Encyclopaedia Index
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|Int High Tech|    0  |   The Expert-System CFD code              |
| Forum BASEL |  ---- |           by                              |
|    1992     |   12  |  Brian Spalding, CHAM Ltd , UK            |
|_____________|_______|___________________________________________|
|                                                                 |
|    This file contains the text panels of a lecture              |
|    delivered in 1992.                                           |
|                                                                 |
|    Live demonstrations were conducted during the lecture, by    |
|    means of PHOENICS running on a portable PC 486.              |
|                                                                 |
|    Viewers who wish to run these for themselves can do so       |
|    by way of library cases: 568, ......                         |
|                                                                 |
|    or active demonstrations: .....                              |
|                                                                 |
|                                                                 |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    1  |   The Expert-System CFD code              |
| Forum BASEL |  ---- |           by                              |
|    1992     |   12  |  Brian Spalding, CHAM Ltd , UK            |
|_____________|_______|___________________________________________|
|                                                                 |
|              Contents                                           |
|                                                                 |
|           *  The message                                        |
|                                                                 |
|           *  The problem                                        |
|                                                                 |
|           *  Some partial solutions                             |
|                                                                 |
|           *  Demonstrations                                     |
|                                                                 |
|           *  Conclusion                                         |
|                                                                 |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    2  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |       The main message                    |
|_____________|_______|___________________________________________|
|                                                                 |
|     * Computational Fluid Dynamics Computer codes are           |
|       being used by an ever-increasing number of specialists    |
|       in OTHER subjects than CFD.                               |
|                                                                 |
|     * CFD-code vendors have therefore provided easy-to-use      |
|       PRE-PROCESSORS for inputting GEOMETRICAL and PHYSICAL data|
|       and POST-PROCESSORS for viewing and interpreting results. |
|                                                                 |
|     * HOWEVER, the results of flow-simulation calculations are  |
|       affected also by NUMERICAL INPUTS, which even specialists |
|       in CFD find it hard to choose optimally.                  |
|                                                                 |
|     * These must therefore be chosen by the code's EXPERT SYSTEM|
|                                                                 |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    3  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  | Some of the NUMERICAL INPUTS to CFD codes |
|_____________|_______|___________________________________________|
|                                                                 |
|   Numerical inputs needed by CFD codes include:-                |
|                                                                 |
|   * The grid: * type: structured, unstructured or multi-block   |
|               * (if structured) cartesian, polar or body-fitting|
|               * fineness (how many nodes?)                      |
|               * non-uniformity (power-law? geometric progession?)
|   * Equations:* type: elliptic, parabolic or hyperbolic         |
|               * terms: convection, diffusion, transient, sources|
|               * schemes: upwind, central, QUICK, hybrid,.....   |
|               * solvers: iterative (which?), matrix-inversion...|
|   * Parameters* iteration-cut-off criteria (eg minimum residual)|
|               * over-relaxation parameters (for each variable)  |
|               * under-relaxation devices (eg false time steps)  |
|   * Initial guesses of field values for steady-state problems   |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    4  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |  Why NUMERICAL INPUTS must be optimal     |
|_____________|_______|___________________________________________|
|                                                                 |
|  Numerical inputs affect:                                       |
|                                                                 |
|  * WHETHER any result is obtained at all                        |
|            (the iterative solution procedure may DIVERGE)       |
|                                                                 |
|  * THE ACCURACY of the result                                   |
|            (coarse grids and slow convergence give POOR accuracy)
|                                                                 |
|  * THE COST of obtaining the result                             |
|           (fine grids and poorly-converging algorithms          |
|            entail very LONG COMPUTER TIMES )                     |
|                                                                 |
|  * THE SIZE of the computer which is needed                     |
|           (parabolic options use much less memory than elliptic)|
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    5  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |  Some of the numerical inputs required    |
|    1992     |   12  |  by the PHOENICS flow-simulation code     |
|_____________|_______|___________________________________________|
|                                                                 |
| To run PHOENICS, it is necessary to specify, FOR EACH VARIABLE: |
|                                                                 |
|    DTFALS, the relaxation parameter;                            |
|    RESREF, the termination criterion based on "residuals";      |
|    LITER,  the maximum number of iterations within the solver;  |
|    ENDIT,  the iteratio-termination or over-relaxation factor;  |
|    ISOLX, ISOLY, ISOLZ, ISOLBK, the frequencies of employment of|
|            the available block-relaxation actions;              |
|    VARMIN & VARMAX, the lower and upper cut=off values;         |
|            and MANY MORE |||||                                  |
|                                                                 |
| Also, there are parameters which govern the maximum number of   |
|   cycles through loops encompassing all variables and slabs.    |
|       ?????   WHAT ARE THE BEST VALUES FOR A GIVEN CASE  ?????  |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    6  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  | Sources of advice on numerical-inputs     |
|_____________|_______|___________________________________________|
|                                                                 |
| 1. Text-book theory. This is almost non-existent. What exists   |
|                      pertains to very simple (non-typical) cases|
|                                                                 |
| 2. Code manuals.     These are more suggestive than instructive.|
|                      Code vendors condense their experience into|
|                      guidance rules, not precise prescriptions  |
|                                                                 |
| 3. User experience   Only users who work intensively on a narrow|
|                      problem range accumulate enough enough     |
|                                                                 |
| 4. Data bases        Past experience of all users could in      |
|                      principle be put into a data base, to be   |
|                      accessed at input time. Who knows of one?  |
| 5. Trial and error   .......                                    |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    7  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   17  |  Two kinds of trial and error method      |
|_____________|_______|___________________________________________|
|                                                                 |
|   1. PRE-SELECTION AND REPETITION (the artillery method)        |
|                                                                 |
|      Many runs are conducted, with different numerical-input    |
|      data, systematically varied.                               |
|                                                                 |
|      The data combinations which maximise accuracy and economy  |
|      are then adopted for further "production runs".            |
