Asymmetric Flow Pattern Research Articles

One-way coupled simulation of FIV in a 7-pin wire-wrapped fuel pin bundle

A common fuel geometry found in sodium cooled reactor designs is the hexagonal lattice of wire-wrapped fuel pins. The wire-wrappers, which replace the more common spacer grids, act as a spacer between the fuel pins, while simultaneously increasing the fluid mixing between subchannels. The induced transverse flow, while good for mixing, increases the force load experienced by the fuel pins, increasing the possibility of problematic flow induced vibrations. The primary source of vibrations is turbulent buffeting, so very small in amplitude, but capable of rupturing the fuel cladding by fretting. The vast majority of research on fretting has been for bare rods with spacer grids. There are several significant ways that wire-wrapped fuel pins differ from bare rods that may have significant impact on the vibrations present. The increased transverse flow and asymmetrical flow pattern may result in larger and imbalanced force loads on the pins. The helical bracing of each pin may result in unique vibration patterns, and gaps may form between pins affecting how they are braced. Simulation allows for the observation of the forces present, how the flow field affects these forces, and how the structure responds and can be repeated for multiple cases for relatively low cost. In the present study, simulation of the flow induced vibrations for a 7-pin wire-wrapped fuel pin bundle is carried out by coupling Nek5000, a spectral element LES/DNS solver with DIABLO, a finite element CSM solver. The small amplitude vibrations allow for the assumption that the structure motion has negligible effect on the fluid flow, permitting the much less computationally expensive one-direction coupling. In order to gather significant force history data for the 2.63 helical pitch structure, a scheme was developed to artificially create force history from the power spectrum density of the one-pitch CFD simulation forces. The vibrations could then be simulated in the structure multiple times varying the contact and gaps between various pins. This allows for the search of worst-case scenarios and informs about the types of fretting that might be present. The simulation geometry is based on an experiment for comparison. However, the viscosity of the simulation was increased by a factor of four to make the simulation computationally feasible. While validation is not possible, comparisons are still valuable.

Control of asymmetric flow over a blunt-nosed slender body

An innovative control device for the asymmetric flows over a blunt-nosed slender body is studied at a high angle of attack (α = 50°). The control device is composed of a micro-particle and a blowing device. Experimental tests are conducted to study the control of asymmetric flow. Particle-image velocimetry, surface oil-flow visualizations, and pressure scanning are used to present the vortical structures, flow separations, and pressure distributions. A blow hole with a diameter of db = 0.005D and a particle with a diameter of dp = 0.006D are placed on both sides of the nose as the micro-blowing perturbation and micro-particle perturbation (MPP), respectively, to control the behavior of the asymmetric vortices. The blowing coefficient (Cμ) through the blow hole is 0–9.42 × 10−5. Results show that a certain and predictable pattern of asymmetric flow is first presented at Cμ = 0, owing to the certain location of MPP on the nose. Subsequently, as Cμ increases, the asymmetric flow is changed into symmetrical first and then into the opposite asymmetric flow pattern. As a result, the corresponding side-force is changed into zero and then the opposite direction. These results indicate that the pattern of asymmetric vortices and their corresponding side-forces are easily managed by changing the blowing coefficient. The evolutions of associated pressure distributions, flow separations, and vortical structures with the increase in blowing coefficient are presented in detail.

Hydrodynamic coefficients of yawed cylinders in open-channel flow

The hydrodynamic response of a cylinder in an open-channel flow is experimentally investigated by considering the effects of horizontal orientation and submergence, the latter ranging from fully submerged to half-submerged cases. Drag and side coefficients are measured with a specific hydrodynamic balance which allows the planar rotation and the variation of submergence of the sample. The instrument design and characteristics are discussed, also estimating the uncertainties of force and hydrodynamic coefficients measurement. The tests are carried out under four submergence ratios (SR, submerged height to water depth), showing that for lower SR the drag and side coefficients increase with respect to tests performed on a fully submerged cylinder, which correspond to the highest SR. The tests performed for a half-submerged cylinder show instead a reduction of the side and drag coefficients, mainly due to the reduced submerged section. The effect of orientation on the two coefficients is different, yet it leads to similar submergence-independent trends. The drag coefficient is maximum for the 90° crossflow configuration, where maximum flow deviation occurs, while the side coefficient is maximum for the 45° orientation, which shows the most asymmetrical flow pattern. Measurements for deep submergence are validated against available literature results, confirming the reliability of the obtained values.

