Tige | EPFL Graph Search

vignette|Schéma représentatif de l'anatomie d'une plante. 1. Appareil caulinaire (6 : tige, 9 : entre-nœud) 2. Appareil racinaire 3. Collet. vignette|L'observation en microscopie à fluorescence de cette coupe transversale montre les caractéristiques anatomiques d'une tige : organe végétal à symétrie axiale, avec des faisceaux cribro-vasculaires constitués de xylème primaire à différenciation centrifuge (5) et de phloème primaire centripète (6). L’absence de formations secondaires et l'endoderme en U (3) caractérisent une tige de Monocotylédone. La présence d'un parenchyme cortical amylifère (2) bien développé, d'un épiderme (1) peu différencié (ni cuticule, ni stomates) et l'absence de tout tissu de soutien caractérisent un rhizome, en l'occurrence celui du muguet. La tige est chez les plantes, l'axe végétatif polarisé, généralement aérien ou souterrain (tubercule caulinaire, rhizome), qui prolonge la racine et porte les bourgeons et les feuilles. La tige se ramifie généralement en br

Hydrodynamics of turbulent flows within arrays of circular cylinders

Ana Margarida Da Costa Ricardo

In rivers riparian vegetation can have not only strong interaction with the flow but also with the ecological services of the aquatic system. In wetlands and along riverbanks often tree-like vegetation with quite stiff trunks can be found. The study of the flow in the space between plant stems is highly relevant since it influences the deposition of suspended sediment and the transport of pollutants or nutrients. With her comprehensive experimental research study Mrs Dr. Ana Margarida da Costa Ricardo carried out a novel detailed spatial characterization, at the inter-stem scale, of the turbulent flow within arrays of emergent, rigid and cylindrical stems, randomly placed with constant and varying density in a flume. Mrs Dr. da Costa Ricardo performed detailed measurement of instantaneous velocities with a 2D Particle Image Velocimetry (PIV) and a 3D Laser Doppler Anemometry (LDA). Part of the data was treated with Double-Averaging Methodology (DAM), used as an up-scaling technique to deal with heterogeneous flows. Mrs Dr. da Costa Ricardo could reveal that the drag force per submerged stem length is not correlated directly with the stem areal number density and the stem Reynolds number. The drag force depends on the longitudinal variation of the stem areal number-density. It could be observed that the decrease of the latter is associated to larger flow resistance and that the drag coefficient increases with the relative roughness, revealing an influence of the bed on the definition of the flow structure. The importance of vortex shedding and unsteady separation of the flow on the stems could be highlighted. Finally Mrs Dr. da Costa Ricardo gives a new insight in the dissipation rate of turbulent kinetic energy (TKE). She could contribute significantly to a better understanding of the complex flow within random arrays of rigid and emergent stems, at the inter-stem scale regarding flow resistance, budget of TKE and computation of dissipation rate of TKE.

EPFL - LCH2014

Turbulent flow within a random array of rigid, emergent stems : Laboratory characterization of the drag coefficient

Ana Margarida Da Costa Ricardo, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Within the bioengineering framework, designing a non-erodible channel is a complex fluid dynamics problem as it involves knowing the drag exerted on the boundary, the drag exerted on the plant stems and the overall friction slope. Most of the existing design criteria employ resistance formulas such as Manning’s, calibrated ad hoc. Moving toward physically based design criteria, progresses have been made in the characterization of 3D flows over irregular boundaries, mainly due to the application of Double-Averaging methods (D-AM), which are a particular form of upscaling. Such methods are especially pertinent for the characterization of the flow within and in the near vicinity of plant canopies.This work is aimed at the determination of the drag coefficient associated to rigid emergent stems in turbulent flows. Specific objectives are i) the detailed characterization and quantification of the flow within vegetated areas susceptible to be simulated by dense arrays of vertical emergent stems and ii) the independent quantification of the forces, per unit bed area, acting on the stems and on the bed boundary. Both objectives concur for a better knowledge of the flow resistance in wetlands and vegetated areas in general. To meet the proposed objectives, conditions similar to those found in nature were reproduced in laboratory facilities, using Particle Image Velocimetry (PIV).Since actual wetlands exhibit patchiness and spatial variability in stem density, the experiments featured a periodic distribution of stem densities with minimum and maximum values of 400 and 1600 stems/m2, respectively. The treatment of the data was done with the Double-Averaging methodology (D-AM), to account for the great spatial variability of the flow. The results reveal that the contribution of form-induced stresses, namely longitudinal and shear stresses is of the order of magnitude of the contribution of Reynolds stresses. Hence, in general, this stresses cannot be neglected. The analysis of form-induced stresses helps to explain the increase of the drag coefficient when the stem density increases.

TPU2013

The terms of turbulent kinetic energy budget within random arrays of emergent cylinders

Ana Margarida Da Costa Ricardo, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

This article is aimed at quantifying and discussing the relative magnitude of key terms of the equation of conservation of turbulent kinetic energy (TKE) in the inter-stem space of a flow within arrays of vertical cylinders simulating plant stems of emergent and rigid vegetation. The spatial distribution of turbulent quantities and mean flow variables are influenced by two fundamental space scales, the diameter of the stems and the local stem areal number-density. Both may vary considerably since the areal distribution of plant stems in natural systems is generally not homogeneous; they are often arranged in alternating sparse and dense patches. The magnitude of the terms of the budget of TKE in the inter-stem space has seldom been quantified experimentally and is currently not well known. This work addresses this research need. New databases, consisting of three-component LDA velocity series and two-component PIV velocity maps, obtained in carefully controlled laboratory conditions, were used to calculate the terms of the TKE budget. The physical system comprises random arrays of rigid and emergent cylinders with longitudinally varying areal number-density. It is verified that the main source of TKE is vortex shedding from individual cylinders. The rates of production and dissipation are not in equilibrium. Regions with negative production, a previously unreported feature, are identified. Turbulent transport is particularly important along the von Karman vortex street. Convective rate of change of TKE and pressure diffusion are most relevant in the vicinity of the cylinders.

American Geophysical Union2014

Turbulent flow within a random array of rigid, emergent stems : Laboratory characterization of the drag coefficient

Ana Margarida Da Costa Ricardo, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

TPU2013

Hydrodynamics of turbulent flows within arrays of circular cylinders

Ana Margarida Da Costa Ricardo

EPFL - LCH2014