Coupled Level Set Research Articles

A validated methodology for the 3D reconstruction of cochlea geometries using human microCT images

Accurate reconstruction of the inner ear is a prerequisite for the modelling and understanding of the inner ear mechanics. In this study, we present a semi-automated methodology for accurate reconstruction of the major inner ear structures (scalae, basilar membrane, stapes and semicircular canals). For this purpose, high resolution microCT images of a human specimen were used. The segmentation methodology is based on an iterative level set algorithm which provides the borders of the structures of interest. An enhanced coupled level set method which allows the simultaneous multiple image labeling without any overlapping regions has been developed for this purpose. The marching cube algorithm was applied in order to extract the surface from the segmented volume. The reconstructed geometries are then post-processed to improve the basilar membrane geometry to realistically represent physiologic dimensions. The final reconstructed model is compared to the available data from the literature. The results show that our generated inner ear structures are in good agreement with the published ones, while our approach is the most realistic in terms of the basilar membrane thickness and width reconstruction.

Simulations of Flow Transitions in a Vertical Pipe Using Coupled Level Set and VOF Method

The level set (LS) and volume-of-fluid (VOF) methods are usually employed to simulate the two-phase flow. However every single method of them will face the mass conservative or accurate issues during the simulation. The coupled level set and volume-of-fluid (CLSVOF) method was not only able to conquer the shortages of the LS and VOF methods but also simultaneously keep the merits of both of the methods. In CLSVOF method the geometry reconstruction technology was employed to realize the coupling between LS and VOF. After the validation of single bubble rising cases, the CLSVOF method was used to simulate the complex transitional two-phase flows in a vertical pipe and the simulation results were compared to experiments.

A Flexible Coupled Level Set and Volume of Fluid (flexCLV) method to simulate microscale two-phase flow in non-uniform and unstructured meshes

An innovative Flexible Coupled Level Set (LS) and Volume of Fluid (VOF) algorithm (flexCLV) to simulate two-phase flows at the microscale on unstructured and non-uniform meshes is proposed. The method combines the advantages of the VOF method in terms of mass conservation and the LS method in terms of accuracy of the surface tension implementation and can handle both 2D and 3D domains discretized by either structured hexaedra or unstructured tetrahedral grids with high aspect ratio elements, thus guaranteeing flexibility and robustness. The method is implemented within the VOF-based OpenFOAM’s solver interFoam, which is retained as the base algorithm for the interface advection, while the surface tension force is calculated by using the level set function reconstructed from the VOF’s fraction. The method is first validated in static flow conditions by simulating a circular bubble at equilibrium and then in dynamic flow conditions by studying a freely bubble rising in both 2D and 3D domains discretized by both structured and unstructured meshes. The proposed flexCLV algorithm is then used to simulate the dynamics of confined bubbles in circular microchannels in the low capillary number regime. 2D and 3D mesh grids with high aspect ratio elements are utilized to discretized the liquid film at the tube’s walls. The numerical results are compared with the available literature and simulations performed with the original interFoam solver in terms of bubble shape and velocity, thickness of the liquid film and amplitude of the bubble tail oscillations. Results compare very well with the experimental measurements and demonstrate the superior accuracy of the coupled flexCLV method with respect to the original VOF method when surface tension and accurate interface representation play a fundamental role. Importantly, the present study also provides a precious insight on the time-dependent patterns appearing on the bubble surface in the visco-inertial regime, which could be here investigated in detail.

Joint Segmentation of Retinal Layers and Focal Lesions in 3-D OCT Data of Topologically Disrupted Retinas.

Accurate quantification of retinal structures in 3-D optical coherence tomography data of eyes with pathologies provides clinically relevant information. We present an approach to jointly segment retinal layers and lesions in eyes with topology-disrupting retinal diseases by a loosely coupled level set framework. In the new approach, lesions are modeled as an additional space-variant layer delineated by auxiliary interfaces. Furthermore, the segmentation of interfaces is steered by local differences in the signal between adjacent retinal layers, thereby allowing the approach to handle local intensity variations. The accuracy of the proposed method of both layer and lesion segmentation has been evaluated on eyes affected by central serous retinopathy and age-related macular degeneration. In addition, layer segmentation of the proposed approach was evaluated on eyes without topology-disrupting retinal diseases. Good agreement between the segmentation performed manually by a medical doctor and results obtained from the automatic segmentation was found for all data types. The mean unsigned error for all interfaces varied between 2.3 and 11.9 μm (0.6-3.1 pixels). Furthermore, lesion segmentation showed a Dice coefficient of 0.68 for drusen and 0.89 for fluid pockets. Overall, the method provides a flexible and accurate solution to jointly segment lesions and retinal layers.

Coupled level-set and volume-of-fluid method for two-phase flow calculations

ABSTRACTIn simulating two-phase flows, the volume-of-fluid (VOF) method has the advantage of mass conservation while with the level-set (LS) method, the surface tension force can be calculated more accurately. In this study, we present a coupling method which combines the advantages of both methods. The volume-of-fluid (VOF) method adopted in the calculation is the conservative interpolation scheme for interface tracking method proposed recently by the authors. Based on the location of the interface calculated from the VOF, the LS function is obtained by solving the equation used in the LS method for re-initialization without needing to solve its advection equation. A high-resolution-bounded scheme within the frame of finite-volume methods is used to solve the re-initialization equation. This scheme is verified by considering a variety of interface geometries. A circular bubble at equilibrium is used to assess the coupled LS and VOF method by examining the spurious currents generated in the bubble. Three-dimensional calculations are conducted to study the rising of a bubble in the quiescent water.

