Formation Of Flow Patterns Research Articles

Advanced neural network prediction and system identification of liquid-liquid flow patterns in circular microchannels with varying angle of confluence

The flow pattern map for liquid-liquid system in a circular microchannel of 600μm was experimentally investigated for a varying confluence angle (10–170 degree) of inlet fluids. The experimental results showing distinguishing nature of transition boundaries were established using graphical interpretation. Investigations were carried out to find a better objective flow pattern indicator for a vast flow pattern observations. Studies on advanced feed-forward back-propagation networks and radial basis networks such as CFN, ANN-FF, ANN-PR, PNN, GRNN and ANFIS showed that GRNN (Generalized Regression Neural Network) showed a better prediction over other prediction techniques with a R2 value of 0.988. The relative impact of input variables such as confluence angle, superficial velocity of dodecane and superficial velocity of water on flow pattern formation was found to be 17.78%, 43.30% and 38.92% respectively. The discrete and continuous time state space models for the system was also developed and interpreted in detail using system identification technique for the better understanding of the system.

Yielding of glass under shear: A directed percolation transition precedes shear-band formation.

Under external mechanical loading, glassy materials, ranging from soft matter systems to metallic alloys, often respond via formation of inhomogeneous flow patterns, during yielding. These inhomogeneities can be precursors to catastrophic failure, implying that a better understanding of their underlying mechanisms could lead to the design of smarter materials. Here, extensive molecular dynamics simulations are used to reveal the emergence of heterogeneous dynamics in a binary Lennard-Jones glass, subjected to a constant strain rate. At a critical strain, this system exhibits for all considered strain rates a transition towards the formation of a percolating cluster of mobile regions. We give evidence that this transition belongs to the universality class of directed percolation. Only at low shear rates, the percolating cluster subsequently evolves into a transient (but long-lived) shear band with a diffusive growth of its width. Finally, the steady state with a homogeneous flow pattern is reached. In the steady state, percolation transitions also do occur constantly, albeit over smaller strain intervals, to maintain the stationary plastic flow in the system.

LDV survey of cavitation and resonance effect on the precessing vortex rope dynamics in the draft tube of Francis turbines

The large-scale penetration of the electrical grid by intermittent renewable energy sources requires a continuous operating range extension of hydropower plants. This causes the formation of unfavourable flow patterns in the draft tube of turbines and pump-turbines. At partial load operation, a precessing cavitation vortex rope is formed at the Francis turbine runner outlet, acting as an excitation source for the hydraulic system. In case of resonance, the resulting high-amplitude pressure pulsations can put at risk the stability of the machine and of the electrical grid to which it is connected. It is therefore crucial to understand and accurately simulate the underlying physical mechanisms in such conditions. However, the exact impact of cavitation and hydro-acoustic resonance on the flow velocity fluctuations in the draft tube remains to be established. The flow discharge pulsations expected to occur in the draft tube in resonance conditions have for instance never been verified experimentally. In this study, two-component Laser Doppler Velocimetry is used to investigate the axial and tangential velocity fluctuations at the runner outlet of a reduced scale physical model of a Francis turbine. The investigation is performed for a discharge equal to 64 % of the nominal value and three different pressure levels in the draft tube, including resonance and cavitation-free conditions. Based on the convective pressure fluctuations induced by the vortex precession, the periodical velocity fluctuations over one typical precession period are recovered by phase averaging. The impact of cavitation and hydro-acoustic resonance on both axial and tangential velocity fluctuations in terms of amplitude and phase shift is highlighted for the first time. It is shown that the occurrence of resonance does not have significant effects on the draft tube velocity fields, suggesting that the synchronous axial velocity fluctuations are surprisingly negligible compared to the velocity fluctuations induced by the vortex precession.

Zonal Flow Patterns: How Toroidal Coupling Induces Phase Jumps and Shear Layers.

A new, frequency modulation mechanism for zonal flow pattern formation is presented. The model predicts the probability distribution function of the flow strength as well as the evolution of the characteristic spatial scale. Magnetic toroidicity-induced global phase dynamics is shown to determine the spatial structure of the flow. A key result is the observation that global phase patterning can lead to zonal flow formation in the absence of turbulence inhomogeneity.

Droplet formation of H2SO4/alkane system in a T‐junction microchannel: Gravity effect

A special system of concentrated sulfuric acid (H2SO4) and n‐hexane was used to study the droplet formation in a glass T‐junction microchannel with H2SO4 as the continuous phase. The effects of capillary number, flow ratio, and viscosity ratio on the droplet formation were investigated. The effect of gravity was explored by changing the flow direction in the microchannel. Results showed that the formation of transition flow pattern from squeezing to dripping is much easier for this special system compared with common aqueous/organic systems. This phenomenon is due to the considerably higher viscosity of H2SO4 than that of common aqueous phase and the higher density difference of the system compared with those of common systems. In addition to capillary number and flow ratio, gravity evidently affects the formation of droplets and flow patterns. The droplet size is smaller than that during the horizontal flow when the flow direction is consistent with gravity. By contrast, flow direction contrary to that of gravity results in larger droplet size than that at horizontal flow. This phenomenon provides guidance on the operation of these special systems in microchannels. Finally, mathematical models of droplet size at different flow patterns have been established, and these models can predict droplet size very well. This study could be helpful to extend the application of microreactors to new working systems. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4564–4573, 2016

A Kinematic Description of the Key Flow Characteristics in an Array of Finite-Height Rotating Cylinders

Experimental data are presented for large arrays of rotating, finite-height cylinders to study the dependence of the three-dimensional (3D) mean flows on the geometric and rotational configurations of the array. Two geometric configurations, each with two rotational configurations, were examined at a nominal Reynolds number of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders and that net time–space averaged transverse and vertical flows exist within the developed flow region of the array. This net vertical mean flow provides an additional mechanism for the exchange of momentum between the flow within the array and the flow above it, independent from the turbulent exchange mechanisms which are also observed to increase by almost a factor of three in a rotating array. As an array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines (VAWTs), this planform momentum flux (both mean and turbulent) is of particular interest, as it has the potential to increase the energy resource available to turbines far downstream of the leading edge of the array. In the present study, the streamwise momentum flux into the array could be increased for the rotating-element arrays by up to a factor of 5.7 compared to the stationary-element arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotating-element arrays compared to stationary-element arrays.

Traffic flow sensitivity to parameters in viscoelastic modelling

ABSTRACTThis paper presents viscoelastic modelling of traffic flows briefly first and then shows numerical simulation results of ring traffic flow sensitivity to model parameters, such as viscoelasticity, average vehicle length, braking distance, and characteristic length used to define relaxation time. It was found that traffic flow pattern formation is dramatically sensitive to viscoelasticity and fundamental diagram which is explicitly impacted by average vehicle length and braking distance, implying that drivers' self-organisation ability is significant in determining the intensity of traffic wave interaction. Since the decrease of characteristic length can lead to a relevant change of traffic relaxation time, the length can play a sensitive role in changing traffic wave structures. Optimisation of traffic regulations is necessary for the well operation of a segmental or ring road.

Three-dimensional numerical simulation of double-diffusive Rayleigh–Bénard convection in a cylindrical enclosure of aspect ratio 2

This paper presented a series of three-dimensional numerical simulations on the double-diffusive Rayleigh–Bénard convection in a cylindrical enclosure filled with the isopropanol/water mixture with an initial mass fraction of 10%. The Prandtl number and the Lewis number of the isopropanol/water mixture were 13.2 and 140.2, respectively. The cylindrical enclosure was heated at the bottom wall and cooled at the top wall. Results show that the ratio of the solutal buoyancy to thermal buoyancy has an important effect on flow pattern formation and bifurcation sequences. For cooperating buoyancy convection, an unsteady flow appears foremost, and then four typical steady flow patterns happen orderly. For opposing buoyancy convection, only five types of oscillatory flow patterns are detected, which are characterized by azimuthal rotation. The critical Rayleigh number at the primary threshold is strongly influenced by concentration gradient, and decreases with the increase of the buoyancy ratio. Different solution branches are denoted in terms of the Nusselt number, which depends primarily on the Rayleigh number and the flow pattern. Furthermore, the several flow pattern coexistence and hysteresis phenomenon of the flow pattern transition in double-diffusive Rayleigh–Bénard convection have also been confirmed.

