### 6.4.1. Verification Experiments

The purpose of verification tests is to ensure the correct implementation of all numerical and physical models in a CFD method. The best verification data would be the analytical solutions for simple cases that enable the testing of all relevant implementation aspects of a CFD code and the models implemented. As analytical solutions are not always available, simple experimental test cases are often used instead.

#### 6.4.1.1. Description

For CFD code verification, convergence can be tested against exact analytical solutions like:

- Convection of disturbances by a given flow

- Laminar Couette flow

- Laminar channel flow.

For the verification of newly implemented models, verification can only in limited cases be based on analytical solutions. An example is the terminal rise velocity of a spherical bubble in a calm fluid.

In most other cases, simple experiments are used for the verification. It is recommended that you compute the test cases given by the model developer in the original publication of the model, or other trustworthy publications. Quite often experimental correlations can be applied, without the need for comparison with one specific experiment. For instance for turbulence model verification, the most frequently used correlations are those for flat plate boundary layers.

#### 6.4.1.2. Requirements

The only requirement for verification data is that they enable a judgement of the correct implementation of the code and/or the models. This requires information from other sources concerning the performance of the model for the test case. Strictly speaking, it is not required that the simulations are in good agreement with the data, but that the differences between the simulations and the data are as expected.

The test suite for model verification must be diverse enough to check all aspects of the implementation. As an example, a fully developed channel flow does not enable a test of the correct implementation of the convective terms of a transport equation. The test suite should also enable testing the correct interaction of the new model with other existing features of the software.

Software verification for physical models should be carried out in the same environment that the end-user has available. Testing of the new features in an expert environment might miss some of error sources, such as the GUI.

Verification cases should be selected before the model is implemented. They must be considered an integral part of the model implementation.

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