Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L25: Vortex Dynamics and Vortex Flows VIII |
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Chair: Robert Krasny, University of Michigan Room: 320 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L25.00001: Investigation of turbulent energy transport by applying POD-LSE complementary method Osamu Terashima, Yasuhiko Sakai, Kouji Nagata, Yasumasa Ito Turbulent energy transport mechanism involving large-scale coherent vortex structures in the self-similarity region of a plane turbulent jet is experimentally investigated. First, a simultaneous multipoint measurement of two velocity components and pressure is performed by using several combined probes consisting of a pressure probe and an X-type hot-wire probe. Then, proper orthogonal decomposition (POD) is applied to both velocity and pressure fields to determine the coherent vortex structure in the jet. Further, a complementary technique of POD and linear stochastic estimation (LSE) is used to reconstruct the spatiotemporal velocity and pressure field of the dominant POD mode. As a result of reconstruction, the coherent structure with counter-rotating vortices staggering to the jet centerline is extracted and it contains approximately 42\% of the total turbulent energy. Finally, the turbulent energy transport caused by the large scale coherent vortex structure is evaluated by using the reconstructed velocity and pressure fields. The results show that the production and pressure diffusion of the turbulent energy in the jet are mainly caused by this coherent vortex structure. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L25.00002: Formation of Small-Scale Vortex Rings from Vortex Pairs Close to the Ground Daniel Asselin, Charles Williamson In this research, we examine the effect of a solid boundary on the dynamics and instabilities of a pair of counter-rotating vortices. An isolated vortex pair is subject to a short-wave elliptic instability and a long-wave Crow (1970) instability. Near a wall, the boundary layer between the primary vortices and the wall can separate, leading to the generation of secondary vorticity. These secondary vortices can be subject to small-scale instabilities (Harris {\&} Williamson, 2012) as they come under the influence of the primary vortices. In contrast, in the present study we are interested in the long-wave Crow instability interrupted by interaction with a wall. This can cause significant axial flow, resulting in a periodic concentration of fluid containing vorticity at the peaks of each wavy vortex tube and a corresponding evacuation of fluid containing vorticity from the troughs. It appears that this axial flow is driven at least in part by the formation of vortex ring-like structures in the secondary vortex as it is deformed by the primary vortex. Furthermore, additional small scale-vortex rings evolve from the secondary vorticity and from the concentrated vortical regions in the primary vorticity. In both cases, these rings cause vorticity to rebound away from the ground. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L25.00003: Vortex wandering in grid turbulence Steffen Pentelow, Stavros Tavoularis The tip vortex of a square-tipped NACA 0012 wing at an angle of attack of $5^{\circ}$ was investigated in a water tunnel. The chord length was $c=180~$mm and the chord-based Reynolds number was 25000. Cases with three free-stream conditions were examined: unobstructed flow with a transverse fluctuation intensity (in the free-stream at the wing-tip plane) $u_2^\prime /U_{\infty} = 2.3\%$; ``small-grid'' turbulence with $u_2^\prime /U_{\infty} = 3.5\%$ and a transverse integral length scale $L_2=0.063c$; and ``large-grid'' turbulence with $u_2^\prime /U_{\infty} = 5.3\%$ and $L_2=0.078c$. Velocity maps were obtained on several transverse planes using stereo particle image velocimetry and three-dimensional, time-dependent vortex wandering was resolved using flow visualisation of fluorescent dye injected into the vortex at the wing tip. The results quantify the effect of turbulence on the amplitude, frequency and wavelength of the vortex wandering motion, as well as on the axial and azimuthal velocity variations within the vortex. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L25.00004: Cutting, Splicing, and Kelvin Waves Martin Scheeler, Dustin Kleckner, William T.M. Irvine Recent experimental advances have allowed us to create, visualize and track vortices of prescribed shape and topology in classical fluids. We study the effect of surgery (cutting and splicing) on the evolution of the geometry and topology of these vortex loops, with a particular focus on the wave-like excitations generated by these operations. We interpret the dynamics of these excitations and the role they play within the broader context of vortex evolution. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L25.00005: Three Dimensional Motions, Kelvin Waves, and Nanoparticle Tracking in Superfluid Helium David Meichle, Daniel Lathrop Liquid Helium becomes a quantum superfluid when cooled below the lambda transition temperature of 2.17 Kelvin. Superfluid helium exhibits interesting macroscopic effects such as zero viscosity; and its flow is irrotational except for the presence of line-like topological phase defects with quantized circulation called quantum vortices. The vortex dynamics can be observed by dispersing tracer particles into the fluid, which become trapped on the vortex cores. Using atmospheric ice particles our group recently observed the excitation and self-similar propagation of helical Kelvin waves on these quantized vortex cores following a vortex reconnection event. This observation of an intrinsically three dimensional phenomenon has motivated the development of a three dimensional imaging apparatus for liquid helium. We will present new data obtained by dispersing fluorescent nanoparticles tracers and our progress towards full three dimensional tracking of quantized vortex dynamics in liquid helium. