A comparison study on the deck house shape of high speed planing crafts for air resistance reduction

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1 csnak, 2014 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~875 pissn: , eissn: A comparson study on the deck house shape of hgh speed planng crafts for ar resstance reducton Chung-Hwan Park 1, Hee-Seung Park 1, Ho-Yun Jang 1 and Namkyun Im 2 1 Research Insttute of Medum & Samll Shpbuldng, Busan, Korea 2 Mokpo Martme Unversty, Mokpo, Korea ABSTRACT: Planng crafts were specfcally desgned to acheve relatvely hgh speeds on the water. When a planng craft s runnng at hgh speed, dynamc pressure on the bottom makes the boat rse on the surface of the water. Ths reduces the area of the snkng surface of the boat to ncrease ar resstance. Ar resstance means the resstance that occurs when the hull and deck house over the surface of the water come n contact wth the ar current. In ths paper, we carred out a CFD numercal analyss to fnd optmal deck houses that decreased ar-resstance on the water when planng crafts are runnng at hgh speed. We fnally developed the deck house shape of hgh-speed planng crafts that optmally decreased ar resstance. KEY WORDS: Ar resstance; Planng crafts; Deck house; Computatonal flud dynamcs (CFD); Resstance reducton. NOMENCLATURE B D R F g LOA P R t Breadth Depth Total drag resstance An external force Gravtatonal acceleraton Length overall Pressure Total resstance Tme VOF Volume of flud C T Total resstance coeffcent S 1 Wetted surface area S 2 Upper deck surface area S 3 Total surface area u The speed of the flud u ' u ' Reynolds stress ρ The densty of the flud μ The vscosty of the flud INTRODUCTION The number of people enoyng marne lesure actvtes has been rapdly ncreasng n Korea owng to a rse n GNI and the ntroducton of the fve-day workweek. As a result of ths, the demand for shps as a maor mean for marne lesure actvtes, therefore, has been contnuously ncreasng. Furthermore, the government recognzed the mportance of the marne lesure ndustry and has tred to foster t as a new growth engne so that t can create added value for assocated ndustres and success- Correspondng author: Namkyun Im, e-mal: Ths s an Open-Access artcle dstrbuted under the terms of the Creatve Commons Attrbuton Non-Commercal Lcense (http://creatvecommons.org/lcenses/by-nc/3.0) whch permts unrestrcted non-commercal use, dstrbuton, and reproducton n any medum, provded the orgnal work s properly cted. Unauthentcated

2 868 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~875 fully accomplsh the natonal polcy of 'low-carbon green growth'. Hgh speed lesure boats, the key equpment for marne lesure actvtes, nclude two knds: planng crafts and hydrofol boats that can reach hgh speed by dynamc methods, whle hover crafts are typcal boats that can reach hgh speed by statc methods (Lee and Lew, 2003). The maor flud dynamc feature of hgh speed planng crafts that occupy more than 80% of all the hgh speed boats s as follows: dynamc pressure between the hull and the surface of the water makes them rse to the surface of the water, reducng ar resstance and makes them run at hgh speed (Park et al., 2012; Savtsky, 1964). Then, the dynamc pressure also generates trm and changes the runnng posture, gvng a large effect to maor performance of the planng crafts (Park, 2008). On the other hand, when the planng craft s runnng, the wnd passng by t gves both frcton and buoyancy; and the ar frcton resstance by the wnd passng by the deck house ncreases the resstance of hgh speed planng crafts, actng as a cause of decreasng the speed of boats. Most research on resstance reducton for plannng lesure boats have concentrated on reducton of wave resstance through the optmzaton of the hull and of frctonal resstance by decreasng the area of snkng surface below the water (Park et al., 2012). However t has been known that a large porton of total resstance at the hgh speed occurs above the water as ar resstance. Accordng to CFD numercal results and wnd tunnel tests, when speed plannng crafts are runnng wth 30 kts and 1-4 degrees of trm, ar resstance occupes up to 30% of total resstance (Km and Hwang, 2010; 2013) Ths paper deals wth the development of deck house shapes that optmally decreases ar resstance above the water when plannng crafts are runnng at hgh speed. We carred out CFD numercal analyss for four types of deck houses to fnd deck houses of hgh-speed planng crafts that optmally decrease ar resstance. AIR RESISTANCE REDUCTION PLANING CRAFTS Desgn of hull forms Hgh speed planng crafts have the hull made of FRPs and ther desgn speed s 40 knots. They have multple chnes of the V shape to smoothly run even when the tde s hgh; they are desgned to have a hgh stem for excellently breakng waves even when the tde s extremely hgh. A maor feature of the hull of hgh speed planng crafts s ther straght body to decrease resstance at hgh speed and to reduce shock load when tde s hgh (Jang et al., 2010; Katayama and Ikeda, 1996; Savtsky, 1981). To reduce the shock load by strppng the bottom and to mprove dynamc performance, two spray strps of trangular shape are attached to the left and rght sdes of the bottom, respectvely (Tanaka, 1991). Table 1 shows the prncpal partculars of the planng hull, the subect of ar resstance reducton, and Fg. 1 shows lnes. Table 1 Prncpal partculars of hgh speed planng hull. LOA (m) Lpp (m) Bwl (m) Depth (m) Engne power Persons 8.76 m 7.24 m 2.40 m 1.20 m 250 hp 6~8 Fg. 1 Lnes of the hgh-speed planng hull. Unauthentcated

