Propeller wake impingement on a strut

He, Moqin (2006) Propeller wake impingement on a strut. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

An unsteady and strong vortical propeller wake can have significant interaction with a body in its race. Since the pod and strut is located downstream an operation propeller for a tractor-type podded propeller, it faces risks of earlier cavitation on the strut, severe vibration of the pod unit, and noise of the propulsor. The aim of this work is to develop a prediction method for the pressure fluctuation on the surface of the pod and strut. -- The study consists of two parts, experimental and numerical. In the experimental study, a podded propeller was tested in a cavitation tunnel. The tip-vortex interacting with the strut was visually investigated, and the strut surface pressure and the propeller shaft loads were measured. By using a data processing procedure, the load and pressure measurements were broken down into time-averaged and phase-averaged components. These components were then linked to the steady and dynamic performance of the propulsor. Pressure measurements showed that the low time-averaged pressure was concentrated in the area around the leading edge of the strut. The lowest pressure was located on the strut suction side near the junction of the strut and pod. However, the largest amplitude of the phase-averaged pressure occurred on the strut leading edge where the tip-vortex impacts the strut. Based on measurement of the propeller shaft loads, the wake impingement effect on the propeller performance was evaluated. The evaluation was made by comparing the shaft loads measured in two tests with and without the installation of the pod and the strut. These showed that wake impingement had no significant effect on the propeller shaft loads, neither on the steady component nor the dynamic components. -- In the numerical study, a Wake Impingement Model (WIM) was developed and inserted into a panel code, PROPELLA. Simulations of the pressure variation on the strut surface were then conducted using the software. The WIM starts from the relaxed and aligned propeller wake. The motion of the wake sheet was traced step by step. For each time step, every wake panel moved from its previous location to a new position. The new position was determined by the product of the time interval and the local induced velocity. When the wake approached the strut, a scheme was applied to keep the wake from penetrating the body surface. To avoid the large numerical disturbance resulting when two dipole panels get too close, the two dipole panels were merged before the calculation of influence coefficients. Numerical simulations on the same tractor-type podded propeller as that in the experimental study were conducted. Although the amplitude of the pressure fluctuation in the tip-vortex/strut interaction zone was under-predicted, comparisons of the numerical results with the experimental data indicated that the simulated pressure was in good agreement over most of the impingement area. Both experimental and numerical studies with the WIM show that WI has insignificant (around 2%) effects on the propeller thrust and torque.

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