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arXiv2013

Stretching and folding processes in the 3D Euler and Navier-Stokes equations

Stretching and folding dynamics in the incompressible, stratified 3D Euler and Navier-Stokes equations are reviewed in the context of the vector $\bdB = \nabla q\times\nablaθ$ where $q=\bom\cdot\nablaθ$. The variable $θ$ is the temperature and $\bdB$ satisfies $\partial_{t}\bdB = \mbox{curl}\,(\bu\times\bdB)$. These ideas are then discussed in the context of the full compressible Navier-Stokes equations where $q$ takes the two forms $q = \bom\cdot\nablaρ$ and $q = \bom\cdot\nabla(\lnρ)$.

math-ph↗

arXiv2013

Frequency lock closed-loop control of a separated flow using visual feedback

In this study, a simple model based closed-loop algorithm is used to control the separated flow downstream a backward-facing step. It has been shown in previous studies that the recirculation bubble can be minimized when exciting the shear layer at its natural Kelvin-Helmholtz instability frequency. In this experiment, the natural shedding frequency is identified through real-time analysis of 2D velocity fields. Actuation (pulsed jet) is then locked on this frequency. If flow characteristics stray too far from a set point, shedding frequency is updated and actuation changed. The present work demonstrates the efficacy and robustness of this approach in reducing recirculation while Reynolds number is randomly varied between 1400 and 2800.

physics.flu-dyn↗

arXiv2013

The Science and Beauty of Fluidization

Particle flows of high particle concentration are important in many fields, including chemical processing, pharmaceutical processing, energy conversion and powder transport. However, despite decades of research and industrial application, the real time behavior of particle flow fields is still not well understood. One of the main reasons is that experimental data is difficult to acquire in such harsh, opaque particle flow environments. In this educational video for the Gallery of Fluid Motion, high speed video acquired with a new, patented high speed particle imaging velocimetry (high speed PIV) technology is presented. This technology was developed by the USDOE National Energy Technology Laboratory (NETL). It is being applied to observe and measure the real time behavior of individual particle motion inside particle flow fields of high particle concentration for the first time. The high speed PIV system records high speed videos of particle motion with excellent spatial and temporal clarity. The high speed videos are analyzed to measure the concentration and the two-dimensional motion (velocity and trajectory) of individual particles. Data sample rates for velocity vectors are in the range of 0.1 to 3 million vectors per second, thereby providing full resolution of the temporal domain of particle velocity. To see and measure particle motion inside the flow fields at high particle concentrations, a custom borescope is inserted into particle flow fields. The particle tracking technique can measure gas and fluid flow if the particles are small enough (have a low enough Stokes number) to follow the gas/fluid flow. In areas of rapid mixing, such as turbulent wakes, particle tracking can provide better spatial resolution that conventional cross-correlation based PIV analysis. Keywords: fluid dynamics video, particle tracking, fluidization

physics.flu-dyn↗

arXiv2013

Discrete Filters for Large Eddy Simulation of Forced Compressible MHD Turbulence

In present study, we discuss results of applicability of discrete filters for large eddy simulation (LES) method of forced compressible magnetohydrodynamic (MHD) turbulent flows with the scale-similarity model. Influences and effects of discrete filter shapes on the scale-similarity model are examined in physical space using a finite-difference numerical schemes. We restrict ourselves to the Gaussian filter and the top-hat filter. Representations of this subgrid-scale model which correspond to various 3- and 5-point approximations of both Gaussian and top-hat filters for different values of parameter $ε$ (the ratio of the mesh size to the cut-off lengthscale of the filter) are investigated. Discrete filters produce more discrepancies for magnetic field. It is shown that the Gaussian filter is more sensitive to the parameter $ε$ than the top-hat filter in compressible forced MHD turbulence. The 3-point filters at $ε=2$ and $ε=3$ give the least accurate results and the 5-point Gaussian filter shows the best results at $ε=2$.

physics.plasm-ph↗

arXiv2013

Microrheology measurements with a hanging-fiber AFM probe

A method to measure the viscosity of liquids at microscales is presented. It uses a thin glass fiber fixed on the tip of the cantilever of an extremely low noise Atomic Force Microscope (AFM), which accurately measures the cantilever {deflection}. When the fiber is dipped into the liquid the dissipation of the cantilever-fiber system increases. This dissipation, linked to the liquid viscosity, is computed from the power spectral density of the thermal fluctuations of the cantilever {deflection}. The {high sensitivity of the AFM} allows us to show the existence and to develop a model of the coupling between the dynamics of the fiber and that of the cantilever. This model {accurately} fits the experimental data. The advantages and draw-backs of the method are discussed

cond-mat.soft↗

arXiv2013

Yield and Solidification of Yield-Stress Materials in Rigid Networks and Porous Structures

