Homogeneity and isotropy in a laboratory turbulent flow

We present a new design for a stirred tank that is forced by two parallel planar arrays of randomly actuated synthetic jets. This arrangement creates turbulence at high Reynolds number with low mean flow. Most importantly, it exhibits a region of 3D homogeneous isotropic turbulence that is significantly larger than the integral lengthscale. These features are essential for enabling laboratory measurements of turbulent suspensions. We use quantitative imaging to confirm isotropy at large, small, and intermediate scales by examining one-- and two--point statistics at the tank center. We then repeat these same measurements to confirm that the values measured at the tank center are constant over a large homogeneous region. In the direction normal to the symmetry plane, our measurements demonstrate that the homogeneous region extends for at least twice the integral length scale $L=9.5$ cm. In the directions parallel to the symmetry plane, the region is at least four times the integral lengthscale, and the extent in this direction is limited only by the size of the tank. Within the homogeneous isotropic region, we measure a turbulent kinetic energy of $6.07 \times 10^{-4}$m$^2$s$^{-2}$, a dissipation rate of $4.65 \times 10^{-5}$m$^2$s$^{-3}$, and a Taylor--scale Reynolds number of $R_\lambda=334$. The tank's large homogeneous region, combined with its high Reynolds number and its very low mean flow, provides the best approximation of homogeneous isotropic turbulence realized in a laboratory flow to date. These characteristics make the stirred tank an optimal facility for studying the fundamental dynamics of turbulence and turbulent suspensions.Comment: 18 pages, 9 figure

Universality in dynamic wetting dominated by contact line friction

We report experiments on the rapid contact line motion present in the early stages of capillary driven spreading of drops on dry solid substrates. The spreading data fails to follow a conventional viscous or inertial scaling. By integrating experiments and simulations, we quantify a contact line friction ($\mu_f$), which is seen to limit the speed of the rapid dynamic wetting. A scaling based on this contact line friction is shown to yield a universal curve for the evolution of the contact line radius as a function of time, for a range of fluid viscosities, drop sizes and surface wettabilities

Characterization of very-large-scale motions in high-Re pipe flows

Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number

Polymer-based photocathodes with a solution-processable cuprous iodide anode layer and a polyethyleneimine protective coating

Organic semiconductors are proven to efficiently drive photoelectrochemical water splitting

Liquid-phase exfoliation of bismuth telluride iodide (BiTeI): structural and optical properties of single-/few-layer flakes

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a two-dimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonication-assisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m(-1) and a Hildebrandt parameter between 19 and 25 MPa0.5. The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of & SIM;3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |chi((2))| & SIM; 1 nm V-1, which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS2)

Velocity measurements in a fiber suspension flow: formation of a fiber network

The aim of the present work is to experimentally study the dynamics of the formation of a fiber network formed from the filtration of a fiber suspension. This is relevant for all industrial applications (e.g. papermaking, productions of composite material, etc.) where a suspension of fibers has to flow through narrow gaps, and the quality of the product depends on the distribution of mass and orientation of the fibers. To study the dynamics of network formation, we developed an experimental setup where the filtration of a fiber suspension through a semi-permeable screen can be studied. In the setup, both the fluid and the solid phase can be visualized. The focus of the present thesis is to study the fluid flow generated during the filtration. Index of refraction matching, image processing and particle image velocimetry have been used to measure the velocity field in the proximity of the resulting fiber network. Experiments with varying fiber length and filtration velocity have been performed. The disturbances generated by the screen and the forming network was found to be confined in a region (boundary region), whose extension varies with time: first, after the formation of the first fiber layers, the extent of the boundary region increases; at later times, the boundary region is thinner. The extent appears to be correlated to the gap size either of the screen (at very early times) or of the fiber network, but independent of the filtration velocity. Fluctuations on a scale larger than a fiber length are also observed during the filtration process. These fluctuations are found to be correlated to the nondimensional number Se that relates the sedimentation velocity of a fiber to the filtration velocity. The governing non-dimensional parameters are derived from the equations. The parameters are used to relate the experimental observations to the dewatering process in papermaking.QC 2010110

