Showing papers in "European Journal of Mechanics B-fluids in 2020"

MHD mixed convection stagnation-point flow of Cu-Al2O3/water hybrid nanofluid over a permeable stretching/shrinking surface with heat source/sink

Abstract: This study focused on Cu-Al2O3/water hybrid nanofluid to address the issue related to mixed convection stagnation-point flow and heat transfer over a permeable stretching/shrinking sheet with the magnetic field and heat source/sink effects. The governing equations of the problem are initially reduced into similarity equations by utilising similarity transformations, prior to being solved numerically using the function bvp4c in MATLAB. As a result, dual (upper and lower branches) solutions were obtained in opposing and assisting flow regions. The outcomes derived from stability analysis indicated that the upper branch solution was stable, while unstable for the lower branch. Additionally, the heat transfer rate of base fluid (water) was higher than the hybrid nanofluid.

Permeability and fluid flow-induced wall shear stress in bone scaffolds with TPMS and lattice architectures: A CFD analysis

Abstract: Fluid flow dynamics within porous scaffolds for tissue engineering play a critical role in the transport of fundamental materials to the cells and in controlling the biocompatibility of the scaffold. Properties such as permeability and fluid flow-induced wall shear stress characterize the biological behavior of the scaffolds. Bioactivity depends on the diffusion of oxygen and other nutritious elements through the porous medium and fluid flow-induced shear stress is known as the dominant mechanical stimulant of cell differentiation and proliferation within the scaffolds. In this study, eight different bone scaffold models with a constant porosity of 80% were designed computationally using the TPMS and lattice-based structures. We investigated the fluid flow within the scaffolds using CFD analysis. The results of the work showed that scaffold architecture has a significant impact on the permeability and that scaffold permeability can vary up to three times depending on the architecture. The scaffolds with the minimal variation in their channel size exhibited the highest permeability. We investigated the distribution statistics of wall shear stress on the walls of the scaffolds and showed that a correlation between the architecture of the scaffolds and the distribution statistics of wall shear stress did not exist. The outcomings of this work can be promising in designing better scaffolds in tissue engineering from a biological point of view.

Heat transfer enhancement with nanofluids in plate heat exchangers: A comprehensive review

Abstract: The application of nanofluids has dramatically increased from the past two decades. Nanofluids have elegantly captivated the attention of researchers nowadays. At present, various papers are being reported dealing with this interesting domain, allied with fascinating applications. However, the nanofluids being captives these days depicts the crucial need to bestow the comprehensive review of nanofluids application in distinct domains. This paper examines the utilization of nanofluids with distinct Plate Heat Exchanger (PHE) geometries. All the reported studies are alienated to two main categories; experimental and numerical. Furthermore, critical information regarding nanoparticle size, base fluids, analytical methods, heat transfer enhancement, flow regime, and pressure drop is presented in a comprehensive table in each section. Also, it was ultimately found that all the studies; analytical, experimental and numerical gave desired and appreciable thermal performance compared to conventional fluids. Author also reported the statistical analysis for the past published papers and the results show the increasing importance of nanofluids application in plate heat exchanger. Most of the studies showed preferred thermal behaviour, heat transfer enhancement, reduction in entropy generation and reduction in exergy destruction compared to the base fluids. An increase in Reynolds number can provide better heat transfer rates. The operating temperature of nanofluids plays a key role in the effectiveness of heat exchanger and heat transfer enhancement. Almost all the studies have demonstrated the preferred nanofluids thermal behaviour in plate heat exchanger, compared to the base fluid but Chevron and Corrugated type geometry of plate heat exchanger gives the appreciable enhancement in Nusselt number.

Spectral quasi linearization simulation of radiative nanofluidic transport over a bended surface considering the effects of multiple convective conditions

Abstract: This article enlightens on the hydrothermal performance of radiative nanofluid flow over a curved stretched surface. The curved surface is coiled inside a circular segment of radius R. The surface has been presumed to be permeable and slippery. Effects of multiple convective conditions i.e. convective conditions at the surface caused by both heat and mass transport are incorporated to explore the outcomes. The permeability characteristic of the surface that affects the hydrothermal integrity of the flow has been discussed in detail. The leading equations are renovated into its non-dimensional form by applying similarity conversion. After then, the spectral quasi linearization method (SQLM) is introduced to solve those foremost nonlinear ordinary differential equations (ODEs). Residual error analysis is depicted to show the convergence rate of SQLM. The impact of the pertinent factors on the flow is illustrated through graphs and tables. Several streamlines and three-dimensional plots are provided to enrich the result section. Results assured that temperature reduces for curvature parameter, but intensifies for both thermal and mass Biot numbers. Nanofluidic motion increases for curvature parameter and declines for slip factor. Curvature parameter and Lewis number provide reduction in mass transfer, whereas thermal and mass Biot numbers, slip parameters provide the enhancement. Heat transfer is enhanced for curvature parameter, thermal and mass Biot number.

