Experimental Investigation of Artificial Aeration on a Smooth Spillway with a Crest Pier

17 Pages·2017·6.23 MB·English

water
Article
Experimental Investigation of Artificial Aeration on a
Smooth Spillway with a Crest Pier
JuanCésarLuna-Bahena,OscarPozos-Estrada*,VíctorManuelOrtiz-Martínezand
JesúsGracia-Sánchez
InstitutodeIngeniería,DepartmentofHydraulicEngineering,UniversidadNacionalAutónomadeMéxico,
Cd.Universitaria,C.P.04510Mexico,Mexico;[email protected](J.C.L.-B.);
[email protected](V.M.O.-M.);[email protected](J.G.-S.)
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:22August2018;Accepted:15September2018;Published:3October2018
Abstract: Crestpiersplacedonoverflowspillwaysinducestandingwavesatthedownstreamendof
themandthesupercriticalflowexpandsafterflowingpasttherearofthepier. Theexpandingflow
fromeachsideofapierwillintersectandformdisturbancesorshockwavesthattravellaterallyas
theymovedownstreamandeventuallyreachthechutesidewalls. Recently,investigationsregarding
crestpiersarerelatedwithartificialaerationonsteppedspillwaystoeliminatetheriskofcavitation
damage. However, there is a lack of studies on standing and shock waves in smooth spillways
concerning the air entrainment into the flow in presence of crest piers. This paper presents the
study of the combined effect on air entrainment of a crest pier and an aerator on the bottom of
a smooth spillway (configuration 1). For comparison, experimental tests were developed in the
spillwaywithoutpier,thatisinpresenceofaeratoronly(configuration2). Theconfiguration1results
showthattheairconcentrationdistributiononthespillwaybottomacrossthewidthandlengthof
thechuteincreasesincomparisonwithconfiguration2,reducingevenmoretheriskofcavitation
damageandenhancingthesafetyofthehydraulicstructure.
Keywords: dam spillway; spillway-gate pier; spillway aerator; air concentration; cavitation;
conductivityprobe
1. Introduction
Spillways are important hydraulic structures that provide adequate safety for dams,
regulatingoutletforfloodcontroloperationswhenreservoirlevelsareabovethespillwaycrest[1].
Nevertheless,whetherthedischargedwaterflowreacheshighvelocitiesassociatedwithlowpressures,
this may cause cavitation damage on the chute spillway bottom if adequate precautions are not
taken[2]. Further,ifthespillwayinvertanditssidewallsaresubjectedtocontinuousremovalofthe
surfacematerial,itcouldseriouslyendangertheintegrityofthehydraulicstructure[3].
Thecavitationdamagecanbemitigatedbyanumberofmethods,forinstance,increasingthe
cavitationindexbyremovingsurfaceirregularities[4]. Likewise,thedamagingeffectsofcavitation
erosionmaybediminishedbyincreasingthecavitationresistanceoftheboundarymaterials,suchas
fibrousconcrete,steellining,epoxyresins,coatingsandvariouscombinationsofthese. Theresults
varyinsuccessbutsteel-fiberreinforcedconcreteappearstobethebestoption[5]. However,inmost
casesitwouldbeuneconomicaltoreplacethematerialinthespillwayforcavitationresistantmaterials.
Forthisreason,otheralternativesshouldbeconsideredtoprotectthespillwaysurfacefromcavitation
damage. Averyeffectiveandeconomicmethodtoavoidcavitationdamageisdispersingaquantityof
airalongthespillwayinvertbyplacingaerationdevicestoprovideartificiallyairwithinhigh-velocity
flowsupstreamofcavitationriddenzones[6,7].
