Experimental investigation of mixing and axial dispersion in Taylor–Couette flow patterns

The flow and mixing in a Taylor–Couette device have been characterized by means of simultaneous particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements. Concentration of a passive tracer measurements was used to investigate mixing efficiency for different flow patterns (from steady Taylor vortex flow to modulated wavy vortex flow, MWVF). Taylor–Couette flow is known to evolve toward turbulence through a sequence of flow instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to internal flow structures. PLIF measurements show clear evidences of different transport mechanisms including intravortex mixing and tracer fluxes through neighboring vortices. Under WVF and MWVF regimes, intravortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while intervortex transport occurs due to the presence of waves between neighboring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We show that hysteresis may occur between consecutive regimes depending on flow history, and this may have a significant effect on mixing for a given Reynolds numbe

Experimental Study of the Impact of Biogenic Macrorugosities on Transport Properties in Benthic Boundary Layers

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Experimental study of enhanced mixing induced by particles in Taylor–Couette flows

Local mixing dynamics was recently investigated experimentally in Taylor–Couettesingle-phase flow, thanks to simultaneous Particle Image Velocimetry (PIV) and PlanarLaser-Induced Fluorescence (PLIF) techniques. The results highlighted the influence of thesuccessive flow bifurcations and the role of azimuthal wave states on the dispersion of dyeinjected in Taylor–Couette flows.The present work extends this study to two-phase configurations with spherical solidparticles. The respective effect of particle size and concentration on the vortices size andtransition thresholds between the various flow regimes has been examined thanks to flowvisualizations and PIV measurements. These hydrodynamic features have been comple-mented with PLIF experiments, that revealed a drastic enhancement of mixing due to thepresence of particles regardless of the flow regime, highlighting the existence of significantparticle-induced mixing in Taylor–Couette flows

Milli-PIV rheology of shear-thinning fluids

Milli-Particle Image Velocimetry (Milli-PIV), which is an application of the standard PIV has been developed to measure the velocity distribution of complex fluids in a millimetric Hele-Shaw cell. A laser sheet is sent perpendicularly to the cell and the camera observes the PIV images via an internal optical prism. We studied, with Milli-PIV, a structural series of shear thinning xanthan fluids at different injection rates. We determined the velocity profiles in the thickness of the flow cell and discussed the accuracy of measurement. Using an inverse method, we calculated the constitutive parameters and showed that they are similar to those measured by rheology. The pressure gradient characteristic of the couple experimental conditions - fluid were also calculated from modeling the velocity profile

3D measurements of inclined vortex rings interacting with a density stratification

Vortex rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid. They are also considered as elementary bricks of turbulence. Their faculty to propagate along their revolution axis by self-induction confers to such structures interesting transport properties, namely, transport of momentum, mass and heat. They are therefore often qualified as good candidates for mixing. From this perspective, the present study addresses the interaction of a vortex ring with a density stratification in order to get a better understanding of the subsequent mixing mechanisms. A new 3D time-resolved technique is used and gives a highlight at short timescale on the 3D vorticity reorganization and at larger timescale on the 3D patterns of internal gravity waves forced by the impacting/penetrating vortex. The influence of the Reynolds number of the vortex ring and its angle of attack relative to isopycnals will be detailed

Experimental Investigation of Interfacial Mass Transfer Mechanisms for a Confined High-Reynolds-Number Bubble Rising in a Thin Gap

Interfacial mass transfer is known to be enhanced for confined bubbles due to the efficiency of the transfer in the thin liquid films between them and the wall. In the present experimental investigation, the mechanisms of gas–liquid mass transfer are studied for isolated bubbles rising at high Reynolds number in a thin gap. A planar laser induced fluores- cence (PLIF) technique is applied with a dye the fluorescence of which is quenched by dissolved oxygen. The aim is to measure the interfacial mass fluxes for pure oxygen bubbles of various shapes and paths rising in water at rest. In the wakes of the bubbles, patterns due to the presence of dissolved oxygen are observed on PLIF images. They reveal the contrasted contributions to mass transfer of two different regions of the interface. The flow around a bubble consists of both two thin liquid films between the bubble and the walls of the cell and an external high-Reynolds-number in-plane flow surrounding the bubble. Mass transfer mechanisms associated to both regions are discussed. Measurement of the concentration of dissolved oxygen is a difficult task due to the nonlinear relation between the fluorescence intensity and the concentration in the gap. It is however possible to accurately measure the global mass flux transferred through the bubble interface. It is determined from the fluorescence intensity recorded in the wakes when the oxygen distribution has been made homogeneous through the gap by diffusion. Assuming a reasonable distribution of oxygen concentration through the gap at short time also allows a measurement of the mass fluxes due to the liquid films. A discussion of the results points out the specific physics of mass transfer for bubbles confined between two plates as compared to bubbles free to move in unconfined flows. VC 2016 American Institute of Chemical Engineers AIChE J, 63: 2394–2408, 201

