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 R40: Focus Session: Reconfiguration II |
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Chair: Mitul Luhar, University of Southern California Room: Sheraton Back Bay D |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R40.00001: On the dynamics of flexible blades in oscillatory flows Mitul Luhar, Heidi Nepf We present an experimental and numerical study that describes the motion of flexible blades, scaled to be dynamically similar to natural aquatic plants, forced by wave-induced oscillatory flows. For the conditions tested, blade motion is governed primarily by two dimensionless variables: the Cauchy number, $Ca$, which represents the ratio of the hydrodynamic forcing to the restoring force due to blade stiffness, and the ratio of the blade length to the wave orbital excursion, $L$. For flexible blades with $Ca>>1$, the relationship between drag and velocity can be described by two different scaling laws. For large excursions ($L<<1)$, the flow resembles a unidirectional current and the scaling laws developed for steady-flow reconfiguration studies hold. For small excursions ($L>>1)$, the beam equations may be linearized and a different scaling law for drag applies. The numerical model employs the Morison force formulation, and adequately reproduces the experimentally measured forces and blade postures. In some cases with $Ca\sim O(1)$, the measured forces generated by the flexible blades exceed those generated by rigid blades. Observations of blade motion suggest that this behavior is related to an unsteady vortex shedding event, which the quasi-steady numerical model cannot reproduce. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R40.00002: Mechanisms and models which govern bending and reconfiguring of trees under water flow action Catherine Wilson, Peter Whittaker A model for predicting the drag and reconfiguration of flexible vegetation under hydrodynamic loading is presented. The model is based on a refined ``vegetative'' Cauchy number to incorporate the magnitude and rate of a tree's reconfiguration. In addition, analysis of data from a tree drag force study conducted at the Canal de Experiencias Hidrodinamicas de El Pardo, Madrid, is also presented. This data enables the analysis of the frontal projected and the side-view areas as well as the bending angle of the main tree stems over a full range of velocities. New physical mechanisms which link tree posture, permeability, and the Reconfiguration number-Cauchy number relationship for various key stages of reconfiguration are proposed. These mechanisms are mainly developed for multi-stem trees in their foliated state. In addition direct comparisons of mechanisms for foliated and defoliated states are also presented. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R40.00003: Reconfiguration of a flexible fiber immersed in a 2D dense granular flow close to the jamming transition Evelyne Kolb, Nicolas Algarra, Damien Vandembroucq, Arnaud Lazarus We propose a new fluid/structure interaction in the unusual case of a dense granular medium flowing against an elastic fibre acting as a flexible intruder. We experimentally studied the deflection of a mylar flexible beam clamped at one side, the other free side facing a 2D granular flow in a horizontal cell moving at a constant velocity. We investigated the reconfiguration of the fibre as a function of the fibre's rigidity and of the granular packing fraction close but below the jamming in 2D. Imposing the fibre geometry like its length or thickness sets the critical buckling force the fibre is able to resist if it was not supported by lateral grains, while increasing the granular packing fraction might laterally consolidate the fibre and prevent it from buckling. But on the other side, the approach to jamming transition by increasing the granular packing fraction will be characterized by a dramatically increasing size of the cluster of connected grains forming a solid block acting against the fibre, which might promote the fibre's deflection. Thus, we investigated the granular flow fields, the fibre's deflexion as well as the forces experienced by the fibre and compared them with theoretical predictions from elastica for different loadings along the fibre. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R40.00004: Effect of herbivore damage on broad leaf motion in wind Nicholas Burnett, Adit Kothari Terrestrial plants regularly experience wind that imposes aerodynamic forces on the plants' leaves. Passive leaf motion (e.g. fluttering) and reconfiguration (e.g. rolling into a cone shape) in wind can affect the drag on the leaf. In the study of passive leaf motion in wind, little attention has been given to the effect of herbivory. Herbivores may alter leaf motion in wind by making holes in the leaf. Also, a small herbivore (e.g. snail) on a leaf can act as a point mass, thereby affecting the leaf's motion in wind. Conversely, accelerations imposed on an herbivore sitting on a leaf by the moving leaf may serve as a defense by dislodging the herbivore. In the present study, we investigated how point masses (\textgreater 1 g) and holes in leaves of the tuliptree affected passive leaf motion in turbulent winds of 1 and 5 m s$^{\mathrm{-1}}$. Leaf motion was unaffected by holes in the leaf surface (about 10{\%} of leaf area), but an herbivore's mass significantly damped the accelerations of fluttering leaves. These results suggest that an herbivore's mass, but not the damage it inflicts, can affect leaf motion in the wind. Furthermore, the damping of leaf fluttering from an herbivore's mass may prevent passive leaf motions from being an effective herbivore defense. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R40.00005: The reconfiguration of broad leaves in strong winds and currents Laura Miller, Alex Hoover, Jeremy Marzuola Flexible plants, fungi, and sessile animals are thought to reconfigure in the wind and water to reduce the drag forces that act upon them. In strong winds, for example, leaves roll up into cone shapes that reduce flutter and drag when compared to paper cut-outs with similar shape and flexibility. Simple mathematical models of a flexible beam immersed in a two-dimensional flow will also exhibit this behavior. What is less understood is how the mechanical properties of a two-dimensional leaf in a three-dimensional flow will passively allow roll up and reduce drag and flutter. In this project, we use computational fluid dynamics and particle image velocimetry to determine how leaves roll up into drag reducing shapes in extreme conditions. Force and flow measurements are taken on real broad leaves and simplified physical models. Corresponding numerical simulations using the immersed boundary method are used to understand which features of the flexible leaves result in proper reconfiguration and drag reduction. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R40.00006: Reconfiguration parameters for drag of flexible cylindrical elements Chapman John, Bruce Wilson, John Gulliver This presentation compares parameters that characterize reconfiguration effects on flow resistance and drag. The drag forces occurring on flexible bluff bodies are different from the drag occurring on rigid bluff bodies due to reconfiguration. Drag force data, collected using a torque sensor in a flume, for simple cylindrical obstructions of the same shape and size but with different flexibility is used to fit drag parameters. The key parameter evaluated is a reference velocity factor $u$ to account for drag reduction due to reconfiguration, similar to a Vogel exponent. Our equations preserves the traditional exponent of the drag relationship, but places a factor onto the drag coefficient for flexible elements, rather than a Vogel exponent arrangement applied to the flow velocity. Additionally we relate the reference velocity factor$ u$ to the modulus of elasticity of the material through the Cauchy Number. The use of a reference velocity factor $u$ in place of a Vogel exponent appears viable to account for how the drag forces are altered by reconfiguration. The proposed formulation for drag reduction is more consistently estimated for the range of flexibilities in this study. Unfortunately, the mechanical properties of vegetation are not often readily available for reconfiguration relationships to the elastic modulus of vegetation to be of immediate practical use. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R40.00007: Enhancing wind turbines efficiency with passive reconfiguration of flexible blades Vincent P A Cognet, Benjamin Thiria, Sylvain Courrech du Pont Nature provides excellent examples where flexible materials are advantageous in a fluid stream. By folding, leaves decrease the drag caused by air stream; and birds' flapping is much more efficient with flexible wings. Motivated by this, we investigate the effect of flexible blades on the performance of a wind turbine. The effect of chordwise flexible blades is studied both experimentally and theoretically on a small wind turbine in steady state. Four parameters are varied: the wind velocity, the resisting torque, the pitch angle, and the blade’s bending modulus. We find an optimum efficiency with respect to the bending modulus. By tuning our four parameters, the wind turbine with flexible blades has a high-efficiency range significantly larger than rigid blades', and, furthermore enhances the operating range. These results are all the more important as one of the current issues concerning wind turbines is the enlargement of their operating range. To explain these results, we propose a simple two-dimensional model by discretising the blade along the radius. We take into account the variation of drag and lift coefficients with the bending ability. This model matches experimental observations and demonstrates the contribution of the reconfiguration of the blade. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R40.00008: Drag reduction by reconfiguration in gorgonians Julien Derr, Annemiek J. M. Cornelissen, Claude Bouchon, Yolande Bouchon, J\'er\^ome Fournier, Lionel Moisan, Pascal Jean Lopez, St\'ephane Douady Gorgonians are polyp colonies over a flexible branched skeleton. Attached to the coral reefs, they are under the continuous oscillations of the swell. We investigate experimentally the drag, under continuous force traction, of {\it Gorgonia Ventalina}, which is particular as its branches are highly reconnected to form a flat net (see fan), perpendicular to the swell, and compare it with another branched species (candelstick). We observe a drag which is linear with speed, indicating a strong reconfiguration, which we also documented by imaging the gorgon shape, and transients showing that the gorgon do not always evolve along quasi-static curves. Depending on the size and shape of the gorgon, we observe different details, from a more rigid small gorgon to a flexible long one. A large gorgon with detached fingers, closing on themselves under the current, presents characteristics surprisingly close to a rigid candlestick one, with not much reconfiguration. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R40.00009: Interfering with the wake of cylinder by flexible filaments Alfredo Pinelli, Mohammad Omidyeganeh This work is the very first attempt to understand and optimize the configuration of flexible filaments placed on the lee side of a bluff body to manipulate flow transitions and bifurcations. It is found that the presence of a sparse set of flexible filaments on the lee side of a cylinder can interfere with the 2D-3D transition process resulting in elongation of recirculation bubble, inhibition of higher order unstable modes, and narrowing the global energy content about a particular shedding frequency. Filaments become effective when spacing between them is smaller than the dominant unstable mode at each particular Reynolds number, i.e. A and B modes. In another study, by a particular arrangement the reconfigured filaments can reduce pressure fluctuations in the wake and drop lift flluctuations significantly ($\simeq 80\%$). [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R40.00010: Catenaries in viscous fluid James Hanna, Brato Chakrabarti Slender structures live in fluid flows across many scales, from towed instruments to plant blades to microfluidic valves. The present work details a simple model of a flexible structure in a uniform flow. We present analytical solutions for the translating, axially flowing equilibria of strings subjected to a uniform body force and linear drag forces. This is an extension of the classical catenaries to a five-parameter family of solutions, represented as trajectories in angle-curvature ``phase space.'' Limiting cases include neutrally buoyant towed cables and freely sedimenting flexible filaments. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R40.00011: Numerical investigation on optimizing blast wave focusing effects for multiple munitions Shi Qiu, Veronica Eliasson The phenomenon of blast wave focusing onto a specified target has been studied. Simulations were performed in which multiple munitions were placed in a circular pattern around a target. The number of munitions was varied through multiple cases while the total energy distributed among all munitions was held constant. Previous research shows that there exits an optimal number of munitions to produce the most extreme conditions at the target while simultaneously reducing collateral damage. Two numerical approaches, inviscid Euler equations and geometrical shock dynamics were used to study the interaction between blast waves in order to further investigate the optimization problem. To generate initial conditions for geometrical shock dynamics simulations on interaction between blast waves, it was found that a transition point between regular reflection and irregular reflection needs to be determined in advance. Both experimental and theoretical investigation is included to study the transition condition. Optimization strategy for focusing blast waves is also discussed. [Preview Abstract] |
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