Post impact droplet hydrodynamics on inclined planes of variant wettabilities

Abstract: Experimental investigations were carried out to elucidate the roles of surface wettability and inclination on the post-impact dynamics of droplets. The maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number (We n ) for superhydrophobic (SH) surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, albeit with the incorporation of modifications for the oblique impact conditions. The energy ratios and elongation factors were also determined for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch-off at the contact line and inclination dependent elongation behavior were observed. On the contrary, the hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and We n , respectively. This is caused by the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Further, the influence of the surface tension (using surfactant solutions, without significantly changing the viscosity) and viscosity (using colloids, without significantly changing the surface tension) for impact on SH and hydrophilic surfaces are probed. The exercise allows better insight on the exact hydrodynamic mechanisms at play during the impact events. Overall, the article provides a comprehensive picture of post-impact dynamics of droplets on inclined surfaces, encompassing a broad spectrum of governing parameters like Reynolds number (Re), Weber number (We), degree of inclination and surface wettability.

Fluid–Structure-Electrophysiology interaction (FSEI) in the left-heart: A multi-way coupled computational model

Abstract: In this study we present a computational model for unprecedented simulations of the left heart in realistic physiological conditions. To this aim, models for the electrical network of contractile muscular fibers (electrophysiology bidomain model), the myocardium mechanics (shells with hyperelastic and orthotropic constitutive relations) and the complex hemodynamics (direct numerical simulation of the Navier–Stokes equations) have been developed and multi-way coupled. The resulting multi-physics model, relying on the immersed-boundary method to cope with the complex fluid–structure interaction, is then validated by replicating the dynamics of the left heart considering simultaneously its atrium and ventricle, with the embedded aortic and mitral valves, and the thoracic aorta where blood is pumped. It is shown that the developed model, when given as input the parameters for the human heart, can reproduce the physiologic velocity and pressure signals obtained by cardiographic diagnostics of real patients.

A BGK model for high temperature rarefied gas flows

Abstract: High temperature gases, for instance in hypersonic reentry flows, show complex phenomena like excitation of rotational and vibrational energy modes, and even chemical reactions. For flows in the continuous regime, simulation codes use analytic or tabulated constitutive laws for pressure and temperature. In this paper, we propose a BGK model which is consistent with any arbitrary constitutive laws, and which is designed to make high temperature gas flow simulations in the rarefied regime. A Chapman-Enskog analysis gives the corresponding transport coefficients. Our approach is illustrated by a numerical comparison with a compressible Navier-Stokes solver with rotational and vibrational non equilibrium. The BGK approach gives a deterministic solver with a computational cost which is close to that of a simple monoatomic gas.

Hydrodynamic features of three equally spaced, long flexible cylinders undergoing flow-induced vibration

Abstract: Three equally spaced flexible cylinders are frequently applied in many engineering fields. The flow-induced vibration (FIV) hydrodynamic features of three such cylinders are not the same as those of an isolated single one and remain unknown. In this paper, the hydrodynamic coefficients for three flexible cylinders subjected to FIV in an equilateral-triangular arrangement with a centre-to-centre spacing of 6 diameters were identified using an inverse analysis method according to the displacement response data obtained from model tests. The lift coefficient, varying drag coefficient and added mass coefficient at the dominant frequency were calculated by decomposing the cross-flow (CF) and in-line (IL) fluctuating forces. Three typical cases of an equilateral-triangular configuration (A, B, and C), which correspond to flow incidence angles of 0 ∘ , 30 ∘ , and 60 ∘ , were studied and discussed (the incidence angle is defined as the angle between the flow orientation and the line linking the centre points of one cylinder and the equilateral-triangular configuration). The hydrodynamic coefficients of the upstream cylinders are insignificantly influenced by the downstream cylinders. In contrast, the wake of the upstream cylinders impose a notable effect on the IL hydrodynamic coefficients of the downstream cylinders. Two robust frequency components ( f v , I L and f v , I L _ 1 ∕ 2 ) were observed in the IL vibrations of the downstream cylinders. The IL hydrodynamic forces at f v , I L have similar features to the classical vortex shedding forces of an isolated flexible cylinder. However, the IL hydrodynamic forces at f v , I L _ 1 ∕ 2 exhibit distinct behaviours that are closely related to the unique response characteristics in the IL direction. In addition, the IL fluctuating force coefficients and varying drag coefficients at f v , I L _ 1 ∕ 2 are relatively small and change slowly with the reduced velocity.

