The Reynolds-stress is governed by following transport equation:
where is dissipation rate of turbulence energy.
and are retained without approximation.
The first referred to as the 'return-to-isotropy' term, the second as the 'rapid' contribution, and the third being a correction accounting for the effects of wall-induced pressure reflections.
For Gibson and Launder's model (1978), The three contributions are modeled as follows:
where
For Speziale, Sarkar and Gatski's model (1991),
where and are
and
The dissipation rate is governed by following transport equation:
which is, formally, identical to that used in the high-Re variant of the model, except that the diffusion process is represented by the GGDH approximation:
The constants appearing in the above equations have the following values: