Numerical Simulation for Stern Flow Characteristics of Twin Skeg Container Vessel A. Maimun, Y.M. Ahmed, Zulhazazi, Sunarsih and Mehdi Nakisa

Transcription

1 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) Numercal Smulaton for Stern Flow Characterstcs of Twn Skeg Contaner Vessel A. Mamun, Y.M. Ahmed, Zulhazaz, Sunarsh and Mehd Naksa Abstract Ths paper employs computatonal tools to eamne the propulsve mprovement of a twn skeg contaner shp by observng the streamlne pattern and wake dstrbuton of propeller plane. A sngle skeg contaner shp was used for the study as a mother shp. The mother shp was then modfed to a twn skeg shp. Arrangement of the twn skeg s based on the dstance between skegs and angle of the skeg. Based on the skeg flow analyss, an optmze twn skeg arrangement was selected. RANSE code Ansys CFX ftted wth Shear Stress Transport (SST) turbulence was used to carry out the smulaton. Grd generator ICEM CFD was used to buld hybrd grds for RANSE code solver. Keywords Contaner shp, CFD, propulsve mprovement, twn skeg. S I. INTRODUCTION HIP effcency and fuel cost are nherently key consderatons for shp owners and shp desgners. Improvement of shp effcency wll lead to a better envronmental control as well as fulfllment of the classfcaton socety requrement. Twn skeg desgn whch ncreases propulsve effcency s advantageous from the pont of vew of hydrodynamc and thus becomng an alternatve desgn. In the study of hull form desgn and shp flow analyss, computatonal method has been wdely used to obtan prelmnary result pror to valdaton through eperment. Solely dependng on the result of eperment mght be relable but t consumes etensve tme and cost. Usng CFD, local flow characterstc whch s dffcult to obtan durng model test wll be readly generated through such method. Computatonal method s applcable to analyse shp resstance, propulson and manoeuvrng. Among past researches that utlse CFD are assessment and predcton of shp-propulson performance [1], [2] and analyss of bare hull, rudder and propeller [3]. In ths work, flow analyss of hull form desgn of contaner shp s presented usng CFD code (CFX). Ths study emphaszed the skeg arrangement to acheve a propulsve mprovement. Self-propulson performance was observed when the geometrcal varaton of skeg dstance and angle were appled on the shp stern. Through nvestgaton, an optmzed skeg arrangement desgn was determned. It was assumed that each skeg varaton was operated by the same dmenson of propeller plane. By neglectng the propeller nfluence n the smulaton, predcton of propulson performance was consdered to be suffcent by nvestgatng the streamlne pattern and velocty dstrbuton nduced on propeller plane. II. DESCRIPTION OF MODEL The study eplored sngle skeg contaner shp whch s modfed to be twn skeg usng Rhnoceros 5.0 by referrng to the skeg desgn of [4]. Table 1 shows the partculars of the model employed whle Fg. 1 and Fg. 2 dsplay the mother shp of a sngle skeg contaner model and a redesgned twn skeg contaner model at dstance of 45.4 percent of breadth of mdshp from longtudnal centerlne respectvely. TABLE 1 PRINCIPLE DIMENSION Wetted Volume Length Beam Draft Surface dsplace Area 3.84 m m m m m 3 Fg. 1 Sngle skeg contaner model A. Mamun s wth the Marne Technology Centre, Faculty of Mechancal Engneerng, Unverst Teknolog Malaysa, Skuda, Malaysa (e-mal: ad@fkm.utm.my). Y.M. Ahmed, s wth the Marne Technology Centre, Faculty of Mechancal Engneerng, Unverst Teknolog Malaysa, Skuda, Malaysa (e-mal: yasser@mal.fkm.utm.my). Zulhazaz was graduated from Faculty of Mechancal Engneerng, Unverst Teknolog Malaysa, Skuda, Malaysa (e-mal: zulhazaz@gmal.com). Sunarsh and Mehd Naksa s a post graduate student at Faculty of Mechancal Engneerng, Unverst Teknolog Malaysa, Skuda, Malaysa (e-mal: bekharunnsa@gmal.com, nmehd5@lve.utm.my) Fg. 2 Twn skeg contaner model 149

