Two-dimensional Numerical Investigation Research Articles

Elastoviscoplasticity intensifies the unstable flows through a micro-contraction geometry

This study focuses on the two-dimensional numerical investigation of complex fluid flows through a micro-contraction geometry in the creeping flow regime, specifically examining elastoviscoplastic (EVP) fluids. These fluids exhibit a combination of viscous, elastic, and plastic behaviors. The governing equations, including mass and momentum, are solved using a finite volume method-based discretization technique. Saramito’s constitutive model is utilized to accurately represent the viscous, elastic, and plastic responses of the EVP fluid. The present results demonstrate significant differences in flow dynamics, such as vortex dynamics and transitions between flow regimes (e.g., steady to unsteady), when compared to simple Newtonian and non-Newtonian viscoelastic (VE) or viscoplastic (VP) fluids. This study reveals that when the yield strain (ϵy) exceeds a critical value, approximately ranging from 0.79 to 0.89, the flow transits from a steady to an unsteady state for the EVP fluids. Importantly, the present study shows that EVP fluids exhibit intensified chaotic flow dynamics and increased instability compared to VE and VP fluids under similar flow conditions. However, the presence of shear-thinning behavior in EVP fluids suppresses this instability. The analysis of local velocity fields and flow deformation in this study highlights the impact on the stretching of fluid microstructure and elastic stresses, which ultimately contribute to the origin of this intensified unstable flow condition for EVP fluids. The finding from this study holds significant potential for enhancing heat or mass transfer rates and mixing efficiency in micro-scale systems, where the prevailing steady and laminar flow conditions often hinder these transport processes.

Two-dimensional high-resolution numerical investigation of eddy effect in artificial rough conduits with different shapes

Subsurface conduit flow is a critical element of investigating groundwater movement in karst regions, and it is still poorly understood due to complexity of subsurface conduit geometries and the nonlinearity of the conduit flux-hydraulic gradient relationship. A significant nature of the flow nonlinearity is the occurrence of different eddies in conduits. To reveal the nonlinear nature of conduit flow, this study investigates the influence of the eddy effect on the hydraulic gradient of different rough conduits based on extensive high-fidelity numerical simulations. Overall, the hydraulic gradients of all types of rough conduits show quadratic relations with an increase in the specific discharge. In addition, the triangular rough conduit has the largest hydraulic gradient, which gradually decreases as the shape coefficient (which is used to quantify rough elements) increases. We have identified the development and evolution of eddies in different types of rough conduits in detail. This study shows that a larger specific discharge leads to a larger eddy area, and the hydraulic gradient changes exponentially with the eddy area ratio within a limited range. Finally, the relationships among specific discharges, eddy area ratio, and hydraulic gradients are constructed and summarized. The results of this investigation are helpful for developing realistic karst groundwater flow and transport models in which subsurface conduit plays a dominant role for conducting groundwater flux and mass transport.

Numerical Study of the Thermal and Hydraulic Characteristics of the SiO2–Water Nanofluid through a Backward-Facing Step

The work of this paper is devoted to the two-dimensional numerical investigation of the turbulent forced convection through backward-facing step. Two cases are taken into account. For the first, the bottom wall is smooth; but for the second case, it is corrugated with a triangular base containing ten corrugations. A uniform heat flux is imposed on the bottom wall. A flow of a nanofluid consisting of silicon dioxide SiO2 nanoparticles dispersed in a base fluid (water) flows through this step.The flow equations were discretized by the finite volume method. For modeling turbulence, the K-ɛ-RNG model was applied to analyze Reynolds number effect, the volume fraction and the size of the particle. Numerical simulations were conducted for various Reynolds between 10,000 and 40,000; volume fractions of nanoparticles which vary from 0 to 5% and particle diameter dp equal to 30; 50 and 70 nm. We noted that the obtained findings are in good conformity with those of the literature.

