Ventilated marine propeller performance in regular and irregular waves; an experimental investigation

Transcription

1 Ventilated marine propeller performance in regular and irregular waves; an experimental investigation G. K. Politis Department of Naval Architecture & Marine Engineering, National Technical University ofathens, GREECE Abstract The effect of the propeller ventilation and waves (regular or irregular) in propeller open water performance is investigated experimentally. Experiments have been performed in the towing tank of the National Technical University of Athens using the open water propeller dynamometer. Two model propellers have been selected with diameter D=0.2 m and pitch to diameter ratio P/D=0.78, 1.23 respectively. Time records of propeller thrust, torque and free surface elevation have then been taken for four different propeller axis depths covering the range from the fully submerged case to the fully ventilated case (i.e. propeller axis at the undisturbed free surface). Measurements have been performed in regular as well as irregular free surface waves and at two different propeller advance ratios Results were given in non-dimensional form showing the effect of the various independent parameters (i.e. shaft depth, wave height, P/D, J) to the propeller thrust and torque coefficients and interesting conclusions were drawn regarding the effect of propeller ventilation in propulsive performance of marine propellers. Symbols index A wave amplitude (= w/2) or significant wave amplitude (=^72) AE/AO propeller expanded blade area ratio D propeller diameter d distance of propeller axis from undisturbed (mean) free surface Kq propeller torque coefficient (=Q/(pNV))

2 176 Computational Methods and Experimental Measurements Kt propeller thrust coefficient (=T/(pNW)) N propeller revolutions P/D propeller pitch ratio Q propeller torque R propeller radius (=D/2) T propeller thrust Tp wave period (regular waves) or peak to peak period (irregular waves) t time Va propeller velocity of advance Z propeller number of blades (w wave height in regular waves (wg significant wave height in irregular waves (t) instantanius free surface elevation at propeller position p fluid density Introduction The problem of ship's speed control and maintenance in rough seas, has been a subject of intensive investigation during recent years, Nakamura and Naito [1]. At sea states exceeding Beaufort 6 and head sea, reductions in speed of 8-10% are usual, while this reduction can be doubled at a sea state corresponding to Beaufort 8. At higher sea states speed losses in the range of 40-50% have been reported. Added resistance and violent ship motions are the main reasons for speed losses in irregular seas. More specifically added resistance is a result of the radiated energy due to dumping waves and additional energy loss due to wind The two effects can result in an increase in propulsive power demand in the range of 40-60%. In case of engine power margin in the range of 15-25%, which is usual in propulsive calculations practice, unintentional speed losses are unavoidable. On the other hand, violent ship motions lead also to unintentional speed losses, through a number of mechanisms as follows: shipping of water and slamming and propeller ventilation. Propeller ventilation occurs when the propeller tip approaches the water free surface. In this case the adverse pressure gradients between the free surface and the propeller suction side result in absorption of air which is then trapped at the blade suction side reducing thus the capability of the propeller to produce thrust and to absorb the engine torque. This violent reduction of the propeller torque demand, leads to an instantaneous engine over-speed and occasionally to a loss of control of the main engine. Unintentional speed losses are thus unavoidable in order to prevent engine damage and reduction of engine life. Thus, in order to control and maintain ship's speed, a number of different problems have to be resolved. In this respect added resistance has to be as small as possible, and this generates the hydrodynamic problem of optimum hull geometry from the point of view of minimization of added resistance. Shipping of water and slamming are also related to the optimum hydrodynamic selection of hull lines. Finally engine's control scheme has to be designed with maximum

