On the air permeability of Populus pit

Sap hydrodynamics in vascular cells of trees seems to be controlled by small membranes called pits. Understanding how the pit junctions regulate the sap flow and stop embolism by cavitation is today a challenging issue. The hypothesis that the pit porosity adjusts the flow under negative pressure and stops the air bubble diffusion need to be validated. In this talk, we will present the experimental results on Populus trees that support the idea that pits operate "passively" in a biological point of view. This work is based on atomic force microscope (AFM) experiments, which have been realised to measure quantitatively the mechanical properties of pits at the nanoscale

Sand transport over a barchan dune

The present work investigates an important and yet unsolved issue: the relationship between the sand flux and the fluid shear stress over a spatially varying bed of particles. It is now recognized that over such a bed, the particle flux is not in equilibrium with the shear stress: there is some lag related to the particle inertia or particle settling. A confident modelling of this relaxation phenomena and the corresponding length scales, is still lacking (Charru, Andreotti and Claudin 2013). This question is investigated here from experiments on barchan dunes in a closed-conduit water flow. From visualizations with a high-speed camera and a tracking algorithm, the particle motion over the whole dune surface is determined: particle trajectories, local velocity and surface density of the moving particles, and local particle flux. The relationship between the local particle flux and local shear stress (estimated from previous analyses) is investigated. Surprisingly, the particle flux appears to be out-ofequilibrium over the whole dune surface, with saturation length much larger than expected

On the large difference between Benjamin's and Hanratty's formulations of perturbed flow over uneven terrain

Flow over an uneven terrain is a complex phenomenon that requires a chain of approximations in order to be studied. In addition to modelling the intricacies of turbulence if present, the problem is classically first linearized about a flat bottom and a locally parallel flow, and then asymptotically approximated into an interactive representation that couples a boundary layer and an irrotational region through an intermediate inviscid but rotational layer. The first of these steps produces a stationary Orr–Sommerfeld equation; since this is a one-dimensional problem comparatively easy for any computer, it would seem appropriate today to forgo the second sweep of approximation and solve the Orr–Sommerfeld problem numerically. However, the results are inconsistent! It appears that the asymptotic approximation tacitly restores some of the original problem’s non-parallelism. In order to provide consistent results, Benjamin’s version of the Orr–Sommerfeld equation needs to be modified into Hanratty’s. The large difference between Benjamin’s and Hanratty’s formulations, which arises in some wavenumber ranges but not in others, is here explained through an asymptotic analysis based on the concept of admittance and on the symmetry transformations of the boundary layer. A compact and accurate analytical formula is provided for the wavenumber range of maximum laminar shear-stress response. We highlight that the maximum turbulent shear-stress response occurs in the quasi-laminar regime at a Reynolds-independent wavenumber, contrary to the maximum laminar shear-stress response whose wavenumber scales with a power of the boundary-layer thickness. A numerical computation involving an eddy-viscosity model provides a warning against the inaccuracy of such a model. We emphasize that the range $k\unicode[STIX]{x1D708}/u_{\unicode[STIX]{x1D70F}} of the spectrum remains essentially unexplored, and that the question is still open whether a fully developed turbulent regime, similar to the one predicted by an eddy-viscosity model, ever exists for open flow even in the limit of infinite wavelength

Determination of the sand flux over a barchan dune under a water flow

The modelling of sand transport by uid ows is crucial in many environmental as well as industrial problems. The present work investigates an important and yet unsolved issue: the relationship between the particle ux and the shear stress exerted by the uid ow at the bed surface

Une histoire de l'Institut de Mécanique des Fluides de Toulouse de 1913 à 1970

En 1913, Charles Camichel crée le laboratoire d'hydraulique de l'institut électrotechnique de l'université de Toulouse, institut qu'il avait lui-même fondé six ans plus tôt. Ce laboratoire acquiert rapidement une grande notoriété pour la qualité de sa recherche expérimentale et pour ses succès dans la transposition à l'hydraulique des concepts et méthodes de la physique et de la mécanique. En 1930, le ministère de l'Air lui associe un institut de mécanique des fluides, et finance une grande soufflerie. Un siècle plus tard, l'IMFT a diversifié son activité dans de nombreux domaines autour d'enjeux scientifiques et d'applications très variés. Il est l'un des plus grands laboratoires de sa discipline à l'échelle internationale. Cet article en retrace l'histoire jusqu'un 1970, en insistant plus particulièrement sur les remarquables contributions scientifiques des trois premières décennie

Subaqueous Barchan dunes in turbulent shear flow. Part 2: Fluid flow.

