Flow For Re Research Articles

Flow past rotating cylinders using deterministic vortex method

This study examines the performance of an in-house code based on a deterministic vortex method on circular and square cylinders rotating with constant velocity. Subjecting a bluff body to rotary about its axis effectively reduces drag forces, suppresses the fluctuating forces, and increases the lift forces (known as the Magnus effect). Rotating cylinders are well known for their use in rotor ships, rotor aircrafts, and wind turbines. This study aims to establish the accuracy of the in-house code for the rotating circle cylinders based on the results from the literature and examine the behavior of the rotating square cylinders (a less-studied case). A square section, as a many-sided shape, can be considered when the behavior of more complex geometries is of interest, such as trajectories of the sediments, or replacing circular sections when autorotation is of importance. Both cylinders are exposed to a uniform flow of Re = 200 and an imposed rotation rate range of 0 ≤ α ≤ 5.5 for the circular, and 0 ≤ α ≤ 5 for the square cylinder. The present numerical tool is able to predict the vortex shedding suppression as a result of forced rotation. For both cases, a systematic increase in the time-averaged lift coefficient (≈25.5 for the circular and ≈26 for the square case when α = 5) and a general descending trend in the time-averaged drag coefficient (≈0.03 for the circular and ≈0 for the square case when α = 5) are detected. The second mode of vortex shedding for the circular cylinder is captured within a narrow range of rotation rates. A noticeable feature of the flow over rotating square cylinders is the emergence of near-field wakes in the near-body region, which develop independently from the wake behind the body. To the best of the authors’ knowledge, this is the first comprehensive study of a rotating body carried out using a deterministic vortex method-based numerical tool.

‘It’s not cardboard, it’s a house’: cartographies of agentic assemblage in the early childhood classroom

ABSTRACT The concept of assemblage, drawing on the posthuman theorisations of Deleuze and Guattari, delineates a dynamic and new materialist approach to an event. In this approach, desires, material agency and (de)(re)territorialisation emerge as key concepts, and open ways to understand the school classroom in early childhood as a territory where lines of flight challenge the boundaries of normative education. This paper focuses on a classroom assemblage and aims to cartography the material relations between human and non-human bodies, where (de)(re)territorialisation forces are constant. We draw on diffractive ethnography in order to think-with-theory, making use of a vignette and a diagram containing its material relations. Our analysis highlights the agentic relations of matter in the assemblage, the role of desire as a dynamic force and the ever-changing flow of (de)(re)territorialisations that emerge in it. This study shows the complexity of material experience in early childhood, where desire and deterritorialisation frame creative and unexpected processes that defy the idea of education and classroom activities as linear processes controlled by adults. On the contrary, the cartography depicted in this research supports an idea of education as a space for the emergence of creative lines of flight, material relations and non-linear meaning-making.

Numerical and experimental investigation of the in-phase and out-phase plasma actuation effects on the wake flow for Re = 1000

The aim is to characterize the role of symmetry pattern on the flow control induced by dielectric barrier discharge plasma actuators. Two DBD actuators are employed for in-phase excitation with duty cycle 50% leading to symmetric wake pattern and out-phase excitation for two pulsing frequencies St=0.2 and St=1 at Re=1000 which forms quasi-symmetric and asymmetric flow structures respectively. The modes deduced from POD and DMD methods represent the competition between symmetric (S) and asymmetric mode (K) in all cases. The harmonic in-phase plasma actuation with 50% duty cycle is the most effective flow control method with lowest power consumption. The harmonic out-phase excitation is not much effective; however adjustment of the actuator position can form the symmetric pattern with minimum power consumption. The superharmonic out-phase plasma actuation reduces the wake region however is not effective in lift reduction significantly and consumes high amount of energy. This indicates that phase difference and plasma location influence on the size and symmetry of the vortical structure. Then, based on the symmetry properties of the designed wake pattern created by frequency and phase difference of the plasma actuator, the effective active flow control can be performed. The numerical results are validated with experiments.

изико-механические свойства керамических покрытий, получаемых короткоимпульсной лазерной наплавкой порошковой смеси на основе бора

Modern repair production that implements traditional technologies for making restorative coatings has lost its relevance, since it cannot provide the necessary physical and mechanical properties of the coatings. The problem is that in modern metallurgy there is no line of repair additive materials with high strength and technological properties. Therefore, the issues of searching for innovative materials and technologies for their application on the surface of worn parts are relevant and require fundamental research. The aim of the work is to study the physical and mechanical properties of ceramic coatings obtained by short-pulse laser treatment. To achieve this goal, ceramic compositions based on boron carbide, additionally doped with boron nitride (0 to 40% BN), magnesium oxide (2 to 10% MgO) and lithium (5 to 25% LiO), were used. The scientifi c novelty of the research lies in the application of selective laser sintering technologies in the formation of thin coatings (up to 200 μm) under conditions of a short-pulse laser refl ow of powder media. To assess the physical and mechanical properties, laboratory research methods were used, in particular, analysis of the microhardness of the created restorative coatings, microstructure and tribological performance indicators using modern techniques and instrumentation. The research results confi rm the possibility of practical implementation of restorative coatings based on ceramic compounds. The multi-layer ceramic coatings (B4C-BN- MgO-Li2O) up to 200 μm thick showed good workability, low coeffi cient of friction (0.18) and the ability to work under high dynamic loads in the absence of intensive lubrication. The structure of the coatings has a dense adhesive zone with no visible defects. The presented results of studying the physic-mechanical properties of ceramic coatings have high research credentials and practical signifi cance, and their application will improve the durability of machine parts in the conditions of their operation.

Numerical investigation on fully-developed heat transfer augmentation and friction factor with dimple shaped intrusion and different radius of curvatures on triangular ducts

The heat transfer phenomenon is moderate in the case of triangular ducts due to the presence of sharp corners, which causes stagnant flow in the corners. Thus, this study focuses on improving heat transfer by converting one of the corners of the duct to a rounded structure having a variable curvature radius value (Rc). Rc varies from 0.67 h (maximum value) to 0.33 h (minimum value), where h is the height of the duct. Heat transfer and airflow have also been tested with dimple-shaped intrusion and rounded corners under varying ranges of relative length in the longitudinal direction (Z/e = 10–18) and relative length in the transverse direction (X/e = 10–18). Application of thermal-based boundary conditions on the turbulent flow for Re = 5600–21,000 facilitates the heat transfer analysis in a duct under steady-state. The commercial software tool, ANSYS Fluent 18.1, is used to simplify-three dimensional Reynolds averaged Navier Stokes equation with the compressible fluid flow by considering the pressure-based solver with the standard K- ε turbulence model. Compared to a typical duct, the rounded corner area shows higher velocity because of the dimple intrusions and rounded corners. Introduction of curvature to the corner opposite to the heat-conducting surface in simple triangular duct results in the increment of heat transfer by 5.71–44.37 %, 3.172–23.76 %, and 1.16–8.27 % for the values of Rc = 0.33 h, 0.49 h, and 0.56 h, respectively. Further the addition of dimple-shaped intrusion in the triangular duct with rounded corner, increases the heat transfer for Z/e = X/e = 18 as visible from the increment in average Nusselt Number by 22.9–91.52 %, 49.41–129.82 %, 31.96–89.26 % and 14.36–85 % for Rc = 0.333 h, 0.49 h, 0.56 h & 0.67 h respectively for Re spanning from 5600 to 21000.

Third-order less oscillatory and less diffusive compact stencil-based upwind schemes, and their applications to incompressible flows and free surface flows

We propose novel third-order less oscillatory and less diffusive compact stencil-based upwind schemes for the approximation of the continuity equation. The proposed schemes are based on the constrained interpolation profile-conservative semi-Lagrangian schemes. An important feature of the proposed schemes is that the interpolation functions are constructed using only variables within one upwind cell (a cell average and two boundary values). Furthermore, the proposed schemes have third-order accuracy and are also less oscillatory, less diffusive, and fully conservative. The proposed schemes are validated through various benchmark problems and comparisons with experiments of two droplets collision/separation and droplet splashing. The numerical results have shown that the proposed schemes have third-order accuracy for smooth solution, and capture discontinuities and smooth solutions simultaneously without numerical oscillations. The proposed schemes can capture the secondary vorticity of lid-driven cavity flow of Re = 7500 with a Cartesian grid of 64 × 64. The numerical results of two droplets collision/separation of We = 40 show that the proposed schemes can reproduce droplets collision/separation with quite coarse grids. These numerical results of droplet splashing have demonstrated that proposed schemes can reduce numerical diffusions well against existing schemes and robust.

