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We investigate the dynamics of bubble propagation in a symmetric bifurcating Y-channel by varying the power-law index (n) of the carrier fluid from 0.3 to 1.5, in the presence of gravity. To characterize the bubble evolution, the unsteady two-phase flow is solved numerically, employing a suitable phase-field model. Based on the flow rate ratio between the upper and lower branch channels and the neck-width evolution, the bubble bifurcation process is divided into three distinct stages, namely, squeezing, transition, and pinch-off. Temporal variation of neck-width demonstrates that the bubble pinch-off is somewhat delayed for shear-thickening (n > 1) fluids, while a shear-thinning carrier fluid (n

In the present work, a numerical analysis of non-Newtonian impinging jets on a flat plate subjected to a constant heat flux is carried out under turbulent flow conditions. By employing the power-law constitutive model to describe the jet’s non-Newtonian behavior, simulations have been performed for two (2D) and three-dimensional (3D) jet flow conditions. For turbulence simulation, SST- $$k\omega$$ turbulence model has been used coupled with continuity and energy equations. Dimensional results have been obtained by varying the inflow conditions 0.5–1.4 m/s and at a constant heat flux of 5000 W/m2. To instill the influences of shear thinning, Newtonian, and shear-thickening fluids, the range of power-law indices (n) is varied from 0.6 to 1.6. The results of the simulations infer that the pseudoplastics fluids are more efficient than Newtonian and dilatant fluids. Increasing power law shows efficacy of the fluids to cool the surface decreases as is observed by the average values of the Nusselt number on the target. It is found that there is a small difference in the final average temperature of the plate after cooling with different fluids, but that owes it to the small heat flux used in the present analysis. It is observed that the higher the velocity of the jet, the better is the cooling. Finally, the energy-saving capabilities of the jets are characterized by altering the rheological characteristics of the jets.

Micro-combustors, which are emerging as portable power sources, have serious flame stabilization issues due to enhanced heat losses. Hydrogen, an eco-friendly alternative to conventional fossil fuels, can be a potential fuel for micro-combustors because of its high calorific value, leading to high energy density. In the present work, numerical simulations of premixed lean (equivalence ratio = 0.5) hydrogen-air flames in a 2 mm wide channel with three different wall materials (glass, steel, and aluminum) were performed. The effects of the wall material on the dynamics of the flames were extensively studied. The walls of the combustor play an important role by conducting heat upstream and facilitating ignition and stabilization of the flame. For different values of wall thermal diffusivity, periodically oscillating flames of varying frequencies ([Formula: see text]) and intermittent bursting flames were observed. Time series analysis and modal decomposition of temperature fields were utilized to quantify the flame dynamics and to identify the dominant structures of the flames. A recurrence analysis using the temperature time series data revealed significant differences in flame dynamics, including period-2 oscillations and intermittency, for different wall materials. The underlying physics behind the periodic oscillations and intermittent bursting has been explained.

We numerically investigate the effect of electrohydrodynamics on a non-Newtonian droplet pair suspended in a Newtonian medium. The leaky dielectric model is implemented to study the response of emulsion drops in an externally applied electric field. Subsequently, the non-Newtonian drop behavior is incorporated using the power law model, whereby three different fluid behaviors are considered for the drops: Newtonian, Shear thinning, and Shear thickening. We validated our numerical model with the available literature data, and the results are in good agreement. The droplets' deformation and net motion are investigated for a range of electrical permittivity ratios of the droplet medium with respect to the surrounding fluid. In this study, four distinct regimes are identified based on the net drop pair motion and the circulation pattern that develops due to the electric stresses inside and around the drops. Furthermore, it is observed that the droplet deformation and their net motion are fastest for the pseudo-plastic drops and slowest for dilatant drops. We devised a simple ratio-based model to understand this behavior. The inferences drawn from this study will help contribute to a better understanding of the behavior of nonlinear fluids under an electric field.

We study the influence of Aspect Ratio ( AR) on three-dimensional wake transition past a thin pitching plate at Reynolds number of 1000 by performing computations for the range [Formula: see text] at pitching frequencies [Formula: see text] and maximum pitching angles [Formula: see text]. For all AR, larger θmax and St promote thrust generation. However, higher AR imparts a stabilizing influence in the wake of the drag regime. For the ranges of AR, the drag-producing wake consisting of horseshoe vortices and bridgelets-type vortex structures, whereas twin-jet type bifurcated wakes with entangled vortices are observed for thrust-generating wakes. At higher AR, the wakes show a two-dimensional signature in the drag regime, whereas a spatial wake transition is observed in the thurst regime. The spanwise wake width shows the effect of wake compression for larger St even at [Formula: see text], which is also substantiated by particle tracking showing wake compression for the thrust cases up to [Formula: see text]. The near wake oscillations are prevalent for higher AR, although the core region remains unaffected by the aiding influence of spanwise instability for larger AR. The time average streamwise velocity for both drag and thrust regimes resembles an apparent feature of the reverse von Kármán vortex street.