Numerical research on kerosene/hydrogen/air rotating detonation engines with discrete injection strategies

The two-dimensional simulation of two-phase rotating detonation engines commonly adopts the nozzle–wall configuration, which is inconsistent with the real configurations in experiments. In the annular slot–nozzle structure, the oxidant is completely injected into the annular gap, whereas the fuel is partially injected into the nozzle hole, which is closer to the real operation condition. In this study, a gas–liquid two-dimensional numerical investigation was conducted to explore a more accurate operating condition with a discrete injection configuration. The effects of the kerosene/hydrogen injection area ratio and hydrogen equivalence ratio on the propagation mode and operating performance were investigated. The simulation results show that the kerosene droplet/hydrogen/air gas–liquid two-phase rotating detonation wave exhibits different propagation modes depending on the hydrogen equivalence ratio. When the hydrogen equivalence ratio (φH2) is 0, the detonation wave cannot be successfully initiated. When φH2 is increased to 0.2 and 0.5, a single-wave propagation mode is generated. When φH2 is 1, a double-wave propagation mode is produced initially in the combustion chamber and is subsequently converted into the deflagration mode. The flow field structure, velocity performance, and propulsive performance in the detonation combustion chamber are analyzed. It was found that an appropriate amount of hydrogen addition to the liquid kerosene detonation can aid in stabilizing the propagation of the detonation wave and improving the performance of the detonation engine.

Effect of multiple sides of heated bluff body and heat transfer in a confined region: a computational study

The rapid increase in technologies, especially computer chips, increases the demand for highly efficient thermal management systems. The present study carries out a two-dimensional numerical investigation on the effect of the number of sides of a heated cylinder in a highly confined fluid domain. The flow is confined to a blockage ratio of 3. The study is conducted for laminar flow with Reynolds numbers being 50, 100, 150 and 200. Six cylinders are chosen; Square, pentagon, hexagon, circle, dodecagon and hexa-decogan. The simulation is carried out using ANSYS FLUENT 18. SIMPLE algorithm is used to solve the Navier–Stokes equation. The flow and heat transfer characteristics are studied from pressure and temperature contour, viscous drag co-efficient, pressure-drag coefficient, total drag co-efficient, pressure co-efficient, Strouhal number and Nusselt number. As the number of sides increased, a significant difference in flow and heat transfer characteristics is observed at the starting, but all slowly started to converge toward the characteristics of the circular cylinder. The dodecagon cylinder shows better flow and heat transfer performance on average when compared to all the other cylinders.

Numerical investigation of shock Mach number effects on Richtmyer–Meshkov instability in a heavy square bubble

This study conducts a two-dimensional numerical investigation of incident shock Mach number effects on the Richtmyer–Meshkov (RM) instability after a shock wave impulsively drives a heavy square bubble. The square bubble is composed of SF6 gas, which is surrounded by nitrogen gas. Five different shock Mach numbers are considered: Ms=1.12, 1.22, 1.4, 1.7, and 2.1. Two-dimensional compressible Euler equations for two-component gas flows are simulated with a high-order modal discontinuous Galerkin solver. For validation, the numerical results are compared with the existing experimental results and are found to be in good agreement. The present results reveal that the incident shock Mach number is critical in the growth of RM instability in a heavy square bubble. The shock Mach number affects flow morphology significantly, resulting in complicated wave patterns, shock focusing, jet creation, bubble deformation, and vorticity generation. The convergent shape of the bubble causes shock focusing, which creates a local high-pressure zone and, as a result, an outward jet generation. It is found that the bubble deforms differently with increasing shock Mach number, and the different shock Mach numbers alter the distance between the shock focusing position and the downstream bubble interface. The effects of shock Mach numbers are investigated in depth through physical phenomena such as vorticity production, kinetic energy, and enstrophy. Finally, the shock Mach number impacts on the time-variations of the shock trajectories and interface structure are thoroughly investigated.

