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ObjectiveThis study aims at developing a pipeline that provides the capability to include the catheter effect in the computational fluid dynamics (CFD) simulations of the cardiovascular system and other human vascular flows carried out with the open-source software SimVascular. This tool is particularly useful for CFD simulation of interventional radiology procedures such as tumor embolization where estimation of a therapeutic agent distribution is of interest.ResultsA pipeline is developed that generates boundary condition files which can be used in SimVascular CFD simulations. The boundary condition files are modified such that they simulate the effect of catheter presence on the flow field downstream of the inlet. Using this pipeline, the catheter flow, velocity profile, radius, wall thickness, and deviation from the vessel center can be defined. Since our method relies on the manipulation of the boundary condition that is imposed on the inlet, it is sensitive to the mesh density. The finer the mesh is (especially around the catheter wall), the more accurate the velocity estimations are. In this study, we also utilized this pipeline to qualitatively investigate the effect of catheter presence on the flow field in a truncated right hepatic arterial tree of a liver cancer patient.

Inlet boundary conditions (BC) are one of the uncertainties which may influence the prediction of flow field and hemolysis in blood pumps. This study investigated the influence of inlet BC, including the length of inlet pipe, type of inlet BC (mass flow rate or experimental velocity profile) and turbulent intensity (no perturbation, 5%, 10%, 20%) on the prediction of flow field and hemolysis of a benchmark centrifugal blood pump (the FDA blood pump) and a commercial axial blood pump (Heartmate II), using large-eddy simulation. The results show that the influence of boundary conditions on integral pump performance metrics, including pressure head and hemolysis, is negligible. The influence on local flow structures, such as velocity distributions, mainly existed in the inlet. For the centrifugal FDA blood pump, the influence of type of inlet BC and inlet position on velocity distributions can also be observed at the diffuser. Overall, the effects of position of inlet and type of inlet BC need to be considered if local flow structures are the focus, while the influence of turbulent intensity is negligible and need not be accounted for during numerical simulations of blood pumps. [ABSTRACT FROM AUTHOR]

Highly transient engine-out emissions imply significant challenges for the optimization and control of automotive aftertreatment systems, motivating studies of the effects of flow pulsations on the system behavior. In this work, an axisymmetric aftertreatment system with a first-order reaction in the monolith section is chosen to demonstrate the role of pulsations on the time-averaged conversion at the exit. Reactive computational fluid dynamics simulations under transient conditions are performed by applying the SST k- ω turbulence model along with a reactant species balance equation and a porous medium description of the catalyst. Four different types of temporal velocity variations (constant, step-like, sawtooth and sinusoidal) are applied at the inlet. Additionally, the corresponding fluctuations driven by a prescribed inlet pressure are also investigated. It was found that the fluctuations in the incoming flow affect the transient response of the monolith, the time-averaged conversion, the evolution of the flow uniformity index and the dispersion downstream of the catalyst. It is also shown that the retention time distribution is modulated by the pulsations and that the mixed-cup conversion span is different for geometrically identical systems having the same velocity span if the fluctuation characteristics are different. In conclusion, simulations of phenomena that depend on time-resolved boundary conditions from experiments require proper characterization of fluctuations present in the real-world systems; otherwise, the method of recreating the signal at the boundary may influence the obtained results. [ABSTRACT FROM AUTHOR]

Abstract Objective This study aims at developing a pipeline that provides the capability to include the catheter effect in the computational fluid dynamics (CFD) simulations of the cardiovascular system and other human vascular flows carried out with the open-source software SimVascular. This tool is particularly useful for CFD simulation of interventional radiology procedures such as tumor embolization where estimation of a therapeutic agent distribution is of interest. Results A pipeline is developed that generates boundary condition files which can be used in SimVascular CFD simulations. The boundary condition files are modified such that they simulate the effect of catheter presence on the flow field downstream of the inlet. Using this pipeline, the catheter flow, velocity profile, radius, wall thickness, and deviation from the vessel center can be defined. Since our method relies on the manipulation of the boundary condition that is imposed on the inlet, it is sensitive to the mesh density. The finer the mesh is (especially around the catheter wall), the more accurate the velocity estimations are. In this study, we also utilized this pipeline to qualitatively investigate the effect of catheter presence on the flow field in a truncated right hepatic arterial tree of a liver cancer patient.

