MTM 144
ASPEN
FLUENT
•
Timeschedule
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Program
Lectures
Teaching assistance
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Fluent INTRO
4h
2h
•
GAMBIT
4h
8h
•
FLUENT
6h
14h
•
ASPEN
6h
4h
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http://sirius.mt.luth.se/~lassew/AO1/mtm144/mtm144.htm
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http://www.luth.se/
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Starting Fluent/Gambit from UNIX
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• Logon using your userID and password
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Toggle K+ / non

KDE Apps / X Terminal to get a x term window
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Or toggle on the low bar the display icon to get a x term window
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Write:
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>module add fluent
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•
gambit
This will start Gambit
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Write:
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>module add fluent
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•
fluent
This will start Fluent
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Your work will be saved in your directory so it’s possible to reach the
files also from PC.
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Starting Fluent/Gambit from PC
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• Logon using your userID and password
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Toggle start and All programs
•
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Choose Fluent Inc. products/ Fluent 6.1/ Fluent 6.1
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Toggle start and run
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• Write
gambit
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PS for the moment it’s
NOT
possible to run gambit on PC, workstations
are needed.
1.2 Program Structure
Example
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2 seconds
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6 seconds
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15 seconds
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27 seconds
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50 seconds
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200 seconds
•
100 seconds
1.3 Program Capabilities
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The
FLUENT
solver has the following modeling capabilities:
•
flows in 2D or 3D geometries using unstructured solution

adaptive
triangular/tetrahedral, quadrilateral/hexahedral, or mixed (hybrid) grids
that include prisms (wedges) or pyramids.
(Both conformal and
hanging

node meshes are acceptable.)
•
incompressible or compressible flows
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steady

state or transient analysis
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inviscid, laminar, and turbulent flows
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convective heat transfer, including natural or forced convection
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coupled conduction/convective heat transfer
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radiation heat transfer
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chemical species mixing and reaction, including combustion
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Lagrangian trajectory calculations for a dispersed phase of
particles/droplets/bubbles, including coupling with the continuous phase
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phase

change models
•
flow through porous media
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multiphase flows, including cavitation
These capabilities allow
FLUENT
to be used for a wide variety of
applications, including the following:
•
Process and process equipment applications
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Power generation and oil/gas and environmental applications
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Aerospace and turbomachinery applications
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Automobile applications
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Heat exchanger applications
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Electronics/HVAC/appliances
•
Materials processing applications
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Architectural design and fire research
1.4.1 Planning Your CFD Analysis
When you are planning to solve a problem using
FLUENT
, you
should first give consideration to the following issues:
•
Definition of the Modeling Goals:
What specific results are
required from the CFD model and how will they be used?
What degree of accuracy is required from the model?
•
Choice of the Computational Model:
How will you isolate a
piece of the complete physical system to be modeled? Where
will the computational domain begin and end? What boundary
conditions will be used at the boundaries of the model? Can
the problem be modeled in two dimensions or is a three

dimensional model required? What type of grid topology is
best suited for this problem?
•
Choice of Physical Models:
Is the flow inviscid, laminar, or
turbulent? Is the flow unsteady or steady? Is heat transfer
important? Will you treat the fluid as incompressible or
compressible? Are there other physical models that should be
applied?
•
Determination of the Solution Procedure:
Can the problem
be solved simply, using the default solver formulation and
solution parameters? Can convergence be accelerated with a
more judicious solution procedure? Will the problem fit within
the memory constraints of your computer, including the use of
multigrid? How long will the problem take to converge on your
computer?
Careful consideration of these issues before beginning your CFD
analysis will contribute significantly to the success of your
modeling effort.
•
1.4.2 Problem Solving Steps
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Once you have determined the important features of the problem you want to
solve, you will follow the basic procedural steps shown below.
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1. Create the model geometry and grid.
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2. Start the appropriate solver for 2D or 3D modeling.
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3. Import the grid.
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4. Check the grid.
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5. Select the solver formulation.
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6. Choose the basic equations to be solved: laminar or turbulent (or inviscid),
chemical species or reaction, heat transfer models, etc. Identify additional
models needed: fans, heat exchangers, porous media, etc.
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7. Specify material properties.
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8. Specify the boundary conditions.
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9. Adjust the solution control parameters.
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10. Initialize the flow field.
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11. Calculate a solution.
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12. Examine the results.
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13. Save the results.
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14. If necessary, refine the grid or consider revisions to the numerical or
physical model.
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Step 1 of the solution process requires a geometry modeler and grid
generator. We will use GAMBIT for geometry modeling and grid generation.
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1.5.1 Single