|                                                                 |
|      Drawbacks:                                                 |
|                                                                 |
|      * The preliminary search can cost much computer- and       |
|        man-time.                                                |
|                                                                 |
|      * The adopted data sets may still not be best for all runs.|
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    8  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  | Two kinds of trial-and-error method       |
|_____________|_______|___________________________________________|
|                                                                 |
|  2. "IN-FLIGHT" ADJUSTMENT (the automatic pilot)                |
|                                                                 |
|    The computer-code is provided with an "expert-system" device |
|    which seeks the optimum set of numerical controls WHILE THE  |
|    COMPUTATION IS IN PROGRESS.                                  |
|                                                                 |
|    The search involves:-                                        |
|    * systematically varying the inputs;                         |
|    * noting their efects on convergence speed;                  |
|    * "homing in" on the optimal set.                            |
|                                                                 |
|    Advantage: The method exploits the fact that the best data   |
|               sets for the START of a calculation are often not |
|               the best for the MIDDLE and END stages.           |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|    9  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |  Examples of in-flight adjustment         |
|_____________|_______|___________________________________________|
|                                                                 |
|   Three examples of the use of the PHOENICS-EXPERT system       |
|   will be presented:-                                           |
|                                                                 |
|   1. Adjustment of the over-relaxation factor in the solution   |
|      of a steady-state HEAT-CONDUCTION problem.                 |
|                                                                 |
|   2. The operation of the same device for calculating the       |
|      potential-flow solution for flow around a MOVING TRUCK.    |
|                                                                 |
|   3. The correction of bad first choices of "false time steps"  |
|      for velocities in a DRIVEN-CAVITY CALCULATION.             |
|                                                                 |
|   The calculations will be performed "live" on a "notebook PC". |
|                                                                 |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|   10  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |       Discussion                          |
|_____________|_______|___________________________________________|
|                                                                 |
|  1. The heat-conduction example.                                |
|                                                                 |
|    * EXPERT raised the over-relaxation factor to higher-than-   |
|      expected values                                            |
|                                                                 |
|    * A better solution was found (ie one with lower residuals)  |
|      in a shorter computer time.                                |
|                                                                 |
|                                                                 |
|   2. Potential flow around a moving truck                       |
|                                                                 |
|    * PHOENICS computes the velocity potential and its gradients.|
|                                                                 |
|    * EXPERT may help to get the solution more quickly.          |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|   11  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |       Discussion of the examples          |
|_____________|_______|___________________________________________|
|                                                                 |
|  2. Potential flow around a moving truck                        |
|                                                                 |
|    * EXPERT has again brought economies.                        |
|                                                                 |
|    * For large problems, computer-time savings are substantial. |
|                                                                 |
|                                                                 |
|  3. The driven-cavity flow                                      |
|                                                                 |
|    * PHOENICS will solve the Navier-Stokes equations for the    |
|      laminar, steady flow in a square box with a moving lid.    |
|                                                                 |
|    * Non-linearity makes the optimal numerical-input parameters |
|      difficult to pre-select. Can EXPERT correct bad choices?   |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|   12  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |           Conclusions                     |
|_____________|_______|___________________________________________|
|                                                                 |
|  * Some preliminary successes of the PHOENICS Expert System     |
|    have been shown.                                             |
|                                                                 |
|  * Many more could have been presented had time permitted.      |
|                                                                 |
|  * The general conclusion is that the search procedures         |
|    which are being employed for "in-flight" adjustment of       |
|    numerical-input parameters are sound.                        |
|                                                                 |
|  * Experience has mainly been confined to "one-at-time"         |
|                                                                 |
|  * When many parameters may adjusted simultaneously, PRIORITY   |
|    PRINCIPLES are required. These are now being developed.      |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|   15  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |   Encore 1.    The principle employed     |
|_____________|_______|___________________________________________|
|                                                                 |
|  * Postulate that there exists a      | rate        ..          |
|    convergence-rate-versus-parameter  |            .  .         |
|    curve as shown.                    |           .   .         |
|                                       |          .     .        |
|  * Vary the parameter systematically  |         .      .        |
|    so as to keep close to the value   |       .         .       |
|    which maximises the speed.         |      .          .       |
|                                       |    .            .       |
|  * Store the information about the    | . .             .       |
|    optimum parameter values for use   |                  .      |
|    in later similar calculations.     |                  .      |
|                                       |                  .      |
|  * Introduce traps to prevent too-    |_________________________|
|    large changes leading to divergence      parameter ---->     |
|_________________________________________________________________|

___________________________________________________________________
|Int High Tech|   16  |       The Expert-System CFD Code          |
| Forum BASEL |  ---- |                                           |
|    1992     |   12  |  Encore 2. A role for parallel computing? |
|_____________|_______|___________________________________________|
|                                                                 |
|   * When multiple processors are available, it is posssible for |
|     each to perform a particular step in the calculation        |
|     procedure with a different set of numerical-input values.   |
|                                                                 |
|   * Each would achieve a different degree of success in reducing|
|     the residual errors.                                        |
|                                                                 |
|   * The solution with the greatest reduction of errors, and the |
|     corresponding numerical-input values, would be adopted      |
|     (for all processors) for the next step.                     |
|                                                                 |
|   * The numerical parameters have such an enormous effect on the|
|     economy/accuracy performance of computer codes, that the    |
|     here-suggested use of parallelism may beat conventional use.|
|_________________________________________________________________|

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