In this paper, we address the issue of threshold yield pressure of yield-stress materials in rigid networks of interconnected conduits and porous structures subject to a pressure gradient. We compare the results as obtained dynamically from solving the pressure field to those obtained statically from tracing the path of the minimum sum of threshold yield pressures of the individual conduits by using the threshold path algorithms. We refute criticisms directed recently to our previous findings that the pressure field solution generally produces a higher threshold yield pressure than the one obtained by the threshold path algorithms. Issues related to the solidification of yield stress materials in their transition from fluid phase to solid state have also been investigated and assessed as part of the investigation of the yield point.

physics.flu-dyn↗

arXiv2013

A new large scale instability in rotating stratified fluids driven by small scale forces

In this paper, we find a new large scale instability displayed by a stratified rotating flow in forced turbu- lence. The turbulence is generated by a small scale external force at low Reynolds number. The theory is built on the rigorous asymptotic method of multi-scale development. There is no other special constraint concerning the force. In previous papers, the force was either helical or violating parity invariance. The nonlinear equations for the instability are obtained at the third order of the perturbation theory. In this article, we explain a detailed study of the linear stage of the instability.

physics.flu-dyn↗

arXiv2013

Imaging the proton concentration and mapping the spatial distribution of the electric field of catalytic micropumps

Catalytic engines can use hydrogen peroxide as a chemical fuel in order to drive motion at the microscale. The chemo-mechanical actuation is a complex mechanism based on the interrelation between catalytic reactions and electro-hydrodynamics phenomena. We studied catalytic micropumps using fluorescence confocal microscopy to image the concentration of protons in the liquid. In addition, we measured the motion of particles with different charges in order to map the spatial distributions of the electric field, the electrostatic potential and the fluid flow. The combination of these two techniques allows us to contrast the gradient of the concentration of protons against the spatial variation in the electric field. We present numerical simulations that reproduce the experimental results. Our work sheds light on the interrelation between the different processes at work in the chemo-mechanical actuation of catalytic pumps. Our experimental approach could be used to study other electrochemical systems with heterogeneous electrodes.

cond-mat.mes-hall↗

arXiv2013

Quantum ideal hydrodynamics on the lattice

After discussing the problem of defining the hydrodynamic limit from microscopic scales, we give an introduction to ideal hydrodynamics in the Lagrange picture, and show that it can be viewed as a field theory, which can be quantized using the usual Feynman sum-over-paths prescription. We then argue that this picture can be connected to the usually neglected thermal microscopic scale in the hydrodynamic expansion. After showing that this expansion is generally non-perturbative, we show how the lattice can be used to understand the impact quantum and thermal fluctuations can have on the fluid behavior.

hep-lat↗

arXiv2013

Numerical evaluation of relative permeability using Johnson--Koplik--Dashen model

We present a numerical study aimed at comparing two approaches to the evaluation of relative permeability curves from 3D binary images of porous media. One approach hinges on the numerical solution of Stokes equations, while the other is based on the Johnson-Koplik-Dashen (JKD) universal scaling theory of viscous frequency-dependent flow [D.~L. Johnson, J.~Koplik, and R.~Dashen, \emph{Theory of dynamic permeability and tortuosity in fluid--saturated porous media}, Journal of Fluid Mechanics \textbf{176} (1987), 379--402.] and the method of maximal inscribed spheres. JKD steady-flow simulations only require the solution of a boundary-value problem for the Laplace equation, which is computationally less intensive than the solution of Stokes equations. A series of numerical calculations performed on 3D pore-space images of natural rock demonstrate that JKD-based estimates are in good agreement with the corresponding Stokes-flow numerical simulations.

physics.flu-dyn↗

arXiv2013

Polytropic process and tropical Cyclones

We show a parallelism between the expansion and compression of the atmosphere in the secondary cycle of a tropical cyclone with the fast expansion and compression of wet air in a bottle. We present a simple model in order to understand how the system (cyclone) draws energy from the air humidity. In particular we suggest that the upward (downward) expansion (compression) of the warm (cold) moist (dry) air follows a polytropic process, $PV^β$= constant. We show both experimentally and analytically that $β$ depends on the initial vapor pressure in the air. We propose that the adiabatic stages in the Carnot-cycle model for the tropical cyclone be replaced by two polytropic stages. These polytropic processes can explain how the wind wins energy and how the rain and the dry bands are produced inside the storm.

physics.ao-ph↗

arXiv2013

Double scouring by turbulent jets downstream of a submerged sluice gate

In this work we study the scour produced by a jet downstream of a submerged sluice gate on a sediment bed of non-cohesive particles. The experiments were performed for various values of sill heights and °uid depths. New regimes were observed in which two holes are simultaneously developed. We identify the origins of the two holes and show that they are produced by different scouring mechanisms. The dependence of the position of the holes with the approach °uid depth and the gate opening were studied and expressed in terms of adequate non-dimensional numbers.

