Simultaneous Measurements of Concentration and Velocity with Combined PIV and planar LIF in Vegetated Open-Channel Flows

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1 River Flow - Dittric, Koll, Aberle & Geisenainer (eds) - Bundesanstalt für Wasserbau ISBN Simultaneous Measurements of Concentration and Velocity wit Combined PIV and planar LIF in Vegetated Open-Cannel Flows Taka-aki Okamoto, Ieisa Nezu & Aki Katayama Department of Civil Engineering, Kyoto University, Kyoto , Japan ABSTRACT: Aquatic plants reduce suspended sediment transport because of te local reduction in bed sear stress. Terefore, it is very important for river environment to reveal te mass and momentum excanges near te vegetation zone. Turbulent diffusion mecanisms in emergent vegetated flows ave recently been investigated by many researcers. However, te effect of te submerged vegetation on te scalar flux as not been fully investigated. So, continuous dye injection experiments were conducted to evaluate te vertical mass transport in open-cannel flow wit rigid vegetation models by canging te vegetation density. In te present study, a combination tecnique between PIV and planar laser-induced fluorescence (LIF) was developed by using two sets of CMOS cameras, to measure te instantaneous velocity and te instantaneous concentration field simultaneously. Keywords: LIF, PIV, Simultaneous measurements, Submerged vegetation, Vertical scalar flux INTRODUCTION Aquatic plants are fundamental components of a natural water environment in rivers and wetlands. Submerged vegetation generates coerent turbulent motions near te vegetation edge and reduces suspended sediment transport because of te local reduction in bed sear stress. It is terefore important for river management to investigate ydrodynamic caracteristics and coerent eddies in opencannel flows wit vegetation canopies. In recent years, te measuring tecniques of PIV and planar laser induced fluorescence (LIF) are extensively used to investigate te mixing processes. For example, Cen & Jirka (999) ave measured te instantaneous concentration field in sallow jet flows by LIF. Tey revealed te overall meandering of te plane jet and te role of largescale turbulence structure in te mixing processes. Crimaldi & Koseff () measured te spatial and temporal structure of an odor plume in turbulent flows by two kinds of LIF tecniques, i.e., full-field planar LIF and single-point LIF probe. Te results sowed a wide range of turbulent structures in detail; te nature of te structure varied significantly in te different regions of plume. Raman & Webster (5) conducted planar LIF measurements in turbulent open-cannel flow and examined te effect of bed rougness on a cemical plume. Tey revealed tat te concentration fluctuation intensity is smaller and decreases faster as te bed rougness increases. On te oter and, turbulence structure and transport mecanism of momentum and mass in te vegetated flows ave been investigated in te past decades. Gao et al. (988) ave proposed a ramp-jump structure and umidity witin and above a terrestrial canopy by using triaxial sonic anemometers and termometer. Tey revealed tat coerent structures consisted of a weak ejection from te canopy top followed by a strong sweep into te canopy. Gisalberti & Nepf (5) conducted continuous dye injection experiments to caracterize vertical mass transport in submerged vegetation flow. Troug te absorbance-concentration relationsip of te Beer Lambert law, a digital imaging was used to provide ig-resolution concentration profiles of te dye plumes. Tey suggested tat vertical mass transport as te strong periodicity and coerent vortices of te sear layer dominate te turbulent diffusion. Reidenbac et al. (7) ave measured finescale mixing and mass transport witin a coral canopy by using planar LIF. Tey sowed tat te action of surface waves, interacting wit te struc- 487

2 Ar-ion Laser Table Hydraulic condition Case Φ H(cm) (cm) H/ U m (cm/s) Re Fr R. - - R R.6 H y..7 Vegetation elements LLS Hig-speed CMOS CCD camera for PIV Log-law zone y V H U x z W Flow Nozzle Control computer Sarp cut-off filter Hig-speed CMOS CCD camera Pulse for LIF generator log p Coerent vortex Mixing-layer zone Vegetation elements Vegetation edge Emergent Wake zone zone Figure Experimental set-up ture of te reef, could increase instantaneous sear and mixing up to six times compared to tat of non-conditional flow. Jamali et al. (8) conducted LIF measurements to investigate te excange flow due to te temperature differences between te vegetation and non-vegetation zones. Tey suggested tat te magnitude of te excange rate