Computation of two- and three-dimensional water hammer flows

ABSTRACTThis paper investigates water hammer flows using two- and three-dimensional (2D and 3D) numerical simulations. The unsteady Reynolds-averaged Navier–Stokes (URANS) equations in conjunction with the k–ω SST turbulence model are employed for the computations. The valve modelling approach is used for 3D simulations, with superior agreement with the experiments. Similar predictions are obtained by 2D simulations and the flow rate reduction curve obtained from the 3D computations. The asymmetric flow patterns induced by the valve are confined within approximately one pipe diameter upstream of the valve. The contributions of inertia and pressure gradient terms are dominant at the instance of pressure wave passage, leading to abrupt changes in the fluid flow parameters. However, the effects of inertia and viscous shear stress terms are significant after the pressure wave passage, resulting in the flow tendency to approach a new steady condition. The viscous and turbulent dissipations are dominant close to and away from the wall, respectively.

Unsteady large-scale flow patterns and dynamic vortex movement in near-field triple buoyant plumes

Unsteady flow patterns of interacting buoyant plumes are important for buoyant ventilation and particularly influence pollutant and heat transports in indoor and outdoor environments. This study reveals fundamental large-scale flow patterns in triple building plumes, investigates vortex moving trends during the pattern transition processes, and explores possible mechanisms of pattern diversity by two-dimensional (2-D) Particle Image Velocimetry (PIV). Total five tests are studied, including three different heat strengths Q (180, 90, and 30 W) and three source layouts characterized by the ratios of source spacing S to source width W (0.2, 0.5, and 1.0).Streamline distributions and axial velocity profiles clearly reveal three fundamental global flow patterns: a right-slanting asymmetrical flow pattern, a left-slanting asymmetrical flow pattern, and an axisymmetric flow pattern. Correspondingly, it indicates four basic transition processes, i.e., right-to-center, left-to-center, center-to-right, and center-to-left transitions (“center” represents the axisymmetric pattern). A novel vortex tracking method, based on lambda-2 (λ2) criterion and principles of the PIV technique, is developed and successfully applied to qualitatively track the vortex moving trends during the transition processes. The regular vortex moving trends are found to be reasonably consistent with the global pattern transition trends.The flow pattern diversity is speculated to be mainly driven by unstable heat source wall flows and downstream swaying motions in this study. These critical unstable motions are considered to probably relate to unstable lateral entrainment and vortex interaction, particularly beside the central plume. Consistently, the regular vortex moving trends are usually observed in and around the central plume.

Fluidumok, áramlási rendszerek és ásványtani lenyomataik összefüggései a Budai Termálkarszton

