Chen Onn Chin and Wei Li
Associate Professor, Research Fellow, School of Civil and Structural Engineering, Nanyang Technological University, Singapore
EERC, School ofCSE, Nanyang Technological University,
Singapore, 639798 65-7905331, 7921650(fax), E-mail: ccochinfgntu.edu.
sg: 65-7906912, 8615254(fax) E-mail: cwlifgntu.edu.sg
Abstract: Ships with more powerful engines and self-propelled berthing capability are widely engaged recently. Severe propeller jet scour on the bed or bank of navigation channels and port structures may occur. In investigating the mechanics of propeller jet scour, it is crucial to find out the velocity distribution of jet flow. In this study model tests were conducted to examine the axial, tangential and radial velocity of jet flow behind a propeller. From the axial velocity distribution, zone of flow establishment and zone with established flow can be distinguished. At the distance of 3D from propeller jet exit, jet flow was fully set up. Because of air motion above the water surface, velocity distribution is normally slightly asymmetrical about the propeller axis in laboratory condition, especially distant from the hub. Although the axial flow is the main part of propeller jet, it should be noticed that tangential velocity of propeller jet is considerable, which can induce to large scour hole under propeller. Radial velocity is relative small comparing with tangential and axial velocity, and the irregularity of radial velocity distribution is due to the strong influence of tangential and axial velocity. Rotation speed of propeller is the decisive parameter for the formation of jet flow, strength of axial and tangential flow will be increased remarkably with larger rotation speed of propeller.
Keywords: propeller, jet, velocity distribution, scour
In continual increment of world marine, ships with more powerful engines are widely engaged. Driven by large-diameter propellers or side thrusters, velocities in propeller jets or side thrusterjets can reach 6 to 8 m/s, and the strong jet flow can last for the distance of several propeller diameters from the exit (Prosser, 1986). Near to such an intense jet flow, sedimentary material can be removed once the threshold velocity touched. The jet can lead to severe erosion on the bed or bank of navigation channels and harbor structures. The impingement of propeller or thrusterjets is more serious where large ships navigate in shallow water with minimum keel clearance, where Ro-Ro ferries berth regularly facing the ramp, and where ships manoeuvre unassisted with bow and stern thrusters.
Scour induced by propeller or thruster wash has become one of the most important issues for maintenance and design of navigation channels and harbor structures. Bergh and Cederwall (1981) reported that the damage induced by propellers was quite severe in 25 quay structures of Swedish harbors. Chait (1987) found scour damage of many ports of South African. To minimize propeller jet attack and protect marine facilities better, a comprehensive understanding of jet flow is necessary. A propeller jet normally can be divided into zone of flow establishment and zone with established flow. Verhey (1983) put forward a method to determine the efflux velocities behind a propeller. Hamill and Johnson (1993) investigated velocity character of propeller jet flow, it was found that the maximum propeller jet velocity in the zone of flow establishment depends on efflux velocity and propeller structure.
Since the complex turbulence in propeller jet flow, it is difficult to probe the velocities of propeller jet distinctly. This study examines the axial, tangential and radial velocity distribution of the propeller jet flow by experiments. From the analysis of jet velocities, characters of velocity distribution in different direction were revealed, and the rotation speed of propeller was ascertained as the main factor for the formation of propeller jet flow.
The experiments were conducted in a laboratory tank with water depth, H of 45 cm (Fig. 1). Three-bladed stainless steel propeller with diameter, D of 210 mm was used to simulate a ship propeller. Speed of the propeller, n ranged from 200 to 350 rpm. A Micro Acoustic Doppler Velocimeter (MADV) was used to measure the velocities of jet flow, U in axial, tangential and radial direction at the distance of x= 0.5D and 3D, where x is the jetwise distance from the propeller. Data was acquired at sampling rates of 50 Hz and the sampling time is 120 seconds. The MADV was mounted on a rigid stem located in vertical central section of propeller.
The axial velocities of propeller jet are plotted in Fig. 2 and Fig. 3, where Zg is the distance from centerline of the propeller hub. It is found that axial velocities at 0.5D are approximately symmetrical about the propeller axis, the maximum velocities occur beside propeller axis, at the radial distance of0.238D from the axis of propeller hub. The axial velocity on the axis of the propeller is relative small. It is obvious that the distance of 0.5D belongs to the zone of flow establishment, where the propeller hub has the significant obstruction effects on propeller jet flow and reduces the velocities in the center of the propeller jet.