Water2018,10,1383;doi:10.3390/w10101383 www.mdpi.com/journal/water
Water2018,10,1383 2of17
The positive effects of air addition to diminish cavitation damage in hydraulic structures are
knownsincemorethansevendecadesago[8,9]andhavebeenappliedformorethan40years.However,
studiesfocusedontheamountofairneededforsuitablechuteprotectionarescarce. Forinstance,
Peterka[10]statedthatbyintroducing1%to2%ofairasalayerofairbubblesnexttothespillwayinvert
cavitation,damageisreduced,while5%to8%entrainedairdamageispracticallyeliminated.Likewise,
other researchers differ from the amount of air needed for cavitation protection. Rasmussen [11]
developedaseriesofexperimentsanddemonstratedthatnocavitationdamageoccurredbyaddinga
0.8%to1%ofairbyvolumedistributedassmallbubbles. SemenkovandLentiaev[12]investigated
thecavitationdamageexperiencedbyconcretesurfacesandfoundthatanairconcentrationofatleast
3%isneededtoavoiddamagingeffects. Pylaev[13]corroboratedthatanairconcentrationof3%is
requiredforavoidingcavitationerosion. RussellandSheehan[14]foundthata2.8%ofairissufficient
forcavitationprotectionofasurfacemadeoutofconcrete. Basedontheabove,itcanbestatedall
authorsagreethatasmallamountofairclosetothespillwayfloordiminishestheriskofcavitation
damageconsiderably.
Investigationsregardingtheaerationofflowsandtheuseofaeratorsonspillwayshavebeen
developed for many decades and there is much literature available in this field [15–23]. Further,
severalresearchershaveinvestigatedmodelandprototypeaeratorsoverthelastfourdecades[24–39].
Nowadays, aerators are extensively used in spillways all around the world to prevent cavitation
damage. Inthesameway,morerecently,between2015and2017,Calitz[40]andKoen[41]studied
the artificial aeration on stepped spillways by using various crest pier designs. They found that
thebullnosepierpracticallyeliminatedtheriskofcavitationdamage,allowingincreasingtheunit
discharge capacity. However, there is a lack of studies on smooth spillways concerning the air
entrainmentinpresenceofcrestpiers.
Itiswellknownthatinspillwayswithcrestpiersastandingwave,alsocalledroostertailoccurs
immediatelyatthedownstreamendofthepierand,moreover,thesupercriticalflowexpandsafter
flowingpasttherearofthecrestpier. Theexpandingflowfromeachsideofapierwillintersectand
formdisturbancesorshockwavesthattravellaterallyastheymovedownstreamandeventuallyreach
thechutesidewalls[42,43]. TheselatterphenomenaareshowninFigure1. Hydraulicandgeometric
characteristicsofstandingwavesgeneratedbypiershavebeeninvestigatedsincetheearlyeightiesof
lastcentury[42,44–51].
Nevertheless,therenoexiststudiesonstandingandshockwavesregardingonairentrainmentto
thebottomofsmoothspillwaystoeliminateorminimizetheriskofcavitationdamage. Thisprompts
theauthorstoinvestigatethecombinedeffectontheaerationprocessofbothacrestpierandanaerator
(configuration1)onthephysicalhydraulicmodeloftheHuitesdamspillwaytoanalyzethebehavior
ofbottomairconcentrationdistributionacrossthewidthandlengthofthechute. Forcomparison,
experimentaltestsweredevelopedinthephysicalmodelwithnopier,thatisinpresenceofaerator
only (configuration 2). From the results obtained with model configuration 1, it can be observed
thatthecrestpiergeneratedanincreaseinthebottomairconcentrationalongthechutebottomin
comparisontotheresultsachievedwithmodelconfiguration2.
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Water 2018, 10, x FOR PEER REVIEW 3 of 18

Figure 1. Standing and shock waves.
Figure1.Standingandshockwaves.
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ias cdeinvtirdaeldw beyd ga ec-esnhtarpale wpieedrg0e.-2s2hmapew pidieer, 02..242m ml ownigd,ew, 2it.4h mav loarniga,b wleihthe iag hvatrfiraobmle 0h.9eimghtto fr2o.5mm 0.a9n md
tmo a2d.5e mou atnodf mwaodoed o.uTtw oof w20o0ohdp. Tcwenot r2i0fu0 ghapl cpeunmtrpifsugwailt hpuamvparsi awbilteh sap veaerdiadbrliev septeoedre dgruilvaet etot hreegwualateter
tdhiesc whaartgere darisecuhsaerdget oarfee eudsethde tom foedeedl .thTeh empohdyesli. cTahlesp pilhlwysaicyaml sopdilellwisayil lmusotdraetle ids iilnluFsitgruarteed2 .in Figure
2. Onthephysicalmodel,theaeratorislocated4.7mdownstreamfromthecrestatthechangeof
slopeinthespillwayfrom39.16◦ to33.29◦. Thedeflectorheight(t)is1.5cm,thedeflectorangle(α)
is4◦ andtheoffsetheight(s)is9cm. Theaerationductshavebeensizedwithtwo11×10cm2 air
ductsinthespillwaysidewalls. Theheightofthesidewallswas1.2m,theaerationductsweremade
ofacrylicandthespillwaybedwasconcrete. TheaerationsystemisshowninFigure3.
