The near wall mechanics of 3D turbulent boundary layers were studied using direct numerical simulation. A fully developed turbulent channel flow was suddenly subjected to a transverse pressure gradient. An explanation for the reduction in the Reynolds stresses, observed in experiments, was provided in terms of the dynamics of near-wall streamwise vortices in turbulent boundary layers was described. It was shown that the effect of three-dimensionality is to change the trajectory of fluid particles in the vicinity of these vortices, and how this change leads to reductions of turbulence stresses. The proposed mechanisms are consistent with numerous statistical data and correlations which are presented. Turbulence, Boundary layers, Simulation.

The various methods for measurement of the six components of the turbulence stress tensor are reviewed, and some of the data on the turbulent shear stress vector are presented to demonstrate the validity of current ideas for.

The IUTAM Symposium on Three-dimensional Turbulent Boundary Layers was suggested by the Gesellschaft für Angewandte Mathe­ matik (GAMM) and sponsored by the International Union of theor­ etical and Applied Mechanics. The symposium was organized by H.H. Fernholz (Hermann-Föttinger-Institut für Thermo- und Fluiddynamik der Technischen Universität Berlin) and E. Krause (Aerodynamisches Institut der RWTH Aachen). After two success­ ful Euromech Colloquia on the same topic in Berlin 1972 and Trondheim 1975 the organizers felt that another meeting should be convened, this time with participants from inside and out­ side Europe. The aim of the symposium has been to bring together scientists who are actively engaged in boundary layer research, both ex­ perimental and theoretical. The scope of the meeting encompass­ ed incompressible and compressible three-dimensional turbulent boundary layers. Special emphasis was laid on economical cal­ culation methods, on measurements of fluctuating quantities and on measuring techniques designed for and applied success­ fully to three-dimensional boundary layers. From among thirty-four papers submitted for presentation, twenty­ six contributions of twenty-five minutes each were selected by the European mernbers of the Scientific Committee. Furthermore there were four invited lectures of forty-five minutes. Short discussions were held directly after each presentation with a long discussion period at the end of each day. The final dis­ cussion on the last day of the symposium was recorded on tape and is presented in a slightly shortened version as the last contribution in this volume.