Abnormal grouping of large eddies in a submerged jet

(1)

ABNORMAL GROUPING Of LARGE EDDIES IN A SUBMERGED JET

SV S.K.A. NAIB, B.Sc. (Eng.) Ph. O., D.I.C., A.C.G.I. * Ji is weIl known that when a turbulent jet Dows

through an enveloping nuid, the surface of diseon- tinuity hreaks up into vortiees whieh are earried alternatively along the surface. The developmenl of these vortiees in the eonrse of nerw of a jet issuing l'rom a submerged sluiee-way and ditIusing into the water downstream has been previously deseribed [1, 2]. The vortiees grenv with distanee and eventually extend to the water surface through

• Principal Lecturer in Finie! l\lcchanics, \Vest Ham Col1cge of Teehnoiogy, London, E.1;'.

the entire height of the eirculation zone. The mixing produced by these vortices simultancously causes the jet to expand until its uppcr limit momcntarily l'caches the l'l'ce surface, whcre i t divides and a stagnation point develops.

In the course of experiments to sIudy the dif- fusion of the jet, it was observed Ihat a partieular kind of unsteadiness developed involving abnormal grouping of large eddies in the Derw, a phenomenon whieh was clearlv observed in semi-suhmerged patterns, as sho\vn" in Figure 1a. Here, two large eddies A and 13 are indicated near the middle of the

282

1/

'l'wo for1115 of no\\" ^dOW11- stream of a submergee! sluice gate. Dcpth upstream of gate

=

^7.5in. Downstrearn e!epth

=

^5.4^in. Exposllres:

0.2 sec.

Denx formes de l'écoulement li l'aval d'une vanne noyée.

Hall/ellr d'l'ail ci l'amont de la vanne

=

^7,5pOilces. Hau- tellr d'l'ml ci l'aval

=

^{5,4 pou-}

ces. Temps de pose: 0,2 s.

Article published by SHF and available at http://www.shf-lhb.org or http://dx.doi.org/10.1051/lhb/1967020

(2)

circulation, ·where normally there would he a continous retu:'n flow, Figure 1 b. The photographs were taken wIth a "Zeiss Ikon" camera \vith the 110w iIlun:inated hy a mercury-vapour discharge lamp flashmg at 100 times a second [il]. The traces were droplets of a new white emulsion discovered hy the author hy mixinga mixture of olive oil and nitrobenzine prepared to the same density of water with a suitahle percentage of willer. Sorne idea of the scale of the photographs may he o1Jtain- ed hy noticing the sluice opening \vhich is 2 in.

deep.

The situation exhibited in Figure 1 is very com- plex and wc ci:n only make the following guess

?bout the physlCal sequence of events lcading to It. Of the two eddies indicated in FiGure 1:::J ..(l-, the one nearer the gate appears to form first; il is pl'ohahly caused hy fluetuation either of the local turbulent stresses or of the pressure. \Vhatever the l'eason may he, once the eddy is initiated, momenta·

r~lya st.ream of fluid emerges l'rom the jet into the cIrculatIon zone and proceeds towards A. This stream ohstructs or retards the back flowing fluid and hence leads partIy to an increase in the volume of fluid in region 13, and parUy to the down-

\vard dellection and so ultimatelv' leads to the formation of the second eddv 13 on the c10wnstream sicle of A, thus creating two 'momentary circulation zones on top of the jet. The life either eddy is, however, short and is comparahle with the tÜ;le il took to clevelop, i:ncl hoth edclies eventually pass downstream and cbsappear, artel' which the normal form of flow with one circulation zone overlvin^rrthe jet, (Fig. 1b), prevails. ' tJ

The above change in form of flow was also studiecl hy photographing the motion of aluminium particles on the surface of "\vater flowin^crb throucrh a. b

sluice opening arranged in an Ahlhorn tank [4]

:vhere a sl:arp-eclgecl gate was arranged with one mch openll1g across two parallel walls placed

LA HOUILLE BLANCHE/N° 3-1967

2/^{Plan view}

of the Ahlhorn tanl, rue en plan de la cuve d'Ahlborn.

longituclinally in the tank in which "\vater llowed l'rom one end to another, Figure 2. The distance s between the two walls was varied l'rom 2 to 8 times the gate opening d. '1'0 avoid the formation of surface waves due to surface tension, the flo\\' was adjusted to he less than :3 inches pel' second at the gate. The surface of watel' was illuminated hy two 1 000 \V lamps, ~mdphotographs were taken hy a camera mounted vertically over the working portion of the tank. Figure 8, showing two of the resulting photographs for s

=

8d, clearl)' confirms the abnol'I11al grouping of large eddie~ and the formation of two momentarv circulation zones on top of the

.i

et. .

References

N·Ull (SILA.). _.. Flow Patterns in a Sudmerged Liquid .Jet ])jffusing Under Gl·avity. Na/ure, vol. 210 (l'Ilay 14, I%G), G!J4.

[2] N.-uB (S.K.A.) .. - Unstcadiness of the Cireulation Pattern in a Confined Liquid .Jet. Na/ure, vol. 212 (November 12, l!)(j(j), 7;)'1.

:Ji NAlli (S.I\.A.). --- Photographic Method for Measurillg Velouty Profiles in a Liquid ,Jet.. The Engineer, vol. 221 (.June 2-1, 1%G), !J(i1.

TI ·WALLIS (lU).). - A Photographie Study of Fluid Flow Betweell Banks of Tubes. l'roc. 1ns/. Mecll. E.,

\'01. 1-12 (1!j:J!)),

,ml.

3/

Two patterns of 110w shown bv the motion of aluminium n:lken in an AhlbOl'n tanlL 'Vidth of now

=

^{8 in.} ^Ex-

posures : 1 sec.

Dell:!' aspects de l'écoulement.

Visualisation ilu ?1IOIwemen/

Ù l'aide de flocons (ralumi- nium dans une Clwe d'Alll- barn. LarfJeur de l'écollle- menl

=

8 ]Jouces. Temps de pose: l s

283,

(3)

ÉDITIONS DU CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE

15[ quai Anatole-France[ PARIS-VW

C.C.P. PARIS 9061-11

PUBLICATIONS

Tél. : 705-93-39

284

DE

L'ANNÉE GÉOPHYSIQUE

Format in-4° coquille, broché

OCÉANOGRAPHIE

fascicule 1

par

GOUGENHEIM A., MARTIN J., LENOBLE J. et ROTSCHI H.

Introduction générale (1955-1959). Utilisation du courantomètre électrique à électrodes remorquées (1955-1956). Spectrophotomètre photoélectrique sous-marin (1957). Sur l'emploi du salinomètre australien du C.S.I.R.O. (1958), 78 pages.

PRIX: 9,50 F

Fascicule Il

par DU BUCQ Cdt

Hydrologie et dynamique des mers. Niveau moyen (Bibliographie), 56 pages.

PRIX: 6 F