Modeling and simulation of blood flow with magnetic nanoparticles as carrier for targeted drug delivery in the stenosed artery

Abstract: A systematic study on targeted drug delivery is carried out in an unsteady flow of blood infused with magnetic nanoparticles with an aim to understand the flow pattern and nanoparticle aggregation in a diseased arterial segment having atherosclerosis. The magnetic NPs(nanoparticles) are supervised by a magnetic field, which is significant for the therapeutic treatment of arterial diseases, tumors, and cancer cells and removing blood clots. Coupled thermal energy equation has been modeled by considering the dissipation of energy that encounters due to the application of the magnetic field and the presence of high viscosity of blood. The simulation technique used to solve the mathematical model is vorticity-stream function formulations in the diseased artery. An elevation in SLP (Specific loss power) is noted in the aortic bloodstream when the agglomeration of nanoparticles is higher. This phenomenon has potential applications in the treatment of hyperthermia. The study focuses on the lowering of WSS (wall shear stress) with increasing particle concentration at the downstream of the stenosis, which depicts the vigorous flow circulation zone. These low shear stress regions prolong the residing time of the nanoparticles carrying drugs, which soaks up the LDL (Low-Density Lipoprotein) deposition. Moreover, an increase in NP concentration enhances the Nusselt number, which marks the increase of heat transfer from the arterial wall to the surrounding tissues to destroy tumor and cancer cells without affecting the healthy cells. The results have a significant influence on the study of medicine to treat arterial diseases such as atherosclerosis without the need for surgery, which can minimize the expenditures on cardiovascular treatments and post-surgical complications in patients.

Investigating lump and its interaction for the third-order evolution equation arising propagation of long waves over shallow water

Abstract: In this paper, we used the Hirota bilinear method for investigating the third-order evolution equation to determine the soliton-type solutions We probe five cases including lump, lump–kink called the interaction between a lump and one line soliton, lump-soliton called the interaction between a lump and two-line solitons, kinky breather–soliton and finally the stripe soliton function only with exponential solution function The theorem along with the proof for the considered problem is given Moreover, to more investigating the moving velocity, the maximum amplitude and also moving pass function are obtained The existence criteria of these solitons in the unidirectional propagation of long waves over shallow water are also demonstrated Different arbitrary parameters received in the solutions help us to discuss the physical interpretation of solutions that can be linked with a large variety of physical phenomena

Merkin and Needham wall jet problem for hybrid nanofluids with thermal energy

Abstract: The classical Merkin and Needham’s wall-jet problem was studied for copper-titanium dioxide/water hybrid nanofluid with the effects of suction/injection and thermal energy. The governing equations were converted into non-linear ordinary differential equations by using proper similarity transformations. These equations were then solved analytically for the hybrid nanofluid flow and temperature distribution in hypergeometric function. Therefore, we have obtained expressions for both the reduced skin friction coefficient and reduced Nusselt number. It was found that the upper-branch and lower-branch solutions are possible only for suction. Further in the case of regular fluid, i.e. the solid volume fractions are zeros, the current results are in a very good agreement with those in the literature. Moreover, it was shown that copper-nanoparticle volume fraction plays a very important role in variation of the velocity. Furthermore, adding nanoparticles of copper to the nanofluid titanium dioxide/water cools/warms the resulting hybrid nanofluid on increasing some of the involved parameters.

On secondary tones arising in trailing-edge noise at moderate Reynolds numbers

Abstract: Direct numerical simulations are carried out to investigate the flow features responsible for secondary tones arising in trailing-edge noise at moderate Reynolds numbers. Simulations are performed for a NACA 0012 airfoil at freestream Mach numbers 0.1, 0.2 and 0.3 for angle of incidence 0 deg. and for Mach number 0.3 at 3 deg. angle of incidence. The Reynolds number based on the airfoil chord is fixed at R e c = 1 0 5 . Flow configurations are investigated where noise generation arises from the scattering of boundary layer instabilities at the trailing edge. Results show that noise emission has a main tone with equidistant secondary tones, as discussed in literature. An interesting feature of the present flows at zero incidence is shown; despite the geometric symmetry, the flows become non-symmetric with a separation bubble only on one side of the airfoil. A separation bubble is also observed for the non-zero incidence flow. For both angles of incidence analyzed, it is shown that low-frequency motion of the separation bubbles induce a frequency modulation of the flow instabilities developed along the airfoil boundary layer. When the airfoil is at 0 deg. angle of attack an intense amplitude modulation is also observed in the flow quantities, resulting in a complex vortex interaction mechanism at the trailing edge. Both amplitude and frequency modulations directly affect the velocity and pressure fluctuations that are scattered at the trailing edge, what leads to secondary tones in the acoustic radiation.

