EXPERIMENTAL STUDY ON DRAG REDUCTION OF A SHIP DUE TO A DRAINAGE SLIT

Transcription

1 Computer Modelling and New Technologies, 213, Vol.17, No. 4, 83-9 Transport and Telecommunication Institute, Lomonosov 1, LV-119, Riga, Latvia EPERIMENTAL STUD ON DRAG REDUCTION OF A SHIP DUE TO A DRAINAGE SLIT A. M. Liu, J. M. Wang School of Transportation, Wuhan universit of Technolog No.14, Heping Street, Wuhan, China Phone: (+86) lamfluid@126.com In order to reduce the ship s resistance, the drag reduction effect is figured out b a drainage slit on a medium or low speed ship in wind tunnel. The double-model was adopted as the testing ship model. Two cases including the original ship model and that with a drainage slit from bow to stern were investigated. The velocit distribution for the ship models were measured b five-holes Pitot tube, wall surface pressure were obtained through the silicon-based micro pressure sensors, and the viscous drag force was measured using the self-made strain gauge as well. The test results showed that the flow velocit of around the ship model with the slit changed not considerabl comparing with the original ship model except near the inlet and outlet of the slit. However, the pressure coefficient decreases obviousl on the bow and varies on the stern; the drag reduction ratio was about 5%. Kewords: drag reduction, pressure measurement, force measurement, velocit measurement 1. Introduction As is well known, when the fluid flows past a blunt bod, because of the boundar laer separation, the average pressure on the rear of the bod is considerabl less than that on the front. Thus, a pressure drag, which composes the most of the total viscous drag in some cases is developed. In the light of these results, we can assume that the total drag mabe decrease if the pressure difference between on the rear of the bod and that on the front reduces b means of some method. The most straightforward wa perhaps is to open a flow passage between the front stagnation point and the rear stagnation point. Whereupon the pressure on the front portion of the bod probabl decreases and that on the rear portion ma increases, therefore the total drag ma be decreased. From the published literatures the flow passage is commonl slit or hole. Iutsu [1] carried out fluid force measurements for a circular clinder with a slit, and ajima and Sano [2] investigated the drag reduction of a circular clinder due to double rows of hole. These researches indicated that the reduction effect is good. Moreover, Tan [3] studied the flow around a hollow circular clinder with several splits and the drag reduction mechanism b using the hdrogen bubble flow visualiation technique. The experimental result demonstrated that the drag force decreased obviousl and the attack of a split can hardl affect the drag of the clinder. As to medium-speed or low-speed ship, especiall to plump ship, the viscous drag can reach an amount up to 8% of the total drag. Accordingl, it is important to reduce the viscous drag as far as drag reduction of ship is concerned. Chen et al. [4] carried out research on drag reduction of a ship due to leading edge hole using numerical simulation method. The results showed that if the position and form of flow passage reasonabl is chosen, the pressure drag decreases drasticall and then the total viscous drag reduction ratio reaches about 2%. Based on the above research results, in order to make sure of the effect of this kind of the drag reduction method, an experimental stud of drag reduction was made in this paper. 2. Experiment Arrangement Experimental research is carried out in wind tunnel. The total drag force, wall surface pressure and velocit distribution for the ship models were respectivel measured at Renolds number of Re= (based on the length of waterline of the testing ship model). The Strain gauge was emploed to measure total drag force. The ship surface pressure distribution was obtained through the pressure sensor. The velocit distribution around the ship model was obtained b the five-hole Pitot tube Ship Model In the present experiment, according to the results of Chen et al. [4], one two-tailed ship with bulbous bow and no rudder were selected as the test ship. For the wind tunnel tests, a model ship was 83

2 made of wood with a scale ratio of 1/97.5, which was.8 m in the length of waterline and.16 m in the molded breadth and.41 m in the molded draft. Table 1 and figure 1 provide the principal particulars and the bod plan of the ship. Two cases including the original three dimensional ship model and that with a drainage slit from bow to stern are shown in figure 2. The inlet of the drainage slit is at the high pressure region of bow, and outlet, below stern transom plate (shown in figure 3). The projection area of inlet in the direction, which is perpendicular to the incoming velocit, equals that of outlet, i.e., the are 11.87% of the ship s maximal cross-sectional area, or.45% of ship wet area. It is important to point out that a double-model is adopted, that s to sa, two same ship models stick together at the water plane. TABLE 1. Principal particulars of the test ship Lpp(m) B(m) Draft(m) Wetted surface area(m2) Prototpe Model Height(mm) Particular Frame 1 15 Oct 21 title Frame 1 9 Oct 21 title Bredth(mm) Figure 1. The bod plan of the ship(mm) Sl it Outlet Inlet Frame 1 15 Oct 21 title Case 1. Original ship model Stern transom plate.34t -.2 Inlet Outlet.1T Propeller shaft Case 2. Ship model with a drainage slit from bow to stern Figure 2. Ship models T.5.5T Figure 3. Inlet and outlet of the slit (T is draught) 84

