Stability of explicit models

A physically reasonable condition for an explicit second order model expressed the fact that increasing (non-dimensional) shear $ \overline{S}$ should lead to increasing vertical shear-anisotropies of turbulence, $ b_{13}$ and $ b_{23}$. It has been shown by Burchard and Deleersnijder (2001) that a violation of this condition may lead to numerical instabilities of the models.

Mathematically, the shear-condition is expressed by

$\displaystyle \dfrac{\partial {(b_{13}^2 + b_{23}^2)^\frac{1}{2}}}{\partial {\o...
...\partial {\tilde{c}_\mu \overline{S}}}{\partial {\overline{S}}}\ge 0 \; , \quad$ (84)

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 $ \alpha_M = \overline{S}^2$. A simpler condition can be obtained, when this equation is solved after terms multiplied by $ d_5$ and $ n_2$, which usually are very small, are neglected.

The resulting approximate condition is

$\displaystyle \alpha_M \le \dfrac{d_0 n_0 + (d_0 n_1 + d_1 n_0) \alpha_N + (d_1...
...pha_N^3}{ d_2 n_0 + (d_2 n_1 + d_3 n_0 ) \alpha_N + d_3 n_1 \alpha_N^2} \quad .$ (85)

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