Computational Fluid Dynamics Simulation Of Flow Research Articles

CFD simulations of turbulent flow and dispersion in built environment: A perspective review

Computational fluid dynamics (CFD), a powerful analysis tool in wind engineering, has been applied to a wide variety of problems over the past few decades. The application of CFD to environmental wind problems in the built environment is much more common than its application to structural wind problems, in both practice and research. This paper briefly reviews the history of urban wind problems and the development of wind engineering. Then, the progress in the applications of CFD to environmental problems is outlined, particularly in terms of the evolution of the studies presented over time at the International Conferences on Wind Engineering. The major achievements of CFD in environmental wind engineering in the last 30 years are discussed in terms of Large Eddy Simulation versus the Reynolds-averaged Navier-Stokes model, appropriate boundary conditions, verification and validation, and best practice guidelines. Furthermore, notable topics in recent applications of CFD to the built environment, including complicated particle (e.g., snow, rain, sand, sea salt) dispersion, are reviewed. Finally, the future direction of the applications of CFD to environmental problems as an advanced design and research tool is discussed.

Prediction of PAN oxidation in a gas turbine bearing chamber using coupled chemical kinetics and CFD simulation of lubricant flow

A Computational Fluid Dynamics (CFD) model, using COMSOL 6.1, was developed in this work to simulate the oil temperature distribution within a bearing housing, to provide a means of predicting the most probable zones for PAN oxidation. Three different zones in the radial direction and two distinct zones in the axial direction were specified with different temperature profiles. It was also found that the rotational speed of the rotor and oil outlet pressure can significantly influence the temperature distribution. Oil inlet temperature was the other factor that had a minor impact on the temperature profile. Furthermore, the highest temperatures were observed in the bulk oil in the area surrounding the rotor. A chemical reaction analysis, which was performed using MATLAB R2022a, was performed to estimate the rate of PAN oxidation. According to the final results; higher rotational speeds increase the rate of oxidation. Moreover, a reduction in revolution speed extends the time required to completely consume the original PAN content. These findings were also used to demonstrate where varnish deposits start to form. Multiple temperature zones were used instead of the average temperature to carefully check the mutual relation between the revolution speed and temperature, and accurately calculate the rate of PAN oxidation reaction.

Changes in velocity distribution produced by paddlewheels in algae raceway ponds

Open raceway ponds (RWPs) are widely used in large-scale algae production. To improve the performance of these systems, it is crucial to understand the flow structure throughout the ponds. Computational fluid dynamics (CFD) simulations have been widely used to study the flow field in RWPs. However, such studies typically require assumptions about the distribution of velocities at the inlet boundary of the domain (which is usually taken as the cross-section a short distance downstream from the paddlewheel). In this study, laboratory measurements were carried out to study the flow structure in the vicinity of a paddlewheel in a raceway pond model. Results showed that the paddlewheel may induce complex changes to the distribution of depth-averaged velocities, including the complex inversion of the velocity gradient across the channel. The measured non-uniform distribution of velocities was then used as the boundary condition of a CFD simulation to understand the effects of common assumptions on the accuracy of CFD simulations of flows in RWPs. Furthermore, observations in a field (full-scale) installation corroborated that the presence of a paddlewheel significantly affects the velocity distribution in its vicinity, further strengthening the argument that such changes to the flow should not be disregarded in numerical simulations of RWPs.

Application of microcatheter shaping based on computational fluid dynamics simulation of cerebral blood flow in the intervention of posterior communicating aneurysm of the internal carotid artery.

