Mathematically, the shear-condition is expressed by

where (76) has been used. Using the equilibrium form of the stability function described in section 4.26, this condition leads to a cubic equation in . A simpler condition can be obtained, when this equation is solved after terms multiplied by and , which usually are very small, are neglected.

The resulting approximate condition is

Burchard and Deleersnijder (2001) showed that using (85) the most well-known models yield numerically stable results. However, for some models like those of Mellor and Yamada (1982) and Kantha and Clayson (1994), the limiter (85) is almost always `active', and hence replaces the actual turbulence model in a questionable way.

Karsten Bolding 2012-12-28