We have performed a three-dimensional numerical simulation to determine the effect of local atmospheric pollution level on the spreading characteristics of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus through ejected droplets during sneezing and coughing in an open space. Utilizing a finite volume-based numerical method, we have performed computations for various ranges of droplet diameters and sneezing speeds. The interactions between the droplets and the suspended particles are considered by taking both hydrophobic and hydrophilic wettability characteristics into account. Our computational results show that the virus-containing droplets partially affect aerosols during the path of their transmission. With the progression of time, the droplet distribution shows an asymmetric pattern. The maximum dispersion of these droplets is found for higher sneezing velocities. The droplets with a diameter of 50 μm travel a larger distance than the larger diameter droplets. We have found that an aerosol with hydrophilic wettability undergoes complete wetting by the disease-containing droplets and therefore is conducive to disease propagation. The droplet engagement duration with aerosol decreases with increase in the sneezing velocity. Our study recommends against using physical exercise centers in a closed environment such as gymnasium and indoor games during the COVID pandemic, especially in a polluted environment. The results from our work will help in deciding proper social distancing guidelines based on the local atmospheric pollution level. They may act as a precursor in controlling further spread of diseases during this unprecedented situation of the COVID pandemic.

In the present research, we address the implications of the pulsating electric field on controlling mass flow-rate characteristics for the time-periodic electro-osmotic flow of a viscoelastic fluid through a microchannel. Going beyond the Debye-Hückel linearization for the potential distribution inside the Electric Double Layer, the Phan-Thien-Tanner constitutive model is employed to describe the viscoelastic behaviour of the fluid. The analytical/semi-analytical expressions for the velocity distribution corresponding to a steady basic part, and a transient perturbed part are obtained by considering periodic pulsations in the applied electrical field. Our results based on sinusoidal pulsations reveal that enhanced shear thinning characteristics of the viscoelastic fluids show higher amplitude of pulsations with the oscillations in the velocity gradients primarily contrived within the Electric Double Layer region. The amplitude of mass flow rates increases with increasing the viscoelastic parameter , whereas, the phase lag displays a reverse trend. The analysis for an inverse problem is extended where the required magnitude of electric field pulsations for a target mass flow rate in the form of sinusoidal pulsations. It is found that with increasing shear-thinning characteristics of the viscoelastic fluid, there is a progressive reduction in the required electric field strength to maintain an aimed mass flow rate. Besides, required electric fields for controlled mass flow with triangular and trapezoidal pulsations are also determined.

We have conducted two-dimensional computations of vortex-induced vibration of an elastically mounted circular cylinder colliding with a rigid wall at a Reynolds number of 300. For wide ranges of reduced velocities, we have identified C (chaotic), P (periodic), and QP (quasi-periodic) vortex shedding modes at two gap and mass ratios. The deflected gap flow induced by the wall shear layer during the descending motion of the cylinder cause stretching of the vortex strands, while both the P and QP modes show the “S”-type shedding signature. The pathlines for the C-mode yield unstable hyperbolic behavior, whereas twisted knots and spirals are seen for the P-mode. Criss-cross interactions with the swirling pathlines are observed in the QP-mode.

Fluid dynamics and heat transfer behaviour are studied past side-by-side square cylinders at incidence in vertical flow arrangement numerically considering laminar, Newtonian, steady, incompressible and two-dimensional flow using Finite Volume Method based ANSYS Fluent solver. Using air (Pr = 0.7) as the working fluid, computations are performed at a representative value of Re = 100. The angle of incidence (alpha) is varied from 0 degrees to 45 degrees in the step of 5 degrees, whereas, the lateral distance between the cylinders is kept constant. To consider all possibilities, cylinders are rotated either clockwise or counter-clockwise, simultaneously or individually, which generates thirty-four different orientations. Buoyancy assisting phenomenon is created by changing the Richardson number (Ri) from 0 to 1 in step of 0.25. Due to the variation of the angle of incidence and its orientations, vortex generation is observed in the buoyancy assisting case at a critical value of Ri. To study heat transfer characteristics, time average and the local Nusselt number are analysed. For a fixed lateral distance between the cylinders, maximum heat transfer is found to occur at an incident angle of 45 degrees. Other important parameters, such as drag coefficient (C-D), lift coefficient (C-L), Strouhal number (St) are also studied. (C) 2018 Elsevier Ltd. All rights reserved.

In this paper, a comprehensive study has been performed to address the excitation of a separated boundary layer near the leading edge due to surface roughness. Experiments are performed on a model airfoil with the semicircular leading edge at a Reynolds number (Rec) of 1.6×105, where the freestream turbulence (fst) is 1.2%. The flow features are investigated over the three rough surfaces with the roughness characteristic in the wall unit of 17, 10.5, and 8.4, which are estimated from the velocity profile at a location far downstream of reattachment. The wall roughness results in an early transition and reattachment, leading to a reduction of the laminar shear layer length apart from the bubble length. It is worthwhile to note that although the large-amplitude pretransitional perturbations are apparent from the beginning for the rough surface, the shear layer reflects the amplification of selected frequencies, where the fundamental frequency when normalized is almost the same as that of the smooth wall. The universal intermittency curve can be used to describe the transition of the shear layer, which exhibits some resemblance to the excitation of the boundary layer under fst, signifying the viscous effect.