The present study aimed to investigate the application of the aneurysm embolization microcatheter plasticity method based on computational fluid dynamics (CFD) to simulate cerebral blood flow in the interventional treatment of posterior communicating aneurysms in the internal carotid artery and to evaluate its practicality and safety. A total of 20 patients with posterior internal carotid artery communicating aneurysms who used CFD to simulate cerebral flow lines from January 2020 to December 2022 in our hospital were analyzed. Microcatheter shaping and interventional embolization were performed according to the main cerebral flow lines, and the success rate, stability, and effect of the microcatheter being in place were analyzed. Among the 20 patients, the microcatheters were all smoothly placed and the catheters were stable during the in vitro model test. In addition, the microcatheters were all smoothly placed during the operation, with a success rate of 100%. The catheter tips were stable and well-supported intraoperatively, and no catheter prolapse was registered. The aneurysm was completely embolized in 19 cases immediately after surgery, and a small amount of the aneurysm neck remained in one case. There were no intraoperative complications related to the embolization catheter operation. Microcatheter shaping based on CFD simulation of cerebral blood flow, with precise catheter shaping, leads to a high success rate in catheter placing, stability, and good support, and greatly reduces the difficulty of catheter shaping. This catheter-shaping method is worthy of further study and exploration.

CFD Simulation of isothermal upward two-phase flow in a vertical annulus using interfacial area transport equation

This work presents a numerical simulation of a vertical, upward, isothermal two-phase flow of air bubbles and water in an annular channel applying a Computational Fluid Dynamics (CFD) code. For this, the Two-Fluid model is applied considering interfacial force correlations, namely: drag, lift, wall lubrication, turbulent dispersion, and virtual mass. The turbulence k-ε model effects and the influence of One-group Interfacial Area Transport Equation (IATE) are taken into account, in this case, the influence of two source term correlations for the bubble breakup and coalescence IATE is analysed. The work assesses whether the code properly represents the physical phenomenon by comparing the simulation results with experimental data obtained from the literature. Six flow conditions are evaluated based on two superficial liquid velocities and three void fractions in the bubbly flow regimen. The annular channel adopted has an outer pipe with an internal diameter of 38.1 mm and an inner cylinder of 19.1 mm. To represent this geometry, a three-dimensional mesh was generated with 160,000 elements, after a mesh sensitivity study. The void fraction distribution, taken radially to the flow section, is the main parameter analysed as well as interfacial area concentration, interfacial gas velocity, and bubble sizes distribution. The CFD model implemented in this work demonstrates satisfactory agreement with the reference experimental data but indicates the need for further improvement in the phase interaction models.

Integration of CFD simulated abrasive waterjet flow dynamics with the material removal model for kerf geometry prediction in overlapped erosion on Ti-6Al-4V alloy

Complex surfaces in difficult-to-machine materials can be milled by maneuvering the abrasive waterjet (AWJ) according to the local part geometry by suitably overlapping the kerf cross-sectional profiles (CPs). The CP of a kerf milled in a single pass (i.e., trench) and overlapped pass (i.e., cavity) erosion by the AWJ are the building blocks that need to be known apriori for developing a jet path strategy to achieve a complex surface. This demands an efficient model for the kerf CP prediction. The present work proposes a novel framework for overlapped kerf CP prediction, wherein the jet characteristics of the AWJ exiting the focusing tube are evaluated from the two-phase flow computational fluid dynamics simulation coupled with the material removal model developed based on energy conservation principles. Towards incorporating the non-linearities observed in the kerf formation from the overlapped experimental trials into the model for CP prediction enhancement, the following are considered: (i) a hypothesis: ‘secondary erosion by the jet on the previously eroded kerf increases material removal’, (ii) the varying jet-material property of the ductile material (Ti-6Al-4V alloy), and (iii) local particle impact angles. Furthermore, the variation in the above mentioned non-linearities is incorporated into the model under the change in the AWJ conditions. From the results, it is observed that the proposed models predicted (i) the trench CP with a mean absolute error (MAE) of 27 μm, and the maximum erosion depth (hmax) with an error < 2%, and (ii) cavity at different overlaps (7% – 82%) and jet traverse rate (3000 mm/min – 5000 mm/min) with a MAE of 47 μm, and the hmax with an error < 6%. The predicted geometry of the CP correlates well with the experimental ones, with an average R2 of 0.98 and 0.95 in single- and overlapped- pass erosions.

Modelling blood flow in patients with heart valve disease using deep learning: A computationally efficient method to expand diagnostic capabilities in clinical routine.