3 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~ Shapes of deck houses for ar resstance reducton In order to desgn the shapes of deck houses that have the least ar resstance for 8 m hgh speed plannng crafts, we nvestgated and analyzed the deck house shapes of actual lesure boats. As a result, most of deck houses for hgh speed lesure boats were dvded nto four types accordng to ther shapes when they were seen n the front and from both sdes: Type 1 and Type 2 wth a long streamlned stem; and Type 3 and Type 4 wth the shapes of square wndows. Then, CFD numercal analyss for these four types of deck houses was carred out to fnd the deck house of hgh speed plannng crafts that has the least ar resstance. The man characterstcs of four types of deck houses that were used n CFD numercal analyss s summarzed n Table 2. Fg. 2 shows a front vew and a sde vew for each type of deck house. Table 2 Prncpal character of the deck houses. Item Deck house shape Edge shape Upper deck surface area (m 2 ) Type 1 Wthout rectangular wndow / Type 2 Wthout rectangular wndow \ Type 3 Wth rectangular wndow / Type 4 Wth rectangular wndow \ Fg. 2 Shapes of deck houses. NUMERICAL ANALYSIS AND SETTING CONSTRAINTS Equatons of moton and numercal analyss To carry out the numercal analyss of the resstance performance of plannng crafts, the commercal applcaton Star- CCM+ based on the fnte volume method was used n consderaton of 3D ncompressble vscous flow. The equatons of moton were the contnuty equaton and Reynolds averaged Naver-Stokes (RANS) equaton. u = 0 (1) ( ρu ) ( uu ) u + = [ μ( t t p u + 2 u ) ( μ )] 3 + ( ρu ' u ') + ρg + F (2) Unauthentcated

4 870 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~875 where ρ s the densty of the flud, t s tme, u s the speed of the flud, P s pressure, μ s the vscosty of the flud, g s gravtatonal acceleraton, and F s an external force. In Eq. (2), u ' u ' s Reynolds stress and the realzable κ ε turbulence model was used. Flow around boats was consdered as two-phase flow that has two dfferent phases and the VOF method was used to trace the boundary surface of the flow. Lattce generaton and boundary condtons The number of lattces for numercal analyss of ar resstance of plannng crafts was 530,000. The separaton of two 3 + adacent lattces that were vertcal to the wall was , whch corresponds to y = Calculaton regons and boundary condtons can be seen n Fg. 3 and the shapes of lattces generated accordng to the coordnate system are shown n Fg. 4. Fg. 3 A schematc of the system. (a) (b) Fg. 4 (a) An overvew of generated grds; (b) grds around the shp. Valdaton for numercal analyss In ths study, a comparatve study was carred out between the results of an experment and CFD numercal analyss n order to valdate the effectveness of the CFD numercal method that was used n ths research. A hgh speed craft wth a scale of 1/10 was adopted for the valdaton of CFD numercal method and a wnd tunnel test was carred out usng the hgh speed craft model (Km and Hwang, 2013). Fg. 5 shows the lnes of valdaton hull and Table 3 ndcates the prncpal partculars. Fg. 6 shows the comparson of ar resstance results between experment and CFD numercal analyss for model and real scale. The results of ar resstance for model scales were converted nto real scale n Fg. 6. The lattce generaton of numercal + analyss for real scale s same wth the method of prevous charter 3.2, whch corresponds to y = 2124 and the lattce + generaton for model scale uses y = 212 for numercal analyss. As shown n Fg. 6, the dfference of ar resstance accordng to the number of lattces s not so remarkable wthn 40 kts of shp s speed. On the contrary, the dfference between tunnel test and numercal analyss becomes bgger as the speed ncreases over 40 kts. The reason s consdered to be due to the wall effect of tunnel test. Unauthentcated

