Detachment of Particles by a Water Flow

for smallish particles, it is sufficient to determine the adhesive forces between these and the surface in order to calculate the flow velocity at which all the adhering particles are removed. For larger particles, the adhesive forces of which are smaller than the weight of the particles, the pulling velocity of the flow Vp may be calculated from other empirical formulas [330-333]. 

Detachment and Pulling of Particles. In a liquid medium the detaching forces of adhering particles depend on the structure of the boundary layer in the same way as in the case of air (see 31). Shulyak [334] made an experimental study of the detachment of particles from a thin plate moving in a stationary medium. 

The following represent the experimental values of the average flow velocities (vav) required to remove particles of diameter 80-500 M, the depth of immersion of the particles in the boundary layer (6 is the thickness of this layer), and the relative detachment velocities  

The relative velocity in the boimdary layer is the ratio of the flow velocity at a level equal to the height of the particle (v det) to the average velocity (v y) of the flow (or in the present case, the velocity of the plate). Tlie relative velocity shows what proportion of the average flow velocity is represented by the velocity at a level equal to the height of the particle. 

It follows from these results that the particles lie in the laminar layer and their detachment is effected at a velocity lower than the mean velocity of the flow (velocity of the plate). 

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In order to test the validity of this equation, the velocity has been determined for the detachment of monodisperse particles by a water stream, and the experimental data have been compared with calculated results. Listed below are values of the water flow velocity required to detach all spherical glass particles, in relation to the particle diameter ... 

In contrast to the case examined previously (see p. 349), in river flow the detachment of particles adhering to the bottom takes place not from a surface, but from a layer of solidly packed particles. In this case, the same as in the case of detachment of particles by air or water flow, we can distinguish critical velocities for detachment of the adherent particles. The minimum critical velocity at which the first particles are detached, in application to riverbed evolution, is termed the nonscouring velocity, and the velocity for first mass movement of the particles is termed the scouring velocity [351, p. 288]. 

Changes in Parameters of Particle Size Distribution After Detachment of Part of Particles. When poly disperse particles are detached by a water stream, there will be changes in the parameters characterizing the particle size distribution, i.e., the median particle diameter d and the standard deviation a. These changes, depending on the position of the plate relative to the direction of flow and the water stream velocity, are illustrated by the following data ... 

The water-treating process can be improved by the introduction of a polyacrylamide. As the polyacrylamide content is increased, the ratio ajb drops off, i.e., the particle adhesion is reinforced and particle detachment by the water flow is diminished, thus improving the filtration quality. According to Mints, the parameter b is determined at the start of the filtration process, i.e., when the first contaminants adhere to the surface of the filter grains [302]. 

Stream channel characteristics and adversely affect aquatic plant and terrestrial life. The erosion effects of water can be minimized by mechanical control, varying the employed irrigation technology, reducing the water flow rates, by waterway vegetation or lining by concrete, stone, or plastics. These factors decrease the abiUty of a water flow to detach and move soil particles along surfaces, and increase the resistance of the soil surfaces to the force of the water flow. 

Effect of the Methods of Dust Deposition and the Properties of the Surface on the Detachment of Particles. It is well known that there are other ways in which plates become dusty as well as by free settling. Dust particles may occur in drops of rain and settle on surfaces at the same time as these. When the drops dry in the air, the adhesion of such particles becomes much stronger (see 26). The following represents some experimental data relating to the efficiency with which particles are removed by a flow of water (i.e., the variation in the coefficient K ) as a function of the method by which the particles are deposited on the surface ... 

Below we present some data relating to the detachment of glass particles by a flow of water (i.e., the variation in the coefficient Km) as a function of the properties of the surface ... 

Simulating the Detachment of Particles. The surface-cleaning factors and particle-size distributions obtained in the large and small troughs respectively after subject to a flow of water (Table VII.2) coincide. We now have to decide how far such results maybe used in order to characterize other cases of the removal of attached particles in a flow of water, for example, with varying depth of flow, plate size, etc., i.e., we wish to know whether the process underlying the detachment of adhering particles by a flow of water may be modeled or simulated. 

Thus we see that the conditions for particle detachment can be expressed in terms of a critical velocity, in terms of the forces acting on the particles from the water stream, and in terms of certain dimensionless quantities characterizing the start of movement for the bed-load particles. It should be noted that there are a number of different points of view as to the causes of particle detachment from the bottom. Particle detachment may take place under the influence of a lifting force generated by the action of the vertical component of pulsating velocities in a turbulent stream, or detachment may be a consequence of unsym-metrical flow around the particles at the bottom. 

In order to verify the validity of this relationship, the velocity corresponding to the detachment of a set of particles of constant size by a flow of water was determined experimentally and the results were compared with theory. The following represents the relation between the water-flow velocity required to detach 100% of the spherical glass particles from a steel surface and the particle diameter ... 

The Mackrle paper [25] took no account of such processes as the autohesion of contaminant particles to each other and their adhesion to a layer of earlier-adhering particles, nor the detachment of adhering particles by the flow of water. These deficiencies are to some extent eliminated in the papers of Mints [345, 346], who in calculating the efficiencies of grainy filters treated the adhesion processes with due allowance for the balance of forces associated with the adhesion or detachment of the adhering particles ... 

The first and second critical velocities of the water and air flows as shown as functions of particle diameter in Fig. XI.7. As we should expect, the first critical velocity for wind erosion exceeds the velocity for erosion by a flow of water, owing to the difference between the adhesive forces in air and water. The broken line shows the change in the value of when the force of interaction between the particles exceeds their weight (see Fig. XI.5 and 31, 37, 38). In this case the first critical velocity produces the detachment of the adhering particles. The value of this velocity may exceed Vj 2> velocity required to produce actual flight... 

With increasing PAD content the ratio a/h falls, i.e., the adhesion of the particles increases and their detachment by the flow of water diminishes, which improves the quality of the filtration. 

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