Non-axisymmetric Homann stagnation-point flow of a viscoelastic fluid towards a fixed plate

Abstract: We have investigated the non-axisymmetric Homann stagnation-point flow of a viscoelastic fluid over a rigid plate. In a recent paper Weidman (2012) has modified Homann’s stagnation point flow and made it non-axisymmetric over a rigid plate. Now if the fluid is non-Newtonian a new family of asymmetric stagnation-point flows arises depending on the shear to strain-rate ratio γ ( = b ∕ a ) and the viscoelastic parameter k . Here a , b are the strain rate and shear rate of the stagnation-point flow. The governing momentum equations are solved numerically using fourth order Runge–Kutta method with shooting technique. The effect of the various parameters on the wall shear stress parameters, the dimensionless velocities, the displacement thicknesses and the velocity distributions are analysed. Numerical results of wall shear stress and displacement thicknesses are compared with their large value behaviours and those behaviours give a good agreement with the corresponding numerical solutions.

Effect of a porous sea-bed on water wave scattering by two thin vertical submerged porous plates

Abstract: A hydrodynamic model, with incorporation of porosity, is considered to investigate oblique water wave scattering by two fully submerged parallel porous plates with the wave propagating over a porous bed in a homogeneous fluid flow with the upper surface exposed to atmosphere. The porous plates are assumed to follow the theory of thin plates and the wave propagation through the porous structure follows porous wave-maker theory. The behavior and properties of the roots of the dispersion relation are analyzed by adopting counting argument and contour plot. Time-harmonic propagating waves propagate with exactly one wavenumber along the free surface for any given frequency. Methods of eigenfunction expansion and least square are employed to acquire the complete analytical solution for interaction of water waves with submerged porous plates. Subsequently the reflection and transmission coefficients as well as the energy loss are computed. Then those are examined corresponding to various values of parameters such as porous-effect parameter, the submergence depth of plates from free surface, angle of incidence, porosity of the sea-bed. Present investigation clearly demonstrates that the wave reflection is of oscillatory nature. It further shows that the occurrence of minima in wave reflection is due to an increase in the inertial effect of the porous plates which dissipate a significant portion of the wave energy. The effect of the porous bed under consideration on surface gravity waves is carried out by introducing various numerical values to the hydrodynamic wave characteristics and it is noticed that a reasonable change in porosity of the sea-bed has a significant impact when the propagating wave encounters the submerged structure. The present approach is expected to be of great significance in designing and construction of different types of effective wave absorbers utilized in sea for studying reflection as well as dissipation of wave energy in coastal regions and hence for the purpose of coastal as well as offshore engineering. The present model is validated by comparing it with some established result.

Numerical investigation on the evolution of forces and energy features in thermo-sensitive cavitating flow

Abstract: This study aims to investigate the evolution of transient force and energy features in a thermo-sensitive cavitating flow around a hydrofoil A cavitation model is extended to investigate the thermal effect, and a shear stress transfer — partially averaged Navier–Stokes turbulence model is adopted to simulate the cavitating turbulent flow The distribution of changed cavitation number is consistent with the cavity shape The evident temperature gradient exists in the vicinity of the closure line, which leads the temperature drop to increasing gradually from the closure line to the centre of the cavity The changed cavitation number also increases according to this rule The force element analysis reveals that vortex structures induced by cavity evolution make the lift and drag forces fluctuation Lift and drag forces increase approximately linearly with the development of sheet cavity When the large attached sheet cavity separates from the suction surface, the lift and drag forces increase substantially Conversely, the collapse of cloud cavity causes a sudden reduction of lift and drag forces About the detailed evolution mechanisms of lift and drag forces, this work also expounds Moreover, the terms in the kinetic energy transport equation are applied to depict the dissipation and transitivity of energy The attached sheet cavity suppresses energy dissipation because of the stable flow field The separation of sheet cavity and the shedding of cloud cavity induce the generation of multiscale vortex structures, and causes the energy loss to increase obviously Combined with the analysis of the lift and drag forces, the evolution of fluid forces on the hydrofoil is also related to the energy loss