In the present study, thermofluidic characteristics of a combined pressure-driven and electrical field mediated thermally fully developed flow of an immiscible Newtonian and a viscoelastic fluid bi-layer in a microchannel have been analyzed. The simplified Phan-Thien–Tanner model with a linear kernel for the stress coefficient function has been utilized to describe the complex fluid rheology for the non-Newtonian fluid. Disparate zeta potentials have been assumed at the interfaces. Accordingly, distinct zeta potential values have been used at the channel walls and interfaces between the fluids to derive the closed-form analytical expressions for the pertinent velocity, stress, and shear viscosity distributions in the fluid layers. For thermally developed flows, the temperature and entropy distributions are obtained along the microchannel for constant wall heat flux boundary conditions. Major findings from our research show that amplification of the viscoelastic parameter designated by the Weissenberg number exhibits an enhancement in the non-dimensional axial velocity, flow rate, and stress magnitudes. Furthermore, the present study indicates that Joule heating and viscous dissipation significantly vary the dimensionless temperature profiles along the fluid bi-layer. The Nusselt number values are found to decrease with the augmentation of the viscoelasticity, Joule heating, and viscous dissipation parameters. The total entropy generation for the fluid layer systems increases with the increasing Joule heating parameter.

In this work, we aim to develop a mathematical model for capillary filling dynamics of electromagnetohydrodynamic flow of non-Newtonian fluids. An axially applied electric field and a transverse magnetic field are considered to elucidate the electromagnetohydrodynamic transport through the microcapillary. Assuming a non-Newtonian power-law obeying fluids, we analyze the transient evolution of the electromagnetohydrodynamic capillary positions by considering the magnitude of the total force balance via finite volume-based numerical formalism. We have highlighted the various rheological regimes in the horizontal capillary through a scaling analysis. For the Newtonian fluids, corresponding inviscid linear Washburn regime is also analyzed and compared with the power-law obeying fluids. Furthermore, we have also derived closed-form analytical expressions for the electromagnetohydrodynamic velocity, pressure gradient, and transient evolution of the capillary positions by using couple stress parameter model to characterize the fluid rheological behaviors. We perform a comparison test of the coupled stress parameter model with the results from the literature for a similar set of fluid rheological parameters. The comparison results are found to be in good agreement.

In this paper, a two-dimensional numerical simulation is carried out to understand the effect of confinement (blockage ratio β ) on fluid flow and forced convective heat transfer characteristics past a blunt headed cylinder. Utilizing air as an operating fluid, flow simulations are carried out for wide ranges of blockage ratios ( 1 10 ≤ β ≤ 1 3 ) and Reynolds numbers ( 60 ≤ Re ≤ 200 ) . The flow characteristics and heat transport are analysed critically for different β . The functional dependence of C D (Drag Coefficient) and C L (Lift Coefficient) on blockage ratio is examined. It has been found that C D reduces with increasing Re, while the Strouhal number and the average Nusselt number show an increasing trend when the blockage ratio is increased. The average Nusselt number also increases with increasing Re .

We characterize, electrokinetically modulated axial pressure driven transport of a power-law fluid through microchannel in the presence of superimposed magnetic field. We obtain solutions for streaming potential, velocity, and temperature fields owing to a combined interplay of the flow rheology, kinematics, influences of finite ion sizes (steric effect), and the electromagnetohydrodynamics. Our results demonstrate that giant augmentations in the energy conversion efficiency and streaming potential field may be achieved for shear thickening fluids in the presence of superimposed magnetic field. Our analysis reveals that heat transfer rate can be amplified by enhancing magnetic field magnitude and the power law behavioural indices. We, further, carried out an exergy analysis for an optimal process design via reducing irreversibilities in terms of “entropy generation analysis”. We found that the total irreversibilities of the system decreases with increasing the values of power law index. We believe that the inferences obtained from the present research may be useful in the design of advanced energy efficient devices, smart sensors, etc., with optimal combinations of power law rheology and magnetohydrodynamic influences.

This paper presents a theoretical analysis of non-Newtonian (power-law obeying) fluid in a narrow confinement subjected to the combined consequences of interfacial electrokinetics, rheology, and superimposed magnetic field. We devote special attention on the exploitation of magnetic field and power-law exponent, in the development of induced streaming potential and thermofluidic energy transfer characteristics over small scales. In an effort to do so, going beyond the Debye-litickel limit, we first derive an expression for streaming potential by invoking the consequences of strong EDL (electrical double layer) interactions in the narrow fluidic passage and finite conductance of the Stern layer. In particular, we solve thermal energy transport equation with an illustrative case of classical uniform wall heat flux boundary and considering the volumetric heat generation effects due to viscous dissipation as well as Joule heating. Our results demonstrate that the applied magnetic field imparts a retarding influence on the induced streaming potential development, whereas, it results in enhancement of heat transfer rate. Moreover, additional influences of power law index show reduction in heat transfer as well as the streaming potential magnitude. We unveil the optimal combinations of power law index and the magnetic field which lead to the minimization of the global total entropy generation in the system. We believe that theoretical results presented in this research will be useful in the development of novel narrow fluidic energy efficient devices under electrokinetic modulation. (C) 2017 Elsevier Ltd. All rights reserved.

A numerical analysis is performed to elucidate the forced convective fluid flow and heat transfer characteristics past a blunt-headed cylinder. Simulations are carried out employing air (Pr = 0.71) as an operating fluid in a Reynolds numbers range 40≤Re≤200. The curvature ratio is varied from 0 to 0.5. The flow and heat transport features are elucidated in detail for different curvature ratios. Important flow parameters such as boundary layer thickness, vortex strength, wake width, drag, lift, skin friction coefficient, pressure coefficient, Strouhal number, and recirculation length are computed. Also, a primary stability analysis has been carried out using the Landau equation, whereas secondary stability analysis has been done implementing dynamic mode decomposition (DMD) to compute the critical Reynolds number at each curvature ratio. Calculating local and time-averaged values of Nusselt numbers, heat transfer characteristics are studied. An entropy generation analysis is done to investigate the effects of corner modification on the efficacy of thermofluid transport characteristics.

