Oscillating Flow Regime Research Articles

Implications of capillarity-wettability interaction on geometrically mediated droplet splitting mechanism

The droplet splitting in a microchannel with different geometrical configurations viz., curved T-junction, flat T-junction and Y-shaped channel is investigated using a level set method for different wettability conditions of the wall surfaces and capillary number. The underlying physics is examined in terms of deformation of droplet, neck thickness, and the time that the droplet spends at the junction. For a Y-shaped channel, the droplet splitting takes place even for a small capillary number due to the presence of sharp bifurcating tip. On the other hand, the hydrophilic surfaces of channel wall yields into droplet splitting as compared to neutral and hydrophobic surfaces. In general, the droplet splitting is seen at higher capillary numbers whereas non-splitting and oscillating flow of droplet near the junction is observed at lower capillary number. The curved and flat T-junctions channels yield almost similar droplet splitting behaviour. In general, the Y-shaped channel with hydrophilic wall surfaces is found to be ideal configuration for droplet splitting to occur. Finally, the splitting, non-splitting and oscillating flow regimes are identified and its dependence on physicochemical parameters like wettability, capillary number and geometrical orientation are discussed. The present study establishes the significance and advancement on the droplet splitting mechanism prior to the previously reported investigations through the consideration of curve T-junction and surface wettability influence.

Dissolved oxygen transfer from oscillatory flows to microbes in a permeable organic sediment bed

We investigate high-Schmidt-number mass transfer of dissolved oxygen (DO) from oscillatory turbulent flows to permeable microbial sediment bed by large eddy simulations. The three-dimensional open channel flow is numerically solved for friction Reynolds number 180 and Schmidt number 373. In this numerical study, an external periodical pressure gradient is applied to drive intermittent turbulence over the streamwise direction. The bottom sediment layer populated with DO-absorbing bacteria is described by a modified Darcy-Brinkman-Forcheimer model which couples a bio-geochemical model. The dependence of mass transfer on amplitude and period is analyzed by means of phase-average statistical quantities. The goal is to investigate oxygen transport dynamics towards an underlying mass-absorbing sediment layer in the oscillating flow regime. The present study reveals that the turbulent diffusion is the major path in DO transport. The bulk velocity, the turbulent kinetic energy, DO concentration, and the diffusion flux all exhibit periodic and quasi-periodic patterns. Phase lags can be observed for these quantities compared with the oscillatory forcing signal. It reveals that the flow field as well as oxygen consumption requires a certain amount of time for feedback on the oscillatory force. A negative correlation exists between the oxygen consumption and DO concentration. Low frequency and high amplitude of oscillations could significantly influence the transfer of momentum and oxygen.

Instabilities and spin-up behaviour of a rotating magnetic field driven flow in a rectangular cavity

This study presents numerical simulations and experiments considering the flow of an electrically conducting fluid inside a cube driven by a rotating magnetic field (RMF). The investigations are focused on the spin-up, where a liquid metal (GaInSn) is suddenly exposed to an azimuthal body force generated by the RMF, and the subsequent flow development. The numerical simulations rely on a semi-analytical expression for the induced electromagnetic force density in an electrically conducting medium inside a cuboid container with insulating walls. Velocity distributions in two perpendicular planes are measured using a novel dual-plane, two-component ultrasound array Doppler velocimeter (UADV) with continuous data streaming, enabling long term measurements for investigating transient flows. This approach allows to identify the main emerging flow modes during the transition from a stable to unstable flow regimes with exponentially growing velocity oscillations using the Proper Orthogonal Decomposition (POD) method. Characteristic frequencies in the oscillating flow regimes are determined in the super critical range above the critical magnetic Taylor number $Ta_c \approx 1.26 \times 10^5$, where the transition from the steady double vortex structure of the secondary flow to an unstable regime with exponentially growing oscillations is detected. The mean flow structures and the temporal evolution of the flow predicted by the numerical simulations and observed in experiments are in very good agreement.

