The Effect of Coherent Waving Motion on Turbulence Structure in Flexible Vegetated Open Channel Flows

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1 River Flow - Dittric, Koll, Aberle & Geisenainer (eds) - Bundesanstalt für Wasserbau ISBN Te Effect of Coerent Waving Motion on Turbulence Structure in Flexible Vegetated Open Cannel Flows Ieisa Nezu and Taka-aki Okamoto Department of Civil Engineering, Kyoto University, Kyoto , Japan ABSTRACT: Most aquatic plants are igly flexible and te waving motions of plants are usually observed. Te downstream advection of coerent vortices causes te coerent waving of aquatic vegetation, so-called te Monami. Te strong oscillations in canopy geometry ave te effects of decreasing te amount of vertical turbulent momentum and mass transport into te aquatic canopies. Terefore, ydrodynamic caracteristics of vegetated canopy open-cannel flows ave been investigated intensively, as well as in terrestrial canopy flows. However, te effects of te plant waving motion on turbulence structure ave not been revealed as yet. In te present study, te illuminated flow images were taken by CMOS camera (4 4pixels) and te simultaneous measurements of water velocity and vegetation motion were successfully conducted by te use of bot PIV and PTV tecniques. As te results, we revealed te effect of waving motion on turbulence structure in flexible vegetated open-cannel flows. Te former part of te present study examines te coerent-vortex formulation zones on te basis of te Adrian et al.() s detection metod. Te latter part focused on te relation between coerent structure and te occurrence of Monami penomena. Keywords: PIV, Turbulence structure, Monami, Flexible vegetation, Coerent vortex INTRODUCTION Most aquatic plants are igly flexible and te waving motions of plants are observed usually. Te organized vortices are generated by te inflection-point instability and govern te momentum and mass transport. Te downstream advection of tese coerent vortices causes te coerent waving of aquatic vegetation, referred to as Monami. Terefore, ydrodynamic caracteristics of vegetated canopy open-cannel flows ave been investigated intensively, as well as in terrestrial canopy flows. Finnigan (979) as investigated te coerent features of te turbulent velocity fluctuations above and witin weat canopies by ot-wire anemometers and found te resonant waving of weat stalks referred to as Honami, wic corresponds to Monami in aquatic waving plants. He revealed ten an important role of gusts on mass and momentum transfer toward canopies from te outer boundary layers. Brunet et al. (994) ave analyzed te distributions of various single-point statistical moments and te velocity spectra in a wind tunnel wit waving weat models by ot-wire anemometers. Tey sowed tat te pressure diffusion term in te turbulent kinetic energy (TKE) equation tends to balance wit turbulent energy diffusion just above and witin te canopies. Nepf & Vivoni () ave conducted turbulence measurements in open-cannel flow wit flexible vegetation models by using bot laser Doppler anemometer (LDA) and acoustic Doppler velocimetry (ADV) and examined te transition between te submerged canopy flow and te emergent canopy flow. Tey suggested tat te flow witin a canopy layer could be divided into two zones: te upper layer is called te vertical excange zone and te lower layer is called te longitudinal excange zone. Py et al. (5) conducted on-site experiments in weat and alfalfa canopy flows and quantified te motion of a large set of plants by applying a modified PIV tecnique. Tey developed a Bi- Ortogonal Decomposition (BOD) analysis of te spatio-temporal velocity fluctuations over te crop canopies and found te large-scale coerent prop 49

2 Ar-ion Laser Table Hydraulic condition Case Classification of Φ H(cm) Re Fr (cm) U m(cm/s) plant motion F.5 Swaying(S) F. Swaying(S) F.4 Monami(M) F4.6 Monami(M) F5.7 Monami(M) F6.4 Monami(M) R.5 Rigid(R) R. Rigid(R) R.4 Rigid(R) R4.6 Rigid(R) R5.7 Rigid(R) R6.4 Rigid(R) y V, v model vegetation U, u x z W, w Water Flow B v H d LLS L v ig-speed CMOS camera Fluorescence marker ( Δ x, Δy) d Non-wake region y LLS Wake region z H Vegetation elements control & storage computer Cannel centerline Figure Experimental set-up agating structures wic scaled wit a typical wavelengt of wind fluctuations over te canopies. Gisalberti & Nepf (6) conducted flume experiments wit rigid and flexible vegetation models and found tat te oscillations of te flexible canopies decreased te amount of vertical momentum transfer. Tey