2 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) III. MATHEMATICAL MODEL Mathematcal descrpton of free surface flow n CFX s based on the homogenous multphase Euleran Euleran flud approach. Both fluds (water and ar) n ths approach share the same velocty and other relevant felds such as temperature, turbulence, etc., whch s separated by a dstnct resolvable nterface. The local equatons governng the moton of unsteady, vscous and ncompressble flud (ether lqud or gas) namely Naver Stokes s gven as t t (1) u 0. 0 u u u u' u' u u 2 ul (2) 3 l where u u u u u 2 ' ' t k t (3) 3 And 2 r ; r u ; r 1 (4) 1 The SST turbulent model was used n CFX code formulated by k k ku k Gk Y (5) k t k u G Y D t t (6) where Γk and Γω, Gk and Gω, and Yk and Yω respectvely represent the effectve dffusvty for k and ω, generaton of turbulence knetc energy and ω due to mean velocty gradents, and dsspaton of k and ω due to turbulence respectvely. Meanwhle Dω represents the cross-dffuson term. IV. COMPUTATIONAL GRID A commercal code, RANSE (ICEM CFD) was used to generate hybrd mesh and the dstance of the frst grd pont of the shp surface was mantaned for each case, y+ 8 whch s wthn a log-law regon. The computatonal doman of the model whch s etended to 1.5L forward, 2.5L backward, 1.5L asde and 1.2L downward under the keel of the shp model has been meshed wth unstructured tetrahedral mesh elements. The ar layer was etended to 0.125L above the stll water surface [5]. Fg. 3 dsplays the computatonal doman used n the smulaton. Smaller element sze was used for the contaner model and unstructured tetrahedral was bult n the regon around the model. The smaller the sze of element used the hgher the model element number produced. Furthermore, small sze of element used on and near the shp hull surface s desgnate to obtan a better result of velocty dstrbuton on propeller plane. In the meantme, prsm layer was bult around the model wth a total of fve layers to produce better free surface effect. Fg. 4 depcts grd of the unstructured tetrahedral and prsm layer of the model. V. COMPUTATIONAL METHOD AND BOUNDARY CONDITION The current study consdered the flat bottom as sea bed wthout natural rregulartes and ecluded the dynamc mesh hence treated the shp model wthout propeller nfluence. Attenton was gven only to starboard sde due to symmetrc propertes of the shp hull. Computaton was run on an 7 processor, 4 core hyper threadng, 3.6GHz and 16 GB RAM wth the effect of free surface and surface wthout wave and current. The water regon wdth was set to fourth of shp lengths as to elmnate the wall effect and no-slp boundary condton was mposed on the hull surface. The statc pressure and ntal locaton of the free surface were accordngly defned by the functon of water volume fracton at the outlet boundary and volume fracton of water and ar at both nlet and outlet boundares. The scalable wall functon was used wth turbulence model whle the reference pressure was set to the atmospherc pressure. The flow was consdered steady n ANSYS CFX calculatons wth utlsaton of fnte volume method and hgh resoluton numercal scheme for dscretzaton process and advecton terms respectvely. The pressure and velocty were correspondngly nterpolated usng lnear and trlner numercal schemes. Fnally, root mean Fg. 3 Computatonal doman used n the smulat 150

3 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) Fg. 4 Grd of unstructured tetradehral and prsm layer on shp model square (RMS) crteron wth a resdual target value of was used to check the convergence of the solutons. VI. VARIATION OF SKEG ARRANGEMENT Two desgn parameters n stern arrangement were selected for varaton.e. dstance between two skegs and the nclnaton angle. These parameters were used to nvestgate the flow pattern n the skeg stern area that may mpact the selfpropulson performance of the shp model. Stern flow characterstcs of three dfferent models namely Shp A, Shp B and Shp C were nvestgated where poston of skeg for each shp model was lsted n Table 2. The dstance between the two skegs was specfed to be 34.5 percent from shp centrelne for frst model and ncreased accordngly about 5 percent for the followng models. Shp TABLE II SKEG DISTANCE OF THE SHIP MODEL Percentage of skeg dstance from centrelne/shp half breadth A 34.5 varaton n stern skeg arrangement appled to the model. VII. RESULTS A. Effect of the Skeg Dstance on the Stern Flow Numercal flow analyss of the model scale shps was performed and computatonal data was acqured usng RANSE code. All models were run at model speed of m/s whch equals to 20 knots full scale shp. Shp A B 40 C 45.5 Shp B Shp C Fg. 6 Streamlne pattern at outer skeg surface Fg. 5 Varaton n stern skeg arrangement For the skeg nclnaton angle, the shp model ndcatng the best flow characterstc was then modfed by makng an nclnaton angle to the neutral poston of the skeg whch s about 7 degree. Fg. 5 provdes the schematc vew of the Numercal streamlne pattern on the outer and nner part of the skeg surface are presented n Fg. 6 and Fg. 7, respectvely. As shown n Fg. 6, the flow pattern of all models was lkely movng towards the propeller plane wthout twstng the streamlne. Boed regon of Shp C ndcates almost consstency n flow pattern at the stern bulb. Smooth dstrbuton as shown by Shp C results n smaller dead water area. On the contrary, Shp A shows nconsstency n ts flow pattern where the flow tends to dense at the stern bulb but less at the upper part. Dfferently, flow lne at the upper part bulb 151