decreases as te canopy drag increases. However, tese above-mentioned previous studies do not offer te detailed information of turbulent diffusion mecanism, because it as been difficult to measure te velocity and concentration fluctuations simultaneously. In te present study, a combination tecnique between PIV and LIF was developed by using two sets of ig-speed cameras to measure te instantaneous velocity and instantaneous concentration field simultaneously. Of particular significance is tat tat te syncronicity allows te direct computation of scalar flux and velocity. Consequently, effects of coerent vortices on scalar flux were examined in detail. EXPERIMENTAL METHOD. Experimental setup and vegetation model Te experimental setup and te coordinate system are indicated in Figure. Te present experiments were conducted in a m long and 4cm wide tilting flume. x, y and z are te streamwise, vertical and spanwise coordinates, respectively. H is te Figure Flow zone model in vegetated open-cannel flow water dept and is te vegetation eigt. Bv and Lv are te neigboring vegetation spacings in te spanwise and streamwise directions, respectively. Te elements of vegetation model were made of rigid-strip plates (=5mm eigt, b=8mm widt and t=mm tickness) in te same manner as conducted in laboratory experiments by Nezu & Sanjou (8). Te time-averaged velocity components in eac direction are defined as U, V and W, and te corresponding turbulent fluctuations are u, v and w, respectively. c ~ ( t) C + c( t) is te instantaneous dye concentration, wic is defined in te same manner as velocity components. In te present study, a combination tecnique between PIV and laser-induced fluorescence (LIF) was developed by using two sets of CMOS cameras (4 4 pixels), to measure te instantaneous velocity components ( u~, v ~ ) and te instantaneous concentration c ~ simultaneously. Te mm tickness laser-ligt seet (LLS) was generated by W Argon-ion laser. Te illuminated flow pictures were taken by two sets of ig-speed cameras wit 5Hz frame-rate and 6s sampling time. ~ Te instantaneous velocity components ( u ~, v ) on te x-y plane were analyzed by te PIV algoritm for te wole flow dept region. Tese PIV metods are te same as used previously by Nezu & Sanjou (8). Te instantaneous concentration field c ~ ( x, y) was quantified by using te planar LIF tecnique. Dye (Rodamine B) was injected troug a mm diameter stainless nozzle. Te 488

3 dye-injection velocity was adjusted to matc te local flow velocity U. Te dye absorbed te green laser ligt and reemitted ligt wit te different longer wavelengts. Under appropriate conditions, te ligt intensity emitted by te dye is directly proportional to te dye concentration and laser ligt intensity. Consequently, te planar concentration field c ~ ( x, y, t) is calculated from te captured images by one CMOS camera (for LIF), using a relevant calibration function.. Hydraulic condition Table sows te ydraulic condition. Te bulk mean velocity U m and te submergence dept H/ were kept constant for all cases, i.e., U m = cm/s and H/=.. Two kinds of experiments were conducted by canging te vegetation density Φ and te vertical position of te nozzle tip y. In tis study, te vegetation density Φ is defined as follows: Aibi i Φ = = ab () S in wic, A i is te frontal area of te vegetation element, b i is te vegetation-element widt and S is te referred bed area. In te present study, te nozzle tip position was canged to examine te turbulent diffusion properties in all tree sub-zones of te vegetated open-cannel flow, as sown in Figure Nepf & Vivoni () and Poggi et al.(4) ave pointed out tat te wole dept region in submerged vegetation flow could be classified into several layers on te basis of te vertical profiles of mean streamwise velocity and Reynolds stress. Nezu & Sanjou (8) found tat te submerged rigid canopy flow can be divided into te following tree sub-zones on te basis of previous experimental results. Wake zone Mixing layer Log law ( y p ) zone ( p y log) ( ) log y H zone ().5.5 Φφ.5 U /U.5.6 U /U.5 uv /U *.6 uv /U * p is te penetration dept defined by Nepf & Vivoni () and log is te lower limit of te log-law zone defined by Nezu & Sanjou (8). In te Wake zone ( y p ), te vertical turbulent momentum transport is negligibly small due to te strong wake effects beind vegetation stems, i.e. te Karman vortex appears predominately. In te Mixing-layer zone ( p y log ), a large-scale coerent vortex is generated near te vegetation edge due to te inflection-point instability and consequently, te vertical turbulent excange contributes largely to te momentum transfer between over- and te