Experimental Investigation of Ba e E ect on the Flow in a...

Transaction B: Mechanical EngineeringVol. 17, No. 4, pp. 241{252c Sharif University of Technology, August 2010

Experimental Investigation of Ba�eE�ect on the Flow in a Rectangular

Primary Sedimentation Tank

H. Jamshidnia1 and B. Firoozabadi1;�

Abstract. In primary sedimentation tanks, short-circuiting enlargement of dead zones and high ow mixing problems are caused by circulation regions (dead zones), which can reduce the optimalsedimentation of particles. For proper design of such tanks, the formation of recirculation zones should beavoided. The provision of a ba�e as a geometrical modi�cation of a tank may in uence the ow �eld forbetter sedimentation. Thus, in this study, velocity measurements were performed by a three-dimensionalAcoustic Doppler Velocimeter (ADV) to investigate ba�e e�ects on the velocity distribution in a primaryrectangular sedimentation tank, quantitatively. E�ects of ba�e positioning were also determined. Mean ow analysis shows how a ba�e can alter the hydrodynamics of the ow �eld. It was quantitatively foundthat the intermediate ba�e not only in uences the ow �eld in its downstream, but also a�ects the owpattern in its upstream. It was found that the ba�e setting and its position relative to the inlet andoutlet in uences the ow �eld and the development of ow. Ba�ed ow may provide better conditions forsedimentation by in uencing velocity pro�les. However, further detailed experimental study is necessaryto fully capture the ba�e e�ect and obtain further insight into the complex ow �eld in a sedimentationtank.

Keywords: ADV; Intermediate ba�e; Sedimentation tank; Primary settling tank; Reverse ow region.

INTRODUCTION

Sedimentation by gravity is a usual and importantprocess in settling tanks to remove inorganic settleablesolids from water and waste water in re�nery plants [1].

Numerous studies show that in order to removesuspended solids with minimum cost, they should beremoved as quickly and e�ciently as possible from thewater. In fact, if the removed solid concentrationsfrom settling tanks are increased in order to increasetreatment e�ciency, the size of water treatment facil-ities, which are located downstream of the clari�ers,can be reduced [2]. According to the investigations ofCamp [3] and Swamee and Tyagi [1], the investmentcosts of settling facilities contribute to a large portion(typically one-fourth to one-third) of the total cost

1. Center of Excellence in Energy Conversion, School ofMechanical Engineering, Sharif University of Technology,Tehran, P.O. Box 11155-9567, Iran.

*. Corresponding author. E-mail: �[email protected]

Received 14 February 2009; received in revised form 12 February2010; accepted 18 May 2010

of treatment plant construction. For that reason,signi�cant savings in both capital and operational costsat various stages of treatment can be expected byincreasing solid removal e�ciency [2]. As a result,increasing removal e�ciency is important.

Many factors can in uence removal e�ciency,including tank hydraulics, which are of great signi�-cance [4]. It is noticeable that the ability of a sedimen-tation tank to remove suspended solids depends on its ow �eld. Therefore, investigating the structure of the ow �eld is of great importance [5].

Settling tanks are divided into two main cate-gories: primary and secondary (�nal) sedimentationtanks. It is worth making a distinction betweenthese types of tank. Primary sedimentation tanks aredesigned to reduce the particulate ow velocity andprovide for the settling of organic solids [6]. Thesludge in these tanks is not activated [6], hence particleconcentration is low and there is not a large di�erencebetween particle sizes [7,8]. As a result, the ow isnot much in uenced by concentration distribution [8].But in a �nal sedimentation tank, the particle con-

242 H. Jamshidnia and B. Firoozabadi

centration in settled sludge is relatively high resultingin signi�cant density e�ects [7,8]. Also, a wide rangeof particles with various sizes can be found [8]. In aprototype test, Anderson compared the ow �elds inprimary and �nal settling tanks of similar geometriesand hydraulic loadings. In the primary clari�ers, ow was observed to be along the surface from inletto outlet [9]. On the contrary, sludge concentrationin the secondary-clari�er inlet resulted in a densitycurrent along the bottom, causing reverse ow at thesurface [7]. Furthermore, in a secondary clari�er,particles occulate, which produces larger particles,causing an increase in particle settling velocity [10].

