Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
Currently Skimming:
An Experimental Investigation of Cavitation Inception and Development of Partial Sheet Cavaties on Two-Dimensional Hydrofoils
An Experimental Investigation of Cavitation Inception and Development of Partial Sheet Cavaties on Two-Dimensional Hydrofoils
Pages 654-669
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 654... ...
ABSTRACT The main results of m e perimental st dy concerning the inception md the development of partial sheet cavities on four Rio denenzoncl hyd of oils are presented The conditions of cavitation inception are measured together with She minim m p~essme coefhcient in some cases The effect of the Rey olds n mber is studied Concerning cavitation development, depending on the cavitation member or the male of incidence various types of cavitation are observed es partial sheet cavities, bubble, fingers, patches or supercavitation patterns For partial sheet cavities, the cavity lengths w re measured on th foils for various conditions of cavitation n mber md male of incidence A attempt to correlate th cavity length date is studied et She end of the pap r INTRODUCTION The physical process associated with She inception md She development of ca itation is complex md basic experiments on two-dimensiomal hyd of oils remain m effective way to study She fundamentals of cavitation md to mderstmd propellers for mst mce Partial sheet ca ity is one of the cavitation patterns that occurs on c twodimensional hydrofoil t picclly near th lecdmg edge it corre ponds to th situation for which c cavity of vapor extends own c Faction of She hyd of oil's surface For inception of partial cavities, the two questions which arise are where md when do cavitation occur ? it is generally acepted that cavitation occurs on c f 11-scale lifting surface et the position of the minim m pressure md when She local minim m pressure falls to or below She vapor pressure of She flowing liquid But m m my cases, particularly on scale models, the incipie t cavitation m mber ti is fo md to be different (often smeller)
|
From page 655... ...
srv4 still recess6 y to st dyc6 ibtion The6imofthispaperistop~esent experimenbl results of 6 procedure of ir~sstigation concermog the mception md the dew410pment of p6 ti61 c6 ities on various hyd of oils Correktions 6 e tudied to provide mpi ic61 formulations for the c6 ity lengths, which may pro s usef I for f sther e smirurtion md comp6 ison with mmmeric61 res 5ts EXPERIMENTAL APPARATUS Test fadlity The experiments w ~e conducted m 6he B ole N6~1e C6 itation Tcmx41 fitted with 6 Im long md h=0 192m wide square cross test section Velocities of up to I 5 m/s md p~v4ssmv4s betw en 30 mb6 md 3 b6 m be 6chie sd The experiments w re conducted on the hyd of oils show on Fig I Th ee of 6hem 6 e of the NACA66 fsmily md h6 4 detemm med using the d6 t6 of V6 lentme (1974) Two of th m h6 s the wme rekti s m6 im m 6hick ess of 6%, but diffe~ent chord lengths of 100 mm 6md 150 mm re pecti dy The thi d has 6 ~elati s m6 im m thick ess of 12% 6md 6 chord length of 100 mm Th fo sth h~s 6m Eppler section E817)
|
From page 656... ...
For partial sheet cavities, the cavity length 1, was estimated from the cavity profiles determined by image processing of visualizations obtained by a laser sheet, formed by a 1W Argon Laser source focused through a cylindrical lens. It consisted of computing the mean value of the gray level of a given pixel from about 20 acquired images and of determining the frontier of the cavity.
|
From page 657... ...
(~ md c~(~ me ms 6~t et c gi on olocity 6he sbtic presYcre u4quiu4d to indu e cavitation must be sig ificmtly lowor 6 m that requi cd to elimim~te it Howo or, the diffeu4D2e c m be exphfin by the process of cc itation detection based on visu~lizations which c m i fer bics on 6he mgle of cavibtion mception it c m be cssumed that the reel cc ibtion i 2eption (micro bu bles not observm le) occu s before it is visuslly detected 56t , ~ Fig.
|
From page 658... ...
As shown, for Re= 0.4x106, glossy longitudinal cavities characterize cavitation inception whereas, for Re = 0.8x106, the inception appears as a narrow spanwise cavitation band. Moreover, the location of cavitation inception moves towards the leading edge for Re = 0.8x106 in accordance with the location of the experimental minimum pressure coefficient.
|
From page 659... ...
~: 1.2 - / 1.0 ~ _ 0.8 0.6 / 0.4 ~ 0.2 _ _~ _ 0.04 -0.2 1 1 1 1 1 · 1 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 10 oc Fig. 13.b Difference between the experimental inception cavitation number and the experimental minimum pressure coefficient as a function of the angle of incidence oc.
|
From page 660... ...
