Detailed Three-dimensional Numerical Simulations Research Articles

Based on Fluent, the numerical simulation of the flow field on the filter disk surface of the rotary filter device

The rotary screen filter is the most typical basic structural element in rotating mechanical devices. It is being widely applied in various fields of industrial production. Using a spinning disc as the research subject, this study utilizes Computational Fluid Dynamics (CFD) techniques to establish a physical model of the filter disc. Various parameters and their corresponding boundary conditions are set, followed by a comprehensive and detailed three-dimensional numerical simulation of the high-speed rotating disc using the Fluent software platform. The objective is to investigate the effects of disc rotation speed and liquid inlet velocity on the surface flow field, as well as the influence of rotational speed on the attachment of particle impurities to the surface. Finally, the computational results will be visualized and thoroughly analyzed.

Topology optimization of regenerative cooling channel in non-uniform thermal environment of hypersonic engine

Regenerative cooling technology is widely used in the active thermal protection system of hypersonic airbreathing engine. However, the overall efficiency of active cooling with limited total coolant quantity needs to be improved due to flow non-uniformity, hydrocarbon fuel pyrolysis and other factors. This paper presents a fluid–structure coupled topology optimization design of the regenerative cooling channel to improve heat transfer efficiency. To further verify the advantages of topology-optimized cooling channels under real operating conditions, detailed three-dimensional numerical simulations were conducted to investigate the supercritical flow and heat transfer processes of hydrocarbon fuel in the topology-optimized channels. Results reveal that compared to the traditional straight cooling channel, the topology-optimized channel has improved the heat transfer efficiency and flow distribution significantly. Many micro-channels perpendicular to the mainstream direction which can be termed as “capillary-like” zone in the optimized channel distribute the inlet fuel to the adiabatic walls, thus significantly improving the overall heat transfer. Remarkable cooling capacity robustness of the topology-optimized cooling channels has been found under different non-uniform heat flux conditions. Comparing to the straight channels, the averaged temperature of the heated wall in the optimized channels can be decreased up to 5.8% and the pressure drop can be decreased up to 20.6%. The uniformity of the fuel temperature distribution is about 2 times better than that of the straight channel. However, although overall heat transfer efficiency is high, complex flow and heat transfer phenomenon occurs in the topology-optimized cooling channel, such as the reverse flow phenomenon induced by adverse pressure gradient near the entrance and the stagnated flow in the “capillary-like” zone near the exit, which impair the local heat transfer and cause the existence of high temperature regions.

Three-Dimensional Numerical Simulation of Creep Crack Growth Behavior for 316H Steel Using a Stress-Dependent Model

Based on detailed three-dimensional numerical simulation, creep crack growth behavior of C(T) specimen with different thicknesses of 316H steel was predicted using a stress-dependent creep ductility and strain rate model. Three regions were observed in the relation of creep crack growth rate versus fracture parameter C ∗ . The C(T) specimen with higher thickness exhibits higher CCG rate. The turning point 1 location from low C ∗ region to transition C ∗ region increases with increasing thickness, while that of turning point 2 seems to be independent of specimen thickness. Based on the finite element results, constraint-dependent turning point 1 location and creep crack growth rate equations were fitted. More accurate and realistic life assessment may be made when the stress-dependent model and the constraint effect were considered for creep life assessments of high-temperature components subjecting to a low applied load.

Alignment statistics of pressure Hessian with strain rate tensor and reactive scalar gradient in turbulent premixed flames

The relative alignment of the eigenvectors of pressure Hessian with reactive scalar gradient and strain rate eigenvectors in turbulent premixed flames have been analyzed for Karlovitz number values ranging from 0.75 to 126 using a detailed chemistry three-dimensional direct numerical simulations database of H2–air premixed flames. The reactive scalar gradient preferentially aligns with the most extensive strain rate eigendirection for large Damköhler number and small Karlovitz number values, whereas a preferential collinear alignment between the reactive scalar gradient with the most compressive strain rate eigendirection is observed in flames with small Damköhler number and large Karlovitz number. By contrast, the eigenvectors of pressure Hessian do not perfectly align with the reactive scalar gradient, and the net effect of the pressure Hessian on the evolution of the normal strain rate contribution to the scalar dissipation rate transport acts to reduce the scalar gradient in the zone of high dilatation rate. The eigenvectors of pressure Hessian and the strain rate are aligned in such a manner that the contribution of pressure Hessian to the evolution of principal strain rates tends to augment the most extensive principal strain rate for small and moderate values of Karlovitz numbers, whereas this contribution plays an important role for the evolution of the intermediate principal strain rate for large values of Karlovitz number. As the reactive scalar gradient does not align with the intermediate strain rate eigenvector, the influence of pressure Hessian contributions to the scalar–turbulence interaction remains weak for large values of Karlovitz number.

