CFD simulation of cooperative AUV motion

Transcription

1 Indan Journal of Marne Scences Vol. 38(3), September 2009, pp CFD smulaton of cooperatve AUV moton Muhamad Husan* 2, Zahurn Samad 1 & Mohd Rzal Arshad 2 1 School of Mechancal Engneerng, Unverst Sans Malaysa, Engneerng Campus, Nbong Tebal, Seberang Pera Selatan, Pulau Pnang, Malaysa [E-mal: zahurn@eng.usm.my] 2 Underwater Robotcs Research Group(URRG), School of Electrcal and Electronc Engneerng Unverst Sans Malaysa, Engneerng Campus Nbong Tebal, Seberang Pera Selatan, Pulau Pnang, Malaysa Tel: ext. 6002, Fax: , [E-mal: rzal@eng.usm.my, husan_pge8078@yahoo.com.my] Receved 14 July 2009, revsed 11 September 2009 Cooperatve AUV performance and effcency s drectly related to ts power effcency. The power consumpton for ths type of underwater vehcle s nfluenced by ts moton requrement snce most of the power s spent for thruster propulson. Drag force s known as the man parameter n resstng the body moton. In the present study, the behavor of ths force s studed by usng computatonal flud dynamc approach (CFD). Two poston arrangement of cooperatve AUV was chosen to study the drag varaton. Frst, the dstance effect between two AUV was nvestgated to represent the basc poston arrangement of cooperatve AUV. Second, the effect of dfferent poston arrangement was also nvestgated. The comparson between dstance and poston arrangement s dscussed n ths paper. Present study elucdates that the dstance behnd the leadng AUV does not gve much effect to the drag force, but the poston arrangement ndcated sgnfcant nfluence. [Keywords: Cooperatve AUV, CFD, hydrodynamc coeffcent] Introducton The concept of multple Autonomous Underwater Vehcles (AUVs), cooperatvely performng a msson, offers several advantages over sngle vehcles workng n a non-cooperatve manner, such as ncreased effcency, performance, robustness and the emergence of new capabltes 1. The recent advances n sensng, communcaton and computaton enable the mplementaton of cooperatve mssons. Multple, hghly autonomous systems are envsoned because they are capable for hgher performance, lower cost, better fault tolerance, reconfgurablty and upgradablty 2. The growth of cooperatve AUV applcaton ncreases the sgnfcant of optmum energy consumpton of AUV. Most of the current AUV platform are powered by batteres that supply energy for sub-systems such of the propulson system and electrcal module. Underwater thruster s commonly used n AUV platform for propulson system. Ths system conssts of the motors and propeller. Many researchers are currently workng on AUV thruster desgn and t s performance 2,3. *Author for correspondence The basc concept of ths type of cooperatve AUV s that only one AUV wll gven the moton plannng capablty. Ths AUV wll lead the other AUV s traectory. Due to the moton of frst leadng AUV whle n operaton, the followng AUV wll have to face the ncomng flud flow. Ths flow totally due to the turbulence generated by the leadng AUV. The dsturbance n flud flow wll lead to changes n the separaton pont of the AUV. Separaton pont s the poston where the transton of flud from lamnar to turbulence occurs. LIU Zhen et. (2008) proved that the turbulent flow occurred around the submerge body wll ncrease the drag force actng on the body 4. The ncrease n the drag force wll ncrease the thrust needed to move the AUV body 3. New separaton pont s predcted to occur early than the leadng AUV. The obectve of ths study s to determne the value of hydrodynamc force actng on the follower body due to dsturbance of flud flow n certan poston arrangement. Also ncluded n ths study s the relatonshp between the dstances of follower AUV nose to the leadng AUV tal. Ths relatonshp s mportant to further dstance settng of the AUV, n terms of mnmzng the energy consumpton.

