PARTICLE-SIZE AND VELOCITY DISCRIMINATION IN A SEDIMENT-LADEN TURBULENT-FLOW USING PHASE DOPPLER ANEMOMETRY

A series of experiments were performed in a mixing box in order: (1) t o investigate the applicability of phase Doppler anemometry (PDA) to d iscriminate fluid and sediment particle sizes and velocities in sedime nt-laden turbulent flows; and (2) to relate the size and amount of sed iment in suspension to the grid-generated turbulence. Natural impuriti es within the water provide excellent ''seeding'' to represent the flu id and can be easily discriminated from spherical glass beads (75-355 mu m) used as sediment. Slight asphericity in the glass beads results in larger grain size ranges determined by PDA compared to the nominal sieved sizes. The mean, root-mean-square and skewness of the vertical fluid velocities increase at higher grid oscillation frequencies but d ecrease with distance from the grid. Similarly, the size and amount of suspended sediment increase with grid oscillation frequency and decre ase with distance from the grid. The suspension of sediment is shown t o be dependent on the magnitude and anisotropy of the fluctuating vert ical component of velocity. Phase Doppler anemometry offers a unique m ethodology to investigate the complex links between the transport of s ediment and the turbulent flow field.