witin-canopies, as discussed by Raupac et al. (996) for terrestrial canopy, and by Nezu & Sanjou (8) for aquatic canopy. log is te lower limit position of te Loglaw zone, in wic turbulence caracteristics are analogous to tose of boundary layers. RESULTS. Mean flow Before examinations of te concentration field, it is very important to examine te mean-flow and turbulence caracteristics in vegetated opencannel flows. Figure sows te vertical distributions of te time-averaged and space- (orizontal) averaged (i.e. double-averaged) streamwise velocity U and Reynolds stress uv for submerged vegetation density ( Φ =.5,.6). Tese values are normalized by te mean velocity at te vegetation-edge U and te friction velocity U *, respectively. Te value of U * was evaluated as te peak of uv in te same way as conducted by Nezu & Sanjou (8). Near te top of te vegetation (=.), bot te local velocity ( y) y vegetation edge U /U Figure Mean velocity and Reynolds stress uv /U * U and its gradient U y increase rapidly, wic induces a strong sear layer and a significant inflection-point instability, as pointed by many researcers. It sould be noticed tat te value of uv attains maximum near te vegetation edge, i.e., =.. Te Reynolds stress decays rapidly in te canopy layer (<) and becomes negligibly small near te cannel bed. Figure also sows tat te values of Reynolds stress decrease wit an increase of te vegetation densityφ. 489

4 .5.5 Φφ =.6 =. Half Widt y vegetation edge Φ =.6 Φ =.5 Φ =. Log-law zone Mixing-layer zone =..5 Half Widt Emergent zone E b b + Figure 5 Distribution of alf-value-widt (x/ =.) (B) =..5.5 Half Widt Figure 4 Contours of time-averaged concentration. Time-averaged concentration Figure 4 sows te contour lines of te timeaveraged dye concentration C ( x, y) for Φ =.6 (dense canopy). Te nozzle tip positions are y / =. (Wake zone),. (Mixinglayer zone) and. (Log-law zone). Te contour values are normalized by te mean concentration at te nozzle tipc. To evaluate te vertical turbulent diffusion quantitatively, te streamwise variations of te alf-value-widt b / + (upper 5 5 (A). c/c. (C) t(s) Figure 6 Time series of instantaneous velocity vectors and concentration field layer) and b / (lower one) as well as te trajectory of te maximum concentration Cmax are also included in Figure 4. It is found clearly tat te plume evolves downstream and te alf-valuewidts b/ + and b / increase linearly along te x-axis, wic is consistent wit te experiments of Webster et al. (). Figure 5 compares te alf-value-widt b/ + and b/ at Lv =. for Φ =.,.5 and.6. Bot of b/ + and b/ become larger at y / =.6 and., i.e., in te Mixing-layer zone, for Φ =.5 and.6. Te vertical turbulent diffusion is smaller at y / =., i.e., in te Wake zone and y / =.4, i.e., in te Log-law zone. In contrast, for te Smoot flat bed (no vegetation, Φ =.), te values of b/ + and b / are negligibly small, irrespective of te nozzle tip position of y /. Tis implies tat in submerged vegetation flows, a large-scale coerent vortex is generated near te vegetation edge due to te inflection-point instability (Figure ) and te vertical turbulent excange contributes largely to te scalar transport in te Mixing-layer zone. Tese results are in good agreement wit Nepf & Vivoni () and Nezu & Sanjou (8). 49

5 It is also observed tat b/ + and b / for Φ =.6 (dense canopy) are greater tan tose for Φ =.5 (sparse canopy) and Φ =. (smoot bed). Nezu & Sanjou (8) pointed out tat te strong sear layer appears near te vegetation edge for dense canopies. Gisalberti & Nepf (5) also demonstrated tat te dense canopies would generate vortices wit a greater rotational speed and tat te iger rates of rotation would result in a more rapid flusing in te canopy layer. Terefore, it is recognized tat te large distributions of te dye concentration are mixed rapidly due to te strong sear layer for Φ =.6.. Instantaneous velocity and concentration field Figure 6 sows te time-series of instantaneous velocity vectors ( u ~, v ~ ) by PIV and te corresponding instantaneous concentration field c ~ ( x, y, t) by LIF. At t =.s, a sweep motion is observed below te canopy top. Te dye concentration is transported into te canopy layer. At t = 6.-7.s, an ejection motion appears and te ig value of concentration is observed above te vegetation edge. At t =.-.s, te sweep motion appears again near te vegetation top. Figure 7 sows some examples of te instantaneous velocity vectors ( u ~, v~ ), wic were ob- t=.s t=4.8s t=7.5s 4 5 Figure 7 Instantaneous velocity vectors u (cm/s) t=.s t=4.8s t=7.5s 4 5 Figure 8 Instantaneous Reynolds stress uv (cm/s) 4 5 y/ t=.s y/ t=4.8s t=7.5s 4 5 Figure 9 Corresponding instantaneous concentration field 4 5. C / C o

