Transient Computational Fluid Dynamics Simulations Research Articles

Myocardial bridging and endothelial dysfunction – Computational fluid dynamics study

Myocardial bridging (MB) is associated with endothelial dysfunction in patients with angina and non-obstructive coronary artery disease. This study aims to determine if there is a link between abnormal blood flow patterns and endothelial dysfunction in patients with MB. Ten patients with MB in their left anterior descending (LAD) artery were selected, 5 of whom had endothelial dysfunction and 5 had no endothelial dysfunction based on their response to acetylcholine. Similarly, 10 patients without MB in their LAD, 5 of whom had endothelial dysfunction and 5 of whom had no endothelial dysfunction, were studied as a control group. Transient computational fluid dynamics simulations were performed to derive wall shear stress (WSS) over the entire vessel including proximal, middle and distal segments. Patients with MB and endothelial dysfunction had lower WSS in the proximal LAD and greater WSS in the mid-LAD than patients with MB but without endothelial dysfunction. When comparing patients with endothelial dysfunction, those with MB had significantly lower shear stress in the proximal LAD (0.32 ± 0.14 Pa (with MB) vs 0.71 ± 0.38 Pa (without MB), p = 0.01) and greater shear stress in the mid-LAD (2.81 ± 1.20 Pa (with MB) vs 1.66 ± 0.31 Pa (without MB), p = 0.014) than patients without MB. Our findings demonstrated that the presence of MB significantly contributes to low WSS and endothelial dysfunction relationship.

TWO-DIMENSIONAL NONISOTHERMAL NUMERICAL ANALYSIS OF SPEED RATIO EFFECTS ON DISPERSION AND DISTRIBUTION IN HIGH-VISCOSITY PARTIALLY FILLED RUBBER MIXING

ABSTRACT Two-dimensional, transient, and nonisothermal computational fluid dynamics simulations are conducted for high-viscosity rubber mixing in a two-wing rotor-equipped partially filled chamber of fill factor 75%. Calculations presented assess the effect of three differential speeds or speed ratios of the two rotors for the rubber mixing process: 1.0 (also called even speed), 1.125, and 1.5. A Eulerian multiphase model, the volume of fluid technique, is employed to simulate two different phases, rubber and air, by calculating the free surface between the two phases, in addition to the main governing equations such as the continuity, momentum, and energy equations. To characterize the non-Newtonian, highly viscous rubber under nonisothermal conditions, the shear rate–dependent Carreau-Yasuda model along with an Arrhenius function are employed. A set of massless particles is introduced into the chamber to calculate several parameters related to dispersive and distributive mixing characteristics. Specifically, the mixing index and maximum shear stress are analyzed for the dispersive nature, whereas cluster distribution index and length of stretch are calculated for investigating the distributive nature of the mixing process. Also, the temporal viscous heat generation rate, a good indication of the temperature rise throughout the domain, which is critical in the process and equipment design, is analyzed here. Results showed that the 1.125 speed ratio was the most efficient in terms of distributive mixing and heat generation.

Optimal shape of thick blades for a hydraulic Savonius turbine

Ever-growing energy demands and unacceptable emissions from fossil fuel combustion are major driving forces for expanding alternative, green energy sources. Hydro-power is one promising ecological alternative to meet these energy requirements. Such systems do not require any weir or dam and, thus, can be employed with minimum ecological impact. However, available designs are not yet suitable as efficient water energy converters, in particular for conditions corresponding to low water speeds (as mostly found in practice), due to low power output. Savonius turbines are particularly robust and cost-efficient, but show a poor efficiency. This article aims at maximizing the output power of a hydraulic Savonius turbine by modifying the blade profile. The main difference in this work compared to previous studies is that the blade shape of the concave and convex sides evolve independently from each other (no constant blade thickness). Twelve geometrical parameters are involved during shape optimization. To obtain optimal conditions, many transient computational fluid dynamics (CFD) simulations are performed using the industrial flow simulation code Star-CCM+, driven by the in-house optimization library OPAL++ relying on evolutionary algorithms. The optimization process takes into account the output power coefficient (Cp) as a target function. Compared to the traditional Savonius turbine, a relative increase of Cp by almost 12% is obtained at a tip speed ratio of 1.1. The performance of the optimal geometry was compared to the standard design over the whole range of operation. This comparison revealed additionally that the performance was improved by almost 15% at a tip speed ratio of 1.2. The performance of the optimal design is superior for the whole range of operation, particularly at a tip speed ratio exceeding 0.8. Finally, it was checked that the optimal design is still self-starting.

