Turbulent boundary layers perturbed by embedded bodies

Experiments conducted since mid-seventies have shown that it is possible to insert bodies in turbulent boundary layers which markedly alter the large scale motions. The resulting reduction in Reynolds stresses and hence skin friction occurs over distances of the order of hundred times the undisturbed boundary layer thickness. Net drag reduction of the order of 20 per cent or less has also been shown to be possible depending on the geometry of the body and the flow conditions. While several technological problems need to be worked out before practical applications can be taken up, the results obtained so far have been regarded as promising enough to justify programmes in at least five countries. This abstract refers to these experiments and their results. References are given in the abstract