4 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) of Shp B s straghtened out. From the pont of vew of shp self-propulson, t can be thus deduced that streamlne dstrbuton over the outer sdes of the stern skeg for each models s to be straghtforward snce t mples less flow separaton n the stern area. Whle for the nner sde of the stern skeg, by referrng to Fg. 7, t can be seen that the streamlnes along the upper part of the stern bulb of Shp A whch possesses the smallest skeg dstance shows twstng pattern and reverse flow whch ndcate large flow separaton n the zone. Ths separaton and the generated dstortng flow over the stern bulb lead to a negatve mpact on the propeller whch s reducng the ncomng water velocty nto the propeller plane. On the contrary, as observed n the boed area of Shp B and Shp C, flow lnes go down towards the shaft hole wthout twstng. Summarly, the skeg dstance appled to these two models mproves the propulson effcency as both can accelerate the flow nto the propeller plane. hull and the water nflow velocty. Wake fracton formula employed s based on Taylor epresson as follows. w v v s a (7) Accordng to the CFD code result, numercal flow characterstcs n the stern regon of the shp models are as demonstrated n Fg. 8. The fgure represents the computatonal wake dstrbuton n the starboard propeller plane of Shp A, Shp B and Shp C respectvely. The velocty dstrbutons presented n the fgure are mportant as to compare the flow characterstcs among the three skeg dstances. v s TABLE III WAKE FRACTION OF THE SHIP MODEL Shp Vs Va Vs-Va w A B C Shp A Shp A Shp B Shp B Shp C Fg. 7 Streamlne pattern at nner skeg surface In addton to the nterrogatons onto the flow characterstcs n the stern skeg area, wake dstrbuton on the propeller plane was also nvestgated. Table 3 summarzes the advance velocty Va and wake fracton w of the three shp models. Keepng the shp speed constant at m/s, the wake fracton values were compared among the models. The values are mportant as to determne the nteracton between the shp Shp C Fg. 8 Dstrbuton of computatonal velocty n the propeller plane The fgure tells that the wake dstrbuton of Shp A s unbalance as for zone I the flow tends to move away to the top left hand sde. Ths can be eplaned by relatng the flow characterstc of the nner skeg surface of Shp A where the streamlne at the top part goes backward (reverse flow) as well 152

