Hydrodynamic Performance Evaluation of an Ice Class Podded Propeller under Ice Interaction

Fluid-structure interaction between an ice sheet on the water surface and a podded R-Class propeller was examined and analyzed in terms of numerical simulation using a newly enhanced unsteady timedomain, multiple body panel method model. The numerical model was validated and verified and also checked against various previous in-house experimental measurements. The simulation was performed in a real unsteady case, that is, the ice piece stands still and the podded propeller moves and approaches the ice piece until collision occurs. Experimental data were taken from a previous cavitation tunnel test program for a bare R-Class ice breaker propeller under open water conditions, for the R-Class propeller approaching a bladeleading-edge contoured large size ice block under the proximity condition, and from an ice tank test program for a tractor type podded/strutted R-Class propeller under open water conditions. Comparison between experimental and numerical results was made. A general agreement was obtained. The magnitude of force fluctuations during the interaction increased significantly at the instant immediately before the impact between the propeller blades and the ice piece. INTRODUCTION Studies of ice effects on navigation have been extensive. Ice propeller interaction is mainly divided into two broad categories: ice induced force fluctuation due to proximity, that is the suction force created between the propeller and the ice in front of it, which is called blockage effect and is hydrodynamic in nature; and, the contact force due the collision impact of ice piece on propeller and then the force on the propeller when it mills the ice [Veitch 1995]. There are some other ice conditions such as propeller blades working in a local flow domain of mixed broken ice particles and fluid. During the mid and late 1990s, some experimental investigations were also performed at the Institute for Ocean Technology and Memorial University of 1 Newfoundland [Doucet 1996]. In parallel with the experiments, a panel method (PM) code, or boundary element method (BEM) code, from NASA, called PMARC, was modified to simulate the ice blockage effect on propeller thrust and torque coefficients under the proximity condition (shaft thrust and torque coefficients versus different fixed gap values) [Bose 1996]. At the same time, an inhouse unsteady panel method code especially targeted for propeller applications was developed [Liu 1996a], based on a boundary element method (BEM) that was developed for oscillating foil applications [Liu 1996b]. This in-house code, PROPELLA, was first used to predict the hydrodynamic effects of ice blockage in terms of the proximity, on shaft thrust, torque and normal force fluctuation of the same R-Class propeller [Liu 1996a, Liu et al. 2000]. Further, PROPELLA was modified to implement a previous ice contact load model [Veitch 1995] for the R-Class propeller. Shaft force fluctuations of several skewed ice class propellers were also obtained and analyzed [Veitch et al. 1997, Doucet et al. 1998]. In a relatively recent work, variable proximity between a wall-shaped ice blockage and the R-Class propeller was numerically modeled [Liu et al. 2005]. In this numerical model, the ice blockage was set to stand still whereas in the previous work the ice blockage advanced with the propeller. That is, in the recent numerical model, the propeller moves with the advance speed, approaching the ice blockage far away. A more recent experimental work was also conducted in-house for a podded ice class propeller with the same blade geometry of the R-Class propeller, except that the podded propeller is 1.5 times the diameter so the new R-Class propeller model was able to fit on a pre-tapered hub [Akinturk et al. 2003 and Wang et al. 2005]. In this experimental work, the ice blockage is a sawn ice in a notch shape, not the same as the wall-shape in the previous studies mentioned above. The current numerical work simulated this most recent experimental condition for a podded Copyright © 2008 by ASME Copyright © 2008 by National Research Council of Canada propeller interacting with ice to observe the transient force fluctuations in this case. PROCEDURE AND METHOD The method used in the current study is a 3-D unsteady low order panel method, in a dynamic object oriented multiple-body from. The arrangement of this form is to set objects moving in a stand-still fluid. The current numerical model was developed to simulate the ice block in front of a puller type podded propeller. Figure 1 shows the interaction scenario: the sawn ice is set to stand still in front of the podded R-Class propeller. As the diameter of the propeller is 300 mm, before the propeller approached the triangle region of the ice block, the distance from the tip of the propeller to the side edge of the ice was 350 mm (1.167R), which is greater than the diameter of the propeller, so the blockage effect of the side edge of the ice was neglected. In numerical simulation, at a time equal to zero, the propeller was aligned with the base of the ice triangle. As the triangle was equilateral with an apex angle of 90°, the vertical distance was the half of the base. Therefore, the initial distance between the propeller plane to the tip of the triangle was 350 mm. The final time step when the zero proximity occurred was when the propeller plane passed the y-axis at which position the propeller plane and the ice edge form an equilateral triangle with a base of 300 mm.