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A five-velocity-component LDV system was designed to make five nearly simultaneous velocity component measurements: coincident instantaneous U , V , W components of the velocity at one point and two V velocity components at two nearby locations. The research is aimed at examining the relations among measured velocity components in order to investigate the near wall turbulence structure of 3D turbulent boundary layers (3DTBLs) and aid in the development of new turbulence models for 3 D subsonic pressure-driven turbulent boundary layer (TBL).To map the relations between the 'sweeps' and 'ejections' of near wall turbulent fluid, two V measurement points can be traversed within a selected domain. In order to map the velocity field for given V measurement locations, a U . V . W measurement point is traversed in a domain defined by the two V measurement points. At the same time, the system can make measurements throughout the whole boundary layer to investigate other phenomena. Data from this system show that the uncertainties are low and the repeatability of measurements is excellent. Data presented include the mean and fluctuation velocities, shear stresses, some of the triple velocity fluctuation correlations and some auto and cross correlation coefficients obtained at one measurement station of a high Reynolds number 3DTBL flow. Five-velocity-component laser-Doppler systemThis system is composed of an optical table, the fibre optic probe, data collection units and data reduction programs.

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