Density Modelling in Mixed Convection Flow in a Vertical Parallel Plate Channel

In this paper, a novel way of modelling the density in buoyancy term of mixed convection flow problem is presented using equation of state and Boussinesq approximation without first-order approximation of density with respect to temperature. The presented density model is used to investigate the laminar mixed convection flow in a vertical parallel plate channel under symmetric constant wall heat flux. The results obtained are compared with the results obtained using first-order approximation of density with Boussinesq approximation, and also compared with the results obtained using variable thermophysical properties with negligible viscous dissipation. Investigation is performed on the basis of flow and thermal fields for Re=150 and 300, Ri=0.1 to 25. It is found that the presented density model produces relatively better results, which is able to describe the case of developing flow under constant heat flux condition that is not evident if Boussinesq approximation with first-order approximation of density is used. An appearance of recirculatory cells when reverse flow takes place is also witnessed in vertical channel flow with constant heat flux boundary condition which was not reported earlier.

A comprehensive study of secondary and tertiary vortex phenomena of flow past a circular cylinder: A Cartesian grid approach

This study envisages undertaking a comprehensive simulation of the flow past an impulsively started circular cylinder with special emphasis on vortex dynamics in the secondary and tertiary level. A recently developed second order spatially and temporally accurate compact finite difference scheme on a nonuniform Cartesian grid has been used to discretize the transient Navier–Stokes equations governing the flow. The grid is generated in such a way that the cylinder boundary passes through the grid points, thus dispensing with the need to use the immersed interface approach on a Cartesian grid. High quality simulations are accomplished for a wide range of Reynolds numbers (Re) from 5≤Re≤10 000 in the laminar regime, including the periodic flow characterized by von Kármán vortex street. The α, β, sub-α, and sub-β phenomena, which are the trademark of the secondary and tertiary vortex dynamics associated with such flows, are studied in detail. Our results are compared with existing experimental and numerical results, and close comparison is obtained in all the cases exemplifying their accuracy. In the process, for the first time, we also provide a tabular documentation of the early stages of the flow for Re≥700.

A segregated spectral element method for the 2D transient incompressible Navier-Stokes equations

In this paper, a spectral element method for the solution of the two-dimensional transient incompressible Navier-Stokes equations is introduced, which combines the segregated velocity-pressure equal-order formulation originated from the SIMPLE (semi-implicit method for pressure linked equations) algorithm. In contrast to previous segregated finite element method based on the SIMPLE algorithm, the pressure equation is derived from the momentum and continuity equations using the element matrices to ensure the convergence of the pressure. High-order element basis functions are adopted to construct the discrete form that presents arbitrary order or accuracy. The validation test with analytical solution verifies the high accuracy of the method. The flow in a lid-driven cavity with different inclination angles and the flow over a backward-facing step are investigated to further illustrate the property of the scheme. The computed results are in excellent agreement with the benchmark solutions, even if fewer grid points are used. The almost periodic solution for the flow of Re=10000 in a lid-driven square cavity is also captured by the present scheme. The numerical results indicate that the proposed high-order method can significantly reduce the number of grid points used in the calculation.

Effect of no-reflow/reflow on P-wave time indexes in patients with acute myocardial infarction undergoing percutaneous coronary intervention

Aim: The aim of this study was to investigate the relationship between no-reflow (no-RF), reflow (RF), and the P-wave time index in patients undergoing percutaneous coronary intervention (PCI) with a diagnosis of acute myocardial infarction (AMI) due to total occlusion. Methods: This study included a total of 272 AMI patients with no-RF (110 patients) and RF (162 patients). Results: The no-RF group had higher values of mean maxPWTpostPCI(94.95 ± 15.61 vs. 117.86 ± 12.06, P

