLU,TURB,T command. The Standard k-e Model and the Zero Equation Turbulence Model are available along with four extensions of the Standard k-e Model. See the ANSYS Theory Reference and the ANSYS CFD FLOTRAN Analysis Guide for more information on the models.

KEYOPT(1) activates multiple species transport, which allows you to track the transport of up to to six different fluids (species) in the main fluid.

Real constants, shown in Table 4.141-1a, are required only if a distributed resistance (Section 4.141.1.2) or a fan model (Section 4.141.1.3) is to be included.

4.141.1.2 Distributed Resistance

A distributed resistance provides a convenient way to approximate the effect of porous media (such as a filter) or other such flow domain features without actually modeling the geometry of those features. It is an artificially imposed, unrecoverable loss associated with geometry not explicitly modeled. Any fluid element with a distributed resistance will have a real constant set number [REAL] greater than 1 assigned to it.

The resistance to flow, modeled as a distributed resistance, may be due to one or a combination of these factors: a localized head loss (K), a friction factor (f), or a permeability (C). The total pressure gradient is the sum of these three terms, as shown below for the X direction.

where:

= is the density (mass/length3)

= is the viscosity (mass/(length*time))

RE = is the local value of the Reynolds Number (calculated by the program): RE = ( V Dh) /

f = is a friction coefficient (calculated by the program): f = a RE-b

If large gradients exist in the velocity field within a distributed resistance region, you should deactivate the turbulence model by setting ENKE to 0 and ENDS to 1.0 in this region.

Non-Newtonian viscosity models also are available for this element. Currently, ANSYS provides a Power Law model, a Bingham model, and a Carreau model.

In addition, ANSYS provides a user-definable subroutine to compute viscosity. The ANSYS Theory Reference and the ANSYS CFD FLOTRAN Analysis Guide describe these models and how to use them. The subroutine, called UserVisLaw, is documented in the Guide to ANSYS User Programmable Features.

4.141.1.3 Fan Model

The fan model provides a convenient way to approximate the effect of a fan or pump in the flow domain. It is an artificially imposed momentum source that provides momentum source terms associated with a fan or a pump not explicitly modeled.

The pressure rise associated with a fan model is given by the pressure gradient times the flow length through the elements with the fan model real constants. For a one-directional fan model, (real constant TYPE=4), three coefficients are input. The pressure gradient can be treated as a quadratic function of velocity, as shown below for the X direction.

V is the fluid velocity and C1, C2, and C3 are the coefficients specified as real constants. For an arbitrary direction fan model (real constant TYPE=5), the three coefficients are the components of the actual coefficients along a coordinate direction. See also the ANSYS CFD FLOTRAN Analysis Guide.

4.141.1.4 Non-Fluid Elements

If the material number [MAT] of the element is greater than 1, it is assumed to be a non-fluid element. Only the energy equation is solved in the non-fluid elements. You can define up to 100 different non-fluid materials. To specify density, specific heat, and thermal conductivity for the non-fluid elements, use the MP command. Temperature variation of the non-fluid properties is permitted, and you specify it via the MP or MPDATA commands. Orthotropic variation also is permitted, with the restriction that the spatial variation is always with respect to the global coordinate system. Note that element real constants have no meaning for non-fluid FLUID141 elements.

Table 4.141-1 summarizes the element input. Section 2.1 gives a general description of element input.

Table 4.141-1 FLUID141 Input Summary

Element Name

FLUID141

Nodes

I, J, K, L

Degrees of Freedom

VX, VY, VZ, PRES, TEMP, ENKE, ENDS

Real Constants

See Table 4.141-1a

Material Properties

Non-fluid: KXX, KYY, C, DENS
Fluid: Density, viscosity, thermal conductivity, specific heat (use FLDATA commands) or MPTEMP and MPDATA.

Surface Loads

HFLUX, CONV, RAD

Body Loads

HGEN, FORC

Special Features

Nonlinear,
Six turbulence models
Incompressible or compressible algorithm
Transient or steady state algorithm
Rotating or stationary coordinate system
Algebraic solvers particular to FLOTRAN
Optional distributed resistance and fan models
Multiple species transport.

KEYOPT(1)

Activates multiple species transport.
0 - Species transport is not activated.
2-6 Number of species transport equations to be solved.

KEYOPT(3)

0 - Cartesian coordinates (default)
1 - Axisymmetric about Y axis
2 - Axisymmetric about X axis
3 - Polar Coordinates


Table 4.141-1a Real constants for FLUID141

Number

Name

Meaning

Units

R1 TYPE

Type of distributed resistance or fan model:

1 = Distributed resistance: isotropic
2 = Distributed resistance: one-directional
3 = Distributed resistance: direction-dependent
4 = Fan model: aligned with a coordinate axis
5 = Fan model: arbitrary direction




-
R2 (Blank)

DIR

(Blank)

for TYPE=1,2,3 - Not used

for TYPE=4 - Fan orientation: 1=X, 2=Y, 3=Z

for TYPE=5 - Not Used


-
R3 K

Kx

C1

C1x

for TYPE=1,2 - Dimensionless head loss / length

for TYPE=3 - Head loss in X direction

for TYPE=4 - Constant term

for TYPE=5 - Vector component of C1 in X dir.