3 Computational Methods and Experimental Measurements 177 flexibility in order to cope with the large propeller torque variations resulting from propeller ventilation. The significance of the propeller ventilation in propeller-engine cooperation has attracted a number of researchers. In reference [2], the results of open water experiments (no waves) on three propellers operating in the partially submerged condition are analyzed in order to study the effect of the independent geometric parameters to the propeller thrust and torque. In reference [3], the experimental results regarding thrust produced by and efficiency of a ventilated supercavitating propeller are presented along with cavity pressure measurements. In reference [4] an attempt is made to classify the various physical mechanisms entering in the problem of combined cavitation-ventilation and affecting the propeller forces. In reference [5], the value of semi-submerged propellers for displacement hulls is investigated. The results of propeller channel tests and selfpropulsion tests for a DD 963 destroyer hull with twin semi-submerged propellers are presented and discussed. In reference [6], the influence of parameter variations on added resistance, the influence of periodic variations in propeller immersion on propulsive performance in head sea and the possibility of reducing the risk for propeller racing due to ventilation by changing the propeller geometry are discussed. In reference [7], the problem of ventilated propellers is investigated and approximate formulae for predicting their performance are developed, based on open water tests data. In reference [8], two problems regarding ventilation of marine propellers, that is, the propeller load fluctuation due to pitching motion and the vibratory shaft force induced by the ventilation, are studied based on open water tests data In reference [9], the steady lateral force and moment set on surface-piercing propellers, which may be inclined relatively to the stream direction, are evaluated. The fully-submerged inclined propeller case is included as a specialization of the formula. Finally Savineau and Kinnas [10] attempt to develop, a non-linear, time-matching, potential based boundary element method to be used for the study of the flow field around a fully ventilated two-dimensional hydrofoil of arbitrary geometry. The effectiveness of the method is discussed. As afirststep in the process of developing an unsteady model for the propeller ventilation, we decide to investigate experimentally the open water performance of two model propellers in regular and irregular waves. The results of such an investigation shall be presented in the next paragraphs. Problem geometry Figure 1 shows the geometry of the problem. A propeller of diameter D and radius R=D/2 operates at a depth d measured from the undisturbed free surface. Distance of propeller tip from the wave through is denoted by L=d-A-R, where A is the wave amplitude. In case of irregular waves similar definitions hold with wave amplitude substituted by the significant wave amplitude. The propeller is travelling at a speed of advance Va, rotating at a number of revolutions denoted by N and developing a thrust T(t) and absorbing a torque Q(t) as a result of the sea irregularity. In case of irregular sea the wave spectrum is determined by a

4 178 Computational Methods and Experimental Measurements significant wave height ^ and a peak to peak period Tp. In case of regular waves a wave height C^ and a period Tp determines the sea state. Figure 1. Problem geometry Planning of experiments Two model propellers have been selected with common diameter, number of blades and expanded blade area ratio and different pitch ratio: D=0.2 m, AE/AOO.628, Z=4, P/D=0.78, Time records of propeller thrust T(t), torque Q(t) and free surface elevation ^(t) have been taken for: four different propeller axis depths d covering the range from the fully submerged case to the fully ventilated case (i.e. propeller axis at the undisturbed free surface): d=(0.05, 0.1, 0.15, 0.35, 0.4) m or d/r=(0.5, 1., 1.5, 3.5, 4.) two different propeller advance ratios J: Propeller with P/D=0.782: J=0.65, 0.75 Propeller with P/D=1.23: J=0.75, 0.8 regular waves: all combinations of four different wave heights with four different wave periods: ^=(0.133, 0.166, 0.2, 0.25) m, Tp=(1.826, 2.136, 2.264, 2.739) sec # irregular waves: all combinations of four different significant wave heights with four different peak to peak periods: ^=(0.133, 0.166, 0.2, 0.25) m, Tp=(1.826, 2.136, 2.264, 2.739) sec Experimental propeller revolutions have been selected to be equal to 17 rps (1020 rpm) in order for the Reynolds number at 70%R to be overcritical (>5*10*). Propeller advance ratios have been selected in order to cover the range of usual propeller hydrodynamic operation (i.e. a range left of the propeller maximum efficiency point). Values of significant wave heights and peak to peak periods have been selected in order to correspond to sea states 6 to 7 for a geometric model scale of 1=30. Thus with the selected model propeller diameter our experiments simulate sea states 6V7 for a full scale ship of 160^200 m. All