The computational modelling of blood flow is known to provide vital hemodynamic parameters for diagnosis and treatment-support for patients with valvular heart disease. However, most diagnosis/treatment-support solutions based on flow modelling proposed utilize time- and resource-intensive computational fluid dynamics (CFD) and are therefore difficult to implement into clinical practice. In contrast, deep learning (DL) algorithms provide results quickly with little need for computational power. Thus, modelling blood flow with DL instead of CFD may substantially enhances the usability of flow modelling-based diagnosis/treatment support in clinical routine. In this study, we propose a DL-based approach to compute pressure and wall-shear-stress (WSS) in the aorta and aortic valve of patients with aortic stenosis (AS). A total of 103 individual surface models of the aorta and aortic valve were constructed from computed tomography data of AS patients. Based on these surface models, a total of 267 patient-specific, steady-state CFD simulations of aortic flow under various flow rates were performed. Using this simulation data, an artificial neural network (ANN) was trained to compute spatially resolved pressure and WSS using a centerline-based representation. An unseen test subset of 23 cases was used to compare both methods. ANN and CFD-based computations agreed well with a median relative difference between both methods of 6.0% for pressure and 4.9% for wall-shear-stress. Demonstrating the ability of DL to compute clinically relevant hemodynamic parameters for AS patients, this work presents a possible solution to facilitate the introduction of modelling-based treatment support into clinical practice.

A semi-implicit slip algorithm for mesh deformation in complex geometries, implemented in OpenFOAM

Many engineering applications of computational fluid dynamics (CFD) comprise extensive movement of objects that necessitate complex dynamic mesh treatments. In particular, the mesh motion process frequently requires a proper slipping of mesh points on highly curved surfaces. The currently available implementation of explicit slip boundary conditions in OpenFOAM fails to allow large deformations of the mesh without severely degrading the mesh quality and inverting some of the cells. Thus, a robust semi-implicit slip algorithm, based on the Laplacian smoothing methodology, is developed in the present work to tackle this issue. The algorithm is in fact performed in two steps, one explicit and one implicit. The OpenFOAM implementation of the algorithm includes different mesh motion solvers and boundary conditions, based on the displacement or velocity of points. The method is first verified using simple, yet relevant, test cases, and it is shown that the developed algorithm significantly outperforms some of the well-known proprietary CFD codes. Then, it is applied to a complex practical CFD case study. An engineering application that requires the features of the developed mesh motion algorithm is the transient operation of Kaplan turbines. These double-regulated machines simultaneously adjust the guide vane and runner blade angles while changing the operating condition. CFD simulations of such transient operations are highly complex, as they involve mesh deformation of the guide vane passage and simultaneous mesh deformation and rigid-body rotation of the runner blade passage. The mesh deformation requires points to slip on the curved hub and shroud surfaces while preserving the cell quality in tiny blade clearances. Therefore, the feasibility of the developed algorithm is evaluated for a load rejection sequence of a Kaplan turbine model. Program summaryProgram Title: Semi-implicit slip mesh motionCPC Library link to program files:https://doi.org/10.17632/wztc26vh7b.1Developer's repository link:https://github.com/salehisaeed/semiImplicitSlipLicensing provisions: GPLv3Programming language: C++Nature of problem: CFD simulations of numerous engineering fluid flows, such as transient operation of hydraulic turbines, involve an immensely complicated mesh motion process consisting of simultaneous mesh deformation and mesh slipping on highly curved surfaces. The available standard mesh motion methodology in OpenFOAM lacks some features to simulate this elaborate mesh motion. The introduced program addresses this problem by developing a new dynamic mesh algorithm.Solution method: The program implements a robust semi-implicit algorithm for slipping the mesh points on curved surfaces. The algorithm includes two steps, namely, an explicit step based on the general slip condition and an implicit step based on the Dirichlet condition. It employs the Laplacian smoothing equations to spread the mesh deformation into the domain. Additionally, a solid-body rotation may be added on top of the deformed mesh, which could be useful for modeling the runner region in transient operation of Kaplan turbines which contains simultaneous mesh deformation and solid-body rotation of the mesh.

Examining the universality of the hemolysis power law model from simulations of the FDA nozzle using calibrated model coefficients.