Inlet boundary conditions (BC) are one of the uncertainties which may influence the prediction of flow field and hemolysis in blood pumps. This study investigated the influence of inlet BC, including the length of inlet pipe, type of inlet BC (mass flow rate or experimental velocity profile) and turbulent intensity (no perturbation, 5%, 10%, 20%) on the prediction of flow field and hemolysis of a benchmark centrifugal blood pump (the FDA blood pump) and a commercial axial blood pump (Heartmate II), using large-eddy simulation. The results show that the influence of boundary conditions on integral pump performance metrics, including pressure head and hemolysis, is negligible. The influence on local flow structures, such as velocity distributions, mainly existed in the inlet. For the centrifugal FDA blood pump, the influence of type of inlet BC and inlet position on velocity distributions can also be observed at the diffuser. Overall, the effects of position of inlet and type of inlet BC need to be considered if local flow structures are the focus, while the influence of turbulent intensity is negligible and need not be accounted for during numerical simulations of blood pumps.

For large eddy simulation, it is critical to choose the suitable turbulent inlet boundary condition as it significantly affects the calculated flow field. In this paper, the effect of different inlet boundary conditions, including random method (RAND), Lund method, and divergence-free synthetic eddies method (DFSEM), on the flow in a channel with a hump are investigated through large-eddy simulation. The simulation results are further compared with experimental data. It has been found that turbulence is nearly fully developed in the case based on the Lund method, not fully developed in the case based on DFSEM, and not developed in the case based on the RAND method. In the flow region before the hump, mean velocity profiles in the case applying the Lund method gradually fit the law of the wall as the main flow moves towards the hump, but the simulation results based on the RAND and DFSEM methods cannot fit the wall function. In the flow region after the hump, cases applying Lund and DFSEM methods could relative precisely predict the size of turbulent bubble and turbulent statistics profiles. Meanwhile, the case based on the RAND method cannot capture the positions of flow separation and re-attachment point and overestimates the turbulent bubble size. From this research, it could be found that different turbulent inflow generation methods have a manifested impact on the flow separation and re-attachment after the hump. If the coherent turbulence is maintained in the approach flow, even though turbulent intensity is not large enough, the simulation can still predict the flow separation and turbulent bubble size relative precisely. [ABSTRACT FROM AUTHOR]

In order for computational fluid dynamics to provide quantitative parameters to aid in the clinical assessment of type B aortic dissection, the results must accurately mimic the hemodynamic environment within the aorta. The choice of inlet velocity profile (IVP) therefore is crucial; however, idealised profiles are often adopted, and the effect of IVP on hemodynamics in a dissected aorta is unclear. This study examined two scenarios with respect to the influence of IVP—using (a) patient-specific data in the form of a three-directional (3D), through-plane (TP) or flat IVP; and (b) non-patient-specific flow waveform. The results obtained from nine simulations using patient-specific data showed that all forms of IVP were able to reproduce global flow patterns as observed with 4D flow magnetic resonance imaging. Differences in maximum velocity and time-averaged wall shear stress near the primary entry tear were up to 3% and 6%, respectively, while pressure differences across the true and false lumen differed by up to 6%. More notable variations were found in regions of low wall shear stress when the primary entry tear was close to the left subclavian artery. The results obtained with non-patient-specific waveforms were markedly different. Throughout the aorta, a 25% reduction in stroke volume resulted in up to 28% and 35% reduction in velocity and wall shear stress, respectively, while the shape of flow waveform had a profound influence on the predicted pressure. The results of this study suggest that 3D, TP and flat IVPs all yield reasonably similar velocity and time-averaged wall shear stress results, but TP IVPs should be used where possible for better prediction of pressure. In the absence of patient-specific velocity data, effort should be made to acquire patient's stroke volume and adjust the applied IVP accordingly. [ABSTRACT FROM AUTHOR]

