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High Fidelity Numerical Investigation of Rotor Wake Dynamics in the Near Field Open Access

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The near wake of a notional submarine propeller was investigated computationally to identify the evolution and interaction of key flow structures as they travel downstream from the propeller plane. The time accurate flow field was solved using a high-fidelity large eddy simulation method with the boundary conditions on the propeller enforced through the immersed boundary method. A series of instantaneous solutions were generated for the unsteady analysis as well as to calculate the phase-averaged solution and turbulence statistics.Specific emphasis was placed on studying the stability of the helical tip vortices generated by each propeller blade. The vortices visually manifested instability modes were illustrated and the unstable behavior was quantified for a range of propeller operating conditions. A dependence on this operating condition was seen for the degree of instability and subsequent location of breakdown for the vortices. Particularly, the stronger vortices of the more highly loaded case were showing greater instabilities and faster breakdown than the weaker vortices of the lightly loaded case.The wakes at different operating conditions were analyzed in detail to investigate the underlying causes of instabilities in the vortices. A strong interaction was found between neighboring blade vortex sheets and tip vortices for the highly loaded case. This interaction was caused by differences in the wake alignment, or the helical pitch angles of the flow structures for each loading condition.Consequences of the instability and eventual breakdown of the tip vortices were illustrated and quantified through turbulence in the wake. Due to the unstable behavior and the breakdown process, the tip vortices become an additional source of turbulence downstream of the propeller plane while the rest of the wake is decaying. Finally, a realistic scenario of disturbed flow upstream of the propeller was studied through a coupled simulation. A notional appendage was designed based on model scale geometry of a submerged body and simulated upstream of the operating propeller. The dynamics of the propeller tip vortices were compared between the disturbed and undisturbed flow scenarios. The consequences of the upstream disturbance were quantified through both the global propeller force coefficients and the detailed evolution of the tip vortices. The appendage was found to have a measurable impact on the instability of the tip vortices while the location of vortex breakdown remained constant.

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