Two-dimensional characterization of particle entrainment over a downstream obstacle

This study conducts a two-dimensional numerical investigation of the particle entrainment in the presence of a large downstream circular obstacle, and aims to deepen the understanding of the entrainment mechanism and propose a dimensionless number to describe the entrainment threshold. With the coupled lattice Boltzmann and discrete element method, direct numerical simulations are conducted for a wide range of obstacle-to-particle radius ratios ( 2 < R o / R p < 8 ) and Reynolds numbers ( 38 ≤ R e ≤ 160 ). The results show that the low-pressure separated vortex due to the obstacle is the fluid structure to entrain the particle by vortex stretching at low R o / R p ( 2 < R o / R p < 3 . 8 ). Based on this mechanism, a dimensionless number defined as the ratio of the vertical drag force to the particle gravity is proposed, which can be understood as a generalized Shields number including the additional effect of the obstacle radius, and it approaches the constant value of 2.5198 at the critical conditions of entrainment. In addition, we find that at high R o / R p ( 5 . 6 ≤ R o / R p ≤ 8 . 0 ), the particle is too far from the separated vortex to undergo sufficient vertical drag force, and thus its inertia should be small enough to ensure the entrainment. In this situation, the entrainment threshold is well described by the Stokes number, which approaches the constant value of 0.2134. At the moderate R o / R p ( 3 . 8 ≤ R o / R p ≤ 5 . 6 ) the drag force of separated vortex and the low particle inertia are both essential to realize the particle entrainment. • Particle entrainment over a downstream obstacle is investigated by LB-DEM. • Two kind of entrainment mechanisms are identified. • Two dimensionless numbers are proposed to describe critical entrainment condition.

A Numerical Study on an Integrated Solar Chimney with Latent Heat Thermal Energy Storage in Various Arrangements

The paper presents a two-dimensional numerical investigation of a solar chimney combined with an absorbing capacity wall in a building’s south façade. The analysis was carried out in Aversa (Italy), in July and December, from sunrise to sunset. The chimney is made up of a converging channel, a vertical absorbing wall, and a two-degree angled glass plate. It is 5.0 m tall, while the height of the channel is 4.0 m, with an inlet of 0.34 m and an outlet portion of 0.20 m. A phase change material with metal foam makes up the thermal energy storage system. Four possible configurations are investigated to identify the ideal hybrid system arrangement in terms of thermal performance; the commercial code Ansys-Fluent is used to solve the problem. The results of the thermal energy storage system with metal foam and PCM are provided in terms of liquid fractions, stored energy, and stream function fields, while the ones of the wall temperature distributions in the channel are reported. Thermal and fluid dynamics behaviours are so evaluated to have some useful indications to improve the system.

Two-Dimensional URANS Numerical Investigation of Critical Parameters on a Pitch Oscillating VAWT Airfoil under Dynamic Stall

In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pitching airfoil to properly comprehend the dynamic stall and aerodynamic forces. The computational software ANSYS Fluent was used to solve the unsteady Reynolds-averaged Navier–Stokes equations. Low Reynolds number flows were modeled using the k-ω shear stress transport turbulence model. Aerodynamic forces, fluid flow structures, and flow separation delay angles were explored as a function of the Reynolds number, reduced frequency, oscillation amplitude, and mean angle of attack. The maximum aerodynamic forces, including lift, drag, and the onset of the dynamic stall, were all influenced by these variables. The critical parameters that influenced the optimum aerodynamic forces and ended up causing dynamic stall delay were oscillation amplitude and mean angle of attack. The stall angle was raised by 9° and 6°, respectively, and a large increment in the lift coefficient was also noted in both cases. Additionally, for the highest Reynolds number, a considerable rise in the maximum lift coefficient of 20% and a 28% drop in drag coefficient were observed, with a 1.5° delay in the stall angle. Furthermore, a significant increase of 33% in the lift force was seen with a rise of 4.5° in the stall angle in the case of reduced frequency.