Flow and heat transfer of nanofluid through a horizontal microchannel with magnetic field and interfacial electrokinetic effects

Abstract: Flow and heat transfer of a nanofluid through a horizontal microchannel in the presence of the magnetic field effects and electric double layer (EDL) is investigated theoretically. For a microchannel with a large aspect ratio, the flow problem is treated as a two-dimensional nonlinear system. The body force generated by the EDL and magnetic field is considered in momentum equation. In order to study the mechanism of nanofluid heat transfer, the nanoparticles distribution and the heat transfer process of nanofluid flow are represented by the Buongiorno’s nanofluid model with the passively controlled nanoparticles distribution at the boundary, which has not been considered in previous microchannel studies. Compared to the so-called active control of nanoparticle volume fraction at the boundary, the current approach makes the model physically more reliable by taking into account of the effect due to varying temperature. The analytical approximations obtained by the homotopy analysis method reveals that both the magnetic field effects and the EDL play significant roles on altering the flow and heat transfer in microchannels. It is also found that the heat enhancement is significantly depend on the Brinkman number and the temperature applied to the wall.

Soluto-thermo-hydrodynamics influenced evaporation kinetics of saline sessile droplets

Abstract: The present article experimentally and theoretically probes the evaporation kinetics of sessile saline droplets. Observations reveal that presence of solvated ions leads to modulated evaporation kinetics, which is further a function of surface wettability. On hydrophilic surfaces, increasing salt concentration leads to enhanced evaporation rates, whereas on superhydrophobic surfaces, it first enhances and reduces with concentration. Also, the nature and extents of the evaporation regimes (constant contact angle or constant contact radius) are dependent on the salt concentration. The reduced evaporation on superhydrophobic surfaces has been explained based on observed (via microscopy) crystal nucleation behaviour within the droplet. Purely diffusion driven evaporation models are noted to be unable to predict the modulated evaporation rates. Further, the changes in the surface tension and static contact angles due to solvated salts also cannot explain the improved evaporation behaviour. Internal advection is observed (using PIV) to be generated within the droplet and is dependent on the salt concentration. The advection dynamics has been used to explain and quantify the improved evaporation behaviour by appealing to the concept of interfacial shear modified Stefan flows around the evaporating droplet. The analysis leads to accurate predictions of the evaporation rates. Further, another scaling analysis has been proposed to show that the thermal and solutal Marangoni advection within the system leads to the advection behaviour. The analysis also shows that the dominant mode is the solutal advection and the theory predicts the internal circulation velocities with good accuracy. The findings may be of importance to microfluidic thermal and species transport systems.

Heat transfer investigation of combined electroosmotic/pressure driven nanofluid flow in a microchannel: Effect of heterogeneous surface potential and slip boundary condition

Abstract: In the present study, electroosmotic and pressure driven Newtonian nanofluid flow in a microchannel with constant temperature boundary condition is studied via Lattice Poisson–Boltzmann method. In order to validate the numerical solution, the computed results are compared with some existing analytical solutions. Then, effects of different parameters such as ratio of pressure velocity to Helmholtz–Smolochowski velocity, slip coefficient, nanoparticles volume fraction and diameter on flow field and heat transfer is examined. The results show that by fixing the electric field and increasing the pressure force, Nusselt number decreases, and it increases by fixing the pressure force and increasing the electric field. Also, increasing the slip coefficient causes the velocity enhancement in the electroosmotic flow. Furthermore, increasing the nanoparticles volume fraction decreases the velocity and increases Nusselt number. Finally heterogeneous surface potential moves the vortices toward the walls and enables controlling the quantity and direction of velocity field.

Secondary flow of second kind in a short channel observed by PIV

Abstract: By using the Stereo Particle Image Velocimetry (PIV) technique we observe the secondary flow of second kind in a corner of a channel of square cross-section. The boundary layer thickness is much lower than the channel size. Therefore, the flow is still developing, not filling the entire channel cross-section, which is the case more widely reported in literature studied in a very long channel. The non-linear secondary effects of interacting boundary layers are observed as a single stream-wise vortex close to the channel corner in case of laminar boundary layers. In the case of turbulent boundary layers, the secondary flow takes shape of a symmetric pair of counter-rotating vortices. This pattern is observable only in the average velocity field, while the instantaneous ones display large amount of vortices, whose spatial distribution close to the corner in dependence on their orientation leads to statistically emergent net vorticity. At the same time, this pattern is reproduced by using a numerical simulation.

