Sherman C. P. Cheung

Biomechanical investigation of pulsatile flow in a three-dimensional atherosclerotic carotid bifurcation model

It is a well-established fact that atherosclerosis in carotid bifurcation depends on flow parameters such as wall shear stress, flow pulsatility, and blood pressure. However, it is still not clearly verified how atherosclerosis can become aggravated when plaque experiences a high level of shear stress during advance stages of this disease. In this paper, fluid and structural properties in idealistic geometries are analyzed by using fluid-structure interaction (FSI). From our results, the relationship among blood pressure, stenotic compression, and deformation was established. We show that a high level of compression occurs at the stenotic apex, and can potentially be responsible for plaque progression. Moreover, wall shear stress and deformation are significantly affected by the degree of stenosis. Finally, through analysis of the FSI-based simulation results, we can better understand the parameters that influence flow through a stenotic artery and plaque aggravation, and apply the knowledge for the enhancement of clinical research and prediction of treatment outcomes.

Flickering Behavior of Turbulent Buoyant Fires Using Large-Eddy Simulation

A numerical study investigating the flickering behavior of a turbulent buoyant fire is conducted using large-eddy simulation to examine coupled turbulence, combustion, soot chemistry, and radiation effects. The three-dimensional, Favre-filtered, compressible mass, momentum, energy, and mixture fraction and its scalar variance conservation equations are closed using the Smagorinsky subgrid-scale (SGS) turbulence model. A two-stage predictor-corrector methodology for low-Mach-number compressible flows is adopted. Formation of large-scale vortical structures is well captured, with the predicted puffing frequency agreeing closely with experimentally determined frequencies. Comparisons of instantaneous, mean, and root-mean-square quantities also show qualitative agreement against other experimental data.

Experimental and numerical study on the hemodynamics of stenosed carotid bifurcation

Numerical simulation is performed to demonstrate that hemodynamic factors are significant determinants for the development of a vascular pathology. Experimental measurements by particle image velocimetry are carried out to validate the credibility of the computational approach. We present a study for determining complex flow structures using the case of an anatomically realistic carotid bifurcation model that is reconstructed from medical imaging. A transparent silicone replica of the artery is developed for in-vitro flow measurement. The dynamic behaviours of blood through the vascular structure based on the numerical and experimental approaches show good agreement.

Numerical simulation and experimental validation of swirling flow in spiral vortex ventricular assist device

Spiral Vortex Ventricular Assist Device (SV-VAD) supports cardiac patients with refractory heart failure. Unfortunately, thrombus formation and risk of stroke due to flow complications may lead to aggravated conditions. The hemodynamics of a continuous flow in the ventricular chamber of a SV-VAD can be analyzed using numerical simulation. Particle image velocimetry and laser Doppler anemometry are utilized for validating the simulated spiral flow in a transparent acrylic SV-VAD replica based on its cross-sectional averaged axial and tangential velocities. After validation, the relationship between swirling flow and blood cell damage is established by evaluating flow effect on thrombosis due to high shear stress. Based on our analysis, stagnancy of the flow within the SV-VAD is insignificant and its low shear stress minimizes hemolysis.

Study of three LES subgrid-scale turbulence models for predictions of heat and mass transfer in large-scale compartment fires

ABSTRACT Numerical assessment was performed to investigate the wall-adaptive features offered by two subgrid-scale (SGS) turbulence models: Wall-Adapting Local Eddy Viscosity (WALE) and Vreman against the Smagorinsky model. The gas temperature and velocity field predictions were enhanced using WALE over Smagorinsky, especially at the flaming and near-wall regions since WALE considers both strain and rotation rates of the turbulent structure and the turbulent viscosity approaches zero at the wall. Conversely, the simulation results by Vreman were under-predicted against the experimental data. The WALE model could notably enhance the simulation accuracy for large-scale compartment fires due to significant improvements of the flow diffusivity modeling.