Two-phase flow patterns in short horizontal rectangular microchannels

The two-phase flow in a short horizontal channel of a rectangular cross-section with the height of 100–500 µm and width of 9–40 mm was studied experimentally. The use of the Schliren and fluorescent methods made it possible to reveal the flow of liquid in the channel and to determine its characteristics quantitatively. The features of the churn, jet and drop flow patterns were studied in details. Two particular regimes that can be distinguished represent formation of immobile drops on the channel walls because of the liquid film or liquid bridges breakage and appearance of mobile drops due to the two-phase flow instabilities. It is found out that formation of various two-phase flow patterns and transitions between them are determined by instabilities of the liquid–gas flow in the side parts of a channel. Frontal instability has been observed during the liquid–gas interaction in the region of liquid output from the nozzle. It is shown that a change in the height and width of the horizontal channels has a substantial effect on the boundaries between the flow regimes. One of the results is that the region of the churn regime increases significantly with decreasing thickness of the channel.

Stochastic-deterministic modeling of bed load transport in shallow water flow over erodible slope: Linear stability analysis and numerical simulation

In this article we propose a stochastic bed load transport formulation within the framework of the frictional shallow-water equations in which the sediment transport rate results from the difference between the entrainment and deposition of particles. First we show that the Saint-Venant-Exner equations are linearly unstable in most cases for a uniform base flow down an inclined erodible bed for Shields numbers in excess of the threshold for incipient sediment motion allowing us to compute noise-induced pattern formation for Froude numbers below 2. The wavelength of the bed forms are selected naturally due to the absolute character of the bed instability and the existence of a maximum growth rate at a finite wavelength when the particle diffusivity coefficient and the water eddy viscosity are present as for Turing-like instability. A numerical method is subsequently developed to analyze the performance of the model and theoretical results through three examples: the simulation of the fluctuations of the particle concentration using a stochastic Langevin equation, the deterministic simulation of anti-dunes formation over an erodible slope in full sediment-mobility conditions, and the computation of noise-induced pattern formation in hybrid stochastic-deterministic flows down a periodic flume. The full non-linear numerical simulations are in excellent agreement with the theoretical solutions. We conclude highlighting that the proposed depth-averaged formulation explains the developments of upstream migrating anti-dunes in straight flumes since the seminar experiments by Gilbert (1914).

The effect of rotational speed on granular flow in a vertical bladed mixer

Granular mixing is an important unit operation used to ensure the uniformity of mixture properties. Unfortunately, the mechanisms of particles' motion and the role of operation parameters remain poorly understood. In this work, granular mixing is studied using computer simulations via DEM (discrete element method). The examined process is the mixing of approximately 42 thousand glass spherical particles with a 2mm diameter in a vertical cylindrical mixer with two opposed flat blades with a 45° rake angle. The effect of different blade rotational speeds (varying from 0.1rpm to 960rpm) on the formation and evolution of flow patterns is investigated. Examining individual particle trajectories shows that the particles exhibit two basic types of periodic movements. The first one, characterized by a higher frequency, is related to the motion of the stirrer. The second one (lower frequency) is associated with recirculations in the vertical plane. A methodology for their detection is proposed. The observed recirculation zones are the secondary flow commonly occurring in liquid mixing cases and also the mechanism of their emergence is similar. Unlike liquids, the mixing of granular systems exhibits a greater diversity of recirculation zones. The tangential motion does not create complex structures in the angular direction, because the height of the granular layer is several times greater than the height of the blade. For this reason, we have focused only on the tangential velocity distribution in the radial and axial directions. Three global particle transport characteristics describing the mixing process in the tangential, axial and radial directions were proposed. Significant changes in the behavior of these variables were used to distinguish six dynamical regimes of granular mixing depending on the blade rotational speed. These regimes are characterized by different flow patterns.