A Two-Dimensional Numerical Investigation of Aerodynamic Noise from an Inclined Plate

A Two-Dimensional Numerical Investigation of Aerodynamic Noise from an Inclined Plate

Two-dimensional numerical pore-scale investigation of oxygen evolution in proton exchange membrane electrolysis cells

Oxygen blocking the porous transport layer (PTL) increases the mass transport loss, and then limits the high current density condition of proton exchange membrane electrolysis cells (PEMEC). In this paper, a two-dimensional transient mathematical model of anode two-phase flow in PEMEC is established by the fluid volume method (VOF) method. The transport mechanism of oxygen in porous layer is analyzed in details. The effects of liquid water flow velocity, porosity, fiber diameter and contact angle on oxygen pressure and saturation are studied. The results show that the oxygen bubble transport in the porous layer is mainly affected by capillary pressure and follows the transport mechanism of ‘pressurization breakthrough depressurization’. The oxygen bubble goes through three stages of growth, migration and separation in the channel, and then be carried out of the electrolysis cell by liquid water. When oxygen breaks through the porous layer and enters the flow channel, there is a phenomenon that the branch flow is merged into the main stream, and the last limiting throat affects the maximum pressure and oxygen saturation during stable condition. In addition, increasing the liquid water velocity is helpful to bubble separation; changing the porosity and fiber diameter directly affects the width of pore throat and the correlative capillary pressure; increasing porosity, reducing fiber diameter and contact angle can promote oxygen breakthrough and reduce the stable saturation of oxygen.

Numerical investigation on vortex-induced vibration energy harvesting of a heated circular cylinder with various cross-sections

This study describes a two-dimensional numerical investigation of the flow and heat transfer around inverted D-shaped cylinders. The effects of thermal buoyancy force over the bluff bodies are considered numerically based on the flow field around circular and semi-circular cylinders. The operating Richardson numbers (Ri) are 0, 0.5, 1, and 1.5 with a Reynolds number (Re) of 100. Five cylinders with a length -to -diameter ratio of L* = (L/D) = 0.5, 0.6, 0.7 0.8, 0.9 are selected to investigate the effect of L* on energy harvesting. The functional dependence of drag coefficient, lift coefficient, resultant force coefficient, dimensionless maximum vibration amplitude, and output power is analyzed and studied. To validate the present study, some of the results are compared with other numerical and experimental investigations, and reasonable agreement is obtained. The results show that in the absence of buoyancy force, the history of the contours, and streamlines are periodic, and vortices created at the back of the body detach similarly from either side of the body; however, this behavior is not seen for other values of the Richardson numbers. Moreover, when the temperature contours are examined, a higher heat transfer is observed when a higher value of the Richardson is prescribed. Finally, the analysis shows that the output power of the bluff bodies under the condition of Ri =1.5 increases by 7.02, 10.4, 15.6, 21.2, and 25.4 as the L* increases, and the maximum extracted power is 26.2 at L* = 1 in relation to not applying buoyancy.

Two-Dimensional Numerical Investigation of Flow and Heat Transfer Around Rounded Triangular Cylinders In Cross Flow

Two-Dimensional Numerical Investigation of Flow and Heat Transfer Around Rounded Triangular Cylinders In Cross Flow

Hydrodynamics of a fish-like body undulation mechanism: Scaling laws and regimes for vortex wake modes

