Experimental Study On The Drag Force Coefficient Of Natural Geotechnical Particle Materials In The Liquid

Calcareous sand is often used as a filling material in construction of artificial islands in South China Sea.The permeability of calcareous sand has significant influence on the consolidation and settlement of soil mass,which affecting the safety and stability of marine structures.Calcareous sand is usually composed of coral,shellfish and other marine biological remains,so its chemical and mineral composition is quite different from terrestrial quartz sand and other geomaterials.The internal and external pores are extremely developed,particle morphology is very irregular and easy to break,which leading to its difference compared with terrestrial quartz sand in mechanical and water properties.The drag force coefficient is an indispensable physical quantity that expresses the interaction between fluid and granule.It is also an important parameter to characterize the permeability of granular soil.At present,the research is very limited on the drag force coefficient of natural geomaterials such as calcareous sand at both home and abroad.In present study,the surface morphology of calcareous sand and quartz sand is analyzed by aspect ratio and circularity,and a modified three-dimensional parameter is used to quantify the shape of two kinds of natural irregular grain materials including calcareous sand and quartz sand.It is found that the average value of ? for quartz sand particles is 0.7,but the average amount of calcareous sand particles is 0.57.The fractal dimension of calcareous sand and quartz sand particles is discussed.It is found that the fractal dimension of calcareous sand is larger than that of quartz sand,which indicates the shape of calcareous sand particles is more complicated.Through the self-designed and processed settling device,a series of settling tests for natural geotechnical and mechanical particle were carried out with individual particles in liquid.The sedimentation process was recorded by a high-speed camera,and the particle sedimentation terminal velocity was obtained by image processing technique.For the experimental results of regular particles,this paper systematically analyzes the errors of the previous models in the aspects of drag force coefficient and grain sedimentation terminal velocity.It is found that the calculation of Brown and Lawler model is more accurate.Then the drag force coefficient CD and the Reynolds number Re are calculated by Ut,and the drag force coefficient of the shape and Reynolds number interval is analyzed for the natural and irregular materials of calcareous sand and quartz sand.The drag force coefficient decreases with the increase of Re and ?.By introducing the shape influence coefficient ?,which can reflect the influence on the CD in different Reynolds numbers region.Finally the semiempirical drag force coefficient model of calcareous sand and quartz sand are fitted respectively including CD,Re and ? parameters.The model can consider the effect of irregular particle shape on the drag force coefficient,and improves the accuracy of soil permeability prediction.It is also important to calculate the consolidation and settlement of calcareous sand in the South China Sea reef filling project.By referring to the research results of Di Felice et al.,The relationship between the calcareous sand particle aggregates and single particle drag force was established and embedded into the CFD-DEM program.Through the free sedimentation tests of single particle in liquid,whose results of the model test are compared and verified,and the validity of the program is verified.It is proved that the model can be used to simulate the terminal velocity more effective for the irregular particles settlement,in which the new model embedded.For the next step,this model will play a good role in the research of seepage characteristics of the geomaterials.