Computational fluid dynamics (CFD) is widely used to predict mechanical hemolysis in medical devices. The most popular hemolysis model is the stress-based power law model that is based on an empirical correlation between hemoglobin release from red blood cells (RBCs) and the magnitude of flow-induced stress and exposure time. Empirical coefficients are traditionally calibrated using data from experiments in simplified Couette-type blood-shearing devices with uniform-shear laminar flow and well-defined exposure times. Use of such idealized coefficients in simulations of real medical devices with complex hemodynamics is thought to be a primary reason for the historical inaccuracy of absolute hemolysis predictions using the power law model. Craven et al.(Biomech Model Mechanobiol 18:1005-1030, 2019) recently developed a CFD-based Kriging surrogate modeling approach for calibrating empirical coefficients in real devices that could potentially be used to more accurately predict absolute hemolysis. In this study, we use the FDA benchmark nozzle to investigate whether utilizing such calibrated coefficients improves the predictive accuracy of the standard Eulerian power law model. We first demonstrate the credibility of our CFD flow simulations by comparing with particle image velocimetry measurements. We then perform hemolysis simulations and compare the results with in vitro experiments. Importantly, the simulations use coefficients calibrated for the flow of a suspension of bovine RBCs through a small capillary tube, which is relatively comparable to the flow of bovine blood through the FDA nozzle. The results show that the CFD predictions of relative hemolysis in the FDA nozzle are reasonably accurate. The absolute predictions are, however, highly inaccurate with modified index of hemolysis values from CFD in error by roughly three orders of magnitude compared with the experiments, despite using calibrated model coefficients from a relatively similar geometry. We rigorously examine the reasons for the inaccuracy that include differences in the flow conditions in the hemolytic regions of each device and the lack of universality of the hemolysis power law model that is entirely empirical. Thus, while the capability to predict relative hemolysis is valuable for product development, further improvements are needed before the power law model can be relied upon to accurately predict the absolute hemolytic potential of a medical device.

Verification and validation of CFD and its application in PWR fuel assembly

Fuel assembly is one of the most important components in Pressurized Water Reactor (PWR). Mixing Vane Spacer Grid (MVG) provides support for the fuel bundle. However, the most important role for MVG is to mix the coolant and decrease the temperature gradient, ultimately enhance the heat transfer and improve Critical Heat Flux (CHF). Hence the performance of MVG are directly related to the safety and economics of PWR. Due to the complexity of MVG and the broad range of thermal hydraulic conditions involved, full scale, time consuming and costly full length rod bundle experiments (usually 5 × 5 or 4 × 4 or 6 × 6) were performed to study the complicated thermal hydraulic phenomenon. Recently, with the development of computer technology, Computational Fluid Dynamics (CFD) has become one of the most popular tools to simulate the flow field of the fuel assembly. In this paper, the method based on CFD to study the MVG performance is reviewed. The main process for CFD in simulating flow field of MVG is divided into two parts. The first part is the verification and validation of CFD codes under single phase-and two-phase flow. The second is the application of CFD for exploring the MVG performance. For the verification of single phase flow CFD simulation, mesh sensitivity, turbulent model effects were examined, the validation including axial pressure drop, lateral temperature distribution, and lateral velocity distribution. For two phase flow verification and validation, the sensitivities of different drag and non-drag forces on void fraction distribution were tested. The experimental data with a tube and an annual flow fields under two phase boiling were chosen to validate the two phase flow simulation and to cover different aspects of internal and external wall boiling effects. The verified and validated CFD codes were then used to study the performance of MVG. Considering the complexity of MVG, a separated effect study approach is presented to evaluate the performance of MVG. Another systematic approach from simple geometry to 5 × 5 MVG model to study the effect of different components, including vane, dimple, and spring were proposed for the simulation of two phase flow. Some indexes were reviewed and used to evaluate the performance of MVG.