Attempts have been made to improve ow con-ditions using two approaches: manipulating the inletstructure, and installation of solid or perforated wallsor ba�es in sedimentation tanks [7]. The constructionof ba�ed channels in water treatment plants with nomechanical parts is considered to be more reliable,requires less maintenance and hence reduces cost [11].

Various studies have been done on the e�ects ofba�es on the ow and hydraulics of settling tanks andthe major �ndings of these studies are presented here.Lyn and Rodi [12] conducted turbulence measurementsby Laser Doppler Anemometry (LDA) in a rectangularlaboratory settling tank and studied the ow �eldsfrom a plane jet impinging on two types of de ector.Experimental studies of Bretscher et al. [13] on velocityand concentration �elds in a rectangular settling tankshowed the e�ectiveness of a central barrier wall.Krebs [14] investigated the e�ect of the inlet andintermediate ba�es on the ow �eld in �nal clari�ers.His research was based on experiments, numericalmodeling and analytical relations. Other experimen-tal investigations performed by Taebi-Harandy andSchroeder [15] on primary sedimentation tanks showedthat the placement of an intermediate ba�e, installedclose to the middle of the tank and extended fromthe bed upward to one-third depth, had no signi�cante�ect on e�ciency. They believed that the discrepancybetween the results of their study and other worksis likely due to the di�erence in ow patterns. Thismeans that if the dominant current is a surface current,a ba�e extending from the top upward may improvethe velocity �eld and, accordingly, the solid removale�ciency [15]. Ahmed et al. [16] studied the e�ects ofthe position and height of the ba�e in a secondarysedimentation tank by placing the ba�e at threedi�erent positions and various heights, qualitatively.The best result was for the case in which an inletba�e of the height of 67% of the total depth wasplaced in the �rst 5% of the channel [16]. In primarysedimentation tanks, short-circuiting, enlargement ofdead zones and high ow mixing problems are causedby circulation regions known as dead zones, which inturn can reduce the optimal sedimentation of particles.

Thus, for proper design of such tanks great care shouldbe taken to avoid the formation of circulation zones.Tamayol et al. [14] simulated the e�ect of a simpleba�e at various positions. Their results showed thatwhen the ba�e is located in the recirculation region,the dead zone (recirculation regions) would be reducedin size. Moreover, if the ba�e is located in an improperposition or if it has an improper height, the particle re-moval performance in the primary sedimentation tankwould be decreased drastically [17,18]. From a reviewof previous research investigations, it is clear thatthere is no universal agreement between researchersover the existence and placing of the intermediateba�e. The lack of detailed measurements of ow �eldcharacteristics in sedimentation tanks was revealedby Lyn and Rodi [12]. Therefore, there is a needto perform an experimental study to determine theproper position and height of an intermediate ba�ein a settling tank.

Considering the complex features of a ow �eldand the lack of detailed experimental data, this studyis undertaken to obtain a better understanding of the ow �eld and to evaluate the in uence of intermediateba�e on the velocity distribution in a rectangularprimary sedimentation tank in a quantitative manner.It was realized that by using an Acoustic DopplerVelocimeter (ADV) the velocity �eld could be obtainednon-intrusively and in a quantitative manner.

EXPERIMENTATION

Experimental Apparatus

An open channel loop at the Fluid Mechanics Lab-oratory of the Mechanical Engineering Departmentof Sharif University of Technology (Iran) that wasoriginally designed to demonstrate the hydraulic char-acteristics and performance of an open rectangular sed-imentation channel, was utilized for the experiments. Aschematic sketch of the experimental set up is shownin Figure 1. As shown, in ow water was taken fromthe laboratory main supply to the cylindrical supplytank and, then, was pumped by the recirculation pumpto a constant head tank. The role of the constanthead tank is to keep the ow rate unchanged duringexperiments. Water moves from this tank to the mainchannel while passing through a ow meter. A multi-layer damping screen was installed at the inlet todistribute the in ow as uniformly as possible, to achieveuniform ow distribution along the inlet width andavoid entrance disturbances.