Forces and torques on a sphere moving near a dihedral corner in creeping flow

Abstract: The low-Reynolds-number flow past a sphere moving near a right dihedral corner made by a stationary and a tangentially sliding wall is considered. Using the superposition principle, the arbitrary motion of the sphere is decomposed into simple elementary motions. Fully-resolved spectral-element simulations are carried out in the frame of reference translating and rotating with the particle such that the velocity on the particle’s surface vanishes. Forces and torques on the sphere are obtained as functions of the particle position near the corner. The data obtained are fitted by closed-form expressions which take into account symmetries of the problem, exact solutions, and asymptotic solutions from lubrication theory. The correlations obtained can easily be implemented in larger-scale one-way-coupled particulate-flow simulations to correct the particle motion near dihedral corners where mere point-particle models break down.

Vortex dynamics and flow patterns in a two-dimensional oscillatory lid-driven rectangular cavity

Abstract: The vortex dynamics in a two-dimensional oscillatory lid-driven cavity with depth-to-width ratio 1:2 has been investigated, covering a wide range of Reynolds numbers and Stokes numbers where this flow is known to be in the two-dimensional regime. Numerical simulations show that the present flow can be divided into four flow patterns based on the vortex dynamics. The regions of these flow patterns are given in the Stokes number and Reynolds number space. For the flow pattern with lowest Reynolds number, there is no transfer of vortices between two successive oscillation half-cycles while for the three other patterns, vortices are carried over from one oscillation half-cycle to the next. For a given Stokes number, the flow pattern appears sequentially as the Reynolds number increases, showing that the transition between the different flow patterns depends strongly on the Reynolds number. However, if the frequency of oscillation is increased (i.e., the Stokes number is increased) for a given Reynolds number, the extrema of the stream function have less time to grow and the center of the primary vortex has less time to move away from the lid. To compensate these effects, the amplitude has to be increased with increasing frequency to maintain the same flow pattern.

A NURBS-based isogeometric boundary element method for analysis of liquid sloshing in axisymmetric tanks with various porous baffles

Abstract: An isogeometric boundary element method (IGA-BEM) based on the non-uniform rational B-splines (NURBS) is firstly performed to investigate the liquid sloshing in axisymmetric tanks with the porous baffles. The proposed method can completely maintain the advantages of the BEM that only the boundary of a domain requires discretization. By applying the NURBS basis functions it can exactly describe the geometry of the boundary. Meanwhile, it can also be obtained better solution field approximation at the domain boundary. Furthermore, as to the axisymmetric geometries of the containers considered in this paper, the 3-D liquid sloshing problems can be effectively reduced to 2-D ones on half of the cross-sections of the containers, which can significantly increase the computational efficiency. Meanwhile, the zoning method is employed in this paper to treat the arbitrary mounted porous baffles, and the Laplace equation is utilized as the governing equation of the potential flow model by assuming the fluid motion to be inviscid, irrotational and incompressible. Additionally, the weighted residual method together with the Green’s theorem is applied to develop the BEM integral equation. The natural sloshing frequencies and dynamic sloshing forces solved by the proposed method are compared with the available literatures and the traditional boundary element method (BEM). Good agreements are observed in the comparisons between numerical results and those of the existing literatures. And higher accuracy and convergence can be achieved by the proposed IGA-BEM method with significantly fewer nodes than the traditional BEM. Moreover, spherical tanks with the coaxial hemispherical, wall-mounted conical or surface-piercing cylindrical porous baffle, ellipsoidal tanks with spheroidal or surface-piercing cylindrical porous baffle, and the toroidal tank with tubular porous baffle are considered to investigate the effects of the porous-effect parameter, radius, length, height, horizontal and vertical semi-axes of the porous baffle on the sloshing characteristics (i.e. dynamic sloshing forces and surface elevations). The results show that the surface-piercing cylindrical porous baffle offers more noticeable suppression on sloshing response than the hemispherical and spheroidal porous baffles. Changing the radius of the tubular porous baffle has almost negligible effect on the sloshing force acting on the toroidal tank. The excitation frequency corresponding to the maximal value of sloshing force can be altered evidently by changing the porous-effect parameter of the porous baffle. In addition, choosing reasonable porous-effect parameter, radius, horizontal semi-axes and relatively larger length, height as well as vertical semi-axes for the porous baffles yields considerable suppression on the sloshing response.