A Budai Termálkarszt (BTK) fluidum-fejlődéstörténetét az ásványparagenezisek tükrében a késő-miocén fedett karbonátos állapottól vizsgáltuk. Ekkor a termikus felhajtóerő vezérelte a felszínalatti vízáramlást. Később, a Budai-hegység kiemelkedésével és a csapadékvíz beszivárgásával megkezdődött a vízszint különbségek által vezérelt vízáramlási rendszerek kialakulása. További változást okozott a Gödöllői-dombság kiemelkedése, mely a Budai-hegység kiemelt területei mellett a keleti medencerész felől is domborzati hajtóerőt biztosított. A meginduló beszivárgás lehetővé tette a fedő sziliciklasztos képződmények nátrium-kloridos vizeinek karbonátos víztartóba történő lejutását. Jelenleg a Budai-hegység és tágabb környezete a BTK áramlási rendszerének nyugati félig fedett részét, míg a Gödöllői-dombságtól a Dunáig tartó terület a rendszer keleti, fedett részét képezi. A BTK összefüggő áramlási rendszerében hidrosztatikus közeli nyomásviszonyok uralkodnak. A rendszer regionális és köztes megcsapolódási területei a Duna mentén húzódnak. A helyi, köztes és regionális áramlási rendszerek különbségeit a nyugati, félig fedett terület forrásai jelzik. A jelenkori vízáramlás domináns hajtóereje a vízszintkülönbségekből fakad, asszimmetrikus jellege a hidrosztratigráfiai helyzet, valamint a keleti és nyugati medencerészek közötti beszivárgáskülönbség következménye. A Duna alatt Ny­–K irányú regionális átáramlás zajlik. A regionális feláramlás hidrotermális komponense NaCl-os medenceeredetű vízzel egészül ki. A meteorikus eredetű, langyos köztes, ill. a hideg helyi vízáramlásokban Mg2+ és SO42- gazdag vizek jellemzőek, a medenceeredtű fluidomok jelenléte itt nem bizonyított. Az eredmények rámutatnak a rendszer hőmérsékleti eloszlását meghatározó folyamatokra, valamint a fedőüledékek hő felhalmozódásában betöltött szerepére. A rózsadombi és a Gellért-hegy előterében található források fizikai-kémiai tulajdonságok szerinti elkülönülését szerkezeti és a két áramlási rendszer különbségei okozhatják. A Gellért-hegynél csak termálvíz lép felszínre, míg a Rózsadombnál langyos és termálvíz megcsapolódás egyaránt zajlik. A különbség a hidrotermális vizek összetételében, eltérő forrásterületre és vízkémiai folyamatokra utal. A déli rendszer szulfát-többlete az evaporit összletekkel, a központi rendszer szulfátja medenceeredetű kén-hidrogénnel hozható összefüggésbe. A rózsadombi langyos meteorikus vizek szulfát forrása a fedő képződménybek piritje lehet. A fluidum-fejlődéstörténeti tanulmány eredményei hozzájárulnak a barlangképződési folyamatok értelmezéséhez.

Periodic and aperiodic flow patterns around an airfoil with leading-edge protuberances

Recently leading-edge protuberances have attracted great attention as a passive method for separation control. In this paper, the effect of multiple leading-edge protuberances on the performance of a two-dimensional airfoil is investigated through experimental measurement of aerodynamic forces, surface tuft visualization, and numerical simulation. In contrast to the sharp stall of the baseline airfoil with large hysteresis effect during AOA (angle of attack) increasing and decreasing, the stall process of the modified airfoil with leading-edge protuberances is gentle and stable. Flow visualization revealed that the flow past each protuberance is periodic and symmetric at small AOAs. Streamwise vortices are generated on the shoulders of the protuberance, leading to a larger separation around the valley sections and a longer attachment along the peak sections. When some critical AOA is exceeded, aperiodic and asymmetric flow patterns occur on the protuberances at different spanwise positions, with leading-edge separation on some of the valley sections and non-stalled condition elsewhere. A combined mechanism, involving both the compartmentalization effect of the slender momentum-enhanced attached flows on the protuberance peaks and the downwash effect of the local stalled region with low circulation, is proposed to explain the generation of the aperiodic flow patterns. The influence of the number of protuberances is also investigated, which shows similar aperiodic flow patterns. The distance between the neighboring local stalled valley sections is found to be in the range of 4–7 times the protuberance wavelength. According to the proposed mechanism, it is speculated that the distance between the neighboring local stalled valley sections is inclined to increase with a smaller protuberance amplitude or at a larger AOA.

