There is a notable lack of research on propeller performance at intermediate Re. In this study, a propeller-driven toy submarine was used to examine thrust performance in silicone oil with varying viscosities to adjust Re. Our results show that for Re ranging from 5 to 150, the toy submarine consistently moves in reverse, regardless of the propeller's rotation direction. By modeling the submarine as a disk-propeller combination, we confirmed that at intermediate Re conditions, the interaction between the submarine body and the propeller causes the thrust direction to reverse even when the propeller rotates forward.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.
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