ShapeMaker: A VR-object creation package

ShapeMaker: a creator of VR objects

What Shape-Maker does
How to use it
Future developments

1. What Shape-Maker does

A fundamental problem

Users of the PHOENICS Virtual-Reality Editor are familiar with the fact that the shapes of objects supplied by CHAM is extremely limited in number, as the following image reveals:

Although this displays only one of several supplied folders, the fact remains that the attempt to provide enough shapes by way of facet-describing .dat files is in principle hopeless; for, all that the VR-Editor can do after importing them is to change the dimensions of the rectangular box which contains them. However, if one of the objects were a hollow cylinder, for example, the ratio of the internal to external diameter would remain the same, no matter how much the object was stretched and squeeezed.

As a consequence, users have in the past been advised to combine two of the available objects in order to create a not-available third. Hollow cylinders, for example, have been created by combining a first solid cylinder with a coaxial second one, of smaller diameter, of which the material is domain fluid.

That many users have been successful in doing so testifies to their admirable ingenuity; but that they were required to exercise it was not something in which CHAM could take pride.

The ShapeMaker solution

CHAM therefore now provides the Shapemaker program which enables users to create objects having the extremely wide range of shapes of which the following image lists about one quarter.

Where, it might be asked, is the previously-mentioned hollow cylinder? Hidden, is the answer, within the class of objects named 'hollow frustrum sector'; for these comprise all solids having a conical exterior surfac, a coaxial conical inner surface, and other faces exposed by two perpendicular-to-axis and two passing-through-axis cutting planes. The following image illustrates what is meant.

Obviously, a hollow cylinder will be created if the inner and outer radii are the same at the top and the bottom, and if the sector angle is made equal to 360 degrees instead of the 120 degrees shown above. These parameters can be chosen interactively by way of menus such as the following:

2. How to use ShapeMaker

Creating objects in advance of use

Users who require very few new objects, for example hollow cylinders only, with no more than 5 discrete diameter ratios, may be content to create these, once-for-all; to store the resulting .dat files in the directories which CHAM has provided; and then to import them into their simulation scenarios in the usual way.

Cites designed by not-very-imaginative architects might need only a few of the 'nonahedron' class of object, seen below:

all with the same roof angle and differing only by way of a few length-to-width ratios. Their CFD specialists too may then adopt the prepare-in-advance strategy.

Creating objects when they are needed

Most users however will wish to be able to decide upon the diameter ratio, roof angle and width-to-height ratio, or the other parameters controllng their shapes of choice, without pre-ordained restrictions. These users must create the objects they want as and when they need them.

There are two ways of so doing:

interactively, and
via command files.

First- or single-instance object creation

If an object is to be created for the first time, or possibly for only one use, the interactive method is preferable; for then the user can see immediately what object is being created, and decide there and then whether what is displayed is what is desired. Activation is effected by way of the shap.bat batch file; and all the appropriate menus then appear which allow the choices to be made.

ShapeMaker can also be activated from within the PHOENICS VR-Editor.

This interactivity necessarily entails specialist-man-time expense; which can not be tolerated when many runs are to be conducted with objects of the same class; and conducted, as is often the case, by persons who are not CFD specialists. In the majority of cases therefore the command-file method must be adopted.

Parameterised-object creation by command

Suppose that it is desired to optimise the design of secondary-surface fins, to be attached to the outsides of tubes in a shell-and-tube heat exchanger. The number of geometrical parameters is not less than 10; and the thought that a human being should be involved in systematically selecting one set after another is absurd. It is arduous work even for computers. But ShapeMaker allows them to do it.

It can to do so because it describes an object in two distinct ways:

firstly via a .dat file which is used by the VR-Editor to display it graphically, and by the PHOENICS 'Earth' solver module to determine which cells are cut by the solid; and
secondly via a (usually much smaller) .geo file which records all the parameters which define the object and its .dat representation.

Moreover, whereas a .dat file is only written, a .geo file can be read also. Indeed the command:

shap fin.geo fin.dat will cause ShapeMaker to:

read from fin.geo the parameters describing the geometry of a fin and then
write a corresponding fin.dat file which expresses its shape by way of facets.

But who or what writes the .geo file? A parameterised Q1 can do so, as will now be described.

How PQ1 writes .geo

PHOENICS Input File Library case V???.htm is a parameterised Q1 devoted to flow in finned-tube banks and it contains the following lines:

mesg(about to write fin.geo
write(>fin.geo,shape    compsfrstr)
write(>>fin.geo,height  :ftkn1:)
write(>>fin.geo,heigh2  :ftkn2:)
write(>>fin.geo,heigh3  :ftkn3:)
write(>>fin.geo,borad   :frou1:)
write(>>fin.geo,birad   :fradin:)
write(>>fin.geo,torad   :frou1:)
write(>>fin.geo,tirad   :fradin:)
write(>>fin.geo,tord2   :frou2:)
write(>>fin.geo,tird2   :fradin:)
write(>>fin.geo,tord3   :frou2:)
write(>>fin.geo,tird3   :fradin:)
write(>>fin.geo,f2pi    0.5)

These, when processed by the PHOENICS Satellite module, cause it to write the following file, fin.geo

shape    compsfrstr
height  7.5E-04
heigh2  7.5E-04
heigh3  1.5E-03
borad   0.035
birad   0.017
torad   0.035
tirad   0.017
tord2   0.02
tird2   0.017
tord3   0.02
tird3   0.017
f2pi    0.5

This is a file which ShapeMaker understands, the first line indicating that the required object is of ShapeMaker's composite-frustrum kind shown below:

and the subsequent lines indicating what (non-default) parameter values it should use.

The numbers are the evaluations of the quantities :ftkn1: etc which must have been declared in the PQ1, their values having been assigned or calculated higher up in that file.

Immediately below the 'WRITE' instruction in the PQ1 there appears the single line:

EXEC(t,shap.bat,fin.geo,fin.dat)

which causes ShapeMaker to read the fin.geo file and write the fin.dat one. The VR Editor can then reveal what has been created thus:

The use of the command mode for creating facetted objects is thus extremely simple; and it allows long sequences of runs to be made, in each of which the shape of the object has been slightly changed, without any human intervention.

How PQ1 places objects

VR-Editor users who are accustomed to having manually to move imported objects into position, and then to resize them, may ask how user intervention can have been totally avoided in this case. The answer is that the writer of the parameterised Q1 had already decided where to place the fin and what were its dimensions; the relevant lines in his Q1 were therefore:

xpos=xaxis-frou1
ypos=0.
zpos=0.> OBJ,    NAME,        FIN
> OBJ,    POSITION,    :xpos:, :ypos:, :zpos:
> OBJ,    SIZE,        :fdmou:, :frou1:, :ftkns:
> OBJ,    GEOMETRY,    FIN

in which the values of the parameters fdmour, etc had already been assigned. Of course these values can vary from run to run as the formulae in the PQ1 dictate.

Placing and sizing by way of ShapeMaker itself.

However, an even simpler method could have been used.. When any composite-frustrum .geo file is created by ShapeMaker, only the first lines in it contain parameters which are specific to its class, as follows:

(c) shape   compsfrstr     Composite frustrum         
height  0.100000       height                                            
heigh2 0.200000       2nd height                                        
heigh3 0.100000       3rd height                                        
borad   1.000000       bottom outside radius                             
birad   0.100000       bottom inside radius                              
torad   0.900000       top outside radius                                
tirad   0.150000       top inside radius                                 
tord2   0.300000       top 2nd outside radius                            
tird2   0.100000       top 2nd inside radius                             
tord3   0.200000       top 3rd outside radius                            
tird3   0.150000       top 3rd inside radius                             
f2pi    1.000000       fraction of 2 pi, (0-1)

But there follow settings which are supplied for all classes of objects; and these include sizes, positions, rotations, etc

nfacets 24             number of facets                                  
cartes  1              1 for cartesian or 0 for polar                    
opaque  1              1 for opaque or -1 for transparent facets         
nxcopy  1              X number of copies of arrayed shape               
nycopy  1              Y number of copies of arrayed shape               
xarray  0.000000       shift in X direction of arrayed object            
yarray  0.000000       shift in Y direction of arrayed object            
xsize   1.000000       X size of placed object                           
ysize   1.000000       Y size of placed object                           
zsize   1.000000       Z size of placed object                           
xpos    0.000000       X position of placed object                       
ypos    0.000000       Y position of placed object                       
zpos    0.000000       Z position of placed object                       
xrot    0.000000       rotation angle about X                            
yrot    0.000000       rotation angle about Y                            
zrot    0.000000       rotation angle about Z                            
tmprt   0.000000       temperature of object                             
initmp  0.000000       initial temperature of object                     
prpno   0.000000       material-property index

The above are the default values, which the author of Library case v??? was evidently content to accept; but he could have simplifed his task by transmiting his size and position decisions by way of his .geo file. Corresponding >OBJ.position and >OBJ,size lines would then be placed in the .dat file; and those in the PQ1 would be de-activated.

3. Future developments

The command mode of using ShapeMaker is comparatively new, having been made possible by changes during 2012 within ShapeMaker itself and within the Satellite, the latter being the WRITE and EXEC features. It is however expected to become the most-commonly-used mode because of the great extension of capability it affords to designers and users of parameterised Q1s.

Further developments now in progress or under consideration include:

Showing how PQ1s can also write .dat files directly, when the number of facets is few;
Increasing the number of commonly-required objects which ShapeMaker can create;
Introducing into ShapeMaker object-combination possibilities by way of the geometrical equivalents of the logical operators:
- .OR. which defines a new shape by claiming as its own all that lies within some other defined shapes;
- .AND. which defines as its space only that which lies within each of the other defined shapes; and
- .AND..NOT. which excludes from its space that which lies within one or more other shapes.
Enabling Shapemaker to export to the PHOENICS solver module files containing grid-cell-edge intersections, so relieving EARTH of the necessity to deduce these, less accurately from the facet descriptions in the .dat files.

In summary, the switch of attention from the single-instance VR-Editor approach to multi-instance PHOENICS-Direct has received a significant boost from enhancements to ShapeMaker; and in particular its ability to operate in command mode.