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Measurement of Hydrodynamic Forces on Gravel Particles in the Erosion Function Apparatus
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Many theoretical methods in the literature have been introduced that require the knowledge of the normal stress and shear stress acting at the contact between the soil particles in the river bed and the fluid (mostly water). However, there are not many practical methods to measure the normal stress and the shear stress. In this study, the goal is to find a way to directly measure and calculate the normal force and drag force that acts on the surface of soil particles during the erosion phenomenon before departure in the Erosion Function Apparatus (EFA). If these stresses caused by hydrodynamic forces can be quantified, this will help improve the current erosion model. During the erosion process, drag forces and the resulting shear stresses develop on the surface at the interface between the soil particles and the eroding fluid. Also the eroding fluid causes a decrease in the normal stress induced on the surface of the soil particle, and due to the turbulence in the water, the induced normal stress and shear stress fluctuate. The erosion phenomenon is proposed to be analyzed using an image analysis technique where the movement of the particles is recorded by a camera. Through derivative calculations, the velocity and then the acceleration of each particle are obtained. Then the forces are obtained by using the mass times the acceleration. One major goal of this study is to find out what roles the hydrodynamic shear and normal forces play in the detachment of the particle from the river bed. The answer to this question can also help to obtain the most practical erosion model. It is concluded that shear stresses tend to be larger for rougher particles, while normal stresses are larger in samples with a bigger mean particle size (D50). Also, it was observed that when the particles are more uniform in size, the displacement of the particles prior to detachment tends to be smaller. The vibration frequency of particle movement also tends to be smaller when the soil is composed of smaller particles. Both normal and shear stresses were observed to be proportionally correlated with the erosion rate. The effect of normal stresses tends to be more prevalent on smaller gravel particles, while the effect of shear stresses seems to play a more prominent role in the erosion of larger gravel particles.
Zhang, Zihan (2018). Measurement of Hydrodynamic Forces on Gravel Particles in the Erosion Function Apparatus. Master's thesis, Texas A & M University. Available electronically from