Rheological studies and numerical investigation of barite sag potential of drilling fluids with thermochemical fluid additive using computational fluid dynamics (CFD)

The growing advancement in drilling technology had necessitated the development of self-destructive mud cake, which is composed of encapsulated thermochemical fluids (TCF) to facilitate dissolution of filter cake. However, segregation of weighting component, commonly the Barite particles, can lead to various operational problems that should be avoided. In this investigation, the segregation potential of Barite particles (“Barite Sag”) in drilling fluids has been investigated. The experimental data from rheological studies have been employed to guide CFD modeling and simulation of multiphase flow of a dense suspension mimicking the conventional oil-based mud (OBM), water-based mud (WBM), and those comprises thermochemical additives viz. OBM_TCF and WBM_TCF. The results revealed that the drillings fluids conform to the shear thinning pseudoplastic behavior within the conditions operated in this study. Notably, the apparent viscosity of the WBM was observed to decrease with increasing temperature between 25 and 50 °C but increased afterwards. Evaluation of gravitational settling characteristics revealed that the conventional OBM might have lower sagging potential, at lower temperature, compared with the conventional WBM, due to higher settling velocity of Barite particles in the later. In comparison, at higher temperature, which corresponds to the conditions of the newly formulated muds (i.e., the OBM_TCF and WBM_TCF), it was found that the WBM_TCF exhibit lower potential for Barite sag due to lower settling velocity of the particles compared with that of OBM_TCF. The reason essentially has to do with higher viscosity of the WBM_TCF. The CFD studies have considered both the hydrodynamic forces and shear induced migration of the particles. Analyses of various simulation results including particle flux, particle mass fraction, mixture viscosity, and the pressure drop, consistently revealed that the WBM_TCF might have lower Barite segregation potentials compared with other types of drilling fluids considered in this study.

Blood flow CFD simulation on a cerebral artery of a stroke patient

The purpose of this paper is to conduct a numerical simulation of the stroke patient's cerebral arteries and investigate the flow parameters due to the presence of stenosis. The computational fluid dynamics (CFD) simulations are based on simplified and realistic cerebral artery models. The seven simplified models (benchmarks) include straight cylindrical vessels with idealized stenosis with variable d/D (0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1). The realistic model of the cerebral artery is based on magnetic resonance imaging (MRI) for patient-specific cerebral arteries. The simulation for the realistic model of the cerebral artery is performed at boundary conditions measured by ultrasonography of the input and the output flow profiles (velocity and pressure). The obtained CFD results of the benchmarks are validated with actual data from the literature. Furthermore, a previous vascular contraction is assumed to be exist and the effect of this contraction area ratio on the blood flow regime is discussed and highlighted. Furthermore, CFD results show that a certain vascular contraction area critically affects the blood flow which shows increasing the wall shear stress WSS at the stenosis site. An increase in the blood velocity and vortex appears after the contraction zone, this lead to vessel occlusion and strokes.Article highlightsThe pressure drop across the arterial contraction is reduced when the area ratio d/D is increased.In some cases, the vortex can prevent blood flow from crossing, this leads to vessel occlusion especially at low d/DThe WSS near the contraction area is high. Increasing the WSS can cause embolism that leads to lead to vessel occlusion.

Experimental and numerical validation of a Francis turbine draft tube designed for mitigation of pressure fluctuations

This paper presents an experimental and numerical investigation of the internal flow in a Francis turbine draft tube previously designed for minimizing pressure fluctuations and energy losses in off-design conditions. The design of the draft tube geometry is based on an original approach combining Design of Experiments and steady/unsteady Computational Fluid Dynamics (CFD) simulations of the draft tube internal flow. The proposed method provides new insight on the influence of the draft tube geometry on the flow dynamic behaviour on one hand and enables the determination of a geometry promoting flow stability and hydraulic performance on another hand. CFD simulations of the internal flow in the final geometry showed promising results in terms of flow stability compared with the initial geometry designed by conventional CFD-aided methods. A reduced-scale model of the prototype machine featuring the final draft tube geometry is finally installed and tested in laboratory. Tests include performance and pressure fluctuations measurements over the complete operating range. The analysis of the results shows that the draft tube flow remains globally stable over the complete part-load range with pressure fluctuations amplitude lower than 1% of the net head. It is also shown that the dominant pressure component at the runner outlet in the draft tube cone is of synchronous nature. The physical mechanisms of excitation are finally highlighted by analysis of unsteady CFD simulation results.

Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl

Abstract The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where a significant premixing is desired. In particular, flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In this work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all the annular flow configurations. Pressures from 3 to 8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow computational fluid dynamics simulation.

Influence of inlet boundary conditions on 3D steady RANS simulations of non-isothermal mechanical ventilation in a generic closure

Inlet boundary conditions at the supply opening play an important role in the accuracy and reliability of computational fluid dynamics (CFD) simulations for indoor airflow. A non-exhaustive overview of past CFD simulations of ventilation flow in generic enclosures indicates that uniform hypotheses of inlet boundary conditions are commonly used. However, due to thermal effects and a complex air supply system geometry, constant values of inlet airflow quantities can be insufficient for an accurate simulation of non-isothermal ventilation flow. In addition, this can lead to biased conclusions on the performance of turbulence models and other computational settings. To assess this issue, a well-documented experiment on non-isothermal mechanical ventilation with detailed information of inlet conditions from the literature is used in this study. Different turbulence models and inlet boundary conditions are validated, including seven steady Reynolds-averaged Navier-Stokes (RANS) models, six methods of imposing inlet air velocity, three methods of specifying air temperature, and a wide range of turbulence quantities. The parametric study demonstrates that in this particular case, i.e., cold air supply from a single round nozzle diffuser close to the ceiling, ω-based turbulence models perform better than the ε-based turbulence models. The impacts of different methods to specify inlet air temperature profile and turbulence quantities on the simulation results are not significant. However, if the inlet airflow direction is not taken into account appropriately, significant deviations are observed between CFD simulations and experimental data.

Continuous production of 3,5,5-trimethylhexanoyl chloride and CFD simulations of single-phase flow in an advanced-flow reactor

BackgroundThe combination of flow chemistry and microreactor technology is emerging in the modern chemical industry. It can link chemical engineering, organic synthesis, and green chemistry, which provides a safety guarantee for dangerous chemical processes. MethodIn view of the advantages of continuous flow synthesis, a more efficient method for the synthesis of 3,5,5-trimethylhexanoyl chloride from triphosgene and 3,5,5-trimethylhexanoic acid catalyzed by N, N-dimethylformamide was obtained by using silicon carbide microreactor, which was reported for the first time. Computational fluid dynamics (CFD) simulation of single-phase flow in an advanced microreactor was carried out with OpenFOAM software. The flow lines, lag zone, velocity distribution, pressure field, and residence time distribution (RTD) were obtained at different flow rates (5–100 mL/min). Significant findingsA continuous flow process for the 3,5,5-trimethylhexanoyl chloride with a 91% isolated yield has been reported. Triphosgene and 3,5,5-trimethylhexanoic acid were used as starting materials to achieve excellent results in the silicon carbide flow reactor, which could tolerate the corrosion of chloride ions at 55 ℃ and 0.8 MPa. In the continuous flow process, based upon the cyclic feed reaction method, the product was obtained with sufficiently high 3,5,5-trimethylhexanoic acid conversion (> 99%) and product 3,5,5-trimethylhexanoyl chloride selectivity (95%). The throughput reached 0.35 kg/h, and the purity of the final product was greater than 90% by distillation, which was in accordance with the needs of production. This new process using more polar tetrahydrofuran as the solvent was time- and cost-effective, and the obtained product had a higher yield, brighter color, and less impurity. With the increase of the flow velocity, the stagnation area and the swirling intensity increased slightly, but the velocity and pressure distribution in each mixing unit were relatively uniform. Additionally, the reactor had symmetric RTDs at all flow rates. Due to the larger flow rate, the Peclet number was increased and the axial dispersion was reduced, with the result of a narrower RTD curve.