2 HUSAINI et al.: CFD SIMULATION OF COOPERATIVE AUV MOTION 347 In order to understand the behavor of the followng AUV whch has been proposed earler, the crtcal consderaton s needed n mesh generaton. LIU Zhen et. (2008) has reported n hs work on flud flow around the aerofol, that the best qualty of mesh wll gve a comparable result to the expermental data. In ths study, a very good qualty of mesh wll ensure that the turbulence can be modeled properly, and close to the real condton 4. Turbulence flow s mportant n ths study because the followng AUV wll be drectly facng the turbulent flow from the leadng AUV platform. Materals and Methods The effect of the wake produced by the leadng AUV moton wll be studed by varyng the leadng and followng AUV dstance. The AUVs were arrange n seres poston arrangement as shown n Fg. 1. The dstance between the leadng and followng AUV wll be vared every 10% from length of the body. Drag of the followng AUV wll be observed for dfferent dstance. In ths study, the angle of attack of the body s set to zero to represent the moton along X axs only. Ths settng s to avod the effect of varaton n flow drecton. There are two types of correspondng drag for the body movng n flud flow, vz. the pressure and the skn frcton drags. The pressure drag refers to the component of force when measured n the drag drecton. Ths force s due to the ntegral of pressure dstrbuton over the body. Followng the d Alambert paradox, when the body s workng n the nvscd flud, ths ntegral should be zero. Ths s because the ntegraton around the closed body s symmetrc 4. However, n real condton, the pressure dstrbuton decreases from the nvscd predcton n the regons of separated flow and consequently, gves to rse nonzero values of the ntegral. The skn frcton drag s the component of ntegral of the shear stresses over the body surfaces, that s measured n the drag drecton. In ths paper, the 2D vscous drag s consdered and gven by equaton (1): Vscous drag = skn frcton drag + pressure drag.... (1) Grd generaton Geometry meshng n ths study was generated by usng Gambt software. The mesh fle wll be mported to FLUENT for numercal study. FLUENT s a commercal CFD package for runnng the complex dfferental equaton by numercal approach. The solver used n FLUENT bascally has been developed by applyng the fnte volume approach for dscretzaton. A 2D mesh was appled to water around the body for ths study. Unstructured trangular mesh was chosen due to the complexty n the poston order of multple AUVs. There were 7197 node and face was generated by ths method. The mesh scheme for the two cooperatve AUVs are ascrbed n Fg. 1. It shows the operaton poston arrangement of the AUVs. The dstance between the AUVs was vared n ths study to determne the varaton of drag. Amt Tyag (2006) defne the three parameter that characterze a computatonal grd s total number of grd ponts, locaton of outer computatonal boundares and mnmum spacng 5. Generally, mnmum spacng s refer to y+ value, a dmensonless parameter representng a local Reynolds s number n the near-wall regon. value s defned by Schlchtng: + y = yu. / v (2) Fg. 1 Doman mesh for dstance 0.2 length. where, = dstance from the wall surface,, = shear stress at the wall, densty and knematc vscosty. The value of dstance of boundary layer regon from the wall can be determned by applyng the flat plate boundary layer theory, the relatonshp can be derved below: + y = y 0.172( )Re L 0.9 (3)

3 348 INDIAN J. MAR. SCI., VOL. 38, NO. 3, SEPTEMBER 2009 Fg. 2 Boundary layer at tal of AUV ( ρ v) = +.( ρvv) =. p +.( ) (5) τ In ths equaton, represent the velocty vector n Cartesan coordnate. P the statc pressure and the stress tensor gven by: = T 2 = µ [( v + v ). vi] τ 3 (6) s molecular vscosty, I the unt tensor and the second term of rght hand sght s volume dlaton. In ths paper k- turbulent model was used. Ths turbulent model s based on Reynolds averagng approach. After applyng Reynolds averagng term, the Naver Stoke equaton can be wrtten n Cartesan Form as : Mass conservaton: ρ ( ρu ) + = 0 x (7) Momentum conservaton: Fg. 3 Cd vs dstance ( ρu ) ( ρu u + 2 u δ 3 x ) p = + x x ] + x ' ( ρu u ' u [ µ ( x ) u + x (8) where, = Reynold s number s based on the body length. In ths paper the estmate mnmum spacng s determned by settng the equaton above solved by settng the value of L s 1.5. Important to note here that the value of s ust estmated based on flat plate theory. The real value of s vares over the surface accordng to the flow n the boundary layer. Fgure 2 show the boundary layer of the mesh geometry. Numercal method The governng equatons for mass and momentum are wrtten as below: Mass conservaton: ρ +.( ρ ) = 0 v Momentum conservaton: (4) Here s the Kronecker delta, and the Reynolds stresses. Ths governng equaton wll be dscretzed by usng Fnte volume approach whch support by FLUENT package. The governng equaton was solved usng the boundary condton as shown n Fg. 3. The teraton was stop at 830 because of convergence crteron has meet. Ths smulaton lmted the convergence crteron to 1E-5. Ths value s comparable enough to defne the accuracy of the soluton. Results and Dscussons The smulaton shows that the drag behavor of seral order AUVs s approxmately constant wth dstance. The drag s not much affected by the seral arrangement. Only small dfference of drag force for varous dstance are reported. Fg. 3 shows the relatonshp between drag and the dstance from the leadng AUV s presented n Fg. 4. The leadng AUV