also revealed tat te coerent vortex consists of a strong sweep on its front side, followed by a weak ejection on its backside, by using a pase average analysis of te waving motion of vegetation elements. Zu et al. (7) conducted PIV measurements in a wind-tunnel model canopy flow to resolve te flow structure over a wide range of scales. Teir quadrant conditional sampling indicated fundamental differences in flow structure, especially between te sweep and ejection events. Recently, Nezu & Sanjou (8) observed te large-scale coerent structure near te vegetation on te basis of LDA and PIV measurements as well as LES computational simulation. Tey compared te experimental results wit tose of terrestrial canopy flows and pointed out te importance of double-averaging metod (DAM), i.e., Figure Illuminated PIV plane by LLS bot time-average and space-average, in te witin-canopy layer. As mentioned above, tese vegetated opencannel flows ave received muc attention for flexible canopies as well as for rigid ones. Flexible canopy flows are more complicated and callenging researc topics tan rigid canopy flows, because te former sould interact wit wavings of vegetations in a complicated and organized manner, wic are called te Monami penomena. However, te detailed information on Monami penomena is not yet available, because flowvisualization tecniques suc as PIV ave not sufficiently been applied to tese vegetated flows in previous studies. Terefore, te present study examined te interactions between turbulence structure and coerent waving motion in submerged canopy flows wit flexible plant models by a combination of PIV and PTV (Particle Tracking Velocimetry). As te results, we revealed te relation between te plant motion and te coerent vortex generated near te vegetation canopies. 4

3 y L δ es.5.5 Flexible uv U * EXPERIMENTAL METHOD Φ p vegetation Figure Reynolds stress distribution Vortex penetrates to te bed K / C D =.9 Type Experimental data R Present data F R Nepf et al.(7) F : Teory of Nepf et al. (7). K / C D =.7. Valid zone of Nepf λ = a Figure 4 Penetration tickness against te vegetation density Figure sows te experimental setup and te coordinate system. Te experiments were carried out in a m long and 4 cm wide tilting flume. Te cannel slope was canged to create uniform flow conditions. x, y and z are te streamwise, vertical and spanwise coordinates, respectively. Te vertical origin, y =, was cosen on te cannel bed. u ~ ( t) U + u, v ~ ( t) V + () v and w ~ t W + w are te instantaneous velocity components in eac coordinate. U, V and W are te time-averaged velocity components and u, v and w are te corresponding turbulent fluctuations. H is te water dept. Te undeflected eigt of a flexible vegetation element corresponds to te rigid vegetation eigt. In contrast, d is te time-averaged value of te fluctuating deflected eigt d () t = Δy of a typical flexible-vegetation element. Te present flexible vegetation was made of =7mm eigt, b=8mm widt and t=.mm tickness OHP film strip seets. Teir flexural rigidity J E I was J= [Nm ], in wic E = stiff modulus and I = inertial moment. Tis value of J was in te same order of magnitude as J=.7-5 [Nm ] of Velasco et al. () for plastic plant models. Te rigid vegetation was modeled as 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). In te present study, two-components of instantaneous velocity vectors ( u ~, v ~ ) and te tip posi- of flexible vegetations were measured simultaneously by using a combination of PIV and PTV tecniques. Te illuminated flow pictures were taken by a ig-speed CMOS camera (4 4 pixels) wit 5Hz frame-rate and 6s sampling time. A laser ligt seet (LLS) was projected into te water vertically from te free surface. Te mm tickness LLS was generated by W Argon-ion laser using a cylindrical lens. Te instantaneous velocity vectors on te x-y plane were calculated by te PIV algoritm for te wole dept region. Te instantaneous motion of te flexible vegetation elements was analyzed by PTV tecnique. Te fluorescence marker was attaced on te corner of te vegetation element, and te LLS was projected on te markers from te free surface (Figure ). Table sows te ydraulic condition. Experiments were conducted wit bot rigid and flexible vegetations on te basis of six flow scenarios, tions ( Δ x, Δy) in wic te vegetation density Φ = ab (a is defined as te total frontal area per vegetation volume V, b is te widt of te vegetation elements) was canged. Te relative submergence dept H/ and te bulk mean velocity U m were kept constant for all cases. In Table, Re HU m /ν is te Reynolds number, and Fr U m /(gh) / is te Froude number. Te classification of plant motions is indicated in Table. S means Gently Swaying (nonorganized waving), and M means Monami (organized waving). RESULTS. Turbulence structure Figure sows te vertical distribution of Reynolds stress uv for flexible vegetation. Te friction velocity U * was evaluated as te peak value of uv (i.e., U * = uv max ), in te same manner as used by Nezu & Sanjou (8). Te Reynolds stress for flexible vegetation attains maximum near te vegetation and te values of uv decrease rapidly in te canopy layer (y/<) as te vegetation density becomes larger. 4

