# Yap correction

### From CFD-Wiki

The Yap correction [Yap. C. J. (1987)] consists of a modification of the epsilon equation in the form of an extra source term, , added to the right hand side of the epsilon equation. The source term can be written as:

Where

is the normal distance to the nearest wall.

This source term should be added to the epsilon equation in the following way:

Where the epsilon equation has been written in the same way as is in the CFD-Wiki article on low-Re k-epsilon models.

The Yap correction is active in nonequilibrium flows and tends to reduce the departure of the turbulence length scale from its local equilibrium level. It is an ad-hoc fix which seldom causes any problems and often improves the predictions.

Yap showed strongly improved results with the k-epsilon model in separated flows when using this extra source term. The Yap correction has also been shown to improve results in a stagnation region. Launder [Launder, B. E. (1993)] recommends that the Yap correction should always be used with the epsilon equation.

## Implementation issues

The Yap source term contains the explicit distance to the nearest wall, . This distance is sometimes difficult to efficiently calculate in complex geometries. In structured grids, the coordinate distance to the nearest wall can be used as an approximation. Otherwise, a brute force calculation must be used which greatly benefits from a multi grid approach. In topologies with domain boundaries that are not walls the problem becomes more complex, because the non-wall boundaries will block the direct path to the wall boundaries. A simple loop over length must now be accompanied by topological path checking. This makes the Yap correction most suitable for use in a structured code where some normal wall distance is readily available. There are several alternative formulations that can be used instead though *(anyone have the references??)*.

When implementing the Yap correction it is common to use it only if the source term is positive. Hence:

## References

**Launder, B. E. (1993)**, "Modelling Convective Heat Transfer in Complex Turbulent Flows", Engineering Turbulence Modeling and Experiments 2, Proceedings of the Second International Symposium, Florence, Italy, 31 May - 2 June 1993, Edited by W. Rodi and F. Martelli, Elsevier, 1993, ISBN 0444898026.

**Yap, C. J. (1987)**, *Turbulent Heat and Momentum Transfer in Recirculating and Impinging Flows*, PhD Thesis, Faculty of Technology, University of Manchester, United Kingdom.