Highly transient engine-out emissions imply significant challenges for the optimization and control of automotive aftertreatment systems, motivating studies of the effects of flow pulsations on the system behavior. In this work, an axisymmetric aftertreatment system with a first-order reaction in the monolith section is chosen to demonstrate the role of pulsations on the time-averaged conversion at the exit. Reactive computational fluid dynamics simulations under transient conditions are performed by applying the SST k-ω turbulence model along with a reactant species balance equation and a porous medium description of the catalyst. Four different types of temporal velocity variations (constant, step-like, sawtooth and sinusoidal) are applied at the inlet. Additionally, the corresponding fluctuations driven by a prescribed inlet pressure are also investigated. It was found that the fluctuations in the incoming flow affect the transient response of the monolith, the time-averaged conversion, the evolution of the flow uniformity index and the dispersion downstream of the catalyst. It is also shown that the retention time distribution is modulated by the pulsations and that the mixed-cup conversion span is different for geometrically identical systems having the same velocity span if the fluctuation characteristics are different. In conclusion, simulations of phenomena that depend on time-resolved boundary conditions from experiments require proper characterization of fluctuations present in the real-world systems; otherwise, the method of recreating the signal at the boundary may influence the obtained results.

Hemolysis is one of the most important issues of blood contacting artificial organs. Computational fluid dynamics, in conjunction with hemolysis models, has been widely used to predict the flow field and hemolysis during the development phase of these devices. It is widely accepted that hemolysis is related to flow field parameters, such as stress and energy dissipation. It is known that inlet boundary conditions such as turbulent intensity have important effects on flow fields. Nonetheless, the influence of inlet turbulent intensity on hemolysis is not yet clear. This study investigates the influence of turbulent intensity on the prediction of flow field and hemolysis in the FDA benchmark nozzle model. Three configurations are investigated: first is the original nozzle model (with a gradual contraction, throat, and sudden expansion); the second is composed only of the throat and sudden expansion; the third one is similar with the second, but with a shorter throat. Four turbulent intensities are considered, ranging from 1% to 20%. For the first configuration, the influence of turbulent intensity on both flow field and hemolysis is very small and limited prior to the gradual contraction, while for the other two configurations, the influence is considerable. The jet breaks down sooner with increased turbulent intensity. Both turbulence dissipation rate and Reynolds stress increase in the throat with increased turbulent intensity, but decrease after the jet breaks down. The influence of turbulent intensity is more considerable for the configuration with a shorter inlet. The influence of turbulent intensity on the prediction of hemolysis is up to 38.5%. This study shows that turbulent intensity can have considerable influence on the prediction of flow field and hemolysis in blood-contacting devices, thus should be taken in account when conducting blood compatibility design. [ABSTRACT FROM AUTHOR]

In previous numerical simulations of local scour, the steady flow is normally adopted as the inlet boundary condition, and the approach flow is always assumed as a homogeneous isotropic turbulence. In the present study, the results turn out to be different from such previous cases, even without obstructions. Under conventional methods, the approach flow decays along the streamwise in the computational domain. Meanwhile, in order to ensure the simulation effect for some major projects such as the local scour around bridge piers and submarine pipelines, appropriate inlet boundary conditions and its self-sustaining character become some of the important factors. Therefore, in the present study, a novel self-sustaining model with the standard k-ɛ turbulence model is proposed to quickly obtain a homogeneous isotropic turbulence. The model parameters are discussed in the light of flume experiment data by sensitivity analysis. To verify the applicability of the proposed model and inlet boundary conditions, numerical simulations in an empty domain are also performed. It is shown that the novel self-sustaining model and corresponding inlet boundary can produce a homogeneous isotropic turbulence, which will help to guarantee fine numerical simulations for scour prediction with prescribed flow conditions. [ABSTRACT FROM AUTHOR]