Axisymmetric free jet of Re → 0 from a round tube

An axisymmetric free jet of Reynolds number Re → 0 from a round tube is investigated in a theoretical way in order to elucidate pressure and velocity fields in the downstream of the tube exit. The Stokes approximation is performed to the Navier–Stokes equations expressed by the spherical coordinate system. The whole flow field is divided into two regions, a closed region consisting of both the tube exit and the hemisphere with the tube radius, i.e. inner region, and a region outside of the hemisphere, i.e. outer region. The pressure and velocity components in the inner region are expanded by a series of unknown functions of the angle θ, multiplied by powers of the radial distance r, whilst those in the outer region by a series of unknown functions of θ, multiplied by inverse powers of r, both of which satisfy the respective boundary conditions at the tube exit and infinity. The inner-region and outer-region pressures are patched together well at an arbitrary point over the hemisphere common to both regions, and the velocity components are done at the same point likewise. The parabolic velocity distribution at the exit is derived from the velocity in the inner region. The jet-ejection pressure and the pressure gradient coefficient are related by one parameter σ and the role of the parameter in the patching is clarified. Stream lines show a typical flow characteristic of the free jet flow for Re → 0 in the outer region.

A segregated spectral finite element method for the 2D transient incompressible Navier–Stokes equations

In this paper, the spectral element approximation and the velocity–pressuredecoupling method implementing the SIMPLE (semi-implicit method for pressure linked equations) algorithm are first combined to form a high-order segregated scheme for the solution of the two-dimensional transient incompressible Navier–Stokes equations. In contrast to previous segregated finite element methods based on the SIMPLE algorithm, the pressure equation is derived from the continuity equation using the element matrices to ensure convergence. High-order element basis functions are adopted, which greatly reduces the number of nodal points used in the calculation. The validation test that has an analytical solution demonstrates the high accuracy and convergence rate of the method. The flow in a lid-driven cavity with different inclination angles and the flow over a backward-facing step are investigated to further illustrate the performance of the scheme. The computed results are in excellent agreement with the benchmark solutions. The almost periodic solution for the flow of Re=10000 in a lid-driven square cavity is also captured by the present scheme.

Numerical Analysis of Liquid Mixing in a T-Micromixer with Taylor Dispersion Obstructions

Passive micromixers are of great importance in biomedical engineering (lab-on-chips) and chemical processing (microreactors) fields. Various hydrodynamic principles such as lamination, flow separation, and chaotic advection were employed previously to improve mixing in passive mixers. However, mixing enhancement due to velocity gradients in the flow, which is known as the Taylor dispersion effect, has been seldom studied. In the present study, thin rectangular slabs oriented in the flow direction are placed in the mixing channel of a T-micromixer. The thin rectangular slabs are referred to as Taylor Dispersion Obstructions (TDOs) as they are designed to create velocity gradients in the flow. The mixing performance of T-micromixer with and without TDOs is estimated in the Re range of 0 to 350. It is observed that there is no effect on mixing in the presence of TDOs in the low Re (0 < Re < 100), as the velocity gradients created in the flow are considerably small. The vortex formed in the flow for Re of 100 to 220 damped the gradients of velocity created in the flow (due to the presence of TDOs) which resulted in negligible improvement in the quality of mixing. However, considerable enhancement in mixing performance is obtained at high Re (250 to 350) with the presence of TDOs in the mixer. The increase in inertial effects at higher Recreated larger gradients of velocity near the walls of TDOs and mixing channel walls and thereby a significant enhancement in mixing performance is obtained due to Taylor dispersion.

Electromagnetic field analysis and cooling system design for high power switched reluctance motor

PurposeThe electromagnetic field and cooling system of a high power switched reluctance motor (SRM) are studied numerically. The geometry of the motor and its main components are established using a computer-aided design software in the actual size. This study aims to evaluate the resulting thermal losses using the electromagnetic analysis of the motor.Design/methodology/approachIn the electromagnetic analysis, the Joule’s loss in the copper wires of the coil windings and the iron losses (the eddy currents loss and the hysteresis loss) are considered. The flow and heat transfer model for the thermal analysis of the motor including the conduction in solid parts and convection in the fluid part is introduced. The magnetic losses are imported into the thermal analysis model in the form of internal heat generation in motor components. Several cooling system approaches were introduced, such as natural convection cooling, natural convection cooling with various types of fins over the motor casing, forced conviction air-cooled cooling system using a mounted fan, casing surface with and without heat sinks, liquid-cooled cooling system using the water in a channel shell and a hybrid air-cooled and liquid-cooled cooling system.FindingsThe results of the electromagnetics analysis show that the low rotational speed of the motor induces higher currents in coil windings, which in turn, it causes higher copper losses in SRM coil windings. For higher rotational speed of SRM, the core loss is higher than the copper loss is in SRM due to the higher frequency. An air-cooled cooling system is used for cooling of SRM. The results reveal when the rotational speed is at 4,000 rpm, the coil loss would be at the maximum value. Therefore, the coil temperature is about 197.9°C, which is higher than the tolerated standard temperature insulation material. Hence, the air-cooled system cannot reduce the temperature to the safe temperature limitation of the motor and guarantee the safe operation of SRM. Thus, a hybrid system of both air-cooled and liquid-cooled cooling system with mounting fins at the outer surface of the casing is proposed. The hybrid system with the liquid flow of Re = 1,500 provides a cooling power capable of safe operation of the motor at 117.2°C, which is adequate for standard insulation material grade E.Originality/valueThe electromagnetic field and cooling system of a high power SRM in the presence of a mounted fan at the rear of the motor are analyzed. The thermal analysis is performed for both of the air-cooled and liquid-cooled cooling systems to meet the cooling demands of the motor for the first time.

