EXPERIMENTAL STUDY ON THE STERN FLOW FIELD OF HYDRAULIC PROPELLED HIGH-SPEED CATAMATAN

  Tang Hongwu, Chen Jian and Xie Linfeng

College of Water Conservancy and Hydropower Engineering, HoHai University

College of Water Conservancy and Hydropower Engineering

HoHai University. 1,Xikang Road, Nanjing 210098, China

Tel: +86-25-3713777 ext.50515, 50970 Fax: +86-25-3731332

E-mail: b3710256@public1.ptt.js.cn

Abstract: In this paper, a quantitative flow visualization technique is used to measure the stern flow field of self-propelled high-speed catamatan. The shortcomings of traditional velocity metre are overcome. On the basis of the measurement of the stern flow field, an analysis on the wake field of fast-moving ship is carried out, and a formula of the corresponding velocity at river bottom is established. Moreover, a local sediment disturbing experiment is done to verify the river-bed variation between scouring and silting of Xiaonan Waterway in Pearl River Delta.

Keywords: hydraulic-propelled, high-speed catamatan, stern flow field, flow visualization

1    PREFACE

With the development of large cargo and passenger vessel, especially of fast-moving passenger ship investigated in this paper, it should be systematically studied how they affect the revetments and bottoms of existing channels. Most concern is focused on the effect that the flow does to channel which is caused by fast-moving ship. Investigation has been carried out most on slow-moving ship, but a little on fast-moving one. However, the later is becoming more important subject with economy’s development. Most design on the protection engineering of canal side slope usually consideres ship wave as the main factor at home and abroad. The part of the side slope underwater and river bottom are not protected. And the flow around ship hull will scour them and destroy the whole protection engineering when the flow velocity is greater than the maximum erosion-resisting velocity[4]. So the measurement of the flow field around fast-moving ship and it’s propeller must be done to give scientific reference to the related developments.

The characters of backflow (caused by fast-moving ship hull) and jet (caused by the propeller) are that flow impacts very soon and it’s instantaneous velocity is great relatively. In addition, the flow velocity under ship hull which mainly includes jet velocity is concerned. But a traditional velocity metre such as common propeller-blade current metre can only measure the average value in 5s (that can’t be shorten any more), and it also interferes ship’s sailing. Furthermore, electromagnetic current metre is easily disturbed by the magnetic field that mainly comes from fast-rotating motor of self-propelled model ship. To settle that difficulty, Hohai university has developed a computer quantitative flow visualization technique by itself[5], which is a non-contact measuring system. It can measure the velocity which closes to instantaneous velocity and it’s distribution in a 0.04s interval, and give visual flow image to show velocity field. According to the characters of self-propelled speedy model ship, and to conquer the defect that contact velocity metre can’t measure the flow velocity around ship hull, an advanced computer quantitative flow visualization measuring system is used to measure and analyze the flow field.

2    MODEL DESIGN AND MEASURING EQUIPMENT

2.1    Test condition and model scale

The model experiment is carried out in a laboratory pond in Hydraulic Research Institute of Hohai University. The pond is of 60m long, 12.5m wide, and 0.6m deep. The width and side slope of the pond’s testing part can be adjusted with the requirement from experimental study, through adjusting the side slope made of the grey plastic board that is supported by movable steel frame. On the basis of laboratory condition, research at home and abroad, and consideration of corresponding scale effect[1], 1:30 is accepted as model scale according to gravity similarity law, to assure the reliability of model test and the accuracy of test data. Tab.1 lists the main parameters of catamatan.

Table 1    Main parameters of catamatan

2.2     The design of model ship and laboratory pond

A hydraulic propelled high-speed catamatan is selected, considering tens of passenger vessels navigating in the rivers of Guangdong province at present and development in future. The model ship is designed and made in Shanghai Ship Research Institute. The total length of laboratory pond is 60m. Considering model ship’s performance, the first 20m of pond is used as model ship’s accelerating section, then the 30m is used as pond’s observation section (in this part, model ship sails at an average velocity), and the rest is used as model ship’s stopping section. The water depth of the pond is adjusted by two submersible pumps. There is a underground observing chamber under the observation section. The root of the chamber (i.e. river bottom) is made of glass which is of 1m long and 6m wide. This window is used to set flow visualization monitoring instrument to observe the flow under model ship when it sails.

2.3    The means of measurement to flow field^[2][3]

The measuring system is illustrated with Fig.1. Composed of chemical solution and paint, the tracer which has the same density to water is sent out in the observation part of tested water body. In sheet light (laser or incandescent light etc.), the image of trace particle image accompanying flow is recorded by computer through the image input interface by CCD image sensor whose exposure time is changeable. Then the flow velocity data can be obtained through processing and analyzing the particle movement trace in the image with software. This job is completed by computer under manual participating. The process is figured by Fig.2.

Fig. 1    Schematic diagram of the experimental setup

3    THE MEASUREMENT RESULTS AND ANALYSIS OF RIVER-BED FLOW CLOSE TO STERN

During the experiment, the flow near river bottom caused by model ship navigation at different velocities, is investigated under three different widthes of channel bottom (i.e. B=50m, 100m, 150m) and three different depthes of water body (that is, D=3m, 6m, 9m). The result shows that the water depth of channel and the velocity of ship are key factors which affect the velocity of the flow near river bottom; the affect of the channel width is negligible; and the peak values from reflected wave’s superposition decrease with the width’s increase. Therefore, among the factors which affect channel flow, water depth of channel and ship velocity are decisive. Fig.3 indicates the hydrographs of the longitudinal and horizontal flow velocity near the bottom of channel under the typical ship velocity, B=50m, D=3m, 6m, and m=3 (m is coefficient of side slope). Fig.4 and 5 respectively express some instantaneous flow vector of the horizontal plane close to river bottom and of vertical plane along ship during model ship sailing at the typical velocity.

