Laminar Model Research Articles

Newtonian and Non-Newtonian Blood Rheology Inside a Model of Stenosis

In order to imitate the atherosclerosis artery disease and determine the key issues, Computational Fluid Dynamics (CFD) is able to play a leading rule in the analysis of flow physics within the clogged arteries, in particular the stenosis artery. The problem of blood flow blockage through the blood vessel has been investigated numerically within a stenosis artery. In this work, a CFD technique was employed to solve the three-dimensional, steady, laminar and non-Newtonian Carreau model blood flow through a stenosis artery using Star-CCM+ software. The shape of stenosis that has been selected is a trapezoidal with two cases (70% and 90% blockage). Shear rate, streamlines, vorticity and importance factor are examined to assess the influence of non-Newtonian model through the test section, the Carreau model was compared with Newtonian model. The clinical significance of the shear rate is reported for the examined cases, observing that the levels of non-Newtonian model are predicted to be higher in the 90% blockage than that observed within the 70%; the same finding as related with the axial velocity and vorticity. The levels of re-circulation areas and vorticity are showed to be enlarged in the Carreau model compared with the case of Newtonian.

Prediction of particle deposition on the walls of a cubical cavity with differentially heated opposed walls using heat and mass transfer laminar mixed convection boundary layer models

In this paper we present a model to predict particle deposition velocities on the isothermal walls bounding a turbulent natural convection flow at moderate Rayleigh numbers. The model is based on the laminar mixed convection characteristics of the velocity and thermal boundary layers near the thermally active walls of the cavity. These characteristics are verified with Direct Numerical Simulations reported elsewhere. The model considers the gravity force, the thermophoretic effect and the Brownian diffusion of small particles. The predicted deposition velocities are compared successfully with those obtained with one of the most popular particle deposition models and with experimental measurements reported in the literature and carried out in a cavity with the same thermal boundary conditions as those considered in this study.

Influence of Prandtl number on bifurcation and pattern variation of non-isothermal annular Poiseuille flow

The relative influence of momentum diffusivity and thermal diffusivity, in terms of the Prandtl number (Pr), on the finite-amplitude instability of a non-isothermal annular Poiseuille flow (NAPF) is analyzed. The limiting value of the growth of instabilities under nonlinear effects is studied by deriving a cubic Landau equation. Emphasis is given especially on studying the impact of the low Prandtl number and the curvature parameter (C) on the bifurcation and the pattern variation of the secondary flow for both axisymmetric and non-axisymmetric disturbances. The finite-amplitude analysis predicts that in contrast to NAPF of water or fluid with Pr ≥ O(1) where the flow is supercritically unstable, the NAPF of low Pr fluids, particularly liquid metals, has shown both supercritical and subcritical bifurcation in the vicinity as well as away from the critical point. The nonlinear interaction of different harmonics for the liquid metal predicts a lower heat transfer rate than those by the laminar flow model, whereas for a fluid with Pr > 2, it is the other way. The maximum heat transfer takes place for the considered minimum value of C. For fluids with low Pr, a probable lower critical Rayleigh number is obtained. The corresponding variation in neutral stability curves as a function of wavenumber reveals that the instability that is supercritical for some wavenumber may be subcritical or vice versa at other nearby wavenumbers. The structural feature of the pattern of the secondary flow under the linear theory differs significantly from those of the secondary flow under nonlinear theory away from the bifurcation point. This is a consequence of the intrinsic interaction of different harmonics that are responsible for the stabilizing or the destabilizing nature of different components in the disturbance kinetic energy balance.

Numerical study of the effects of laminar flame speed on flame dynamics in the early stages of flame propagation in two-dimensional half open tubes

The effect of laminar flame speed on the flame dynamics in two-dimensional half open tubes is investigated through a two-dimensional laminar combustion model. Several single-step reaction mechanisms are established to alter the corresponding laminar flame speed. The simulation results show that the similarity of flame propagation only holds during the acceleration phase. In the flame acceleration phase, the flame shape and flame tip position corresponding to each dimensionless moment are almost the same. This similarity gradually disappears after the flame touches the wall. When the flame decelerates, the periodic oscillation of the flame tip velocity is related to the laminar flame speed. The higher the laminar flame speed, the longer the corresponding dimensionless oscillation period. The destruction of this similarity is also reflected in the correlation between the flame surface area and the growth rate of burnt gas volume. In addition, the dimensionless evolution correlation of the flame surface area during the acceleration phase is given in this paper. The moving speed of the flame skirt at different laminar flame speeds and the time interval of flame inversion in the two-dimensional half open tubes are also presented.

