Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Simulation of UUV Recovery Hydrodynamics
Simulation of UUV Recovery Hydrodynamics
Pages 833-847
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From page 833... ...
Grant (Naval Undersea Warfare Center, USA) ARSTlRACT: A novel medhod to compute th 3-D unsteady hyd odynamics with application to undersea vehicles is presented This approach solves the vo ticity equation, which is derived fiom the moment m equation of the No ergot es equation Most problems of No y interest Evolve Compressible flow, which c m be described m terms of She vo ticity alone Velocity is m integ cl quantity of She mstmtaneous vorticity field Specific geometries me represented using surface source Ed vo tex panels whose trength is prescribed to satisfy She no-slip Ed no-flux bo mdary conditions Vorticity is diffused fiom the vortex sheets onto She body surface to mci tam c vorticity balance Vorticity in the flow is specified et pomts Ed the vorticity et my oth r point m th field is obtained vie Imear i temoktion Imtemohtion is perfommed by conshucting tetrahed c using D launcy hiaDguiarizatioa Tetrahed c provide She control vol me to Meg ate over to obtain th velocity Ed She cow ctivity of the control pm ts pro ides c basis to conshuct derivatives A BcldwimLomax eddy iscosity model was implemented mto the sol non algorithm to model turbulent flow effects This method was Hen validated for two disparate flow cases flow past m mmanned undersea vehicle UW et Rey olds n mbers of one million Ed unsteady flow development pest c cone Attached flow pest She WV was compared with mpi iccl turbule t flat pate results Quality of the flow pest c cone was compared with data obtained wish e perime hi date Validation of f is method allows for c subsequent simulation of c UW recovery problem INTRODUCTION: Undersea vehicle hyd odynamics pose sigmifc mt challenges for the computation of complex, f ee-dimemiorul unsteady flow field.
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From page 834... ...
es w 11 es 6he second order Lcpkcia ~teg ction of the Biot-Savart i teg cl provides 6he velocities A oth r difficulty in t~ectmg high R y olds n mber flowis aco mting for tmbulence Traditional vortex methods have relied on mdom walk ad oth r tochc tic medhods to simohte 6he high R y olds number tubuent flow Thus far, most deterministic solutions for viscous flow have been limited to low R y olds number kminar cases In 6he present method, c deterministic aproah utilizmg 6he full viscous equ~tions was desi ed Therefore, c tmbulence model has been i trodu d to acoumt for 6he hck of spaticMesolution required to properly t~ect fully tubulent flow diectly A Bcldwm-Lomax tubulence model was implemented mto th viscous soluion methodology to better model 6he tmbuent flow charateristics To th athor's k owledge, 6his h~s yet to be implemented in my vorticity based solution methodology This pcper includes um tecdy compubtiorud date collected for flow pc t c UW et c Rey olds number of one million md um tecdy flow pest c cone for c R y olds number of 50,000 Attahed tubuent boumdary hy r flow over th WV is compared with tubulent velocity profiles obbined fiom empiriccl ~esults by Spcldmg md Coles (see White, 1974) Time averaged wake velocity dab for flow pc t c cone is compared with dab prese ted by Calve t (1967)
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From page 835... ...
Initial Volume Vorticity Distribution: After the surface panels are defined, it is required to initialize the volume vorticity beginning with the surface vorticity. Nodal vorticity values are located at the surface body points used to define the geometry.
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From page 836... ...
ol6 ~t (12) BaldwimLomax Turbudenee Model: Since spati61 resolution conshamts du to th computatiomd co t of the c61cuation prohibit di ect m meric61 simulation of high Rey olds number flow past the bodies of i te~est, 6 B6 Idwm-Lom6 tu bUIk e model w6 s implemented i to th code This tubuik e model is summ6 ik d by Wilcox (1993)
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From page 837... ...
The reason for this is that the layer of vorticity is very thin so there is little difference in the velocity generated by the infinitely thin sheet and the thin vorticity elements connected to the surface. Initial Volume Vorticity Distribution, Euler Layer and Point Creation: After the surface panels are defined, it is required to initialize the volume vorticity on a set of points.
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From page 838... ...
Reynolds number can then be set as an independent variable and for these cases was set to 1,000,000 for flow past a UUV and 50,000 for flow past a cone. The UUV was defined using 550 surface points resulting in 1100 panels (Figure 5a)
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From page 839... ...
- _ o 0.10 0.09 t~ 0.08 8 0.07 0.06 ~ 0.05 am- ~o 0 . 0 4 ~ 0.03 Z 0.02 0.01 0.00 Figure 7: Instantaneous vector plots in the x-y plane of the turbulent flow past a UUV.
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From page 840... ...
and the turbulent flow case (Re = 50,0004. Velocity vectors are colored based on the wake zvorticity and the surfaces are colored based on the vorticity magnitude.
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From page 841... ...
Figure 15: Velocity vector plots at the tetrahedra centroids depicting the unsteady flow development past a cone/docking UUV from t = 0.0 - 0.5.
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From page 842... ...
Figure 17: Velocity vector plot comparison of flow along the centerline in the x-y plane for Xuuv = 0.2 and the UUV center vertically displaced - 0.05. The surfaces are colored based on the surface pressure and the velocity vectors are colored based on the z-vorticity component.
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From page 843... ...