Computers and turbulence

Abstract: This paper briefly reviews the influence that the rapid evolution of computer power in the last decades has had on turbulence research. It is argued that it can be divided into three stages. In the earliest (‘heroic’) one, simulations were expensive and could at most be considered as substitutes for experiments. Later, as computers grew faster and some meaningful simulations could be performed overnight, it became practical to use them as (‘routine’) tools to provide answers to specific theoretical questions. More recently, some turbulence simulations have become trivial, able to run in minutes, and it is possible to think of computers as ‘Monte Carlo’ theory machines, which can be used to systematically pose a wide range of ‘random’ theoretical questions, only to later evaluate which of them are interesting or useful. Although apparently wasteful, it is argued that this procedure has the advantage of being reasonably independent of received wisdom, and thus more able than human researchers to scape established paradigms. The rate of growth of computer power ensures that the interval between consecutive stages is about fifteen years. Rather than offering conclusions, the purpose of the paper is to stimulate discussion on whether machine- and human-generated theories can be considered comparable concepts, and on how the challenges and opportunities created by our new computer ‘colleagues’ can be made to fit into the traditional research process.

About analytical ansatz to the solving procedure for Kelvin–Kirchhoff equations

Abstract: In this paper, absolutely new analytical ansatz for solving Kelvin–Kirchhoff equations has been presented. The aforesaid approach was formulated first in Ershkov (2017) for solving Poisson equations; furthermore, a new type of the solving procedure for Euler–Poissonequations (rigid body rotation over the fixed point) is implemented here for solving momentum equation of Kelvin–Kirchhoff. The system of equations of Kelvin–Kirchhoff problem has been explored with regard to the existence of an analytic way of presentation of the analytical solution. A new and elegant ansatz is suggested in this publication whereby, in solving, the momentum equation is reduced to a system of three linear ODEs of 1st order in regard to the three components of the velocity of the spherical particle (dependent on time t). In this premise, a proper elegant partial solution has been obtained due to the invariant dependence between temporary components of the solution. We conclude that the system of Kelvin–Kirchhoff equations has not the analytical presentation of solution (in quadratures) even in case of zero components of fluid force, influencing on the motion of the particle.

Numerical simulation of a novel non-uniform electric field design to enhance the electrocoalescence of droplets

Abstract: The behavior of binary droplet collision, exposed to an external electric field is studied numerically. Based on a fundamental understanding of the role of the electric field in the coalescence of aqueous drops in an immiscible oil phase, it is possible to optimize the design and operation of these external stimulators. In this research, numerical modeling of electrocoalescence is implemented, using Computational Fluid Dynamics (CFD) methods. Level set method is used to evaluate water droplet evolution in an oil phase under a novel concentric semi-elliptic non-uniform electric field configuration. The observations show that the shape of applied non-uniform fields can remarkably alter the electrocoalescence performance in a nontrivial way, which is sensitive to the drop-medium system. In addition to the shape of the medium, some other parameters such as droplets initial distance, initial skew angle of the droplets, applied voltage amplitude and oil viscosity are analyzed in the present paper. Our results can be useful in designing an optimum electrode configuration, achieving an efficient electrocoalescence.

CFD simulations of an isolated cycling spoked wheel: Impact of the ground and wheel/ground contact modeling

Abstract: Spoked wheels are the most frequently used wheel type in road cycling competitions and their aerodynamic optimization is crucial for cyclist performance. The aerodynamic performance of wheels is generally analyzed by wind tunnel tests or CFD simulations for isolated wheels. There is a large number of options to model the wheel/ground contact in CFD simulations, including different clearances between tire and ground and different heights of solid contact patches (step). However, it is unclear to what extent these modeling options influence the CFD results. The present paper systematically analyzes the impact of these options on the computed forces and moments of an isolated cycling spoked wheel and elucidates the flow behavior around this wheel for zero yaw conditions. The wheel drag coefficient for the cases where the ground is included in the simulations using a clearance or a step is 1.0% and about 1.8% lower compared to the case without ground, respectively, whereas the rotational moment is about 2.0% lower for all the wheel/ground contact modeling approaches compared to the case without ground. The gap clearance ( ≤ 20 mm) and step height ( ≤ 10 mm) should be kept minimal to avoid a significant influence on the forces and moments. In addition, the presence of the ground influences the flow behavior in the lower section of the wheel including the pressure distribution on the exterior of the wheel. This study is intended to help researchers and manufacturers to perform accurate CFD simulations of cycling spoked wheels and to optimize their aerodynamics.

