Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G24: Aerodynamics II |
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Chair: Daniel Maynes, Brigham Young University Room: 30E |
Monday, November 19, 2012 8:00AM - 8:13AM |
G24.00001: How Cristiano Ronaldo performs his knuckleball? Caroline Cohen, Baptiste Darbois Texier, David Quere, Christophe Clanet A soccer ball kicked at very low spin can exhibit a zigzag trajectory. Along its straight path, the ball deviates laterally from about 0.2 m, that is to say one ball diameter. One zig zag happens as the ball travelled about 15 m. As the deviation direction seems unpredictable, this effect is highly annoying for goalkeepers. That why Cristiano Ronaldo and many soccer players are looking for this phenomenon. Those trajectories called knuckleballs are also observed on volleyball and baseball. We study experimentally indoor knuckleballs for different balls varying from soccer balls to smooth spheres. We show that knuckle effect doesn't derive from ball deformations at foot impact or ball seams. Actually, aerodynamic lift forces on a smooth sphere are fluctuating and are responsible for knuckleballs. From this study, we deduce side force intensity exerted on smooth spheres and sport balls for typical game Reynolds number ($Re \sim 10^4 - 10^6$). Finally we discuss required conditions to observe a knuckleball on the sport field. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G24.00002: Nuances between flags fluttering in horizontal and vertical flows Emmanuel Virot, Pascal Hemon, Xavier Amandolese When placed in a constant-velocity flow, a flag starts suddenly to flutter above a critical flow velocity. It appears that the same flag hanged horizontally or vertically exhibits different behaviors. The critical velocity is higher in the vertical case, suggesting an important role of gravity. To investigate the influence of length, width and thickness of paper flags, we perform experiments in both horizontal and vertical wind tunnels. The involvement of elasticity, fluid friction-induced tension or gravity-induced tension on the flag flutter will be discussed. The flag can be also a mean to harvest the wind kinetic energy: in a preliminary approach, we measure and discuss periodic forces that a horizontal flapping flag produces on his shaft. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G24.00003: Why historical east deviation experiments are so difficult to perform? Baptiste Darbois Texier, Caroline Cohen, David Quere, Christophe Clanet From the 17th to the 19th century, a big deal was to prove Earth rotation existence. For this purpose, numerous experimental physicists from Borelli in 1668 to Reich in 1832 tried to measure an eastward deviation of a falling sphere. Reich performed 106 falling experiments on a 158.5 m deep mine pit. The mean eastward deviation of its experiments is 2.8 cm. This value corresponds exactly with the theoretical one predicted by Laplace and Gauss expression at Freiberg latitude where experiments were conducted. While Reich took extreme precautions to perform its experiments, the dispersion on its results is very important. Actually aerodynamic lift forces on a smooth sphere made its free fall non perfectly straight. We understand Reich's results dispersion considering fluctuating lift forces intensity on a smooth sphere at those Reynolds numbers ($Re \sim 10^4 - 10^5$). This study provides a criterion above which we can distinguish between lift force and Coriolis deviation during a free fall experiment. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G24.00004: Linearized boundary conditions at a rough surface Paolo Luchini Linearized boundary conditions are a common numerical tool in any flow problems where the solid wall is nominally flat but the effects of small roughness of height $\epsilon$ are being investigated. Typical are receptivity problems in aerodynamic transition prediction or turbulent flow control. However, two distinguished mathematical limits have to be considered: a ``shallow" limit, where the linearized boundary condition properly applies, involving a family of surfaces that become smoother and smoother as $\epsilon\rightarrow 0$, and a ``small" limit, more closely representative of usually encountered roughness, whose family of surfaces remain geometrically similar to themselves (in particular, retain their slope) as $\epsilon\rightarrow 0$. A connection between the two limits will be established through an analysis of their asymptotic behaviour. As a result, the correct effect of the surface in the ``small'' limit, obtained through a numerical solution of the Stokes equation, will be recast as an equivalent linearized boundary condition modified by a suitable {\it protrusion coefficient} (related to the {\it protrusion height} used years ago in the study of riblets). Quantitative numerical examples of such protrusion coefficients will be provided. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G24.00005: On the lift induced drag in viscous flows Renato Tognaccini, Claudio Marongiu, Makoto Ueno As stated by Spalart ({\it JFM}, 2008): ``An ambition which will have to wait is a rigorous definition of induced drag in viscous flows.'' The idea that there is a link between the aerodynamic force and the Lamb vector, defined as the cross product of fluid vorticity and velocity dates back to Prandtl. Saffman (``Vortex Dynamics,'' 1992) and, more recently, Wu J.-Z. {\it et al.} ({\it JFM}, 2007) suggested an expression of the lift induced drag in terms of vortex force (the volume integral of the Lamb vector). In this paper we analyze the {\em steady} incompressible flow around a 3D lifting body at high Reynolds numbers. The suggested connection between vortex force and induced drag is discussed in detail. In particular, a rigorous definition of the lift induced drag in viscous flows without ambiguities is proposed. A numerical experiment: the analysis of the flow around an elliptic wing will confirm the theoretical analysis. The aerodynamic force and its lift and drag components are computed by integration of the Lamb vector field as obtained by a numerical solution and will be compared with classical expressions. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G24.00006: Drag and near wake characteristics of flat plates normal to the flow with fractal edge geometries Jovan Nedic, Bharath Ganapathisubramani, Christos Vassilicos Past results have suggested that the coefficient of drag and shedding frequencies of regular polygon plates all fall within a very narrow band of values. In this study, we introduce a variety of length-scales into the perimeter of a square plate and study the effects this has on the wake characteristics and overall drag. The perimeter of the plate can be made as long as allowed by practical constraints with as many length-scales as desired under these constraints without changing the area of the plate. A total of eight fractal-perimeter plates were developed, split into two families of different fractal dimension all of which had the same frontal area. It is found that by increasing the number of fractal iterations, thus the perimeter, the drag coefficient increases by up to 10\%. For the family of fractal plates with the higher dimension, it is also found that when the perimeter increases above a certain threshold the drag coefficient drops back again. Furthermore, the shedding frequency remains the same but the intensity of the shedding decreases with increasing fractal dimension. The size of the wake also decreases with increasing fractal dimension and has some dependence on iteration without changing the area of the plate. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G24.00007: Slipping through the water: A study of superhydrophobic hydrofoils Robert Daniello, Kierstin Del Valle, Jonathan Rothstein Superhydrophobic surfaces which are chemically hydrophobic with micron or nanometer scale surface features have been studied for their ability to produce a slip interface which has been shown to affect drag, separation, lift, and vortex dynamics. In this talk, we will consider an experimental study of the effect of slip on lift, drag and stall of hydrofoils with a slip-producing superhydrophobic coating. Direct force measurements of lift and drag will be presented for a series of superhydrophobic and no-slip hydrofoils over a range of Reynolds numbers 3500$<$\textit{Re}$<$35000 and angles of attack from 0 to stall. Effects of slip on the boundary layer, separation and stall will be considered with particle image velocimetry. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G24.00008: Flow over slippery liquid-infused porous surfaces Brian Rosenberg, Gilad Arwatz, Jessica Shang, Alexander Smits Slippery liquid-infused porous surfaces (SLIPS) demonstrate remarkable liquid repellency in addition to high pressure stability and rapid self-healing [T.S. Wong et al., Nature 2011]. The SLIPS surface consists of a thin lubricating film, locked in place on a nano-textured membrane, that permits mobility of the surrounding liquid at the interface. The relaxation of the no-slip boundary condition, in addition to their robustness in high-stress environments, means that these surfaces have promise to reduce drag in engineering flows. Here, we investigate the response of SLIPS in the laminar, transitional, and turbulent flow regimes. Experiments are performed in a Taylor-Couette apparatus, with water as the working fluid, over a wide Reynolds number range and with varying lubricant viscosities. We assess the skin friction properties of SLIPS surfaces and compare it to those of untreated surfaces. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G24.00009: Drag Control through Wrinkling on Curved Surfaces Denis Terwagne, Pedro Reis We present the results of an experimental investigation on the wrinkling of positively curved surfaces and explore their use towards drag reduction applications. In our precision desktop-scale experiments we make use of rapid prototyping techniques to cast samples with custom geometry and material properties out of silicone-based rubbers. Our structures consist of a thin stiff shell that is chemically bonded to a thicker soft substrate. The substrate contains a spherical cavity that can be depressurized, under controlled volume conditions, to compress the ensemble structure. Under this compressive loading, the initially smooth outer-shell develops complex wrinkling patterns. We systematically characterize and quantify the morphology of the various patterns and study the phase diagram of the system. We consider both geometric and material quantities in the parameter space. Moreover, since the wrinkling patterns can be actuated dynamically using a pressure signal, we systematically characterize the aerodynamic behavior of our structures in the context of fluid drag reduction. An added advantage of the novel mechanism we introduce is that it allows for both dynamic switching and tuning of the surface morphology, thereby opening paths for drag control. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G24.00010: Vorticity Transport on a Rotating Blade Craig Wojcik, James Buchholz The development of the leading-edge vortex (LEV) is investigated on the suction surface of a rectangular flat plate undergoing a starting rotation in a quiescent fluid for angles of attack between 25 and 45 degrees. For blade aspect ratios of 2 and 4, the LEV is shown to be compact and quasi-stationary at inboard regions of the blade, consistent with the results of some other recent investigations. A salient feature of this flow is a region of opposite-sign vorticity generated on the blade beneath the LEV which is observed to become partially entrained into the LEV. A detailed vorticity transport analysis on the LEV has revealed that the resulting annihilation of vorticity is an important mechanism regulating LEV circulation, and therefore its stability. A parametric study is discussed, which elucidates the roles of shear layer vorticity flux, spanwise flow, vortex tilting, and annihilation on the evolution of LEV circulation with changes in azimuthal position, blade aspect ratio, spanwise position, and Reynolds number. [Preview Abstract] |
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