Comparative Performance of Thermoacoustic Heat Exchangers with Different Pore Geometries in Oscillatory Flow. Implementation of Experimental Techniques

Heat exchangers (HXs) constitute key components of thermoacoustic devices and play an important role in determining the overall engine performance. In oscillatory flow conditions, however, standard heat transfer correlations for steady flows cannot be directly applied to thermoacoustic HXs, for which reliable and univocal design criteria are still lacking. This work is concerned with the initial stage of a research aimed at studying the thermal performance of thermoacoustic HXs. The paper reports a detailed discussion of the design and fabrication of the experimental set-up, measurement methodology and test-HXs characterized by two different pore geometries, namely a circular pore geometry and a rectangular (i.e., straight fins) pore geometry. The test rig is constituted by a standing wave engine where the test HXs play the role of ambient HXs. The experiment is conceived to allow the variation of a range of testing conditions such as drive ratio, operation frequency, acoustic particle velocity, etc. The procedure for estimating the gas side heat transfer coefficient for the two involved geometries is described. Some preliminary experimental results concerning the HX with straight fins are also shown. The present research could help in achieving a deeper understanding of the heat transfer processes affecting HXs under oscillating flow regime and in developing design optimization procedures.

Intensification of liquid-liquid mass transfer by oscillation in a high-throughput microextractor

A highly efficient microextractor with large fluxes was designed based on oscillation to improve its extraction performance. The flow patterns and mass transfer performance of the microextractor were experimentally investigated. The influence of Reynolds number on these metrics of a liquid-liquid system was discussed. From the flow visualization experiment, three different flow patterns were observed: stratified flow, single oscillating flow, and full oscillating flow. It was found that the mass transfer performance was closely related to the conversion of the flow patterns. In the stratified flow, the extraction efficiency decreased with an increase in Reynolds number, while in the single oscillating flow and fully oscillating flow regimes, the extraction efficiency increased with increasing Reynolds number. When the flux of the aqueous phase was 45mL/min with a corresponding residence time of 0.027s, an extraction efficiency of 98.9% was achieved. Furthermore, the overall volumetric mass transfer coefficient of the oscillating extractor was about two to three orders of magnitude higher than that of traditional liquid-liquid macro-scale extractors and one magnitude higher than that of other liquid-liquid micro-scale extractors. The high efficiency, high throughput, and short residence time gives the oscillating extractor a competitive advantage over other liquid-liquid extractors.

A Numerical Study of Oscillating Peristaltic Flow of Generalized Maxwell Viscoelastic Fluids Through a Porous Medium

This article presents a numerical study on oscillating peristaltic flow of generalized Maxwell fluids through a porous medium. A sinusoidal model is employed for the oscillating flow regime. A modified Darcy-Brinkman model is utilized to simulate the flow of a generalized Maxwell fluid in a homogenous, isotropic porous medium. The governing equations are simplified by assuming long wavelength and low Reynolds number approximations. The numerical and approximate analytical solutions of the problem are obtained by a semi-numerical technique, namely the homotopy perturbation method. The influence of the dominating physical parameters such as fractional Maxwell parameter, relaxation time, amplitude ratio, and permeability parameter on the flow characteristics are depicted graphically. The size of the trapped bolus is slightly enhanced by increasing the magnitude of permeability parameter whereas it is decreased with increasing amplitude ratio. Furthermore, it is shown that in the entire pumping region and the free pumping region, both volumetric flow rate and pressure decrease with increasing relaxation time, whereas in the co-pumping region, the volumetric flow rate is elevated with rising magnitude of relaxation time.

Hydrodynamic parameters of mesh fillers relevant to miniature regenerative cryocoolers

Directional hydrodynamic parameters of two fine-mesh porous materials that are suitable for miniature regenerative cryocoolers were studied under steady and oscillating flows of helium. These materials included stacked discs of #635 stainless steel (wire diameter of 20.3 μm) and #325 phosphor bronze (wire diameter of 35.6 μm) wire mesh screens, which are among the commercially available fillers for use in small-scale regenerators and heat exchangers, respectively. Experiments were performed in test sections in which pressure variations across these fillers, in the axial and lateral (radial) directions, were measured under steady and oscillatory flows. The directional permeability and Forchheimer’s inertial coefficient were then obtained by using a Computational Fluid Dynamics (CFD)-assisted method. The oscillatory flow experiments covered a frequency range of 50–200 Hz. The results confirmed the importance of anisotropy in the mesh screen fillers, and indicated differences between the directional hydrodynamic resistance parameters for steady and oscillating flow regimes.