physics.flu-dyn↗

arXiv2013

Nonlinear Acoustics -- Perturbation Theory and Webster's Equation

Webster's horn equation (1919) offers a one-dimensional approximation for low-frequency sound waves along a rigid tube with a variable cross-sectional area. It can be thought as a wave equation with a source term that takes into account the nonlinear geometry of the tube. In this document we derive this equation using a simplified fluid model of an ideal gas. By a simple change of variables, we convert it to a Schrödinger equation and use the well-known variational and perturbative methods to seek perturbative solutions. As an example, we apply these methods to the Gabriel's Horn geometry, deriving the first order corrections to the linear frequency. An algorithm to the harmonic modes in any order for a general horn geometry is derived.

physics.flu-dyn↗

arXiv2013

Thermoconvective structure dynamics in horizontal Homeotropic Nematics films heated from above

Homeotropic Nematic Liquid Crystal heated from above present convective Rayleigh-Benard instability for applied thermal gradients greater than $Δ$Tc. This threshold increase with the intensity of applied external magnetic field parallel to the initial molecular orientation (vertical). We studied the different patterns which exist near to and away from the threshold, such as hexagons and squares, and the transitions between them. The study of patterns evolution away the threshold shows coexistence between patterns and a global system dynamics. Also we studied how this general dynamics change with the inclination of the cell, where for very small angles (1 degree) the hexagons and squares are transformed into rolls.

physics.flu-dyn↗

arXiv2013

Turbulent spots in channel flow: an experimental study Large-scale flow, inner structure and low order model

We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot. Special attention is paid to the large-scale flow surrounding the spot. We show that this large-scale flow is an asymmetric quadrupole centred on the spot. We measure the time evolution of the turbulent fluctuations and the mean flow distortions and compare these with the predictions of a nonlinear reduced order model predicting the main features of subcritical transition to turbulence.

physics.flu-dyn↗

arXiv2013

Numerical modeling of ion transport in a ESI-MS system

Gas and ion transport in the capillary-skimmer subatmospheric interface of a mass spectrometer, which is typically utilized to separate unevaporated micro-droplets from ions, was studied numerically using a two-step approach spanning multiple gas dynamic regimes. The gas flow in the heated capillary and in the interface was determined by solving numerically the Navier-Stokes equation. The capillary-to-skimmer gas/ion flow was modeled through the solution of the full Boltzmann equation with a force term. The force term, together with calculated aerodynamic drag, determined the ion motion in the gap between the capillary and skimmer. The three-dimensional modeling of the impact of the voltage applied to the Einzel lens on the transmission of doubly-charged peptides ions through the skimmer orifice was compared with experimental data obtained in the companion study. Good agreement between measured and computed signals was observed. The numerical results indicate that as many as 75% ions that exit from the capillary can be lost on the conical surface of the skimmer or capillary outer surface due to the electrostatic force and plume divergence.

physics.flu-dyn↗

arXiv2013

Attractor Control Using Machine Learning

We propose a general strategy for feedback control design of complex dynamical systems exploiting the nonlinear mechanisms in a systematic unsupervised manner. These dynamical systems can have a state space of arbitrary dimension with finite number of actuators (multiple inputs) and sensors (multiple outputs). The control law maps outputs into inputs and is optimized with respect to a cost function, containing physics via the dynamical or statistical properties of the attractor to be controlled. Thus, we are capable of exploiting nonlinear mechanisms, e.g. chaos or frequency cross-talk, serving the control objective. This optimization is based on genetic programming, a branch of machine learning. This machine learning control is successfully applied to the stabilization of nonlinearly coupled oscillators and maximization of Lyapunov exponent of a forced Lorenz system. We foresee potential applications to most nonlinear multiple inputs/multiple outputs control problems, particulary in experiments.

nlin.CD↗

arXiv2013

Effect of slip on circulation inside a droplet

Internal recirculation in a moving droplet plays an important role in several droplet-based microfluidic devices as it enhances mixing, chemical reaction and heat transfer. The occurrence of fluid slip at the wall, which becomes prominent at high shear rates and lower length scales, results in a significant change in droplet circulation. Using molecular dynamics (MD) simulations, the presence of circulation in droplets is demonstrated and quantified. Circulation is shown to vary inversely with slip length, which is a measure of interface wettability. A simple circulation model is established that captures the effect of slip on droplet circulation. Scaling parameters for circulation and slip length are identified from the circulation model which leads to the collapse of data for droplets with varying aspect ratio (AR) and slip length. The model is validated using continuum and MD simulations and is shown to be accurate for droplets with high AR.

physics.flu-dyn↗