Water 2018, 10, x FOR PEER REVIEW 4 of 18
Water2018,10,1383 4of17
Water 2018, 10, x FOR PEER REVIEW 4 of 18

Figure 2. Physical model of the Huites Dam spillway.
On the physical model, the aerator is located 4.7 m downstream from the crest at the change of
slope in the spillway from 39.16° to 33.29°. The deflector height (t) is 1.5 cm, the deflector angle () is
4° and the offset height (s) is 9 cm. The aeration ducts have been sized with two 11 × 10 cm2 air ducts

in the spillway sidewalls. The height of the sidewalls was 1.2 m, the aeration ducts were made of
Figure 2. Physical model of the Huites Dam spillway.
acrylic and the spillwayF bigedu rwea2s. Pcohnycsriceatel.m Tohde ealeorfattihoenH syusitteesmD ias mshsopwilnlw ina yF.igure 3.
On the physical model, the aerator is located 4.7 m downstream from the crest at the change of
slope in the spillway from 39.16° to 33.29°. The deflector height (t) is 1.5 cm, the deflector angle () is
4° and the offset height (s) is 9 cm. The aeration ducts have been sized with two 11 × 10 cm2 air ducts
in the spillway sidewalls. The height of the sidewalls was 1.2 m, the aeration ducts were made of
acrylic and the spillway bed was concrete. The aeration system is shown in Figure 3.

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several decades. Physical modelling is a mature, proven tool and its results can be used with high
severaldecades. Physicalmodellingisamature,proventoolanditsresultscanbeusedwithhigh
confidence. During the modeling of free-surface flows the Froude law is usually used, nevertheless,
confidence. Duringthemodelingoffree-surfaceflowstheFroudelawisusuallyused,nevertheless,
the modeling of high-speed air-water two-phase flows using only the ratio of inertial and
themodelingofhigFhi-gsupree e3d. Gaeiorm-wetartye orf ttwheo s-ppilhlwasaey flaeorwatisonu ssyinstgemo nwliytht mheodrealt dioimoefnisnioenrst.i alandgravitational

forces,aswellasthegeometricsimilaritybetweenmodelandprototypeisimpossible,sincetheair
transportPihnysmicoald seclaslei sspaiflflewcateyd mboydeslcs ahleaveef fbeecetns ubseecda utos esismuurlfaatcee atnedn siniovnesteixgpatree saseeradtewd iftlhowths efoWr eber
several decades. Physical modelling is a mature, proven tool and its results can be used with high
number(W)isoverestimated,whiletheinternalflowturbulencerepresentedbytheReynoldsnumber
confidence. During the modeling of free-surface flows the Froude law is usually used, nevertheless,
(R)isunderestimated[54–56]. Inthesameway,anadequatereproductionoftheairconcentrationsand
the modeling of high-speed air-water two-phase flows using only the ratio of inertial and
airtransportintwo-phasemodelflowsispossibleiflimitationsintermsofWandRarerespected[57].

PfisterandHager[58]concludedthattoachievereliableairconcentrationsforhigh-speedair-water
mixture flows using the Froude similitude, minimum values of R = 1 × 105 and W = 110 must be
Water 2018, 10, x FOR PEER REVIEW 5 of 18
gravitational forces, as well as the geometric similarity between model and prototype is impossible,
since the air transport in models is affected by scale effects because surface tension expressed with
the Weber number (W) is overestimated, while the internal flow turbulence represented by the
Reynolds number (R) is underestimated [54–56]. In the same way, an adequate reproduction of the
air concentrations and air transport in two-phase model flows is possible if limitations in terms of W
Water2018,10,1383 5of17
and R are respected [57]. Pfister and Hager [58] concluded that to achieve reliable air concentrations
for high-speed air-water mixture flows using the Froude similitude, minimum values of R = 1 × 105
respectedinthephysicalmodel. Therefore,itcanbestatedthatthemodelscale1:21usedduringthe
and W = 110 must be respected in the physical model. Therefore, it can be stated that the model scale
presentinvestigationisconsideredlargeenoughtoavoidscaleeffects,becauseitsatisfiestheminimum
1:21 used during the present investigation is considered large enough to avoid scale effects, because
ReynoldsandWebercriteria. WithinthesectionexperimentsaresummarizedthevaluesoftheFroude,
it satisfies the minimum Reynolds and Weber criteria. Within the section experiments are
ReynoldsandWebernumbers.
