Research On The Straw-soil-rotary Blade Interaction Using Discrete Element Method

After harvesting of the crops, the residues (straw) are incorporated into soil to increase soil organic carbon storage and reduce soil erosion and decrease environmental pollution and improve straw utilization. With the development of returning straw to field in China,the objective that tillage machine treats with has become the soils with high straw content or the mixture of soil and straw. The design of tillage machine does not have theoretical foundation because there is a lack of the knowledge of soil-tillage machine interaction.Understanding of the mechanical properties of such mixed soils and the interaction of soil-tillage machinery in detail can provide a theoretical base for designing and optimizing of related tillage machines.Soil-tool interaction is usually characterised by forces at the soil-tool interface and movement of soil, this interaction has become straw-soil-tool interaction because of the existence of straw. This paper will investigate the interaction by analyzing the soil and straw movement and blade force. Firstly, the effect of straw on soil shear strength is studied.Secondly, an IT225 rotary blade was used to study the straw-soil-tillage tool interaction in indoor soil bin. Thirdly, the simulation models of soil-rotary blade interaction and straw-soil-rotary blade interaction based on EDEM were established. Fourthly, the field investigation of straw-soil-rotary blade interaction was done.The major research work and results are summarized as following:1. Based on the direct shear test of soil, the remolded straw-soil sample was opted to study the type of straw and straw density on the shear strength of soil. There were high correlations observed between field and remolded samples using WS for the cohesion and internal friction angle. The remolded soil samples with different straw densities represented an increasing trend of soil shear properties, the cohesion represented a linear trend, whereas the internal friction angle showed a quadratic trend. By overlapping the cohesion and internal friction angle curves, it was observed that both RS-soil and WS-soil samples have same cohesion and internal friction angle at straw densities of 0.63% and 0.46%. As a whole, the increase in shear properties of soil due to WS incorporation was higher than those of RS.2. A study of straw-soil-rotary blade interaction was conducted under the controlled conditions in indoor soil bin through soil and straw displacement, torque of rotary blade and straw burial. The results showed that though the straw mixture could be used to study straw displacement, single-sized straw would be better than straw mixtures for soil displacement. Higher rotavator kinematic parameter resulted in larger soil and straw movement as soil and straw being spread in a larger area. Straw burial could be used as an indicator of movement in vertical direction; lower rotavator kinematic parameter buried less straw than higher one. The maximum blade torque increased with increasing the rotational speed, the blade torque in soil with straw covered was higher with 15% than that in soil.3. The simulation model of straw-soil-rotary blade interaction using distinct element method (DEM) could be hypothesized to provide a better understanding of the straw movement. The straw displacement both in soil bin and simulation increased with increasing rotational speed of blade. Moreover, the displacement in forward direction was larger than that of side direction at all rotational speeds in both experiment and simulation.The relation equation between relative error and rotational speed can be used to predict the experimental values, and the error between the predicted ones and experimental ones were 8.7% and 9.3% for forward and side displacements. Microscopic movements of straw were analyzed by tracing three specific straw particles. The movement of straw near sidelong edge or lengthwise edge was affected by cutting edges, the straw located at inner side of sidelong edge slipped off along the border of sidelong edge when rotary blade started to cut the soil while the straw near lengthwise edge was pushed to move along the lengthwise edge at the initial stage of soil cutting and later was tossed upward. The movement of straw away from cutting range was only affected by soil disturbance. The straw particles near lengthwise edge or transition edge could be buried directly during tillage.4. The experimental results and simulation results of soil movement were compared,and the mesocopical movement of soil in three directions were also analyzed. The results showed that the average error of soil displacement between simulation results and experimental results were 24.9% for soil forward displacement while 15.3% for soil side displacement. The soil forward and side displacement in experiment increased with increasing rotational speed of blade, the forward displacement was larger than the side displacement. The displacement of shallow soil was the largest, and then middle soil and deep soil had the minimum displacement. The closer the soil to the rotational point was, the larger the forward and side displacements of soil were. For the particles in tillage scope, the percent of particles which moved to the opposite direction are 26.2%、72.1% and 48.4% for shallow soil, middle soil and deep soil, respectively. Most soil particles moved backward in horizontal direction during tillage process. The direction of side force and side displacement depended on the situation that the soil particle lay in the left or right side of the lengthwise edge axis. If the soil lay in the left side of the lengthwise edge axis, the side displacement was towards the left and vice versa. The soil particle moved downward with the rotary blade at the beginning of soil cutting, and later it slipped from the border of blade and being tossed up.5. The paper studied the forces and torque requirements of a single blade working in soil and soil with straw covered, rotational speed of blade was also taken into account using DEM modeling and indoor experiment. The comparison between experimental and simulation torque showed that the experimental torque was higher than that of simulation with 19.1% in soil with straw covered and 16.3% in soil. Straw affected the 3-D forces and torque requirement during tillage and the values in soil with straw covered was always higher than those in soil. The resultant force, forward and side force in two soil conditions all increased from nearly 0 N to a maximum values and then decreased to approximate 0 N.The vertical force increased from nearly 0 N to a maximum and then decreased to 0 N in upwards direction, later another rising and falling period occurred in the opposite direction.The maximum horizontal force and vertical force during tillage increased with increasing rotational speeds while there was no clear trend of variance for side force. The higher blade force in soil with straw covered in the initial stage was attributed to the existence of straw.6. The field experiment was done to investigate the straw-soil-rotavator interaction by studying the soil and straw movement, straw burial, and soil breakage at three rotational speeds. The results showed that the lateral and forward displacements of both straw and soil increased with the rotational speeds. Forward displacement of soil and straw was significantly larger than that of lateral displacement. The addition of straw increased the cohesion and internal friction angle of soil, thus strengthened the soil. The penetration resistance of shallow soil (0-50 mm) decreased significantly after tillage operation, while the resistance of middle soil (50-100 mm) only decreased significantly under higher rotational sppeed and the resistance of deep soil (100-150 mm) did not change significantly at all. The mean clod diameter was smaller at higher rotational speeds, while the rate of soil breakage increased with increasing rotational speeds. The study has also shown that the rotational speeds had positive effects on the straw burial. The rate of straw burial increased with the increasing rotational speeds. By comparing the field experiment, indoor experiment and simulation results of soil and straw displacement, it was found that both the soil and straw displacement in field experiment were smaller than those in indoor experiment because of different soil environment. Moreover, simulation model could also be used to simulate soil and straw displacement in field to some extent.