An Experimental Study of Settling Velocity in Turbulence Using the Digital Particle Image Velocimetry Technique

In this paper, turbulence influence on the settling behavior of fine sand particles was investigated by conducting experiments in a water tank with turbulence generated by a grid oscillating in the vertical direction. Digital Particle Image Velocimetry (DPIV) system with enhanced shadow processing function was employed to conduct simultaneous velocity measurements of individual fine sand particles driven by gravity and the ambient fluid in the confined turbulent environment. The spherical shape and diameter (3.0-6.0 mm) of fine sand particles were tested with different oscillating frequencies. The turbulence effects on the relative settling velocity were expressed in terms of different dimensionless parameters, such as Stokes number, Richardson number and dimensionless turbulence length scale. The two main findings were observed from the experimental investigation: (i) the turbulence decays followed the power law; and (ii) the integral length scale increases linearly with the distance from the grid. The flow formation near the grid was also experimented in detail with the advantage of the planar measurements. The results showed that shear flow perceptibly exists near the vertically oscillating grid and homogeneity of the turbulence can only be achieved at a distance from the vertically oscillating grid greater than about three mesh sizes.