Experimental Study on Propeller Wake Dynamics in Self Propulsion by Towed Underwater PIV

Experimental Study on Propeller Wake Dynamics in Self Propulsion by Towed Underwater PIV Measurement

Jeonghwa Seo1, Bumwoo Han2, Seongtaek Park3, Shin Hyung Rhee1,3*

ISROMAC 2017

International Symposium on Transport Phenomena and Dynamics of Rotating Machinery

Hawaii, Maui

December 16-21 2017

Abstract

A series of particle image velocimetry (PIV) measurements were conducted in a towing tank to analyze propeller wake dynamics of a very large crude oil carrier model in self-propulsion condition. Froude number and Reynolds number of the test condition based on the model length and towing speed were 0.142 and 2,320,000, respectively. By the horizontal wake field measurements using two-dimensional PIV, the difference in wake dynamics between the port and starboard sideswere investigated. The propeller produced greater thrust on the starboard side, because the clockwise-rotating propeller, combined with the non-uniform wake, generated higher pressure and greater acceleration compared to those on the port side. In the results, wake characteristics, such as vorticity of the tip vortices, axial velocity gradient, and wake velocity, were quantified for comparison between two sides.At the center of the tip vortex, local maxima of turbulence kinetic energy and corresponding local turbulence structure were found. In addition, rudder effects on the propeller wake development were also investigated. The pressure imbalance between two sides made the propeller wake to bend over to the port side where thrust was small and pressure was low. However the presence of the rudder blocked the skewed flow to some degree. The hub vortex progress was disturbed by the rudder leading edge andstrong turbulence appeared where the hub vortex met the rudder leading edge.

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Keywords

Propeller Wake—Particle Image Velocimtery—Turbulence Structure

1Research Institute of Marine Systems Engineering, Seoul National University, Seoul, Korea

3Hyundai Heavy Industries Co., Ltd., Seoul, Korea

2Departmentof Naval Architecture and Ocean Engineering, Seoul National University, Seoul, Korea

*Correspondingauthor:

INTRODUCTION

Propeller wake dynamics is very complex, as it contains various fluid dynamic phenomena: periodic development of vortical structures, local pressure fluctuation, turbulent boundary layer, and cavitation. Particle image velocimetry (PIV) is the most renowned experimental flow visualization method to investigate propeller wake field.

Flow visualization methods have been applied to a propeller working in hull wake [1, 2], as well as open water condition. In addition, Seo et al. [3] performed wake field measurement of a fully-appended model ship in self propulsion condition in a towing tank.

The present study aims toinvestigate the rudder effects on the propeller wake working behind a ship, by using a two-dimensional (2D) PIV system. By comparing the propeller wake field with and without a rudder, effects of the rudder on the propeller wake was identified.

1.EXPERIMENTAL SETUP

A scaled model of KVLCC2 hull and KP458 propeller were used [4]. The scale ratio of the test model was 1/100, and the length between perpendiculars (LPP) and propeller radius (R) were 3.2 m and 0.049 m, respectively.Froude number and Reynolds number of the test condition based on LPP were 0.142 and 2,320,000, respectively. At the self-propulsion point, the propeller revolution rate was 12.5 RPS.

A towed underwater 2D PIV system was used to acquire phase-averaged propeller wake field at z/R = 0. The size of the area was 0 < x/R < 4 and -1.5 < y/R < 1.5, as shown in Figure 1.Total six different measurements at different horizontal locations were conducted to complete the flow field measurement.

Figure 1.Fields of View and the Measurement Plane: Case with Rudder (left) and without Rudder (right).

Acquired particle images were analyzed by multiple cross correlations with decreasing interrogation window size [12]. The size of the interrogation window was 32 ×32 pixels with 50% overlap for the last step. The distance between the vectors were around 1 mm.

2.RESULTSANDDISCUSSION

Figure 2 shows Galilean decomposition results with the transfer speed of 1.08 U. By the Galilean decomposition, swirling motion of the tip vortices could be identified. The tip vortices preserved at x/R = 3.0 on the port side, where strong tip vortices were generated.

Figure 2.Galilean Decomposition Results of the Propeller Wake without the Rudder

Figures 3 and 4 show turbulence kinetic energy and vorticity contours, respectively. At the tip vortices center which were identified by vorticity distribution, turbulence strength were also intensive. Besides, hub vortex curved to the port side, due to difference of the angle of attack onto the propeller blade.

By applying the rudder, the propeller wake stream did not bend to port side as the case without rudder and flowed straightly. Besides, hub vortex was not detected in the results. In this case, the hub vortex progress was disturbed by the rudder leading edge and the hub vortex headed for the direction out of the measurement plane.

Figure 5.Axial Velocity Contours of the Propeller Wake without the Rudder

Figure 5.Axial Velocity Contours of the Propeller Wake without the Rudder

3.CONCLUSIONS

In this study, the wake field of a propeller working behind a model ship was measured using a towed underwater 2D PIV system. By the measurement, characteristics of the propeller wake and rudder effects could be identified. The rudder disturbed the hub vortex progress and prevented the propeller wake to curve to the port side.

REFERENCES

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