Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HZ: Instability: General II |
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Chair: Lou Kondic, New Jersey Institute of Technology Room: Hyatt Regency Long Beach Regency F |
Monday, November 22, 2010 10:30AM - 10:43AM |
HZ.00001: Dynamics of Coherent Structures in Localized Turbulence in a Pipe Jerry Westerweel, Dirk-Jan Kuik The transition to turbulence in pipe flow is still not completely understood. Recently it was shown that localized turbulent structures (puffs) can survive for hunderds of pipe diameters (or integral time scales) and then suddenly disintegrate. Questions that emerge are: Why is the turbulence localized? What mechanism is required for puffs to sustain itself? What changes in the structure of a puff when it suddenly decays? For the investigation a high resolution DNS is used. The high resolution is required to resolve the localized high energy peaks, which were observed in earlier experimental investigations. We use a stereoscopic planar PIV measurement as initial condition for the DNS and continued the time evolution at Re=1900. The first observation is that the velocity of the structures is higher than the bulk velocity at Re=1900 as opposed to the Re=2500 case, which is in agreement with experimental observations. The peaks in in- plane kinetic energy are reproduced in the DNS, and can be associated with hair-pin vortices. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HZ.00002: Spatiotemporal chaos and infinite-lifetime turbulence in pipes and channels Dwight Barkley, David Moxey Lifetime measurements of localized states have been the focus of many recent studies of transitional turbulence. We argue that the transition to infinite-lifetime turbulence must be understood as a transition to spatiotemporal chaos, similar to directed percolation (although the transition may not be strictly DP). While such arguments were first made many years ago, we report evidence substantiating this view from direct numerical simulations of long pipes. We also report work on modeling these phenomena. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HZ.00003: Global instabilities of the flow over a backward-facing step Daniel Lanzerstorfer, Hendrik C. Kuhlmann The three-dimensional linear stability of the two-dimensional, incompressible flow over a backward-facing step is considered. The geometry is varied covering an expansion ratio from 0.091 to 0.975. The basic flow becomes unstable to three different three-dimensional modes depending on the expansion ratio. An energy-transfer analysis is used to understand the nature of the instability. In the limit of vanishing step height the critical mode is stationary and the amplification process is caused by a Kelvin-Helmholtz-type instability. For high expansion ratios the basic flow features a wall-jet structure and becomes unstable due to centrifugal forces with respect to an oscillatory mode. For intermediate expansion ratios an elliptic instability mechanism is identified and the instability characteristics change continuously with the expansion ratio. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HZ.00004: Structural changes of laminar separation bubbles induced by global linear instability Daniel Rodriguez, Vassilis Theofilis Global modal linear instability analysis considers three- dimensional disturbances superimposed upon (essentially non- parallel) two- or three-dimensional basic flows. Here two- dimensional (BiGlobal) analysis of laminar separation bubbles embedded in a flat-plate boundary layer is performed. Results obtained show the presence of a stationary three-dimensional eigenmode, which is unstable for a finite range of spanwise wavenumbers, while the same steady basic flow is stable against two-dimensional disturbances of the Kelvin-Helmholtz/Tollmien- Schlichting class. Critical-point theory shows that 2D flow is ``structurally unstable'' and the presence of any 3D disturbance, like the aforementioned global mode will alter the complete topological description regardless of the disturbance amplitude. Critical-point theory is used here in order to characterize the different topological bifurcations exerted by global instability on the steady laminar two-dimensional bubble: a spanwise modulation of the separated region appears, eventually leading to the breakdown of the recirculation region into independent cellular structures, highly resemblant to the patterns observed experimentally on stalled airfoils. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HZ.00005: Pinning of rotating waves in systems with imperfect $SO(2)$ symmetry Francisco Marques, Alvaro Meseguer, Juan M. Lopez, Rafael Pacheco Experiments in small aspect-ratio Taylor-Couette flows have reported the presence of a band in parameter space where rotating waves become steady non-axisymmetric solutions (a pinning effect) via infinite-period bifurcations that previous numerical simulations were unable to reproduce. Here we present numerical simulations that include a small tilt of one of the endwalls, simulating the effects of imperfections that break the $SO(2)$ axisymmetry of the problem, and indeed are able to reproduce the experimentally observed pinning of the rotating waves. A detiled analysis of the corresponding normal form shows that the problem is more complex than expected, and the complete unfolding is of codimension six. A detailed analysis of different types of imperfections indicates that a pinning region surrounded by infinite-period bifurcation curves appears in all cases. Complex bifurcational processes, strongly dependent on the specifics of how the symmetry is broken, appear very close to the intersection of the Hopf bifurcation and the pinning region. The numerical and theoretical results agree with the previous experimental studies. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HZ.00006: The attractor manifold of the flow past a circular cylinder for $Re=100$ Iago C. Barbeiro, Julio R. Meneghini, J.A.P. Aranha The flow past a circular cylinder in its two-dimensional nonstationary r\'egime is concerned, in the vicinity of $Re=100$. At this point it shows the behavior of a self-sustained oscillator with a simple attractor, the periodic solution. This study proposes a methodology to build the attractor manifold from one picture of the solution inside the attractor using the spectral structure of the linearized evolution operator (a reduced subset of its eigenspace). The aim is to project the Navier-Stokes equations onto this manifold to obtain a nonlinear reduced model of Galerkin type for the phenomenon. The numerical scheme is based on a Finite Element Method discretization using Taylor-Hood elements and results will be presented at the time of the meeting. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HZ.00007: Experiments in the stability of basic two-dimensional flows Paul W. Fontana, Edward C. Titmus, Adrian Kirn Two-dimensional flows have different stability behavior than their three-dimensional counterparts due to enstrophy conservation, but they have not been studied as thoroughly in experiments. We present data from quasi-two-dimensional flow experiments suggesting that basic shear flows show instability not predicted by theory, while square-votex-lattice flows are more stable than predicted by linear theory. To allow proper quantitative comparisons between experiments and theory we have developed new techniques for quantifying and distinguishing kinematic viscosity and Ekman friction. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HZ.00008: Measurement Techniques: Viscosity and Surface Drag in Quasi-Two-Dimensional Flows Edward C. Titmus, Adrian T. Kirn, Paul W. Fontana The effects of kinematic viscosity and linear drag are both significant in many quasi-two-dimensional (Q2D) flows in nature and the laboratory. These effects, however, are difficult to measure and to distinguish from one another. We demonstrate precise, independent measurement of both kinematic viscosity and linear drag using decay rates of vortices of varying scales in a Q2D experiment involving soap films in a circular Couette cell. As expected, we have found both the kinematic viscosity and the linear drag to depend inversely on film thickness. The approach can be generalized to apply to other configurations and experiments. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HZ.00009: Experimental Exploration of the Dispersion Relation of Rayleigh-Taylor Instability Marie-Charlotte Renoult, Chia-Ling Chen, Sameh Ferjani, Pierre Carl\`{e}s, Charles Rosenblatt Investigating arbitrary initial configurations in the Rayleigh-Taylor instability presents an experimental challenge due to the difficulty controlling the initial interface shape. To overcome that, we pioneered in 2006 the use of magnetic levitation. In our current set-up, the denser paramagnetic fluid is levitated above the lighter fluid, using a quasi-homogeneous magnetic force. In order to modulate the static interface shape as desired, magnetically permeable elements such as straight pieces of magnetically permeable wire are added in the device. The initial interface thus is no longer flat and the destabilization of the interface is observed after the magnetic field is turned off. We will show our recent results of a systematic exploration of the dispersion relation of the instability by using straight segments of wires of different lengths. We will concentrate on the linear growth rate calculation with two different independent approaches and point out the consistency of the two methods. Beyond the verification of a known theory, this first attempt paves the way for a new systematic exploration of non-linear growth and mode couplings in the Rayleigh-Taylor instability for more original distributions of initial perturbations. [Preview Abstract] |
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