Using EMPHASIS for the Thermography-Based Fault Detection in Photovoltaic Plants

In this paper, an Efficient Method for PHotovoltaic Arrays Study through Infrared Scanning (EMPHASIS) is presented; it is a fast, simple, and trustworthy cell-level diagnosis method for commercial photovoltaic (PV) panels. EMPHASIS processes temperature maps experimentally obtained through IR cameras and is based on a power balance equation. Along with the identification of malfunction events, EMPHASIS offers an innovative feature, i.e., it estimates the electrical powers generated (or dissipated) by the individual cells. A procedure to evaluate the accuracy of the EMPHASIS predictions is proposed, which relies on detailed three-dimensional (3-D) numerical simulations to emulate realistic temperature maps of PV panels under any working condition. Malfunctioning panels were replicated in the numerical environment and the corresponding temperature maps were fed to EMPHASIS. Excellent results were achieved in both the cell- and panel-level power predictions. More specifically, the estimation of the power production of a PV panel with a shunted cell demonstrated an error lower than 1%. In cases of strong nonuniformities as a PV panel in hotspot, an estimation error in the range of 9–16% was quantified.

Thermal management of high-power LED module with single-phase liquid jet array

Next generation high-power Light–Emitting Diodes (LED) require specialized cooling systems for ensuring performance and reliability. The ability of a multi-jet single-phase liquid cooling system (jet diameter 400 µm) for thermal management of the high-power LED based luminaire is investigated and successfully demonstrated for a 300 W nominal power module. Operating pressure of the jet system is varied in the range of 0–8 bar, leading to coolant water flow rates varying from 65 ml/min to 782 ml/min (Re = 3833–46119). With the proposed multi-jet cooling system, the experiments show the possibility of maintaining the module surface temperature well below 70 °C for substrate level heat flux up to ~125 W/cm2. We show that the system can still maintain safe operating temperature, without loss of luminous efficiency for input power up to 130% of the nominal design power. Supporting detailed three-dimensional numerical simulations of the conjugate module heat transfer inside the LED package volume are provided. It is concluded that single-phase array of water-based jet flow is an excellent potential option for thermal management of high-power LED luminaire.

Evaluating deformation modes of sandwich serpentine structures for high stretchability

Assembling sandwich serpentine structures with soft functional materials creates an effective way to high-performance stretchable devices. In order to achieve better design criteria for the application of serpentine-shaped stretchable systems, this paper explores the mechanics governing the deformation mechanisms of the sandwich serpentine structures. Firstly, detailed three-dimensional (3-D) numerical simulations were conducted to well capture the deformation modes. Secondly, a theoretical model was developed within the simplified 1-D formulation. Numerical and theoretical investigations reveal the important features of the underlying materials (including the representative low-modulus elastomers) and mechanics aspects of the polymer/metal/polymer sandwich serpentine interconnects system with strain-isolating layers, and their dependence on key design variables (including the material selection and thickness of both the isolation layer and the serpentine clad). Three distinct deformation modes of the sandwich serpentine structures were observed and discussed: 1) Global buckling; 2) Local wrinkling; and 3) Planar bending. By theoretically analyzing the wrinkling wavelengths, we propose a simple design criterion for the strain-isolating layers that indicates a transition from local wrinkling to global buckling, with the increasing thickness of the isolation layer. Results will be useful to mechanical design of stretchable systems with high levels of the elastic stretchability.

Nonlinear numerical simulation of punching shear behavior of reinforced concrete flat slabs with shear-heads

This paper examines the structural response of reinforced concrete flat slabs, provided with fully-embedded shear-heads, through detailed three-dimensional nonlinear numerical simulations and parametric assessments using concrete damage plasticity models. Validations of the adopted nonlinear finite element procedures are carried out against experimental results from three test series. After gaining confidence in the ability of the numerical models to predict closely the full inelastic response and failure modes, numerical investigations are carried out in order to examine the influence of key material and geometric parameters. The results of these numerical assessments enable the identification of three modes of failure as a function of the interaction between the shear-head and surrounding concrete. Based on the findings, coupled with results from previous studies, analytical models are proposed for predicting the rotational response as well as the ultimate strength of such slab systems. Practical recommendations are also provided for the design of shear-heads in RC slabs, including the embedment length and section size. The analytical expressions proposed in this paper, based on a wide-ranging parametric assessment, are shown to offer a more reliable design approach in comparison with existing methods for all types of shear-heads, and are suitable for direct practical application.

