The effect of an upstream hull on a propeller in reverse rotation

Propeller crashback is an off-design operating condition where a propeller rotates in the reverse direction. Experiments (Bridges 2004, T e c h R e p. MSSU-ASE-04-1, Department of Aerospace Engineering, Mississippi State University) have shown that the presence of an upstream hull significantly increases the side force on a propeller in crashback below an advance ratio of J = -0.7. Large-eddy simulation (LES) is performed for a propeller with and without a hull at two advance ratios, J = -1.0 and J = -0.5. LES reproduces the experimentally observed behaviour and shows good quantitative agreement. Time-averaged flow fields are investigated for a qualitative understanding of the complex flow resulting from the interaction of the upstream hull with the propeller blades. At J = -1.0, two noticeable flow features are found for the case with the hull - a recirculation zone upstream in the vicinity of the propeller and a vortex ring much closer to the propeller. In contrast, at J = -0.5, there is a much smaller recirculation zone which is further upstream due to the increased reverse flow. As a result, the hull does not make much difference in the immediate vicinity of the propeller at J = -0.5. For both advance ratios, side force is mainly generated from the leading-edge separation on the suction side. However, high levels of side force are also generated from trailing-edge separation on the suction side at J = 1.0.