1.Introduction

1. What GENTRA is and does

   GENTRA is a software "add-on" for the PHOENICS suite of CFD programs which provides particle-tracking facilities. The name GENTRA stands for GENeral TRAcker.

   GENTRA can therefore simulate the motion of particles (the "particulate or disperse phase") through a fluid (the "continuous phase"). GENTRA takes into account the effect of the fluid velocity, temperature, turbulence, etc on the particles; and, conversely, the effect of the presence of particles on the continuous phase is also considered.

   The data specification and solution procedure for the continuous phase and the particles are carried out by separate modules:

   • PHOENICS is used to specify the data pertaining to the continuous phase. This can be done using any of the means available in PHOENICS (ie, PIL commands, a library case or a menu). The continuous phase thus specified is then solved by EARTH in the usual way.
   • GENTRA is used to specify the data corresponding to the particulate phase. This can be done using the GENTRA menu (a part of the general PHOENICS-VR Main Menu which writes the GENTRA data at the end of the Q1 data-file), or by using in the Q1 file a subset of PIL variables specific to GENTRA. The particulate phase thus specified is then solved by GENTRA.

   Of course, the solution procedures for the continuous phase and particles have to interact. Details of such interaction can be found in this manual in Section 1.4.2.

   It should be pointed out that PHOENICS has already a "built-in" two-phase capability, which uses Eulerian transport equations to represent the two interacting phases (Spalding, 1980). Similarly to single-phase flows, these equations are solved by discretising the space into computational cells, by integrating the equation over each cell (thus obtaining an algebraic equation) and by solving the resulting system of algebraic equations.

   The approach of GENTRA is a different one: while the continuous phase is treated as above, the particles are represented by Lagrangian equations, which are integrated to yield the particle trajectory and the particle properties along this trajectory. This approach is based on the PSI-Cell method of Crowe, Sharma and Stock (1977).

   The standard two-phase PHOENICS can therefore simulate particulate flows; however, the GENTRA approach should be preferred by users who:-

   • Need detailed information on the particle trajectory (eg, because particle impingement on obstacles has to be accurately predicted).
   • Need to simulate accurately particle-obstacle interactions (such as bouncing, sticking or flash vaporisation).
   • Have to consider simultaneously a range of particle sizes, temperatures, densities etc. (In these circumstances, the two-phase approach of PHOENICS would yield an average value for each particle property without any distribution information).
   • Want to avoid numerical diffusion in the particulate phase, to which the Eulerian two-phase approach is prone, and of which the Lagrangian approach of GENTRA is free.

2. Features of GENTRA

The salient features of GENTRA are:-

(a) In respect of pre-processing:-

• Three alternative user interfaces for data input (menu, command language and FORTRAN);
• Full on-line help facility in the menu;
• Library of examples and test cases.

(b) In respect of modelling features:-

• GENTRA can track particles in all the co-ordinate systems supported by PHOENICS (i.e., Cartesian, cylindrical-polar and general curvilinear (BFC)).
• GENTRA can solve both steady and transient problems.
• GENTRA can model particles of six kinds:

(a) "Tracers" (or "lazy particles"), which simply follow the continuous-phase flow-field;

(b) "Beams" (or "stubborn particles"), which have a constant velocity, regardless of the continuous-phase flow field;

(c) Particles in isothermal flow;

(d) Particles experiencing heat exchange with the continuous phase;

(e) Melting/solidifying particles;

(f) Vaporising droplets.

• GENTRA will automatically detect internal walls and obstacles, and provides options for:

(a) Particle bouncing, with a user-specified restitution-coefficient;

(b) Particle adhesion to the wall;

(c) Withdrawal of the particle;

(d) Flash vaporisation of the particle (if the particle is a droplet).

(c) In respect of post-processing:

• GENTRA can generate input files for the PHOENICS post-processors VR-Viewer, PHOTON and Autoplot:
• For VR-Viewer and PHOTON, it generates "trajectory" files, which record the trajectory of the particle as it moves through the domain.
• For Autoplot, it generates "history files", which record the evolution of the particle properties with time.

3. Limitations of GENTRA

As well as capabilities, GENTRA has also limitations. Appendix A of this guide provides a list of the main limitations known to the GENTRA Development Team at CHAM. The contents of the list changes as known limitations are removed, and new ones found. An updated list of the limitations affecting your version of GENTRA (if different from the one described in this manual) is available through the GENTRA Input Menu (Help and information panel). See Section 2.4 of this Guide for details.

4. How GENTRA fits in

Section 1.1 above classified GENTRA as a PHOENICS "add-on’’. The present section describes in more detail how GENTRA is related to the rest of PHOENICS.