4 U c /U Present data U c /U Monami (H/ =.) U c /U deflected vegetation ( y = d ) Finnigan (979) Honami (H/ = ) penetration of momentum toward te canopy layer is smaller for flexible canopy tan rigid one.. Detection of coerent eddies In te present study, te convection velocity U c can be evaluated directly from two-point spacetime correlation function C uu wit no use of Taylor s frozen turbulence ypotesis, as follows: 4 U c /U Figure 5 Distribution of convection velocity Tese results are consistent wit te rigid vegetation data of Nezu & Sanjou (8) To evaluate te vertical momentum transfer toward te cannel bed quantitatively, Nepf & Vivoni () defined te penetration dept p as te elevation of % of te maximum Reynolds stress. Te values of p are indicated in te legend of Figure. It is observed tat te values of p become larger as te vegetation density increases. Tis implies tat te penetration of te Reynolds stress into te canopy becomes smaller for te denser canopies. Te momentum transfer from te over-canopy layer toward te bed is obstructed by deflected vegetation elements. Nepf et al. (7) also defined te K-H vortex penetration scale δ, as follows: ( ) y= e U δ e = () U y δ e implies te mixing-layer tickness, in wic te inflection point appears at te vegetation. Assuming a turbulence energy balance of large-scale eddies, Nepf et al. (7) proposed te following relation in a range of λ = a =. to.. δ e K = C a D U c =U () K is te constant value and C D is te drag coefficient. Figure 4 compares te penetration tickness δ e for Rigid and Flexible canopies. Te values of δ e decreases wit an increase of te vegetation density λ = a for bot Rigid and Flexible canopies, wic is consistent wit te Reynolds stress properties in Figure. It is confirmed tat te penetration scale δ e obeys te teoretical curve of Eq.() in a range of λ =. to.. Te values of δ e become larger for Rigid canopy tan for Flexible one. Consequently, te Δx U c = max () τ in wic, Δ xmax is te streamwise distance between te reference point and te movable point, at wic te correlation becomes maximum wen te time lag τ passes. Figure 5 sows te vertical distribution of te convection velocity U c (y) for Monami canopy, normalized by te time-averaged mean velocity U(y). For a comparison, Figure 5 also sows te flexible canopy data of Finnigan (979). Te convection velocity U c of mean eddies is always larger tan te local mean velocity U, and te value of U c /U attains at te flexible vegetation. Te value of U c /U increases rapidly witin te canopy (y/<.), wic is in good agreement wit terrestrial canopy data by Finnigan (979). He revealed tat wen te vortex passes, tere is a positive perturbation in te velocity field witin te canopy. Te positive velocity perturbation may deflect te flexible vegetation elements and, as te results, generates te coerent waving motion of plants (so called Honami for terrestrial canopy flows and corresponds to Monami for aquatic ones). Figure 6 sows some examples of instantaneous velocity vectors ( u ~, v ~ ) at te time t =.s,.8s and.67s for Monami canopy ( Φ =.6). Te contours of te streamwise turbulent fluctuations, u( x, y, t) u ~ U, are sown by color rainbow. Hig-speed fluid parcel and low-speed fluid parcel are seen clearly, and tese large-scale organized parcels are convected downstream. Parcels A and B in Figure 6 correspond to ejection motions, in wic te low-speed fluid is lifted up toward te free surface. On te oter and, parcels C and D correspond to sweep motions, in wic te ig-speed fluid is inrused into te canopy. On te basis of te Adrian et al.() s detection metod of bursting penomena in boundary layers, te present study tried to examine coerent eddies. Figure 7 sows te time-series of instantaneous velocity vectors in te PIV plane, wic are subtracted by a vortex convection velocity U c at te vegetation. Te value of U c was evaluated from te two-point space-time correla- 4