A compact streamfunction-velocity scheme for the 2-D unsteady incompressible Navier-Stokes equations in arbitrary curvilinear coordinates

A streamfunction-velocity formulation-based compact difference method is suggested for solving the unsteady incompressible Navier-Stokes equations in the arbitrary curvilinear coordinates, in which the streamfunction and its first derivatives as the unknown variables are utilized. Numerical examples, involving the boundary layer problem, a constricted channel flow, driven polar cavity flow and trapezoidal cavity flow problem, are solved by the present method. Numerical results demonstrate the accuracy of the proposed scheme and exhibit the numerical capability to simulate the flow problems on geometries beyond rectangular. For driven polar cavity flow problem, the results show that the flow for Re = 5 000 is not steady but time-periodic, and the critical Reynold number (Rec) for the occurrence of a Hopf bifurcation is given.

Utilidad de la angiografía intracoronaria con microcatéter para evitar fenómeno de no reflujo en la angioplastia primaria

Introduction: The acute coronary occlusion associated with a high valuation of morbimortalidad in the acute infarction of the myocardium with elevation of the ST segment (STEMI), represents a challenge for the interventionist, due the unknown amount of thrombus load and its characteristics beyond the occlusion (microvascular obstruction (MVO), distal stenosis), which leads to complicate the primary angioplasty (PTCA) and to the no refl ow phenomenon (NFP), which has an overall incidence reported from 20 to 40%. Objectives: To evaluate the effi cacy of the distal coronary angiography with microcatheter, that leads to an individualized therapeutic strategy, with the purpose of prevent the NFP in the accute coronary occlusion in STEMI treated with PTCA. Material and methods: 70 patients were included with STEMI. We performed coronary angiography with microcatheter distal to the obstruction, and the therapeutic strategy was chosen in accordance with fi ndings: stenosis, thrombus amount or MVO. After the PTCA were performed, we evaluated the NFP incidence. Results: Report of 70 patients, the majority men (80%), class Killip Kimbal I (95 %), with disease of the right coronary artery in 57%, and anterior descending coronary artery in 37%. Distal thrombus was present in 47% of the studied patients. An obstruction length of more than 30 mm was observed in 21% of the cases, also we observed dissection of the arteries in a 21%, MVO was presented in 41%, and distal stenosis in 25%. Used therapeutic strategies: It was applied medicated stent in 81% of the patients, long balloon in 93%, and thrombolytic therapy in 30%. We observed a NFP incidence of 18.6 %. Conclusions: The diagnostic and therapeutic approach for STEMI that this study recommends, diminishes the NFP in comparison with reported. An obstruction > 30 mm is an independent factor for NFP, therefore in these cases we recommend use the described diagnostic approach and long balloon.