Experiments were conducted in a rectangularopen channel 8 m � 0.2 m � 0.4 m in length, widthand height (x, y and z with x = 0 at the upstream end,y = 0 at the center of the channel and z = 0 at the bed),respectively, with a smooth bottom. A rectangular

Ba�ed Flow in a Rectangular Primary Sedimentation Tank 243

Figure 1. Schematic sketch of the experimental setup.

bottom feed slot with a height of h0 = 0:01 m extendingthroughout the full width of the channel provided theinlet gate. The depth of water was controlled by asharp-edged weir of a height of 32 cm located at thedownstream end of the channel. A typical photographof the installed ba�e is shown in Figure 2.

Velocity Measurement Using Acoustic DopplerVelocimeter (ADV)

Flow velocity components in a 3D direction at eachspatial point of the channel were measured by athree-dimensional Nortek Acoustic Doppler Velocime-ter (ADV). It is noted that all the measurements havebeen performed on the vertical central plane and, as a

Figure 2. A photograph of installed ba�e.

result, the 2D ow can reasonably be assumed on thisplane.

The ADV operates based on the principles of aDoppler shift of a sound wave re ected from movingparticles suspended in the water [4] and is considered aproper choice for velocity measurements [19]. The ADVemits a burst of sound energy of a known frequencythat is re ected back to the ADV probe by movingparticles within the uid stream. Consequently, a shiftin frequency of the backscattered sound energy, knownas a Doppler shift, occurs. Flow speed is proportionalto the magnitude of this shift. The probe of theADV consists of a central transmitter surrounded bythree equally spaced receivers [20]. ADV is capable ofmeasuring velocity accurately using a sampling volumeas small as 0.254 cm3 [20]. The sampling volume islocated at the intersection of the transmit and receivesbeams from either 50 or 100 mm away from the probe(in this study 50 mm away from the sensor) to provideundisturbed measurements. As a result, the ADV iscapable of measuring the velocity components reliablyin a non-invasive way [20,21]. A schematic of ADVtransducers and its operation method is illustrated inFigure 3.

The ADV is more advantageous compared withother forms of velocity measurement such as propeller,electromagnetic or hot-�lm sensors [20]. Furthermore,the ADV method is highly accurate and is not sensitiveto water quality, which allows for a wide range ofapplications [21].

In our experiments, data were collected for about20 to 30 s at each point at the maximum availablesampling rate of 25 Hz. The 3-D velocity range is2.5 m/s, and the velocity output has no zero-o�set [21].


Figure 3. Schematic of ADV transducers and itsoperational method.

Since downward-looking ADV probes were used, ve-locity measurements were performed from the top ofthe channel and continued by dipping the probe to thechannel bottom. About 14 positions were measured toget the representative velocity pro�le. These series ofexperiments were carried out to obtain instantaneousvelocities and time-averaged velocity pro�les.

Figure 4 shows the channel and the test sections.Two ADV probes were mounted separately on a carrierwith 1.1 m distance. Thus, measurements at eachpair of sections ((x = 1:5 m, 2.6 m), (x = 3:5 m,4.6 m) and (x = 5:5 m, 6.6 m)) were performedsimultaneously. The probes were easily transferablein vertical and horizontal directions. The ADV signalconditioning module was connected to a personal PCduring measurements. Real-time observation of thevelocity time series was made possible (in tabular andgraphical forms) by data acquisition software suppliedby the manufacturer.

It should be noted that due to a low concentrationof scattered particles, ADV measurements are noisywhen using clean tap water and measuring is morechallenging. In order to overcome this problem, seedingparticles provided by the Nortek Company were addedas tracers to the tap water. The density of these

Figure 4. Schematic of open channel and measurementsections.

particles was almost the same as water density andtheir average diameter was about 10 �m. For uniformdistribution, seeding particles were mixed properlywith an adequate amount of water and then wereinjected in the inlet region upstream of the dampingscreen (see Figure 1).

