Propeller can't propel at Intermediate Reynolds Numbers - numerical simulation*

Microrobots hold significant potential in biomedical applications, such as targeted drug delivery and minimally invasive surgery. Advances in microrobots have spurred interest in their propulsion mechanisms, particularly for propellers operating at low Reynolds numbers (Re). While extensive research has explored propeller performance at high Re in aviation and maritime contexts, the behavior of propellers at intermediate Re remains underexplored. In this study, we use a simplified model comprising a propeller and a disk to investigate why a toy submarine's propeller, when rotated forward, causes the submarine to move backward within the Re range of 5 to 130. Through 3D numerical simulations, we demonstrate that at high Re, the jet behind the propeller is well-defined. As Re decreases, the jet flow transitions to a configuration with an increasing angle to the axis and significant inward flow behind the propeller. This shift signifies a growing dominance of centrifugal forces. Additionally, a negative pressure region around the propeller expands towards the disk with decreasing Re. At lower Re, this negative pressure creates a substantial backward force on the disk, counteracting the forward thrust from the propeller and resulting in an overall backward force.