This paper investigates the path selection of bubbles suspended in different power-law carrier liquids in microfluidic channel networks. A finite volume-based numerical method is used to analyze the two-dimensional incompressible fluid flow in microchannels, while the volume of fluid method is used to capture the gas–liquid interface. To instill the influences of shear thinning, Newtonian, and shear-thickening fluids, the range of power-law indices (n) is varied from 0.3 to 1.5. We have validated our numerical model with the available literature data in good agreement. We have investigated the nonlinearity in the hydrodynamic resistance which arises due to single-phase non-Newtonian fluid flow. The path selection of a bubble in power-law fluids is examined from the perspective of velocity distribution and bubble deformation. We have found that the bubble indeed goes to the channel with a higher flow rate for all power-law fluids, but interestingly it did not always take the shorter route channel at a junction for n = 0.3. Our results suggest that long channels need not be more resistant for every fluid and that the longest arm becomes the least resistant resulting in the bubble leading into the long arm at a junction for shear-thinning fluid. We have proposed a deterministic model that enables predicting the second bubble path in a single bubble system for any location of the first bubble. We believe that the present study results will help design future generation microfluidic systems for efficient drug delivery and biomedical and biochemical applications.

We investigate thermofluidic transport phenomena and entropy generation for combined electroosmotic and pressure-driven flows through narrow confinements, subjected to spatially varying non-uniform magnetic field. Going beyond the Debye-Huckel limit, we consider the size effects of the ionic species (steric effect) to analyse magnetohydrodynamic flow and heat transfer characteristics. We demonstrate that a confluence of the steric interactions with the degree of wall charging (zeta potential) may result in heat transfer enhancement, and overall reduction in entropy generation of the system under appropriate conditions. In particular, it is revealed that a judicious selection of spatially varying magnetic field strength may lead to an augmentation in the heat transfer rate. It is also inferred that incorporating non-uniformity in distribution of the applied magnetic field translates the system to be dominated by the heat transfer irreversibility. The novel scope of the current research lies in the state-of-art design of advanced micromechanical industrial smart-sensors, actuators, and biomedical devices. (C) 2016 Elsevier Ltd. All rights reserved.

The present study analyzes the transport characteristics and associated instability of a forced convective flow past a semi-circular cylinder at incidence with a downstream circular cylinder. Considering air as an operating fluid, unsteady computations are performed for the ranges of incidence angles ϕ and Reynolds numbers (Re) (0° ≤ ϕ ≤ 90°, 60 ≤ Re ≤ 160 ). The numerical model is adequately validated with the available experimental and numerical data from the literature. It is found that the presence of the upstream semi-circular cylinder at various incidence angles yields a rotational effect on the flow structures that evolve from the downstream circular cylinder. The modulation of the incidence angle reveals three separation regimes of the shed-vortex structures, which shows wake confluence. The dependencies of the coefficient of drag C D and the root mean square values of the lift coefficient C L , r m s on the angles of incidence are examined for both of the cylinders. The frequency of vortex shedding increases with increasing ϕ and attains its peak value at ϕ ∼ 30°. The forced convective heat transfer for the semi-circular cylinder decreases with increasing ϕ, whereas a contrasting trend is observed for the circular cylinder until ϕ ∼ 45°. The global stability analysis through the dynamic mode decomposition shows a stabilizing flow situation for the present range of operating parameters.

We have performed direct numerical simulations of flow past a tapered circular cylinder during the early transition to three dimensions for two successive taper ratios (TR) of 20 and 12.5. Our results indicate the random occurrence of vortex splits and dislocations as the topology of the shedding signature. In particular, we observe oblique cellular shedding with multiple spanwise patterns and oppositely oriented oblique cells in the shed structure. Unlike flow imposed shear, the vortex formation length becomes sensitive to the taper ratio, which removes oblique frequency waves noticed for lower shear rate. The local Strouhal frequency (Stz) at the higher TR case exhibits a decreasing trend with remarkably smaller finite jumps at the cell boundaries and is found close to uniform cylinder flow. The wavelet analysis reveals the narrowing of the spectrum at a lower TR. A higher TR case shows a distinctly regular and evenly spaced spectrum which does not reach the maximum Stz, making it a rare event. The present results show that tapering causes the appearance of a secondary motion, which completely reverses at the downstream cylinder wake. Our numerical calculations show that pressure has an indirect role in the growth of the secondary instabilities, where isobars align along with the taper profile. The geometrically induced shear promotes greater mixing in the near wake, and we found that the maximum cross-stream velocity never exceeds 10% of the mean flow even with the steepest TR. The streamwise growth of the defect layer is slower for increasing TR and reaches an early saturation. Although the velocity deficit is higher at the steepest TR, it causes a delay in the momentum recovery along the streamwise direction. The shape factor for the lower TR case shows a delay in the laminar–turbulent transition. Finally, our global stability analysis results employing dynamic mode decomposition revealed a nonlinear dynamical system with spanwise dissipation of the dynamic modes.