Precision and Double

Precision Solvers
•
Both single

precision and double

precision versions of FLUENT are available
on all computer platforms. For most cases, the single

precision solver will be
sufficiently accurate, but certain types of problems may benefit from the use
of a double

precision version.
Control volume
•
Mesh/grid created in
GAMBIT
Using the conservation laws for mass, momentum and energy for each CV
(stationary conditions)
0
mass into the CV must also exit the CV
dm
dt
( )
momentum can be changed by the action of
forces and it's conservation equation i
s
Newton's second law of motion
stands for time, for mass, for veloci
ty and for forces acting on the f
d m v
f
dt
t m v f
luid in CV
0
energy into the CV must also exit the CV
dE
dt
•
Integration of these equations on the individual control volumes to
construct algebraic equations for the discrete dependent variables
(``unknowns'') such as velocities, pressure, temperature and so on.
•
Linearization of the discretized equations and solution of the resultant
linear equation system to yield updated values of the dependent
variables.
•
FLUENT
provides three different solver formulations:
•
segregated
•
coupled implicit
•
coupled explicit
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The segregated and coupled approaches differ in the way that the
continuity, momentum, and (where appropriate) energy and species
equations are solved: the segregated solver solves these equations
sequentially (i.e., segregated from one another), while the coupled
solver solves them simultaneously (i.e., coupled together). Both
formulations solve the equations for additional scalars (e.g., turbulence
or radiation quantities) sequentially. The implicit and explicit coupled
solvers differ in the way that they linearize the coupled equations.
•
22.1.1 Segregated Solution Method
•
Using this approach, the governing equations are solved sequentially (i.e., segregated from one
another). Because the governing equations are non

linear (and coupled), several iterations of the
solution loop must be performed before a converged solution is obtained. Each iteration consists of
the steps outlined below:
•
1. Fluid properties are updated, based on the current solution. (If the calculation has just begun,
the fluid properties will be updated based on the initialized solution.)
•
2. The
u
,
v
, and
w
momentum equations are each solved in turn using current values for pressure
and face mass fluxes, in order to update the velocity field.
•
3. Since the velocities obtained in Step 2 may not satisfy the continuity equation locally, a
``Poisson

type'' equation for the pressure correction is derived from the continuity equation and the
linearized momentum equations. This pressure correction equation is then solved to obtain the
necessary corrections to the pressure and velocity fields and the face mass fluxes such that
continuity is satisfied.
•
4. Where appropriate, equations for scalars such as turbulence, energy, species, and radiation are
solved using the previously updated values of the other variables.
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5. When interphase coupling is to be included, the source terms in the appropriate continuous
phase equations may be updated with a discrete phase trajectory calculation.
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6. A check for convergence of the equation set is made.
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These steps are continued until the convergence criteria are met.
•
U

