AbstractData on stream and spanwise variation in four representative transitional boundary layers is presented, the test cases encompassing free transition, transition due to a two-dimensional trip wire, with laminar and with turbulent re-attachment, and transition due to isolated spherical roughness elements.
The concept of statistical similarity in transition regions has been confirmed for the cases of free transition and for transition due to a two-dimensional trip wire, with the streamwise mean intermittency distribution described by the normal distribution function. Significantly, the condition of "transition at the wire" is found to have a very small but well defined intermittency growth region.
Transition due to isolated roughness elements is however seen to have a fundamentally different character. The elements induce large amplitude fluctuations, related to vortex shedding, which degenerate downstream to become random, three-dimensional, turbulent fluctuations in the developing wakes, with only mild recourse to an intermittent type of breakdown.
Spanwise variation in the layer is related to inconsistencies in the turbulent spot source density and occurrence frequency, and can result in substantial non-uniformity of the boundary layer if the transition region is long.
The resulting low Reynolds number turbulent boundary layer exhibits a moderate increase in the additive constant in the law of the wall, and is characterised by a deeper penetration of the transverse intermittency distribution. The choice of transition agent however, has no apparent influence on the rate at which the developing turbulent boundary layer approaches self- preserving conditions.
A general relation, based on local boundary layer parameters, has been developed for the estimation of the transitional skin friction coefficient and has been tested via momentum balance principles.
The skin friction relation has also been employed in a general integral prediction technique, for the incompressible transitional boundary layer in two-dimensional, arbitrary pressure gradient flows.
|Date of Award||Jan 1979|