The experimental study of water management in the cathode channel of single-serpentine transparent proton exchange membrane fuel cell by direct visualization

The water existence is essential to preserve membrane of PEMFC well-hydrated but accumulation of excessive water in channel leads to flooding and performance loss. In current research, the water management in cathode channel of a single-serpentine transparent PEMFC is studied for different stoichiometry, RH and temperature using direct visualization and image processing. An algorithm is introduced to quantify and classify the water and flow patterns in the cathode channel. The water coverage ratio is defined as measurement of liquid water accumulation in the total and local areas of channel. It is found that accumulation of water in the elbows is significant. Most of liquid water accumulates in latter channel rows near to gas outlet. The cathode stoichiometry has more effect on water management and cell performance than anode's. The effect of water coverage on cell performance is investigated. The time durations for first formation of important two-phase flow patterns in cell water management as fog/mist, slug and plug are calculated to obtain the time-scale of two-phase flow and water management. The superficial velocities of moving slugs and plugs are studied.

Heat Transfer and Flow-pattern Formation in a Cylindrical Cell with Partially Immersed Heating Element

A non-Boussinesq weakly compressible-ϐlow numerical model is used to study the temperature and ϐluid velocity distribution in a cylindrical tank with a heating element partially immersed in a temperature dependent ϐluid. This conϐiguration is commonly found in immersion heaters and electrolytic cells where the heating element does not extend along the entire axial length of the cylindrical vessel. A very particular vortex pattern arises inside the cell which stratiϐies the domain dividing it into two regions of different temperatures. With the increase in aspect-ratio, the Nusselt number at the surface of heating element increases, while the Nusselt number at the inner surface of cylindrical tank decreases.

Stable Vortex Generation in Liquid Filled Wells by Mode Conversion of Surface Acoustic Waves

The formation of stable vortex flow pattern has been observed at liquid filled aluminum wells of 15 to 30mm diameter when Lamb waves are excited on the bottom of the wells by piezoelectric transducers operated at a frequency of 1MHz. The shape of the vortex pattern changed with the position of the transducer. Strong differences in mixing times were observed between water and ethanol when the filling level was changed and a remarkable reduction of mixing time was achieved by the addition of a small amount of detergent to water at small filling levels. Besides mixing of liquids thermal equilibration within a liquid volume was accelerated by acoustic streaming.

Development of a mechanistic foam model for alkanolamine-based CO2 absorbers fitted with sheet-metal structured packing

A foam model was developed for the alkanolamine-based CO2 absorber fitted with sheet-metal structured packing. The model was built upon the principles of fluid flow pattern, column hydrodynamics, and foam formation mechanism and was verified with the experimental foaming data obtained from a 0.10m (I.D.) absorber fitted with Mellapak 500.Y. Experimental results show that liquid velocity has an apparent effect on foam volume, whereas superficial gas velocity has a negligible effect at a low liquid velocity and a small effect at higher liquid velocities. Simulation results show that the model has the capacity for predicting foam volumes within the absorber with an average absolute deviation (AAD) of 16.3%, for determining possible foam sites and process conditions where foaming is likely to occur, and for evaluating foaming impacts on process throughput. More foam tends to occur at the absorber top than the bottom and causes no significant reduction in process throughput. The presence of degradation products and corrosion inhibitors induces more foam volumes in the absorber.

Computational Modeling of the Hydraulic Jump in the Stilling Basin with Convergence Walls Using CFD Codes

The stilling basin has been accepted to be the most powerful hydraulic structure for the dissipation of the flow energy. The size and geometry of the stilling basin affect the formation of flow patterns, which can be influ- ential for hydraulic performance of the whole system. The NazlooDaminIranwasselectedasthestudyarea.TheUSBR II stilling basin was conducted for four convergence angles (5 ◦ ,7.5 ◦ ,10 ◦ , and 12.5 ◦ ). The convergence walls cause the jump to stabilizeinside thebasin and cause the energy loss to increase in the stilling basin. To simulate the hydraulic jump in the convergence stilling basin conditions in this region, a free surface computational fluid dynamics (CFD) numerical model has been applied. The commercially known software, FLOW-3D, was applied to numerically solve the Navier- Stokes equations for solution domains, namely the shout, the stillingbasin and thedownstream of dam, and to estimatethe turbulence flow, the standard k-e and RNG models was used. These models are based on the volume-of-fluid method, and they are capable of simulating the hydraulic jump. The cal- culated results such as the pressure, the velocities, the flow rate, the surface height air entranced, the kinetics energy, the kinetics energy dissipated, and the Froude number were compared with the scale model data where available. This allowed a comparison for the use of CFD as a cost-effective alternative to physical models. The physical model and CFD model results showed good correlations.

