Appendix A; Known Limitations of GENTRA

This entry provides a summary of those limitations of GENTRA known to the GENTRA Development Team at CHAM. The list is not intended to be exhaustive; but the main limitations are nevertheless listed and, where possible, alternatives are suggested.

Most of these limitations will be removed in future versions of the software.


Only right-handed XYZ and IJK systems are allowed.

Two-dimensional BFC grids are treated internally as having a uniform thickness in the third direction equal to XULAST, YVLAST or ZWLAST as appropriate. (Note that this is an INTERNAL arrangement that does not affect the user, other than that trajectories will only appear correct if viewed head-on with no perspective.)


GENTRA cannot be used in combination with the PHOENICS-CHEMKIN interface without user intervention. The reason is that GENTRA operates in SI units, whereas CHEMKIN employs cgs units.


In BFC cases, only cyclic boundaries which are coincident in physical space may be used.


GENTRA will not recognise obstacles that are represented by FIXVALing the velocity components to zero, as is the case at the surface of solids in conjugate heat-transfer problems. However, in such problems a property index field (PRPS) is stored, the value of which denotes the material or fluid in each cell. PRPS values greater than a certain number (99 by default) represent solid materials and GENTRA tests the PRPS field if it is stored to determine the presence of solid obstructions.

This method cannot be applied to locate cell faces which have been blocked by fixing the velocities to zero (and which would therefore represent thin plates).

However, users can represent these obstacles by, in addition to FIXVALling the velocities to 0, using a porosity of 0.999 and alter accordingly the porosity threshold of GENTRA in the BOUNDARY CONDITIONS section. A porosity of 0.999 will then be recognised by GENTRA as an obstacle, while leaving the domain virtually unblocked for the diffusion of the continuous phase.

Fine-Grid Embedding

GENTRA is not compatible with the use Fine-Grid Volume objects.


GENTRA is not compatible with the GCV or CCM forms of BFC, in single or multi-block form.


The out-of-core device of PHOENICS cannot be used with GENTRA.


GENTRA does not work with the parabolic solution-procedure of PHOENICS (PARAB=T).


Particle-to-particle effects (such as particle collision and droplet coalescence) are not considered.


In cylindrical-polar grids, the PHOENICS variable RINNER (which specifies the inner radius of the computational domain) must be 0 if GENTRA is used.

Annular geometries may be represented by specifying RINNER=0.0, and setting the dimension of the first radial cell (IY=1) to be the required inner radius of the annulus.

The cells at IY=1 should then be blocked with porosities (i.e. CONPOR(0.0,CELL,1,NX,1,1,1,NZ)).


Turbulence modulation (the effect of the presence of particles on the continuous-phase turbulence) is not considered.


GENTRA has not yet been used in conjunction with the Eulerian-Eulerian two-phase capabilities of PHOENICS (ONEPHS=F). However, there is no built-in limitation in this respect. (Note that, if used in two-phase mode, phase-1 variables would be used as representing the continuous-phase flow-field.)


Volume-displacement effects (ie, the volume occupied by the disperse phase and therefore unavailable to the continuous phase) are not considered in GENTRA.


GENTRA will automatically detect internal and boundary walls; however, all moving walls will be interpreted as being stationary for the particles.

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