Numerical Stability of Partitioned Approach in Fluid-Structure Interaction for a Deformable Thin-Walled Vessel

Added-mass instability is known to be an important issue in the partitioned approach for fluid-structure interaction (FSI) solvers. Despite the implementation of the implicit approach, convergence of solution can be difficult to achieve. Relaxation may be applied to improve this implicitness of the partitioned algorithm, but this commonly leads to a significant increase in computational time. This is because the critical relaxation factor that allows stability of the coupling tends to be impractically small. In this study, a mathematical analysis for optimizing numerical performance based on different time integration schemes that pertain to both the fluid and solid accelerations is presented. The aim is to determine the most efficient configuration for the FSI architecture. Both theoretical and numerical results suggest that the choice of time integration schemes has a significant influence on the stability of FSI coupling. This concludes that, in addition to material and its geometric properties, the choice of time integration schemes is important in determining the stability of the numerical computation. A proper selection of the associated parameters can improve performance considerably by influencing the condition of coupling stability.

A Review of Population Balance Modelling for Isothermal Bubbly Flows

In this article, we present a review of the state-of-the-art population balance modelling techniques that have been adopted to describe the phenomenological nature of isothermal bubbly flows. The main focus of the review can be broadly classified into three categories: (i) Numerical approaches or solution algorithms of the PBE; (ii) Applications of the PBE in practical gas-liquid multiphase problems and (iii) Possible aspects of the future development in population balance modelling. For the first category, details of solution algorithms based on both method of moment (MOM) and discrete class method (CM) that have been proposed in the literature are provided. Advantages and drawbacks of both approaches are also discussed from the theoretical and practical viewpoints. For the second category, applications of existing population balance models in practical multiphase problems that have been proposed in the literature are summarized. Selected existing mathematical closures for modelling the “birth” and “deat...

Capturing the Pulsation Frequency of a Buoyant Pool Fire using the Large Eddy Simulation Approach

A numerical model using the large eddy simulation (LES) approach with considerations of turbulence, combustion, soot chemistry and radiation effects is presented and employed to capture the pulsation frequency of a turbulent buoyant pool fire. Numerical results from the present model are validated and compared against the experimental data and predictions from another LES field model–fire dynamic simulator (FDS). Quantitative comparisons were performed between the experimental data and time-averaged results of the two LES models. The centreline temperature and velocity from the present model were found to be in good agreement with the experimental data and those of FDS results. The puffing effect of the buoyant plume was appropriately captured by the present model while the predicted pulsation frequency agreed well with the experimentally measured frequency. FDS appeared, however, to yield higher pulsation frequency, which exceeded the range of experimental observations.

Investigation of the Influence of Elevated Pressure on Subcooled Boiling Flow—Model Evaluation Toward Generic Approach

Modeling subcooled boiling flows in vertical channels has relied heavily on the utilization of empirical correlations for the active nucleation site density, bubble departure diameter, and bubble departure frequency. Following the development and application of mechanistic modeling at low pressures, the capability of the model to resolve flow conditions at elevated pressure up to 10 bar is thoroughly assessed and compared with selected empirical models. Predictions of the mechanistic and selected empirical models are validated against two experimental data at low to elevated pressures. The results demonstrate that the mechanistic model is capable of predicting the heat and mass transfer processes. In spite of some drawbacks of the currently adopted force balance model, the results still point to the great potential of the mechanistic model to predict a wide range of flow conditions in subcooled boiling flows.

Study of Isothermal Vertical Bubbly Flow Using Direct Quadrature Method of Moments

In the numerical study, investigation of bubbly flow requires deep understanding of complex hydrodynamics under various flow conditions. In order to simulate the bubble behaviour in conjunction with suitable bubble coalescence and bubble breakage kernels, direct quadrature method of moments (DQMOM) has been applied and validated instead. To examine the predictive results from DQMOM model, the validation has been carried out against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. Numerical results showed good agreement against experimental data for the local and axial void fraction, bubble size distribution and interfacial area concentration profiles. Encouraging results demonstrates the prospect of the DQMOM two-fluid model against flow conditions with wider range of bubble sizes and rigorous bubble interactions. Moreover, moment sensitivity study also has been carried out to carefully assess the performance of the model...