Evaluation of the hemolysis and fluid dynamics of a ventricular assist device under the pulsatile flow condition

When the rotary blood pumps are used as ventricular assist devices, the pump flow rate will have a pulsatile component even at a constant impeller rotational speed due to the remaining beating of the natural heart. However, previous studies on the in vitro hemolysis evaluation of a rotary blood pump have always been conducted under steady states and this pulsation was not taken into account. In this study, the hemolysis in a centrifugal blood pump is evaluated under the pulsatile flow condition in vitro. The required time-varying flow rate is obtained by conducting a system simulation of the pump-assisted cardiovascular system, and realized by controlling a pulsation unit in the experiments. The results of our tests indicate a significant increase in hemolysis under the pulsatile flow condition compared with the non-pulsatile condition. To reveal the flow characteristics responsible for the higher hemolysis, transient computational fluid dynamic simulations are then performed. This study suggests that traditional hemolysis evaluation under the steady states may not fully represent the hemolytic performance in the clinical use. For the ventricular assist pumps at the design stage, eliminating the concern about the extra hemolysis under the pulsatile condition will be helpful for the subsequent in vivo experiments.

3D numerical investigations of the effect of fill factor on dispersive and distributive mixing of rubber under non‐isothermal conditions

Three‐dimensional, non‐isothermal, transient computational fluid dynamics simulations are conducted for rubber mixing with a set of two‐wing rotors in a partially filled chamber. The main objective is to analyze the effect of different fill factors of rubber on dispersive and distributive mixing characteristics by simulating 15 revolutions of the rotors rotating at 20 rpm. 60%, 70%, 75%, and 80% are the four different fill factors chosen for the study. An Eulerian multiphase model is employed to simulate two different phases, rubber and air, and the volume of fluid technique is used to calculate the free surface between two phases, in addition to the continuity, momentum and Energy equations. To characterize non‐Newtonian, highly viscous rubber, shear rate and temperature dependent Carreau‐Yasuda model has been used. A set of more than 3,600 massless particles are injected after a certain period of time to calculate several quantities in terms of dispersive and distributive mixing. Both the Eulerian and Lagrangian results showed that, fill factors between 70% and 80% presented the most reasonable and efficient mixing scenario, thus exhibiting the best dispersive and distributive mixing characteristics combined. POLYM. ENG. SCI., 59:535–546, 2019. © 2018 Society of Plastics Engineers

A numerical study on flow patterns inside an electrical submersible pump (ESP) and comparison with visualization experiments

This paper presents a numerical study of flow pattern recognition inside the rotating impeller of an electrical submersible pump (ESP) using the transient multiphase Computational Fluid Dynamics (CFD) simulations. The calculation domain is constructed based on the previous experimental facility for visualizing flow patterns in an ESP (Barrios (2007)). The high-quality structured mesh comprising hexahedral grids is generated using multi-block technique in ANSYS ICEM. For CFD simulations, the realizable k-ε turbulence model with volume of fluid (VOF) and Eulerian-Eulerian multiphase models is successfully implemented in ANSYS Fluent solver. The sliding mesh technique is applied to interfaces where rotating and stationary parts interact. By incorporating the same boundary conditions as Barrios experimental study, three flow cases with constant gas flow rates and varying liquid flow rates are selected to conduct numerical simulations. The comparison of simulation results with Barrios’ observations shows that the Eulerian-Eulerian multiphase model is superior to VOF model for simulating gas-liquid two-phase flow in a rotating ESP. The single-phase simulation results match catalog curves, which validates the numerical methodology. For two-phase simulations, the simulated flow patterns using Eulerian-Eulerian model agree well with visualization experiments. Different flow patterns prevailing inside the rotating ESP impeller are captured. At low gas flow rate, bubbles are dispersed in liquid phase, and the flow pattern is categorized as dispersed bubble flow. As gas flow rate increases, bubbles can accumulate and coalesce, causing large gas-pocket formation leading to intermittent/slug flow. Transient multiphase CFD simulation is an efficient and reliable tool to predict flow patterns inside ESPs.