Three-dimensional (3D) direct numerical simulations are carried out for shear flows past a circular cylinder in the early 2D (two-dimensional)–3D transition regime. The effect of incoming shear is buried in a source term employing a velocity transformation. Wake transition events are inspected for both planar and span-wise shear flows. Parallel, time-mean-symmetric shedding for planar shear and oblique vortex shedding for span-wise shear are observed with a near wake roll-up vortical structure. Vortex splits and dislocations are found without any order in time for moderate shear, while they give way to visibly higher levels of instabilities at higher shear rates. Mode “B” instabilities are noted for planar shear, while opposite streamwise vortices align in parallel horizontal layers for span-wise shear. Local Strouhal frequency (Stz) drops inside a span-wise cell for span-wise shear with finite jumps across cell boundaries. Wavelet multiresolution analysis indicates a strong flushing effect, triggered by vortex dislocations, which gives rise to a new frequency event. The dominant span-wise mode indicates periodic forcing of mode “A” instabilities at a rate close to the inverse of local Strouhal number. In contrast, the streamwise velocity modes result in a global span-wise similarity. Intrinsic secondary instabilities play a vital role in span-wise shear cases. The addition of planar shear makes the downstream defect layer nearly span-wise-invariant. However, the velocity defect is entirely controlled by the span-wise shear. The momentum thickness exhibits streamwise growth, similar to the Blasius profile. The shape factor of such profiles indicates a delay in laminar–turbulent transition for span-wise shear.

The present work aims at studying the laminar flow and heat transfer characteristics of a Newtonian fluid over a row of semi-circular cylinders placed in uniform cross-flow configuration. The effects of spacing to diameter (gap) ratio, varied from 1 to 10, on the flow and heat transfer patterns have been studied at a Reynolds number of 100 for air as the working fluid. There has been no significant interaction in the flow at a spacing ratio greater than 4 and each semi-circular cylinder behaved more or less like the situation of a single cylinder. At lower gap ratios, the flow becomes more complicated due to the shear layer interactions that exist in the flow. The streamline and isotherm contours have been presented and discussed in detail for various gap ratios. The variation in flow behavior is explored further by studying the drag coefficient and lift coefficient signals of various semi-circular cylinders. The presence of a secondary frequency at low gap ratios that has adverse effect on the drag coefficient has been confirmed. The variation of measured global quantities, namely the drag coefficient, the Strouhal number and the Nusselt number have been studied and discussed in detail.

In the present study, the heat transfer characteristics of thermally developing magnetohydroclynamic flow of nanofluid through microchannel are delineated by following a semi analytical approach. The combined influences of pressure driven flow, electroosmotic transport and magnetic field is taken into account for the analysis of the complex microscale thermal transport processes. Solutions for the normalized temperature distributions and the Nusselt number variations, considering the simultaneous interplay of electrokinetic effects (electroosmosis), magnetic effects, Joule heating and viscous dissipation are obtained, for constant wall temperature condition. Particular attention is paid to assess the role of nanolluids in altering the transport phenomena, through variations in the effective nanoparticle volume fractions, as well as the aggregate structure of the particulate phases. It is observed that magnetohydrodynamic effect reduces advective transport of the liquid resulting in gradual reduction of heat transfer. Increase in nanoparticle volume fraction shows decrease in heat transfer. Similar effects are observed with increase in aggregate sizes of the nanoparticles. The effect of the nanofluids on system irreversibility is also studied through entropy generation analysis due to flow and heat transfer in the microchannel. Total entropy generation is found to be dominant at the thermally developing region of the microchannel, whereas it drops sharply at the thermally developed region. Presence of nanoparticles in the base fluid reduces the total entropy generation in the microchannel, thereby indicating decrease in thermodynamic irreversibility with increasing nanoparticle volume fraction. (C) 2014 Elsevier Ltd. All rights reserved.

For the case of solidification of a bottom cooled binary alloy, the magnetohydrodynamic stationary and oscillatory convective stability in the mushy layer is investigated analytically using normal mode linear stability analysis. In the limit of large Stefan number (S-t), a near-eutectic approximation with large far field temperature is considered in the present research. To ascertain the instability in the mushy layer, the strength of the superimposed magnetic field is so chosen that it corresponds to a given mush Hartmann number (Ha(m)) of the problem. The results are presented for various values of mush Hartmann numbers in the range, 0

The present paper presents fluid flow and mixed convective heat transfer characteristics past a semi-circular cylinder at incidence with a tandem square cylinder in cross flow. Using air ( Pr = 0.71 ) as an operating medium, numerical simulations are performed for the range of Reynolds numbers, 10 ≤ Re ≤ 45 and incidence angles, 0 0 ≤ α ≤ 180 0 . The effect of thermal buoyancy is brought about by varying the Richardson number in the range 0 ≤ R i ≤ 2 . The mathematical model is firstly validated with the experimental and numerical results from the literature and found to be in good agreement. The steady separated flow is observed to become unsteady periodic under the superimposed thermal buoyancy. Furthermore, apart from thermal buoyancy effect, angle of incidence is found to play a pivotal role in bringing hydrodynamic instabilities and thereby vortex shedding for such steady mixed convective flows. Functional dependence of drag ( C D ) , lift ( C L ) , and moment ( C M ) coefficients on the combined influence of α and R i , is explored and analysed in detail. Additionally, other global quantities, such as local and average Nusselt number distribution, Strouhal number ( S t ) are determined with respect to the various ranges of parameters considered in the present investigation.

