Perturbations to unidirectional turbulent flow due to small lateral mean velocity gradients

The solution for the stress transport turbulence model equations for the situation where the flow is unidirectional with gradients in the lateral plane is now commonly applied to flows with weak secondary currents in closed ducts, open channels and rod bundles in nuclear reactor channels. Here perturbations to the unidirectional solutions are studied by solving the exact equations using an iterative procedure. The equations contain also some small derivatives formerly neglected. The justification as well as the limitation of the use of the algebraic stress modelling is discussed. Modifications for weak lateral motion are suggested as well.