Competing Flow Between Partial Circulatory Support and Native Cardiac Output: A Clinical Computational Fluid Dynamics Study.

Partial circulatory support is a promising concept for the treatment of heart failure patients. A better understanding of induced hemodynamic changes is essential for optimizing treatment efficacy. Computational fluid dynamics (CFD) is an alternative method to gain insight into flow phenomena difficult to obtain in vivo. In 10 patients implanted with a Circulite Synergy Micro-pump (HeartWare, Framingham, Massachusetts) (a continuous flow partial circulatory assist device connecting the left atrium to the right subclavian artery), transient CFD simulations were performed. Patients were divided into two groups depending on their cardiac output (CO; high CO group: 5.5 ± 1.1 L/min, low CO group: 1.7 ± 0.7 L/min). The partial assist device provided a supporting flow of 1.5 ± 0.8 L/min. Support was highest at diastole and decreased during systole because of a collision of the blood flows from the partial assist device and the CO. Reversed flow counteracting the flow of the device was significantly higher for the high CO group (mean flow in peak systole: -2.18 ± 1.08 vs. 0.23 ± 0.59 L/min; p = 0.002) showing an inverse correlation between CO and amount of reversed flow during peak systole (R = -0.7; p < 0.02). The flow collision lead to higher total pressures at the point of collision and consequently in the Circulite outflow graft. The CFD simulations allow quantifying hemodynamic alterations in patients with partial support consisting of a flow collision, thereby reducing effectiveness of the circulatory support. Partial support in heart failure patients alternates their hemodynamics not only in providing support for the circulation but also inducing unfavorable changes in flow patterns.

A computational framework for adjusting flow during peripheral extracorporeal membrane oxygenation to reduce differential hypoxia

Peripheral veno-arterial extra corporeal membrane oxygenation (VA-ECMO) is an established technique for short-to-medium support of patients with severe cardiac failure. However, in patients with concomitant respiratory failure, the residual native circulation will provide deoxygenated blood to the upper body, and may cause differential hypoxemia of the heart and brain. In this paper, we present a general computational framework for the identification of differential hypoxemia risk in VA-ECMO patients. A range of different VA-ECMO patient scenarios for a patient-specific geometry and vascular resistance were simulated using transient computational fluid dynamics simulations, representing a clinically relevant range of values of stroke volume and ECMO flow. For this patient, regardless of ECMO flow rate, left ventricular stroke volumes greater than 28 mL resulted in all aortic arch branch vessels being perfused by poorly-oxygenated systemic blood sourced from the lungs. The brachiocephalic artery perfusion was almost entirely derived from blood from the left ventricle in all scenarios except for those with stroke volumes less than 5 mL. Our model therefore predicted a strong risk of differential hypoxemia in nearly all situations with some residual cardiac function for this combination of patient geometry and vascular resistance. This simulation highlights the potential value of modelling for optimising ECMO design and procedures, and for the practical utility for personalised approaches in the clinical use of ECMO.

Two-step method for radiative transfer calculations in a developing pool fire at the initial stage of its suppression by a water spray

A procedure based on two-step method is suggested to simplify time-consuming spectral radiative transfer calculations in open flames containing scattering particles. At the first step of the problem solution, the P1 approximation is used to calculate the divergence of radiative flux, and it is sufficient to determine the flame parameters. The second step of solution is necessary to obtain the radiation field outside the flame, and this can be made independently using the ray-tracing procedure and the transport source function determined at the first step. Such a splitting of the complete problem results in much simpler algorithm than those used traditionally. It has been proved in previous papers that the combined two-step method is sufficiently accurate in diverse engineering applications. At the same time, the computational time decreases in about two orders of magnitude as compared with direct methods. An axisymmetric pool fire at the initial stage of fire suppression by a water spray is considered as the case problem. It is shown that evaporating small water droplets characterised by a strong scattering of infrared radiation are mainly located in regions near the upper front of the flame and one can observe the scattered radiation. This effect can be used in probe experiments for partial validation of transient Computational Fluid Dynamics (CFD) simulations.

