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General Remarks
===============

In addition to the codes described below, this directory contains
also the file 'caffauns.f', which is identical to 'caffac.f', except
that it contains many detailed comments regarding extension to
unstructured grids. Since there are already several free or low-cost
grid generators for unstructured grids available on internet, we
intend to provide an unstructured version of CAFFA-code in the near
future. Meanwhile, interested readers may find the comments in the file
'caffauns.f' useful if they want to perform this extension themselves.

Another extension that is planed for the near future is the addition
of moving grid and compressibility effects...

M. Peric, July 1999



======================================================================
CAFFA -- Computer-Aided Fluid Flow Analysis; Version 1.5, Sept. 1998
======================================================================


FEATURES
--------

This directory contains grid generator, flow solver, and post-processor
for a single-block, non-orthogonal, boundary-fitted grid. It is based
on the methodology described in Chapter 8. Many comments are included to
make the adaptation and extension easier.

H-type, C-type and O-type grids with the following boundary conditions 
can be generated and used: inlet, outlet, wall, and symmetry. The cuts
in O- and C-grids are sorted in a list like the other boundary faces;
however, these faces are treated as inner cell faces, i.e. the routines 
which calculate surface integrals for the faces of inner CVs are applied 
(see routines FLUXUV and FLUXM, and the lists for inlet and outlet, for 
which these routines are also used). Only the solver needed
a minor modification to update the residuals along cuts; the
treatment is as in the case of parallel computing (see Chapter 11 and
references on parallel computing). When the method is extended to
block-structured grids, block-interfaces can be treated in exactly the
same way; in other words, the flow solver is set for block-structured
grids already -- it only needs another pointer (block ID in addition 
to grid level) and the grid generator has to be extended to generate
grids block by block and search all interfaces (boundary type 10) for
matching pairs. See paper by Lilek et al., An implicit finite-volume
method using non-matching blocks of structured grid, Numer. Heat
Transfer, Part B, vol. 32, pp. 385-401 (1997) for details on the
extension to block-structured grids.

SIMPLE algorithm is used, and the non-orthogonality is neglected in the
pressure-correction equation ('caffa.f'), or taken into account interactively
('caffac.f'). When the non-orthogonality effects are neglected, one has to
use lower values of under-relaxation parameters for pressure;
we suggest to use 0.1 for strongly non-orthogonal grids and
0.2 otherwise for all steady flows. In the case of unsteady flows, the
under-relaxation parameter for pressure may be increased as far as to 1.0,
depending on the time step size (higher values for smaller time steps).
For small time steps, the under-relaxation parameters for the velocity
components may also be increased as far as to 1.0. The non-orthogonality 
effects is (in 'caffac.f') taken into account by performing a second corrector 
step, as described in Sect. 8.8. However, for all but extremely 
non-orthogonal grids (angles below 30 deg. and stretched cells), the 
simplified method will work fine. It may be helpful to solve the pressure-
correction equation more accurately in the case of unsteady flows, i.e.
to reduce the tolerance, say to 0.01, and to increase the allowed number
of inner iterations for p'-equation to 20-30.

In the sub-directory "examples" there are several subdirectories which include 
input files for several examples. Three of them refer to test cases for 
which detailed solutions are presented in the paper by Demirdzic, Lilek and 
Peric: "Fluid Flow and Heat Transfer Test Problems for Non-Orthogonal 
Grids: Bench-Mark Solutions", Int. J. Numer. Methods Fluids, Vol. 15, 
pp. 329-354 (1992). One case is the lid-driven flow in an inclined cavity 
(angle 45 deg.; files in the subdirectory 'cav45l'); the other is 
buoyancy-driven flow in the same cavity (files in 'cav45b'); 
the third is buoyancy-driven flow around a hot cylinder in a square channel 
with cold side walls and adiabatic upper and lower wall (files 
in 'buocyl'). There are also input files for a steady flow around a
foil-like body which requires C-type grid (files in 'foil00')
two sets of files for a flow around a circular cylinder, which
requires O-type grid (one for steady flow, files in 'cylstd',
and one for unsteady flow, files in 'cyluns'), a set of
files for a channel with one 180-degree turn and one 90-degree turn
(files in 'chanel'), and a set of files for a model of a damp diffuser, 
with an axi-symmetric geometry (files in 'damdif'). There are also two 
versions of grid for the flow around a circular cylinder with a splitter 
plate in the wake ('splitt' and 'split1'). These test cases demonstrate 
the applicability of the code to different flow types. 

For each problem, there are input files for grid generator (extension
'gin'), for flow solver (extension 'cin'), and for post-processor
(extension 'pin').

The 'param.ing' file specifies dimension parameters for the grid generator.
If these parameters are large enough, one does not have to recompile the
code to generate different grids (the present version is good for all
test cases; for finer grids, the dimensions may have to be increased --
a message is issued if the dimensions are too small).

The grid generator creates for each grid a new file 'param.inc', which
goes into flow solver. It has to be recompiled for each new grid. The
post-processor is dimensioned for one grid only, and these dimensions
should be big enough for the finest grid. It does not have to be recompiled;
present version is good for all test cases and all grid levels. If
necessary, change the dimensions in the file 'param.inp' and recompile
the file 'plot.f'.

A file 'float.inc' has been included to enable switching between single
and double precission computations. All codes should be compiled using
the same version of this file, since otherwise the format of files
with grid and results data will not be exchangable between the codes.

It would make sense to put all '*.ing', '*.f', and '*.inc' files in a special 
directory, in which also the compiled grid generator and post-processor
should reside. When solving a particular problem, one should create
a directory and copy to it 'user.gen'-file and one of the existing '*.cin' 
files, as these need to be adapted to each new problem (see below).
The modified 'user.gen'-file should be re-named 'user.f'. Upon generating
the grid, one should compile the file 'caffa.f' from the special directory,
using '*.inc' files from the same directory and the 'user.f' and
'param.inc' files from the current directory. Since this arrangement
depends on the operating system being used, this is left to each user
to organize as it best suits her or him.


HOW TO GENERATE THE GRID
------------------------

The grid generation is described for the case requiring C-grid. One has
to first compile the grid generator, i.e. the file 'grid.f'; once this
is done, the grids can be generated for other problems without 
re-compilation, as long as the dimensions set in 'param.ing' are not 
exceeded. If this does happen, a warning is issued.

How to compile the code depends on the compiler used. Here is the
command which works on most Unix or Linux based machines:

   f77 grid.f -o grid

The executable file is named 'grid', so to run the grid generator, one
has to just type:

   grid

The user is first asked to enter the problem name; it has to have six
characters. For running one of the test cases provided with the code,
one has to type the appropriate name as indicated above.

The next question concerns the input of data: the user is asked whether
the input comes from the keyboard or not. When generating a new grid,
data is usually input from keyboard, i.e. interactively. If the answer 
is no, the program opens the file 'name.gin' and reads all input data
from it ('name' is the six-character problem name). That is what happens
when one of the supplied problems are used. When the data is read from
a file, the echo goes to the unnamed file attached to unit 1 ('fort.1'
on most Unix-machines). Since it is identical to the file used to read
the data from, it can be deleted.

In an interactive grid generation session, a descriptive prompt is
printed for each data input required. These are self-explanatory and will
not be described here in detail -- only a short description is given
below; in case of doubt, have a look at the code portion from which the 
prompt is issued -- it should be obvious what it means.

The grid generator allows one to specify for each block side a number of
lines, and to enter for each line:

- coordinates of line begin and end;
- number of segments (cell faces), line type and boundary type
  (line types:     1 - straight; 2 - circle; 3 - arbitrary, specified 
                   point by point.
   boundary types: 1 - inlet; 2 - outlet; 3 - symmetry; 4 - isothermal
                   wall; 5 - adiabatic wall; 10 - O and C cuts.)
- size of the first segment, expansion factor.

