Machine transplanting as an advanced technology in rice cultivation has been widely adopted. Nevertheless, under extreme wind conditions, transplanted rice seedlings can float and then die off due to external forces generated by wind and water waves. Studying the mechanical characteristics of these seedlings is essential for understanding the underlying mechanisms of rice seedling floating. In this study, the Volume of Fluid (VOF) method is applied to study the flow mechanisms of water waves around rice seedlings. It is found that the pressure differential between the windward and leeward wave sides of the rice seedlings changes during wave diffraction, resulting in unsteady drag forces. The drag force reaches its maximum value when the wave crest propagates to the rice seedlings. Under the condition where waves do not break, increasing wave height does not alter the fluctuation pattern of drag force but leads to a larger drag force amplitude. When the wave height increases sufficiently to cause wave breaking, additional loads are generated, resulting in a significant increase in drag amplitude and exciting new high-frequency disturbances. Increasing wavelength reduces the drag force amplitude but raises its fundamental fluctuation frequency and introduces new low-frequency pulsations. As the rice seedlings tilt, the altered flow mechanism has an uncertain effect on the variation in drag force amplitude and generates new excitations with other frequencies. It indicates that increased wave height, decreased wavelength, and rice seedling inclination may all elevate the risk of rice seedlings lodging. These results could provide valuable references for the prevention of floating rice seedlings.

To meet the operational criteria for mechanical transplanting of young seedlings in paddy-straw-made bowl-seedling trays, a new seedling needle and seedling pushing device have been developed. This paper presents the design process and analyzes the effects of key parameters, including the angle of inclination of flat edge, width of flat edge, length of flat edge, wedge angle, and width of seedling pushing device for new seedling needle and seedling pushing device. Optimization parameters of new seedling needle and seedling pushing device have been validated by orthogonal experiments. The optimized values, such as a flat edge inclination angle of 35°, a flat edge width of 10 mm, a flat edge length of 64 mm, a wedge angle of 60°, and a seedling-pushing device width of 8 mm, satisfy the operational requirements for mechanically transplanting young seedlings in paddy-straw-made bowl seedling trays. The results demonstrate significant improvements in transplanting efficiency within Chinese agricultural systems.

Machine transplanting, as an advanced production technology, is widely used for rice cultivation. However, the transplanted rice seedlings may float over and die under the stress of external force in extreme windy weather. In order to reveal the mechanism of floating rice seedlings, it is vital to investigate their force characteristics. In this study, steady-state simulations of the flow around the rice seedlings are conducted. It is found that the drag force of rice seedlings is generated by the positive pressure of the windward side and the negative pressure of the leeward side. Both the drag force and the moment are proportional to the square of wind speed, regardless of the wind direction angle or the inclined angle of rice seedlings. The drag force coefficient exhibits a complex pattern with the wind direction angle, but decreases as the inclined angle increases. The moment arm of the drag force initially increases and then decreases with increasing wind direction angles, while it decreases with larger inclined angles. It indicates that the variation in wind direction angle may increase the risk of lodging in rice seedlings, whereas the inclined angle does not have such an effect. The results of this study provide a valuable reference for preventing the floating of rice seedlings.