A comprehensive two-dimensional numerical investigation has been undertaken to calculate the energetic cost of propulsion and the various flow transitions of a fish-like body undulation mechanism based on a National Advisory Committee for Aeronautics 0012 hydrofoil. This covers a wide range of Strouhal (0≤St≤1.4) and Reynolds (500≤Re≤5000) numbers from simulations based on a level-set function immersed-interface method. It is found that the time-averaged thrust coefficient displays a quadratic relationship with increasing St, and increases significantly with Re. Additionally, the time-averaged input power coefficient exhibits a cubic dependence with increasing St but is independent of Re. Both St dependences agree with those previously observed experimentally and numerically for an oscillating foil; however, for similar ranges of governing parameters, comparisons suggest that the body undulation mechanism possesses a higher propulsive efficiency. The St∝Re−0.19 scaling for the drag-to-thrust transition is consistent with that found for a wide variety of fish and birds in nature. Interestingly, for cases with an undulation wave-speed below the free-stream speed, the time-averaged drag coefficient is found to be higher than that of a stationary hydrofoil at the same Re. Furthermore, the time-averaged input power coefficient is negative, indicating the potential for the undulation mechanism to extract energy from the free-stream. Eight different wake patterns/transitions are documented for the parameter space; these have been assembled into a wake-regime parameter-space map. The present findings should aid in predicting and understanding different hydrodynamic forces and wake patterns for undulating kinematics.

Lattice Boltzmann analysis for electro–thermo-convection with a melting boundary in horizontal concentric annuli

In this paper, we perform a two-dimensional numerical investigation into the electro–thermoconvection with a melting boundary in horizontal concentric annuli filled with a dielectric phase change material. The whole set of coupled equations is solved by lattice Boltzmann method: Navier–Stokes equations, electrohydrodynamics (EHD) equations, and the energy equation. It is found that there exist three regimes during melting with EHD, namely, diffusive regime, thermal convection regime, and electroconvection regime, and the augmentation of melting heat transfer is due to the radial electroconvective flow induced by Coulomb force in the third regime. Moreover, the continuous melting of solid leads to the dynamical transition between the different flow patterns of electro–thermo-convection, as well as the interesting evolutions of temperature and charge density distribution. In different regimes of melting, the liquid fraction fl and Nusselt number Nu follow different power laws. In detail, before the onset of radial flow motion, fl scales as Fo1/2 whereas Nu scales as fl−4/5 (Fo represents the Fourier number), and in the electroconvection dominated regime, we have fl ∼ Fo and Nu ∼ fl0.

Behaviour of granular matter under gravity-induced stress gradient: A two-dimensional numerical investigation

Gravity is the most important load source in mining and geotechnical engineering, which causes both the stress level and stress gradient inside geomaterials. Different from the stress level, the influence of gravity-induced stress gradient on the behaviour of the material is still unknown. An in-deep study on it will help to promote the understanding of material behaviour, especially for those cases related to unconventional gravity such as terrestrial ng physical modelling and extraterrestrial resource exploitation (g is the terrestrial gravitational acceleration). In this study, a high-order homogenization for granular materials is proposed at first, in which the stress gradient is drawn into the constitutive representation by adopting a representative volume element (RVE). The consolidation and shear strength behaviour of RVE are then investigated by performing numerical biaxial tests. The results show that all the compressibility, shear strength, shear stiffness, volumetric deformation, and critical state behaviour show a stress gradient dependence. A coupling between stress gradient, stress level, and material properties is also observed. These observations suggest that, besides the stress level, extra attention needs to be paid to material responses related to stress gradient during engineering practices.

Heat Transfer Performance of Uni-Directional and Bi-Directional Lid-Driven Cavities Using Nanoparticle Enhanced Ionic Liquids (NEILS)

Two-dimensional numerical investigation has been carried out using an in-house CFD code to obtain the heat transfer performance of normal, uni-directional and bi-directional lid-driven cavity. The 2D in-house code is based on single phase approach for studying steady state laminar flows. Pure water (PW), ionic liquid (IL): 1-ethyl-3-methylimidazolium methanesulfonate; [C2mim][CH3SO3], ionic liquid mixtures (PW + IL) and alumina (Al2O3) nanoparticle enhanced ionic liquids (NEILS) are used in the present investigation. The thermophysical properties of all the heat transfer fluids used in the present numerical investigation were obtained from the experimentally measured data available in the literature. The obtained results showed that uni-directional lid-driven cavity exhibits 68.78 % higher Nuavg value at Re = 1900 in comparison with normal lid-driven cavity for pure water. The use of [C2mim][CH3SO3] ionic liquid also enhances the heat transfer performance by exhibiting 115.48 % higher Nuavg value in comparison with pure water at Re = 1900. Moreover, 15 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL gives 21.2 % and 161.16 % higher heat transfer performance in comparison with ionic liquid and pure water respectively at Re = 1900. Furthermore, it is also observed that 10 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL shows similar heat transfer performance like ionic liquid at all Re.