5 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~ Fg. 5 Lnes of valdaton hull. Table 3 Prncpal partcular of valdaton hull. LOA (m) Lpp (m) Bwl (m) Depth (m) Scale :10 Fg. 6 Comparson of ar resstance. RESULTS AND DISCUSSIONS For the same draft and flow as the hgh speed planng crafts at the speed of 40 knots, we carred out a numercal analyss of the resstance performance of the four types of deck houses. As a result of the numercal analyss based on free surface, the total resstance of the hull and deck houses was obtaned. Then, t was assumed that the total resstance coeffcent could be estmated by the area of surfaces, snce t was dffcult to separately calculate the resstance of the upper deck surface and the wetted surface of the hull due to the characterstcs of the numercal analyss program. Several vsual results such as velocty vector, waveform around the hull are shown at frst and then the comparson of the resstance for each type s explaned. Unauthentcated

6 872 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~875 The dstrbuton of waveform around the hull Fg. 7 shows the dstrbuton of the waveform around the hull. The dstrbuton of the waveform by the bottom of the hull for planng crafts showed smlar patterns both n the stem and stern. For dsplacement forms that belong to commercal shps, the shoulder wave was formed n the stem; for hgh speed planng crafts, the varatons of the waveform was remarkable n the transom of the stern. Fg. 7 Comparson of waveform patterns. The dstrbuton of velocty vectors around the hull Fg. 8 shows the dstrbuton of the velocty vectors around the hull. As shown n ths fgure, the results of a numercal analyss of the four types of deck houses showed that the magntude of velocty vectors at the end of the deck houses and the transom n the stern decreased especally around the hull and deck houses. The magntude of velocty vectors n the secton between the deck house and the upper deck also decreased remarkably. Ths s supposed to be caused by eddy flow that was generated by statonary flow accordng to the shape of the deck houses. The Type 1 deck house, among the four types of deck houses, showed the smallest area where velocty vectors decreased at the rear n the stern. Accordng to ths result, t could be expected that the resstance s relatvely small n Type 1 compared to Type 3 and Type 4. Fg. 8 Comparson of velocty vectors. Unauthentcated

7 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~ The dstrbuton of speed around the hull Fg. 9 shows the dstrbuton of speed around the hull. As seen n the dstrbuton of velocty vectors around the hull, t reduced the area where the magntude of velocty at the end of the deck house and the transom n the stern decreased for four types of deck houses. The magntude of velocty n the secton between the deck house and the upper deck also remarkably decreased. The change of magntude of velocty between the deck house and the upper deck was remarkable especally n Type 3 and Type 4 compared to Type 1. In addton, the area of the boundary where speed decreased was relatvely large n Type 3 and Type 4 compared to Type 1. In consderaton of the fact that the resstance generally ncrease n proporton to the area of the boundary where speed decreased, t s expected that total resstance n Type 1 would be relatvely small compared to Type 3 and Type 4. Fg. 9 Comparson of the dstrbutons of speed. The dstrbuton of streamlnes around the hull Fg. 10 shows the dstrbuton of streamlnes around the hull. The dstrbuton of streamlnes around the hull showed the same results as that of velocty vectors of the four types of deck houses: t generated eddy flow n the secton between the deck house and the upper deck. As shown n Fg. 10, the occurrence of eddy flow n Type 1 deck house s less than other types. It may cause the less change of the velocty n the secton between the deck house and the upper deck. Fg. 10 Comparson of streamlnes dstrbuton. Unauthentcated

8 874 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~875 Comparson of resstance and dscussons Table 4 shows the quanttatve data of CFD numercal analyss of the four types of deck houses for hgh speed planng crafts. In addton, the results of the numercal analyss of resstance for each shape of the deck houses were shown n Fg. 11. In the results of the numercal analyss of the four types of deck houses wth the same hull and wetted surface, the Type 1 deck house wth the long streamlned stem showed less resstance than other types: the resstance of the wetted surface of the hull was reduced by 1.6% ~ 2.5%; and that of the upper deck surface due to ar resstance was reduced by 5% ~ 12%. Table 4 Comparson of total resstance for four types of the deck houses. Item Symbol Type1 Type2 Type3 Type4 2 Wetted surface area S ( ) m 2 Upper deck surface area S ( ) m Total area S 3 = S1 + S Total resstance R (N) 27,505 27,948 28,188 28,024 Total resstance C T coeff. 2 R/ 0.5ρ VA S E E E E-03 Increasng rate of C T to Type 1 % Fg. 11 Comparson of resstance coeffcents. Accordng to the dstrbuton of streamlnes n Fg. 10, the occurrence of the eddy flow for the Type 1 deck house was less than other types snce the change of the velocty n the secton between the upper deck and deck house was smaller than the other types. The area of the boundary where speed decreased and the change of magntude of velocty between the deck house and the upper deck was relatvely small n Type 1 compared to Type 3 and Type 4 accordng to Fg. 9. The smlarty between vsual graphc results and quanttatve data of CFD can be found. As shown n Table 4, the shape of the Type 1 deck house was verfed to have the least amount of ar resstance. The reason for ths can be explaned as that the occurrence of the eddy flow for the Type 1 deck house was less than other types snce the change of the velocty n the secton between the upper deck and deck house was smaller than the other types. Therefore Type 1 deck house wth long streamlned stem s recommended for hgh speed planng crafts n order to reduce ar resstance. Besdes t s desred to avod deck houses lke Type 3 and 4, because they have short streamlned stem whch Unauthentcated