We investigate analytically the combined consequences of electromagnetohydrodynamic forces and interfacial slip on streaming potential mediated pressuredriven flow in a microchannel. Going beyond traditional Debye-Huckel limit, we first derive a closed-form analytical solution for velocity field by considering nonlinear electrical potential distribution, wall slip effects, externally imposed transverse magnetic field, and laterally applied electric field in the plane of flow. The effects of electrical double-layer (EDL) formation and the consequent interfacial phenomena are critically examined under such situations. An expression for induced streaming potential in the microchannel is deduced considering EDL formation and the consequences of finite conductance of the immobilized Stern layer. This simplified analytical expression is later on critically assessed against three-dimensional simulation paradigm of streaming potential mediated flows, which is a first effort of this kind. We demonstrate that flow rate increases progressively with increasing surface potential and eventually approaches to a limiting value. Combination of electromagnetohydrodynamic effect with liquid slip is shown to amplify the flow rate, even at lower values of surface potential. Our study brings out the possibility of achieving an optimum flow rate by judicious application of combined electromagnetohydrodynamics. The present analysis has significant consequence in the design of advanced microfluidic devices with improved efficiency and functionality.

This paper critically analyzes, for the first time, the effect of nanofluid on thermally fully developed magnetohydrodynamic flows through microchannel, by considering combined effects of externally applied pressure gradient and electroosmosis. The classical boundary condition of uniform wall heat flux is considered, and the effects of viscous dissipation as well as Joule heating have been taken into account. Closed-form analytical expressions for the pertinent velocity and temperature distributions and the Nusselt number variations are obtained, in order to examine the role of nanofluids in influencing the fully developed thermal transport in electroosmotic microflows under the effect of magnetic field. Fundamental considerations are invoked to ascertain the consequences of particle agglomeration on the thermophysical properties of the nanofluid. The present theoretical formalism addresses the details of the interparticle interaction kinetics in tune with the pertinent variations in the effective particulate dimensions, volume fractions of the nanoparticles, as well as the aggregate structure of the particulate system. It is revealed that the inclusion of nanofluid changes the transport characteristics and system irreversibility to a considerable extent and can have significant consequences in the design of electroosmotically actuated microfluidic systems.

In this paper, we have proposed a theoretical analysis to investigate buoyancy driven convection of nanofluids in an infinitely long channel under superimposed magnetic field. We derive closed form analytical solutions for the magnetohydrodynamic flow and temperature field under two distinctive wall boundary conditions. Proceeding further ahead, we also present an analysis for the total entropy generation due to magnetohydrodynamic fluid friction and heat transfer irreversibilities. Utilizing water based Al2O3 nanofluids, results are shown for the following range of conditions as 0 ⩽ Ha ⩽ 50, 0 ⩽ ϕ ⩽ 4% and 103 ⩽ Gr ⩽ 105. It is revealed that magnetohydrodynamic effect reduces flow strength. Likewise the case of the velocity profiles, magnetic effect reduces the magnitude of temperature distribution. Total entropy generation shows decreasing trend when the volume fraction of the nanofluids is increased. Increasing nanoparticle size results in increasing total entropy generation and the Bejan number.

The mixed convective flow stability of nanofluids past a square cylinder is investigated by Dynamic Mode Decomposition (DMD). The energy content in the individual modes for Cu–water nanofluids is found to be higher than that of Al2O3–water nanofluids. DMD results showed the fact that Cu–water nanofluids have more small-scale structures of higher frequency modes compared to that of Al2O3–water nanofluids. The most dominant temporal dynamic mode corresponds to the lower-frequency eigenvalue λ = ( 0.99374 , ± 0.1117 ) for Al2O3–water nanofluids and λ = ( 0.99451 , ± 0.10464 ) for Cu–water nanofluids. Energy content in the mean flow of the base fluid at Richardson number of −0.5 is found to be maximum compared to that of nanofluids.

The present work simulates the buoyancy driven mixed convective flow and heat transfer characteristics of water-based nanofluid past a square cylinder in vertically upward flow using a SUPG (Streamline Upwind Petrov–Galerkin) based finite element method. Nano sized copper (Cu) and alumina (Al 2 O 3 ) particles suspended in water are used with Prandtl number ( Pr ) = 6.9. The range of nanoparticle volume fractions ( ϕ ) considered is 0 ⩽ ϕ ⩽ 20%. Computations are carried out at a representative Reynolds number ( Re ) of 100. Effect of aiding and opposing buoyancy is brought about by considering the Richardson number ( Ri ) range −0.5 ⩽ Ri ⩽ 0.5. Al 2 O 3 – water and Cu– water nanofluids show suppression of vortex shedding at Ri ⩾ 0.15. Vortex shedding process is initiated and a completely new phenomenon is discovered when the nanofluid solid volume fraction, ϕ , is increased. For Al 2 O 3 – water nanofluids, at Ri = 0.15, completely periodic vortex shedding is found for ϕ ⩾ 10%. For Cu– water nanofluid, shedding is observed for both Ri = 0.15 and Ri = 0.5. At Ri = 0.15 shedding is found at ϕ ⩾ 5%, whereas at Ri = 0.5 it is at ϕ ⩾ 15%. A new expression for convective instability of nanofluids is derived to calculate critical nanofluid solid volume fraction. The local Nusselt number increases with increasing ϕ . At a fixed ϕ , the time averaged local Nusselt number is higher for Cu– water nanofluids, as compared to Al 2 O 3 – water nanofluids. The average Nusselt number ( Nu avg ) increases with the concentration of ϕ . Cu– water based nanofluids show higher magnitudes of Nu avg compared to Al 2 O 3 – water nanofluids.