Effect of a Single Leading-Edge Protuberance on NACA 634-021 Airfoil Performance

Leading-edge protuberances on airfoils or wings have been considered as a viable passive control method for flow separation. In this paper, the aerodynamic performance of a modified airfoil with a single leading-edge protuberance was investigated and compared with the baseline NACA 634-021 airfoil. Spalart–Allmaras turbulence model was applied for the numerical simulation. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foil decreased and the decline of the lift coefficient became mild. The poststall performance of the modified airfoil was improved, while the prestall performance was declined. Asymmetric flows along the spanwise direction were observed on the modified airfoil, and the local region around one shoulder of the protuberance suffered from leading-edge separation at prestall angles of attack, which may be responsible for the performance decline. At poststall angles of attack, the attached flows along the peak of the protuberance with a sideward velocity component would help improving the total performance of the airfoil. Experimental visualization methods, including surface tuft and smoke flow, were performed, and the asymmetric flow pattern past the protuberance was successfully captured. This specific phenomenon may be largely related to the formation of the biperiodic condition and other complicated flow patterns induced by multiple leading-edge protuberances. The formation mechanism and suppression method of the symmetry breaking phenomenon should be investigated more deeply in the future to guide the practical application of this passive control method.

Effect of vertical length on asymmetric flow and inclusion transport in vertical-bending continuous caster

A large eddy simulation (LES) model has been developed to simulate the transient fluid flow, heat transfer and solidification processes in various vertical-bending continuous casting casters with 0 to 5m of vertical length. The LES with dynamic sub-grid model is used to calculate the transient turbulent flow inside the liquid pool. The enthalpy-porosity approach is used to simulate the heat transfer and solidification of steel in the caster. A criterion is developed using the user-defined functions to model the motion and entrapment of inclusion in the caster based on the Lagrangian approach. The 1/4 and 1/2 inclusion band defects located in a steel plate are found by the UT method, and the distribution of inclusions is intermittent and asymmetric. The transient and asymmetric flow pattern has been obtained in a one-third-scale vertical-bending type water model of mold. The asymmetric flow inside the liquid pool is identified a periodical behavior. The oscillating motion of asymmetric flow inside the mold is induced by the turbulence, rather than the curve shape of the caster. The maximum impact depths of the lower recirculation flow for various casters are almost around 2.8m below the top surface of the mold. The time for inclusion removal from top surface can be divided into three periods: 3.5–6.5s, 6.5–30s, and 30s after injection. The vertical length has a less effect on the removal ratio of inclusions from the top surface, but determines the entrapment positions of inclusions.

Oxygen and nitrogen gases mixing in T-type micromixers visualized and quantitatively characterized using pressure-sensitive paint

This study investigated the mixing between oxygen and nitrogen gases in T-type micromixers by using the experimental technique known as pressure-sensitive paint (PSP). Both mixing qualities and flow fields for oxygen and nitrogen gases in T-type micromixers were quantitatively visualized in great detail. T-type micromixers with a width of 550μm and a height of 125μm were selected for the investigation. The mixing quality in a 10-mm-long mixing channel can reach 75% at the channel exit for Reynolds (Re) numbers of 13.9–36.7 [Peclet (Pe) numbers of 10.5–27.6]. Under these low Re and Pe conditions, mixing was dominated by diffusion and longer retention time resulted in higher mixing quality. When the Re number was set to 376.2 (Pe=252.8), convection increased in the mixing channel, and the mixing quality dropped to 34.95% at channel exit. However, when the Re number was increased to 596.1 (Pe=428.6), the engulfment flow regime was identified as an asymmetric flow pattern in the mixing channel. The mixing quality at a Re number of 596.1 was approximately 30% higher than that at a Re number of 376.2. Commercial CFD software, ANSYS CFX, was used to simulate the flow fields inside the T-type micromixers, and the numerical data was compared with experimental PSP measurements. Favorable agreement has been established for the cases measured before the engulfment flow regime started. In addition, the flow fields and mixing quality levels of oxygen and nitrogen gases with asymmetric inlet conditions were obtained. The asymmetric flow pattern, which was similar to the engulfment flow regime, was triggered more easily, and an agitated flow improved the mixing quality in the T-type micromixers. The feasibility of PSP measurement in microscale gas mixing was demonstrated and validated in this study. Future studies can experimentally investigate micromixer designs by using various active or passive actuators to obtain detailed information regarding flow fields and the evolution of mixing quality.

