The entrainment or resuspension of particles in air or water is a very common phenomenon. It is determined by the fluctuating lift force that a panicle, which is lying on a surface, experiences by the turbulent flow over the surface.This lift force is one of the four force components experienced by the particle which are-in addition to lift-weight, adhesion and drag force. Here we consider only very small particles that are completely embedded in the viscous sublayer of a turbulent boundary layer. For the measurementof the lift force on these particles no experimental technique was available. Therefore we have developed a new technique for measuring this lift force and its fluctuations. It is capable of measuring fluctuating lift forces of around N. A particle is glued on top of a cantilever: a focused laser beam is used to measure the deflection of the cantilever. The experimental set-uD and the measurement technique are explained and some results are shown.

Cross correlation between signals from a wall probe and a hot-wire probe indicates that large structures, inclined to the wall at approximately 20 °, exist in a rough-wall boundary layer as reported earlier by Brown and Thomas and by Head and Bandyopadhyay for a smooth wall.

The acquisition and digital storage of analogue turbulent signals is optimized with respect to minimal data handling. A best anti-aliasing filter and optimum settings of the analogue-to-digital converter are found by simulating a turbulent signal digitally and calculation of the distortion under different circumstances.

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