Numerical assessment of reinforced concrete members incorporating recycled rubber materials

This paper is concerned with the inelastic behaviour of reinforced concrete beam-column members incorporating rubber from recycled tyres. Detailed three-dimensional nonlinear numerical simulations and parametric assessments are carried out using finite element analysis in conjunction with concrete damage plasticity models. Validations of the adopted nonlinear finite element procedures are carried out against experimental results from a series of tests involving conventional and rubberised concrete flexural members and varying levels of axial load. The influence of key parameters, such as the concrete strength, rubber content, reinforcement ratio and level of axial load, on the performance of such members, is then examined in detail. Based on the results, analytical models are proposed for predicting the strength interaction as well as the ductility characteristics of rubberised reinforced concrete members. The findings permit the development of design expressions for determining the ultimate rotation capacity of members, using a rotation ductility parameter, or through a suggested plastic hinge assessment procedure. The proposed expressions are shown to offer reliable estimates of strength and ductility of reinforced rubberised concrete members, which are suitable for practical application and implementation in codified guidance.

Three-dimensional shock-sulfur hexafluoride bubble interaction

The evolution of shock-sulfur hexafluoride (SF6) bubble interaction is investigated using a detailed three-dimensional numerical simulation. The influences of the end wall distance on the bubble evolution are analyzed by using the high-resolution simulations. The results show that vorticities mainly emerge at the interfaces of the shock wave and the SF6 bubble, and a downstream jet is formed, owing to the impingement of the high pressure in the vicinity of the downstream pole of the bubble and the induction of nearby vorticities. Besides, the big vortices of the SF6 bubble could interact with the walls in the y-direction to increase the bubble volume. When the end wall distance is shortened, a short and wide downstream jet is formed, owing to the untimely interaction of the reflected shock wave with the distorted SF6 bubble. Also, a new upstream jet emerges behind the impingement of the reflected shock wave, and there is no interaction between the distorted SF6 bubble and the wall in the y-direction until a very late time. From a quantitative point of view, the discrepancy between the bubble volume and effective bubble volume is larger in the case with a long end wall distance, which has enhanced vorticities and strengthened bubble-wall interaction. Moreover, the reflected shock wave has a dominant compression effect on the distorted SF6 bubble evolution for the two cases with different end wall distances, but for the case with a longer end wall distance, the bubble-wall interaction has a more significant influence than the influence of vorticities on the bubble volume increase. The computational results demonstrate the three-dimensional effects of shock-SF6 bubble interactions, which have not been seen in previous two-dimensional simulations.

Simulation of Intermittent Flow Development in a Horizontal Pipe

In this study, detailed three-dimensional (3D) numerical simulations of intermittent multiphase flows were carried out to investigate the slug initiation process and various features of intermittent flows inside a horizontal pipe. Air and water are used as working fluids. The domain used for simulations is a 14.4 m long pipe with 54 mm inner diameter. The volume of fluid (VOF) model was used to capture the air/water interface and its temporal evolution. Using the developed computational fluid dynamics (CFD) model, the slug formation and propagation along horizontal circular pipe were successfully predicted and studied comprehensively. Slug length and the frequency of slug formation, as two main features of intermittent flow, were used to validate the model against experimental results and available correlations in the literature. Three-dimensional numerical simulation of intermittent flow proved to be a powerful tool in tackling limitations of experiments and providing detailed data about various features of the intermittent flow. The effect of gas and liquid superficial velocities on the liquid slug and elongated bubble length was explored. Moreover, the study revealed new findings related to the elongated bubble shape and velocity field in the slug unit.

Rocking response of inverted pendulum structures under blast loading

We investigate the dynamics of inverted pendulum structures under fast-dynamic excitations arising from an explosion. We model blast actions using established empirical models and best-fit interpolations of existing experimental tests. We focus attention on pure rocking response mechanisms. We present moment balance equations and overturning conditions. Inspired by previous works in the frame of earthquake engineering, we derive new analytical, closed-form solutions for the rocking response and the overturning domain of slender blocks due to explosions.The analytical findings and assumptions are validated through existing experimental tests and detailed three-dimensional numerical simulations, which consider the full interaction between the blast waves and the structure. We show that unilateral rocking response and overturning are predominant mechanisms compared to sliding and up-lifting.Finally, we develop design charts to be used as a straightforward decision making tool for determining the critical stand-off distance between the explosive source and the target in order to prevent overturning. Our model can be easily applied for the design of protective devices to preserve artefacts, secure buildings and humans, as well as for devising energy absorbing systems based on rocking.