6 .5.5 Non-conditional Half widt.5 Non-conditional (H=) (H=) (H=) (H=) Half widt C/C.5 max Half widt 4 5 tained by PIV. Te contours indicate te magnitude of turbulent fluctuations u (x,y,t) for te streamwise velocity component. Figure 8 sows te contours of te instantaneous Reynolds stress u(t)v(t) at te same time. It is recognized tat large values of te instantaneous Reynolds stress are located near te vegetation edge and tese locally-ig distributions of Reynolds stress correspond well to ejection and sweep motions of Figure 7. Tese results imply tat te coerent motions govern te momentum transfer near te canopies significantly. Tese noticeable features for submerged canopies are in good agreement wit Nezu & Sanjou (8). Figure 9 sows te corresponding simultaneous concentration field c ( x y, t) C / C max Figure Contour of conditional sampling concentration. 49 Figure Distribution of conditional sampling concentration, measured by LIF. Figure 9 reveals te ig spatial variability of te instantaneous concentration field. At t=.(s), a sweep motion, i.e., te downward vectors, appears near te vegetation edge. It is observed tat te large distribution of te instantaneous concentration is transported downward ( < ) v by te sweep motion. At t=4.8(s), an ejection motion is observed and causes te local increase of te concentration above te canopy. At t=7.5(s), te ejection motion is transported downstream and te oter sweep motion appears at te upstream side. Te individual filaments of te dye concentration are observed clearly and te igly intermittent nature of te plume is exibited, wic is consistent wit Raman & Webster (5). Tese images also sow tat te locally-ig distributions of dye concentration correspond well to tese coerent structure zones. Tese results provide more evidence tat te large-scale coerent motions dominate te vertical mass and momentum transport in vegetated flows..4 Conditional analysis To examine te effects of te coerent motions on te vertical turbulent diffusion quantitatively, quadrant analyses were performed. Figure (b) and (c) sow te contours of te concentration C E and C S at y / =. for φ =.6 (dense canopy) by using conditional sampling of te motion (u<, v>) and te motion (u>, v<), respectively. Te contour of te time-averaged concentration (non-conditional) C is also included for comparison in Figure (a). It is evident from Figure (b) tat C E as te peak value above te vegetation edge and te values of C E are larger tan C (non-conditional) over te canopy. Tis implies tat te ig-value of te dye concentration is transported to te above-canopy layer by te motion, wic is consistent wit Figs.4 and 5. Wereas, C S becomes larger tan C below te vegetation edge and te motion transports te dye concentration into te witin-canopy layer.

7 .5 Φφ =.6, y / = Φφ =.5, y / = L -. uc / u' c'. v Figure Turbulent scalar flux (y /=.).5 uc and vc Non-conditional (H=) (H=) ( ,.548) (.854, (H=).548) ( ,.548 (H=) ( ,.548 ( L -.. v vc / v' c' consistent wit Figure. Tese results reveal tat te large distributions of te dye concentration are mixed more rapidly witin te canopy due to te stronger wake effects beind vegetation stems, and consequently, te peak value of C S decreases more significantly in te witin-canopy layers uc UC Figure Vertical distribution of conditional sampling turbulent scalar flux (φ =.6) Figure sows te vertical distributions of te concentration C E and C S at y / =. for Φ =.6 by using conditional sampling of te s and s. To exclude te lessorganized motion, a tresold level H, so-called te ole-value is used. Te values of te concentration C E ( H = ) and C S ( H = ) are greater tan tose of C E ( H = ) and C S ( H = ). Tis indicates tat muc organized and motions would transport larger-values of te dye concentration. It is also observed tat te value of C S decreases more rapidly tan tat of C, wic is δ c (Non-conditional) δ () c E 49.5 Turbulent scalar flux Of particular significance in tis study is te turbulent scalar flux of a passive tracer. Te PIV-LIF measurements afford us to calculate te local covariance between te concentration and velocity fluctuations, uc and vc. Figure sows te turbulent scalar flux uc and vc for dense ( Φ =.6) and sparse ( Φ =.5) canopies. Te RMS values of te streamwise velocity, u, and te concentration, c, are used to normalize te flux quantities. Te turbulent flux uc as te positive values and vc as te negative values above te vegetation edge (>.). Te large peak values of uc and vc are located in te over-canopy region. Tis indicates tat te ig distribution of concentration (c>) is transported upward by te ejection motion (u<, v>). In contrast, uc ave te negative values witin te canopy layer, wic implies tat te sweep motion (u>, v<) transports te dye concentration into te canopy layer (c>). It sould be noticed tat te values of te turbulent scalar flux uc and vc are greater for dense canopy tan for sparse canopy, wic is