summarized the values of the Froude, Reynolds and Weber numbers.
2.2. Instrumentation
2.2. Instrumentation
During the investigation, measurements of air concentration have been performed by means
During the investigation, measurements of air concentration have been performed by means of
of a double-tip conductivity probe. Conductivity probes have been used since the 1960s for
a double-tip conductivity probe. Conductivity probes have been used since the 1960s for different
differentmeasurementapplicationssuchasthedetectionofairconcentration[19,59,60]andvelocity
measurement applications such as the detection of air concentration [19,59,60] and velocity
measurements in two-phase flows [61]. Moreover, the probes have been perfected and used in a
measurements in two-phase flows [61]. Moreover, the probes have been perfected and used in a
numberofinvestigations. Atpresent,itisfeasibletousethemtomeasure,aircontent,bubblevelocity,
number of investigations. At present, it is feasible to use them to measure, air content, bubble velocity,
distribution of the number of bubbles, bubble count rate, air/water chord size, particle size
distribution of the number of bubbles, bubble count rate, air/water chord size, particle size
distributions,interfacialvelocity,turbulentintensity,air–waterturbulenttime,specificinterfacearea,
distributions, interfacial velocity, turbulent intensity, air–water turbulent time, specific interface area,
distributionsofchordlength,concentrationoftheinterfacialareaandbubblesize[62–64]. Insome
distributions of chord length, concentration of the interfacial area and bubble size [62–64]. In some
cases,itisalsopossibletoestimatethebubblevelocityintwo[65]andthreedirections[66].
cases, it is also possible to estimate the bubble velocity in two [65] and three directions [66].
Thedouble-tipconductivityprobeusedduringtheinvestigationwasequippedwithtwoidentical
The double-tip conductivity probe used during the investigation was equipped with two
conductivity sensors or tips with an inner diameter of 0.8 mm and installed inside a cylindrical
identical conductivity sensors or tips with an inner diameter of 0.8 mm and installed inside a
stainless-steelbodythatfacilitatesmeasurementsasitprovidesrigiditytotheinstrument. Theprobe
cylindrical stainless-steel body that facilitates measurements as it provides rigidity to the instrument.
producestwovoltagesignals,oneforeachsensor. Thesignalswillindicatewhetherthetipspierce
The probe produces two voltage signals, one for each sensor. The signals will indicate whether the
airbubblesareatcontactwithwater,duetothedifferenceinelectricalconductivitybetweenwater
tips pierce air bubbles are at contact with water, due to the difference in electrical conductivity
(5mS/m)andair(almostzero). Theadvantageofusingthismeasuringdeviceliesintheeaseofrepair
between water (5 mS/m) and air (almost zero). The advantage of using this measuring device lies in
incaseoffailure,sincethepartsarereadilyavailableandtheelectroniccomponentcostandthecosts
the ease of repair in case of failure, since the parts are readily available and the electronic component
forthemanufacturearelow. Thisspeedsuprepairswhileatthesametimeprovidesreliabledatafor
cost and the costs for the manufacture are low. This speeds up repairs while at the same time provides
evaluatingaircontent. TheprobeisshowninFigure4.
reliable data for evaluating air content. The probe is shown in Figure 4.

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postprocessing. Thedataacquisitioncardusedallowsthecaptureofupto40,000samplespersecond,

thisfrequencyisdividedbythenumberofsensorsinstalled. Thereaderinterestedintheworking
principlesofconductivityprobesisreferredtoChansonandCarosi[63].