CFD modelling of a countercurrent staged fluidised bed

Abstract The bed flow regimes and the hydrodynamics in a 2D countercurrent staged fluidised bed were simulated using Computation Fluid Dynamics (CFD). Based on the two-fluid theory, an Eulerian–Eulerian approach coupled with kinetic theory of granular flow (KTGF) for the solid phase was applied. The predictions were compared with the experimental results of Kuo and Cheng (2006). The calculated and experimental results both showed a non-growth flow regime, a dilute flow regime, an oscillating flow regime, a bubbling flow regime and a flooding flow regime on the perforated plate at different operating conditions. The values of the predicted pressure drop across the bed at steady state for the dilute flowing regime and the bubbling flowing regime agreed quantitatively well with the previous experimental results, although the time to reach the steady state was different. The calculated flow regime map as functions of the gas velocities and solids feeding rates agreed qualitatively with the experiments.

A non-stationary signal correlator for on-line transit time estimation

The on-line reliable estimation and monitoring of transit time in BWRs is investigated. The interactive activation-competition artificial neural network is employed for providing an estimate of the current transit time directly at each incoming pair of signal values. As a result, transit time monitoring is accomplished for both constant and oscillating flow regimes, i.e. for both stationary and non-stationary signals. The proposed approach is robust to the presence of local and global component as well as to the addition of white uncorrelated noise.

Conditional density dependence: The flow trigger to expression of density-dependent emigration in bay scallops

Understanding how biological interactions vary under different water flow regimes is important in systems that are known to exhibit fluctuations in flow due to storms, tidal forcing, or variation in habitat structure. We performed a series of experiments to determine the migratory response of bay scallops, Argopecten irradians concentricus Say, to changes in intraspecific density under two different, tidally oscillating flow regimes in the field: 0–0.10 m s−1 and 0–0.28 m s−1. Despite the bay scallop's ability to swim actively and achieve unaided bursts of motion covering distances on the scale of 0.5 m, the expression of density-dependent emigration was dependent on the presence of higher flows. In the low-flow regime, there was negligible emigration during 24 h at any density level (12, 25, or 62 m−2); however, emigration rate increased with density (0% at 12 m−2, 60% at 25 m−2, and 71% at 62 m−2) under the higher flow regime. These results demonstrate that flow can facilitate biological interactions, creating conditional density dependence.

The effect of oscillatory flow on crossflow microfiltration of beer in a tubular mineral membrane system – Membrane fouling resistance decrease and energetic considerations

We have investigated the consequences due to the changes in hydrodynamics above the membrane surface brought about by an oscillatory flow in the crossflow microfiltration (CFMF) of beer on a tubular mineral membrane. Experimental results in oscillatory flow filtration were analysed in terms of membrane resistance to filtration and energy consumption and compared with steady flow filtration. Two types of beers were used: a clarified beer composed of colloids and macromolecular material and a rough beer containing in addition yeast cells. Oscillatory flow was found to decrease membrane fouling resistance (up to 100%) in rough beer filtration in the presence of a yeast cell cake layer on the membrane surface, whereas it has no effect in clarified beer filtration in the presence of membrane clogging. The detrimental effect of transmembrane pressure on membrane resistance (at Δ P>1 bar) has been emphasized in both oscillatory and steady flows. The time-average hydraulic power dissipated by friction in the filtration module, in relation with the absolute value of the time-average flow rate in oscillatory flow, was found to be systematically higher than for steady flow filtration. However, the hydraulic energy per unit volume of permeate in the microfiltration of rough beer under oscillatory flow was close to that in steady flow at a time-average tangential velocity of 3 m/s. By considering the specific energy (per m 3 of permeate) related to the kinetic energy applied to fluid in oscillatory and steady flow modes, the system by gas compression in oscillatory flow led to a reduction of specific energy ranging from 15% to 40%. Finally the ratio of hydraulic power consumed in oscillatory and steady flow was compared with a theoretical calculation based on the assumption that the oscillating flow regime is quasi-steady.