When applying time-accurate computational fluid dynamics, large-eddy simulation (CFD LES) to room ventilation, a common problem is that the supply jets do not model properly if there is no realistic turbulence in the inflow boundary. In this paper we introduce a novel, easy-to-use method of generating a turbulent inlet boundary condition for simulating a radial diffuser. The basic idea is to apply a time-independent velocity distribution with realistic spatial statistics, and to add temporal fluctuations by rotation. The method is simple to use, requires only one extra parameter, and does not necessitate large storage files. It needs a momentary velocity distribution on the inflow surface with the desired volume flow rate obtained from a time-accurate internal simulation of the diffuser alone. In a room-scale simulation, this velocity distribution is put in rotating movement in order to mimic a turbulent inlet boundary. The method is verified by comparing the outcome with published measurements and with a simulation containing the interior of the diffuser. [ABSTRACT FROM AUTHOR]

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. • Detailed analyses of influence of inlet boundary conditions in CFD simulations. • Validation of RANS CFD simulations with measurement data from the literature. • ω-based turbulence models perform better than the ε-based turbulence models. • The inlet flow conditions can have a very strong effect on overall flow pattern. • Care should be taken in validation studies when inlet conditions are not known. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the effects of varying the system inlet and outlet boundary conditions were investigated based on three-dimensional computational fluid dynamics (CFD) simulation inside a high solid particle flux circulating fluidized bed riser. Four possible system inlet boundary conditions were considered with varying the gas inlet channel size at 25% (case 1), 50% (case 2), 75% (case 3) and 100% (case 4) of the maximal diameter compared to the system channel (0.050, 0.100, 0.150 and 0.200m, respectively). The solid volume fraction inside the system decreased as the gas inlet channel size increased. Varying the system outlet boundary conditions by changing the outlet radius from 0.025m to 0.050, 0.075 or 0.100m revealed that the solid volume fraction inside the system increased with decreasing outlet radii. These results compare well to those experimental and two-dimensional CFD simulations. In addition, the effects of mesh sizes or computational cells, time-averaged ranges and computational fluid dynamics simulation dimensions were discussed. [Copyright &y& Elsevier]

Abstract: The prescription of turbulent inflow boundary condition for large-eddy simulation (LES) is critical to obtain accurate predictions. In this paper, a stochastic model for generating inflow turbulence is implemented and compared to the method of generating inflow turbulence with random fluctuations. The LES studies of the effect of inflow boundary conditions are performed using a semi-implicit Legendre spectral element method. Two different backward-facing step configurations are simulated at Reynolds numbers 5000 and 28,000 utilizing the technique of implicit LES with no sub-grid model. Frequency analysis is performed at various points in the boundary layer and in the shear layer to study the effect of inflow boundary condition. The spectral method requires correlated velocity input data generated by the stochastic model for predicting the correct transition to turbulence at Re =5000, while the random fluctuations predict a delayed transition to turbulence. At Re =28,000, the two techniques of generating inflow turbulence give similar results. In addition, it is also observed that at Re =5000, the semi-implicit method requires small time step size to correctly predict transition to turbulence. The large time step size leads to laminarization of the flow in the inlet channel. This effect of the time step size is not observed in the case of Re =28,000. [Copyright &y& Elsevier]