Influence of gas expansion on the propagation of a premixed flame in a spatially periodic shear flow

It has been previously demonstrated that thermal gas expansion might have a role in boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], inducing local flow-reversal in the boundary layer's low-velocity streaks on the reactants’ side of the flame and facilitating its upstream propagation. We perform a two-dimensional numerical investigation of the interaction between a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged streamwise velocity streaks with alternating regions of high and low velocities found in turbulent boundary layers in the vicinity of the walls. The parametric study focuses on the amplitude and wavelength of the periodic shear flow and on the gas expansion ratio (unburnt-to-burnt density ratio). With the increase of the amplitudes of the periodic shear flow and of the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of curved premixed flame velocity. The flame shape that is initially formed by the oncoming periodic shear appears to be metastable. At a later stage of the flame propagation, the flame shape transforms into the stationary one dominated by the Darrieus-Landau instability.

Effect of ignition parameters on detonation initiation using toroidal shock wave focusing

For fast detonation initiation, a novel compact initiator based on shock wave focusing has been proposed in our work. Two-dimensional numerical investigation was performed to understand the fast detonation initiation mechanism in hydrogen-air mixtures. Detailed evolution processes, namely, flame acceleration, shock wave focusing and detonation initiation were analyzed. Moreover, effects of ignition energy and energy release time on detonation initiation have been discussed thoroughly. The numerical results indicate that flame acceleration process can be divided into three stages: a) curling deformation of flame with low speed, b) tensile deformation of flame with medium speed, c) steady propagation of flame with high speed. After shock focusing, a highly compressed and heated region triggers the rapid detonation initiation. Further analysis indicated that detonation initiation time increases with the decrease of ignition energy. And the reason is mainly due to initial ignition delay differences controlled by ignition energy. In particular, once ignition energy is smaller than 12.5 mJ, mixtures will be no longer ignited. Besides, with the increase of energy release time, both the run-up time and distance of detonation initiation increase. The rapid release of energy can not only make mixtures ignited earlier, but also enhance initial flame velocity. As a result, when energy release time is reduced from 200 μs to 50 μs, the run-up time and distance are decreased by 50% and 32.5%.

Flow confinement effect and heat transfer around square cylinder with downstream control cylinders

A two-dimensional numerical investigation on flow past a heated square cylinder at different confinements has been carried out in the presence of two control cylinders placed on the rear side of the cylinder near the wake to control the shedding phenomena. The confined flow has been simulated with a blockage ratio from 4 to 8 (i.e., ratio of the computational domain height to the diameter of the square). The effects of Reynolds number (Re) range 100 ≤ Re ≤ 150 on various flow and heat transfer characteristics are studied and analyzed in the form of streamline plots, variations of pressure and viscous drag coefficients, Strouhal number (St), total pressure drop and Local and average Nusselt number. Different varieties of flow pattern were observed under laminar conditions with different combinations of Re, Blockage ratio (BR), size of the control cylinder and placement of the control cylinder. The St for geometry with control cylinder of size d—30% decreases by 6.5% when shifted from L = 0.6 to L = 1.6 for least BR 4 and maximum Reynolds number 150. The pressure difference decreases by 10.2% for further control cylinder position from 0.6 to 1.6, size from 20 to 30% at BR 4 and Re 100. The control cylinder closer to the bluff body maintains the pressure difference in increasing BR. The convective heat transfer is higher for L = 1.6 for lower BR. An increase of 8.6% observed for Re 150 at d—30%. The increase in BR reduces the effect of the positioning of the control cylinder and its size becomes the decisive factor.