Effect of the Inclination Angle on Premixed Flame Dynamics in Half-Open Ducts

The propagation of premix methane/air flames in long half-open ducts with different inclination angles θ between the sidewall and the horizon was numerically investigated using the laminar model. The numerical result was compared with the experimental and theoretical ones to validate the numerical model. The results show that the numerical results are in good agreement with them. The investigation provides the basic understanding of the effects on changing the shape of the ducts to promote the premixed flame combustion. For methane/air, the position where the flame front begins to concave is pushed back with the increase in angle θ. The so-called “tulip” flame even disappeared, if the angle θ is bigger than one certain value. Moreover, the flame propagation speed and pressure are enhanced as the angle θ increases. In addition, the numerical simulation indicates that the burning gas creates an eddy near the tip of the flame, altering the flow field and causing the tulip flame to appear. However, with the angle θ increased, the flame propagation is restrained by the change in sidewalls, resulting in the different flow patterns to suppress the formation of tulip flames and promote flame combustion.

A-priori and a-posteriori studies of a direct moment closure approach for turbulent combustion using DNS data of a premixed flame

The direct moment closure (DMC) approach for modeling the filtered chemical source terms in large eddy simulation of turbulent combustion is comprehensively assessed by using direct numerical simulation data of a turbulent premixed flame with a skeletal chemistry. In this approach, closure is achieved by linearizing the chemical source terms using the Taylor series expansions. Different from traditional method, the exponential term is treated as a single variable in the expansion, and the filtered chemical reaction rate can be readily modeled with the first-order and second-order moments, plus higher-order correlations. The a-priori investigations demonstrate that the DMC model is able to reasonably represent the chemical reaction rates, and the second-order correlation of reactive scalars can be well modeled with the algebraic gradient-type model. Compared to the first-order moment closure, the second-order one makes a significant improvement on the predictions. These improvements are also observed in the a-posteriori study in which the premixed flame can be better resolved by the second-order moment closure model. Consequently, the second-order moment closure model provides an alternative for turbulent combustion modeling and is recommended to be used as an improvement to the laminar chemistry closure model. The sensitivity of the DMC approach to the grid size needs further investigation.

Effect of turbulence modeling on hydrodynamics of a turbulent contact absorber

A computational fluid dynamics (CFD) study is conducted to find a suitable two equation turbulence model for accurate prediction of hydrodynamics of an inhouse turbulence contact absorber (TCA) at high gas and liquid velocities. Based on the multi-fluid Eulerian approach, hydrodynamics of TCA is simulated by incorporating three turbulence models i.e. standard k–ε model, RNG k–ε model and SST k–ω model in ANSYS Fluent®. The solid phase stresses were closed by using the kinetic theory of granular flows (KTGF). TCA hydrodynamics parameters; expanded bed height and bed pressure drop were used to compare the results of this study with experimental data and also with earlier numerical study published with laminar viscous model. It was found that the RNG k–ε model predicted the bed height and pressure drop better than its counterparts. To accurately find the effects of secondary phase turbulence, two RNG k–ε model options i-e. per phase and dispersed were also evaluated. The results show that the “per phase” option of RNG k–ε model produced the expanded bed height and pressure drop in close agreement with available experimental data at similar operating conditions.

Numerical simulation of water entry of an inclined cylinder

Water entry of cylindrical structures is a complex multiphase flow problem involving a three-phase medium interaction that occurs instantaneously. It is a nonlinear and unsteady process, which the physical understanding of mechanism in the aspect of evolution of cavity dynamics and multiphase flow field is yet to be fully understood. In the present work, a three-dimensional numerical model of six degrees of freedom (DOF) based on the shear stress transport (SST) k−ω eddy viscosity model was established to capture the complicated characteristics of the vertical water entry of an inclined cylinder at various entry velocities. A volume of fluid method for tracking air-water interface and a combined dynamic fluid body interaction and overlapping grid technology were used to simulate the complex motion. The numerical model with laminar and turbulent model was verified against the published experimental data of cavity evolution and motion characteristics. A parametric study was conducted to study the translational and rotational characteristics of the structure due to the effect of water entry velocity. The investigation revealed the secondary closure phenomenon during the water entry. The asymmetrical multiphase flow field showed that its dynamic characteristics change with space and time under the effect of cavity evolution.