5 .5 vegetation Zone.5 Zone Ejection(A) vegetation Sweep(C) Zone Zone vegetation Ejection(B).5.5 Zone Zone t=.67s x / L t=.67s v.5.5 Zone d Zone vegetation Zone vegetation Zone.5 Zone Sweep(D).5 Zone x / L ~ v tion analysis, as as already been discussed in Figure 5. Te ( u~ U, v ~ c ) vectors sow vortexlike parcels, wic are labeled by a, b, c and d in te figure. Tese parcels are in good agreement wit te parcels A, B, C and D, respectively, in Figure 6. Tis implies tat te vortexlike parcels a and b correspond to te ejections, wile te vortex-like parcels c and d correspond to te sweeps. In Figure 6, te uniform momentum zones are separated by and-drawn lines and labeled Zones, and (Adrian s metod), as follows: Zone: -5< ( u ~ U ( ) ) U c * <- Zone: -< ( u ~ U ( ) ) U c * < Zone: < ( u ~ U ( ) ) U c * <5 Te streamwise velocity component usually canges significantly between zones, but remains rougly constant witin a zone. Of particular significance is tat te lines pass troug te cores of coerent vortices. Suc a remarkable coincidence between te vortex core and te boundary between uniform-momentum zones suggests an important association between coerent vortex and uniform-momentum zones. Tese results are in.5 Zone Zone t=.s t=.8s -5 u (cm/s) Figure 6 Instantaneous velocity vector 5 φ =.6 4 (Monami).5 Zone vegetation.5 Zone c Zone Zone t=.s t=.8s a -5 ( u Uc( ) U* 5 Figure 7 Instantaneous velocity vector subtracted by eddy convection velocity at vegetation good agreement wit Adrian et al. () in boundary layers. Figure 8 sows te corresponding instantaneous vorticity Ω, wic is defined as v~ u~ Ω = (4) x y Figure 8 contains many regions of concentrated vorticity along te boundaries between te uniform-momentum zones. Some of te weaker fluctuations may be noise in te vorticity measurements, and so we will concentrate on only te strongest regions. Te regions labeled a, b, c and d contain te concentrated vorticity. Figure 8 conforms te association between te vortex and uniform-momentum zones, and sows tat te coerent vortices produce te regions of large vorticity Ω.. Interaction between flow field and vegetation motion Importantly, te Monami is visible as te downstream propagation of a localized region of forward plant deflection. Terefore, coerent vortices may induce periodical oscillations in te b

6 .5 Zone vegetation.5 Zone t=.8s Zone.5 vegetation t=.67s.5 Zone t=.s Zone Zone Zone vegetation Zone.5 Zone Ω (/s) Figure 8 Contour of instantaneous voriticity c d a streamwise and vertical velocity components near te vegetation. Figure 9 (a) sows an example of te power spectrum of te streamwise velocity fluctuations u(t) at y/=. (above te canopy) for te Swaying and Monami canopies. For te Swaying canopy, no significant spectral peak property is observed. In contrast, for te Monami canopy, a predominant spectral peak f p occurs near. Hz. Te value of. Hz for te streamwise velocity spectrum is in good agreement wit te value of te teoretical frequency f KH of Kelvin-Helmoltz (K-H) instability proposed by Ho & Huerre (984) in pure mixing layers, wic is given by. U f KH = (5) θ in wic, U = average velocity of mixing layer and θ = momentum tickness of mixing layer. Tis good agreement between f p and f KH was obtained in te oter studies for Monami canopies. Ikeda & Kanazawa (996) and Gisalberti & Nepf () observed coerent eddies over flexible vegetation canopies and ave also obtained te same conclusions. In contrast, Figure 9 (b) sows te power spectrum of te fluctuations d (t) of te flexible vegetation eigt for Swaying and Monami canopies, wic were calculated from te present PTV data. Te vibration frequency of flexible vegetation b 44 elements is in good agreement wit tat of te velocity spectral peak f p and te K-H frequency f KH for Monami canopy. Tese results imply tat te coerent waving motion of vegetation elements is associated wit te K-H instability and te largescale coerent vortices in te mixing-layer zone generate te Monami motion of flexible vegetations. In terrestrial canopy flows, Py et al. (6) investigated te origin of te instability mecanism by coupling te wind and canopy dynamics. Tey found tat te predominant frequency of wind velocity over te waving canopies matces closely wit te free-vibration frequency, i.e. te natural frequency of te weat plants, and tat te flow instability mecanism over plant canopies is driven by a mixing-layer-type instability. Te natural frequency f N of te present flexible vegetation elements was measured using a metod proposed by Py et