Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Control of the Turbulent Wake of an Appended Streamlined Body
Control of the Turbulent Wake of an Appended Streamlined Body
Pages 342-354
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From page 342... ...
ad He pow ring characteristics mch as sh It speed, thrust ad pow r When cavitation or hydkoacoustic studies are concerned, it also becomes impo tat to simulate He th ee dimensional di tribution of velocities, ad perhaps the turbulent flow prope ties in the propeller plan Several means of altering He flow over the hull in order to simulate R y olds n mber similarity have been st died ad tested in different laboratories Bo mdary layer blowing has been selected ad implemented on a model tested in the GTH The design of f is set up is briefly described R sults of LDV measurements a presented which show how the blowing system modifies He dishibution of velocities in a ve y effective rmnmer The characteristics of She wakes generated are analyzed (wake fraction, harmonic content) m pa icular with respect to the effect of apendages A medhod for analyzing LDV measurements in order to estimate the turbulence in the flow is outlined ad aplied to She measurements pe formed on She fterbody Fmally, She effect of She ch tinges m wake on She steady ad un teady pe forma es of a propeller a presented INTRODUCTION Tests conducted at model scale m naval hydkody comics are co fronted with She problem of R y olds n mber similarity which camot be met, even ff large test facilities are used This similarity problem requi es the use of extrapolation methods adapted to She different flows: ship resista e ad propulsion, flows on lifting surfaces ad propellers, separated flows, sheet or bubble cavitation, vortex cavitation, etc The variety of difficulties which arise f om She differences m he is very challengmg to She experimental hydkody amici t These issues a not easily solved by CFD either because of the large R involved (107 to 105)
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From page 343... ...
to f 11 scale wake di trrbutions More recent work Evolving She effect of the propeller has shown how he effects m She nom mat wake are m odffied by the propeller action More recently similar investigations have been performed on a submarine model derived from the SUBOFF shape ad on She twm screw vessel operated by SACLANT (the Allia e) This worked sponsored Through a WEO EUCLID prog am is focused on evaluating the Hi it of CFD to capture flow details such as he ad propeller effects The motivation m this work was to amply this methodology to submarine work m order to evaluate He he effects on bare ad Upended submarine hull forms As far as She wake f action is concerned, different empirical formula have been developed for different types of ships which enable She exhaolation of the model scale effective wake f action to full scale These are based on statistical analysis of families of ships for which reasonably good full scale data is available For submarines such data bases are limited in She mmmber of ships ad when dealing with novel geometries, empirical methods ape of no help it is fhffefore essential to oh am a realistic wake at m odel scale to determine the full scale design point The goal of this work was to develop ad demonstrate She effectiveness of a bo mdary layer conhol method, to study She characteristics of the wake of She body at different he ad with different levels of boundary layer control for different co figurations (with or without apendages)
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From page 344... ...
and the resulting velocity profiles were analyzed to identify the best compromise between jet velocity, slot width and position. One constraint in this optimization was to reduce the flow rate as much as possible so as to reduce the mechanical problems associated with the integration of the ducting in the model and the support strut.
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From page 345... ...
0 0,25 0,5 0,75 1 1,25 u/Vo Figure 3.c: Velocity in the propeller plane Figure 3: Measured radial velocity profiles for 4 blowing flow rate (Vo=5 m/s)
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From page 346... ...
are also shown in the propeller disk to illustrate the difference in velocity profiles due to the difference in Reynolds number. Clearly the range of velocity profiles which can be achieved cover this difference in Re and most likely much larger differences in Re, i.e.
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From page 347... ...
EFFECT OF TAIL PLANES ON THE WAKE DISTRIBUTION In order to analyze in further detail the relative effects of blowing and tail planes, wake maps were performed for the different configurations. Hence, the effect of tail planes can be identified by subtracting the bare hull velocity profiles from the wake maps measured with tail planes.
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From page 348... ...
Although the wake map data showed some effect of blowing, the harmonic analysis in the form presented here shows considerable influence of the harmonic content even for minor blowing flow rates (Cql)
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From page 349... ...