9 Int. J. Nav. Archt. Ocean Eng. (2014) 6:867~ cause eddy flow and relatvely consderable velocty change n the secton between the deck house and the upper deck. As a result, resstance s expected to reach hgh value compared to Type 1 deck house. CONCLUSIONS Usng the Star-CCM+ program based on the fnte volume method as one of the computatonal flud dynamcs, we carred out numercal analyss of total resstance of the four types of deck houses n the same velocty and draft n order to develop deck houses of hgh speed plannng crafts that have the mnmum ar resstance. The results are as follows: 1) To develop deck houses wth the mnmum ar resstance for hgh speed planng crafts, numercal analyss was carred out by varyng slope of the end of Type 1 and Type 2 deck houses wth a long streamlned stem and Type 3 and Type 4 wth the shape of square wndows. 2) Results of numercal analyss showed that the Type 1 deck house wth a long streamlned stem had less ar resstance than other types: the resstance of the wetted surface of the hull reduced by 1.6%~2.5%; and that of the upper deck surface due to ar resstance reduced by 5% ~ 12%. 3) These results are supposed to be due to as follows. Eddy flow for the Type 1 deck house was less than other types snce the change of the velocty n the secton between the upper deck and the deck house and at the end of the transom n the stern was smaller than other types. The flow around the hull and the deck house was relatvely smooth for the Type 1 deck house snce the behavors of the flow around the hull showed that the secton of velocty change (delay) was less than other types. To reduce the ar resstance of hgh speed planng crafts, therefore, Type 1 deck houses wth the long streamlned stem are recommended for the desgn of hgh-speed planng crafts REFERENCES Km, Y.S. and Hwang, S.K., An expermental study on ar resstance of the planng boat. Proceedngs of the Annual Sprng Meetng SNAK, Je-u, 3-5 June 2012, pp Km, Y.S. and Hwang, S.K., A study on ar resstance and greeenhouse gas emssons of an ocean lesure planng boat. Journal of the Korean Socety of Marne Envronment and Energy, 16(3), pp Jang, D.W., Park, C.H. and Jn, S.H., A basc study on the sea model test technques for hgh speed planng boat. Journal of Navgaton and Port Research, 34(8), pp Katayama, T. and Ikeda, Y., A study on unstable rollng nduced by ptchng of planng crafts at hgh advance speed. Journal of the kansa Socety of Naval Archtectects, 225, pp Lee, C.J. and Lew, J.M., A study on the development of the seres for sem-planng hull and the predcton of the resdual resstance coeffcents. Proceedngs of the Annual Autumn Meetng SNAK, Gyeong-u, October 2003, pp Park, C.H., Comparsons of research characterstcs of the hgh-speed planng craft wth respect to the number of bottom steps. Journal of Navgaton and Port Research, 32(8), pp Park, C.H., An, N.H., Jang, H.Y. and Kwon, Y.W., A study on the characterstc of moton and resstance performance from the body plan of plannng lesure boat at low speed. Journal of The Korean socety for power system and engneerng, 16(4), pp Park, C.H., Jang, H.Y. and Park, H.S., A study on the deck house optmzaton of hgh-speed planng craft for ar resstance reducton. Proceedngs of the Annual Autumn Meetng the Korean Socety of Marne Envronment and Safety, Wan-do, November 2012, pp Savtsky, D., Hydrodynamc desgn of planng hulls. Marne Technology, 1(1), p Savtsky, D., Status of hydrodynamc technology as related to model tests of hgh-speed marne vehcles. The Unted Kngdom: Marne Transportaton, Brtsh Martme Technology. Tanaka, H., Cooperatve resstance tests wth geosm models of a hgh-speed sem-dsplacements craft. Journal of the Socety of Naval Archtects of Japan, 169, pp Unauthentcated