al. (6), resulting in f N =.7 Hz, te value of wic is in fairly good agreement wit te peak frequency f p =. Hz of Monami. Figure 9 (b) also confirms tat te present classification of (a) S u ( f ) ( m s) (b) 4x - 4x - x - x - x - x - - 5x -4 6x -5 Spectrum peak S d ( f ) 5x -5 Re = 8 ( m s) Re = 4x -5 x -5 x -5-5 Swaying a=7.6(/m), H/=. Monami Velocity.. Swaying Spectrum peak y/=. Re = 8 Re = Monami f (Hz) Vegetation elements.. f (Hz) Figure 9 (a) Spectrum of streamwise velocity component u(t) and (b) spectrum of deflected eigt of vegetation d(t) for flexible vegetation te Monami and Swaying by eyes is reasonable in comparison between tese two spectra of vegetation fluctuations. No direct measurements of te Monami penomena ave been conducted quantitatively as yet. To examine te coerent waving motion of flexible vegetation, te space-time correlation C dd (x,τ) between te fluctuations of te deflected vegetation-element eigts d (x,t) and d (x +x,t+τ) were analyzed as follows:

7 C ( x, ) ( x, t) d ( x + x, t + τ ) ' ( x ) ' ( x x) d τ = (6) d d d d + in wic, (x ) = reference point of vegetation and (x +x) = movable reference point. d means its r.m.s. value and τ = time lag. C dd (x,τ) for Swaying and Monami canopies are sown in Figure. Te x-axis plane is normalized by te streamwise spacing of vegetation elements L v. So, te integer of x/l v indicates te position of flexible vegetation and tus te distance lag between two vegetation elements. Te time lag τ was selected to τ=. s,.6 s and. s. It is evident from Figure tat te large values of C dd (x,τ) are observed significantly near te reference point (x/l v =.) for Monami canopy. Te correlation peak is convected wit an increase of te time lag τ. Tese results indicate tat te waving motions of flexible vegetations are more organized and tis coerent waving motion is convected in te downstream direction for Monami canopy tan for Swaying one. Py et al. (5) ave conducted similar measurements of flexible canopy motion to detect te local displacements of te crop surface by PIV. Tey also recognized suc a largescale organized motion of vegetation from spacetime decomposition using te BOD analysis. In contrast, for Swaying canopy (Figure b), te correlation values C dd (x,τ) are muc smaller tan tose for Monami canopy. Te convection of te correlation peak is not observed significantly for Swaying canopy, wic is coincident wit te spectral analysis sown in Figure 9 (b). Tis Reference point (a) Monami: a=7.6(/m), H/=., Re=8 C d τ =. (s) d.8 τ =.6(s).6 τ =. (s) Reference point (b) Swaying: a=7.6(/m), H/=., Re= τ =. (s).8 C τ =.6 (s) dd.6 τ =. (s) Figure Space-time correlations of deflected vegetation eigt for (a) Monami and (b) Swaying suggests tat te vegetation elements are independently waving eac oter, i.e. witout organized motions for Swaying canopy. 45 Table Comparison of flow properties between rigid and flexible vegetation flows Rigid Flexible Mean velocity U inflection Yes Yes Sear instability Large Small Flow resistance Large Small Mixing-layer velocity profile Yes Yes Reynolds stress Sarp peak at te vegetation Milder peak Larger peak value Smaller peak value (Monami) Penetration of momentum Large Small Mean-eddy scale L x L x = (Large) L x =. (Small ) Quadrant Reynolds stress RSi Larger contribution of sweep Smaller contribution of sweep Convection velocity U c U c =-.U U c =.U (Monami) Coerent structure Large Small Mass transport Large? Small? 4 DISCUSSION As mentioned above, te flexibility of te canopy and te presence of Monami affect te mean flow and turbulence structure in te submerged vegetation flow. Table compares several flow properties between Rigid and Flexible vegetation flows. Te mean velocity profiles ave an inflection point at te vegetation (y= for rigid canopy and y= d for flexible one), and in te mixinglayer zone, te values of U are approaced to te yperbolic tangent profile bot for Rigid and Flexible vegetation flows. In te Rigid vegetation flow, te mean velocity U witin te canopy is reduced more significantly and te strong sear layer is produced near te vegetation. Consequently, te large-scale coerent vortices at te vegetation dominate te vertical momentum transport. Relative to te Rigid canopy, te values of Reynolds stress uv witin te Monami canopy become smaller and te Monami canopy decreases te turbulent momentum transport troug te sear layer. Te diminised momentum excange suggests tat te coerent vortices become weaker and smaller in te presence of Monami. Tis suggestion is conformed by te observed mean-eddy lengt scale L