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 L12: Vortex VI |
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Chair: Jochen Kriegseis, University of Calgary Room: 26B |
Monday, November 19, 2012 3:35PM - 3:48PM |
L12.00001: Flow Structure on a Delta Wing of Moderate Sweep Angle During and After Pitch-Up Maneuver Alper Celik, Ilhan Ozturk, Habib Can Tunc, Mehmet Metin Yavuz The flow structure over a moderate sweep angle delta wing is investigated during and after the pitch-up maneuver and compared to the corresponding stationary wing results. The effects of pitch-up rate and Reynolds number on flow structures and their transformations are also studied. Dye visualization is used for qualitative studies and particle image velocimetry is used for quantitative analysis. At early stages of the maneuver the transformation of flow is initiated by the intertwinement of the dual vortex structure.Increasing the angle of attack results in disappearance of the vortex located closest to the leading-edge of the wing which results in a single, large scale, leading-edge vortex that undergoes a distinctive form of breakdown. It is found that the motion of the wing creates a significant time lag on the development of the flow pattern, when compared to stationary wing. In the relaxation period, the vorticity concentrations become more diffuse and elongated as they move towards the plane of symmetry, and away from the surface of the wing. All these features and transformations occur irrespective of values of pitch rates and Reynold's number. On the other hand, it is seen that the lag of flow pattern is a function of pitch rate and Reynold's number. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L12.00002: The suppression of wing rock of wing/body configurations Xueying Deng, Yankui Wang The suppression and flow mechanism of wing rock for wing/body configurations with high and low swept wing at high angles of attack have been studied. The results of low swept wing model reveal that wing rock patterns are dominated by body asymmetric vortices which are strongly dependent on circumferential location $\theta$ of micro-perturbation on the body tip. There are three types of free roll patterns: limit cycle at $\theta$=0$^{\circ}$ or 180$^{\circ}$; irregular oscillation at $\theta$=0$^{\circ}$ or 270$^{\circ}$; tiny roll pattern, if $\theta$ is at the other positions. For low swept model a technique of suppressing wing rock by rotating tip perturbation was developed with higher frequency than one of free roll motion. For the model with high swept wing another technique of suppressing wing rock was developed, where two pairs of asymmetric vortices (one pair from body and the other from high swept wing) will dominate the wing rock. If two pairs of asymmetric vortices are in phase coincidence, it makes wing rock stronger while two pairs of vortices are in phase reversal the wing rock becomes weaker. Based on this fact the perturbation at body tip can be adjusted to suppress the wing rock. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L12.00003: Vortex Interactions on Plunging Airfoil and Wings Azar Eslam Panah, James Buchholz The development of robust qualitative and quantitative models for the vorticity fields generated by oscillating foils and wings can provide a framework in which to understand flow interactions within groups of unsteady lifting bodies (e.g. shoals of birds, fish, MAV's), and inform low-order aerodynamic models. In the present experimental study, the flow fields generated by a plunging flat-plate airfoil and finite-aspect-ratio wing are characterized in terms of vortex topology, and circulation at Re=10,000. Strouhal numbers (St=fA/U) between 0.1 and 0.6 are investigated for plunge amplitudes of h$_{o}$/c = 0.2, 0.3, and 0.4, resulting in reduced frequencies (k=$\pi $ fc/U) between 0.39 and 4.71. For the nominally two-dimensional airfoil, the number of discrete vortex structures shed from the trailing edge, and the trajectory of the leading edge vortex (LEV) and its interaction with trailing edge vortex (TEV) are found to be primarily governed by k; however, for St$>$0.4, the role of St on these phenomena increases. Likewise, circulation of the TEV exhibits a dependence on k; however, the circulation of the LEV depends primarily on St. The growth and ultimate strength of the LEV depends strongly on its interaction with the body; in particular, with a region of opposite-sign vorticity generated on the surface of the body due to the influence of the LEV. In the finite-aspect-ratio case, spanwise flow is also a significant factor. The roles of these phenomena on vortex evolution and strength will be discussed in detail. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L12.00004: Aircraft wake two-vortex system at turbulent equilibrium Gregoire Winckelmans, Ivan De Visscher, Laurent Bricteux We consider a two-vortex system (2VS) started from a 2-D initial condition of given energy (two opposite sign vortices, each with an algebraic circulation profile and with a relatively tight core, $r_c$, compared to the distance, $b_0$, center to center: $r_c/b_0=0.05$). The 2VS is submitted to a very weak and realistic atmospheric turbulence background (of energy $<0.01$ that of the 2VS) so that it is excited to go unstable. The flow then generates, by non-linear interactions, instabilities and much more turbulence and eventually reaches a statistical equilibrium: a 2VS still with tight cores, with significant turbulence in the vortex oval, yet still laminar in the inner part of the cores, and which slowly decays in time. This state of equilibrium is quite universal (as confirmed by various sensitivity analyses). It is then of great importance to the physics and modeling of fully formed aircraft wake vortices and is characterized: spectrum, vorticity field, circulation profile and core size of the vortices in cross-planes and for the mean (i.e., longitudinal average). The two-scales Proctor-Winckelmans profile model is also compared to the data: it fits well the inner part $0 < r/b_0 < 0.04$ and the outer part $0.16 < r/b_0 < 0.5$ of the profile, but is still poor in between. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L12.00005: Force Production from Near- and Far-Field Vortices on Flapping Wings Hui Wan, Zongxian Liang, Haibo Dong Aerodynamic performance is closely correlated with the vortex formation and vortex structure of a flapping plate. The aerodynamic forces experienced the plate can be obtained from the surface integration of the pressure and shear stress distribution, or alternatively from the time rate of change of the vortex moment in the flow field Direct Numerical Simulation (DNS) is first conducted to generate the flow field information including both velocity and vorticity. Then the aerodynamic forces obtained from the above two methods are calculated and compared. The roles of near-field and far-field vortex structures on force generation are studied. Both two-dimensional plate and low aspect-ratio plate will be discussed. This work is supported by NSF CBET-1055949. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L12.00006: Modeling the dynamics of four-vortex bluff body wakes Saikat Basu, Mark Stremler, Teis Schnipper, Anders Andersen Vortex-shedding bluff bodies frequently generate wakes consisting of regular groupings of four vortices. We model such wakes as an integrable two-dimensional Hamiltonian system of point vortices by assuming spatial symmetries based on the vortex arrangements observed in experiments. The model demonstrates a number of dynamic modes that we classify using a bifurcation analysis of the phase space topology. In contrast to the standard von Karman street, very few initial conditions lead to relative equilibria in which the vortex configuration moves with invariant size and shape. Scaled comparisons of the model with experiments conducted in a flowing soap film support the model results. The richness of the results reveals a variety of patterns that can occur within the class of `2P' wakes. Following the approach of von Karman, we estimate the drag and lift forces exerted by such wakes on a bluff body. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L12.00007: Effect of Relative Submergence on the Flow Structure in the Wake of Wall-Mounted Spherical Obstacle Seyed Hajimirzaie, Achilleas Tsakiris, James Buchholz, Athanasios Papanicolaou To understand the flow around submerged boulders in open channels, a study of a spherical obstacle on a rough bed in shallow open channel is conducted. In steep mountain streams, relative submergence (d/H, d being flow depth and H obstacle height) is introduced. In this study, through the use of PIV and thermal anemometry, the flow field surrounding a wall-mounted sphere with diameter D=5cm in two different relative submergences has been investigated on a smooth plate as a boundary layer (BL) as well as on a rough bed in shallow open channel flow. Flow patterns (velocity, vorticity) in the streamwise symmetry plane are different between the open channel and BL flow. Streamwise features are observed in the mean wakes of the sphere. In high relative submergence, an upwash (base structure) is observed in the wake of sphere in open channel flow but not in BL tests. The horseshoe vortex is not observed in the wake of the sphere in either case. Dye visualization, spectral analysis and cross-correlation show in d/H$\le $1, the wake in both cases has the appearance of a Karman vortex street, while in d/H $>$ 1, the wakes are quite symmetric. A weak dominant shedding frequency is observed in BL experiments with a Strouhal number of St=fsD/U$\approx $0.35. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L12.00008: Wake dynamics and hydrodynamic forces on a perforated circular plate in cross-flow Francisco Huera-Huarte The cross-flow past a perforated plate is known to become steady, if certain critical porosity or number of holes is imposed to the plate. This happens because the air bleed in the near wake, disrupts the vortex street formation behind the plate, and leads to suppression of the near wake shear layer interaction, forcing the instabilities to take place further away from the disk. This phenomenon is accompanied by a drag reduction. It is not clear however, what is the effect of the porosity distribution used in the plate, neither the effect of the angle of attack on the wake dynamics and the force coefficients. The experimental apparatus consists of an acrylic model in which different number and configuration of holes can be used. The disk hangs upside down from a 2-axis balance, in a way that it is being exposed to a uniform water current generated in a free surface channel. Angles of attack, porosity and its distribution on the disk, can be easily changed. Measurements of force coefficients for different angles of attack, and porosities have been taken. Digital Particle Image Velocimetry (DPIV) has been used to quantify the wake and to investigate the flow structures past the disk. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L12.00009: Influence of diameter ratio and aspect ratio on wake development of a dual step cylinder Serhiy Yarusevych, Chris Morton A dual step cylinder is composed of a large diameter cylinder (D) attached to the mid-span of a small diameter cylinder (d). In a uniform cross flow, vortex shedding occurs from the small cylinder, above and below the large cylinder. The characteristics of the shed vortices are similar to those found in the wake of a uniform circular cylinder. However, wake characteristics of the large cylinder are influenced significantly by the geometric parameters of the model, namely, the ratio between the large and small cylinder diameters (D/d) and the large cylinder aspect ratio (L/D). The present work investigates the flow past dual step cylinders for Re$_{D}$ = 2100, 0.2~$\le $~L/D $\le $ 3, and 1.33 $\le $ D/d $\le $ 2.67. Experiments have been completed in a water flume facility employing Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) systems, as well as hydrogen bubble flow visualization. The results show that the following three distinct large cylinder wake topologies can be observed for the investigated ranges of L/D and D/d: (i) shedding of uniform spanwise vortices, (ii) shedding of highly deformed three-dimensional vortices, and (iii) no distinct vortex shedding. Complex vortex interactions taking place in the wake of the large cylinder are investigated for the identified flow regimes. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L12.00010: ABSTRACT WITHDRAWN |
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