Figme 11: Histog am of time betw en scamp le arrival times This type of random sampling precludes He direct use of classical algorithm to estimate the spectral pow r densities Two methods have been considered: intemolationmethods direct atocovaria e calculation The first method is not relict le a d the second one was implemented because it only uses atocovaria e calculations ad does not require the estimation of data pomts betw en measured pomts The method used relies on He truncation of the actual arrival time to the resolution of the clock of the data Requisition system Hence He data is m the form of a regularly sampled signal with missing data points The pectral pow r density of He velocity fluctuations is obtained by the Fourier transform of He atocovaria e of this signal This method is robust a d effective but it is limited in f equency to a value move which the noise in He pseudo signal is larger than the signal Figme 12 presents f ee examples of t rbulence spectra measured in He wake with ad without blowing ad with ad without propeller The hori ontal line in He plot represents He detection limit of He medhod The results presented here show Hat the t rbulence levels are slightly low r when He blowing >! tem is active ad when the wake is smaller in size However, when the propeller is operating, He turbulence levels are much low r due to the aceleration of the flow This data clearly shows Hat when turbulence quantities ape requi ed, the effect of He propeller camot be ignored Figmre 12 a: Turbulence spectr m (without propeller, CqO)
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From page 350... ...
In order to asses the effect of wake fraction on propeller performance, the measured nominal wake fractions based on the LDV measurements were used to correct the advance ratios from a behind condition to an estimated open water performance JO = JO ~ (1- An)
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From page 351... ...
RpssellEditepr Viscous D ag R duction in Bopmdarv Layers Prog ess in Ashonatics ad Aeronautics, Vol 123, A AA, D Bushnell et J Hefp r, edited s Lapchle G., Gurney G., "Laminar Boundary Layer St i i it on a Heated Underwater Body", Techmical Memorandum, Applied R search Laoratory, PSU/ARL-TM-83-157, Javier 1983 Jessup S., Remmers K., et Berberich W., "Comparativ cavitation p rt arm once evaluation of a naval m face ship prop tier, ISLE 1993, Cavitation h ception, pp 51-62 Nobach H., Mfdler E., Tropea C., "Efficient estimation of pow r pecrral density from laser Doppler a mometer data", E periments in Fluids 24 (1998 )
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From page 352... ...
As discussed by the mthors, the spatially nommnifomm flow Ed the temporal variations are of primary cop rn for the propeller i flow For this reason, ARL Pemm State has utili ed shorter models Ed m some cases sheens are added to the nose of the body to gee rate She predicted me m wake However, the issue of turbulent en rgy distribution Ed flowheld harmonic content as z fun non of Rey olds mmmber remains z critical issue for cavitation mdhyd oacoustic pe formance Hz d oacoustic performance is very sensitive to flow features Ed m my st dies have shown f is relationship Two mterestmg experiments that relate appendage wake feat es to noise have l en conducted by ARL Pemm State Ed Brooktield Ed Waltz (2) Two experiments to investigate blowing from the trailing edge of m appendage have been conducted at ARL Pemm State The wind tum I experiment of z conhol-su face-like appendage show d the ability of hailing-edge blowing to gee rate z moment mless wake, for z nonlifting, thee-dimensiopal al foil At m mgle-of.rttack of ten degrees, z completely moment mless wake using trailing-edge blowing could not be generated, becmse of the asymmetry of She original wake A dual-slot co figuration for zdaptmg hailing-edge blowing to z lifting foil was then evaluated in z second experiment, trailing~dge blowing on five stationary struts located up Ream of z li - e-t lad d f m was UK 0 porffed With z total flow rate f ough ail the blowing holes equal to 0 7% of the flow rate th ough She Ian itself, significant reduction in the radiated noise at the blade-passing frequep y Ed its integer multiples was demonstrated The reductions ringed fi m 15-24 dB for the first four harmonics Ed from 3-9 dB for the p xt th ee harmonics Brookfield Ed Waltz (2)
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From page 353... ...
DISCUSSION Michael B Wilson David Taylor Model Basin, Carderock Division, NSWC This is an interesting investigation and implementation of a slot blowing boundary layer control concept aimed at manipulating the velocity distribution of a model scale turbulent wake at the plane of the propeller.
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From page 354... ...
In our case She model is 4 4 m long The water mm I em p revere was et around 24° during the tests Then She r mge of model Rey olds mmmbers was: 2 4~107 to 5 8* l07 All tests w re performed with the same model Ed the same blowing system The blowing section fomms c I mm thick slot around the cacular periphery of the body Ed is set et c 10° Ogle with respect to the body surface The slot is located et cutout 65% down tream of She nose of the model As explained in She paper, the characteri tics of the slot w re found to be the be t compromise betw en jet velocity, slot width Ed position m order to minimi e the flow mte es much es possible Ed to obtain monotonous velocity profiles in She propeller plane This optimization was perfommed with 2D Reynolds Average Navier Stokes RANS)
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