An acoustic model of a Helmholtz resonator under a grazing turbulent boundary layer

Acoustic models of resonant duct systems with turbulent flow depend on fitted constants based on expensive experimental test series. We introduce a new model of a resonant cavity, flush mounted in a duct or flat plate, under grazing turbulent flow. Based on previous work by Goody, Howe and Golliard, we present a more universal model where the constants are replaced by physically significant parameters. This enables the user to understand and to trace back how a modification of design parameters (geometry, fluid condition) will affect acoustic properties. The derivation of the model is supported by a detailed three-dimensional direct numerical simulation as well as an experimental test series. We show that the model is valid for low Mach number flows (M = 0.01-0.14) and for low frequencies (below higher transverse cavity modes). Hence, within this range, no expensive simulation or experiment is needed any longer to predict the sound spectrum. In principle, the model is applicable to arbitrary geometries: Just the provided definitions need to be applied to update the significant parameters. Utilizing the lumped-element method, the model consists of exchangeable elements and guarantees a flexible use. Even though the model is linear, resonance conditions between acoustic cavity modes and fluid dynamic unstable modes are correctly predicted.

Preliminary design and performance analysis of a radial inflow turbine for a large-scale helium cryogenic system

Large-scale helium cryogenic systems are widely used in superconducting systems, nuclear fusion engineering, space exploration and large scientific engineering, etc. However, its energy efficiency is quite low due to the extremely low operation temperature. The helium turbine constitutes the most vital component of a large-scale helium cryogenic system. Thus, it is essential to develop a high efficiency helium turbine in order to improve the energy efficiency of the cryogenic helium system. In this paper, a high speed radial micro turbine with the splitter blade was designed for a 40 L/h helium liquefier with Claude cycle. The turbine is designed for inlet and outlet temperatures of 14.4 K and 9.4 K respectively. The design speed of the turbine is 223570 rpm due to the small mass flow rate and impeller diameter. A one-dimensional mean line optimal design approach for radial inflow turbine is adopted in this study. Furthermore, detailed three-dimensional viscous numerical simulations are conducted in order to verify the one-dimensional optimal approach in design condition and predict the performance of the designed turbine in off-design conditions. The results indicate that the optimal helium radial inflow turbine for the design and off-design conditions can be designed through applying the proposed analysis method.

Numerical Simulation of the Droplet Formation in a T-Junction Microchannel by a Level-Set Method

To satisfy the increasingly high demands in many applications of microfluidics, the size of the droplet needs accurate control. In this paper, a level-set method provides a useful method for studying the physical mechanism and potential mechanism of two-phase flow. A detailed three-dimensional numerical simulation of microfluidics was carried out to systematically study the generation of micro-droplets and the effective diameter of droplets with different control parameters such as the flow rate ratio, the continuous phase viscosity, the interfacial tension, and the contact angle. The effect of altering the pressure at the x coordinate of the main channel during the droplet formation was analysed. As the simulation results show, the above control parameters have a great influence on the formation of droplets and the size of the droplet. The effective droplet diameter increases when the flow rate ratio and the interfacial tension increase. It decreases when the continuous phase viscosity and the contact angle increase.

Analytical and numerical evaluation of electron-injection detector optimized for SWIR photon detection

Recent results from our electron-injection detectors as well as other heterojunction phototransistors with gain suggest that these devices are useful in many applications including medical imaging, light detection and ranging, and low-light level imaging. However, there are many parameters to optimize such structures. Earlier, we showed a good agreement between experimental results and our models. In this paper, we provide detailed analytical models for rise time, gain, and dark current that very accurately evaluate key parameters of the device. These show an excellent agreement with detailed three-dimensional numerical simulations. We also explore a figure of merit that is useful for low-light-detection applications. Based on this figure of merit, we examine the ultimate sensitivity of the device. Furthermore, we explore the effects of variations in some of the key parameters in the device design and present an optimum structure for the best figure of merit. Our models suggest ways to improve the existing devices that we have, and may be a guideline for similar phototransistors.