5 Computational Methods and Experimental Measurements 179 the experiments have been performed at the experimental towing tank of the Department of Naval Architecture and Marine Engineering, of the NTUA, as part of a diploma thesis of Mrs Vassiliou [11]. Discussion of experimental results The results of all measurements (regular or irregular) have been plotted as time records of propeller thrust coefficient iq(t), torque coefficient Kq(t) and free surface elevation %). Figures 2, 3 and 4 are representative time records for the propeller performance in regular waves. Figure 2 refers to ^=0.133 in, d=0.05 m, Tp= sec. Figure 3 refers to ^=0 133 m, d=0.1 m, Tp=2.739 sec. Figure 4 refers to (w=0.2 m, d=0.35 m, Tp=2.136 sec. All three figures refer to the propeller with P/D=1.23. Careful observation of those figures shows that there are generally three different patterns in propeller-free surface unsteady interaction. In thefirstpattern (fig. 2) significant part of the propeller disk emerges out of the fluid surface. As a result the propeller performance is entirely dominated by blade ventilation and racing and the resulting Kt(t), Kq(t) and (t) are characterized by in phase variation. In the third pattern (fig. 4) the propeller disk is far from the free surface. As a result the propeller unsteady performance is entirely dominated by the wave induced velocity field, resulting in Kt(t), Kq(t) variations in 180 out of phase with ((t). Finally in the second pattern (fig. 3) part of the propeller disk emerges out of the fluid surface while another part is submerged deeply enough in order for the wave induced velocity field to interact with the blade flow. As a result the measured Kt(t), Kq(t) are partially in phase and partially 180 out of phase with ((t). In addition to the time records a systematic investigation has been carried out regarding the effect of propeller shaft depth and wave amplitude in average propeller thrust and torque. In this respect different plot types have been systematically tried out. As a result it was found that the parameter L/D, figure 1, greatly unifies the average propeller performance in both regular and irregular waves. Figures 5,6,7 and 8 present average thrust and torque coefficients and corresponding amplitudes as functions of L/D for the propellers with P/D=0.782 (fig. 5, 6) and P/D=1.23 (fig. 7, 8) in regular waves. Figures 9, 10, 11 and 12 present average thrust and torque coefficients as functions of L/D for the propellers with P/D=0.782 (fig. 9, 10) and P/D=1 23 (fig. 11, 12) in irregular waves. Furthermore we have superimpose the 'no wave' ventilated propeller performance in each of the previous figures for comparison purposes. Considering the results of the previous figures we observe that figure 5 is representative of the propeller performance in regular waves while figure 9 is representative of the propeller performance in irregular waves. Thus we shall concentrate on those two figures. Regular waves (fig. 5): At larger L/D values, i.e. tip far from wave trough, the average thrust is nearly equal to the no wave thrust as expected. In the same region the amplitude of thrust varies depending from both shaft depth and wave

