Revista Recursos Hdricos

DOI:10.5894/rh34n2-5
O texto deste artigo foi submetido para reviso e possvel publicao em outubro de 2013, tendo sido aceite pela Comisso de Editores Cientficos Associados em outubro de 2013. Este artigo parte integrante da Revista Recursos Hdricos, Vol. 34, N 2, 55-67, novembro de 2013.

Caracterizao do escoamento turbulento em canais com vegetao emersa rgida

Characterization of turbulent flow within boundaries covered by rigid and emergent vegetation

Ana M. Ricardo¹, Mrio J. Franca², Rui M.L. Ferreira³

---

1 -Associado APRH /// Doutoranda /// CEHIDRO, Instituto Superior Tcnico, Universidade de Lisboa, Portugal & Laboratory of Hydraulic Constructions, EPFLausanne, Switzerland
2 -Associado APRH /// PhD, Research and Teaching Associate /// Laboratory of Hydraulic Constructions, EPFLausanne, Switzerland
3 - Dr, Professor Auxiliar /// CEHIDRO, Instituto Superior Tcnico, Universidade de Lisboa, Portugal

---

RESUMO
Os escoamentos no interior de zonas povoadas de hastes rgidas e emersas apresentam grande heterogeneidade, pelo que a sua caracterizao requer uma formulao que incorpore explicitamente essa variabilidade espacial.
Este trabalho pretende caracterizar e quantificar o escoamento no interior de zonas com vegetao rgida e emersa, bem como quantificar as foras, por unidade de rea, que o escoamento exerce nas hastes e no fundo do canal. Para tal, realizaram-se dois ensaios laboratoriais com diferentes densidades de hastes, nos quais se mediram campos de velocidades instantneas com o sistema de medio Particle Image Velocimetry (PIV). No tratamento dos dados foi aplicada a metodologia de mdia dupla espcio-temporal. Desenvolveu-se, ainda, um modelo terico para o clculo da fora aplicada nas hastes e respectivo coeficiente de resistncia.
Os resultados obtidos mostram que as tenses dispersivas no so, em geral, desprezveis face s tenses de Reynolds. Concluiu-se, tambm, que o aumento das tenses dispersivas normais longitudinais permite explicar, parcialmente, o aumento do coeficiente de arrastamento das hastes com o aumento da densidade de hastes.

Palavras-chave: Vegetao emersa rgida, resistncia ao escoamento, PIV, metodologia de mdia dupla espcio-temporal

ABSTRACT
The main characteristic of the flow through rigid stems is the great spatial variability that exists in the inter-stem space, requiring a formulation of the momentum and mass conservation equations that take into account such variability.
This work is aimed at characterizating and quantificating the flow within vegetated areas susceptible to be simulated by dense arrays of vertical emergent stems as well as at quantificating the forces, per unit bed area, acting on the stems and on the bed boundary.
To meet the objectives, two experimental tests were carried out, with different densities of stems, to acquire velocity fields with a Particle Image Velocimetry system (PIV). The treatment of the data was done with the Double Averaging methodology (DAM). A theoretical model for the calculation of the drag force exerted on the stems and respective coefficient was developed.
The results reveal that the contribution of form-induced stresses is of the order of magnitude of the contribution of Reynolds stresses. The analysis of form-induced stresses helps to explain the increase of the drag coefficient when the stem density increases.

Keywords: Rigid and emergent vegetation, flow resistance, PIV, Double-Averaging Methodology.

 

Finnigan, J. (2000) –Turbulence in plant canopies. An. Re. Fluid Mechanics, 32, 519 - 571.

Ferreira, R.M.L., Ferreira, L.M., Ricardo, A.M.& Franca, M.J. (2010) - Impacts of sand transport on flow variables and dissolved oxygen in gravel-bed streams suitable salmonid spawning, River Research and Applications, 26 (10), 414–438.

Ferreira, R. M. L., Ricardo, A. M. & Franca, M. J. (2009) - Discussion of ’Laboratory investigation of mean drag in a random array of rigid, emergent cylinders’ by Heydi M. Nepf and Yukie Tanino, Journal of Hydraulic Engineering, vol. 134, n. 1, 2008. Journal of Hydraulic Engineering,135 (8).

Franca M.J., Ferreira R.M.L. & Lemmin U. (2008) - Parameterization of the logarithmic layer of double-averaged streamwise velocity profiles in gravel-bed river flows. Advances in Water Resources, 31 (6), 915-925.

Gimnez-Curto, L.& Corniero Lera M (1996) – Oscillating turbulent flow over very rough surfaces. J. Geophys. Res., 101, 20745 - 20758.

James, C. S., Birkhead, A. L., Jordanova, A. A. &; O’sullivan, J. J. (2004) – Flow resistance of emergent vegetation. Journal of Hydraulic Research, 42 (4), 390 - 398.

Kadlec, R. H. (1990) - Overland flow in wetlands: Vegetation resistance. Journal of Hydraulic Engineering, 116 (5), 691 - 705.

Lopez, F. & Garca, M. (1998) - Open-channel flow through simulated vegetation: Suspended sediment transport modelling. Water Resources Research, 34 (9), 2341 - 2352.

Nepf, H. (1999) – Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research, 35 (2), 479–489.

Nepf, H. M. & Vivoni, E. R. (1999) –Turbulence structure in depth-limited, vegetated flow: transition between emergent and submerged regimes, in Conference Proceedings of the 28th International IARH Conference. Graz, Austria.

Nikora, V., Goring, D., Mcewan, I. & Griffiths, G. (2001) - Spatially averaged open-channel flow over a rough bed. Journal of Hydraulic Engineering, 127 (2), 123 -133.

Raupach, M. R. (1992) - Drag and drag partition on rough surfaces. Boundary-Layer Meteorology, 60, 375 - 395.

Raupach, M. R., Coppin, P. A. & Legg, B. J. (1986) - Experiments on scalar dispersion within a model plant canopy part I: turbulence structure. Boundary Layer Meteorology, 35, 21 - 52.

Ricardo, A. M. (2008) – Caracterizao do escoamento turbulento em canais com vegetao emersa rgida. Aplicao ao estudo da resistncia hidrulica. Tese de Mestrado, Instituto Superior Tcnico - Universidade Tcnica de Lisboa, Portugal.

Tanino, Y. & Nepf, H. M. (2008) – Laboratory investigation of mean drag in a random array of rigid, emergent cylinders. Journal of Hydraulic Engineering 134 (1),34 – 41.

White, B. L. & Nepf, H. M. (2003) - Scalar transport in random cylinder arrays at moderate Reynolds number. Journal of Fluid Mechanics, 487, 43 - 79.

Yalin, MS (1972) – Mechanics of sediment transport. 290p. Pergamon Press, Oxford. ISBN: 0 08 021162 3

.