Experiment and modeling of the rice winnowing process: granular segregation method from an ancient era

Rice winnowing, a process of separating small and large rice grains by air flow, has been practiced since ancient human eras, especially in societies where rice is the main source of carbohydrates (in Asia, Africa, and Latin America). Indeed, this process includes rich scientific rules but was never documented by past societies. Here, we report experimental and simulation investigations of granular segregation by winnowing. Undulating a tray during the winnowing process generates air circulation above the tray. Simulation results showed that the outward component of the air velocity was very large near the tray surface when the tray approached the horizontal position after completing one undulation cycle. This air motion generated forces to falling grains, the accelerations of which are different for grains of different sizes, resulting in separation of grains based on sizes, where the smaller grains were thrown far away toward the tray outer edge. The simulation results can explain well the experimental observations either conducted by manual winnowing or by using a generated air circulation. These results could be useful for designing a new method for separating grains based on size or density. These results also prove that the habits practiced by society in the past are scientific phenomena that have not necessarily been scientifically documented.