| Due to the discrete nature and the nonlinear dissipation,granular matter often exhibits some complex and unique properties under the action of external forces,which makes it has gradually become the forefront of research in mechanics,physics,and many engineering applications.Granular matter in a statically stacked state usually exhibits obvious solid-state characteristics,and has a certain strength and resistance to shear.When an object moves within it,the granular matter will yield and flow,and the object will also be subject to the force generated by the collision or friction of surrounding particles,which is obviously different from the resistance or drag it receives in the liquid.This dissertation mainly aims at the basic problem of the rotational motion of objects in granular matter.Based on the discrete element method and corresponding multi-scale statistical methods,we have studied the resistance characteristics of the rotor in granular matter during continuous rotation and analyzed the dynamic behavior of local particles after the system yields,and then try to establish a scaling law between the resistance and the depth of the rotor.Based on this,the influence of rotor shape and rotation speed on the drag characteristics is discussed.The main work is as follows:Firstly,we establish the discrete dynamic model of a disk-shaped rotor rotating around its axis in granular matter and systematically study the interaction between the rotor and particles.The change characteristics of the torque received by the rotor from the initial rotation to the steady stage were obtained,and the force characteristics of different surfaces of the rotor and the evolution law with time were analyzed.To explore the dependence of resistance on depth,based on a series of simulation results,we established a scaling law between the torque experienced by the rotor during steady state and its embedded depth in the medium.Secondly,the influence of the rotation on the characteristics of the stress distribution when the rotor is rotating in the granular matter is studied by means of parameter analysis.Furthermore,the dynamic behavior of surrounding particles during the rotation is analyzed,and the local volume fraction in the area around the rotor,the contact between particles and the rotor,and the characteristics of particle migration are discussed.Based on the energy spatial distribution of particles after rotation,we found a dumbbell-shaped excitation region centered on the rotor,and further discussed the geometric characteristics of this excitation region and its influence on the formation of the steady state of the system.In addition,the anisotropic characteristics of the stress distribution in the system caused by the rotation of the rotor are analyzed,and the influence mechanism of the formation of arched structures around the rotor on its appearance in the low pressure region is discussed.Finally,the effects of the shape of the rotor on its resistance in rotation are studied based on different shaped rotor models consisting of cylinders.According to the evolution characteristics of the torque,vertical force,and number of contacts of the disk and cylinder rotors with the rotation speed,the influence of the rotor shape on the interaction between the rotor and the particles was analyzed.Based on this,the influence of the rotation speed on the resistance characteristics of the rotor and the motion state of the surrounding particles is analyzed,and the cause of the resistance difference caused by the rotation speed is further explored.Based on the established mechanical model and effective numerical calculation method,this dissertation systematically studies the interaction process between the object that rotates and the granular matter.These quantitative studies are a very important way to deeply understand the resistance characteristics of mechanical equipment under continuous or reciprocating motion in a discrete medium.The study of related issues,on the one hand,reveals the complex interaction process between moving objects and discrete media,and can also provide theoretical basis for the related equipment's operating performance,working efficiency and life prediction and structural optimization design in different environments. |
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