1/L 1/L M/L2t2 M/L2t2
R4 C

Cx

C2

C2x

for TYPE=1,2 - Permeability

for TYPE=3 - Permeability in X direction

for TYPE=4 - Linear coefficient

for TYPE=5 - Vector component of C2 in X dir.

1/L2 1/L2 M/L3t M/L3t
R5 Dh

Dhx

C3

C3x

for TYPE=1,2 - Hydraulic diameter

for TYPE=3 - Hydraulic diameter for X direction

for TYPE=4 - Quadratic coefficient

for TYPE=5 - Vector component of C3 in X dir.

L L M/L4 M/L4
R6 a

ax

(Blank)

for TYPE=1,2 - Coefficient of Reynold's number, used in friction factor calculations

for TYPE=3 - Coefficient a in X direction

for TYPE=4,5 - Not Used



-
R7 b

bx

(Blank)

for TYPE=1,2 - Exponent of Reynold's number, used in friction factor calculations

for TYPE=3 - Exponent b in X direction

for TYPE=4,5 - Not Used



-
R8 (Blank)

FLDIR

Ky

(Blank)

C1y

for TYPE=1 - Not Used

for TYPE=2 - Flow direction: 1=X, 2=Y, 3=Z

for TYPE=3 - Head loss in Y direction

for TYPE=4 - Not Used

for TYPE=5 - Vector component of C1 in Y dir.

- - 1/L - M/L2t2
R9 (Blank)

Cy

(Blank)

C2y

for TYPE=1,2 - Not Used

for TYPE=3 - Permeability in Y direction

for TYPE=4 - Not Used

for TYPE=5 - Vector component of C2 in Y dir.

- 1/L2 - M/L3t
R10 (Blank)

Dhy

(Blank)

C3y

for TYPE=1,2 - Not Used

for TYPE=3 - Hydraulic diameter in Y direction

for TYPE=4 - Not Used

for TYPE=5 - Vector component of C3 in Y dir.

- L - M/L4
R11 (Blank)

ay

(Blank)

for TYPE=1,2 - Not Used

for TYPE=3 - Coeff. of Reynold's no. in Y dir.

for TYPE=4,5 - Not Used



-
R12 (Blank)

by

(Blank)

for TYPE=1,2 - Not Used

for TYPE=3 - Exponent of Reynold's no. in Y dir.

for TYPE=4,5 - Not Used



-
R13 (Blank)

Kz

(Blank)

C1z

for TYPE=1,2 - Not Used

for TYPE=3 - Head loss in Z (swirl) direction

for TYPE=4 - Not Used

for TYPE=5 - Vector comp. of C1 in Z (swirl) dir.

- 1/L - M/L2t2
R14 (Blank)

Cz

(Blank)

C2z

for TYPE=1,2 - Not Used

for TYPE=3 - Permeability in Z (swirl) direction

for TYPE=4 - Not Used

for TYPE=5 - Vector comp. of C2 in Z (swirl) dir.

- 1/L2 - M/L3t
R15 (Blank)

Dhz

(Blank)

C3z

for TYPE=1,2 - Not Used

for TYPE=3 - Hydraulic diameter in Z (swirl) dir.

for TYPE=4 - Not Used

for TYPE=5 - Vector comp. of C3 in Z (swirl) dir.

- L - M/L4
R16 (Blank)

az

(Blank)

for TYPE=1,2 - Not Used

for TYPE=3 - Coeff. of Reynold's no. in Z (swirl) direction

for TYPE=4,5 - Not Used



-
R17 (Blank)

bz

(Blank)

for TYPE=1,2 - Not Used

for TYPE=3 - Exponent of Reynold's no. in Z (swirl) direction

for TYPE=4,5 - Not Used



-

4.141.2 Output Data

The solution output associated with the element takes the form of nodal quantities. Additional intermediate properties and derived quantities supplement the degrees of freedom. See the ANSYS Basic Analysis Procedures Guide for ways to view results.

Table 4.141-2 describes quantities that are output on a nodal basis. Some quantities are not output if the relevant options are not activated. Once an option is used, the relevant DOF quantities are always stored. For example, if a temperature field has been obtained and upon restart the energy equation is no longer to be solved, the temperatures are stored anyway. You control the storage of derived properties such as effective viscosity by issuing the FLDATA5,OUTP command.

