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Circu lar BellmouPre ssu res - -Lef tPr es su re s --Right Conduit--Effect of Approach Floo r E l

EntrancesP r e s s u r e s - -CenterP r e s s u r e s - -CenterEntrancesHead Differential a

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DEPARTMENT OF THE INTERIORBUREAU O F RECLAMATIONOffice of Assistant Commissioner Rep or t No. Hyd-47'0

and Chief Engineer Compiled by: D,, ColgateDivision of Engineering La bo ra tor ies Checked by: W'. P. SimmonsHydraulics Branch Reviewed by: W. E. WagnerDenver, Colorado Submitted by: H. M. Mart inAugust 31, 1962Subject: Aerodynamic mode l stud ies of the outlet wo rks intake st ru ct ur efor Tw in Buttes Dam--San Angelo Project, Texas

PURPOSETe sts we re made to develop cavitation-free entran ces and to determineloss es fo r the three-b arrelled inlet structu re of the outlet works.

CONCLUSIONS1. The circular bellmouth entran ces initially proposed produced cavita-tion pre ssu re s on so me of the flow surfa ces when the .outlet gates (o r agate) we re fully opened and the r es er vo ir elevation was high (Fi gur es 4,6, 7 , arid 8) .2. The sev ere subatmospheric p res su res found in the circu lar bellmouthswer e relieved by disrup ting the flow with sp oil er s at the entrances (Fig-ure s 5 3 and 10). The spoil ers presented c onstruction problems that pre -cluded their use on this structisre.3. Subatmospheric pre ss ur es that occ urred on the crown of the ci rcul arentr anc es were relieve d by lowering the approach floor (Figure 9). How-ev er, the amount of relie f was inadequate, and lowe ring the approachfloor in the field was imprac tical.4. La rge r c i rc ular bellmouths with less rapid curvatures were pr e-cluded by the des irab ility of maintaining the conduits on 20-foot cen ters .5. Rectangular bellmouth entran ces with watertight stoplog slot cov ersproduced posit ive pr es su re s on all flow sur face s fo r any rese rvo ir eleva-tion and any combination of gate openings (Figures .ll, 12, 15, and 16).The recta ngula r entr anc es allow retaining the 20-foot spacing betweenconduits.6. If water is perm itted to en te r the conduit through the stoplog slot sduring outlet operations, cavitation pr es su re s will exis t on the tunnelroofs just downstream from the slot (Figu re 14). Us e of covers overthe top openings of the sl ot s will prevent t his difficult>*.

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second. 2 1 ~e le i i se s Feate r than 600 cubic fee t pe r second, and up7'tothe maxrmum of 35,700 cubic feet per second, a r e co ~ tr o l le d y thethree 1 2 - by 15-foot top s eal radia l regulating gates. The conduits ar eplaced a s close together a s possible for prac tical and economic reasons.At the h ighest d ischarges , f l o ~ ~elocities up to 63 feet pe r second willoccur in the circ ul ar conduits. Correspondingly high velocitie s willoccur within and ne ar the bellmouth inlet s tru ctu re wher e the flow exper i-ences appreciable turning and rapid accelerations. These factors tend toinduce subatmospheric pressures on the boundaries and to produce damag-ing cavitation. Also, unsymm etrical relea ses, which might at some timebe nec es sa ry due to a gate malfunction, could aggrava te the seve re pr es -su re problems to produce even mo re dangerous conditions. Model studiesof the inlet str uc tu re we re there fore made to determine the operating condi-tions and to develop the design mos t appropriate f or construction.

THE MODELIn studies where the syst em flows completely full, low velocity a i r maybe used a s a test fluid for hydraulic structures without introducing appre-ciable error. 31 Aerodynamic models a r e c haracter ized by simplicity ofconstruction,-ease of obtaining data , and economy. The laboratory studyon the Twin Buttes intake structure concerned a deeply submerged, com-pletely filled struc ture , and was made using ai r as the testing fluid.The model was constructed on a line ar scale of 1:23.25 (Fig ure 4). Itincluded an approach apron, the triple-bellmouth entranc e str uc tu re ,and three parallel circular conduits representing 70 feet of the proto-type tunnels. The conduits term inate d in a 200-cubic-foot plenumcha mber which was connected to the inlet of an air blower. Thus, airwas drawn through the bellmouth entran ces f rom the atmosphere. Aircould be drawn through one of the 8 -inch-diameter tunnels a t a velocityof 150 feet pe r second, o r through a ll thr ee tunnels a t 53 feet per second.Gates w ere provided a t the stoplog slot s of each conduit s o the individualtunnels could be shut off o r opened as desired.In the preliminary design, cir cul ar bellmouths wer e used (Figu res 4and 5). The bellmouth stlrfaces were formed of smoothly screed ed con-cre te, the approach apron was made of plywood, and the re ma inde r ofthe model was constructed of sheet metal. Pr es su re taps (piezom eters)were located in regions where adverse pr es sur es we re considered pos-sible. In part icul ar, thes e regions included su rfa ce s of the bellmouthslying on the horizontal and ve rti cal centerlines where flow contractionswould be most severe.m y d r a u l i c Model Studies of the 2 - by 2-Foot Twin Buttes RegulatingGate, " Labor ato ry Rep ort No. Hyd-476 by H. T. Falvey.3 /"Model Te st s Using Low Velocity Air" by J. W. Ball, Transactions,ASCE, Vol. 117, 1952, Paper No. 2517.