velocity is calculated in all CV, then v

velocity is calculated in all CV …..
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22.1.2 Coupled Solution Method
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The coupled solver solves the governing equations of continuity, momentum, and (where
appropriate) energy and species transport simultaneously (i.e., coupled together). Governing
equations for additional scalars will be solved sequentially (i.e., segregated from one another and
from the coupled set.
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1. Fluid properties are updated, based on the current solution.
•
2. The continuity, momentum, and (where appropriate) energy and species equations are solved
simultaneously.
•
3. Where appropriate, equations for scalars such as turbulence and radiation are solved using the
previously updated values of the other variables.
•
4. When interphase coupling is to be included, the source terms in the appropriate continuous
phase equations may be updated with a discrete phase trajectory calculation.
•
5. A check for convergence of the equation set is made.
•
These steps are continued until the convergence criteria are met
.
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SUMMARY
•
The segregated approach
solves for a single variable field (
p
) by considering all cells at the same
time. It then solves for the next variable field by again considering all cells at the same time, and so
on.
•
The coupled implicit
approach solves for all variables (
p
,
u
,
v
,
w
,
T
) in all cells at the same time.
•
The coupled explicit
approach solves for all variables (
p
,
u
,
v
,
w
,
T
) one cell at a time
FLUENT
stores discrete values of the variables at the cell centers (
c
0 and
c
1 in Figure).
However, face values are required for the convection terms and must be interpolated from
the cell center values. This is accomplished using an upwind scheme.
Upwinding means that the face value is derived from quantities in the cell upstream, or
``upwind,'' relative to the direction of the normal velocity.
FLUENT
allows you to choose
from several upwind schemes: first

order upwind, second

order upwind, power law, and
QUICK.
22.2.1 First

Order Upwind Scheme
When first

order accuracy is desired, quantities at cell faces are determined by assuming
that the cell

center values of any field variable represent a cell

average value and hold
throughout the entire cell; the face quantities are identical to the cell quantities. Thus when
first

order upwinding is selected, the face value is set equal to the cell

center value of in the
upstream cell
22.2.3 Second

Order Upwind Scheme
When second

order accuracy is desired, higher

order accuracy is achieved at cell faces
through a Taylor series expansion of the cell

centered solution about the cell centroid.
22.2.7 Under

Relaxation
Because of the nonlinearity of the equation set being solved by
FLUENT
, it is necessary to
control the change of
f
. This is typically achieved by under

relaxation, which reduces the
change of
f
produced during each iteration. In a simple form, the new value of the variable
within a cell depends upon the old value, , the computed change in ,
Df
, and the under

relaxation factor,
a
, as follows:
f
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22.19.1 Judging Convergence
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There are no universal metrics for judging convergence. Residual definitions that are useful for one
class of problem are sometimes misleading for other classes of problems. Therefore it is a good idea to
judge convergence not only by examining residual levels, but also by monitoring relevant integrated
quantities such as drag or heat transfer coefficient.
•
For most problems, the default convergence criterion in
FLUENT
is sufficient. This criterion requires
that the scaled residuals decrease to 10

3 for all equations except the energy and P

1 equations, for
which the criterion is 10

6.
•
At the end of each solver iteration, the residual sum for each of the conserved variables is computed
and stored, thus recording the convergence history. This history is also saved in the data file.
•
The residual Rf computed by
FLUENT
's segregated solver is the imbalance summed over all the
computational cells
P
. This is referred to as the ``unscaled'' residual
•
In general, it is difficult to judge convergence by examining the residuals.
•
FLUENT
scales the residual using a scaling factor representative of the flow rate of
f
through the
domain.
•
For the continuity equation, the unscaled residual for the segregated solver is defined as
•
The segregated solver's scaled residual for the continuity equation is defined as
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The denominator is the largest absolute value of the continuity residual in the first five iterations.
•
22.19.2 Step

by

Step Solution Processes
•
One important technique for speeding
convergence for complex problems is to tackle the
problem one step at a time. When modeling a
problem with heat transfer, you can begin with the
calculation of the isothermal flow. To solve
turbulent flow, you might start with the calculation
of laminar flow. When modeling a reacting flow,
you can begin by computing a partially converged
solution to the non

reacting flow, possibly
including the species mixing. When modeling a
discrete phase, such as fuel evaporating from
droplets, it is a good idea to solve the gas

phase
flow field first. Such solutions generally serve as a
good starting point for the calculation of the more
complex problems. These step

by

step
techniques involve using the
Solution Controls
panel to turn equations on and off.
Example 1
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