ThestaffoftheDepartmentofElectricalandElectronicsEngineeringoftheInstituteofEngineering
constructedtheconductivityprobeandconductedpreliminaryairconcentrationmeasurementsto
Water2018,10,1383 6of17
verify its accuracy. They mounted the probe inside a test rig with the needles parallel to the flow
directionpointingdownward. Themeasuringsectionwasaverticalcirculartubemadeofacrylicwith
aninnerdiameterof10cmandlengthof2m. Clearwaterwasusedasworkingfluidthatcirculates
upwards moved by a centrifugal pump, by means of a compressor air was injected to produce a
verticalconcurrentbubblyflow. Theprobewasdisplacedfromthewalltothecenterofthepipeatfive
axiallocationstoregisterairconcentrationatdifferentpoints. Itwasfoundthattheaccuracyofthe
probeindetectingairconcentrationswasabout±5%.
The flow depths were measured perpendicular to the spillway bottom, by using an acoustic
metallicsensor±0.1mmprecise,oncethepointofthesensorwasincontactwiththewatersurface,
theelectronicsoundsystememittedawhistleandthenthemeasurementwastaken.
The spillway discharge was measured in two ways: (1) Ultrasonic flowmeters PrimeFlo-T,
Model RXG 845 with an accuracy of ±0.5% were installed on the discharge piping of the pumps.
(2)Adigitalwaterlevelsensor(accuracy±0.05%,operatingrange: from1.0mto30.0m)waslocated
upstream of the spillway crest in the tank. The measured water levels together with the equation
proposedbytheUSBR[52]toobtainthewaterdischargeoveranogeecrestwereusedtocalculatethe
discharge. Thedifferencebetweenthetwomethodsusedtomeasurethedischargeflowwasaround
3%.
2.3. Experiments
Thephysicalmodelwasusedtoregisterbottomairconcentrationdatawithandwithoutcrest
pier. Themaininterestisintheairclosetothefloorofthespillwaychute,wherecavitationdamageis
aconcern. Inthechuteofthephysicalmodel,theairconcentrationmeasurementsweremadeateight
differentcross-sections(1to8),13cmupstreamoftheaeratorandatlocations15cm,45cm,80cm,
120cm,160cm,200cmand240cmdownstreamoftheaerator,respectively. Ateachcross-section,
the air content was registered at five equidistant points (A to E) located between each other at a
distanceof28.5cm. Theairconcentrationwastaken3mmabovethespillwaybottom. Themeasuring
sectionsareshowninFigure5.
Water 2018, 10, x FOR PEER REVIEW 7 of 18

FFiigguurree 55.. MMeeaassuurriinngg sseeccttiioonnss..
The tests were performed with flow rates of 500 L/s, 1000 L/s and 1500 L/s, corresponding to
1010 m3/s, 2020 m3/s and 3030 m3/s in the prototype. The sampling rate of the data acquisition was set
at 20 kHz per channel and the total sampling time was 40 s. Further, in order to corroborate that the
scale effects were not substantial in the physical model, the approach flow Froude number Fo =
vo/(gLref)0.5, the approach flow Reynolds number Ro = (voLref)/ν and the approach flow Weber number
Wo = vo/(σ/ρ Lref)0.5, with Lref = ho for each test were obtained, where Lref is the reference length, ho is
the approach flow depth, vo is the approach flow velocity, ν = 1.0 × 10−6 (m2/s) is the kinematic viscosity
of water, σ = 0.00739 (kg/m) is the surface tension of water and ρ = 101.97 (kg·s2 m4) is the density of
water. It is important to highlight that the approach flow depths were practically the same for each
water discharge for both model configurations. These flow depths were measured at a distance of 2.4
m from the vertical upstream face of the crest, that is at the tail part of the pier. The values of the
dimensionless numbers obtained for each test confirms that scale effects can be negligible in the
physical model (Table 1).
Table 1. Hydraulic conditions and dimensionless numbers.