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L25.00006: Adaptive particle methods for barotropic vorticity dynamics on a rotating sphere Peter Bosler, Lei Wang, Christiane Jablonowski, Robert Krasny We present an adaptive particle method for barotropic vorticity dynamics on a rotating sphere. The flow map is represented by Lagrangian particles, organized into either triangular icosahedral panels or cubed-sphere quadrilateral panels. The particles carry vorticity and the panels are used to obtain quadrature weights in the point vortex approximation of the Biot-Savart integral. Adaptive panel refinement and remeshing are applied to maintain accuracy and reduce computational cost. Examples include Rossby-Haurwitz waves, Gaussian vortices, and perturbed jet dynamics. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L25.00007: Knotted Vortices: Entropic Lattice Boltzmann Method for Simulation of Vortex dynamics Fabian Boesch, Shyam Chikatamarla, Ilya Karlin Knotted and interlinked vortex structures in real fluids are conjectured to play a major role in hydrodynamic flow dissipation. Much interest lies in determining their temporal stability and the mechanism through which knots dissolve [1-3]. Kleckner and Irvine [1] recently have shown the existence of such knotted vortices experimentally by accelerating hydrofoils in water. In the present work we employ the entropic lattice Boltzmann method (ELBM) to perform DNS simulations of the creation and dynamics of knotted vortex rings inspired by the experimental setup in [1]. ELBM renders LBM scheme unconditionally stable by restoring the second law of thermodynamics (the Boltzmann H-theorem), and thus enables simulations of large domains and high Reynolds numbers with DNS quality [4-5]. The results presented in this talk provide an in-depth study of the dynamics of knotted vortices and vortex reconnection events and confirm the existence of trefoil knots in silicio for the first time.\\[4pt] [1] Klecker et al, Nature Physics, 9 (2013)\\[0pt] [2] Kida et al, Annu. Rev. Fluid Mech., 26 (1994)\\[0pt] [3] Ricca et al., J. Fluid Mech., 391 (1999)\\[0pt] [4] Karlin et al., Europhys. Lett. 47 (1999)\\[0pt] [5] Chikatamarla et al, Phys. Rev. Lett. 97, 010201 (2006) [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L25.00008: Numerical simulation of the fluttering instability using a pseudospectral method with volume penalization Thomas Engels, Dmitry Kolomenskiy, Kai Schneider, Joern Sesterhenn A new numerical scheme for the simulation of deformable objects immersed in a viscous incompressible fluid is presented. The 2d Navier-Stokes equations are discretized with a Fourier pseudo-spectral scheme. Using the volume penalization method arbitrary inflow conditions can be enforced, together with the no-slip conditions at the boundary of the immersed flexible object. The present work extends the penalization method to account for moving deformable objects while avoiding numerical oscillations in the hydrodynamic forces. For the solid part, a simple 1d model, the non-linear beam equation, is employed. The fluid and solid parts are coupled with a fast explicit staggered scheme. The fluttering instability of a slender structure immersed in a free stream is studied and three distinct states are obtained: stability of the initial condition or maintenance of an either periodic or chaotic fluttering motion. A detailed parameter study for different Reynolds numbers and reduced free-stream velocities is presented. The dynamics of the transition from a periodic to a chaotic state is investigated. The results are compared with those obtained by an inviscid vortex shedding method and by a viscous linear stability analysis, yielding for both satisfactory agreement. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L25.00009: Formation and destabilization of Kelvin-Helmholtz billows in stably stratified turbulence Yoshifumi Kimura, Jackson Herring We investigate the formation and destabilization of Kelvin-Helmholtz billows in stably stratified turbulence using the pseudo-spectral DNS of the Navier-Stokes equations under the Boussinesq approximation with $2048^3$ grid points. Our method is to integrate the equations from the zero total energy initial condition with horizontal forcing imposed in a narrow wave number band. In the course of developments, the horizontal spectra first show a single steep power-law ($k^{?4,5}$, where $k$ is the horizontal wavenumber), and then the tail part of the spectrum begins to rise to show the Kolmogorov-type slope ($k^{?5/3}$). From the viewpoint of vortex formation, we first observe that many wedge vortices are produced which move horizontally (like dipoles) in random directions. As time goes on, the wings of the wedges become thinner and thinner while translating, and finally detach to be almost independent vortex layers. This thinning mechanism makes the vertical shear stronger and eventually the local Richardson number small enough to produce Kelvin-Helmholtz billows. We will demonstrate that the transition in the horizontal energy spectra has a close relation with the destabilizing process of the Kelvin-Helmholtz billows. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L25.00010: Regenerative growth due to axial flow induced by vortex-turbulence interaction Eric Stout, Fazle Hussain Direct numerical simulations of a vortex column embedded in fine scale homogeneous, isotropic turbulence reveals an inviscid mechanism for induction of axial flow on the column. Vortex threads, produced outside the column during vortex-turbulence interaction, are shown to drive the mechanism of axial flow generation. Oppositely oriented threads radially separate by self-induction, hence causing net axial flow. At computationally accessible Reynolds numbers (Re$\equiv $vortex circulation/viscosity$=$10 000), the axial flow due to a pair of oppositely signed vortex threads outside the column increases both with Re and time. At high Re, the axial flow can increase sufficiently to render the vortex column unstable by the well-known q criterion. The vorticity field reveals that axial flow is another mechanism, perhaps more dominant than the parent-offspring hairpin vortex scenario (Hussain, Pradeep {\&} Stout JFM 2011), of regenerative energy growth -- likely to be important for implementing breakup of aircraft trailing vortices. [Preview Abstract] |
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