Study On The Motion Characteristics Of Suspended Particles

The mechanics of sediment-laden flow has been one of the most important subjects in hydraulics and river dynamics. The behaviors of each phase and their interaction are significant in the understanding of the intrinsic mechanism of sediment transport. In this paper, analytical and experimental study was carried out to investigate the motion characteristics of suspended particles.By performing Fourier transformation, the relationship between the turbulent intensity of the particle and that of the carrier fluid was obtained which makes clear that particle size, specific gravity, velocity gradient, and the characteristic frequency of energy-containing eddies are four basic influencing parameters. It is proven theoretically that under certain conditions, the stream-wise and transverse particle turbulent intensities exceed those of the carrier fluid.Based on the equations of two-phase flow, the presence of fine sediment suspension is shown to attenuate the turbulent intensity of the water. The extent ofattenuation is proportional to the parameter under some assumptions inopen channel flow.The lack of a sound theoretical basis for predicting the behavior of coarse particles means that experiments play a critical role. A PTV system was developed which allows particle velocities across a flow plane to be measured at a single instant. PTV presents a new efficient technique in hydraulic measurement.A series of experiments were conducted in a water flume under uniform flow conditions using two kinds of particles: light plastic particles( s= 1.056 , D=0.1~1.5mm) and heavy glass particles ( s=2.65, D=0.1-0.3 mm). The motion characteristics of the suspended particles were obtained by statistical analysis, including vertical concentration distribution, mean velocity profile, turbulent intensity, velocity correlation, and the probability density distribution of the velocity fluctuations. Attempt has also been made to interpret the turbulent characteristics in terms of particle properties and turbulence structure. The vertical concentration distribution can be described fairly well by the Rouse equation. For light particles, 1 when observed within the whole water depth. The relationship between theoretical and measured values of the suspension falls to the second approximation to the solution of the diffusiontheory. The concentration distribution becomes uniform near the bed due to dispersive forces. When observed in the region of y/h > 0.2, 1 , For heavyparticles (D 0.3mm) , 1 in the whole flow. The mean velocity profile follows the log-law, with K 0.39, B 4.0 which have no discernable relation with the diameter and specific gravity of the particles. In the near bed region, the turbulent intensities are elevated above those of water at a rate proportional to particle size due to collisions and velocity gradient. While in the upper region, the turbulent intensities of water exceed those of the particles. Light particles possess higher turbulent intensities than the heavy ones of the same size. The correlation -uv/u2 of light particles with diameter of 0.1mm is similarto the Reynolds stress in clear water flow. For larger and heavy particles, deviation occurs. The correlation coefficient of velocity fluctuations -uv/u'v' takes the shape of a reverse "C". The correlation decreases with the increase of particle size. The coefficients of skewness(Cs) and kurtosis(Ce) have the feature of Cs<0, Ce? for the streamwise velocity fluctuations and Cs>0, Ce>3 for the vertical. The joint probability density distribution (PDF) of the velocity fluctuations falls along the directions of -u + v and+w-v especially for fine particles. The characteristic of PDF is consistent with large-scale structures in turbulence.