Discharge (L/s) ho (m) vo (m/s) Fo Ro Wo
500 0.042 6.96 10.84 2.92 × 105 167.55
1000 0.086 6.80 7.41 5.85 × 105 234.25
1500 0.128 6.85 6.11 8.77 × 105 287.88
2.4. Experimental Observations
Figure 6 shows a series of photographs to illustrate the flow pattern for the two investigated
configurations. For the configuration 1, the standing wave or rooster tail develops directly
downstream of the pier and the shock waves travel laterally reaching the chute sidewalls only for the
water discharges of 1000 L/s and 1500 L/s. The angle between the shock waves has a slightly increase
as the flow rate increases. Likewise, it can be seen an enhancement of the length, width and height of
the standing wave with increasing discharge. In the case of the shock waves, they grow in width and
height and their length remains practically constant. Further, in Figure 6a–f, it can be observed the
Water2018,10,1383 7of17
Thetestswereperformedwithflowratesof500L/s,1000L/sand1500L/s,correspondingto
1010 m3/s, 2020 m3/s and 3030 m3/s in the prototype. The sampling rate of the data acquisition
wassetat20kHzperchannelandthetotalsamplingtimewas40s. Further,inordertocorroborate
thatthescaleeffectswerenotsubstantialinthephysicalmodel,theapproachflowFroudenumber
F =v /(gL )0.5, the approach flow Reynolds number R = (v L )/ν and the approach flow
o o ref o o ref
Weber number W =v /(σ/ρL )0.5, with L = h for each test were obtained, where L is the
o o ref ref o ref
reference length, h is the approach flow depth, v is the approach flow velocity, ν = 1.0 × 10−6
o o
(m2/s) is the kinematic viscosity of water, σ = 0.00739 (kg/m) is the surface tension of water and
ρ=101.97(kg·s2·m4)isthedensityofwater. Itisimportanttohighlightthattheapproachflowdepths
werepracticallythesameforeachwaterdischargeforbothmodelconfigurations. Theseflowdepths
weremeasuredatadistanceof2.4mfromtheverticalupstreamfaceofthecrest,thatisatthetailpart
ofthepier. Thevaluesofthedimensionlessnumbersobtainedforeachtestconfirmsthatscaleeffects
canbenegligibleinthephysicalmodel(Table1).
Table1.Hydraulicconditionsanddimensionlessnumbers.
Discharge(L/s) ho(m) vo(m/s) Fo Ro Wo
500 0.042 6.96 10.84 2.92×105 167.55
1000 0.086 6.80 7.41 5.85×105 234.25
1500 0.128 6.85 6.11 8.77×105 287.88
2.4. ExperimentalObservations
Figure 6 shows a series of photographs to illustrate the flow pattern for the two investigated
configurations.Fortheconfiguration1,thestandingwaveorroostertaildevelopsdirectlydownstream
of the pier and the shock waves travel laterally reaching the chute sidewalls only for the water
dischargesof1000L/sand1500L/s. Theanglebetweentheshockwaveshasaslightlyincreaseas
theflowrateincreases. Likewise,itcanbeseenanenhancementofthelength,widthandheightof
thestandingwavewithincreasingdischarge. Inthecaseoftheshockwaves,theygrowinwidthand
heightandtheirlengthremainspracticallyconstant. Further,inFigure6a–f,itcanbeobservedthe
airentrainedbythechuteaerator,aswellasthestandingwavesonthesidewallsduetotheupperjet
nappeimpingingonthelowpartoftheaerationducts.
Itwasnoticedforthetwoconfigurationsandallflowratestestedthataregionofclearwater
wasobservedwherethewaterentersthespillway. Forthetestswithnopierthefreewatersurface
issmoothandglassyupstreamoftheaeratorandimmediatelydownstreamoftheaeratorthewater
suddenlytakesonamilkyappearanceduetotheairentrainment. Likewise, theairisdistributed
uniformlyovertheentirespillwaywidthwithminimuminterferencetothewaterflowthroughthe
chute. Conversely,whenemployingthepier,itintroducesturbulenceattheoverflowcrest. Aturbulent
boundarydevelopsastheflowpassesoverthepier,Figure6a–cclearlyshowstheseparationpointand
theflowseparationthatforminasymmetricalfashion,aswellasthelongitudinalvorticesdownstream
ofthecrestpier.
Water 2018, 10, x FOR PEER REVIEW 8 of 18
Waairte ren20t1r8a,i1n0e,d13 b83y the chute aerator, as well as the standing waves on the sidewalls due to the upp8eofr 1j7et
nappe impinging on the low part of the aeration ducts.