Modelling of two-phase flow in a minichannel using level-set method

Today there is a great interest in micro-scale multiphase fluid flow. In the paper, the numerical simulation of two-phase flow inside 3 mm minichannel was carried out. The liquid- gas interface was captured using the level-set method. During the calculation, the stabilization and reinitialization of level set function was performed in order to obtain the proper accuracy of the simulation. Incompressible Navier-Stokes equations were solved using the COMSOL Multiphysics® on a two-dimensional mesh. The process of formation of different two-phase flow patterns in the minichannel has been investigated. During the simulation it has been analysed three flow patterns: the bubbly flow and two kinds of slug flow with short and long slugs. It has been shown that unsteady flow at the inlet of the minichannel is responsible for the chaotic character of changes of the slug and bubble sizes. Such unsteady flow modifies the distance between the bubbles and slugs. It has been shown that for the low water inlet velocity the two-phase flow pattern becomes more stable.

Velocimetry Measurements of a Scramjet Cavity Flameholder with Inlet Distortion

Particle image velocimetry measurements were made along the center plane of a scramjet cavity flameholder to analyze simulated inlet flow distortion in the direct-connect test environment. Mach 3 nonreacting tests examined an oblique shock impinging upon locations in- and upstream of the cavity, including cases with wall-normal air injection upstream of the cavity to simulate fuel injection. Addition of flow distortion altered the size and shape of the primary recirculation region within the cavity by deflecting the bounding shear layer: the recirculation region was compressed by shock impingement upstream of the cavity, and shock impingement on the cavity itself expanded it. Air injection upstream of the cavity thickened the shear layer and produced a stronger effect on velocity direction than magnitude, preventing the formation of a large-scale recirculation region in two of the three shock locations studied. Flow distortion and upstream air injection both increased flow unsteadiness, with the greatest increases occurring in the shear layer and above the cavity closeout ramp. Additionally, results suggest the formation of spanwise secondary flow patterns that may account for flow nonuniformities observed in prior studies. This work presents the first velocimetry characterization of a scramjet cavity flameholder under distorted-flow conditions.

Compressible Taylor–Couette flow – instability mechanism and codimension 3 points

AbstractTaylor–Couette flow in a compressible perfect gas is studied. The onset of instability is examined as a function of the Reynolds numbers of the inner and outer cylinder, the Mach number of the flow and the Prandtl number of the gas. We focus on the case of a wide gap, with radius ratio 0.5. We find new modes of instability at high Prandtl number, which can allow oscillatory axisymmetric modes to onset first. We also find that onset can occur even when the angular momentum increases outwards, so that the classical Rayleigh criterion can be violated in the compressible case. We have also considered the case of counter-rotating cylinders, where the $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}m=0$ and $m=1$ modes can onset simultaneously to give a codimension 2 bifurcation, leading to the formation of complex flow patterns. In compressible flow we also find codimension 3 points. The Mach number and the critical inner and outer Reynolds numbers can be adjusted so that the two neutral curves for the $m=0$ and $m=1$ modes touch rather than cross. Complex codimension 3 points occur more readily in the compressible case than in the Boussinesq case, and they are expected to lead to a rich nonlinear behaviour.

Flow patterns and volume fractions of phases during liquid–liquid two-phase flow in pipe bends

This paper presents the results of research into hydrodynamics of horizontal two-phase liquid–liquid flow in pipe bends. The conducted research involved the observation of flow pattern formation and determination of volume fraction of the specific phases which were identified in the flow. On the basis of the results of experimental flow map was created for such flow and a method of calculating volume fractions of the phases was established. The testing was conducted in 180° pipe bends with the internal diameters of 0.016, 0.022, 0.030m and the respective radius of curvature equal to 0.110, 0.154 and 0.210m. The media applied in the measurements were machine oils Iterm 12 and LAN 15 as well as water.