Sea-unammned aerial vehicle takeoff characteristics analysis method based on approximate equilibrium hypothesis

In this paper, transient multiphase flow computational fluid dynamics simulations based on volume of fluid model are conducted for a sea-unmanned aerial vehicle. The approximate equilibrium hypothesis is implemented after estimating the acceleration in the vertical direction. The complete configuration model and hull model are employed in simulation to predict the aerodynamic and hydrodynamic forces separately for different demands of aerodynamic and hydrodynamic computational fluid dynamics predictions and computing efficiency. In takeoff characteristics analysis, the computational fluid dynamics simulations are conducted as inputs for piecewise interpolation method. The calculated results show that the sea-unmanned aerial vehicle takeoff characteristics are totally different from a conventional aircraft. The drag-peak at hump speed is the obvious feature of the sea-unmanned aerial vehicle/seaplane. In most cases, if a sea-unmanned aerial vehicle will takeoff successfully as long as it can pass the drag peak. The takeoff distance and time calculated by piecewise interpolation method match the experimental data within 7% deviation. The accuracy is acceptable for conceptual design stage of a sea-unmanned aerial vehicle/seaplane. The results are applicable to consultation in choosing takeoff field or choosing powerplant.

Nitrogen Injection in Molten Aluminum in a Tank Degasser

The injection of nitrogen in molten aluminum through a static impeller in a tank degassing unit is studied. Using basic principles of fluid mechanics, it is analyzed the influence of the nozzle diameter on the bubble diameter and the mean residence time of the bubbles in the molten aluminum. By means of transient isothermal 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions and the tank degasser dynamics is studied. Finally, an adiabatic CFD simulation is carried out in order to elucidate the changes in the molten aluminum temperature due to the injection of nitrogen at ambient temperature. This simulation shows that molten aluminum does not suffer drastic temperature reductions given that, in spite that the nitrogen is fed at ambient temperature, the mass of nitrogen is relatively small compared with the mass of aluminum.

Prediction of growth of jet fuel autoxidative deposits at inner surface of a replicated jet engine burner feed arm

A low thermally stable jet A-1 fuel was heated up to 115 h under engine representative condition in a simulated burner feed arm with cylindrical shape using “Aviation Fuel Thermal Stability Test Unit (AFTSTU)”. The local growth of surface carbonaceous deposits was measured by the use of three K-type thermocouples which were inserted in discrete locations along the inner surface of heated tube. Subsequently, the deposited tube was sectioned and prepared for “Scanning Electron Microscopy(SEM)” to visualise the circumferential profile of the carbonaceous deposit. This helped to identify the axial profile of deposit thickness by integration of the average deposit thickness at five cross sections along the tube. Using the temperature rise data, a one dimensional, analytical heat transfer model was used to calculate local deposit thickness as a function of time.A transient fuel dependent, two stage chemical kinetic model was developed to simulate the growth of deposit as a function of time, temperature, and fuel chemical composition. In this model, the growth of deposit was assumed to be equal as a geometrical displacement in radial direction. Such a displacement is explicitly calculated using an initial rate of deposition(steady state at t = 0) and accelerating deposition rate(at t=t+Δt) to account for the non linear rate of deposition over the time period of thermal exposure. The concentration of insoluble species and initial rate of deposition as well as temperature at the adjacent layer to the heated surface were obtained through a steady state “computational fluid dynamics(CFD)” simulation using pseudo detailed mechanism of fuel autoxidation by Kuprowicz et al. (2007). The rate parameters of the accelerating deposition were optimised by the application of pattern search method with respect to the calculated deposit thickness from one dimensional heat transfer model. Eventually, the optimised model was used with a transient CFD simulation with dynamic mesh using Ansys Fluent commercial package to predict the growth of deposit in a reactive flow medium. The predicted results are in good agreement with the experimental data obtained from visualised deposit.