4 or n -6 = = = , ~ A_ ~ ~ ~ ~ · ~ ~ ._ .__ Cavitation inception _ ~ partial cavity _ Inception of pulsating partial cavities _ · pulsating partial cavity O bubbles _ Xfingers _ ~ pulsating fingers _ · patches _ ·supercavitation ~ ASIA ~ . 2 2,5 inception ()
|
From page 661... ...
md m "u iqu " pow r how, describi g the partial cavity development whatever the studied foil, is not determined Wish a md n as scaling parameters, the con hymn does not take i to account of a characteri tic of th foils A fir t characteristic of a given foil is the male of zero aft, .-, md particularly the difference (co .-.,) which cm be considered also to be proportional to The lifi coefficient Fig 19 shows the chatty lenzlh databy mtrodu ing th difference (co m)
|
From page 662... ...
Al (1) with m exponent which is close to -2 his conception apple s to be w 11 representative of the cavity development on the tested foils for: I < a < 3 5, 3 5° < Or < 6° md as long as She cavity lengths do not exceed half She foil chord length 100 old 10 100 GI(cc e a)
|
From page 663... ...
it is foumd to be larger th m fhe themeticcl valu dedu cd from comput~tion for m m iscid flow How v r, the su fa pressu e coefficient, dedu cd of v locity mesurements on two foils, indicate th~t the minimum p~essme coefficient is close to fhe ip eption cavit~tion number e pepted when c large separated region is observ d at fhe foil lecdmg edge Cop erning cavitation dev lopment, acordmg to fhe cavitation number or fhe mgle of ip idence, various types of cavitation are observ d es particl sheet cavities, bubble, fingers, patch s or supe pavit~tion patterns For p pti~l shet cavities, the cavity length w re mesu cd on th foils for various conditions of cavitation number md mgle of ippidep e A cttempt to correlate th cavity length date is studied et the end of the pcper 7his study indicates th~t fhe cavity lengths tend to evolv es ((fi(o P ((~)
|
From page 664... ...
20, N°4, pp 215-223 Katz, J., 1984, "Czvit~tion phenomenz withm regions of flow separation", Jou pal of Fluid Mech mics, Vol. 140, pp 397-436 Kawapamd, Y., Kato, H., apd Yamaguchd, H
|
From page 665... ...
Clearly, the coordinates for the NACA 66 with 12% relative Thick ess are given in table I It is right that our experiments for the 66 series foil w re carried out for lift coefficient greater thm the design Ifft coefhcient How ver, this corresponded to the situation for which partial sheet cavities occurred Table 1 Coordinates of the NACA66 foil se tion
|
From page 666... ...
AUTHOR'S REPLY Concerning the recall in She larger discrepancy betw en She experiment Ed predictions with negative Ogles of attack conditions compared to positive Ogles of attack, Pigures 5 Ed 11) , w have to say that no pecu liar attention was paid on this point How ver, the effect of c separation bubble that could be m me impo t mt for negative Ogles bee mse of the shape of She foil c m be assumed How ver, m of her effect es the co finement effect (t m I walls)
|
From page 667... ...
AUTHOR'S REPLY Concerning ~ separation bubble during the LDA measmements near the leading edge of the NACA66 hyd of oils, we have to say that velocity measurement w re carried out only on the NACA66 with 12% rektive thickmess No observable negative velocities w re detected which should be the condition for ~ separation bubble to exi t So the presence of ~ separation bubble was not clear How ver, the cavitation inception formed of ~ thin p mwise bubble b md seemed to Indicate the presence of ~ separation bubble Concemmg the extent md appearance of the cavity which is i fluenced by She chmge in the Rey olds mmmber Fhotogmphs of cavitation patterns showed that such ~ h msition is rented to ~ tr msition pomt on the ca vity so face, which moved fo ward to She leading edge when the Rey olds mmmber Increased it c m be assumed that this c tnt:e related to ~ h msition from ~ long separation bubble to ~ short separation bubble when She Rey olds mmmber so e. How ver, the effect of the turbulence intensity (as rightly suggested by the discusser)
|
From page 668... ...
cm be computed This was done fiom m interpolation of She velocity data together with c spatial numerical derivation in Xe Ed Ye directions (note that ~ es c superscript corresponds to non-dimensiom~l values scaled by the foil chord length Ed She i flow velocity) it must be pointed out that, in some cases, bee mse one of She laser beams intersected to the foil surface, She vertical component of the velocity v was not measly at lea Thus, in that cases, estimated only Through the second term of Equ I A example of the calculation is given in Fig I showing She lines of con d mt sp mwise vorticity for m Ogle of attack 10° cone pondmg to She case for which c flow separation was clearly detected it c m be observed c streamwise stret bed vorticcl shuctme exlendmg from the leading edge up to about xt = 02 along the boundary of the separated legion (depicted by the dashed line on Fig 1)
|
From page 669... ...
As shown, in that case, there is a good agreement between the inception number and the opposite of the minimum pressure coefficient taking into account of the vertical flow. Although, this result can appear as a rough estimation, it indicates that the vertical flow involved in the case of a flow separation can play a very important role in predicting cavitation inception.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.