Experimental Methods

Experiments were performed to investigate the e�ect ofa ba�e on the ow structure at di�erent ow rates in arectangular sedimentation open channel by measuring ow velocities. The experiments were conducted intwo phases. In the �rst phase of the experiments, thevelocity of the pure water ow was measured at the owrate of Q = 42 l/min. This ow rate was the maximumestablished ow rate that could be achieved and, thus,was termed as full ow rate. In the second phase, thesame procedure has been used for half of the full owrate (e.g. Q = 21 l/min). This lower ow rate wasselected to obtain a better understanding of the e�ectof ow rate and inlet Froude number.

In each phase of the experiments, two intermedi-ate ba�e con�gurations (one or none) were used. Theheight of the ba�es was �xed to almost one fourthof the total water depth, hb = 8 cm. To achieve abetter investigation of the in uence of the intermediateba�e on the hydraulics of the ow, its location, denotedby xb which is a structural factor, was changed twice.They were placed individually at xb = 1.2 m and xb =4 m from the inlet and are indicated in Figure 4 by b1and b2, respectively. They are termed as inlet regionba�e (b1) and intermediate ba�e (b2) respectively. Itshould be noted that the placements were not donesimultaneously.

The channel details, ba�es and measurementsections have been schematically shown in Figure 4.

Table 1 summarizes ow conditions in the currentexperiments. The value of important non-dimensionalquantities, such as Reynolds (Re) and Froude (Fr)numbers, at the inlet and in the channel has been illus-trated for two ow rates, respectively. The Reynoldsand Froude numbers are de�ned by the followingrelationships:

Re = UchLch=�; (1)

Fr = Uch=(gLch)0:5; (2)

in which Lch and Uch are characteristic length andvelocity, respectively. At the inlet, Re and Fr numbersare de�ned in terms of inlet height (hO) and inlet bulkvelocity. � is the kinematic viscosity of the water atT = 20�C. The Reynolds number inside the channelis calculated based on the average bulk velocity insidethe channel and the hydraulic diameter of the channel.


Table 1. Experimental conditions.

Q (l/min) Re(In the Channel)

Re(Inlet)

Fr( In the Channel)

Fr(Inlet)

Ba�e Position (m)from Inlet

42 3168.2 3486 0.0056 1.117 No ba�e

42 3168.2 3486 0.0056 1.117 1.2

42 3168.2 3486 0.0056 1.117 4

21 1584.1 1743 0.0028 0.558 No ba�e

21 1584.1 1743 0.0028 0.558 1.2

21 1584.1 1743 0.0028 0.558 4

Although the inlet Froude numbers are greater thanone (supercritical ow) and less than one (subcritical)for cases of full and half ow rate, respectively, theyare very small inside the channel and, thus, the e�ectof free surface can be neglected. Despite this smallnumber, the similitude of Froude was used, similarto Lyn and Rodi [12]. In the absence of sedimenttransport or density di�erences, the Froude and Hazennumbers have no signi�cant e�ect.

The inlet Re number indicates that the inlet ow is turbulent. Similar to the work of Lyn andRodi [12] the traditional hydraulic modeling criteriahas been borrowed from pipe studies and Re > 2000was considered the criterion for turbulent ow. Thus,the values of Re number in the channel indicate that theoperating ow remains turbulent, regardless of ba�eposition or value of Fr number.

RESULTS AND DISCUSSION

A typical time series of the streamwise velocity compo-nent, u(t), measured by ADV at one point during theexperiments has been shown in Figure 5.

It is noted that the ADV data acquisition programmakes it possible to monitor the correlation values andSignal to Noise Ratio (SNR) for each ADV receiversto observe and make sure of the quality of data duringdata acquisition.

One of the disadvantages associated with ADVsis the appearance of spikes caused by the aliasing of

Figure 5. A typical one-point time series of streamwisevelocity measurement.

the Doppler signal in the ADV time series [19], thusto obtain reliable results, the data should be checkedbefore any analysis and interpretation. It is noticeablethat these spikes can be reduced or eliminated byadjusting probe operational parameters but cannot beavoided totally [22]. Considering this constraint, analgorithm should be used to detect and eliminate thesespikes.

All the spikes were removed by freeware WinADVcomputer program written by Tony L. Wahl for analy-sis and post processing of ADV datasets [23].