Experimental Studies of Complex Flows through Image-Based Techniques

This thesis deals with the development of experimental techniques for the study of complex flows inspired to a large extent by the papermaking process. In particular one part of this thesis is devoted to the development of laboratory experiments based on index-of-refraction matching and imaging techniques to study the behavior of dilute and concentrated suspension of elongated particles. Another part is aimed at exploring the potential of the synergy between experiments and numerical simulations to access quantities otherwise not-measurable in complex flows. Highspeedimaging experiments have been specifically designed for this purpose. The first of the Refractive IndexMatching (RIM) experiment was aimed at studying the flow generated during the filtration of a fiber suspension using Particle Image Velocimetry (PIV) and pressure drop measurements. The experiments were performed in a vertical laboratory filtration device. Index of refraction matching of fibers and fluids allowed measurements to be performed in the proximity and, to some extent, in the forming network during filtration. The area over which the forming network induces velocity gradients has been measured and have been found to be independent of the Reynolds number but dependent on the fiber length and the structure of the network. Analysis of the flow scales in the proximity of the network showed that the signature of the mesh used to filter the suspension is never completely suppressed as the network thickness increases. Also, pressure drop measurements over a static fiber network have been performed. A linear dependence of the pressure drop with the basis weight (mass of fibers in the network per unit area) and a non-dimensional filtration resistance independent of filtration velocity and network thickness (if network compressibility is accounted for) was found. These findings can help explain characteristics that are observed on paper sheets and help improvede watering efficiency. The second RIM experiment was aimed at measuring the interactions of Taylorscale elongated particles with turbulence. RIM particles with embedded tracers and Stereoscopic PIV were combined to simultaneously measure fluid phase and particle velocity. The novelty of this technique is that it allows to measure the three-dimensional angular velocity vector of arbitrarily shaped particles. This technique was applied to study the interaction of neutrally buoyant ellipsoidal particles with stationary homogeneous isotropic turbulence. The results were compared to the case of spherical particles. The main result is that both spherical and ellipsoidal particles provide enhancement of the small scales and reduction of the large scales at volume concentrations as low as 0.1%. However, the reduction of the large scales was much more evident for spherical particles. These results highlight the fact that particle elongation introduces different mechanisms of turbulent modulation as compared to the spherical particles. The first of the high-speed imaging experiments was to provide a database for test and validation of a CFD-based flow observer for complex flows. For this purpose time resolved measurements of a turbulent confined jet have been performed with high-speed PIV. The measurements have been used both as a feedback signal and as a reference for the evaluation of a CFD-based estimator for complex flows. Furthermore, based on the measurements Kalman filters have been designed and implemented in the observer. The experimental data have also been used to compare two modal decompositions, namely Proper Orthogonal Decomposition and Dynamical Modal Decomposition and evaluate their ability to describe the global behavior of complex flow. The second of the high-speed imaging experiment was applied to study spreading of a droplet on a solid surface. These experiments have been performed with extremely high time-resolution (140000 fps), over a range of parameters (in terms of droplet viscosity, equilibrium contact angle and droplet size) larger than any other experiment reported in the literature in a single work. By combining the experiments and direct numerical simulations a dissipative mechanisms arising from the contact line movement has been identified and the corresponding macroscopic coefficient has been measured.iQC 2011051

Numerical investigation on the dynamics of a high-performance motorcycle equipped with an innovative hydro-pneumatic suspension system

A hydro-pneumatic suspension system that was recently proposed as a new solution to improve the performance of wheeled road vehicles, particularly motorcycles, is analyzed. The innovative device is primarily based on a hydro-pneumatic spring and a coil spring arranged in series, which generate a strongly non-linear behavior, and in particular a strongly regressive spring rate (i.e. spring rate is reduced with increasing load). This research aims at an initial assessment of the actual benefits and/or drawbacks provided by this device. Because of the strong non-linearities introduced by this suspension device, multibody simulations are an essential tool to do a realistic performance evaluation of such system, as standard linearization techniques would fail to capture the full dynamics. Multibody models are implemented to evaluate the dynamic response of the hydro-pneumatic system as compared to a standard suspension (i.e. linear spring-rate). As a first stage of the research, this work focuses on the in-plane dynamics, and in particular on preliminary simulations of the motorcycle in straight running at constant speed. The load fluctuations on the rear wheel (grip) as well as the velocity of the suspended mass under two different road inputs (comfort) at various speeds are analyzed. Results show that, although the hydro-pneumatic suspension has a higher resonance frequency at static equilibrium, the comfort level with respect to the softer suspension is basically unchanged, while the hydro-pneumatic suspension shows a great potential in terms of reduction in load fluctuations on the rear wheel, thus enhancing grip. Furthermore, the hydro-pneumatic device shows high sensitivity to hydraulic preload

Analysis of time-resolved PIV measurements of a confined co-flowing jet using POD and Koopman modes

Modal analysis by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of experimental data from a fully turbulent flow is presented. The flow case is a turbulent confined jet with co-flow, with Reynolds number based on the jet thickness of Re=10700. Experiments are performed with time-resolved Particle Image Velocimetry (PIV). The jet is created in a square channel with the confinement ratio is 1:5. Statistics of the flow are presented in terms of mean and fluctuating fields. Analysis of spatial spectra and temporal spectra reveal the presence of dominant wavelengths and frequenciesembedded in broad-band turbulent spectrum. Frequencies in the shearlayer migrate from St ≈ 1 near the jet inlet to St &lt; 0.1 at 18 jet thickness downstream. This flow case provides an interesting and challenging benchmark for testing POD and DMD and discussing their efficacy in describing a fully turbulent case. At first, issues related to convergence and physical interpretation of the modes are discussed, then the results are analyzed and compared. POD analysis reveals the most energetic spatial structures that are related to the flapping of the jet; a low frequency peak (St = 0.02) is found when the associated temporalmode is analyzed. Higher order modes revealed the presence of fasteroscillating shear flow modes combined to a recirculation zone near the inner jet. The flapping of the inner jet is sustained by this region. A good agreement is found between DMD and POD; however, DMD is able to rank the modes by frequencies, isolating structures associated to harmonics of the flow.QC 20110214</p