Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Ship Wake Detectability in the Ocean Turbulent Environment
Ship Wake Detectability in the Ocean Turbulent Environment
Pages 687-703
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From page 687... ...
in the Davidson Lctorctory towing tmk on ship-wrke detection by mecs xmg t xbulence m-situ The fheoretical crurly is of fhe ship-wrke txbulence uses th shear-fiee model, seff-modeling md Kokmogorov's hypodhesis for th pmpose of clos xc The envi ommentcl t xbulence in the oce m upper kyer hcs beff~ formokted by the css mption of hori ontally miform hyd ody rmic fleld md k ~ g o p model mder fhe :xistence of smface we ss md its breking BcY4d on k 8 tmbulent closmv4 c couple 3-D non-stecdy m mericcl model (wrke + upper kyx) which mcludes fhe waws brerking h~s been sed, in fhe shear-flv4e cpprocch, to cxry o t m mericclly fhe ship-wrke detectability m the ocem txbule t envi omment Bodh of the crurlyticcl md m mericcl remits show fhe 3-D truct xc of fhe ship wrke for dffferent wind conditions cod ship peeds, md the detection ~a ge on the oce m s xface md fhe detectability in depfh of fhe ship wak in terms of ship parrmetxs mddhe wmd speed The experimentcl study is destir~sd to srify fhe th oreticcl md m mericcl prediction on detectability of tmbulent ship wrke mder dffferent experimentcl conditiom The wrke t xbulence sigmfficmtly exceds fhe m~tmal lew41 of fluctuatiom in th t mk md fhe v~brction noise produced by the towing sy tem The wrke t xbulent spech ms he s w 11 exp~essed Kolomogov's r mg The ship-wrke txbulence is w 11 detectable md Kolmogorov's rmge cm be identified e sn for fhe mo t ~emote location of the probe (~ I O L~, L~ is fhe ship ienf h)
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From page 688... ...
The tubule t upper hy r of th oce m hcs c complex verticcl shu tme defned by i flu nces of dffferent phy iccl mech misms such es energy cod momentum h msfer, th presence of su face waves md their breckmg, fhe tu bule t energy produ tion by me m she flow, fhe wave motions of th fluid, md fhe effect of fhe Coriolis force In conbast to ctmo pheric boumdary hyers over I md, whe~e me m velocity she is th mcin sou e of tubulence energy, th tubulence in the upper hy r of th ocem is gowxned not only by me m velocity she, but clso by su face waves Th t mspo t of momentum, hect, moistme md sclt occu s across fhe ci -see i terface cod is effected by fhe oce m surface waves The~efore suface waves phy m importmt role in fhe ci -see interaction ystem The tmbulent motion m the upper oce m is c highly pecffm example of tmbulence in c liquid whose fie su face is su ject to wmd frictioa Th result of fhis cction is fhe fcrmation of wa~s, pme d if cunents mdtubulence, whichlecdto sho g ve ticclmi mg of fhe surface kyer ~ conbc t to boumdary hy rs et c solid wall whe~e me m velocity sh ar is fhe mcm sou e of fhe tubue t energy, th tmbulence in th upper ccc m is govemed m mmy ~e pects by th rutu e of waves The totcl me m ctmospheric tress not only indu es oce m cu rents th ough th action of th shear t~ess clone but also supplies momentum to g owing su face waves A part of fhe momentum md energy is tr m fened di~ectly fiom the wind to d if cunents, while moth r put goes i to surface waves Wmd waves contain c considemble cmou t of momentum md energy md fhey redi tobute fhe momentum md energy over g ect distances md supply energy to d fft cu rents md tmbulence by th ir brecking Th wave breaking mectes c highly tmbulent envi omment withm fhe top few meters of th ocecn, md th wave dissipation by the breaking i tensffies tubulence in fhe oce m mi cd hy r ~renarm et cl, 1992) Wave b~eaking pro id s c mechmism for injection of both moment m md tmbulent kinetic energy fi om th mface winds to th water E perimentcl ~esults mdicate th~t fhe rektive energy th~t is lo t from th wave motion du to c smgle breaking lies between 102 md 10 ~ ( M Iville md Rcpp, 1985)
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From page 689... ...
, where y is She hori onbl t msverse coordinate For She wake problem, the time t c m be convened to She longitudinal coordinate x, She di lance fiom She ship in th di ection along She wake, by t msformi g x = Ust The change of kinetic energy, kw, in time md space w may describe withm She hamessork of She lliE equ tion in c she free approximation, where se f-modeling md Kolmogorov's hypotheses are used for the pu pose of closme The Meg ction gives She solution in th following form r (t)
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From page 690... ...
Wb s, Tw The moment m fl~ Sw cbm be expressed bS b combim~tion of sww 6he moment m flux for 6he Wb g owth, bmd Twb . 6he moment m fl~ produced by Wb breaking Smce Twb finully goes to the wind curtent, 6he tott I moment m fl~ to the wind c trent inthe tpper Ot bm, [c.