x. Te streamwise and vertical lengt scales Lx and L y are larger for te Rigid canopy tan for te Flexible canopy. Te quadrant analysis implies tat te momentum transfer is dominated by sweeps witin te canopy and te contributions of sweep events are larger for Rigid vegetation flows. Te Monami is observed wen te convection velocity U c of coerent-eddy is significantly greater tan te mean velocity U at te vegetation, as pointed out by Gisalberti & Nepf (). Te convection velocity U c for Monami canopy attains (-.)U at te flexible vegetation and tis value is in te same order of magnitude as U c =.U for Rigid canopy. Tese results suggest tat wen te canopy is waving, te instantaneous drag distribution may

8 not be so contributory to te vortex. Consequently, te vortex structure may lose coerence in te presence of Monami. In particular, effects of flexible canopy on scalar transports, suc as various water quality, dissolved gases (O and CO ) and sediment transport, are not fully investigated as yet. Some researcers suggest tat submerged flexible canopies may protect te coastal bed by reducing friction forces near te water-sediment interface, tus, directly protecting te bed from erosive processes. It is furter necessary to investigate turbulence and sediment transport witin te canopy by simultaneous measurements of velocity and scalar concentration including water temperature. 5 CONCLUSIONS In te present study, te simultaneous measurements of water velocity and vegetation motion were successfully conducted by te use of a combination of PIV and PTV. As te results, we revealed te relation between turbulence structure and te occurrence of Monami penomena. Significant results obtained in tis study are as follows:. We compared te penetration tickness for Rigid and Flexible canopies. Te penetration of momentum toward te canopy layer is smaller for flexible tan rigid canopies.. On te basis of te Adrian et al.() s detection metod, te present study tried to examine coerent-vortex formulation zones. A good coincidence between te center of te coerent vortex cores and te boundary between te uniform-momentum zones clearly demonstrates an association between coerent vortex and uniform-momentum zones.. To examine te coerent waving motion of flexible vegetation quantitatively, te spacetime correlation between te fluctuations of te deflected vegetation-element eigts was analyzed. Te large values of spacetime correlation were observed significantly near te reference point for Monami canopy. Tis indicates tat te waving motions of flexible vegetations are more organized. velocity statistics, Boundary-Layer Meteorology, 7, 95-. Finnigan, J.J Turbulence in waving weat, Mean statistics and Honami, Boundary Layer Meteorology, 6, 8-. Gisalberti, M. and Nepf, H.. Mixing layers and coerent structures in vegetated aquatic flows, J. of Geopysical Res., 7, ---- Gisalberti, M. and Nepf, H. 6. Te structure of te sear layer in flows over rigid and flexible canopies, Environ. Fluid Mec., 6, 77-. Ho, C.M. and Huerre, P.984. Perturbed free sear layers, Ann. Rev. Fluid Mec.,6, Ikeda, S. and Kanazawa, M Tree-dimensional organized vortices above flexible water plants, J. of Hydraulic Eng.,(), Nepf, H. M., Gisalberti, M., Wite, B. and Murpy, E. 7. Retention time dispersion associated wit submerged aquatic canopies, Water Resource Researc, 4, W44 Nepf, H. M. and Vivoni, E. R.. Flow Structure in Dept-limited, Vegetated Flow, J. of Geopysical Res., 5, Nezu, I., and Sanjou. M. 8. Turbulence structure and coerent motion in vegetated canopy open-cannel flows, J. of Hydro-environment Researc,, 6-9. Py, C., de Langre, E., Moulia, B. and H emon, P. 5. Measurement of wind-induced motion of crop canopies from digital video images, Argic. Forest Met.,, - 6. Py, C., de Langre, E. and Moulia, B. 6. A frequency lock-in mecanism in te interaction between wind and crop canopies, J. Fluid Mec., 568, Velasco, D., Bateman, A. Redondo, J. and Demedina, V.. An open cannel flow experimental and teoretical study of resistance and turbulent caracterization over flexible vegetated linings, Flow Turbulence and Combustion, 7, Zu, W., van. Hout, R. and Katz, J. 7. On te flow structure and turbulence during sweep and ejection events in a wind-tunnel model canopy, Boundary-Layer Meteorol., 4, 5- REFERENCES Adrian, R.J., Meinart, C.D. and Tomkins, C.D.. Vortex organization in te outer region of te turbulent boundary layer, J. of Fluid Mec, 4, -54. Brunet, Y., Finnigan, J.J. and Raupac, M.R A wind tunnel study of air flow in waving weat: single-point 46