Sediment transport is one key process in sedimentation problems. The dynamics of sediments in rivers will lead to an ecological change of the catchment. Among all complicated processes in sediment transport, the settling dynamics of solid particles in the water contribute to the water quality and geomorphology of a river channel directly. This paper aims at an investigation of the interaction between particle movement and water flow, with application to hydro-environment situations such as transport of fine sand sediment in natural streams. The Stokes number in this type of application is much smaller than unity at which enhancement of particle settling velocity by strong ambient turbulence has been observed in many numerical and laboratory studies. The present laboratory study is conducted to study the settling of sub-millimeter sized heavy particles in water with a relatively weak grid-generated turbulence. The two-phase flow is measured with the whole-field imaging techniques of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). A two-camera PIV/PTV technique is used to obtain the instantaneous settling velocities of the solid particles and the turbulent water velocity field. Phase separation is based on an effective optical distinction of the two light scattering signals with fluorescent tagging on the solid particles. With this technique, it is found that in most cases of grid-generated turbulence the settling velocity of the heavy particles is slightly lower than the still water value. The falling heavy particles are also found to cause additional turbulence in the water with low ambient turbulence intensities. At one particular case of grid turbulence, the particles are found to fall with a depth-averaged settling velocity evidently higher than the still water value. The instantaneous turbulent fluid motions and particle trajectories tend to support the fast tracking effect to be related to the higher particle falling velocities.

泥沙输送是解决泥沙问题的关键过程之一。河流中沉积物的动态将导致流域的生态变化。在沉积物运输的所有复杂过程中，水中固体颗粒的沉降动力学直接影响河道的水质和地貌。本文旨在研究颗粒运动与水流之间的相互作用，并将其应用于水环境中，例如自然流中细沙沉积物的运输。这种类型的应用中的斯托克斯数远小于一，在许多数值和实验室研究中均已观察到因强环境湍流而提高了颗粒沉降速度的统一性。目前的实验室研究是为了研究亚毫米大小的重颗粒在水中的沉降，而网格产生的湍流相对较弱。两相流通过粒子图像测速（PIV）和粒子跟踪测速（PTV）的全场成像技术进行测量。使用两摄像机PIV / PTV技术获得固体颗粒的瞬时沉降速度和湍流水速度场。相分离是基于两个光散射信号的有效光学区别以及在固体颗粒上的荧光标记。通过这种技术，发现在大多数情况下，由网格产生的湍流中，重颗粒的沉降速度略低于静水值。还发现掉落的重颗粒在水中具有较低的环境湍流强度而引起额外的湍流。在格栅湍流的一种特殊情况下，发现颗粒以深度平均沉降速度下降，该沉降速度明显高于静止水值。瞬时湍流运动和粒子轨迹倾向于支持快速跟踪效果，这与较高的粒子下落速度有关。