While at 3D the axial velocities are asymmetrical about the propeller axis. Air motion above the water surface can cause horizontal movement of the water flow, which contribute mainly to the phenomenon that axial velocities above the propeller axis is higher than that below the propeller axis. The phenomenon is not prominent at 0.5D because of the rotating effect of the propeller, while as the velocities decay with distance the phenomenon becomes more obvious. Irregular air motion above the water surface can also be reflected by the slight deviation of the axial velocity near to the free surface at 3D. Deviation near to the bed can be attributed to the presence of big particle on the bed. The maximum axial velocity occurs nearby to the axis of the propeller hub, which indicates the propeller jet is fully set up at the distance of 3D.
The tangential velocities of propeller jet at 0.5D are plotted in Fig. 4. The rotating effect of propeller causes water to move spirally in the direction of flow. The spiral expands as it moves away from the axis, boundary of the spiral is an area of zero axial velocity named as slipstream. Rotating movement of the propeller causes the tangential velocity to be negative above the propeller axis and be positive below the propeller axis.
Fig. 5 shows a typical plot of radial velocities distribution. Ideally the radial velocity should be positive above the propeller axis and negative below it. The irregularity of the graph is due to the strong influence of the tangential and axial velocities as the magnitude of the radial velocity is comparably small.
From the comparison of Fig. 2, Fig. 4 and Fig. 5, it can be drawn that axial velocity is the dominant sector, but the tangential velocity is not neglectful at the exit of propeller jet. The maximum axial velocity reaches 100 cm/s, while the maximum tangential and radial velocity is nearly 40 and 15 cm/s, respectively. Velocities of axial, tangential and radial will decease in the direction of jet flow considerably, for example, the maximum axial velocity nearly halves from the distance 0.5D to 3D.
Rotation speed of propeller plays the most important role in deciding propeller jet flow. With the increment of propeller speed, the maximum axial and tangential velocities of the jet increase greatly, but the velocities at the edge of propeller jet are seldom changed. In radial direction, effect of rotation speed is complicated for the relative small jet velocities. The largest rotation speed of350rpm induced to negative crest of radial velocities under the propeller axis. Whereas, the medium rotation speed produced the largest radial velocity at the propeller axis, which indicates the effects of strong turbulence within the propeller jet flow.
In this study, flow of propeller jet was examined in particular experiments. The zone of flow establishment and zone with established flow is identified from the axial velocity distribution. It is found that at the axial distance of 3D, zone of established flow is entirely set up. Different from circular jet flow, velocities in tangential and radial direction of propeller jet have certain influence on scour of bank and bed. This study reveals that flow velocities in tangential direction is nearly 40 percents of axial flow, which can induce to considerable scour hole under the propeller when the ship manoeuvre in shallow water channels. Rotation speed of propeller is the crucial factor for the formation of jet flow. In spite of vigorous turbulence and orderless variation of radial velocity distribution, Strength of propeller jet will be enhanced with the increment of rotation speed of propeller.
References
[1] Bergh H, Cederwall K (1981). Propeller Erosion in Harbors, Bulletin No. TRITA-VBI-107, Hydraulics Laboratory, Royal Institute of Technology, Stockholm, Sweden.
[2] Chait, S (1987). Undermining of Quay Walls at South African Ports Due to the Use of Bow Thrusters and Other Propeller Units, P.I.A.N.C., Bulletin No 58, 1987, pp 107-110.
[3] Hamill, GA and Johnson, HT (1993). The Decay of Maximum Velocity within the Initial Stages of a Propeller Wash, Journal of Hydraulic Research, IAHR, Vol 31, No 5, pp 605-613
[4] Prosser, MJ (1986). Propeller Induced Scour, Reports of The British Ports Association, RR 2570, February, 1986, ppl-33
[5] Verhey, H J (1983). The Stability of Bottom and Banks Subject to the Velocities in the Propeller Jet behind Ships, Delft Publication, No 303, Delft Hydraulics Laboratory, Netherlands.
Fig. 1 Experimental set-up
Fig. 2 Measured axial velocity distribution with different speed of rotation at x=0.5D
Fig. 3 Measured axial velocity distribution with different speed of rotation at x=3D
Fig. 4 Measured tangential velocity distribution with different speed of rotation at x=0.5D
Fig. 5 Measured radial velocity distribution with different speed of rotation at x==0.5D