6 180 Computational Methods and Experimental Measurements amplitude. More specifically it is increased with either increasing wave amplitude (L/D decreases) or decreasing shaft depth (L/D decreases). This is the region where wave induced velocities govern the propeller unsteady performance. At lower L/D values ventilation is the governing phenomenon and average thrust is a function of L/D alone. Comparing average thrust at this region with the no wave thrust we observe that the no wave thrust underestimates the average propeller thrust. Thrust amplitude shows similar performance at this region since it depending mainly from the L/D and to less a extend from the propeller loading Irregular waves (fig. 9): At larger L/D values, the average thrust is nearly equal to the no wave thrust. This is the region where wave induced velocities govern the propeller unsteady performance. At lower L/D values ventilation is the governing phenomenon and average thrust is mainly a function of L/D and secondary of significant wave height (denoted by FFM/3 in the figure) and shaft depth. Comparing average thrust at this region with the no wave thrust we observe that the no wave thrust underestimates the average propeller thrust. References 1. Nakamura S., Naito S., "Propulsive performance of a container ship in waves", J. S. N. A. Kansai Japan, Hadler J.B., Hecker R., "Performance of Partially Submerged Propellers", 7* Symposium, ONR, August Hecker R., Crown D. E., "Ventilated Propeller Performance", Department of the Navy Naval Ship Research and Development Center, Washington DC, June Kruppa C. Testing of partially submerged propellers', Proceedings of the 13* ITTC, report of the cavitation committee, 1972, 5. Rains DA, "Semi-Submerged Propellers for Monohull Displacement Ships", SNAME PROPELLERS Minsaas K., Faltinsen O, Persson B., "On the Importance of Added Resistance, Propeller Immersion and Propeller Ventilation for Large Ships in a Seaway", PRADS Guoqiang W, Dashan I, Meiliang C, Zhenbang S., "Propeller Air Ventilation and Performance of Ventilated Propeller", PRADS 1989, Vol Nishikawa E., Ushida M., "An Experimental Study on the Ventilation of Marine Propeller and its Effects on the Propeller Performance and Shaft Force", PRADS 1989, Vol Vorus W. S., "Forces on Surface- Piercing Propellers with Inclination", JSR. Vol.35, No 3, September Savineau C, Kinnas S., "A Numerical Formulation Applicable to Surface Hydrofoils and Propellers", Vassiliou P. 'Experimental investigation of the propeller performance in regular and irregular waves at ventilated conditions', Diploma thesis, National Technical University of Athens, 1998.

7 Computational Methods and Experimental Measurements , ,500 3, Figure 2. w=0.133 m, d-0.05 m, Tp=1.826 sec t.ooo Figure 3. ^=0.133 m, d=0.1 m, Tp sec Figure 4. Cw=0.2 m, d=0.35 m, Tp sec

8 182 Computational Methods and Experimental Measurements Kt, no wave, J=0,65 Poly. (Kt aver, J = ) Poly. (Kt aver, J=0,65) Figure 5. Regular wave performance, P/D=0.782, Kt aver/ampl 10Kq 10Kq aver, amp, J = Kq, no wave, J = 0, Kq aver, J = 0,65 10Kq amp, J=0,65 10Kq, no wave, J = 0,65 'Poly. (10Kq aver, ) 'Poly. (IQKq aver, J = 0,65) Figure 6. Regular wave performance, P/D=0.782, 10 Kq aver/ampl Figure 7. Regular wave performance, P/D=1.23, Kt aver/ampl

9 Computational Methods and Experimental Measurements 183 lokqaver, J = 0,6 10Kq amp, J = 0,6 10Kq, no wave, J = 0,6 lokqaver, J = 0,8 10Kq amp, J = 0,8 10Kq, no wave, J = 0,8 'Poly. (lokqaver, J=0,6) Poly. (lokqaver. J=0,8) Figure 8. Regular wave performance, P/D=1.23, 10 Kq aver/ampl 0,4887, 0,4887, J = 0,4887, H ave, 0,6497, J = 0,4875 0,6497, 0,6497, J = 0,6497, H 1 = 0.65 Figure 9. Irregular wave performance, P/D=0.782, Kt aver J -# -* + a -A ^ o H" /3 0, H* /3 0, , H* /3 0,2 = 0 46 /3 0,25 487* 64 H* /3 0,133 64, H* /3 0, /3 0,2 = , H* /3 0,25 OKq, no w ave = ,5 Figure 10. Irregular wave performance, P/D=0.782, 10 Kq aver

10 184 Computational Methods and Experimental Measurements 0,6, 1-1*1/3=0,133 J = 0,6, H*1/3=0,166 J=0,6, H*1/3=0,2 J = 0,6,H"1/3=0,25 ave, J=0,6 0,8, HM/3=0,133 0,8,H*1/3=0,166 0,8.HM/3=0,2 J = 0,8,HM/3=0,25 Figure 11. Irregular wave performance, P/D=1 23, Kt aver Figure 12. Irregular wave performance, P/D=1.23, 10 Kq aver