It should be noted that for calculating the time-averaged velocities at each point, �rst the time seriesof the velocity at that point has been �ltered. Filteringcan be done by removing low correlation or low SNRvalues. In general, if the instantaneous velocity valuesare not removed due to poor correlation (less than 0.7),a bias can be introduced into both velocity values andturbulent statistics. In this study, a high percentageof the measured data had a high correlation and theremoval was achieved with a high degree of reliability.

In all velocity distribution graphs in this paper,the horizontal axis represents the ratio of time-averagedstreamwise velocity (U) to average bulk velocity (UAve)in the channel, and the vertical axis is the ratio ofheight at each measurement point (h) to the total ow depth (H = 34 cm). It is noticeable that dataacquisition near the free surface was not possible,because if the downward-looking probe of ADV wereout of water, the change of sound velocity in the airand water would lead to poor quality data. Thus,the measurements could be performed only up toh=H = 0.8.

Furthermore, to obtain reliable results and to un-derstand the accuracy of the results, each experimentwas repeated several times under identical conditions,and the range of time-averaged velocity data at eachpoint was studied. Typical results of the arithmeticaverage and the range of variation of time-averagedvelocity values between their maximum and minimumis shown at each spatial point in Figure 6. This rangeof velocity variation shows the scattered velocity ateach point. The amount of scatter near the bottom


Figure 6. Range of velocity variation with respect toheight.

is observed to be wider compared to the other points.This can be described as a result of the probableinterference of the sampling volume with respect to thechannel bed. In general, the scatter was higher near thechannel bed and near the free surface.

Additionally, to verify the accuracy of the results,the arithmetic average of time-averaged streamwisevelocities were compared with the arithmetic averageof their maximum and minimum values at each point.A typical result is shown in Figure 7. As shown, theresults of both types of averaging are very close to eachother and proved to be reliable.

The time-averaged streamwise velocity pro�les at6 di�erent sections from the inlet and in the absence ofa ba�e are illustrated in Figures 8 and 9 for cases offull and half ow rate, respectively.

Comparison of velocity pro�les along the channelin the absence of a ba�e (Figures 8 and 9) showsthat the velocity is negative at the bottom part of the

Figure 7. Comparison of arithmetic average and averageof max and min velocity.

�rst section (immediately after the inlet, x = 1.5 m).Negative velocity values at the inlet region and near thebed indicate the existence of a vortex due to the shearlayer between the inlet jet ow and the uid inside thechannel, which is produced as the result of a noticeablevelocity gradient.

The di�erence in the vortex intensities and, con-sequently, vortex sizes at x = 1.5 m for di�erent owrates can be seen from Figures 8a and 9a. In the caseof half ow rates, the inlet jet velocity is half thatof the full ow rate and, accordingly, the intensity ofthe corresponding vortex due to the velocity gradientbetween the uid layers is much smaller compared tothat in a full ow rate. For the case of half ow rate,because the vortex size and its intensity are smaller, thevelocity pro�les achieve their developed shapes sooner,from x = 2.6 m (which may seem to be better for thepurpose of sedimentation), whereas in the case of full ow rate this fact is observed from x = 4.6 m.

At a distance from the inlet, ow tends to bemore developed. The �rst reason (which can be seen inFigures 8 and 9) is that although part of the velocitypro�les at x = 1.5 m have negative values, the othervelocity pro�les in the �ve other measuring sectionshave positive values over the entire ow depth and,therefore, ow travels toward the outlet. Secondly,at a distance from the inlet, velocity pro�les tend tobecome closer to their expected developed shape; thismeans that they tend to �t themselves to the boundaryconditions of the channel, such as the e�ect of the bed'swall and free surface.

The results of time-averaged streamwise velocitypro�les for channels with an inlet region ba�e at xb= 1.2 m (under full and half ow rates) are shownin Figures 10 and 11, respectively. As shown, partof the velocity pro�le at x = 1.5 m has negativevalues because this location (x = 3:5; hb = 1:5 m)corresponds to the immediate downstream of the ba�e.Speci�cally, the negative values in the case of full ow rate indicate a reverse ow region downstreamof the ba�e. This phenomena is similar to the ow downstream of a backward-facing step, whichincludes reverse ow region and reattachment point. Inaddition, the magnitude of velocities at the upper partof this section increased due to a reduction in crosssectional area and an increase in the kinetic energyof the uid. After x = 1.5 m, similar to the case ofno ba�e, no negative velocity can be observed andvelocity pro�les tend to become more uniform at thedownstream of the ba�e. This uniformity may bemore favorable towards having a tank with better owconditions for sedimentation.