Effective elastic moduli of core-shell-matrix composites

The effective Young’s modulus and Poisson’s ratio of spherical monodisperse and polydisperse core-shell particles ordered or randomly distributed in a continuous matrix were predicted using detailed three-dimensional numerical simulations of elastic deformation. The effective elastic moduli of body-centered cubic (BCC) and face-centered cubic (FCC) packing arrangements of monodisperse microcapsules and those of randomly distributed monodisperse or polydisperse microcapsules were identical. The numerical results were also compared with predictions of various effective medium approximations (EMAs) proposed in the literature. The upper bound of the EMA developed by Qiu and Weng (1991) was in good agreement with the numerically predicted effective Young’s modulus for BCC and FCC packings and for randomly distributed microcapsules. The EMA developed by Hobbs (1971) could also be used to estimate the effective Young’s modulus when the shell Young’s modulus was similar to that of the matrix. The EMA developed by Garboczi and Berryman (2001) could predict the effective Poisson’s ratio, as well as the effective Young’s modulus when the Young’s modulus of the core was smaller than that of the matrix. These results can find applications in the design of self-healing polymers, composite concrete, and building materials with microencapsulated phase change materials, for example.

Effects of different geometric structures on fluid flow and heat transfer performance in microchannel heat sinks

Abstract In the present study, fluid flow and heat transfer in microchannel heat sinks with different inlet/outlet locations (I, C and Z-type), header shapes (triangular, trapezoidal and rectangular) and microchannel cross-section shapes (the conventional rectangular microchannel, the microchannel with offset fan-shaped reentrant cavities and the microchannel with triangular reentrant cavities) are numerically studied with computational domain including the entire microchannel heat sink. Detailed three-dimensional numerical simulations are useful in identifying the optimal geometric parameters that provide better heat transfer and flow distribution in a microchannel heat sink. Results highlight that flow velocity uniformity is comparatively better for I-type and poor for Z-type. The flow distribution is found to be symmetrical for I-type. It is seen from the header shapes analysis that the rectangular header shapes provides better flow velocity uniformity than the trapezoidal and triangular headers. The fluid flow mechanism can be attributed to the interaction of the branching of fluid and the friction offered by the walls of the header. Effects of microchannel cross-section shapes emphasize that the microchannel with offset fan-shaped reentrant cavities and the microchannel with triangular reentrant cavities of the heat sinks enhance the heat transfer compared to the conventional rectangular microchannel. The heat transfer mechanism can be attributed to the jetting and throttling effect, the additional flow disturbance near the wall of the reentrant cavities and the form drag of the reentrant cavities. The heat sink C has better heat transfer characteristic for q v = 150 ml/min and is able to prolong the life of the microelectronic devices.

Three-dimensional LES modeling of induced gas motion under the influence of injection pressure and ambient density in an ultrahigh-pressure diesel injector

Detailed three-dimensional numerical simulations have been performed to analyze gas motion generated by dispersion of non-evaporating ultrahigh-pressure diesel spray. Fuel–air mixing is studied under different injection pressures and ambient densities, based on Eulerian–Lagrangian scheme. Large eddy simulation (LES) turbulence model along with an advanced primary breakup scheme have been applied to model the two-phase flow. The freeware code OpenFOAM is implemented to study the effects of the injection pressure and ambient density on gas motion. To evaluate the capability of the LES method in the simulation of gas motion in a high-pressure injection chamber, the obtained results were validated against published experimental data. Also, spray characteristics have been presented and induced gas velocities compared with experimental data. There are temporal variations in the gas movement around the spray. Based on the reported literature, gas velocity field around the spray is divided into three different zones: quasi static zone, gas recirculation zone, and head vortex zone. Accordingly, induced gas motions that are affected by the spray tip velocity are different in these areas. 3-D plots of spray liquid volume fraction and gas velocity vectors have also been presented. Overall, numerical results have displayed good agreement with the experimental data. It is concluded that LES is quite capable of reproducing the turbulent structures in gas field, mainly in ultrahigh-pressure injection.

Identification of striking distance considering the velocity ratio and competition among the leaders

The striking distance is an important parameter to design a reliable lightning protection system. To protect an object against direct strikes, an improved leader progression model combined fractal geometry and the leader initiation criteria proposed by M. Becerra is introduced to determine the striking distance on a given structure. Both the deterministic and stochastic features of the lightning leader are considered and more detailed three-dimensional numerical simulation of the leader discharge is achieved due to the application of extended grid. Some of the basic methods and strategies are outlined and the influence on protection zone caused by leader velocity ratio is analyzed. Moreover, a comparative analysis of the results obtained through the improved model, the rolling sphere method and the Cooray model is performed. The results indicate that the presented method is closer to the natural lightning and the selection of the breakdown point is actually a complex competitive process among numerous upward leaders.