Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M40: Focus Session: Reconfiguration I |
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Chair: Frederick Gosselin, Ecole Polytechnique de Montreal Room: Sheraton Back Bay D |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M40.00001: Reconfiguration of a flexible flat plate under snow loading Frédérick Gosselin, Emmanuel de Langre Snow and wind constitute two of the main sources of mechanical loading on terrestrial plants. Plants bend and twist with large amplitude to bear these loads. For the past ten years, various authors have sought to decompose the problem of plant reconfiguration under fluid flow into its fundamental mechanical ingredients by studying the reconfiguration of simple flexible structures such as beams, plates, rods and strips. Here, we adopt a similar approach to these studies and consider the snow interception of a flexible flat plate. We performed two sets of experiments on thin flexible rectangular plates supported at their center: in the first one, a plate was subjected to real snowing events; in the second one, a plate was loaded with glass beads acting as a granular media similar to snow. Moreover, a theoretical model coupling the Elastica formulation to a loading with a set angle of repose is developed. The model is found to be in good agreement with the experiments on glass beads. Asymptotic scaling laws can be found similarly to the Vogel exponents of reconfiguring structures. For the real snow loading, it is found that the cohesive force in snow which is highly dependent on the snow temperature complicate things greatly. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M40.00002: Large Deformation of an Elastic Rod with Structural Anisotropy Subjected to Fluid Flow Masoud Hassani, Njuki Mureithi, Frederick Gosselin In the present work, we seek to understand the fundamental mechanisms of three-dimensional reconfiguration of plants by studying the large deformation of a flexible rod in fluid flow. Flexible rods made of Polyurethane foam and reinforced with Nylon fibers are tested in a wind tunnel. The rods have bending-torsion coupling which induces a torsional deformation during asymmetric bending. A mathematical model is also developed by coupling the Kirchhoff rod theory with a semi-empirical drag formulation. Different alignments of the material frame with respect to the flow direction and a range of structural properties are considered to study their effect on the deformation of the flexible rod and its drag scaling. Results show that twisting causes the flexible rods to reorient and bend with the minimum bending rigidity. It is also found that the drag scaling of the rod in the large deformation regime is not affected by torsion. Finally, using a proper set of dimensionless numbers, the state of a bending and twisting rod is characterized as a beam undergoing a pure bending deformation. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M40.00003: Numerical simulation of the dynamics of a flexible cantilevered plate subjected to a perpendicular or a parallel fluid flow Fabien Sansas, Eric Laurrendeau, Frederick Gosselin We focus on the dynamic deformation of a cantilevered flexible plate immersed in a fluid flow. The following two-dimensional numerical study is based on a large deformation beam model solved by finite difference. The fluid is computed by an in-house Arbitrary Eulerian-Lagrangian (ALE) compressible CFD solver. After a validation and verification procedures confirming second order accuracy, two different cases are examined. The first case serves as a validation exercise for the coupling procedure with the flow parallel to the plate: its leading edge is clamped and the trailing end is free. This case models a flapping flag for which the stability of the plate as a function of its mass and flow velocity are investigated. Different vibration modes are compared to previous numerical and experimental results. The second case is that of a plate clamped at its middle, the flow being perpendicular to its initial shape. The plate deforms by bending in the flow direction. Streamlining and projected area reduction lead to fluid forces reduction but, at some point, dynamic instability occurs. Preliminary results of this instability phenomena are presented, namely the various dynamic behaviours and the trade-offs between streamlining and instability. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M40.00004: Influence of blade motion on mass flux to a model seagrass blade Jiarui Lei, Heidi Nepf Seagrass and other freshwater macrophytes can acquire nutrients from surrounding water through their blades. While we anticipate that blade motion and reconfiguration may impact mass flux at the blade surface, this topic is an area of open discussion and research. We seek to better understand the interaction of individual blades with both unidirectional and oscillatory flows and how this interaction impacts mass flux. The degree of reconfiguration can be quantified by two dimensionless numbers, the Cauchy number, \textit{Ca}, and the buoyancy parameter, $B$. For unidirectional currents ($U)$, a theoretical model for the transfer velocity ($K)$ was constructed assuming the boundary layer on the blade surface remained laminar and developed like that over a flat plate, which predicts $K\propto U^{\mathrm{0.5}}$. When the blades were bent-over, the model predicted the measured flux well; when the blades remained upright, the flux to the blade diminished relative to the model. Preliminary wave experiments show that blade motion increased with wave amplitude, and that there are two distinct regimes. In the first regime (\textit{Ca}\textless 15), the maximum reconfiguration was associated with the peak velocity (wave crest), so that the blade velocity leads the wave velocity by 90 degrees. The second regime occurred when \textit{Ca}\textgreater 15. In this regime, the phase difference was approximately zero and the blade moved passively with the wave. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M40.00005: Beds of reconfigurable angled hairs rectify Stokes flows Jose Alvarado, Jean Comtet, Anette Peko Hosoi Biological tissues such as intestines, blood vessels, kidneys, and tongues are coated with beds of passive, elongated, hair-like protrusions such as microvilli, hyaluronans, primary cilia, and papillae. Stresses from fluid flows can bend deformable hairs, but this reconfiguration can in turn affect confined fluid flows. We investigate this elastoviscous coupling by developing a biomimetic model system of elastomer hair beds subject to shear-driven Stokes flows in a Taylor-Couette geometry. We characterize this system with a theoretical model which shows that reconfiguration of hair beds is controlled by a single elastoviscous number. Hair bending results in an apparent shear thinning because the hair tip lowers toward the base and thus widens the gap through which fluid flows. When hairs are cantilevered at an angle subnormal to the surface, flow against the grain bends hairs away from the base and thus narrows the gap. Beds of reconfigurable angled hairs can thus give rise to an asymmetric flow impedance at arbitrarily low Reynolds number and could therefore function as a microfluidic rectifier. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M40.00006: Wave-forced reconfiguration of a 2D artificial canopy Sylvie Barsu, Delphine Doppler, Nicolas Rivière, Michel Lance Blades inside aquatic vegetation canopies show collective motion when submitted to a water flow. Coherent deformation waves might be observed under given flow conditions, which might enhance mass and sediment transfers between the canopy and surrounding flow, thus impacting the plants development. However, most studies have been focused on the flow velocity while the cover motion has been far less studied. Here we present experimental results about the dynamic reconfiguration of a single array of PVC blades in a wave flume. The oscillations of the blades are imaged while the water level is separately measured using resistive probes. A delayed coherent wave motion is observed within the canopy, as a response to the oscillatory flow. The associated transfer function (amplitude, phase, wave speed) is built by correlating blade displacements and water local velocity time series. The canopy-flow interaction is then modelled by a simple linear damped oscillator chain whose parameters are deduced from experiments. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M40.00007: Drag reduction of flexible beams in shear flow Tristan Leclercq, Emmanuel de Langre Flexible systems bending in steady flows are known to experience a lesser drag compared to their rigid counterpart. This effect can be quantified by the Vogel exponent $\nu<0$ such that the total drag force on the structure increases as $U^{2+\nu}$ instead of the classical quadratic drag-velocity relationship. In this work, an analytical expression of the Vogel exponent of cantilever beams in cross-flow is derived by dimensional analysis, in the case of shear flow with vertical self-similarity. Numerical simulations are also performed and show excellent agreement. The results of the self-similar case provides insight regarding the scaling of drag with respect to the magnitude of the flow in more complex situations. The example of reconfiguration in a Blasius boundary layer is discussed. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M40.00008: Drag reduction by reconfiguration of a full tree in a wind tunnel Emmanuel de Langre, Loic Tadrist, Tristan Leclercq, Pascal Hemon, Xavier Amandolese, Marc Saudreau, Andre Marquier, Graham Knapp, Olivier Flamand The results of drag measurements performed on a full 3 m-tall cherry tree in an atmospheric wind tunnel are presented. The drag on the trunk alone is shown to increase quadratically with the velocity of the flow, as expected, but the drag on the whole tree with branches and leaves follows a smaller power law with velocity, after the reconfiguration of most leaves. The transition from the quadratic law to a linear increase of the drag of the leaves with the magnitude of the flow is observed. Data is also obtained on moment loading on the base of the tree showing also an effect of the reconfiguration. Finally, these results are compared with current models of drag reduction by reconfiguration. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M40.00009: Impacts of the Reconfiguration of Flexible Plants on the Structure of Turbulence and Dispersion of Particles Ying Pan, Elizabeth Follett, Marcelo Chamecki, Heidi Nepf, Scott Isard The effect of a canopy of sufficient density on the flow can be parameterized as a distributed drag calculated as the product of the square of velocity, the canopy density and a drag coefficient. Field and laboratory experimental data suggest that the reconfiguration of flexible plants leads to a power-law dependence of the drag coefficient on velocity. For large-eddy simulation (LES) resolving the canopy layer, we represent the effect of reconfiguration by modeling the drag coefficient as a constant when velocity is low and a power-law function of velocity when velocity is above a threshold. For a constrained mean vertical momentum flux at the canopy top, changing the power-law exponent (known as the Vogel number) has negligible effects on LES predictions of the total vertical momentum flux. However, skewness of velocity components, the strength of sweeps and ejections and the fractions of vertical momentum flux transported in different event quadrants are highly sensitive to changes in the Vogel number. These changes in the structure of turbulence have profound impacts on the dispersion of particles within and above the canopy. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M40.00010: Reconfiguration of tree architecture under the effect of wind, competition for light, and annual growth Christophe Eloy In general, trees have self-similar architectures with longer and thicker branches near the roots. Yet, branch segments grown each year always have approximately the same length. This hierarchy of branch lengths and the whole self-similar characteristics results in fact from a continuous process of growth of new branches and shedding of old ones. To assess how such a process affects tree architecture, a functional-structural mechanically-based model of virtual trees is developed. In this model, trees grow into fractal structures to promote efficient photosynthesis in a competing environment. In addition, branch diameters increase in response to wind-induced loads. The results of this model suggest that most self-similar characteristics of trees can be explained by considering that tree are growing structure able to resist mechanical loads due to wind efficiently. [Preview Abstract] |
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