The velocity pro�les for an intermediate ba�eat xb2 = 4 m under full and half ow rates areshown in Figures 12 and 13, respectively. Again, afterthe inlet (x= 1.5 m), the velocity pro�le is partially


Figure 8. Time-averaged streamwise velocity pro�les with no ba�e (Q = 42 l/min).

Figure 9. Time-averaged streamwise velocity pro�les with no ba�e (Q = 21 l/min).

negative for a full ow rate. The negative velocity atthis section indicates the existence of a vortex nearthe bottom part of the channel due to the inlet jet ow resulting from a velocity gradient. This �ndingis similar to testing results with no ba�e. In both�gures, because of the presence of the intermediate

ba�e, a considerable increase in the velocity andkinetic energy is observed. Furthermore, downstreamof the ba�e, the velocity pro�les tend to be muchmore uniform than the previous ones, since this sectionis located far downstream of the ba�e (almost at adistance of 10hb2 from the ba�e). With increasing


Figure 10. Time-averaged streamwise velocity pro�les with ba�e at xb1 = 1.2 m (Q = 42 l/min).

Figure 11. Time-averaged streamwise velocity pro�les with ba�e at xb1 = 1.2 m (Q = 21 l/min).

distance from the ba�e, water ows slower. As aresult of the suction of the outlet weir, the shape ofvelocity pro�les at the last two sections will not be souniform.

It is noted that velocity pro�les after x = 2.6 mtend to be more uniform for half ow rate, whereas the

uniformity in velocity pro�les for full ow rate occursafter x = 3.5 m. In general, results show that velocitypro�les tend to develop faster in cases of half ow raterather than those of full ow rate.

Figure 14 shows velocity pro�les at x = 1.5 mfor cases of full ow rate. The bottom part of all


Figure 12. Time-averaged streamwise velocity pro�les with ba�e at xb2 = 4 m (Q = 42 l/min).

Figure 13. Time-averaged streamwise velocity pro�les with ba�e at xb2 = 4 m (Q = 21 l/min).

these pro�les shows negative velocities. As discussedbefore, for the case of no ba�e and intermediate ba�e,the negative values indicate the existence of a vortexrecirculation region. But negative velocities for thecase of an inlet region ba�e (xb = 1.2 m), are dueto the existence of the ba�e and hence the reverse

ow at its downstream has bigger size and intensity(Figure 15). It prevents the velocity pro�les fromachieving their fully developed shapes very quickly.As a result, the inlet region ba�e seems to have aconsiderable in uence on the ow �eld at x = 1.5 m,which is located at 3:5hb downstream of the ba�e.


Figure 14. Time-averaged velocity pro�les at x = 1.5 m(Q = 42 l/min).

Figure 15. Time-averaged streamwise velocity pro�les atx = 2.6 m (Q = 42 l/min).

In Figure 15, the velocity pro�le, in the presenceof an intermediate ba�e, has overtaken the velocitypro�le of a case of no-ba�e in the bottom part of thechannel. This increase in velocity can be attributed toa fast uprising ow at the intermediate ba�e. Thus,an intermediate ba�e not only in uences ow �eld inits downstream but also a�ects the ow pattern in itsupstream.

Therefore, in general, it is concluded quantita-tively that ba�e position and operating ow ratein uence the ow �eld in various ways and attentionshould be paid to their proper design criteria.

In Figure 16, just behind the intermediate ba�e,the velocity pro�le achieves its maximum in the upperpart of the channel. In this section, velocity pro�lesthat are related to the full ow rate, in cases of noba�e and inlet region ba�e, achieve their maximumin the bottom part of the channel. Also, an obviousreduction of velocity gradient near the channel bed isobserved behind intermediate ba�e. Consequently, inthe presence of this ba�e, a decrease in bed frictionoccurs, which might be favorable (at this section) forsedimentation behind the ba�e.