This is useful when one block side contains both circular and straight
lines (arbitrary lines are also possible, but all grid points have to
be specified on such a line; no spline interpolation is included yet for 
refinement of such boundary lines). Also, one can cut a single straight or
circle line into several pieces and specify different expansion
factors for different parts. Note that, if zero is specified for both 
segment size and expansion ratio, the line will be uniformly subdivided. 
If the size of the first segment is non-zero, it will be used to calculate 
the value of the expansion factor and the typed value for it is ignored.
In order to specify the expansion factor (or contraction, if the value
is less than unity), one has to type zero for the segment size and
the desired value of the expansion factor. For a line consisting of a
circle segment, one has to specify -- in addition to the line begin and
line end -- the coordinates of a third point on a line (or of the center 
point if the circle is closed). Also, one has to specify the angle (measured 
counterclockwise from the direction of the positive X-axis) corresponding
to line begin and line end. The angles do not have to be exact, since
they are only used to calculate coordinates of the points between 
the end-points, but should be close to the real ones. The angle can
either increase or decrease, and be either positive or negative; it
is only important that the angle variation from line begin to line
end is correct. Thus, for a full circle all these specifications for
FIS (angle at line start) and FIE (angle at line end) are
correct: 0 and 360; 90 and 450; 360 and 0; -90 and 270, etc.
 
On each block side, the lines are sorted in the increasing order of 
the grid index, i.e. from west to east or from south to north. The 
grid generator is meant to run interactively when a new grid is generated,
but it also creates an echo file which can be later edited to improve 
the grid quality or change the fineness. Unfortunately, the user has
to specify the number of CVs in each direction in advance; if these
do not match the total number of segments on all lines on one side, a
warning is issued.

Care is needed when generating C- and O-type grids. The cut-line
has to be specified twice, but the number of segments and their size
must be the same. For O-grids, this is easy to ensure, since the cut-line
appears on the opposite sides of the block -- either once on west and
once on east, or once on south and once on north side. One needs than to
enter the same coordinates for line begin and end, the same number
of segments, and the same expansion factor or the size of the first
segment. In the case of C-grids, the situation is more complicated.
The cut-line appears twice on the same side of the block (e.g. north
in the example 'foil00'), but the beginning and end are interchanged;
also, because the numbering is reversed too, the expansion factor on
one line is the reciprocal value of the expansion factor on the other
line. If the grid points on the two cut-lines do not fall to the same
locations (within some tolerance), the program will not be able to
identify the cell faces along the cut and a warning will be issued
(INTERFACE PAIR NOT FOUND...). If that happens, check the line data
carefully. It is unlikely that the tolerance EPSREL will need to be
modified in the routine OCFIND of 'grid.f', but it is there that faces
are paired.

Depending on the values of logical parameter LPRINT, grid data is or is not
printed in the file 'name.got'. If the input was from keyboard, than an
echo file 'name.gin' is created so that the session can be re-run. This
file can be edited in order to change parameters if desired. Finally, 
the file 'name.grd' is created, to which the grid data needed by the 
flow solver is written. Also, postscript files showing grid of each level
are created; these are named grid1.ps, grid2.ps, grid3.ps, etc. One can 
view them with ghostview or ghostscript or another postscript preview 
program, or one can print them on a postscript printer. The program also
creates the file 'param.inc', which contains proper dimensions of all
variables for the flow solver; since the flow solver may have to run
for a longer time, it makes sense to set the dimensions just right.


