Model evaluation
Checking the model
STAR-CD offers a variety of tools to help assess the accuracy and effectiveness of all aspects of the model building process. In performing the modelling stages discussed previously, the user should therefore take advantage of these facilities and check that:
1. The mesh geometry agrees with what it is supposed to represent. This is greatly facilitated by the built-in graphics capabilities that allow the mesh display to be
(a) rotated, (b) displaced, (c) reduced, (d) enlarged.
This enables the user to look at the mesh from any viewpoint, with the view showing the correct three-dimensional perspective. Frequent mesh displays during the mesh generation stage are very useful for verifying the accuracy of what is being created and are therefore strongly recommended, particularly for complex-geometry problems. It is best if such geometries are subdivided into convenient parts that can be individually meshed and then checked visually.
2. Materials of different physical properties occupy the correct location in the mesh. This can be checked visually by using the built-in colour differentiation scheme. Alternatively, each material’s mesh domain can be plotted
individually. Precise values of specified properties can be checked via the screen printout.
3. Boundary conditions are correct, by producing special mesh views that show (a) boundary location,
(b) boundary type,
(c) a schematic of the conditions applied (e.g. inlet velocities).
More complete information on specified boundary values can be obtained from the screen printout.
4. The initial conditions should also be checked, particularly for transient problems and initial fields specified through user subroutines, by running the STAR-CD solver for zero iterations/time steps and plotting the relevant field variables.
Checking the calculations
Having completed the model preparation, the next task is to run the STAR-CD solver and check the results of the numerical calculations. These results are presented in various ways, details of which are given in the Post-Processing User Guide. Briefly, printouts and/or plots can be produced of the following:
• Field values of all primary variables at interior and boundary nodes.
• Interpolated values of the above quantities at arbitrary, user-specified points or surfaces within the solution domain.
Model evaluation
• Surface heat and mass transfer coefficients and forces; also values of the dimensionless coordinate y+ for near-wall mesh nodes.
• Global quantities such as total force components (e.g. drag, lift) on submerged bodies and their dimensionless counterparts, overall energy balances, etc.
It is important to examine this information carefully to verify that the calculations have been properly set up and are producing sensible results. In particular, the user should ensure that:
• The interior fields are examined for plausibility and similar checks made on global quantities.
• For turbulent flow calculations, the near-wall node y+ values are within the recommended range (30-100) in regions where adherence to this constraint is important. In the case of calculations with a two-layer model, checks should be made that the mesh is sufficiently dense within the near-wall layer.
• The magnitude of numerical discretisation errors (spatial and, where relevant, temporal) is assessed and arrangements made for their reduction to acceptable levels, if necessary.
Of the above tasks, the last is currently the most difficult, for it is not possible to achieve it by a simple calculation. What is required are the following:
• A reliable means of evaluating the discretisation errors. At present, this is accomplished by repeating the calculations with finer meshes and smaller time steps (strictly, these should be done independently) and noting regions of appreciable change in the solution.
• Strategies for altering the mesh or time step to reduce errors. These adjustments are made manually.
Ideally, the error correction process should continue until the changes fall to acceptable levels. In practice, this approach may not be feasible, especially for three-dimensional problems involving complex geometries, due to the large preparation and computing overheads.
An alternative way of gaining some insight into the presence of spatial truncation errors is to change the spatial discretisation scheme and note the effect on the solution. The second-order options, or blends thereof, available in STAR-CD will usually produce the lowest numerical errors.
Introduction
Chapter 2 BASIC STAR-CD FEATURES
Introduction
The main aim of this part of the manual is to provide users, whether experienced or not in the application of general-purpose computational continuum mechanics codes, with advice on effective ways of setting up and running a basic continuum mechanics model using STAR-CD. The reader is, however, expected to have gone throughChapter 1 and the material in the Methodology volume.
All aspects of user interaction are handled by pro-STAR, the pre- and
post-processing subsystem of the STAR-CD suite. As a pre-processor, pro-STAR is the means by which the user defines the
• geometry,
• calculation mesh,
• boundary conditions,
• initial conditions,
• fluid and solid material properties,
• analysis controls,
which uniquely determine the problem to be solved. As a post-processor, pro-STAR can
• read and re-format the various data files produced by the analysis,
• manipulate the data read in,
• produce extensive and easily comprehensible printouts,
• summarise information on the calculated results,
• draw sophisticated 3-D graphical images,
• animate those images,
• draw graphs of various calculated quantities.
Both pre- and post-processing operations are served by an extensive set of plotting facilities, enabling rapid visualisation of even the largest models, plus on-line context sensitive help that provides detailed information on usage.
pro-STAR is a combined command-, menu-, and process panel-driven program.
The choice of working interface is entirely up to the user and depends on
• whether the available terminal can accept and display graphical input and output,
• whether the host computer’s operating system supports a windowed, graphical user interface (GUI) environment,
• user preference and level of experience with STAR-CD.
GUI facilities are available for UNIX, Linux or Windows implementations of STAR-CD using the OSF Motif graphics environment. They consist of two basic types:
1. Graphical tools such as drop-down menus, dialog boxes, push-buttons, sliders, etc. to assist users in specifying the desired pro-STAR actions. These facilities are arranged around the main pro-STAR window, or have their starting point located somewhere on that window. Their purpose and best way of using them are explained throughout this volume.