8 consistent wit Figure 5. Tese results reveal tat te strong sear layer near te vegetation edge generates te large-scale coerent vortices for dense canopy and larger distributions of te dye concentration are transported toward te over- and witin-canopies more significantly. Figure sows te vertical distribution of turbulent scalar flux uc at Lv =. for dense canopy ( Φ =.6) by using conditional sampling of te s and s. Te bulk mean velocity U m and te time-averaged concentration C are used to normalize te flux quantities. Te turbulent scalar flux uc as te larger positive values witin te canopy layer tan te uc Non conditional, wereas te values of uc become larger above te vegetation edge (=.-.). Vertical penetration of turbulent scalar flux into te canopy is a measure of a region witin te canopy, wic actively excanges mass. Te extent of tis region may be defined as te point witin te canopy at wic te turbulent scalar flux uc becomes negligibly small. In Figure, δ c is te penetration tickness of turbulent scalar flux. δ c () is significantly larger tan δ c (Non-conditional). Tis implies tat te motions dominate te vertical penetration of scalar flux into te canopy. Tis conclusion is supported by te conditional sampling concentration (Figure ). 4 CONCLUSIONS Tis study conducted PIV & LIF measurements to reveal te effects of te submerged vegetation on scalar flux in open-cannel flows wit rigid vegetations. Te nozzle tip position was canged to examine te turbulent diffusion properties in all tree sub-zones of te vegetated open-cannel flow. Te significant results obtained in tis study are as follows:. Te alf-value-widt becomes larger in te mixing-layer zone and te vertical diffusion is smaller in te wake zone and in te log-law zone. Tis implies tat a large-scale coerent vortex is generated near te vegetation edge and te vertical turbulent excange contributes largely to te scalar transport in te mixinglayer zone.. On te basis of LIF&PIV data, te effect of te coerent motion on mass transport was examined quantitatively. Te conditional analysis reveals tat te large distributions of dye concentration are transported toward te above-canopy and te witin-canopy regions by ejection and sweep motions, respectively.. Te PIV-LIF measurements afford us to calculate te local covariance between concentration and velocity fluctuations. Te values of te turbulent scalar flux are greater for dense canopy tan tose for sparse canopy. Tese results reveal tat te strong sear layer near te vegetation edge generates te large-scale coerent vortices for dense canopy and larger distributions of te dye concentration are transported toward te over- and witin- canopies more significantly. REFERENCES Cen, D., and Jirka, G.H LIF study of plane jet bounded in sallow water layer. J. Hydraul. Eng., 5, Crimaldi, J. P. and Koseff, J. R.. Hig-resolution measurements of te spatial and temporal scalar structure of a turbulent plume, Experiments in Fluids,, 9-. Gao, W., Saw, R.H. and Paw, K.T Observation of organized structure in turbulent flow witin and above a forest canopy. Boundary-Layer Meteorology, 47, Gisalberti, M. and Nepf, H. 5. Mass transfer in te vegetated sear flows. Environ. Fluid Mec., 5(6), Jamali, M., Zang, H. and Nepf, H. 8. Excange flow between canopy and open water, J. Fluid Mec., 6, Nepf, H., and Vivoni, E. R.. Flow structure in deptlimited vegetated flow, J. of Geopys. Res.5, Nezu, I., and Sanjou. M. 8. Turbulence structure and coerent motion in vegetated canopy open-cannel flows. J. of Hydro-environment Res.., 6-9. Poggi, D., Porpotato, A. and Ridolfi, L. 4. Te effect of vegetation density on canopy sub-layer turbulence, Boundary-Layer Meteorology,, Raupac, M.R., Finnigan, J.J., Brunet, J.J, 996. Coerent eddies and turbulence in vegetation canopies: te mixing layer analogy, Boundary Layer Meteor., 78, 5-8. Raman, S., and Webster, D.R. 5. Te effect of bed rougness on scalar fluctuations in turbulent boundary layers, Experiments in Fluids, 8, Reidenbac, M.A., Koseff, J.R, and Monismit, S.G. 7: Laboratory experiments of fine-scale mixing and mass transport witin a coral canopy, Pysics of Fluids, 9, 757. Webster, D.R., Raman, S., and Dasi, L.P.. Laserinduced fluorescene measurements of a turbulent plume, J. Eng. Mec., 9,