HOW TO CALCULATE FLUID FLOW
---------------------------

Once the grid is generated, the flow solver needs to be compiled.
Before this is done, the file 'user.f' needs to be adapted. It contains
routines which specify boundary conditions and printout of specific
quantities. Three example files are provided: 'user.gen' as a generic
case, 'user.cav' that is specific for closed cavity flows, and 'user.cyl',
which is specific to the flow around cylinder (where pressure
and shear stress distribution around cylinder and the total forces
are printed; the total forces are recorded for each time step in a file
'name.to1' for the grid level 1, 'name.to2' for the grid level 2, etc.).
***NOTE: using a wrong file 'user.f' when compiling the programm may
cause a lot of trouble, and this happened to me (M. Peric) many times!!!

Compilation of the flow solver can be done on most machines by typing:

   f77 caffa.f -o caffa

The executable file is named 'caffa'. Before running the code, one has
to prepare the input file, which has to be named 'name.cin' (where
'name' is again the six-character problem name given when generating
the grid). One can use one of the delivered files as an example and edit it
to create the appropriate file for the problem being solved. At the end
of each line the names of variables being set are listed; see routine 
INIT for a description of their meaning. 

Note that, if the version with multiple pressure corrections is used 
(NPCOR > 1), the under-relaxation factors for both velocities and pressure 
can be increased from current settings (which are set for the code 'caffa.f').
For example, one can use 0.8 and 0.2 as under-relaxation factors for
velocity and pressure instead of 0.7 and 0.1 in cases 'cav45l', 'cav45b'
and 'chanel' when using 'caffac.f'. This results in faster convergence.
When the grid is non-smooth, one can set NIGRAD to a value greater than 1,
so that the correction given by Eq. (8.35) is invoked, leading to a higher
accuracy of gradients and convective terms and to smooth pressure contours.

Once the file 'name.cin' is set-up, one can run the flow solver by typing:

   caffa

The user is prompted to enter the problem name; again, this has to be
the six-character name given when generating the grid.

On a Pentium 166 processor, all steady flow cases run (on all grids) in
a minute or so. The result is an output file ('name.out'), from which the 
convergence (or divergence) can be analyzed by looking at residual levels and 
variable values at a monitoring location after each outer iteration;
also, some other information will be printed out, like forces (shear
and pressure) and heat fluxes at walls, as specified in the routines 
SOUT and TOUT of the file 'user.f'. What is now included in samples of
this file is meant to demonstrate how additional information (which is often 
needed for comparison purposes) can be obtained. Note that profiles at
any X- or Y-cross-section can be produced by the post processor.

Other files are also created: some are meant to enable re-start, the others
are for post-processing. Re-start is possible for both steady and
unsteady flow problems. In the case of a steady flow (logical variable
LTIME=F in the input data file, and LWRITE=T), specify the grid level
from which to re-start. If, for example, the specified maximum number
of outer iterations for the third grid ( LSG(3) ) was not sufficient
to reach convergence, but solutions on levels one and two have
converged, just set LREAD=T and specify value 3 for KIN (line
two in the input data file). The solutions from all three grids will
be read in from the result files named 'name.re1', 'name.re2' and
'name.re3', and the computation will be continued on the third grid. 
The file 'name.re3' will in this case be overwritten by the new 
solution (if in doubt whether the new solution will be good, save the
old file under a different name before re-running the code). In
the case of an unsteady flow computation, the results needed for
re-start are written in the same way as for the steady flow problems
but include also the old values (from one but last time step). Again,
one can re-start calculation on any grid -- the computation will continue
in time. Note: if one wants to continue computation in time on the
say second grid level, but does not want to discard previous results
on the finer grids, one should specify zero as the number of iterations
for all finer grids. In that case the execution will stop after the
specified number of time steps is performed on the grid level two.
One can then re-start the computation again, this time specifying the
level 3 from which to re-start. This provides the flexibility in
deciding which simulation should be continued. 