5 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) as due to some flow separaton occurred. By comparng zone I of the three shp models, Shp C ehbts hgher velocty flow whch s represented by the colour contours. B. Effect of the skeg angle on the stern flow Another Skeg angel of 4 degree was modfed from the selected canddate Shp C whch orgnal skeg desgned wthout any angle. The new 4 degree skeg angle shp was assgned as Shp D. Applyng the same procedure, eploratons were conducted to the new assgned shp and the flow pattern and wake dstrbuton were then compared wth Shp C. The result of the wake dstrbuton s dsplayed n Fg. 9. The fgure ndcates that the velocty of wake dstrbuton of the two planes shows less dfference. Furthermore, as the wake value of the orgnal selected desgned Shp C s lower than the modfed Shp D, the earler was absolutely won over the latter and thus s fnally selected as the best desgn canddate. Shp C Shp D Fg. 9 Dstrbuton of computatonal velocty nto propeller plane of Shp C and Shp D C. Comparson between sngle skeg and optmzed stern skeg For the purpose of such comparson, Shp C was chosen to be compared wth the mother shp whch s the sngle skeg contaner shp assgned as Shp E. Table 2 provdes the wake fracton of the shps assessed. It s clearly shown that Shp C possesses lower wake value wth hgher water nflow velocty. TABLE IV WAKE FRACTION FOR SHIP C AND SHIP E Shp Vs Va Vs-Va w C E Fgure 10 fnally compares the velocty dstrbuton on the propeller plane of twn and sngle skeg shp models of Shp C and Shp E respectvely. As dsplayed by the fgure, at zone 11 Shp C ndcates wder area of hgh velocty. However, comparng the top part of the propeller plane, Shp E shows low of flow velocty especally n regon 4 and 5 whle shp C shows a better flow of velocty n regon 6 and 7 about m/s. Shp C Shp E Fg. 10 Dstrbuton of computatonal velocty nto propeller plane of Shp C and Shp E VIII. CONCLUSION Two hydrodynamc characterstcs that can be effectvely used to measure propulsve performance are straghtforward flow lnes along the stern bulb over the nner and outer skeg surface and the wake fracton of the propeller plane. Numercal analyss has been etensvely used n the assessment of the complcated shp stern flows that mpact the propulsve effcency of twn skeg shps. Effects of the varaton of skeg dstance and angle on the behavour of stern flow have been nvestgated to optmze the stern hull form of twn skeg shps n vewpont of selfpropulson performance. Investgaton was started wth sngle skeg contaner shp whch was modfed to be twn skeg. The streamlne and wake fracton were then compared among the varaton of skeg dstance. Fnally streamlne and wake fracton of the selected twn skeg model was compared wth sngle skeg one through the same computatonal method. The selected twn skeg model shows better wake fracton about w= Mnor change n stern hull form of the twn skeg model appears and although t s a small fracton compared to the total hull geometry t mght result n crtcal change n stern flow and propulson performance. REFERENCES [1] J. Km, I.R. Park, K.S. Km, and S.H. Van, RANS computaton of turbulent free surface flow around a self-propelled KLNG carrer, J SNAK, vol. 42, no. 6, pp , [2] [2] H.T Km., H.T Km, and S.H. Van, Numercal analyss of flow characterstcs of a twn-skeg contaner shp wth varaton of stern hull shape, J SNAK, vol. 44, no. 6, pp , [3] [3] J.J. Km, K.J. Pak, Y.C. Lee, and H.T. Km, A numercal study of turbulent vscous flow around a self-propelled shp, n Proceedng of the annual sprng meetng SNAK., pp [4] [4] J.E. Cho, K.-S. Mn, J.H. Km, S.B. Lee, and H.W. Seo, Resstance and propulson characterstcs of varous commercal shps based on CFD results, Ocean Engneerng, vol. 37, pp , [5] [5] Y.M. Ahmed. Numercal smulaton for the free surface flow around a comple shp hull form at dfferent Froude numbers, Aleandra Engneerng Journal, vol. 50, pp ,

6 Internatonal Journal of Mnng, Metallurgy & Mechancal Engneerng (IJMMME) Volume 3, Issue 3 (2015) ISSN (Onlne) A. Mamun s a professor at Unverst Teknolog Malaysa. He specalzes manly n the feld of Marne Vehcles/Structures Dynamcs usng CFD, Tme doman smulatons and epermental work. He had taught, conducted research and consultancy work n the sad feld and had publshed over 50 papers n conferences and ournals. Y.M. AHMED s a Senor Lecturer at Unverst Teknolog Malaysa. He gves lecture and conduct research n the feld of Hydrodynamcs, Dynamc Marne Vehcle, Sea Keepng and Shp Resstance Propulson. He s specalze n computatonal flud dynamc usng varous computatonal method. Zulhazaz was graduated from Unverst Teknolog Malaysa n Mechancal Engneerng-Marne Technology. He s currently a research assstant at Marne Technology Centre Sunarsh s a Post Graduate Student at Unverst Teknolog Malaysa. She s currently studyng marne propeller hydrodynamcs towards development of manoeuvrng model n restrcted waterways. She has eperence n the area of Marne Propulson Plant, Marne Engneerng and Systems, and Fuzzy Mult Crtera Decson Makng Method. Mehd Naksa s PhD student n Faculty of Mechancal Engneerng- Unverst Teknolog Malaysa(UTM). He s currently studyng on propeller performance and shp hydrodynamcs characterstcs n dfferent water ways geometres numercally and epermentally as a member of hydrodynamc research group n marne technology center (MTC) under Prof. Dr. Ad Mamun. Hs eperence s hydrofol catamarans, propeller performance and shp maneouvrablty n restrcted waters usng Ansys commercal code smulaton. 154