FFiigguurree6 6.. FFllooww ppaatttteerrnnssf foorrb bootthhc coonnfifgiguurraatitoionnssw witihtht htheet hthrereeet etestseteddw waatetrerd disicshcharagregse:s(: a()a)5 05000L L//ssw wiitthh
ppieierr,,( b(b))1 1000000L L//ss wwiitthh ppiieerr,,( c(c))1 1550000L L//ss wwiitthh ppiieerr,, ((dd))5 50000L L//ss wwiitthhoouutt ppiieerr,, ((ee)) 11000000 LL//ss wwiitthhoouutt ppiieerr,,
((ff))1 1550000L L//ss wwiitthhoouutt ppiieerr..
It was noticed for the two configurations and all flow rates tested that a region of clear water
was observed where the water enters the spillway. For the tests with no pier the free water surface is
Water2018,10,1383 9of17
3. ResultsandDiscussion
Theresultsobtainedduringthepresentstudyarediscussedwithinthissectionwithregardtothe
statedobjectiveoftheinvestigation,whichwastoanalyzethecombinedeffectonairentrainmentofa
crestpierandanaeratoronthebottomofasmoothspillway.
Figure7showsthemeasuredbottomairconcentrationacrossthechutewidthatsections1to8for
thetwomodelconfigurationsandallthetesteddischarges. Theconfiguration1resultsshowthatthe
airWcoatnerc 2e0n18tr, 1a0t,i ox nFOiRs sPlEiEgRh RtlEyVhIEiWgh eratsection1. Theaverageairconcentrationforthewaterdis1c0h oaf 1r8g es
of500L/s,1000L/sand1500L/swas0.333%,0.315%and0.298%,whiletheaverageairconcentration
during all the experiments developed in model configuration 1, it was observed for the three water
in model configuration 2 was 0.283%, 0.267% and 0.283% for the same discharges. The growth in
discharges that the length, width and height of longitudinal vortices, standing and shock waves
concentrationisduetotheearlyonsetofairentrainmentintotheflowbythestandingwaveandthe
experienced small variations, resulting either in an increase or decrease in air entrainment. It might
longitudinalvorticesgeneratedbythecrestpier. Thisphenomenonhasalsobeenobservedforstepped
be another reason why the bottom air concentration was not symmetrically distributed across the
spillways[40,41].
chute width.

Figure 7. Bottom air concentration across the chute width at the eight sections, (a) 500 L/s with pier,
Figure7.Bottomairconcentrationacrossthechutewidthattheeightsections,(a)500L/swithpier,
(b) 1000 L/s with pier, (c) 1500 L/s with pier, (d) 500 L/s without pier, (e) 1000 L/s without pier, (f)
(b)1000L/swithpier, (c)1500L/swithpier, (d)500L/swithoutpier, (e)1000L/swithoutpier,
1500 L/s without pier.
(f)1500L/swithoutpier.
Figure 8 shows the development of the bottom air concentration of model configuration 1 for
Inthesameway,forallthetestsinbothmodelconfigurationsthehighbottomairconcentration
the test runs performed with the three flow rates, at the eight measuring sections considered
atsection2isduetothelargeamountofairenteringintotheflowthroughthelowerjetinthecavity
previously. The air concentration values at the spillway floor were obtained by averaging
zone. Themeasuredairconcentrationsrangingfrom49.42%to99.95%. Itisworthnotingthatonlypart
measurements registered at the bottom, at the chute width at the five equidistant points (A to E)
ofthisairisbeingentrainedintotheflow,becauseofthebottomrollersatthedownstreamendofthe
located between each other at a distance of 28.5 cm. Likewise, these values are compared with those
cavityzonetrapanimportantportionoftheair[67,68]. Further,fromallthegraphs,itcanbeseenthat
obtained in model configuration 2.
The graphs indicate that bottom air concentration varied in a continuous fashion along the
spillway length. Although the distribution of air concentration follows a trend similar, the data
obtained during the tests in model configuration 1 show a greater overall aeration in comparison
with the results achieved with model configuration 2, which is more evident at measuring sections
downstream of the aerator for all water discharges. Further, it can be seen that as the water discharge
and the flow depth increase a smaller percentage of air reached the spillway floor.