Unsteadiness and transition to turbulence in woven spacer filled channels for Membrane Distillation

To characterize the performance of Membrane Distillation (MD) modules, channels filled with woven spacers were investigated by Computational Fluid Dynamics (including Direct Numerical Simulations and the use of the SST k-ω turbulence model) and by parallel experiments with Thermochromic Liquid Crystals. The cases considered here regard mutually orthogonal filaments with a spacer pitch to channel height ratio P/H=2, two spacer orientations θ with respect to the main flow (0° and 45°), and bulk Reynolds numbers Re from ∼200 to ∼2000, an interval of great interest in practical MD applications. For both values of θ, CFD predicted steady-state flow for Re up to ∼300, and chaotic flow for Re larger than ∼400. In the intermediate range Re≈300-400, periodic flow regimes were predicted for both orientations. These regimes were of particular interest and complexity, as they exhibited a slow global oscillation of the flow superimposed on high order harmonics corresponding to fast local oscillations. Experiments confirmed the appearance of unsteadiness for Re>∼300. Heat transfer and friction were little affected by unsteadiness and exhibited a smooth behaviour with Re. The agreement with the experimental results was good using DNS, and acceptable using RANS.

Effect of Slit Inclusions in Drag Reduction of Flow over Cylinders

The flow over bluff bodies is separated compared to the flow over streamlined bodies. The investigation of the fluid flow over a cylinder with a streamwise slit has received little attention in the past, however there is some experimental evidence that show for turbulent regime it reduces the drag coefficient. This work helps in understanding the fluid flow over such cylinders in the laminar regime. As the width of the slit increases the drag coefficient keeps on reducing resulting in a narrower wake as compared to what is expected for flow over a cylinder. In this work we have used two different approaches in modelling a 2D flow for Re=10 to compare the results for CFD using finite volume method (ANSYS FLUENTTM) and Lattice Boltzmann methods. In all cases cylinders of circular cross section have been considered while slit width changing from 10% to 40% of the cylinder diameter. . It will be shown that drag coefficient decreases as the slit ratio increases. The effect of slit size on drag reduction is studied and discussed in detail in the paper. We have also made comparison of the results obtained from Lattice Boltzmann and finite volume methods.

Stress Analysis and Design Optimization for Low-<inline-formula> <tex-math notation="LaTeX">$k$ </tex-math></inline-formula> Chip With Cu Pillar Interconnection

Cu pillar technology can cater for high I/O, fine pitch, and miniaturization requirements compared with wire bonding and conventional solder flip-chip technologies. However, chip-package interaction of Cu pillar and low-k chip is a critical challenge during assembly process due to stiffer Cu pillar structure compared with conventional solder bump. Thermo-compression bonding (TCB) process was adopted and used for fine pitch Cu pillar assembly on Cu/low-k chip to reduce the package warpage and low-k stress. In this paper, a novel TCB process modeling methodology using a 2-D axisymmetric finite-element model with global-local technique was demonstrated by considering step-by-step process conditions. The TCB modeling method was validated by the experimental data. The simulation results show that TCB process results in much lower package warpage and low-k stress compared with conventional reflow (RF) process. Based on the proposed TCB modeling method, the comprehensive parametric studies were conducted to optimize TCB process and Cu pillar design for package reliability improvement, including bonding process conditions, Cu pillar structure design, package geometry, and packaging material selection. RF process-induced package warpage and low-k stress were also simulated for comparison. The final package and assembly solution was successfully achieved based on the suggestions and recommendations provided by the simulation results.

Unsteady heat transfer and flow structure of a row of laminar impingement jets, including vortex development

Considered is a cylindrical channel with a single row of 10 aligned impinging jets, with exit flow in the axial direction at one end of the channel. For the present predictions, each jet is laminar with a Reynolds number of 200. An unsteady FLUENT solver is employed for predictions of flow characteristics within and nearby the 10 impingement jets. Instantaneous, local surface Nusselt numbers oscillate as time progresses, such that the local Nusselt number signature from one individual vortex diminishes in intensity, as the opposite vortex in the same pair then produces an increase in the local, instantaneous Nusselt number signature. Spectrum analysis of different flow quantities shows frequencies associated with laminar jet and vortex oscillations, and evidence more orderly flow for Re=200, without the chaos and broad-band mixing associated with the turbulent flow when Re=15,000. Laminar flow spectra also evidence increased flow unsteadiness as cross-flows accumulate within the impingement channel with streamwise development as Z/D increases. In some cases, this increased unsteadiness manifests itself through the formation of multiple spectral peaks, in place of single peak spectra. Similar with turbulent jet arrangements, the unsteady, local static pressure gradient variations along interfaces between laminar jets and cross flow are also a key flow feature, which is connected to the initiation and development of the Kelvin–Helmholtz instability induced vortices.