## 1 Introduction
Its primary application fields are drag reduction and wear resistance. The application targets are the subsoiler, opener, bulldozer and similar implements. At present, there are few reports in the literature about drag reduction of potato digging in other countries. Domestic research on potato harvesters mainly focuses on the design and structural optimization of the whole machine. Various types of potato harvesters have been developed, but the research on potato digging for drag reduction has just started. Aiming at the problem of excessive resistance of potato digging under sandy loam soil in the Northwest China, [PERSON] et al. [1] designed a potato bionic digging shovel based on the contour of the mole cricket's claws, and obtained the conclusion that the use of bionic technology can achieve drag reduction. For solving the problem that the shovel of the cassava harvester has difficulty entering the soil during the harvesting process, a bionic digging shovel was designed by [PERSON] et al. [2] based on imitating the shape of the toe claws of the oriental mole cricket, and the structure and strength were analyzed. In southern China, the traditional potato digging shovel could not leak soil well during the operation process, which caused great digging resistance. To deal with that problem, [PERSON] et al. [3] designed a bionic digging shovel based on the wild boar's arch nose structure by reverse engineering technology. Statics analysis of the bionic shovel was carried out and the results show that, under the same preset conditions, the stress and deformation of the bionic digging shovel are the smallest compared with the traditional shovel, which has a certain reference value. [PERSON] et al. [4] designed a bionic digging shovel based on the front paws of mole crickets as a bionic prototype. Its drag reduction characteristics were studied by discrete element method (DEM). The results show that the bionic digging shovel has better digging performance than traditional shovels. However, there is no relevant design or research on bionic applications of the potato digging shovel for clayey soil in Northeast China. The application of physical or field tests for designing agricultural machinery components is not the first choice because it will result in a significant increase in the cost and length of the designing cycle [12]. During the design and optimization of modern soil tillage tools, the simulation of the interaction between the tools and the soil has usually been performed by using computer software for the first step. Discrete element method (DEM) is a numerical method used to model the mechanical behavior of granular materials. It is especially suitable for numerical methods that simulate the dynamic behavior of granular media [13,14]. Due to its unique advantages, the discrete element method has been used by many researchers to design soil tillage components [15, 16, 17, 18]. [PERSON] et al. [19] designed a serials of subsoiling tools by mimicking the features of bear claws, and they developed a numerical model to simulate the interaction between the bear claw and soil using DEM. [PERSON] et al. [20] used a Hertz-Mindlin (no slip) model in EDEM (Engineering Discrete Element Method; software for bulk material simulation) to simulate and analyze the interactions between the bionic subsoilers which were inspired by the placoid scale rib structure of shark skin and the soil. It is worth mentioning that the Hertz-Mindlin (no slip) model is not the best choice for simulating soil. At present, for the discrete element model of soil particles, researchers primarily use the embedded model based on Hertz-Mindlin in the EDEM software to model and simulate soil particles. [PERSON] et al. [13,15,17,21,22,23] used the Hertz-Mindlin and hysteretic spring contact models to study the stress and plastic deformation of soil particles in the presence of cohesive force and noncohesion. [PERSON] et al.