改进颗粒组构力学模型模拟筒仓卸粮成拱细观机理

Simulation of meso-mechanism of silo unloading grain aching based on improved particle composition mechanical model

现有研究表明筒仓卸粮成拱和粮食的内外摩擦密切相关,但现行相关离散元模拟采用单一圆形颗粒,模拟粮食的真实接触面积要小很多,不能客观反映卸粮过程粮食的摩擦情况,也较难还原卸粮成拱现象的细观动态过程。该文针对离散元模拟中圆形颗粒的内摩擦力小于真实粮食内摩擦力的缺陷,在已有PFC离散元程序基础上,添加了黏度系数较大的微型颗粒模拟粉尘,建立了一种改进颗粒组构力学模型,采用几何方法判断圆形颗粒间的接触情况,推导出基本单元间力-位移关系。基于典型事故案例和室内试验成果,采用建立的颗粒组构力学模型模拟了卸粮成拱动态过程中圆形大颗粒间以及圆形小颗粒与仓壁之间的力-位移关系。研究表明在卸粮过程中,切应力在剪切位移达到0.3mm的过程中,迅速提高,达到最大值切应力的60%,所得的切应力位移图与其应力特征曲线与试验成果基本吻合。以试验结果曲线各点值为标准值,改进后模拟结果曲线值的标准差相比改进前减小37%,说明曲线相似度更高,利用该模型可更加客观反映筒仓卸粮成拱的动态细观机理。该文提出的改进颗粒组构力学模型,不仅可用于模拟卸粮成拱机理模拟,而且对于模拟散颗粒流动特性、散体-仓壁相互作用机理都具有一定借鉴意义。

The silo has the advantages of small area,good sealing conditions,low circulation costs,low cost and etc.It is currently the leading warehouse type of grain storage in China.Most silos still use gravity-based discharging methods,however,arches often form during silo discharging,it causes the pressure that should have been taken from the bottom of the warehouse to the wall of the warehouse.As a result,cracks may form in the silo walls causing serious damage.In addition,when there are workers above the grain surface or when the arch is manually broken,the collapse of the arch will not only result in personal injury or death of the grain surface staff,but also may damage the warehouse structure and facilities in the warehouse.In recent years,with the development of building technology,the silo volume has been continuously increasing,the diameter and height of the silo have also become larger and larger,and the related accidents caused by unloading and arching silos have also become more frequent Silo arch accidents in Daqing 2013,Lanxi 2015,in Heilongjiang province,and in Jing County in 2016 in Anhui province,for example,have resulted in the destruction of some silo facilities and casualties.Existing studies have shown that silo unloading and arching are closely related to the internal and external friction of grain.However,in those studies,a single circular particle is used in the current discrete element simulation.The real contact area of simulated grain is much smaller.As such,the simulation cannot objectively reflect the grain of the unloading process.The frictional situation is also more difficult to restore the mesoscopic dynamic process of unloading grain arching.In this paper,based on the existing PFC3D discrete element program,the micro particle simulation dust with larger viscosity coefficient was added to create an improved particle for the defect that the internal friction force of the circular particle in the discrete element simulation was smaller than that in the real grain.We used geometric methods in the structural mechanics model to determine the contact between the circular particles and derives the force-displacement relationship between the basic units.Based on typical accident cases and laboratory test results,the established particle structure mechanics model was used to simulate the force-displacement relationship between circular large particles and between small round particles and walls in the dynamic process of unloading grain and arching.The displacement map of shear stress and its stress characteristic curve were basically consistent with the experimental results.By using this model,the dynamic meso-mechanism of unloading and arching silos can be objectively reflected.The improved particle structure mechanics model proposed in this paper can not only be used to simulate the unloading and arching mechanism simulation,but also has certain reference value for the simulation of the characteristics of loose particles flow and the mechanism of the interaction between the bulk and the silo wall.