As shown in Figure 17, in the last section, thepattern of ow is almost the same in all cases. Thissection is located near the outlet weir and due to the



suction of ow toward the outlet, velocity pro�les havea similar pattern and the ow seems to be under thein uence of the outlet.

Thus, ow pattern can be in uenced by ba�esetting and its position relative to inlet and outlet.Consequently, care should be taken in their propersettings.

SUMMARY AND CONCLUSIONS

The conclusions drawn from this experimental studyare summarized below:

1. ADV has been successfully applied to measure the ow velocity in a rectangular primary sedimen-tation open channel. The results were used toassess the e�ect of a geometrical modi�cation of thechannel on the ow in the presence or absence of anintermediate ba�e at di�erent streamwise sections.Two di�erent ow conditions and ba�e positionswere also studied.

2. At the inlet region and in the absence of a ba�eor in the presence of an intermediate ba�e, thenegative mean streamwise velocities near the chan-nel bed indicate the existence of a vortex that isof a bigger size and intensity at higher Reynolds


numbers. But immediately downstream of the socalled inlet region ba�e (xb = 1.2 m), the existenceof a reverse ow region is observed with di�erentsize and intensity, depending on the ow rate. Itseems to in uence the development of the ow andconsequently, the ow structure in the channel.

3. On the whole, experimental results show that ba�ecan alter the hydrodynamics of a ow �eld indi�erent ways. Analysis of the mean ow showshow a ow structure changes from upstream todownstream of the ba�e at various positions. Im-mediately downstream of the ba�e, a reverse owregion was found based on the negative values ofmean streamwise velocity pro�les. It seems thatthis recirculation zone is of importance when theReynolds number is high enough. It was alsofound quantitatively that the ba�e can changethe distribution of velocity at its downstream byincreasing ow velocity and kinetic energy. Ba�ealso in uences the velocity gradient near the bed ofthe channel.

4. Ba�e provision may have the possibility of provid-ing better conditions for sedimentation by in uenc-ing velocity pro�les. Positioning of a ba�e in themiddle of the channel (xb=L = 0.5) may improvethe ow �eld at its downstream by modifying thevelocity gradient near the channel bed. Positioningof an inlet region ba�e might be bene�cial at lowReynolds numbers, but further investigations arenecessary.

ACKNOWLEDGMENTS

The authors of this paper are grateful to all whoprovided useful assistance. We especially o�er ourgratitude to the Center of Excellence in Energy Conver-sion (CEEC) at the School of Mechanical Engineering,and to the Research Deputy of Sharif University ofTechnology who supported this research. We alsoexpress our thanks to the respected reviewers of thepaper for their valuable comments.

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BIOGRAPHIES

Hamidreza Jamshidnia was admitted to Azad Uni-versity and graduated in March 2005 with a BS in Me-chanical Engineering with a focus on Fluid Mechanicsand Heat Transfer. He was then accepted at Sharif Uni-versity of Technology and awarded a MS in MechanicalEngineering (Energy Conversion) in September 2007.His thesis title was \Experimental investigation of owpattern in a rectangular sedimentation open channel"using an ADV (Acoustic Doppler Velocimeter). Sub-

sequently, he was awarded a Japanese GovernmentScholarship in 2007 and joined Hokkaido University toconduct his PhD studies in ow measurement. Hisresearch has been particularly focused on the studyof multidimensional characteristics of complex spatio-temporal ow structure around a standing ba�e byapplication of a UVP (Ultrasonic Velocity Pro�ler) andis presently in the �nal stages of completion.

Bahar Firoozabadi is associate professor in theschool of mechanical engineering at Sharif Universityof Technology, Tehran. Her research interests areFluid Mechanics in Density Currents, presently focus-ing on Bio uid Mechanics, and Porous Media. Shereceived her PhD in mechanical engineering also atSharif University. She teaches Fluid Mechanics andGas Dynamics for undergraduates, and Viscous Flow,Advanced Fluid Mechanics, Continuum Mechanics andBio uid Mechanics for graduate students.

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