Experimental characteristics of oblique shock train upstream propagation

The structure and dynamics of an oblique shock train in a duct model are investigated experimentally in a hypersonic wind tunnel. Measurements of the pressure distribution in front of and across the oblique shock train have been taken and the dynamics of upstream propagation of the oblique shock train have been analyzed from the synchronized schlieren imaging with the dynamic pressure measurements. The formation and propagation of the oblique shock train are initiated by the throttling device at the downstream end of the duct model. Multiple reflected shocks, expansion fans and separated flow bubbles exist in the unthrottled flow, causing three adverse-pressure-gradient phases and three favorable-pressure-gradient phases upstream the oblique shock train. The leading edge of the oblique shock train propagates upstream, and translates to be asymmetric with the increase of backpressure. The upstream propagation rate of the oblique shock train increases rapidly when the leading edge of the oblique shock train encounters the separation bubble near the shock reflection point and the adverse-pressure-gradient phase, while the oblique shock train slow movement when the leading edge of the oblique shock train is in the favorable-pressure-gradient phase for unthrottled flow. The asymmetric flow pattern and oscillatory nature of the oblique shock train are observed throughout the whole upstream propagation process.

Lattice Boltzmann simulations of three-dimensional incompressible flows in a four-sided lid driven cavity

Numerical study on three-dimensional (3D), incompressible, four-sided lid (FSL) driven cavity flows has been conducted to show the effects of the transverse aspect ratio, K, on the flow field by using a multiple relaxation time lattice Boltzmann equation. The top wall is driven from left to right, the left wall is moved downward, whereas the right wall is driven upward, and the bottom wall is moved from right to left, all the four moving walls have the same speed and the others boundaries are fixed. Numerical computations are performed for several Reynolds numbers for laminar flows, up to 1000, with various transverse aspect ratios. The flow can reach a steady state and the flow pattern is symmetric with respect to the two cavity diagonals (i.e., the center of the cavity). At Reynolds number = 300, the flow structures of the 3D FSL cavity flow at steady state with various transverse aspect ratio, i.e., 3, 2, 1, 0.75, 0.5 and 0.25 only show the unstable symmetrical flow pattern. The stable asymmetrical flow pattern could be reproduced only by increasing the Reynolds number that is above a critical value which is dependent on the aspect ratio. It is found that an aspect ratio of more than 5 is needed to reproduce flow patterns, both symmetric and asymmetric flows, simulated by using 2D numerical models.

Density effect on the mixing efficiency and flow modes in T-shaped micromixers

Flow patterns and mixing of liquids with different densities in T-shaped micromixers are numerically investigated at Reynolds number range from 1 to 250. The density ratio of the mixing media varies from 1 to 2; its effect on the flow structure and the mixing is studied. The dependences of the mixing efficiency and the pressure difference in this mixer on the density ratio and the Reynolds number are obtained. It is shown that the density ratio has a considerable effect on the flow structure, especially before the transition from the symmetric to the asymmetric flow pattern.