The Jobname.PFL file provides additional output. This file contains periodic tabulations of the maximum, minimum, and average values of the velocities, pressure, temperature, turbulence quantities, and properties. The file also records the convergence monitoring parameters calculated at every global iteration. The Jobname.PFL file also tabulates the mass flow at all the inlets and outlets and the heat transfer information at all the boundaries.

A wall results file (Jobname.RSW) contains information associated with the boundary faces of wall elements. Average pressure, temperature, shear stress, Y-plus values and wall heat fluxes are stored, along with vectors denoting the normal direction from the surface (Normal Vector) and the direction of the velocity immediately adjacent to the wall (Tangent Vector).

An optional residual file (Jobname.RDF) shows how well the current solution satisfies the implied matrix equations for each DOF.

Table 4.141-2 uses the following notation:

A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL] (see Section 2.2.2). The R columns indicate the availability of the items in the results file (R), a Y indicates that the item is always available, a number refers to a table footnote which describes when the item is conditionally available, and a - indicates that the item is not available.

Table 4.141-2 FLUID141 Element Output Definitions

Name

Definition

R

VX:

Velocity in the X direction (Cartesian coordinates)

Velocity in the radial direction (Polar coordinates)

Velocity along axis of symmetry (Axisymmetric about X)

Velocity in the radial direction (Axisymmetric about Y)

Y
VY:

Velocity in the Y direction (Cartesian coordinates)

Velocity in the tangential direction (Polar coordinates)

Velocity in the radial direction (Axisymmetric about X)

Velocity along the axis of symmetry (Axisymmetric about Y)

Y
VZ:

Velocity in the swirl direction (Axisymmetric problems)

8
PRES:

Relative Pressure

Y
ENKE:

Turbulent kinetic energy

2
ENDS:

Turbulence dissipation rate

2
TEMP:

Temperature

1
DENS:

Nodal fluid density

8
VISC:

Nodal fluid viscosity

8
COND:

Nodal fluid thermal conductivity

8
SPHT:

Nodal fluid specific heat

8
EVIS:

Effective viscosity (includes effects of turbulence)

8
ECON:

Effective thermal conductivity (includes the effects of turbulence)

2
CMUV:

Turbulent viscosity coefficient

2
TTOT:

Stagnation (Total) Temperature (Only relevant to compressible analyses)

7
HFLU:

Heat Flux at external surface nodes (per unit area)

1
HFLM:

Heat Transfer (film) coefficient at external surface nodes

1
STRM:

Stream Function (2-D)

Y
MACH:

Mach Number (must be requested if incompressible)

6
PTOT:

Stagnation (Total) Pressure

Y
PCOE:

Pressure Coefficient

3
YPLU:

Y+ a turbulent law of the wall parameter

3
TAUW:

Shear Stress at the wall

3
SP0n:

Mass fraction of species n, where n = 1 to 6 (FLOTRAN). If a species is given a user-defined name [MSSPEC], use that name instead of SP0n.

4
LMDn:

Laminar mass diffusion coefficient for species n, where n = 1 to 6.
(Not relevant unless species defined.)

3
EMDn:

Effective mass diffusion coefficient for species n, where n = 1 to 6.
(Not relevant unless species defined.)

2
1. Available if thermal is on.

2. Available if turbulence is on.

3. Must be requested.

4. Available if species defined.

5. Available if property is variable.

6. Available if compressible.

7. Available if compressible and thermal.

8. Available if swirl is turned on.

9. For solid material elements in FLOTRAN, when nodes are connected only to solid nodes, the column for the density (DENS) label within the Jobname.RFL results file, stores the product of the solid material's density and its specific heat.

4.141.3 Assumptions and Restrictions

The element must not have a negative or a zero area. You must define the connectivity of an element with the nodes in counterclockwise order. The element must lie in the X-Y plane. When triangles are formed by duplicating the third node, the FLOTRAN element will ignore the duplicate node and treat nodes I, J, and K. Only linear elements are supported.

You cannot use FLUID141 with any other ANSYS elements. Not all ANSYS commands are relevant to the use of FLUID141. A section in the ANSYS CFD FLOTRAN Analysis Guide documents these command usage restrictions.

FLOTRAN CFD analyses are highly nonlinear. In some cases, convergence is difficult to achieve and requires the use of stability and relaxation parameters. Highly turbulent cases may benefit from preconditioning (the initialization of the flow field with a laminar analysis), particularly if a coarse finite element mesh is being used.

You must determine if use of the turbulence and/or compressible option is warranted. The turbulence option requires a fine mesh near the walls and a fine mesh is recommended near any regions where shock waves occur. If the larger gradients occur in regions with the coarsest mesh, rerun the problems with adjusted meshes.

The following assumptions have been made in the formulation:

4.141.4 Product Restrictions

There are no product-specific restrictions for this element.