Shock waves generated by sudden expansions of a water jet

Direct shadowgraph with parallel light combined with high-speed recording has been used to analyze the water jet of a cutting machine. The use of image processing allowed observing sudden expansions in the jet diameter as well as estimating the jet velocity by means of the Mach angle, obtaining velocities of about $$500\,\hbox {m}\,\hbox {s}^{-1}$$ . The technique used here revealed the development of hydrodynamic instabilities in the jet. Additionally, this is the first reporting of the onset of shock waves generated by small fluctuations of a continuous flow of water at high velocity surrounded by air, a result confirmed by a transient computational fluid dynamics simulation.

Modelling the generation of temperature inhomogeneities by a premixed flame

The response of a laminar, premixed flame to perturbations of upstream equivalence ratio is investigated and modelled, with emphasis on the generation of ‘entropy waves’, i.e. entropic inhomogeneities of downstream temperature. Transient computational fluid dynamics simulations of two adiabatic lean methane-air flames of different Péclet numbers provide guidance and validation data for subsequent modelling. The respective entropy transfer functions, which describe the production of temperature inhomogeneities, as well as transfer functions for the variation of the heat release, are determined from the computational fluid dynamics time series data by means of system identification. The processes governing the dynamics of the entropy transfer functions are segregated into two sub-problems: (1) heat release due to chemical reaction at the flame front and (2) advective and diffusive transport. By adopting a formulation in terms of a mixture fraction variable, these two sub-problems can be treated independently from each other. Models for both phenomena are derived and analysed using simple 0- and 1-dimensional configurations. The heat release process (1) is represented by a fast-reaction-zone model, which takes into account variations of the specific heat capacity with equivalence ratio in order to evaluate the magnitude of downstream temperature fluctuations with quantitative accuracy. For the transport processes (2), two types of models based on mean field data from the computational fluid dynamics simulation are proposed: A semi-analytical, low-order formulation based on stream lines, and a state-space formulation, which is constructed by Finite Elements discretisation of the transport equation for mixture fraction. Model predictions for the entropy transfer functions are found to agree well with the computational fluid dynamics reference data at very low computational costs.

Impact of Side Branches on the Computation of Fractional Flow in Intracranial Arterial Stenosis Using the Computational Fluid Dynamics Method

Computational fluid dynamics (CFD) allows noninvasive fractional flow (FF) computation in intracranial arterial stenosis. Removal of small artery branches is necessary in CFD simulation. The consequent effects on FF value needs to be judged. An idealized vascular model was built with 70% focal luminal stenosis. A branch with one third or one half of the radius of the parent vessel was added at a distance of 5, 10, 15 and 20 mm to the lesion. With pressure and flow rate applied as inlet and outlet boundary conditions, CFD simulations were performed. Flow distribution at bifurcations followed Murray's law. By including or removing side branches, five patient-specific intracranial artery models were simulated. Transient simulation was performed on a patient-specific model, with a larger branch for validation. Branching effect was considered trivial if the FF difference between paired models (branches included or removed) was within 5%. Compared with the control model without a branch, in all idealized models the relative differences of FF was within 2%. In five pairs of cerebral arteries (branches included/removed), FFs were 0.876 and 0.877, 0.853 and 0.858, 0.874 and 0.869, 0.865 and 0.858, 0.952 and 0.948. The relative difference in each pair was less than 1%. In transient model, the relative difference of FF was 3.5%. The impact of removing side branches with radius less than 50% of the parent vessel on FF measurement accuracy is negligible in static CFD simulations, and minor in transient CFD simulation.

Shape optimization of a Savonius wind rotor with different convex and concave sides

Abstract This paper introduces an optimization procedure of a modified Savonius rotor with different convex and concave sides to maximize the power efficiency. A series of transient computational fluid dynamics (CFD) simulations are performed to find the peak coefficients of power (Cp) of each blade geometry. Then, a global response surface model is created according to the Kriging Method, which defines the relationship between optimization objective Cp and the design parameters. A Particle Swarm Optimization (PSO) algorithm is applied to finding the optimal design based on the response surface model. The optimal Cp is 0.2580 and is 4.41% higher than the traditional design. Comprehensive comparisons of torque, power and flow structures between the optimal and the traditional designs are performed to illustrate the mechanism of how the blade shapes improve the rotor performance. It is find found that the optimal blade shape has stronger tip vortices and recovery flows, which contributes to an increase in the performance of the rotor.