Experimental investigation of mixing efficiency in particle‑laden Taylor–Couette flows

This paper reports on original experimental data of mixing in two-phase Taylor–Couette flows. Neutrally buoyant particles with increasing volume concentration enhance significantly mixing of a passive tracer injected within the gap between two concentric cylinders. Mixing efficiency is measured by planar laser-induced fluorescence coupled to particle image velocimetry to detect the Taylor vortices. To achieve reliable experimental data, index matching of both phases is used together with a second PLIF channel. From this second PLIF measurements, dynamic masks of the particle positions in the laser sheet are determined and used to calculate accurately the segregation index of the tracer concentration. Experimental techniques have been thoroughly validated through calibration and robustness tests. Three particle sizes were considered, in two different flow regimes to emphasize their specific roles on the mixing dynamics

Investigation of the three-dimensional turbulent near-wake structure past a flat plate by tomographic PIV at high Reynolds number

This paper reports an experimental investigation of the high-Reynolds number turbulent flow pasta thin flat plate with sharpuntapered edges,by means of tomographic PIV. The experiments,carried out in the S4 wind tunnel of IMFT, have quantified the three-dimensional coherent vortex structures, by means of 3-D Proper Orthogonal Decomposition and reconstruction. The interaction of themostenergetic coherent structures with the random turbulence is discussed. Furthermore, Proper Orthogonal Decomposition (POD), analysis allowed evaluation of three-dimensional phase-averaged dynamics that quantified the vortex shedding mechanism as well as the influence of higher modes associated with the finer-scale turbulence

Coherent and turbulent processes in the bistable regime around a tandem of cylinders including reattached flow dynamics by means of high-speed PIV

The turbulent flow around two cylinders in tandem at the sub-critical Reynolds number range of order of 105 and pitch to diameter ratio of 3.7 is investigated by using time-resolved Particle Image Velocimetry (TRPIV) of 1 kHz and 8 kHz. The bi-stable flow regimes including a flow pattern I with a strong vortex shedding past the upstream and the downstream cylinder, as well as a flow pattern II corresponding to a weak alternating vortex shedding with reattachment past the upstream cylinder are investigated. The structure of this “reattachment regime” has been analyzed in association with the vortex dynamics past the downstream cylinder, by means of POD and phase-average decomposition. These elements allowed interconnection among all the measured PIV planes and hence analysis of the reattachment structure and the flow dynamics past both cylinders. The results highlight fundamental differences of the flow structure and dynamics around each cylinder and provide the ‘gap’ flow nature between the cylinders. Thanks to a high-speed camera of 8 kHz, the shear-layer vortices tracking has been possible downstream of the separation point and the quantification of their shedding frequency at the present high Reynolds number range has been achieved. This issue is important regarding fluid instabilities involved in the fluid–structure interaction of cylinder arrays in nuclear reactor systems, as well as acoustic noise generated from the tandem cylinders of a landing gear in aeronautics

Mixing by bubble-induced turbulence

This work reports an experimental investigation of the dispersion of a low-diffusive dye within a homogeneous swarm of high-Reynolds-number rising bubbles at gas volume fractions α ranging from 1% to 13 %. The capture and transport of dye within bubble wakes is found to be negligible and the mixing turns out to result from the bubble-induced turbulence. It is described well by a regular diffusion process. The diffusion coefficient corresponding to the vertical direction is larger than that corresponding to the horizontal direction, owing to the larger intensity of the liquid fluctuations in the vertical direction. Two regimes of diffusion have been identified. At low gas volume fraction, the diffusion time scale is given by the correlation time of the bubble-induced turbulence and the diffusion coefficients increase roughly as α 0.4. At large gas volume fraction, the diffusion time scale is imposed by the time interval between two bubbles and the diffusion coefficients become almost independent of α. The transition between the two regimes occurs sooner in the horizontal direction (1%6<α<63 %) than in the vertical direction (3%6<α<66 %). Physical models based on the hydrodynamic properties of the bubbleswarm are introduced and guidelines for practical applications are suggested