In this article, we investigate the combined consequences of magnetohydrodynamic forces and interfacial slip on the heat transfer characteristics of streaming potential mediated flow in narrow fluidic confinements by following a semianalytical formalism. Going beyond the celebrated Debye-Htickel linearization, we obtain a closed form analytical expression for velocity and induced streaming potential through the consistent description of finite conductance of the immobilized Stern layer. We report an augmentation in the streaming potential field as attributable to the wall slip activated enhanced electro-magnetohydrodynamic transport of the ionic species within the EDL. In particular, we demonstrate the key role of induced streaming potential in altering thermal transport and Nusselt number variation considering the concurrent interplay of hydrodynamic slip lengths, magnetic effects, viscous dissipation, and Joule heating. We also show the implications of Stern layer conductivity and magnetohydrodynamic influence on system irreversibility through entropy generation analysis due to fluid friction and heat transfer. Finally, our results have significant scientific and technological consequences in the novel design of future generation energy efficient devices and could be useful in further advancement of theory, simulation, and experimental work. (C) 2016 Elsevier Ltd. All rights reserved.

In this work, a numerical study was carried out to characterise the hydrodynamics and heat transport in a five-strand asymmetric billet caster tundish. Employing a three-dimensional computational fluid dynamics model, several numerical experimentations were carried out to capture the exact physics of complex fluid flow and thermal transport in the tundish. Plant practices were simulated by considering seven distinct cases and analysed in detail. Tundish filling operation was tried out with volume of fluid methodology. The effects of thermal buoyancy, its effect on thermofluidic heat transfer and the abrupt change in thermal gradients at different zones in the tundish flow domain were explained after detailed analysis. Refractory heating simulations were performed, and results were analysed in detail. This is the first time that such an exhaustive numerical study on an asymmetric tundish has been reported.

Wake dynamics and forced convective heat transfer characteristics past a semi-circular cylinder at incidence have been investigated numerically. Utilizing air as an operating fluid computations are carried out for wide ranges of the Reynolds number (80 ⩽ Re ⩽ 180) and angle of incidences (0 ⩽ α ⩽ 180°). Angle of incidence reveals three flow separation zones. Structure properties of shear layer and vortex motions on each flow separation zones are analyzed critically. Functional dependence of drag (CD), lift (CL), and moment (CM) coefficients on the angle of incidence is explored and analyzed in detail. Increase in angle of incidence increases streamline curvature. A structural similarity is observed between the contours of vorticity and the corresponding isotherms. Strouhal number shows a decreasing trend up to certain values of α and thereafter it increases marginally. A new correlation of Strouhal number as a function of Re and α has been established for the present range of Reynolds numbers. At the singularity points a sudden jump in local Nusselt number distribution is observed. The trend of variation of average Nusselt number with α is similar to that of Strouhal number variation. The average Nusselt number is found to vary as Re 0.529 ( 1 + α ) - 0.0476 .

This paper investigates the buoyancy driven mixed convective flow and heat transfer characteristics of water-based nanofluid past a circular cylinder in cross flow using a SUPG based finite element method. Nano sized copper particles suspended in water is used with Prandtl number (Pr) = 6.2, and the range of solid volume fractions 0 ⩽ ϕ ⩽ 25 % are considered. Computations are carried out for the range of Reynolds number 80 ⩽ Re ⩽ 180 . Effect of aiding and opposing buoyancy is bought about by considering two representative Richardson numbers of 1 and −1. Increase in nanoparticle loading show symmetric vortex structure distributions and have minimal effect of negative buoyancy. Width of the flayers of thermal energy reduces with increasing nanoparticle volume fractions. Increase in solid volume fraction show reduction in thermal boundary layer thickness and increased thermal gradient at the cylinder surface. The local and average Nusselt numbers are found to increase with increasing Re and ϕ. The average Nusselt number at Ri = 1 and Ri = −1 are found to vary as 0.947Re0.550(1 − ϕ)−1.6253 and 0.881Re0.559(1 − ϕ)−1.7349 respectively. It is observed that the presence of nanoparticles imparts a counter balancing force to that of buoyancy force. This tries to minimize the effect of buoyancy force and stabilizes the flow. This is, perhaps, the first time that such behavior for the nanofluid is being reported.

This paper investigates the combined effect of Prandtl number and Richardson number on the wake dynamics and heat transfer past a circular cylinder in crossflow using a SUPG based finite element method. The computations are carried out for 80 0 ⩽ Ri ⩽ 2 . The results have been presented for both forced and mixed convection flows. In the case of forced convection, crowding of temperature contours with reduced spatial spread is observed for increasing Prandtl numbers. The local and average Nusselt numbers are found to increase with increasing Reynolds number and Prandtl number. The average Nusselt number and Colburn factor are found to vary as Re0.548 Pr 0.373 and Re−0.452, respectively. The extrapolated results of the average Nusselt number for low and high Reynolds numbers are found to match quite well with the available results in literature. Effect of Prandtl number shows various interesting phenomena for the mixed convective flows. Increasing the Prandtl numbers resulted in decreasing deflection and strength in the wake structures. The effect of baroclinic vorticity production during vortex shedding has been demonstrated at the vicinity of the cylinder and near wake. The Strouhal number is found to decrease with increasing Prandtl number, in the case of buoyancy induced flow. The effect of increasing Prandtl number is manifested as the stabilizing effect in the flow. This is, perhaps, the first time that such behavior for the Prandtl number is being reported. Additionally it is observed that the average Nusselt number decreases with increasing Richardson number.

