Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A14: Instabilities, Turbulence and Nonlinear Flows |
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Sponsoring Units: DFD Chair: Guenter Ahlers, University of California, Santa Barbara Room: 315 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A14.00001: Search for the ``ultimate state" in turbulent Rayleigh-B\'enard convection for Rayleigh numbers up to $4\times 10^{13}$ and Prandtl numbers near 0.8. Guenter Ahlers, Denis Funfschilling, Eberhard Bodenschatz Measurements of the Nusselt number $Nu$ over the Rayleigh-number range $10^{10} < Ra < 4\times 10^{13}$ for N$_2$ (Prandtl number $Pr = 0.72$) and SF$_6$ ($Pr = 0.78$ to 0.82) are reported. They were made at pressures up to 15 bars and near-ambient temperatures for a cylindrical sample of height $L=2.2$ m and diameter $D = 1.1$ m in a new High-Pressure Convection Facility (HPCF) constructed at the Max Planck Institute for Dynamics and Self-Organization in G\"ottingen, Germany. The data can be represented well by a power law with an effective exponent of 0.31. They do not show the transition to an ``ultimate regime" reported by Chavanne et al. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A14.00002: Large-scale circulation and Nusselt number in turbulent rotating Rayleigh-B\'enard convection. Jin-Qiang Zhong, Richard Stevens, Herman Clercx, Detlef Lohse, Guenter Ahlers We present measurements of the large-scale circulation (LSC) and the Nusselt number $Nu$ of turbulent Rayleigh-B\'enard convection in a cylindrical cell of aspect ratio 1 and rotated about a vertical axis at a rate $\Omega$. The side-wall temperatures at eight equally spaced azimuthal positions in the horizontal mid- plane were fit to a cosine function that gave the azimuthal LSC orientation $\theta(t)$ ($t$ is the time), the temperature amplitude $\delta(t)$, and the rms amplitude $\delta T(t)$ of the fluctuations about the fits. The LSC precessed in an azimuthal direction opposite to that of the imposed rotation. The precession rate $\omega= d\theta/dt$ showed a sharp transition at a Rossby number $Ro^* \simeq 2.5$. As $\Omega$ increased, $<\delta(t)>_t$ decreased and $<\delta T(t)>_t$ increased beginning at $Ro^*$. At $Ro^*$ $Nu$ began to increase with increasing $\Omega$. At high $Ro$ $| \omega |$ was proportional to but much smaller than $\Omega$. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 8:36AM |
A14.00003: Geometry of turbulence: a stroll through 61,506 dimensions Predrag Cvitanovic, John F. Gibson, Jonathan Halcrow We propose to use a hierarchy of exact unstable invariant solutions of the Navier-Stokes equations -- corresponding to the recurrent coherent structures observed in experiments -- to construct a description of the spatio-temporally chaotic dynamics of turbulent fluid flows as a walk through the space of such structures. This description should allow us to obtain quantitative predictions of transport properties of fluid flows such as bulk flow rate and mean wall drag. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A14.00004: Lagrangian and Eulerian Turbulence: intermittency and Universality Luca Biferale We present the result of a high resolution numerical simulations of homogeneous and isotropic turbulence at $R_{\lambda} \sim 600$. We discuss a phenomenological bridge-relation able to capture intermittent fluctuations in inertial and viscous scales for both Eulerian and Lagrangian ensembles. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A14.00005: Large-eddy simulation of swirling reacting flows Marcel Ilie Turbulent, swirling flows are encountered frequently in various chemical engineering processes. In combustion processes swirling flames are of interest due to the fact that provide enhanced mixing and reduce the pollutants formation. The challenge in understanding turbulent swirling flows stems mainly from the complexity of the flow field which is subject to vortex breakdown, recirculation and flow instability. In general the flow instabilities arise at high swirl numbers and can be used to control the performance of combustors. In the present study a large-eddy simulation (LES) approach with Smagorinsky eddy viscosity subgrid scale model is used to predict the swirling flame. The conserved scalar mixture fraction-based thermo-chemical variables are described using the steady laminar flamelet model. The present study shows that LES together with a laminar flamelet model provides a good prediction of the structure of turbulent swirling flames. Also LES captured very well the complex flame structures involving vortex breakdown which leads to swirl-induced recirculation zones, flow instability, and the occurrence of localized extinction. Also, the present study shows that the formation of an elongated recirculation (bluff-body stabilized) zone is strongly dependent on the swirl number and the ratio of momentum in the swirling annulus and central fuel jet. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A14.00006: Methods to Approach Velocity Data Reduction and Their Effects on Conformation Statistics in Viscoelastic Turbulent Channel Flows Gaurab Samanta, Antony Beris, Robert Handler, Kostas Housiadas Karhunen-Loeve (KL) analysis of DNS data of viscoelastic turbulent channel flows helps us to reveal more information on the time-dependent dynamics of viscoelastic modification of turbulence [Samanta et. al., J. Turbulence (\textit{in press}), 2008]. A selected set of KL modes can be used for a data reduction modeling of these flows. However, it is pertinent that verification be done against established DNS results. For this purpose, we did comparisons of velocity and conformations statistics and probability density functions (PDFs) of relevant quantities obtained from DNS and reconstructed fields using selected KL modes and time-dependent coefficients. While the velocity statistics show good agreement between results from DNS and KL reconstructions even with just hundreds of KL modes, tens of thousands of KL modes are required to adequately capture the trace of polymer conformation resulting from DNS. New modifications to KL method have therefore been attempted to account for the differences in conformation statistics. The applicability and impact of these new modified KL methods will be discussed in the perspective of data reduction modeling. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A14.00007: Study of Influence of Rapid Pressure in MHD Turbulence Saikishan Suryanarayanan, Aarthi Sekaran Turbulence, under the influence of magnetic field is characterized by anisotropy. Relatively limited work has been done in understanding and modeling magnetohydrodynamic (MHD) turbulence.The rapid distortion theory (RDT), which has been employed to study hydrodynamic turbulence, is a limiting case where the gradients of the mean velocity are very high compared to the gradients of the fluctuating field. When analyzed in a spectral framework, this leads to the independent evolution of each Fourier mode. RDT has been used to understand production and more importantly the ``rapid'' part of the pressure strain redistribution, as the other terms in the Reynolds stress evolution equation become negligible in the rapid distortion limit. Earlier work attempts to characterize the effect of the rapid pressure based on the geometry of the symmetric part of the mean velocity gradient tensor. This work deals with the application of RDT to MHD turbulence. The application of Elsasser variables reorganizes the MHD equations in a form similar to conventional Navier-Stokes. The current work is a numerical study of the Elsasser variable evolution equation in the rapid distortion limit and attempts to understand the role of the rapid magnetic pressure in the evolution of the Reynolds stresses for different mean distortions and magnetic fields. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A14.00008: Anisotropic Particles in Fluid Flow Monica Kishore, Nicholas T. Ouellette, Jerry Gollub Anisotropic particles are common in natural flows. In previous work [1] the dynamics of neutrally buoyant finite-sized spherical particles with Stokes numbers up to 0.08 were examined in 2D flows with Reynolds numbers of 72-220. Here, we extend this work to neutrally buoyant, high-aspect-ratio anisotropic particles of mm to cm length in a 2D cellular flow. The particle trajectories and orientations are tracked simultaneously with the underlying velocity field, which is measured using much smaller tracer particles. These methods allow us to compare the relative velocity and orientation of anisotropic particles to various features of the flow field. We find, for example, that the long axes of the particles preferentially align with the instantaneous direction of maximum compression, and that this alignment increases with particle aspect ratio. [1] N.T. Ouellette, P.J.J. O'Malley, and J.P. Gollub, Phys. Rev. Lett. 174504 (2008). [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A14.00009: Statistics of preferential particle concentration in free-surface Jason Larkin, Walter Goldburg, Mahesh Bandi Particles floating on a turbulent surface of water cluster into temporally complex patterns. We experimentally study the statistics of this preferential particle concentration for various Reynolds numbers, for both transient and steady-state dynamics. The probability density function for particle concentration exhibits a power-law with an exponential cut-off. We will discuss our preliminary analysis as to how this distribution depends upon the Reynolds number and the spatial-scale $r$ at which the system is coarse-grained. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A14.00010: Vortex Street behind an Oscillating Wire on a Soap Film Aaron Meyer, Ildoo Kim, X.L. Wu A von K\"arm\"an vortex street, a periodic array of vortices behind a bluff body is normally characterized by a single frequency $f_{0}$ at which the vortices shed. In this study, von K\"arm\"an vortex streets are generated on a 2D soap film using a glass-covered metal wire in a static magnetic field. When the wire is driven with electric current to make an oscillatory motion with frequency $f_{e}$, transverse to the mean flow, vortices shed at a frequency $f'$ differs from $f_{0} $. It is seen that with oscillation, $f_{0}$ is suppressed, $f'/f_{e}$ becomes a rational number, and vortices are rearranged to form an exotic spatial structure. This ``frequency- locking'' phenomena show some features of the sine-circle map, but the relevancy to the physical system is not clear. When the amplitude of the oscillation is large enough, the system becomes chaotic. In this chaotic regime, the energy power spectrum resembles that of 2D decaying turbulence. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A14.00011: An Anomalous Behavior in Vortex Shedding in a Flowing Soap Film ILDOO KIM, X.L. Wu It is generally believed that von K\"arm\"an vortex street is characterized only by Reynolds number $Re=UD/\nu$, where $U$ is the mean flow speed, $D$ is the size of the body which generates the vortex street, and $\nu$ is the kinematic viscosity. In this study, we present experimental data in a flowing soap film showing that changing $U$ with fixed $D$ and changing $D$ with fixed $U$ are not equivalent to each other, suggesting that $Re$ alone is not sufficient to characterize vortex shedding by a bluff body. The velocity of eyes of the vortices relative to the mean flow, normalized by $U$, increases when we increase $D$, but decreases when we increase $U$. It is also found that the longitudinal spacing between the eyes is a linear function of $D$, but independent of $U$. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A14.00012: Rayleigh-Taylor Instability in Nonlinear Optics Shu Jia, Jason W. Fleischer We demonstrate, theoretically and experimentally, an all-optical Rayleigh-Taylor instability. By applying a polar (Madelung) transformation to the nonlinear Schr\"{o}dinger equation for paraxial beams, we identify fluid density with light intensity and fluid velocity with the gradient of the optical phase. Pressure is obtained by using a self-defocusing nonlinearity in a photorefractive crystal, while acceleration is created by imposing a refractive index gradient. In this way, we are able to control the effective gravity, pressure, and input density ratio. The perturbed interface at the output is then studied as functions of these parameters. Observations of the characteristic spatial period show excellent agreement with analytical calculations from perturbation theory. In this case, wave diffraction, rather than viscosity or surface tension, sets the scale for long-wave growth. Further, we show that compressibility effects are important and demonstrate that care must be taken regarding shock-wave formation. The results hold for any Schr\"{o}dinger fluid, e.g. superfluids and quantum plasma, and lay the foundation for a variety of fluid-inspired instabilities in nonlinear optics. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A14.00013: Dispersive shock waves with negative pressure Wenjie Wan, Dmitri Dylov, Christopher Barsi , Jason Fleischer Dispersive shock waves (DSWs) arise from nonlinear wave breaking and mode dispersion and are a fundamental type of fluid behavior. In normal fluid systems, the pressure is positive and repulsive, so that the underlying particles resist compression. Examples include water, plasma, and optical beams with self-defocusing nonlinearity. However, there are systems in which the interactions are attractive, resulting in an effectively negative pressure. Here, we demonstrate that dispersive shock waves can arise in these negative-pressure systems by considering the equivalent optical problem with self-focusing nonlinearity. Using partially-coherent light, to prevent the competition of modulation instability, we experimentally observe DSWs formed in a self-compressive beam in a photorefractive crystal. We characterize the nonlinear speed and profile of the DSWs and show that statistical de-phasing by the incoherent beam causes an effective Landau damping of the waves. Observations are supported both by analytic theory and numerical simulation. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A14.00014: Convective instability in pipe flow through a sudden expansion James Seddon Flow through a sudden expansion in a pipe has been the subject of a lot of recent scientific interest. The geometry occurs in many industrial processes, from heat exchangers to combustion chambers, and is closely related to the physiological problem of flow through a stenosis. The inlet flow from the upstream pipe is Poiseuille, which forms a central jet surrounded by a recirculating eddy in the expanded downstream pipe. Recently we showed that this kind of flow passes through a symmetry breaking bifurcation before the onset of both intermittent and fully periodic time-dependent effects. We have now investigated the intermittency in more detail and find that the flow becomes convectively unstable. A wave packet emerges from the laminar state and grows to a maximum size of several diameters before decaying. [Preview Abstract] |
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