3 2.2. Wind Tunnel The measurement of mean velocit components and pressure were carried out in a single circuit wind tunnel with open test section. The open diameter is 1 meter and the length of the test is 2 meter. Blockage coefficient, defined as the ratio of sectional area of the model and the wind tunnel, was less than 1.6% for the model ship. Because test section of the wind tunnel is open, no correction is made for the blockage effect. The turbulence intensit of the wind tunnel is less than 1%. For the wind tunnel tests, a double-model ship was used. The model ship was fixed in the middle of the test section of wind tunnel. A traversing mechanism driven b three servomotors with a precision of 1 mm for each direction was installed for efficient and accurate positioning of the measurement probes. Experimental facilities and double-model are given in figure 4. In the wind tunnel tests, similarit law of Re was complied. That is to sa, Renolds number in the wind tunnel tests must be equal to that in navigating conditions. This means the wind velocit must be ver high. is ver difficult in the wind tunnel tests. While from Qin [5] experimental results Frame 1 15 However, Oct 21 ittitle we can see that the viscosit drag coefficient is not changed basicall when Renolds number is larger UL than Since the wind speed was 32 m/s, the resulting Renolds number ( Re ) was On the other hand, the aim of the experiment is mainl that the drag reduction mechanism of a ship due to a slit is studied b comparison of the drag between case 1 and case 2. Therefore, as long as the Renolds number of these two cases is the same, the experimental requirements are satisfied. Here, L is the length at design waterline of the prototpe and the model ship, respectivel. is the kinematic viscosit of water or air. All measurements were carried out in the fixed condition. It is noteworth that the average temperature of the tunnel air during the experiment was 5 oc, although it changed slightl ever da. Propeller shaft Inlet Slit Outlet Strut bar Strain gauge Figure 4. Sketch of experimental facilities and double-model 3. Experimental Results 3.1. Local Mean Velocit Measurements For the measurements of the three-dimensional mean velocit field around stern and bow region, a five-hole Pitot tube was utilied. The diameter of pressure hole is.25mm and that of Pitot tube, 2.5mm as well. The measuring range of velocit is m/s~5m/s and the accurac is ±.25%FS. The fixed coordinates of ship are designated (x,, ), as shown in figure 2. The origin of the coordinates is located at the intersection of the midship and waterplane. Where x denotes the downstream direction, is transverse and to starboard, and is the upward direction. x coordinate is nondimensionalied b the length between perpendiculars (Lpp) of the model ship, but and coordinate are nondimensionalied b the draft of the model ship. For the sake of understanding how the velocit near the inlet and outlet of the slit change, velocit 85

4 measurements for the original ship were carried out at 4 cross sections, while the same measurements with a drainage slit were performed at the same sections. Section 1 (x/lpp =-.58) and 2 (x/lpp =-.49) is located at upstream and downstream of the slit inlet respectivel, while section 3(x/Lpp =.46) and 4(x/Lpp =51), upstream and downstream of the slit outlet. Two dimensional velocit vectors at different cross sections are given in figures These figures indicate that the velocit of the cross section change ver slightl at bow and stern after opening the Frame slit 1 except 15 Oct near 21 the inlet and outlet of the slit. Because the inlet and outlet projection area perpendicular to the incoming velocit are the same, the velocit of the outlet varied slightl comparing with that of inlet, that has to sa, the difference between the velocit of the outlet and that of ambient flow around the bottom of the ship is not considerable. Therefore, the change of the wake velocit is slight after opening the slit. Furthermore, due to the propeller shaft, circumfluence exists in the wake of case 1 as well as case 2. In addition, the area of the circumfluence is becoming larger and larger along downstream direction. In general, the flow velocit is not changed obviousl after opening the slit Frame 1 15 Oct Figure 5. Velocit vector at section 1 (x/lpp=-.58, no slit) Frame 1 15 Oct Figure 6. Velocit vector at section 1 (x/lpp=-.58, with slit) Bulbous bow Figure 7. Velocit vector at section 2 (x/lpp=-.49, no slit) 86

5 Frame 1 15 Oct Frame 1 15 Oct Slit Bulbous bow Figure 8. Velocit vector at section 2 (x/lpp=-.49, with slit) Profile of hull Frame Oct Propeller shaft Figure 9. Velocit vector at section 3 (x/lpp=.46, no slit) Slit Figure 1. Velocit vector at section 3 (x/lpp=.46, with slit) 87