Experimental study and modeling of oscillating flow of high density polyethylenes

The influence of flow rate and die geometry on the observable flow rate/pressure relationship of a linear high density polyethylene is investigated using a capillary rheometer. The experimental results are applied to an adapted version of the relaxation–oscillation model proposed by Molenaar and Koopmans for describing the oscillating flow regime. The current model allows for a quantitative description of the hysteresis cycle in the oscillating flow regime in terms of the main experimental variables, such as imposed flow rate, reservoir (barrel) volume, and material compressibility.

Flow in an Annular-Conical Passage.

An experimental study has been conducted on the flow behavior in an annular-conical passage. Experiments are carried out for three different cone apex angles (β=60°, 90°, 120°). For each apex angle two cases of different passage spacing are studied. The emphasis is placed on the transition from laminar to self-sustained oscillating flow regime in which the critical Reynolds number is investigated in relation to the apex angle and passage spacing. It is found that the critical Reynolds number depends on both the cone apex angle and passage spacing. For the self-sustained oscillating regime the Strouhal number is evaluated also with respect to both the passage spacing and cone apex angle and the dependence of the Strouhal number on the Reynolds number is determined.

Distortions of polymer melt extrudates and their elimination using slippery surfaces

An overview is given of extrudate distortions, flow fields and flow curves observed for flexible polymer melts, when they are extruded using axisymmetric or two-dimensional dies. Die walls with high surface energy and low-energy slippery surfaces are considered, as well as the influence of wall roughness. The connection between flow curves and the flow field structure is explained. Details of the extrudate distortions are given, focusing on the physical phenomena common to all polymer chemical species. A description of the second oscillating flow regime is included. Recent results on ideal and on slippery surfaces are reviewed. An application of slippery surface extrusion to several commercial polyethylenes is presented. It is shown that cracks and flow oscillation distortions can be completely eliminated, and very significant quality and productivity improvements can be obtained using slippery surfaces.

Foraging to the rhythm of ocean waves: porcelain crabs and barnacles synchronize feeding motions with flow oscillations

Suspension-feeding activity patterns of four benthic crustacean species (2 porcellanid crabs and 2 balanomorph barnacles) were studied in simulated wave-action (oscillating flow in a laboratory flow tank). The hypothesis that the frequency of rhythmic suspension-feeding motions observed in oscillating flow depends on the frequency of flow oscillation was tested. Also, we examined the effect of flow oscillation frequency on time allocation to food-particle-trapping versus non-trapping components of feeding behavior, as an indication of how the energetic gain-to-cost ratio of feeding varies with flow regime. A newly developed flow pattern generator, consisting of computer programs interfaced with a flow tank motor, was used to produce various oscillating flow regimes (0.16–0.65 Hz. 6.2–12.1 cm·s −1 maximum velocity). Laser-optics, fiber optics, and video equipment were used to record behavior and to determine water flow velocities non-intrusively, by tracking the video-recorded movement of back-lit particles suspended in flowing seawater. All animals tested rhythmically reoriented suspension-feeding fans in oscillating flow so that the cup-shaped fans faced concave-upstream. At all test frequencies, the frequency of the cyclic motions of feeding append ages matched the frequency of flow directional oscillation precisely. Also, all species behaviorally anticipated changes in flow direction, by beginning to reorient feeding fans to changing flow direction before the water itself had reversed flow direction (i.e the animals responded when water was decelerating). This anticipatory behavior allowed animals feeding in wave-action to avoid missing full fan extension during part of the peak velocity range of an upcoming pulse of flow, as would have happened if they had waited until flow direction changed to reorient appendages. That animals actively track wave frequency so precisely with their feeding fan movements suggests than, in natural habitats, wave frequency is a critical factor for certain consumers that affects the energetics of feeding activity. Video analysis showed that as flow oscillation frequency decreased, all animals spent a greater fraction of feeding time trapping suspended food particles with fans fully extended into flow, and a corresponding smaller fraction of time engaged in the muscular activity of removing trapped particles and reorienting feeding fans. These changes suggest an increase in energetic feeding gain-to-cost ratio with a decrease in flow oscillation frequency.