Scattering to Higher Harmonics for Quarter-Wave and Helmholtz Resonators

The nonlinear response of acoustic resonators is investigated over a broad range of frequencies and amplitudes. Helmholtz resonators with a symmetric neck and an asymmetric neck, respectively, as well as quarter-wave resonators are considered. Describing functions for impedance and the reflection coefficient of a Helmholtz resonator at various sound pressure levels are determined from compressible flow simulation and validated against experimental data. The particular focus of the present study is the nonlinear scattering to higher harmonics. For the Helmholtz resonator with a symmetric neck, a distinct pattern in the amplitudes of the higher harmonics is observed, where the odd harmonics dominate the response, whereas the even harmonics are almost negligible. Such an “odd-harmonics-only” pattern, which was observed previously in an experiment at the orifices, is explained by a quasi-steady analysis based on the Bernoulli equation, assuming a symmetric flow pattern at the neck. For the Helmholtz resonator with an asymmetric neck, it is observed in computational fluid dynamics simulations that even harmonics contribute noticeably to the resonator response, such that the odd-harmonics-only pattern is less pronounced. For the markedly asymmetric geometry of the quarter-wave resonator, the second harmonic is dominant and the odd-harmonics-only pattern vanishes completely. The quasi-steady analysis is extended successfully to also describe nonlinear scattering to higher harmonics for asymmetric configurations and flow patterns. Overall, the scattering to higher harmonics remains on a moderate level, even at very high excitation levels for the Helmholtz resonator configurations. For the quarter-wave resonator, the scattering is more pronounced and contributes perceptibly to the response at high excitation amplitudes.

LBM-LES Simulation of the Transient Asymmetric Flow and Free Surface Fluctuations under Steady Operating Conditions of Slab Continuous Casting Process

Transient flow structures in a continuous casting mold can strongly influence the slag entrainment in liquid steel and the bubbles capture in the initial solidified shell, both of which are associated with the quality of the final product. This paper presents a numerical study of the turbulent flow with a top free surface in the continuous casting mold at a meso-scale level by a three-dimensional combined approach of Free Surface Lattice Boltzmann Method and Large Eddy Simulation (FSLBM-LES). The validity of the model is verified by the good agreement between the calculated results and the measurements from various water experiments in terms of the flow velocity and free surface profile. The mathematical model is then used to reveal the transient and spatiotemporal asymmetric characteristics associated with the transient flow field and the free surface fluctuation, although the steady state operation is considered during the continuous casting process. The results show that the locations of the jets of liquid steel from the two out ports of the Submerged Entry Nozzle (SEN) always fluctuate alternatively within a certain range, and periodically deviate from the design angle of the SEN within the same time period. The oscillating behavior of the jets promotes the asymmetric flow patterns and multi-scale vortices at both sides of the SEN. By introducing the Q-criterion in the results analysis, the formation, development, and shedding of the coherent structure (CS) of the turbulent flow are quantitatively characterized. The interaction between the transient flow patterns and the fluctuations of the top free surface as well as the evolution of the transient profile and velocities of the free surface are also demonstrated. The results obtained from the current study suggest that the FSLBM-LES model offers a promising way to study the complex flows and related transfer phenomena in the continuous casting process.

Three-dimensional numerical modelling of flow field in shallow reservoirs

Shallow flows can play a significant role in sediment management for dam reservoirs (e.g. sedimentation in shallow reservoirs and free-flow flushing operation). When shallow flow emerges with symmetric or asymmetric patterns, the flow domain exhibits complex three-dimensional (3D) features (e.g. helical flows). This study focuses on the numerical modelling of the velocity field in shallow reservoirs with varying geometries and also varying bed conditions (i.e. flat and misshaped beds). A fully 3D numerical model using the finite-volume method was utilised to reproduce the 3D flow velocity field. The experimentally measured surface velocity in all cases and velocity profiles in one case were used to validate the model. The numerical results showed that a slight disturbance in the inflow boundary condition results in a steady asymmetric flow pattern in reservoirs with a higher defined shape factor, but does not affect the flow pattern in reservoirs with a lower defined shape factor. Nonetheless, the simulated and measured flow velocity fields are reasonably consistent in all cases. These results can be used to optimise the design of sand traps or water storage facilities, and also to optimise sediment management in existing reservoirs.