Water2018,10,1383 10of17
bottomairconcentrationdiminisheswithdistance. Amassiveairdetrainmentisobservedbetween
sections2and3. Notethatforthethreewaterdischargesandbothmodelconfigurationstheaverage
airconcentrationdecreasessignificantlyatsection3,rangingfrom3.10%to23.52%. Thesignificantair
concentrationdecreasecanbeassociatedwithjetnappeimpactonthespillwaybottom.
Inaddition,thedataregisteredatsections4to8showthatthelowbottomconcentrationsprevail
furtherdownstream. Likewise,forallthetestscarriedoutinmodelconfiguration1thebottomair
concentration is maintained almost constant at sections 4 to 8 with averaged values of about 1%.
Itmightbeexplainedbythefactthatbytheimplementationofthepieratthecrestofthespillwaythe
turbulenceintensitywasincreased,generatingintensecollisions,deformationsandthecollapseof
theairbubbles. Theresultisthedelayedintheriseofthebubblesthatremainmoretimeintheflow,
asdescribedpreviouslybyVolkart[69]. Conversely,inthecaseofmodelconfiguration2thebottom
airconcentrationdecreaseswiththedistance.
From the results, it can be seen that the implementation of the pier, increased the bottom air
concentrationduetothisstructureinducesartificialaerationintotheflowalongthespillway. Likewise,
itisimportanttohighlightthatconfiguration1resultsshowthattheairconcentrationdistribution
onthespillwaybottomacrossthewidthofthechuteisnon-uniform(Figure7a–c). Thepatternof
air concentration along the spillway was similar for the three tested discharges. On the contrary,
the results recorded during the configuration 2 tests show that the bottom air concentration is
distributeduniformlyovertheentirechutewidthwithminimuminterferencetothewaterflowacross
thechutecross-section(Figure7d–f),asdescribedpreviouslybydifferentresearchers[17,18,29,70].
It is believed that the non-uniformity pattern of the bottom air concentration downstream of
thepierandtheaeratoriscreatedbytheturbulenteffectswithintheflowduetothepresenceofthe
longitudinalvortices,standingandshockwavesthatentrainairintotheflowatthesurfacealongits
path. FromtheresultsinFigure7a–c,itcanbeobservedthatthecenterofthespillwaybottomwasnot
aeratedtothesamepercentageaswerethespillwaybottomsides,whichwasexpectedsincetheshock
wavestravellaterallyastheymovedownstream. Likewise,itisimportanttohighlightthatduringall
theexperimentsdevelopedinmodelconfiguration1,itwasobservedforthethreewaterdischarges
thatthelength, widthandheightoflongitudinalvortices, standingandshockwavesexperienced
smallvariations,resultingeitherinanincreaseordecreaseinairentrainment. Itmightbeanother
reasonwhythebottomairconcentrationwasnotsymmetricallydistributedacrossthechutewidth.
Figure8showsthedevelopmentofthebottomairconcentrationofmodelconfiguration1forthe
testrunsperformedwiththethreeflowrates,attheeightmeasuringsectionsconsideredpreviously.
Theairconcentrationvaluesatthespillwayfloorwereobtainedbyaveragingmeasurementsregistered
atthebottom,atthechutewidthatthefiveequidistantpoints(AtoE)locatedbetweeneachotherata
distanceof28.5cm. Likewise,thesevaluesarecomparedwiththoseobtainedinmodelconfiguration2.
The graphs indicate that bottom air concentration varied in a continuous fashion along the
spillway length. Although the distribution of air concentration follows a trend similar, the data
obtained during the tests in model configuration 1 show a greater overall aeration in comparison
withtheresultsachievedwithmodelconfiguration2,whichismoreevidentatmeasuringsections
downstreamoftheaeratorforallwaterdischarges. Further,itcanbeseenthatasthewaterdischarge
andtheflowdepthincreaseasmallerpercentageofairreachedthespillwayfloor.
Figures9–11showthebottomairconcentrationpresentedascontourplots,thetransitionsbetween
thecolorscanbeinterpretedasequalairconcentrationlines. Thecolorscalefortheairconcentration
contourplotsareindicatedbythecolorlegendwiththeboundarybetweenwhite(100%ofair)and
black(0.1%ofair). Thecontourplotswereobtainedbyinterpolatinglinearlytheairconcentration
valuesregistered3mmabovethespillwayflooracrossthewidthofthesevenmeasuringsections
downstreamoftheaerator.