Abstract: In the context of patient-specific cardiovascular applications, hemodynamics models (going from 3D to 0D) are often limited to a part of the arterial tree. This restriction implies the set up of artificial interfaces with the remaining parts of the cardiovascular system. In particular, the inlet boundary condition is crucial: it supplies the impulsion to the system and receives the reflected backward waves created by the distal network. Some aspects of this boundary condition need to be properly defined such as the treatment of backward waves (reflected or absorbed) and the value of the imposed hemodynamic wave (total or forward component). Most authors prescribe as inlet boundary condition (BC) the total measured variable (pressure, velocity or flow rate) in a reflective way. We show that with this type of inlet boundary condition, the model does not produce physiological waveforms. We suggest instead to prescribe only the forward component of the prescribed variable in an absorbing way. In this way, the computed reflected waves superpose with the prescribed forward waves to produce the total wave at the inlet. In this work, different inlet boundary conditions are implemented and compared for a 1D blood flow model. We test our boundary conditions on a truncated arterial model presented in the literature as well as on a patient-specific lower-limb model of a femoral bypass. We show that with this new boundary condition, a much better fitting is observed on the shape and intensity of the simulated pressure and velocity waves. [Copyright &y& Elsevier]

Abstract: A study of the influence of the inlet boundary condition on simulation results is presented. Experimental measurements and simulations of the flow over a hydrofoil in a closed circuit cavitation tunnel were performed. Using the LDA method the velocity profile, 365mm upstream of the hydrofoil, was measured. PIV method combined with the LIF technique was used for the determination of the velocity field in the vicinity of the hydrofoil. Static pressure at 38 positions on the surface of the hydrofoil was measured. The CFD code Fluent was used for the simulations. A RANS approach with the k–ε RNG turbulence model was applied. Two simulations of the fluid flow were performed – one with a uniform (ideal) velocity profile at the inlet of the computational domain and the other with the experimentally determined velocity profile. The simulation with the ideal boundary condition produced rather poor results with a considerable discrepancy to the experimental measurements. On the other hand the simulation produced more accurate results when the real (measured) boundary condition was used. The influence of the boundary condition could be seen in x- and y-velocity component in the vicinity of the hydrofoil, in c p distribution on the surface of the hydrofoil and in the evolution of the velocity profiles downstream of the inlet. It was concluded that it is essential to either use the real (measured) velocity profiles for the inlet boundary condition or to assure that the real upstream flow is almost ideally uniform, if an accurate simulation result is required. [Copyright &y& Elsevier]

Uniformity of coal and air has always been an important problem in the operation of large-scale CFB boilers. The selection of inlet boundary conditions is close related to the simulation accuracy, but seldom considered in the previous literatures. In this paper, a new method to estimate the distributions of coal supply was proposed, which was used as the boundary condition together with the experimental data to carry out combustion simulations on a 600 MW supercritical CFB boiler by using the Computational Particle Fluid Dynamics (CPFD) method. The simulation results were validated against the critical measured data including solid concentration, gas/solid circulation flow rate, gas temperature, oxygen concentration, etc., indicating that the model using the proposed boundary conditions could predict the combustion characteristics more accurately than that with common uniform boundary conditions. Based on the simulation results, the influence of air/coal distribution on the combustion in the furnace under different loads was investigated, a series of non-uniform problems were also found and analyzed. The research method and results of this paper can provide meaningful reference for the combustion simulation and operation optimization of large-scale CFB boilers. • A new method to calculate the distribution of coal feeding rate was proposed. • Systematic inlet boundary conditions were established for numerical simulations. • Gas-solid flow and combustion were simulated in a large-scale CFB boiler in 3D. • The feasibility of CPFD method was verified by the field test results. • A series of non-uniformity phenomena in the furnace were analyzed. [ABSTRACT FROM AUTHOR]