The files which contain results for post-processing are named 'name.x',
where extension 'x' is the data set number. For steady flows, this is 
the grid number (1, 2, 3, etc.). For unsteady flows, data is saved for
each NOTT-th time step. When this is done for the first time, the data
set gets the number 1; each time it is saved again, the data set number
is incremented by one (variable ICONT in 'caffa.f'). Note that, if
unsteady flow is computed on several grids in one run, the results
of coarser grids are over-written. If the results from all grids should
be post-processed, one should first finish simulation on one grid
(set the number of outer iterations on finer grids to zero), than 
re-start computation on that grid for one time step while setting an
appropriate number of iterations on the next finer grid, so that the
results are extrapolated from the coarse grid to initialize the
computation on the next finer grid (if this is not desired, provide
the correct initial values in the routine BCIN of the file 'user.f');
one time step will be calculated on that grid, and the re-start file
will be written. Thereafter, one can re-start the computation on that
grid again by changing the re-start level (KIN) in the input file
('*.cin'), where also the number of time steps to be calculated
should be changed from one to a desired number.

Convergence criterion may need adaptation. For most flows, the residual
levels will be of the order of unity at the beginning so the value of
SORMAX around 0.001 is appropriate to terminate outer iterations.
Should in an application the initial level be much lower, then the
value of SORMAX should also be lowered. The residual levels should
drop by three to four orders of magnitude (except in an unlikely
event that the initial fields already satisfy all equations). Even 
when a solution from one fine grid is used to initiate calculation
on a still finer grid, the residuals jump considerably, although the
variables do not change much. A value of 10^4 is used as SLARGE to
stop unnecessary iterations if the solution process starts diverging;
if the initial residuals happen to be of the order of 10^3, they may
temporarily rise above this value without meaning that divergence is
occurring; just to make sure re-run with SLARGE increased two orders of 
magnitude. The ALFA parameter of the Stone's solver is set at 0.92; 
this is usually good for all problems (although not the optimum value).
Note also that the components of the gravity vector, GRAVX and GRAVY, bare
a sign, depending on the direction of gravity relative to X and Y
coordinate (GRAVY=-9.81 if Y is directed upwards).

When solving unsteady problems, time marching is performed first on the
coarsest grid. If the boundary conditions are steady, the routine BCIN
is called only once at the beginning of the first time step; in case that
the boundary conditions vary in time, the logical switch INIBC should be
set TRUE in the routine BCIN, and the variation of the boundary data
should be programmed there (BCIN is part of the problem-dependent code
contained in the file 'user.f'). When all time steps on one grid
are done, the solution continues on the next grid level with time reset
to zero. However, the latest solution from the preceding grid is 
extrapolated to the next finer grid; this is useful when periodic flows
are studied, like the vortex shedding in the flow around circular
cylinder. In the case that the solution on the refined grid should
start from a specific initial field, this should be programmed in the 
BCIN-routine, so that the extrapolated solution from the coarser grid 
is overwritten. For periodic flows, however, it is helpful to start from
the coarse grid solution since it may take long computation time before
a fully periodic stage is obtained when starting from zero fields (or a
disturbance may be necessary). The file 'cyluns.cin' specifies that
500 time steps are to be run on the first and second grid level only 
(LSG set to zero for the third grid). No true periodic state is obtained
on the first grid; it is too coarse and the central-differencing probably
leads to some disturbances. On the second grid, periodic solution is 
obtained if computations are continued long enough, but the amplitudes of 
oscillations of lift and drag are still too small. On the third grid,
reasonable values are obtained. However, in order to obtain accurate 
Strouhal number and drag and lift coefficients, one needs another 
refinement (the fourth grid with 192 x 128 CV). Also, the outer boundary 
should be displaced further from cylinder, since the boundary conditions 
there are not exact (just repeat the simulation at a doubled distance 
and compare the results -- a nice exercise for a student!). Use the 
procedure described above to continue simulation on finer grids. When 
starting from extrapolated coarser grid solution, the development time 
before the periodic state is achieved will be much shorter on all 
subsequent grids (few oscillation periods will suffice). 