4 HUSAINI et al.: CFD SIMULATION OF COOPERATIVE AUV MOTION 349 Fg. 4 Pressure coeffcent contour, (Graph 2) Graph CP VS poston, CP contour plot. always has the greater drag compared to the succeedng AUV (Fg. 4). Ths s because the leadng AUV faced the maxmum pressure caused by maxmum velocty drop. The man contrbuton for total drag s pressure drag. The dfferent n drag between leadng and follower can be explaned by pressure coeffcent plot as shown n Fg. 5. The varaton of Pressure Coeffcent (CP) was plotted along x axs n Fg. 5. The maxmum pressure occurs at or stagnaton pont of leadng AUV (Fg. 5). The blue plot represents the CP behnd the leadng AUV. From there, the CP value ncreases untl the new peak. Ths peak s the stagnaton pont of the follower AUV. The value of ths peak s lower than the CP value at leadng AUV. Ths condton occurs because of the varaton of velocty along the X axs. At the body of both AUVs, the CP dstrbuton s almost the same. The CP value s dependent on the shape poston arrangement of the body, and locaton on the body. For ths the shape and poston arrangement, the body s dentcal. The CP contour around AUV body s presented n Fg. 6. The smulaton studes reveal that the drag behavor of AUV s not affected much by the dstance wth leadng AUV. However, the drag force of AUV s reduced by the presence of the leadng AUV. The man contrbuton of leadng AUV n terms drag reducton, t can reduce the velocty of flud. So that the followng AUV only facng the smaller flud velocty rather than front AUV. Ths small velocty lead to small velocty drop at stagnaton pont. Ths condton leads to reducng the pressure drag. Second poston arrangement of AUVs was smulated to understand the drag behavor of AUV body by dfferent poston. In ths study, three AUVs were arranged as shown n Fg. 7. The smulaton has carred out for the velocty of AUVs n 2m/s for both arrangements. Table 1 shows the result of each arrangement. Cd1 and Cd2 n Table 1 represent the value of drag coeffcent for poston arrangement one and two respectvely. The value for Cd of leadng and follower n poston arrangement one s smaller than poston arrangement 2 (Table 1). Important to note here that second poston arrangement ncrease the value of Cd for leadng and follower AUVs. But the Cd value dfferent between leadng and follower for poston arrangement two s decrease compare to poston arrangement one. Table 1 Cd value of the poston arrangement. AUV Cd1 Cd2 Leadng Follower

5 350 INDIAN J. MAR. SCI., VOL. 38, NO. 3, SEPTEMBER 2009 Fg. 5 Velocty contour for poston arrangement 2. Fg. 7 Boundary condton for the computatonal grd. n poston arrangement one always lower than leadng drag. For the second poston arrangement, ths order lead to ncreasng the drag value for leadng and the succeedng AUVs. Studes may be conducted to optmze ths poston arrangement for decrease the drag. Ths result shows that the poston arrangement of AUV order n cooperatve s mportant factor for ncrease the effcency of AUV operaton. The optmum poston arrangement must be explore ncrease the cooperatve AUV performance. Ths work endured that a new poston arrangement must be study to fnd the effect of other poston arrangement to the AUV drag. Fg. 6 Velocty contour for poston arrangement 1 The Fg. 5 reveals that the succeedng AUV faces the nlet velocty drectly and the leadng AUV not much slowdowns the velocty. Compared to velocty contour of poston arrangement one n Fg. 6, the velocty around the nose of the follower s low due to leadng effect. So that the pressure drop around the body s ncrease and the value of pressure drag also ncrease. Increasng n Cd value of leadng wll reduce the effcency of cooperatve operaton n term of energy savng. Concluson The study comprses, the dstance between leadng and follower AUVs gve an effect to the CD value. he dfferent n poston arrangement, lke poston arrangement one and two also gve the sgnfcant dfferent n Cd value. But the drag for follower AUV Acknowledgements We are very grateful to Natonal Oceanographc Drectorate (NOD) and Mnstry of Scence, Technology and Innovaton (MOSTI), Malaysa, for provdng fundng to pursue our research n underwater system technology. References 1 Erfu Yang, Dongbng Gu; 2007, Non lnear Formaton- Keepng and Moorng Control of Multple Autonomous Underwater Vehcles. IEEE/ASME Transactons on Mechatroncs, 12(2). Pp Sarkar T., Sayer P.G. & Fraser S.M., 1997, A Study of Autonomous Underwater Vehcle Hull Forms Usng Computatonal Flud Dynamcs. Internatonal Journal For Numercal Methods n Fluds, 25 pp Ja Qulng & Guangwen L, 2007, Formaton Control and Obstacle Avodance Algorthm of Multple Autonomous Underwater Vehcles (AUVs) Based on Potental Functon and Behavor Rules. Proceedngs of the IEEE Internatonal Conference on Automaton and Logstcs August, pp.18-21

6 HUSAINI et al.: CFD SIMULATION OF COOPERATIVE AUV MOTION Zhen Lu, Beom-Soo Hyun, Moo-Rong Km & J-Yuan Jn, 2008, Expermental and Numercal Study For Hydrodynamc Characterstcs of an Oscllatng Hydrofol. Journal of Hydrodynamcs, 20(3) pp Tyag Amt & Sen Debabrata, 2006, Calculaton of Transverse Hydrodynamc Coeffcents Usng Computatonal Flud Dynamc Approach. Ocean Engneerng, 33 (2006) pp