• The digital filter method is used to generate synthetic turbulent inflow data. • The PODFS method is used to compress this data for three test cases. • The PODFS compressed data produces similar results to the original data. • The PODFS method is 2.5 to 8.5 times faster than other methods. • The code used to generate the PODFS/digital filter data is shared and explained. In the past the digital filter technique has been used to successfully generate inflow turbulence for a number of academic and industrially relevant reacting and non-reacting flows. Weaknesses of the method include the requirement that the filter be computed over a structured mesh and can require extremely long computation times in cases where the turbulent length scales or filter widths are large as compared to the mesh spacing. Once computed, the inflow data may be saved and reused, however tens-of-thousands of time steps worth of data must be saved, copied and re-loaded into memory for each new computation and temporal interpolation must be used if the time step is adjusted. The newly developed PODFS (proper orthogonal decomposition Fourier series) method uses proper orthogonal decomposition (POD) to compress this large data set into a handful of modes that are optimally chosen to represent the high energy turbulent structures while a Fourier series representation of the temporal components of the POD modes means that the data becomes continuous, periodic and flexible in terms of time step. The PODFS is first applied to a set of inflow data generated using the digital filter method and used to simulate a turbulent planar jet with the results compared against a DNS simulation. A practical application of the method is then demonstrated where it is used to generate inlet turbulence from a time averaged URANS profile for a swirl stabilised combustor case. In this case, two PODFS models are linearly combined, one that represents acoustic forcing from downstream and one that represents turbulent fluctuations. This highlights a feature of the method which is its ability to represent different flow phenomena using the linear addition of two or more PODFS models. A subsequent LES calculation shows that the method results in the correct penetration of the airflow jets whilst neglecting the inlet turbulence results in the incorrect jet penetration depth. [ABSTRACT FROM AUTHOR]

The gas-liquid intermittent flow regime is the dominant flow pattern, which significantly occurs in the oil and gas industries. The intermittent flow is challenging in the hydrocarbon transportation systems and separators operation. The present investigation aims to comprehend the behavior of intermittent flow characteristics using experimental and numerical methods simultaneously. Superficial velocities of water and air were set in ranges of (0.18–0.84 m/s) and (1.22–4.9 m/s), respectively. The research was conducted in a horizontal pipe with a diameter of 44 mm and 147D long to study slug frequency, slug length, translational slug velocity, pressure drops, as well as slug's initiation location using static pressure data and high-speed camera tools. In this regard, the volume of fluid (VoF) method and SST k-ω model were also applied to specify film thickness, maximum Taylor bubble velocity, and to grasp behaviors of the flow field in more detail. The results revealed that liquid superficial velocity has more effects on slug frequencies than gas superficial velocity. On the other hand, slug initiation location, as well as slug fully developed length, is affected by the gas superficial velocity more than liquid superficial velocity. Moreover, both the gas and liquid superficial velocities have the same effects on translational slug velocities. The obtained experimental results were also employed to define a distinctive inlet boundary condition in the numerical simulation, which leads to a 65 percent reduction in the computational domain. • Determining the slug characteristics using local static pressure data, as well as high-speed camera images. • The VoF method and SST k-ω model were applied to ascertain the liquid film thickness and maximum Taylor bubble velocity • Define a new periodic inlet boundary condition for the numerical simulation according to slug's parameters. • An inlet boundary that is compatible with experimental data reduces 65 % executing run time and the computational domain. [ABSTRACT FROM AUTHOR]

• Coupling of 2D flow field and chemical reaction in fixed bed continuum model. • Efficient solution of momentum balance by staggered finite difference grid. • Pseudo-homogeneous reactor model accounting for 2D dispersive transport. • Critical assessment of artificial boundary condition at inlet domain discontinuity. • Pellet shape and radial porosity profile have strong impact on reactor performance. In combination with computationally demanding, but steadily developing discrete particle simulation techniques, new areas of application for the fixed-bed continuum models, like in silico determination of transport parameters, become accessible. Within this study, we present a pseudo-homogeneous continuum model fully accounting for 2D fluid flow, heat and species dispersion in axial and radial direction and apply it to four configurations of packed beds. Application of different discretization techniques for fluid flow as well as heat and species transport provide an efficient and robust solution strategy. Beds of ideal spheres exhibited deviating conversion behavior due to strong oscillation in the radial flow profile. Moreover, the importance of adequate coupling of fluid flow and chemical reaction is highlighted by a variation of reaction stoichiometry. This theoretical evaluation of the continuum approach provides valuable insights on the interplay of fluid flow and chemical reaction in fixed-bed reactors. [ABSTRACT FROM AUTHOR]