Large-scale heating effects on global parameters for flow past a square cylinder at different cylinder inclinations

The combined effects of heating and cylinder inclination on mixed convective flow past a square cylinder subjected to high heating is investigated numerically using a non-Oberbeck-Boussinesq model. A two-dimensional, unsteady, laminar, viscous and compressible flow model is utilized. In order to account for the large-scale heating effects; variations in transport and thermo-physical properties are considered. The flow parameters such as Reynolds number (Re), Prandtl number (Pr), Mach number (M), Froude number (Fr) and free-stream inclination (α) are kept fixed as Re=100,Pr=0.71,M=0.1,Fr=1.0 and α=0o, respectively. The cylinder inclination (ϕ) is varied from 0o to 45o in a step of 10o, while heating levels (ϵ) defined as (Tw−T∞)/T∞ is varied from 0 to 1, in a step of 0.2. The governing equations, in strong conservative form, transformed into body fitted curvilinear co-ordinates, are solved numerically using particle velocity upwind (PVU-M+) scheme employing finite difference method. The effects of cylinder inclination and heating on global parameters such as drag coefficient, lift coefficient, moment coefficient, Strouhal number and Nusselt number are presented. It is observed that for any heating level, mean drag coefficient increases with increase in cylinder inclination. Also at fixed ϕ, mean drag coefficient increases with heating. It is found that in steady flow regime increase in ϕ, decreases the heat transfer rate. In unsteady flow regime, increase in ϕ, increases the heat transfer rate. It is also observed that at fixed cylinder inclination mean Nusselt number increases with increase in heating level.

Analysis of turbulence and blocking effects on loading capacity for elementary texture cells of infinite width under water lubrication

PurposeThe loading mechanism of textures considering turbulence has not been fully covered. This paper aims to investigate the effect of turbulence on the textured loading capacity under water lubrication and to analyze the causes of the turbulence effect.Design/methodology/approachComputational fluid dynamic models with different textured shapes are established after validation. The transition shear stress transport (SST) model, which is suitable for predicting the transition process of fluid from laminar state to turbulent state, is adopted in the present study. To illustrate the effect of turbulence, the loading capacity of textures predicted by transition SST model and laminar model is compared.FindingsThe loading capacity is higher after considering turbulence because more lubricant enters into textures and the flow rate of lubricant to textured outlet increases. There exists an optimal textured depth ratio and density for loading capacity and the change of flow state would not affect the optimal values. The degree of fluid blockage at textured outlet has a dominant influence on loading capacity. As the textured shape changes to triangle or ellipse from rectangle, the vortices at the textured bottom move forward and the blockage at a textured outlet is enhanced, which makes loading capacity improved under the action of blocking effect.Originality/valueThe enhancement of the blocking effect is found to be crucial to the improvement of textured loading capacity after considering turbulence. Present research provides references to understand the loading mechanism of textures under turbulent conditions.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2020-0149/

Assessment of PIV performance in validating CFD models from nasal cavity CBCT scans

ObjectiveThe aim of our study was to investigate how well Particle Image Velocimetry (PIV) measurements could serve Computational Fluid Dynamics (CFD) model validation for nasal airflow. Material and methods: For the PIV measurements, a silicone model of the nose based on cone beam computed tomography (CBCT) scans of a patient was made. Corresponding CFD calculations were conducted with laminar and two turbulent models (k-ω and k-ω SST). ResultsCFD and PIV results corresponded well in our study. Especially, the correspondence of CFD calculations between the laminar and turbulent models was found to be even stronger. When comparing CFD with PIV, we found that the results were most convergent in the wider parts of the nasal cavities. ConclusionPIV measurements in realistically modelled nasal cavities succeed acceptably and CFD calculations produce corresponding results with PIV measurements. Greater model scaling is, however, necessary for better validations with PIV and comparisons of competing CFD models.