In addition to the field values stored for post-processing, some
additional data may be stored for each time step in the file 'name.to?',
where ? is the grid level. For example, in the case of an unsteady flow
around a circular cylinder, the file 'cyluns.to3' contains time, 
pressure force in X and Y direction and shear force in X and Y direction.
This enables plotting of time variation of drag and lift forces. Any
data written to this file must be prepared in the routine TOUT of the
file 'user.f', which is called at the end of each time step. The data
in this file can be plotted using 'gnuplot'-software, which is available
on each Unix-computer. An example is:

    gnuplot
    plot 'cyluns.to2' using 1:2 with line
    ....

to plot the variation of pressure force on cylinder for the second
grid, connecting points by line.


HOW TO PLOT RESULTS
-------------------

The post-processor (file 'plot.f') needs to be compiled only once (with
sufficiently large dimensions). The procedure is as for other codes, e.g.:

f77 plot.f -o plot

The executable file is named 'plot'. When it is run, the user is asked to
specify problem name first; this is the six-character name given to the
problem under consideration when generating the grid and used for flow
calculation. If one wants to include some reference data (e.g. from
experiments or somebody else's results) in the profiles plotted at given
X=const or Y=const, one has to give the name of the file containing the
reference data. If such data is not used, give some dummy name after
prompted to do so. To see how the reference data should be prepared,
see comments in the program 'plot.f'.

As noted above, during computation of flow, several result files may be
created (one for each grid level in the case of steady flows, or one after
each NOTT time steps in unsteady computations). The code includes a loop
which is passed up to 100 times, unless an earlier exit is forced; each
pass plots data from one data set, and the user is asked to give the data
set number. Therefore, if the results from third grid are to be processed,
enter 3; the file 'name.3' will then be opened.

Prior to executing the post-processor, the file 'name.pin' has to be
set-up; it contains definitions of what is to be plotted. One can also
modify the code to enter this data from keyboard. Use one of existing
files and modify it accordingly. What each entry means is obvious from the
comments given in the code (see file 'plot.f').

The result are postscript files. 'grid?.ps' contains plot of grid on
level '?'; 'isob?.ps' contains isobars, 'pres?.ps' is the color-filled plot
of pressure distribution. 'isot.ps' are isotherms, 'temp?.ps' is the 
color-fill of temperature; finally, 'strl?.ps' are streamlines, 'strc?.ps' 
are color-filled streamlines. For isolines, one can choose whether to use
color or black lines. Which isolines are plotted depends on the control
parameters in the file 'name.pin'. In the case that the zooming option
is used, the whole domain is processed, but shown is only what fits the 
A4 paper size. The bounding box can be edited to a smaller or larger
portion when including the figure in another document (don't forget to
switch the clipping option on in order to get rid of the unwanted region).
An example is shown in the file 'sample.ps', produced by using Latex
(see file 'sample.tex').

One can easily extend the code to plot other fields. For example, one
can easily calculate vorticity from the velocity field (include the routine
'grad(...)' from 'caffa.f' to calculate velocity gradients, and the calculation
of vorticity is trivial), store it in the array FI and just copy the section
of code which plots isotherms to produce isolines and color fill for
the vorticity... One can also adapt it to process measurement data, or to
get three-dimensional plots of a scalar quantity.

The postscript files can be pre-viewed before printing using Ghostscript
or Ghostview (available on all Unix systems). The code can also be easily
adapted to show the figures interactively on the screen (using say X11
graphics); it is left to each user to adapt the source code to her or his
own needs...


CLOSING REMARKS
---------------

A version of this code which incorporates multigrid acceleration of the outer
iterations is included in the directory 'mg'.

Two versions incorporating two turbulence models are included in the directory
'2dgt' (only as single-grid versions).

If absolutely necessary (e.g. a bug is found), contact:
 
peric@schiffbau.uni-hamburg.de 

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Hamburg, September 10, 1998
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