Fluid–Structure Interaction study of carotid blood flow: Comparison between viscosity models

Abstract: Blood is a complex biological fluid and has elements in its composition, such as erythrocytes, which give it a non-Newtonian behavior. Typically, when carotid blood flow is studied, this aspect is frequently ignored and blood is modeled as a Newtonian fluid, with constant viscosity. This work demonstrates a Fluid–Structure Interaction study of carotid blood flow, with the comparison of two distinct viscosity models (Newtonian and Carreau). The results of the Independent Samples Kolmogorov–Smirnov statistical test show that the velocity in the center of geometry of the is similar for both viscosity models (p = 1.00; p = 0.71; p = 1.00) in the three vessels of the bifurcation Wall Shear Stress is dependent on the blood viscosity (p = 0.027), and is considerably higher for the Carreau model. The displacement of the arterial wall is reliant on the fluid pressure, and it is not influenced by the blood viscosity.

Flow/noise control of a rod–airfoil configuration using “natural rod-base blowing”: Numerical experiments

Abstract: When the vortices shedding from an upstream rod impinge upon the surface of a downstream airfoil, the resultant vortex–body interaction noise can be significant. The use of “natural rod-base blowing” to reduce this interaction noise is investigated numerically using a 3D hybrid computational aeroacoustics approach. The natural blowing is generated through an internal slot that interconnects the stagnation and base region of the rod. Numerical simulation is performed for a straight blowing at blowing rates (BRs) between 6.4% and 16.3% and an oblique blowing with slot–incidence angles ( θ ) between 0 ° and 15 ° , respectively. Far-field noise evaluation demonstrates that the natural rod-base blowing under tested BRs can reduce significantly the noise emission, whereas loses its efficiencies gradually with the increase of angle θ . The most effective case is the straight blowing ( θ = 0 ° ) at BR ≥ 13.6% where the far-field tonal noise associated with the steady-periodic von Karman vortex shedding is annihilated. The changes in transient flow structures indicate that the natural blowing gradually attenuates the von Karman vortex shedding when BR increases, resulting in a mitigation of vortex–body​ interaction on the airfoil leading surface and hence a reduction of the unsteady lift. At BR=13.6%, as the angle θ reaches 15 ° from below, a new von Karman vortex street reforms in the flow field, leading to another tonal noise. Linear stability analysis based on a local concept of absolute/convective instability suggests that the absolutely unstable region in the near-wake of the rod elongates first and then shrinks with the increase of BR, while the local absolute growth rate decreases monotonously.

A combined experimental and computational study of flow-blurring atomization in a twin-fluid atomizer

Abstract: A twin-fluid pneumatic injector system with coaxial liquid and air streams mixed slightly upstream of the outlet, is studied in a cross-flow arrangement to understand its working principle. The gap between the orifice and the liquid tube, where the air and liquid cross-flow occurs, is known to determine the modes of working of the atomizer for the given operating flow rates of air and liquid. The two distinct modes, namely the flow focusing (FF) and flow blurring (FB) are observed in the present study, and the transition from FF to FB mode is explored using optical visualization. The characteristics of the internal two-phase flow mixing that occurs due to backflow recirculation in FB mode are also investigated by performing large eddy simulation (LES) using the volume of fluid (VoF) approach. The results of the numerical simulation are validated with experimental observations for the length of backflow recirculation observed in the liquid tube. The present study demonstrates the development of multiple microjets, which explains experimental observations of the lack of core jet and the almost uniform distribution of radial Sauter mean diameter (SMD) reported in the literature. The high-resolution backlit image of a scaled-up transparent injector with a 6 mm orifice diameter validates the numerical results that describe the internal two-phase flow.

Accuracy assessment of the Non-Ideal Computational Fluid Dynamics model for siloxane MDM from the open-source SU2 suite

Abstract: The first-ever accuracy assessment of a computational model for Non-Ideal Compressible-Fluid Dynamics (NICFD) flows is presented. The assessment relies on a comparison between numerical predictions, from the open-source suite SU2, and pressure and Mach number measurements of compressible fluid flows in the non-ideal regime. Namely, measurements regard supersonic flows of siloxane MDM (Octamethyltrisiloxane, C 8 H24O 2 Si 3 ) vapor expanding along isentropes in the close proximity of the liquid–vapor saturation curve. The model accuracy assessment takes advantage of an Uncertainty Quantification (UQ) analysis, to compute the variability of the numerical solution with respect the uncertainties affecting the test-rig operating conditions. This allows for an uncertainty-based assessment of the accuracy of numerical predictions. The test set is representative of typical operating conditions of Organic Rankine Cycle systems and it includes compressible flows expanding through a converging–diverging nozzle in mildly-to-highly non-ideal conditions. All the considered flows are well represented by the computational model. Therefore, the reliability of the numerical implementation and the predictiveness of the NICFD model are confirmed.