Experimental and theoretical evaluation of hydrogen cloud explosion with built-in obstacles

This work is aimed at evaluating hydrogen cloud explosion with built-in obstacles experimentally and theoretically. The flame front evolution is captured using infrared filtering technology and the explosion overpressure at four monitoring points is measured using free-field sound pressure sensor. The laminar flame model and turbulent flame model are established to theoretically predict maximum explosion overpressure in advance. The results demonstrated that the flame acceleration of hydrogen cloud explosion with built-in obstacles is attributed to mutual promotion of flame instabilities and obstacle-induced turbulence. The effects of built-in obstacles on maximum flame front velocity is relatively limited and maximum value of flame wrinkling factor is ΞΔ,max = 3.64. As the distance between pressure sensor and ignition source increases, the maximum explosion overpressure, positive overpressure impulse and absolute value of negative overpressure impulse are decreased monotonously. The positive overpressure and negative overpressure are mainly affected by steel pipe number in vertical and horizontal direction, rather than length of steel pipe cross-section. The velocity of explosion overpressure propagating in the air is c = 350.88 m/s. The maximum explosion overpressure should be significantly underestimated by laminar flame model, the turbulent flame model could be used to satisfactorily predict maximum explosion overpressure at four monitoring points.

Development of a roller hearth furnace simulation model and performance investigation

The roller hearth furnace is numerically investigated to analyze and optimize its performance. It is essential to apply the temperature control model and the material transport model for the furnace simulation, which enables the energy consumption and productivity calculation. When it comes to temperature control models, a total of five models are considered: constant temperature method, constant heat flux method, on-off control, proportional-integral-differential (PID) control, and on-off and PID control. For material transport models, high viscosity laminar flow model, dynamic mesh model, and source term model are developed and evaluated. As a result, a new heating furnace simulation is developed with the on-off and PID control model and the source term model. This simulation model can reduce the computational cost significantly while maintaining the accuracy. The developed model cannot only analyze the results of design changes, but also the results caused by the change in many variables, such as the speed of blank, the setting temperature, material change, and emissivity. Therefore, this simulation model can be used to suggest solutions for energy efficiency and productivity of existing furnaces through various case studies.

Numerical Investigation on the Optimum Thermal Design of the Shape and Geometric Parameters of Microchannel Heat Exchangers with Cavities.

Due to the large surface-area-to-volume ratio, microchannel heat exchangers have a higher heat transfer rate compared with traditional scale heat exchangers. In this study, the optimum microchannel cavity with high heat transfer and low flow resistance is designed to further improve microchannel exchangers’ thermal performance. A three-dimensional laminar flow model, consisting of Navier–Stokes equations and an energy conservation equation is solved and the conjugate heat transfer between the silicon basement and deionized water is taken into consideration. The impact of the shape, aspect ratio, size and spacing of the cavity on the thermal performance of microchannel exchangers are numerically investigated, respectively. The results indicated that the cavity on the sidewall can enhance heat transfer and reduce flow resistance simultaneously, and cavities with a relatively small expansion angle and streamlined edge could enhance thermal performance the most. Based on the conclusions, a new cavity shape is proposed, and the simulation results verify its excellent thermal performance as expected. Furthermore, investigation is performed to figure out the optimum design of the new cavity and the optimal geometric parameters of the cavity under different flow conditions have been obtained in principle for microchannel exchangers’ design.

Evaluation of air curtain and emergency exhaust system for smoke confinement of an enclosure

This paper analyses the various configuration for the emergency ventilation system consisting of an air curtain and exhaust fan to manage the dispersion of heat and toxicity inside an enclosure for different fire scenarios. Employing this ventilation system is helpful to secure an escape route or extinguishing the fire. The air curtain discharge velocity is 10, 18, and 30 m/s and its discharge angle is set to be 0°,15°, and 30°. Three values of 0.5, 1, and 2m3/s are considered for the exhaust flow rate. Computational Fluid Dynamics simulations have been conducted with a steady laminar flamelet model and buoyant standard k−ε turbulence model to assess the performance of the air curtain and emergency exhaust system for smoke confinement. Evaluation of an optimized state of the emergency ventilation system is provided by the Taguchi method due to computational limitations in solving various cases for the ventilation system configuration. Results show that for each location and heat release rate of fire, it is possible to manage heat, CO, and CO2 dispersion with the adjustment air curtain parameters and emergency exhaust system. Finally, the optimum state of the system is reported for each scenario, and the temperature and concentration of toxic gas have been compared. Results show that for the 200 kW fire, 2 m3/s exhaust flow rate with air curtain discharge velocity of 18 m/s and 15° would be adequate. However, 500 kW fire requires 30 m/s with 30° discharge velocity to contain the fire inside the compartment.

