Some notes on three-dimensional turbulent boundary layer data and turbulence modelling

The main results of the three-dimensional turbulent boundary layer experiments carried out up to now are discussed. The emphasis is on the direction of the measured turbulent shear stress, which generally differs from the direction of the velocity gradient. In a number of cases (but not all cases) the behavior can be understood when assuming a large time lag in the downstream development of the turbulence. Turbulence modelling is generally based on the turbulent transport equations. It will be shown that these equations may give large differences between turbulent shear stress and velocity gradient direction, when assuming a negative correlation between the fluctuating static pressure and the excess kinetic energy due to the turbulence.