GENTRA has a pre-processing and a processing part, which are dealt with in the following sub-sections.

1. Pre-processing

The pre-processing part involves the preparation of the GENTRA input, which consists of particle data, solution-control data and output-control data. It can be accomplished in several alternative ways:

(a) By using the GENTRA Input Menu;

(b) By using a set of special PIL commands (the GENTRA PIL);

(c) By loading a case from the GENTRA Input Library.

The pre-processing side of GENTRA uses the general PHOENICS VR environment.

For the benefit of experienced PHOENICS users, it will be pointed out here that all the GENTRA information is sent from the Q1 file to EARTH through the "transfer" arrays RG, IG, LG, and CG.

2. Processing

The processing part of GENTRA takes the form of a collection of FORTRAN subroutines, which are attached to the PHOENICS flow-computing program, EARTH.

GENTRA, which solves the equations for the disperse phase, is called by PHOENICS between the "sweeps" of the computational domain that PHOENICS performs to solve the continuous phase. GENTRA then tracks the particles as they move through the computed flow-field, calculating in the process the interphase interactions (i.e., the transfer of momentum, mass, enthalpy, etc) between the phases. These interaction terms are, after leaving GENTRA, incorporated as sources in the continuous-phase equations for the next PHOENICS sweep.

Since the newly introduced sources are likely to alter the flow field used by GENTRA to track the particles in the first instance, several iterations of the processes PHOENICS-sweep/GENTRA-tracking will normally be needed to obtain a converged solution.

For the benefit of readers with some knowledge of the structure of EARTH, it will be pointed out here that GENTRA is attached to PHOENICS as a Ground-station subroutine that calls, in turn, all of the modules of GENTRA. Most of these modules are delivered in closed (i.e., binary) code; but the Ground station itself (called GENTRA) and a special user-accessible module (GENIUS) are provided in open source.

5. About this Guide

1. The purpose of this Guide, and its intended readership

This Guide has been designed to serve both as a user's guide and reference manual for GENTRA users.

GENTRA, as indeed PHOENICS, has been created to cater for the needs of users with several degrees of knowledge or experience:

• New and occasional users who want to access the system will find the GENTRA Input Menu to constitute a "fast lane"; for it can be used to set up problems with very little knowledge of PHOENICS or CFD. Chapter 2 of this document (The GENTRA Input Menu) has been provided as a guide. Its reading is, however, by no means compulsory, for most of the information it conveys is also available as on-line help to menu users.
• Expert users, or users with unusual needs, will want to know exactly what GENTRA does, and how it does it, and may wish to ascertain whether the system can be expanded, and how. For this second class of users, a "reference" element has been included in this guide, notably Chapters 3 (The GENTRA PIL), 5 (The GENTRA FORTRAN) and 6 (The GENTRA Equations).

A basic knowledge of PHOENICS and its structure is assumed in this Guide.

2. How this Guide is organised

After this introductory chapter, the reader will find the following chapters and appendices:

• Chapter 2 (The GENTRA Input Menu), which is intended mainly for beginners, describes how to use the GENTRA menu to set up particle-tracking problems.
• Chapter 3 (The GENTRA PIL) explains how more experienced users can avail themselves of a sub-set of PIL variables that circumvent the need to use the menu.
• Chapter 4 (Running GENTRA-EARTH) explains how to execute the flow-solving program, EARTH.
• Chapter 5 (The GENTRA FORTRAN) explains how experienced users can use the open-code area of GENTRA-EARTH to supplement the built-in numerical and physical features.
• Chapter 6 (The GENTRA Equations) contains a complete listing of the mathematical models embodied in GENTRA, and some details of the numerical techniques used for the solution of the equations.
• Appendix A lists the main limitations of GENTRA.
• Appendix B lists the GENTRA-PIL variables.
• Appendix C lists the user-accessible variables in the FORTRAN of GENTRA.
• Appendix D is a compilation of the GENTRA run-time errors.
• Appendix E has a listing of the Q1 file resulting from the worked example in Chapter 2.
• Appendix F has a listing of the contents of the GENTRA Input Library.
• Appendix G contains a listing of the user-accessible subroutine GENIUS.
• Appendix H is a glossary of the terms used in this manual.
• Appendix I is a list of references.
• Appendix J contains a list of the nomenclature.
• Finally, Appendix K provides information on setting up GENTRA.

6. Conventions used in this Guide

   The following conventions are used in this Guide:-

   • The bold typeface is used for menu titles, menu options and also for PIL variables.
   • The Courier typeface is used for FORTRAN variables and commands.
   • A star * is used in Chapter 2 to indicate comments and instructions addressed to readers who are following the worked example presented in that chapter.

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