.—Cmx —Cmy —Cmz Non-Dimensional Time Figure 20: Unsteady force and moment coefficients for the UUV with the origin at 0.125 UNSTEADY UUV FORCES o.3 T XU UV = 0.5 _ 0.1 o- , -0.1 - . -0.2-0.3 0 04 T 0 03 002 0 01 n -0 01 -0 02 -0 03 -0 04Non-Dimensional Time UNSTEADY UUV MOMENTS Xuuv = 0.5 Non-Dimensional Time Figure 21: Unsteady force and moment coefficients for the UUV with the origin at 0.5 Cp to 0.0 at t = 0.35 which correlates with boundary layer ingestion of the wake vorticity.
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From page 844... ...
The increase in normal force correlates with an increase in pitch moment suggesting a nose down pitch moment with downward forces proximal to the UUV nose. DISCUSSION: Unsteady Wake Development: The velocity vector plots displayed the initial unsteady flow development aft of the docking cone and the resultant impact of the wake on the docking UUV.
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From page 845... ...
7his cltered trajecto y could be sigmfficmt m terms of dockmg th UW Futme flow cclcoktiom sho 5d focus on th se types of phenomemr CONCLUSIONS: A novel Lag mgim vorticity method hcs been p~esented to compute 6he mstecdy hyd odyr~smics cssocisted with r~l mmarmed mdersec vehicles Since 6he flow is mcompressible, 6he pressure term is not ~equi~ed for sohtion leavmg c velocityworticity form 5ction ~ fact, since 6he velocity is c di~ect integ al qDmtity of th vo ticity, this medhod demonshates th~t 6he m tecdy flow cm be descobed by the vorticity clone 7he Lag mgim m~tme of 6he calcoktion ~equi~es only th~t the diffusion temm be solved e plicitly The cdvection temm is cutomaticclly mchded since 6he points me moved wi6h the loccl flow The dfff sion velocity concept is clso used to move 6he points mto regions of 13 :D:ro vorticity This avoids 6he dffhc 5ty of e tablishmg empty points to dfff se 6he vo ticity onto The diffusion eqDrtion was modffied accordingly md solved st ecch time step fffects of g id resohtion md validation of 6he velocity cclcohtion md dfff sion clgorithm w ~e conducted by comparmg comp htiork~l results for crurlyticcl sohtions for Hill's sphericcl vort:x md c col mnar vortex The mstecdy tmbulent flow pc t c WV et Re = 1,000,000 md the unstesdy flow development m th wske of th cone w re bodh inve tigated to d momtmte 6he effectiveness of 6he curre t medhod Mem tmbulent bo mdary Izyer velocity proflles cg eed quite w 11 with empiriccl res 5ts fiom Spcldmg md Cole Turbule t fluctuations cg ed s mrisingdy w 11 for o'but not es w 11 for v' md w' This was expected due to lak of surface ~esol tioa The fact 6~t turbulent qDmtities are comp ted et cll demon trstes the pote tial of 6his method to compute turbulence di~ectly Unstecdy flow past c cone show d 6he vortex ri g shuctme produced Initiclly, th vortex ring was symmehic but developed csymmehies as 6he flow developed Th Isminar flow case show d c much mme coherent vortex compared wi6h the turb 5ent vortex This is exactly as i tended md was the recson for impleme ti g c turbulence model Consequently, 6he tmbulent flow res 5ts for 6he wake velocity proflles w re in much better cg ement wi6h experimental test cases Simoktion of UW ~ecove y hyd ody smics di plcy d some intere tmg flow fleld phenomena Th m tecdy flow development in 6he wske of 6he cone show d th vort:x ring tructme produced Imticlly, 6he vo tex ring was mmetric but developed c mmetries es th flow developed As 6he csymmet y was mmifested, sheddmg of vorticity was observed The shed vorticity did not cppear as w 11-formed vortex rmg but es convoluted hai pin vort:x shuctmes Proximity of th WW sffected 6he way m which th vorticity was shed For cases whe~e 6he UW was close to th WV, th separcted vorticity region betw en th WV md th cone mcintained cohe~ent vortex ring shuctmes whose csymmetry varied with time As the WV was plaed dow tresm, the vo tex rmg shuctme becime elongated md ellipticclly shaped Here, defimte 'ch mks' of vorticity w re shed which impacted 6he WV As th wske of the cone impacted th WV, sigmfficmt trmsients in loccl pressme were observed The~e was c reduction m stagnation p~essme et 6he nose of 6he WV md mmecse m p~essme et the point of maxim m ~adius of 6he WV Pressure fluctuations w re observed es the vorticity domirurted wske impacted the nose of the WV Cclculations et midbody show d fluctDrtiom m pressure es w 11 Ew~n though 6here was k ge pressure fluctuations, i teg cted force computations show d little fluctuations during mitial wake impact Only es the flow becime developed w re sigmflcmt fmce fluctuations seen Also, the mcgmit de of the force fluctuations rapidly diminished, es the WV was pkced dow tresm Verticcl pkcement of th UW produced cltered wske structure md mstecdy loadmg on 6he WW Placing the WV below 6he cone clte~ed 6he vort :x wake mte~action wi6h th UW md clso produced negative normcl forces md nose dow pitch moment For this case, sim~soidal fluct stiom in
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From page 846... ...
R "E periments on 6he low-speed flow past cop s," Joppul of Flpid M chmics, Vol.
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From page 847... ...
(: n the f rst t me step, the no s ip, noflux bounds y conditions are satisfied with the su face vortex sheet. The surface v or icity is then set so that cDdV=7dA where to is the surface vor icity, 7 is the surface vortex sheet strength, dV is the volume of the elements connected to the surface node and dA is the area of the panels connected to the su face node.
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