Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L35: Turbulence and Instabilities |
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Sponsoring Units: DFD Chair: Greg Voth, Wesleyan University Room: 210B |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L35.00001: Information theory used to study the turbulent cascade Rory Cerbus, Walter Goldburg The simplest picture of turbulence is the one imagined by Richardson and Kolmogorov. In their theory there is a cascade of energy from large scales to small scales. Here we present a new tool to study this picture using simple ideas borrowed from information theory. We use the conditional entropy (conditioned uncertainty) of velocity fluctuations on one scale conditioned on another larger or smaller scale. By varying the scale of the velocity fluctuations used in the conditioning, we can test both direction and locality. The ideas are tested on both experimental and numerical simulation data. This tool can be applied to 3D turbulence or 2D turbulence and because it has nothing to do with the Navier-Stoke's equation, it can be used for any other system where there is a similar cascade-like phenomenon. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L35.00002: Mutually independent cascades in anisotropic soap-film turbulence Chien-chia Liu, Gustavo Gioia, Pinaki Chakraborty Computational, experimental and field data amassed to date indicate that in 2D turbulence the spectrum of longitudinal velocity fluctuations, $E_{11}(k_1)$, and the spectrum of transverse velocity fluctuations, $E_{22}(k_1)$, correspond always to the same cascade, consistent with isotropy, so that $E_{11}(k_1) \propto k^{-\alpha}$ and $E_{22}(k_1) \propto k^{-\alpha}$, where the ``spectral exponent'' $\alpha$ is either 5/3 (for the inverse-energy cascade) or 3 (for the enstrophy cascade). Here, we carry out experiments on turbulent 2D soap-film flows in which $E_{11}(k_1) \propto k^{-5/3}$ and $E_{22}(k_1) \propto k^{-3}$, as if two mutually independent cascades were concurrently active within the same flow. To our knowledge, this species of spectrum has never been observed or predicted theoretically. Our finding might open up new vistas in the understanding of turbulence. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L35.00003: The spectral link in mean-velocity profile of turbulent plane-Couette flows Dongrong Zhang, Gustavo Gioia, Pinaki Chakraborty In turbulent pipe and plane-Couette flows, the mean-velocity profile (MVP) represents the distribution of local mean (i.e., time-averaged) velocity on the cross section of a flow. The spectral theory of MVP in pipe flows (Gioia \emph{et al.}, PRL, 2010) furnishes a long-surmised link between the MVP and turbulent energy spectrum. This missing spectral link enables new physical insights into an imperfectly understood phenomenon (the MVP) by building on the well-known structure of the energy spectrum. Here we extend this theory to plane-Couette flows. Similar to pipe flows, our analysis allows us to express the MVP as a functional of the spectrum, and to relate each feature of the MVP relates to a specific spectral range: the buffer layer to the dissipative range, the log layer to the inertial range, and the wake (or the lack thereof) to the energetic range. We contrast pipe and plane-Couette flows in light of the theory. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L35.00004: Reinforcement of steady streaks for consecutive transition delay Sohrab S. Sattarzadeh, Jens H. M. Fransson Miniature vortex genrators (MVGs) are recently proven efficient as passive control devices to delay the turbulence transition on a flat plate boundary layer by modulating the base flow in the spanwise direction, through generating steady streamwise elongated streaks, and hence reducing the skin-friction drag\footnote{Shahinfar, S., Sattarzadeh, S. S., Fransson, J. H. M., Talamelli, A. {\emph{Phys. Rev. Lett.}} {\bf{109}}, 074501, 2012.}. As the MVGs are localized in the streamwise direction, a shortcoming of the passive laminar control is the recovery of the two-dimensional boundary layer which force the control effects to fade away. In the present study we show that by placing a second array of MVGs downstream of the first one the streamwise extent of the control can be prolonged by reinforcing the steady streaks in the streamwise direction. The reinforced passive control strategy results in consecutive turbulence transition delay with obtaining a net skin-friction drag reduction of 65$\%$, for the present measurement conditions, compared to the smooth plate boundary layer. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L35.00005: Numerical study of turbulent transport at high Reynolds number in Richtmyer-Meshkov instability in an ICF like geometry Pooja Rao, Jeremy Melvin, Ryan Kaufman, Hyunkyung Lim, Yan Yu, James Glimm, David Sharp We study mixing in high Reynolds (Re) number flows in numerical simulations of a Richtmyer-Meshkov instability in an idealized ICF geometry. We propose that LES simulations of these turbulent flows have an underlying dependence on the numerical algorithm and this possible non-uniqueness emphasizes the need for parameter free models to allow extrapolation from validation of Re in the experimental range. Using the front-tracking code FronTier (validated for Rayleigh-Taylor data at Re=35k) in combination with the dynamic subgrid-scale models proposed by Germano, we achieve a parameter free model to allow for this extrapolation step. Under this simulation framework, we discuss the properties of the mixing and document the sensitivity of the subgrid terms to the numerical algorithm. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L35.00006: Measuring the orientation and rotation rate of 3D printed particles in turbulent flow Greg Voth, Stefan Kramel, Brendan Cole The orientation distribution and rotations of anisotropic particles plays a key role in many applications ranging from icy clouds to papermaking and drag reduction in pipe flow. Experimental access to time resolved orientations of anisotropic particles has not been easy to achieve. We have found that 3D printing technology can be used to fabricate a wide range of particle shapes with smallest dimension down to 300 ?m. So far we have studied rods, crosses, jacks, tetrads, and helical shapes. We extract the particle orientations from stereoscopic video images using a method of least squares optimization in Euler angle space. We find that in turbulence the orientation and rotation rate of many particles can be understood using a simple picture of alignment of both the vorticity and a long axis of the particle with the Lagrangian stretching direction of the flow. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L35.00007: Lattice-Boltzmann Simulation of Tablet Dissolution in Complex Hydrodynamic Environment Jiaolong Jiang, Ning Sun, Taeshin Park, Glen H. Ko, Dilip Gersappe Using the Lattice-Boltzmann method, we developed a 3D model to study the tablet dissolution process in a complex hydrodynamic environment involving spatially varying velocity and shear forces. The results show that a turbulent flow is formed in the region above the tablet, which has been obtained by visualization experiments. The dissolution profiles were obtained by incorporating detailed kinetics, showing good agreement with case studies from literature. After studying the influence of the paddle speed and the size of the system, we simulated the dissolution process for multicomponent tablets. Our results indicate how the hydrodynamic environment would affect the dissolution process by changing the local concentration of components near the tablet as well as by the particle erosion under high fluid velocity. Since the code was successfully parallelized, the simulation for comparatively large systems is possible now. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L35.00008: Three Dimensional Characterization of Quantum Vortex Dynamics in Superfluid Helium David Meichle, Daniel Lathrop Vorticity is constrained to line-like topological defects in quantum superfluids, such as liquid Helium below the Lambda transition. We have invented a novel method to disperse fluorescent nanoparticles directly into the superfluid which become trapped on the vortex cores, providing optical tracers. Using a newly constructed multi-camera stereographic microscope, we present data dynamically characterizing vortex reconnections and the subsequent emission of Kelvin waves fully in three dimensions. Statistics of thermally driven counterflow will be compared in 3D to previous measurements in projection. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L35.00009: The Importance of Nonlocal Terms in Superfluid Turbulence Rena Zieve, Owen Dix Simulations of vortex motion in superfluid helium based on the Biot-Savart law plus vortex reconnections can model homogeneous superfluid turbulence. However, the quantitative properties of the turbulent tangle are disturbingly sensitive to details of how the computations are carried out, and in some cases the tangle degenerates unphysically into an arrangement of parallel straight vortices. These problems have been attributed to the reconnection procedure, to the periodic boundary conditions used for most calculations, and to the localized induction approximation (LIA) which often replaces the (non-local) Biot-Savart integral. Previous work using numerical calculation of the complete Biot-Savart integral does not show the same issues as the LIA calculations, but these are time-consuming calculations. We show here that numerical integration over a relatively small region can suffice, as long as the size of the region exceeds the typical intervortex spacing. This result explains why the non-local contribution has a strong effect. It contributes an attraction between nearby vortices that ultimately leads to reconnections, which prevent the vortices from settling into an array of parallel lines. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L35.00010: Study of Grid Turbulence in Superfluid He$^4$ in a Large Square Channel Jihee Yang, Gary G. Ihas, William F. Vinen Studying quantum turbulence in superfluid helium can lead us to a deeper understanding in classical turbulence. We study grid-generated turbulence in liquid helium in the temperature range 1.4 K-2.1 K for homogeneous and isotropic turbulence (HIT). Using a conventional second sound attenuation method, the decay of vorticity ($\omega$) is observed in a long, square cross-section channel. Theories assume that energy is injected on the scale of the grid mesh size, and predict that when the energy containing eddies are growing, the vorticity decays as $\omega\sim$ t$^{-11/10}$ or $\omega\sim$ t$^{-17/14}$. When they saturate at the channel size, the vorticity begins decaying as $\omega\sim$ t$^{-3/2}$. Previous experiments have been performed in 1 cm$^2$ square channels, with a limited range of mesh sizes\footnote{M. R. Smith, R. J. Donnelly, N. Goldenfeld, and W. F. Vinen, Phys. Rev. Lett. 71, 2583 (1993).; S. R. Stalp, Ph.D. dissertation, University of Oregon 1998.; L. Munday, Ph.D. dissertation, Lancaster University 2014.}. We have used a larger channel and various mesh sizes to investigate grid mesh size effects and decay before saturation. A novel phase and amplitude locked feedback system ensures fast, stable attenuation data without disturbances from temperature fluctuations. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L35.00011: Observation of the Stratorotational Instability in Flow between Rotating Concentric Cylinders Ruy Ibanez, Harry L. Swinney, Bruce Rodenborn We study the stratorotational instability in a Taylor-Couette system with a radius ratio $\eta = {r_{o}}/{r_{i}} = 0.877$. The system is vertically stratified with a constant buoyancy frequency, $N = \sqrt{-({g}/{\rho_{o}})( {\partial \rho}/{\partial z})}$. We determine when the flow becomes unstable as the ratio of the outer to inner cylinder rotation rates, $\mu = {\Omega_{o}} /{\Omega_{i}}$, is decreased from unity (solid body rotation), for Reynolds numbers $Re = {\Omega_{i}r_{i}(r_{o}-r_{i})}/{\nu}$ ranging from 450 to 4000 and ${N}/{2\pi} = 0.3$ to $1.0$ Hz. The axial and azimuthal frequencies, obtained from spatiotemporal spectral analysis of digital movies, yield the observed modes at different $Re$ and $\mu$ for fixed $N$. We find for sufficiently large buoyancy frequency, ${N}/{2\pi} > 0.5$ Hz, the stratorotational instability occurs even above the $\mu = \eta$ stability limit obtained from theory developed in the Boussinesq (small $N$) approximation [cf. the review by D A Shalybkov, {\it Physics Uspekhi} {\bf 52}, 915 (2009)]. The frequencies we obtain for the azimuthal modes are close to multiples of the average frequency of rotation of the cylinders, while the axial wavelengths are found to vary linearly with Froude number, $Fr = {\Omega_{i}}/{N}$. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L35.00012: Stability analysis of an electrified liquid jet in the presence of an externally coflowing liquid Venkat Gundabala, Vikrant Modi Traditionally, electrospinning and electrospray are carried out with either air or vacuum as external medium. Recently, it has been shown that electrospray can be successfully implemented in the presence of an external flowing liquid. We envisage that implementation of electrospinning process in the presence of an external liquid coflowing with the electrospinning solution will allow greater control on the fiber deposition and morphology. In the present work, to gain fundamental understanding on the behaviour of an electrified liquid jet in the presence of an externally coflowing liquid, we perform stability analysis on the system. The classical Rayleigh-Plateau instability and an electrically induced axisymmetric instability were identified. The effect of the viscosity, velocity, and permittivity of the external liquid on the two instabilities was studied. It was found that both the growth rate and the critical wavenumbers were strongly influenced by the above parameters. An operating diagram predicting the transition from drop generation mode to fiber generation mode as a function of external liquid properties is generated. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L35.00013: An adaptive selective frequency damping method Bastien Jordi, Colin Cotter, Spencer Sherwin The selective frequency damping (SFD) method is used to obtain unstable steady-state solutions of dynamical systems. The stability of this method is governed by two parameters that are the control coefficient and the filter width. Convergence is not guaranteed for arbitrary choice of these parameters. Even when the method does converge, the time necessary to reach a steady-state solution may be very long. We present an adaptive SFD method. We show that by modifying the control coefficient and the filter width all along the solver execution, we can reach an optimum convergence rate. This method is based on successive approximations of the dominant eigenvalue of the flow studied. We design a one-dimensional model to select SFD parameters that enable us to control the evolution of the least stable eigenvalue of the system. These parameters are then used for the application of the SFD method to the multi-dimensional flow problem. We apply this adaptive method to a set of classical test cases of computational fluid dynamics and show that the steady-state solutions obtained are similar to what can be found in the literature. Then we apply it to a specific vortex dominated flow (of interest for the automotive industry) whose stability had never been studied before. [Preview Abstract] |
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