Effects of contamination and erosion at the leading edge of blade tip airfoils on the annual energy production of wind turbines

Contamination and erosion at the leading edge of blade tips adversely affect the annual energy production (AEP) of wind turbines. Obtaining pertinent quantitative data would help in the efficient planning of operations and maintenance of these turbines. In this study, to quantitatively analyze the effects of contamination and erosion on the aerodynamic performance of a blade tip airfoil (NACA 64-618) and in turn on the AEP loss of wind turbines, transient computational fluid dynamics simulations and AEP calculations were performed for a 5-MW National Renewable Energy Laboratory wind turbine. The simulations indicated that depending on the severity of the conditions, contamination and erosion conditions reduced and increased the lift and drag coefficients, respectively, of the blade by up to 53% and 314%. Contamination and erosion conditions reduced AEP by 2%–3.7%.

Thermal performance of a concrete cask: Methodology to model helium leakage from the steel canister

Concrete cask storage systems used in dry storage allocate spent fuel within containers that are usually filled with helium at a certain pressure. Potential leaks from the container would result in a cooling degradation of fuel that might jeopardize fuel integrity if temperature exceeded a threshold value. According to ISG-11, temperatures below 673K ensure fuel integrity preservation. Therefore, the container thermal response to a loss of leaktightness is of utmost importance in terms of safety.In this work, a thermo-fluid dynamic analysis of the canister during a loss of leaktightness has been performed. To do so, steady-state and transient Computational Fluid Dynamics (CFD) simulations have been carried out. Likewise, it has been developed two methodologies capable of estimating peak fuel temperatures and heat up rates resulting from a postulated depressurization in a dry storage cask. One methodology is based on control theory and transfers functions, and the other methodology is based on a linear relationship between the inner pressure and the maximum temperature. Both methodologies have been verified through comparisons with CFD calculations.The period of time to achieve the temperature threshold (673K) is a function of pressure loss rate and decay heat of the fuel stored in the container; in case of a fuel canister with 30kW the period of time to reach the thermal limit takes between half day (fast pressure loss) and one week (slow pressure loss). In case of a 15% reduction of the decay heat, the period of time to achieve the thermal limit increase up to a few weeks. The results highlight that casks with heat loads below 20kW would never exceed the thermal threshold (673K).

Numerical Simulations of a VAWT in the Wake of a Moving Car

Wind energy generated from the wake of moving cars has a large energy potential that has not yet been utilized. In this study, a vertical axis wind turbine (VAWT) was used to recover energy from the wakes of moving cars. The turbine was designed to be planted by the side of the car lane and driven by the wake produced by the car. Transient computational fluid dynamics (CFD) simulations were performed to evaluate the performance of the VAWT. The influence of two main factors on the performance of the VAWT, the velocity of the car and the gap between the car and the rotor, were studied. The simulations confirmed the feasibility of this plan, and in the tested cases, the VAWT was able to generate a maximum energy output of 100.49 J from the wake of a car. The results also showed that the performance of the VAWT decreased with the velocity of the car, and the increased gap between the car and the VAWT.

Nanofluid-based cooling of cylindrical lithium-ion battery packs employing forced air flow

Efficient cooling strategies are very important issues in thermal management of lithium-Ion battery system and these techniques should provide cost effective and energy saving solutions for temperature rise of the system during battery operation. A pseudo 2D electrochemical model has been used to investigate the electrochemical and thermal condition of commercial 18650 Lithium-Ion battery. This analysis was compared to previous experimental studies and results showed that the model predicts the temperature rise and thermal power generation very well. Results from the electro-thermal analysis were used to examine a cooling method. In this strategy, the cylindrical Li-ion cell is submersed within a thin cylindrical tank containing water-Al2O3 nanofluid. Air flow is employed to remove the heat from the system during discharge process. After testing this method for a single cell with various sizes of the secondary cylinder, critical and thermally dangerous arrangements of Li-ion cells were analyzed employing the new technique. 3D transient Computational Fluid Dynamics (CFD) simulations were used to see the effect of presence of the secondary cylinder on temperature rise. Results revealed that the new method reduces the maximum temperature of the cells efficiently.