We report an experimental study of the turbulent flow above a barchan dune in a channel, from particle image velocimetry measurements, for Reynolds numbers ranging from 9000, just below the threshold for particle motion, up to 24 000, where the dune moves. Two calculations of the speed-up over the dune are compared, the usual ‘same-elevation’ and the more relevant ‘Lagrangian’, showing that the latter is smaller by a factor of two. The two-layer structure of the flow disturbance – an essentially inviscid outer layer and a turbulent inner layer of thickness δi – is assessed. In the outer layer, streamline curvature is shown to be responsible for half of the Lagrangian speed-up, from the comparison of the velocity measurements with two Bernoulli calculations. In the inner layer, detailed measurements of the velocity and stresses are provided, down to γ+ ≈ 1, and the momentum budget is discussed. The Reynolds shear stress decreases monotonically towards the dune surface, according to the standard mixing-length closure, whereas the total shear stress increases strongly in the viscous sublayer. Along the dune surface, the shear stress increases up to the crest where it reaches twice its unperturbed value. A good estimate of the surface stress is provided by a parabolic fit of the inner velocity profile matching the outer flow at γd ≈ δi. Doubling the Reynolds number, the surface shear stress and the speed-up decrease by ∼30 %. The implications of these results on the dune motion, presented in Part 1 of this study (Franklin & Charru, J. Fluid Mech., vol. 675, 2011, pp. 199–222), are finally discussed

Droplet entrainment from a single wave propagating in a stratified air-water pipe flow

We investigate the distribution of sizes and velocities of droplets detached from a single roll wave propagating in a pipe. The wave is induced by a pulse in liquid flowrate in a stratified air-water flow. Due to the shear applied on the crest of the wave by the faster gas stream, droplets are formed and entrained by the gas. Detection and tracking of the droplets is performed by image processing. The distributions of sizes and velocities of the droplets are determined in the center of the pipe for various distances to the roll wave and compared to those measured for higher gas flowrate when droplets are produced from all the roll waves formed naturally at the gas-liquid interface without external forcing

L’Institut de Mécanique des Fluides de Toulouse : 100 ans d’histoire

L’institut de mécanique des fluides de Toulouse (IMFT), issu d’un laboratoire d’hydraulique créé en 1913, a célébré son centenaire en 2016 - année marquant ses 50 ans d’association au CNRS

Dynamics of a slowly-varying sand bed in a circular pipe

The long wave-length dynamics and stability of a bed of sand occupying the lower segment of a circular pipe are studied analytically up to first-order in the small parameter characterizing the slope of the bed. The bed is assumed to be at rest, with at most a thin sand layer (the bedload) moving at the sheared interface. When the sand bed is plane, with depth independent of position z along the axis of the pipe, the velocity of the liquid is known from previous studies of stratified laminar flow of two Newtonian liquids (the lower one with infinite viscosity representing the sand bed). When the depth of the sand bed varies with z, secondary flows develop in the cross-sectional (x, y) plane, and these are computed numerically, assuming that the sand bed remains a straight horizontal line in the cross-sectional plane. The mean shear stress acting on the perturbed sand bed is then determined both from the computed secondary flows and by means of the averaged equations of Luchini and Charru. The latter approach requires knowledge only of the flow over the unperturbed, flat sand bed, combined with an accurate approximation of the distribution of the perturbed stresses between the pipe wall and the sand bed. The perturbed stresses determined by the two methods agree well with each other. Using these stresses, it is then possible to apply standard theories of bed stability to determine the balance between the destabilizing effect of inertial (out-of-phase) stresses and the stabilizing effects of gravity and relaxation of the particle flux, and various examples are considered

Quasilaminar regime in the linear response of a turbulent flow to wall waviness

The linear response of the wall-shear stress of a turbulent flow to wall waviness is analyzed in the context of a comparison between existing experiments, direct numerical simulations, and analytical approximations. The spectral region where the response is largest is found to be amenable to a simplified quasilaminar analysis. The end result is a parameterless description of this phenomenon that completely captures its physics in a single analytical formula, a Padé approximation of the response function