Laminar Burning Velocity Model Based on Deep Neural Network for Hydrogen and Propane with Air

The aim of the study was to develop deep neural network models for laminar burning velocity (LBV) calculations. The present study resulted in models for hydrogen–air and propane–air mixtures. An original data-preparation/data-generation algorithm was also developed in order to obtain the datasets sufficient in quality and quantity for models training. The discussion about the current analytical models highlighted issues with both experimental data and methodology of creating those analytical models. It was concluded that there is a need for models that can capture data from multiple experimental techniques with ease and automate the model design and training process. We presented a full machine learning based approach that fulfills these requirements. Not only model development, but also data preparation was described in detail as it is crucial in obtaining good results. Resulting models calculations were compared with popular analytical models and experimental data gathered from literature. The calculations comparison showed that the models developed were characterized by the smallest error with regards to the experiments and behaved equally well for variable pressure, temperature, and equivalence ratio. The source code of ready-to-use models has been provided and can be easily integrated in, for example, CFD software.

Fluid Drag Reduction by Magnetic Confinement

A solid interface with a viscous liquid flow results in large drag, related to frictional forces. Reducing the drag by more than a few tens of percent remains elusive. Here, we use magnetic forces to stabilize a ferrofluid encapsulating a transported immiscible viscous liquid. This liquid-in-liquid flow exhibits drag reduction from 80% to more than 99%, tuneable by the viscosity ratio between the two liquids, over a range where the transported liquid has a viscosity larger or smaller than that of the encapsulating liquid. Our findings are explained by a laminar flow model that matches data for our whole range of experimental conditions.

Improved Methods of Laminar Flamelet Model for Compressible Flow

To improve the computational performance of the laminar flamelet model for compressible flow (the compressible flamelet model), two formulations of the heat flux term are proposed: form1 achieves efficient calculation of the spatial gradient of the mass fraction of each chemical species, and form2 eliminates the dependence of the calculation process on the number of chemical species. Based on these formulations, three methods are proposed that use linear interpolation (lerp) or an artificial neural network (ANN) for their flamelet tables: form1-ann, form2-lerp, and form2-ann. First, it will be shown that the accuracy of the ANN in the proposed form1-ann and form2-ann methods is sufficient for numerical simulations. Then, to evaluate the form2-lerp and form2-ann methods and show that they can greatly improve the computational performance of the conventional method, numerical simulations will be conducted for the scramjet test-engine combustor of the German Aerospace Center, DLR. The calculation time of the form2-lerp method is reduced by about 0.878 times, and the memory usage is increased by about 2.97 times compared with the values for the conventional method. The calculation time of the form2-ann method is reduced by about 0.946 times, and the memory usage is reduced by about 0.508 times compared with the conventional method.

Dynamic mechanism of crater formation induced by inclusion during intense pulsed ion beam irradiation

Crater is a general defect appeared on metallic target after intense pulsed ion beam (IPIB) treatment, which greatly limits the utilization and development of this technology. Understanding the mechanism of crater formation is of great significance for IPIB application. It is reported that inclusion in metallic target is associated with crater formation. In this work, the inclusions exposed to vacuum environment on target surface were found to lead to crater formation during 2.0 J/cm2 IPIB irradiation via a positioning observation experiment on high speed steel. To investigate the dynamic mechanism of crater formation, thermal conduction model and laminar flow model were performed successively. The flow field analysis revealed that surface tension coefficient difference between molten inclusion and molten matrix is the primary reason to drive melt flow and generate crater. In the subsequent evolution of the crater, surface curvature also plays an important part.

Numerical Investigation to Derive Correlations for Hydrodynamic Entrance Length in Very Low Reynolds Number Regime in Rectangular Microchannels

Abstract A three-dimensional laminar flow model was used for 37 Reynolds numbers (0.1, 0.2…1, 2…10, 20…100, and 200…1000) through six rectangular microchannels (aspect ratios: 1, 0.75, 0.5, 0.25, 0.2, and 0.125) to develop correlations for hydrodynamic entrance length. The majority of the Reynolds numbers are in the low regime (Re < 100) to fulfill the need to determine the hydrodynamic entrance length for microchannels. Examination of the fully developed flow condition was considered using the velocity or fRe criteria. Numerical results from the present simulations were validated by comparing the fRe results. Two new correlations were developed from a vast amount of numerical data (222 simulations). The velocity criterion correlations predict entrance length with a mean error of 4.67% and maximum error of 10.28%. The fRe criterion generated better correlations and were developed as a function of aspect ratio to predict entrance length with a mean error less than 2% and maximum error of 5.75% for 0.1 ≤ Re ≤ 1000 and 0 ≤ α ≤ ∞.