6 Frame 1 15 Oct Figure 11. Velocit vector at section 4 (x/lpp=.51, no slit) Pressure Measurement Figure 12. Velocit vector at section 4 (x/lpp=.51, with slit) In order to explore the mechanism for the drag reduction, the ship surface pressure distribution was obtained through the silicon-based micro pressure sensors. The pressure measurement ranges from Pa to1pa and the accurac is ±.5%FS. The diameter of pressure hole is 1mm. Considering the greater change of pressure just happen near the inlet and outlet of the slit, 8 pressure holes are set up at bow and 6 pressure holes, at stern of the ship bod surface, where is close to the inlet and outlet of the slit(shown in figure 13). In addition, here the waterline is =-.27. The location of the experimental measuring point is listed table 2. Experimental results of pressure coefficient of Cp are given in figure 14. TABLE 2. The location of the measuring point (=-.27) bow stern Points No x/l pp

7 Cp Cp measuring point; 2.slit Figure 13.Location of the measuring point (=-.27) As a whole, pressure of eight points at bow decreases along x direction, but at stern increases graduall, primaril because that velocit is becoming larger and larger with the increasing of the ship cross section area at bow along x direction, while the contrar is at stern. Except for three points (NO.1 and 2 at bow, NO.6 at stern) where are ver close to the inlet or outlet of the slit, Cp of measuring point changes slightl after the slit is opened. The Cp (point NO.1) on the bow of original ship model is close to.7. However, Cp at bow of case 2 decreases to about.25, which that pressure coefficient at bow reduces clearl due to the slit. However, the change of Cp at stern before and after opening the slit is less than that on bow. Therefore, the pressure difference between bow and stern reduces due to the slit. Consequentl, the conclusion that the pressure drag decrease can be made Measuring point at bow experimental results(without the slit) experimental results(with the slit) Measuring point at stern experimental results(without the slit) experimental results(with the slit) Figure 14. Cp of the 1 measuring point 3.3. Viscous Drag Measurement In order to make sure of the effect of drag reduction due to the slit, viscous drag measurements are performed. The self-made strain gauge was emploed to measure drag force. The force measurement ranges from g to 1Kg and the accurac is ±. 1%FS. Experimental drag of two cases is 166.1g and 157.5g, and the corresponding coefficient C f is.76 and.72 respectivel, so the reduction rate which is defined as follows is about 5%. M f (1) ( C fi C f ) / C f Here, subscript and 1 represent the data of case 1 and case 2 respectivel. After opening the slit the wet area increase, the friction drag ma be increased. However, the pressure drag decrease obviousl, i.e., the decreasing of the pressure drag is larger than increasing of friction drag. Thus, the end result is that the total viscous drag force decrease. This also is the reason of the drag reduction due to a drainage slit. 4. Conclusions The following conclusions can be made: (1) Because the difference between the velocit of the outlet and that of ambient flow around the bottom of the ship is not considerable, the change of the wake velocit is slight after opening the slit. (2) The change of Cp at stern after opening the slit is less than that on bow. Therefore, the pressure difference between bow and stern reduces due to the slit. Consequentl, the pressure drag decreases. (3) The test results show that the drag force of the ship model with drainage slit decreases. The drag reduction ratio is greater than 5%. 89

8 (4) The free surface is not considered during carring out the experiment, which is not consistent with the actual conditions. Therefore further research work should be done. It can be well-recognied that in the financial models of human behaviour outlined above the compan acts as an active plaer. Thus, the active sstem under consideration is a single-level one. Acknowledgements The present research was supported b the Natural Science Foundation of Hubei Province (Grant No.211CDB248) and National Natural Science Foundation of China (Grant No.NSFC ). References 1. Iutsu, N. (1993) Fluid Force Measurements for a Circular Clinder with a Slit, Nagare, 12, ajima,., Sano, O. (1996) A Note on the Drag Reduction of a Circular Clinder due to Double Rows of Hole, Fluid Dnamics Research, 18, Tan, G., Wang, J., Li, Q. (21) Drag Reduction Technique of Clinder and Mechanism Research, Journal of Beijing Universit of Aeronautics and Astronautics, 27(6), (In Chinese) 4. Cheng, K., Wang, J., Wei, W. (212) Numerical Simulation on Leading Edge Holes for Drag Reduction and Flow Field Characteristics of the Craft. Journal of Ship Mechanics, 16(5), (In Chinese) 5. Qin, J. (27) Numerical Simulation of Viscous Flow Around Low-speed Full Form Ship. Wuhan: Wuhan Universit of Technolog Press. Received on the 21 st of June 213 9