Numerical results of mixed convective heat transfer from two identical cylinders in a uniform upward flow has been demonstrated at Re = 100. The effect of aiding and opposing buoyancy is brought about by varying Richardson numbers. A stabilized SUPG based finite element technique has been used. Results are discussed for 20 cases by varying the locations of second cylinder with respect to the fixed location of the first cylinder. The effect of buoyancy on force coefficients, Strouhal number and Nusselt number is investigated. Under the same buoyancy induced field the cylinders are found to oscillate at the same frequency. For a particular cylinder spacing, vortex shedding is observed up to Ri = 0.25. Maximum heat transfer is found at the front face of second cylinder. It is found that hydrodynamic instabilities grow and flow shows chaotic phenomena when the system is severely influenced by thermal buoyancy. It is for the first time that such behavior for the Richardson number at this tandem configuration is being reported.

This paper demonstrates the vortex shedding process behind a heated cylinder in a cross-flow at low Reynolds numbers under the influence of thermal buoyancy. The simulations were performed using an SUPG-based finite element technique. The range of Reynolds numbers was chosen to be 10–45. The flow was steady in the absence of thermal buoyancy. The eddy length and the separation angle were computed for the steady separated flow in the above range of Reynolds numbers. The results were in agreement with those reported in the literature. The Nusselt number distribution around the heated cylinder was also computed in the above range of Reynolds numbers for forced convective flows. The results compared fairly well with available experimental results. The effect of superimposed thermal buoyancy in the same range of Reynolds numbers was studied for various Richardson numbers. The steady separated flows become unsteady periodic in the presence of superimposed thermal buoyancy. For the unsteady periodic flows, the Strouhal numbers were computed. The separation angles and average Nusselt number for such unsteady flows were found to vary with time.

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied using large-eddy simulation (LES). A series of airfoils of constant thickness with rounded leading edge are employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case, an infinite row of the model airfoils). This setup is a simplified representation of the rotor–stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of disturbances, becomes turbulent, and finally reattaches forming a separation bubble. In the presence of oncoming wakes, the characteristics of the separated boundary layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase-averaged results illustrate the periodic behavior of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of the boundary layer by convective wakes forming coherent vortices, which are being shed and convect downstream. Further, the transition of the separated boundary layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations, where viscous effect is substantial.

The present work investigates the mixed convective flow and heat transfer characteristics past a triangular cylinder placed symmetrically in a vertical channel. At a representative Reynolds number, Re = 100, simulations are carried out for the blockage ratios beta = 1/3; 1/4; and 1/6. Effect of aiding and opposing buoyancy is brought about by varying the Richardson number in the range -1.0 0: 75 is observed at beta = 1/3.

The entropy generation due to mixed convective heat transfer of nanofluids past a rotating circular cylinder placed in a uniform cross stream is investigated via streamline upwind Petrov–Galerkin based finite element method. Nanosized copper (Cu) particles suspended in water are used with Prandtl number (Pr) = 6.9. The computations are carried out at a representative Reynolds number (Re) of 100. The dimensionless cylinder rotation rate, α, is varied between 0 and 2. The range of nanoparticle volume fractions (ϕ) considered is 0 ≤ ϕ ≤ 5%. Effect of aiding buoyancy is brought about by considering two fixed values of the Richardson number (Ri) as 0.5 and 1.0. A new model for predicting the effective viscosity and thermal conductivity of dilute suspensions of nanoscale colloidal particles is presented. The model addresses the details of the agglomeration–deagglomeration in tune with the pertinent variations in the effective particulate dimensions, volume fractions, as well as the aggregate structure of the particulate system. The total entropy generation is found to decrease sharply with cylinder rotation rates and nanoparticle volume fractions. Increase in nanoparticle agglomeration shows decrease in heat transfer irreversibility. The Bejan number falls sharply with increase in α and ϕ.

In this paper, a three-dimensional transient macroscopic numerical model is developed for the description of transport phenomena during laser surface alloying. In order to make accurate estimates for the species composition distribution during the process, the addition of alloying elements is formulated by devising a species generation term for the solute transport equation. By employing a particle-tracking algorithm and a simultaneous particle-melting consideration, the species source term is estimated by the amount of fusion of a spherical particle as it passes through a particular control volume. Numerical simulations are performed for two cases. The first case corresponds to aluminium as alloying element on a nickel substrate, while the second case is for alloying nickel on aluminium substrate. It is observed for the latter case that the melting of the alloying element is not instantaneous, and hence it cannot be modelled as a species mass flux boundary condition at the top surface. The predicted results are compared with experiments, and the agreement is found to be good.

In this paper, a numerical investigation is performed to study the mixed convective flow and heat transfer characteristics past a square cylinder in cross flow at incidence. Utilizing air (Pr = 0.71) as an operating fluid, computations are carried out at a representative Reynolds number (Re) of 100. Angles of incidences are varied as, 0 degrees

The present work demonstrates entropy generation due to laminar mixed convection of water-based nanofluid past a square cylinder in vertically upward flow. Streamline upwind Petrov–Galerkin (SUPG) based finite element method is used for numerical simulation. Nanosized copper (Cu) and alumina (Al2O3) particles suspended in water are used with Prandtl number (Pr) = 6.2. The range of nanoparticle volume fractions considered is 0–20%. Computations are carried out at a representative Reynolds number (Re) of 100 with Richardson number (Ri) range −0.5