Fig. 3    The typical hydrographs of water movement in the area close to river bottom

Fig. 4    The typical flow vector diagrams of the horizontal plane close to river bottom

Fig. 5    The typical flow vector diagrams of the vertical plane close to river bottom

Least square method is used to analysis of the test data statistically. The maximum flow velocity (V_max) close to river bottom, when there is no inflow and  ( , V_s is ship velocity), can be given by

                   where

From the experiment, it can be concluded that the flow velocity near river bottom, which caused by ship sailing at higher critical velocity, is very little and can’t scour river-bed commonly on the condition of water depth greater than 6m, but it’s effect is great on the condition of water depth less than 6m; especially when ship sails at critical velocity, the flow velocity at river bottom is quite high.

From Fig.4, it can be found that the maximum flow velocity near river bottom takes place under the stern, and this is agreeable to up-and-down law of sailing ship. When ship descends, the surplus depth decreases; and this results in the flow velocity under ship hull increases. During ship navigating, the flow direction under ship hull is firstly the same with the one of ship navigation, but it is quickly changed under the stern in backflow; then the flow oscillates to and fro for stern wave and decays step by step. On the condition of smaller width of water surface, there are wave reflection and waves superposition, which intensifies the water movement. The kinds of water movement varies with ship velocity. It varies slightly and lasts shortly when ship sails at a speed lower than the critical velocity; but it does most oppositely when ship sails at the critical velocity. When ship sails at a speed higher than the critical velocity, because of the stem wave very small there is no stem flow which direction is the same with the ship navigation’s; and the surface velocity of stern flow is large for it is interfered by the jet from the propeller in high speed; then there is backflow under the stern and it scours river bottom very weakly. In this situation, the angle between peak line of ship wave and shoreline is small, 17°or so. This is, the wave’s propagation direction is nearly vertical to bank and stronger reflection wave takes place. Water body is strongly disturbed because of the superposition of the reflection waves from two sides of channel. Especially the reflection will last longer if wave peak is steeper, and this phenomenon takes place more easily on the condition of little water depth. According to test data, it can be concluded that the flow velocity at river bottom will do slightly when ship velocity increases. For example, on the condition of D=3m, the maximum velocity at river bottom will be 0.86m/s when ship velocity is 12m/s, but the former will be 1.01m/s when the later is 15.06m/s. However, this situation appears weaken with the water depth increase; in other word, river bottom is disturbed more weakly.

Fig.5 indicates how the velocity of vertical plane along ship distributes under the typical ship velocity on the condition of D=3m and 6m. From it, it can be found that flow velocity is the greatest under the stern, then wave action appears strongly in water surface, and wake takes place at river bottom. Furthermore, the velocity decreases with water depth’s increase.

4    CONCLUSIONS

Several opinions are obtained by the experimental study on stern’s flow field of high-speed catamatan.

(1) Using self-propelled high-speed catamatan is more close to the real situation than usually using towing basin in test. It can over-all express the characters of ship navigation relatively. Through the local river-bed disturbing experiment of Xiaonan Waterway in Pearl River Delta, the rule of the flow velocity at river bottom is confirmed and this conclusion is creditable.

(2) The rule of the wake at river bottom, which is caused by high-speed catamatan sailing is agreeable to ship wave’s. That is, wake velocity increases with F_d increase when ; it reaches to the maximum when ; and it decreases step by step to a stable value when . Water movement which is caused by ship sailing, is stronger on the condition of smaller section factor of channel; but it weakens when D>6m, thus it has less effect on river bottom’s erosion and the maximum flow velocity is relatively large at slope foot. In a word, the effect decreases with water depth increasing. In the experiment, the maximum flow velocity at river bottom reaches to 1.0m/s and the maximum velocity at slope foot reaches to 1.20m/s (D=3).

(3) The difficulty of how to measure the stern’s flow field of self-propelled model ship is solved by using the quantitative flow visualization technique. This gives a good start to further investigation of ship fluid dynamics.

References

[1]    Wang Shuitian 1980(4), 1981(1-3) Study on ship wave. Waterway and harbor. In China.

[2]    Xu Youren & Tang Hongwu & Zhou Haoxiang 1992 Real-time acquisition and quantitative analysis of flow image traced by hydrogen bubble through computer. Journal of Hohai university. Vol.20 No.3. In China.

[3]    Tang Hongwu & Yan Zhongming & Zuo Dongqi 1994 Digital image technique’s application in mixed flow field of double jet. Journal of Hohai university. Vol.22 No.5. In China.

[4]    Baxin A.M. etc. 1987 Fluid dynamics on shallow-water ship. Energy sources publishing house. Translating in China, from Russia.

[5]    Tang Hongwu Ect., Particle image velocimetry technique and its application of free vortex at vertical intake, J. of Hydraudynamics, Ser.A,Vol.14,No.1