### 13.2.2. Software Limitations

There are several potential sources of errors and uncertainties in all CFD simulations, all of which are extremely relevant in turbomachinery applications using a throughflow program, such as Vista TF. The standard categorization of such errors is into the following groups:

- Numerical errors

- Model errors

- Application uncertainties

- User errors

- Software errors

These are briefly described next.

Numerical errors decrease the quality of simulations and can be reduced but not eliminated. The control of numerical error is largely a matter of adequate grid design, appropriate numerical discretization, and adequate level of convergence. An example in Vista TF is the approximation of the streamlines by a piecewise parabola through 3 points. In a throughflow calculation the so-called model errors probably outweigh all other sources of error. These are related to the fact that the equations that are solved do not really describe the real flow particularly adequately (in this case the solution is for inviscid, circumferentially-averaged mean values on widely spaced grid lines). Application uncertainties may be only partly in your control and are related to the detail with which aspects of the geometry are accurately known or specified. Typical examples would be a lack of knowledge about the fillet radii in the geometry for a blade row calculation, or not having correct information about the inlet boundary conditions for a specific calculation. User errors relate to incorrect use of the program, such as making use of incorrect control parameters to control the calculation. With regard to software errors, every reasonable precaution to ensure the accuracy and reliability of the Vista TF program has been taken. However, when using the program, especially for a critical design, you should first complete an appropriate validation and calibration process. A good turbomachinery analysis procedure dictates that any program, including Vista TF, must be thoroughly tested with non-critical data before there is any reliance on it.

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