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Stoplog slot covers. In the firs t tes ts of the rectangular entranc es,cavitation pr es su re s we re found on the cro wns of the tunnels just down-stream from the stoplog slots (Figure 14). It was appa rent that theseadverse pressures were caused by flow ente rin g the conduit through theslots. When the tops of the slot s we re se ale d to prevent this flow, theadver se p ress ure s were e l- iminated (Figure 14). Watertight stoplogslot covers were used in a l l subsequent tes ts .

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- -----.I- - - - - -in their p rope r design. pr es su re measurements showed that for maximumflow through the outlets and maximum water su rfa ce elevation a pressuredifferential of 54. 2 feet of wate r acted acr os s the cove rs (F igure 17). This 'differential produces a downward hydraulic force of about 97,500 pounds oneach stoplog cover.Pressure conditions. The tes t showed that the pres sur es on the flow s u r -fac es would be positive fo r all combinations of outlet operation (F igu res 15and 16). The expected drop in pre ss ur e gradient was noted in the vicinityof the stop log sl ot s at the crown and at the sidewalls, but this drop wassm all and the pre ss ure s remained positive.Entrance head loss. The head lo ss from the re se rv oi r to the downstreamends of the transitio ns was determined for the rec tangula r entrances.When all thr ee outlets were operating the head lo ss in each inlet was0.1 16 (v2 2g), where V is the velocity in the 15-1 12 -foot ci rc ul ar conduit(F igu re 18). When only one outlet was operating , its head los s was0.118 (v2 2g).

SUMMARYStudies showed that cir cu lar bellmouth entr ances (Figure 4) would beunsatis factor y fo r Twin Buttes outlet works due to the existence of cavita-tion pressures on some of the flo:~~urfa ces during operation at maximumdischarge. La rge r ci rcular bellmouths with les s rapid curvatures w ereprecluded by the nece ssit y of keeping the thr ee tunnels clos e together.Rectangular bellmouth entran ces allowed the s am e tunnel spacings and,with water tight stoplog sl ot cov ers , produced positive, and hence sa ti s-factory, pressures throughout the system. The rectangular entrances ar erecommended for the outlet works intake struc ture.

\A .

6

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Report Hyd 470

A. Preliminary Design

B. Floor lowered.3 feet, and 2-foot-diameter cylindricalpiers and 4-foot-diameter semicylindrical brow andside piersTWIN BUTTES DAM

OUTLET WORKS NTAKE STRUCTURE'Circular Bellmouth Entrances1:23.25 Scale Model

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R I G H T C O N D U I T

u3@--7itII /R I G H T C O N D U I Ti.

40 304WIWa e e3 20InInWaP

10

7'

0

R O W GINVERT

R O W FLEFT S IDE

3 Outlets Open 0---6R~qhtCenter Ooen +-4Left - Rtght Open ~---------lRlght Open -

SA N ANGEL0 PROJECT. TEXASTWIN B U T T E S D D , ~- OUTLET WORKS INTAKE ,STRUCTURE ,

CIRCULAR BELLMOUTH 'ENTRANCESPRESSURES, RIGHT CONDUIT

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FRONT ELEVAT I ON _i_l

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Report Hyd 470

A. Recommended Design

B. General view of the modelTWIN BUTTES DAMOUTLET WORKS INTAKE STRUCTURE

Rectangular Entrances and Test Facilities- P:23.25 Scale Model

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S I N A N G E L0 P R O J EC T . T E X A ST W I N B U T T E S D A MO U T L E T WORKS I N T A K E STRUCTURE .RECTANGULAR BELLMOUTH ENTRANCES, RECOMMEN DED DESIGNPRESSURES, CENTER AND LEF T CONDUITS

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