3D CFD simulation and analysis of transient flow in a water pipeline

Abstract A three-dimensional (3D) computational fluid dynamics (CFD) approach is developed to elaborate the water-hammer pipe flow and 3D detailed dynamic characteristics of a closing ball valve. The proposed CFD approach considers the water compressibility and the viscous sublayer, which are sometimes neglected in previous studies. Comparisons of the CFD results, the measured pressures and the one-dimensional results, demonstrate that the current 3D CFD approach better reproduces the experimental pressure oscillations while helping to visualize the associated physical processes and to further explore the 3D transient characteristics. The mean velocity distributions in the radial direction significantly change as the pipe transient progresses, which is closely associated with transient shear stress. Mean velocity variations at the valve during the closing process undergo three distinct stages: slight change, then drastic reduction, and finally slowing down. Head loss coefficient and discharge coefficient of the valve change as the valve closing time shortens.

Comparison of ultrasound vector flow imaging and CFD simulations with PIV measurements of flow in a left ventricular outflow trackt phantom - Implications for clinical use and in silico studies

In this study we have compared two modalities for flow quantification from measurement data; ultrasound (US) and shadow particle image velocimetry (PIV), and a flow simulation model using computational fluid dynamics (CFD). For the comparison we have used an idealized Quasi-2D phantom of the human left ventricular outflow tract (LVOT). The PIV data will serve as a reference for the true flow field in our setup. Furthermore, the US vector flow imaging (VFI) data has been post processed with model-based regularization developed to both smooth noise and sharpen physical flow features. The US VFI flow reconstruction results in an underestimation of the flow velocity magnitude compared to PIV and CFD. The CFD results coincide very well with the PIV flow field maximum velocities and curl intensity, as well as with the detailed vortex structure, however, this correspondence is subject to exact boundary conditions.

Computational fluid dynamic simulation of gas-liquid flow in rotating packed bed: A review

The rotating packed bed (RPB) has been widely used in gas-liquid flow systems as a process intensification device, exhibiting excellent mass transfer enhancement characteristics. However, the complex internal structure and the high-speed rotation of the rotor in RPB bring significant challenges to study the intensification mechanism by experiment methods. In the past two decades, Computational fluid dynamics (CFD) has been gradually applied to simulate the hydrodynamics and mass transfer characteristics in RPB and instruct the reactor design. This article covers the development of the CFD simulation of gas-liquid flow in RPB. Firstly, the improvement of the simulation method in the aspect of mathematical models, geometric models, and solving methods is introduced. Secondly, new progress of CFD simulation about hydrodynamic and mass transfer characteristics in RPB is reviewed, including pressure drop, velocity distribution, flow pattern, and concentration distribution, etc. Some new phenomena such as the end effect area with the maximum turbulent have been revealed by this works. In addition, the exploration of developing new reactor structures by CFD simulation is introduced and it is proved that such new structures are competitive to different applications. The defects of current research and future development directions are also discussed at last.

CFD Simulation of Solid and Non-Newtonian Fluid Two-Phase Flow in Anaerobic Digesters

A CFD model is used to study solid suspension in anaerobic digesters.Simulations of solid and non-Newtonian fluid two-phase flow are performed.Minimum impeller speed for avoiding solid accumulation in the digester is proposed.Major issues arising from the simulation results are discussed.Abstract. A principal goal of solid-liquid flow modelling is to predict the minimum power consumption for avoiding solid accumulation in stirred reactors. In this study, a computational fluid dynamics (CFD) technique was used to characterize solid suspension in anaerobic digesters with mechanical mixers where the liquid phase exhibited non-Newtonian fluid behavior. The commercial CFD code Fluent 16.0 was utilized to solve the multiphase flow. The impeller torque and cloud height as a function of impeller speed in a small tank were validated against the experimental data from the literature. The simulation results demonstrate that the impeller torques from CFD modeling match perfectly with those from measurements, and that the predicted cloud heights agree reasonably well with the measured ones. The solid-liquid flow in a large egg-shaped digester with a mechanical draft tube mixer was modeled where the impeller speed ranged from 400 to 750 rpm at 50 rpm increments. A combination of tangent line and uniformity index methods was used to predict the minimum impeller speed to maintain solid suspension in the digester. Moreover, the major issues arising from results were discussed. Keywords: Anaerobic digester, Computational fluid dynamics, Multiphase flow, Non-Newtonian fluid, Solid suspension.