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From page 691... ...
shows th~t th Wb breking play bm importamt role m the formmg 6he upper kyer t xbulence ~egime It is b3 mmed that th s xib e Wb s bre fully or blmo t dev loped, bmd 6he wind condition chmges slow enough to adapt 6he steady bpproacb This bwomption reduces the n mb x of urdmown s aLb e ww qw O it giv s Iw = IWb bmd qw =qWb ~ 6he cbse of dev loped Wb s, 6he moment m flux produced by Wb breakmg mby hav same order of magmitude wi6h the moment m flux fiom the btmo ph re Therefore, the simple t hypodhesiS m be made b3 p~u~ = Tw = ~Wb The4 th moment m flux to the wind cmrent [c becomes ~c = ~wb = pau~a = PwU~w (17) The tmbulent kinetic energy, k, in the oceam upper Iby r mby hbV dismepb ies with regmbr t xbule t models becbmse the potentibl motion due to 6he 3mfb e Wb s has vxy shong impact on dy dmic behavim Vb io d bttempts to deriv the equation of t xbule t kinetic energy with presence of 6he pote tibl Wb component m the mdom v locity feld of the ppx oceam hbV been undertaken by Benilov (1973, 1997b)
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From page 692... ...
The detection range xd can be obtained from the turbulent kinetic energy of the ship wake along the x axis and the surface turbulent kinetic energy of the ocean upper layer. Introducing the notation that kw is the kinetic energy of the ship wake along the x axis, which gives the maximum value in the cross-section of the wake, and ke is the surface kinetic energy of the environmental turbulence, the detection range is a solution of the equation k = k .
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From page 693... ...
The ship wake identification below the ocean surface can be made by comparing the turbulent kinetic energy of the wake and environmental ocean turbulence.
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From page 694... ...
—10 6Ua . The turbulent kinetic energy of environmental turbulence for the weak wind case is constant through the depth of a statistically steady layer where the Coriolis force is not effective, which is called the 3.
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From page 695... ...
. Equi-Kinetic Energy Isolines.
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From page 696... ...
The model speed corresponds to the actual ship speed 20 knots, and the surface waves correspond to the actual developed wind waves for the wind speed about 12 m/s. It can be seen that the wake turbulence for that location still significantly exceed the level of the environmental noise and has the well identified Kolmogorov's range.
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From page 697... ...
Turbulent spectrums in the wake cross section at the distance L = 4.2 m from the model. model speed Us =68 calls.
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From page 698... ...
Turbulent spectrums in the wake cross section at the distance L = 13 m from the model. model speed Us =202 calls.
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From page 699... ...
Turbulent spectrums in the wake cross section at the distance L = 13 m from the model. model speed Us =6X cm/s.
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From page 700... ...
G imshow, Spri ger Verkg NewYork,pp 95-101 Hoekshc M, J Th Ligtelijn, 1991, "Macro wake fectu es of r mge of ships", Techmiccl R port 410461 I-PV, Mar it ime ~ e search Instit te Nether kmds , Wcgemogen, The Netherl mds Hoffmm Klms A, 1989, "Computatiom~l Fluid Dynamics For E gineers", A Publication of E gi ering Edu ction System, Austm, Texas Kitcygorodskiy, S A md Yu Z
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From page 701... ...
A M Y~dom, 1965, "Statistical Hyd omechmics: Turbulence M chmics Part 1", (1967, Part 2) Moscow, Fi matgi, pp 639 Fff6h prmtmg, 1987, Stati ticcl Fluid Mech~mcs: Mechmms of Turbulence Vol.
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From page 702... ...
appear to be unique Ed constit te c valuable contribution to She literature I have two questions for She mthors Concerning the turbulent wake of the ship clone, it would be useful ff the mfhors would id tify th rektive conh~butions of ship hull form; propulsor; Ed ship generated waves (which may break) to the total turbulent wake field Visual examination of the surface wake aft of c ship gives the impression Nat the propulsor wake may dominate so Nat the particulars of She ship geometry may only be import mt m defming the propulsor thrust Ed the concentrated kinetic energy it imparts into c diameter of fluid which is subst mticlly smeller th m She beam of the ship In my event it would be used I to under t Ed She mcke-up of the term kw in their Equation (26)
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From page 703... ...
c 3D shucture of She ship wake depending on impo t mt controlling parameters of the source Ed en- nomnent he experimental study with c propelled ship model m the Ictomtory tank, closing the paper, should, on mthors' opinion, verify the developed theory Ed mmmericcl cclcubtions in fact She wake turbulence spectra are rather thoroughly measured by c hot fit anemometer across Ed along She t mk behind the model he anemometer sensitivity mm out ~~1lic~ ml. high to measure very week turbulence si-rLtls even against She background of specially em mated mu facewaves All the th ee approaches usedby mthors of the reviewed paper do not overlap some compact concept of the ship turbulent wake in She retl oce m en- nomnent to m equal extend but rasher complement each other For example, no layered struct re of turbulence, developed analytically, is show evidently in Humeri 31 Ed laoorcto y experiments However, the method of presentation used in She paper has ow cdv mesa es to make perhaps the pattern of studied phenomenon m ore voluminous Some questions c m also arise, when recdmg the article First, it ..-m~ld be desuable to take the see wave directionality into account m the model of upper turbulence, in parallel to the wake axial scale, Though this task is not too simple Also She technical formulas derived c m be readily simplified, for example, by lowering the accuracy in mmmericcl factors Ed exponents, es well es by exp mdmg or approximating some terms, et Second, on my opinion, it is import mt to widen the description of mmmericcl model Ed the presentation of calculated data, m particular, to show both the simplifications based on the developed theory Ed c possible restructuring of the wake es it penetrates Through She waveperturbed zone mto deeper Icyers let the experimental section, it is not clear how w re He su face waves excited in He tank, Ed in general She extensive wave modeling would be of crucial importance in view of the main problem posed m She work An exphrmtion of He experiment scaling seems also to be not superfluous Besides, c general accent on mmmerics Ed experiments (maybe m oral presentation)
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