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.

The settling behavior of solid particles through fluids is of primary importance for many industrial fluid flow applications, environmental multiphase-flow applications and engineering applications. The behavior of settling particles is also important in a variety of applications from environmental to medical. However, in such applications particle settling in a non-Newtonian power law fluid is an issue of interest to many industrial applications, including chemical, food, pharmaceutical and petroleum industry. Settling behavior of particles also occurs in many practical applications of engineering fields such as petroleum, mining or even process engineering. In wells drilling operation, slurry flow of drill-mud with the drilled cuttings in transport process is important application. In most cases, fluid flow is turbulent; hence the quiescent case of particle settling velocity cannot be applied. In natural systems, there are many situations where turbulence generated at one location in a fluid causes mixing across a density interface some distance away. Digital Particle Image Velocimetry (DPIV) has been applied to a wide range of flow problems, varying from the flow over an aircraft wing in a wind tunnel to vortex formation in prosthetic heart valves. DPIV algorithms based on cross-correlation can be implemented in a matter of hours, while more sophisticated algorithms may require a significant investment of time. Application of DPIV in the education system is used in teaching fluid mechanics to students studying fluid mechanics in design, engineering and science including physiology. DPIV has become an essential measurement technique in fluid mechanics laboratories both in research institutes and industry. Papers related to DPIV have represented approximately half of the total papers presented in the Lisbon International Symposia on Applications of Laser Techniques to Fluid Mechanics held since 2000. This success, boosted by the progress in laser technologies as well as electronic image recording, can be explained by the large quantity of information that can be recorded instantaneously and simultaneously for a reasonable implementation effort, compared to other measurement techniques. It is also related to the development of several commercial systems that have made the technique easily available worldwide for a very large variety of applications, ranging from microfluidic scales (~ a few hundred microns) to large fields (~ 1 m) in wind tunnels. This spectacular development has been largely supported through collaborative networks such as EUROPIV or PIV Challenge, set up to foster international cooperation and organize worldwide comparison of algorithm performances.

Mechanical Engineering Journal, Digital Particle Image Velocimetry (DPIV) technique, Settling velocity, Flow measurement, Oscillating grid turbulence.