Natural convection in a cavity surface mounted with discrete heaters and subjected to different cooling configurations

ABSTRACTSteady-state laminar natural convection in a rectangular cavity surface mounted with two identical heaters on the bottom wall is studied numerically. Natural convection is studied for three cooling configurations: (1) cooling at the left vertical wall, (2) cooling at both the left and right vertical walls, and (3) cooling at the top horizontal wall. The cold walls are cooled isothermally while the other cavity walls are kept adiabatic. The effects of Rayleigh number and cavity inclination on natural convection for the above-mentioned three configurations are studied in detail, and the cooling performances of the configurations are compared in terms of the maximum component temperature. Both the wall cooling and the top wall cooling performances show asymmetric flow and thermal patterns for high Rayleigh numbers, despite the geometric and thermal symmetry of the configurations. The cooling performance of left wall cooling is found to be highly sensitive to cavity inclination. Correlations for dimensionless maximum temperature are presented.

Large Eddy Simulation of Transient Flow and Inclusions Transport in Continuous Casting Mold under Different Electromagnetic Brakes

A mathematical model has been developed to analyze transient fluid flow and inclusions transport in a slab continuous casting mold, considering the effects of electromagnetic brake (EMBr) arrangement and magnetic field strength. Transient flow of molten steel in the mold is calculated by using the large eddy simulation. The electromagnetic force is incorporated into the Navier–Stokes equation. The transport of inclusion inside the mold is calculated using the Lagrangian approach based on the transient flow field. The predicted results of this model are compared with the measurements of the ultrasonic testing of the rolled steel plates and the water model experiments. The transient asymmetrical flow pattern and inclusion transport inside the mold exhibits satisfactory agreement with the corresponding measurements. With electromagnetic brake effect, the velocities around the braking region are significantly suppressed, and the recirculating flow in the lower part drops and tends to develop a plug-like flow. The EMBr arrangement has an insignificant effect on the overall removal fraction of inclusions, especially for larger inclusions. The inclusion removal rate for the flow-control mold (FCM arrangement) reduces instead compared with no EMBr, especially for smaller inclusions.

Viscosity effect on the flow patterns in T-type micromixers

Flow patterns and mixing of liquids with different viscosities in T-type micromixers are numerically investigated on the Reynolds number range from 1 to 250. The viscosity ratio of the mixing media varied from 1 to 2; its effect on the flow structure and the mixing is studied. The dependences of the mixing efficiency and the pressure difference in the channel on the viscosity ratio and the Reynolds number are obtained. It is shown that the viscosity ratio has a considerable effect on the flow structure before and after transition from the symmetric to the asymmetric flow pattern. The self-similar behavior of the asymmetric flow pattern is established.

Influence of an adjacent droplet on fluid convection inside an evaporating droplet of binary mixture

Abstract We study the fluid convection inside two adjacent evaporating droplets of binary mixture. Micro-PIV technique has been used to measure the velocity field inside the droplet. The experimental observation on internal convection inside droplet has been explained using the simulation results of evaporative flux distribution and concentration field inside the droplet. The interaction between the two adjacent evaporating droplets has been studied by varying the separation distance between the two droplets. Evaporation from the droplet induces buoyancy driven Rayleigh convection inside the droplet. Fluid convection inside a single droplet shows a symmetrical flow pattern with two recirculating bubbles in X–Z and Y–Z wall normal planes. This behavior is attributed to the symmetric evaporative flux distribution on the droplet surface and the resulting symmetric concentration field inside the droplet. Presence of an adjacent evaporating droplet leads to asymmetric evaporative flux distribution on the droplet surface due to the influence of the neighboring droplet on the free stream mass fraction. This asymmetric evaporative flux distribution on the droplet surface results in asymmetric concentration field inside the droplet. As a result, the fluid convection in two droplets configuration shows asymmetric flow pattern with one recirculation bubble in X–Z wall normal plane and two recirculation bubbles in Y–Z wall normal plane.