A Coupled Level Set and Volume of Fluid method for automotive exterior water management applications

Motivated by the need for practical, high fidelity, simulation of water over surface features of road vehicles a Coupled Level Set Volume of Fluid (CLSVOF) method has been implemented into a general purpose CFD code. It has been implemented such that it can be used with unstructured and non-orthogonal meshes. The interface reconstruction step needed for CLSVOF has been implemented using an iterative ‘clipping and capping’ algorithm for arbitrary cell shapes and a re-initialisation algorithm suitable for unstructured meshes is also presented. Successful verification tests of interface capturing on orthogonal and tetrahedral meshes are presented. Two macroscopic contact angle models have been implemented and the method is seen to give very good agreement with experimental data for a droplet impinging on a flat plate for both orthogonal and non-orthogonal meshes. Finally the flow of a droplet over a round edged channel is simulated in order to demonstrate the ability of the method developed to simulate surface flows over the sort of curved geometry that makes the use of a non-orthogonal grid desirable.

A new coupled level set and volume-of-fluid method to capture free surface on an overset grid system

We present a new coupled level set and volume-of-fluid (CLSVOF) method for free surface flow simulations on an overset grid system. The coupled method takes advantages of the strengths of the level set (LS) method and the volume-of-fluid (VOF) method, and is superior to either single method. The novelty of the present method lies in that we develop the methodology for an overset grid system of embedding, overlapping and moving structured grids. The new methodology accurately captures interface and greatly preserves mass on an overset grid system by demonstrating the 3D sphere advection test. The method is coupled to a well validated Reynolds-Averaged Navier–Stokes incompressible flow solver. The method is validated with the dam-breaking flow interacting with a 3D obstacle (square structure/circular cylinder) by comparing the numerical results with available experimental and numerical studies. The water impact of a sphere case is further performed to demonstrate the capabilities of the new method on a complicated moving overset grid system.

Numerical investigation of the pool film boiling of water and R134a over a horizontal surface at near-critical conditions

ABSTRACTSaturated pool film boiling over a flat horizontal surface is investigated numerically for water and refrigerant R134a at near-critical conditions for wall superheats (ΔTSup) of 2 K, 5 K, 8 K, 10 K, 15 K, and 20 K. The flow is considered to be laminar and incompressible. The governing equations are solved using a finite volume method with a collocated grid arrangement. For capturing the interface in two-phase boiling flows, a Coupled Level Set and Volume of Fluid (CLSVOF) with a multidirectional advection algorithm is used. Both single-mode and multimode boiling models are used for the numerical investigation to understand the effect of computational domain sizes on flow and heat transfer characteristics. In the case of water, the evolution of interface morphology shows the formation of a discrete periodic bubble release cycle occurring at lower Jacob numbers, Jav ≤ 10.2(ΔTSup ≤ 8 K), and the generation of jets of stable vapor film columns occurs at higher Jav ≥ 12.7 (ΔTSup ≥ 10 K). In the case of R134a, for all the Jav values considered in this study (0.163 ≤ Jav ≤ 1.63), the formation of a discrete periodic bubble release is observed. The results show that multimode boiling model should be used to understand the flow characteristics better. The magnitude of average Nusselt number obtained from the multimode film boiling model is lower than that of the single-mode film boiling model. The Nusselt numbers obtained from the present numerical studies are also compared with the available semiempirical correlations.

Numerical Investigation of Various Fuel Injection Angles on Interaction in Cold Kerosene-fueled Supersonic Flow

The incident shock wave strongly affects the transversal injection field in cold kerosene-fueled supersonic flow, possibly due to its affecting the interaction between incoming flow and the fuel through various operation conditions. Optimum selection of fuel injection parameters indicates optimum interaction between incoming flow and fuel. Based on three dimensional Couple Level Set & Volume of Fluids (CLSVOF) approach, a detailed Computational Fluid Dynamics model of a cold kerosene-fueled scramjet combustor is developed in this study. Next, the effects of various injection angles on the interaction between incident shock wave and transversal cavity injection are addressed. The injection angles are specified from 45̊ to 135̊ in 45̊increments when other operation parameters, i.e. the injection diameter, velocity and pressure drop are all constant. The CLSVOF-based predictions are focused on penetration height, span-wise expansion area, angle of shock wave and sauter mean diameter (SMD) distribution of the kerosene droplets. Our findings show that the penetration depth, span-wise angle and expansion area of the transverse cavity jet all increased with the injection angle, and that the kerosene droplets are more prone to breakup and atomization at the outlet of the combustor for the injection angle of 45̊. This study demonstrates the effectiveness of CLSVOF modeling for better understanding of kerosene-fueled supersonic mixing and scramjet combustor design improvement.

Numerical study of the interactions and merge of multiple bubbles during convective boiling in micro channels

Multi bubbles interaction and merger in a micro-channel flow boiling has been numerically studied. Effects of mass flux (56, 112, 200, and 335kg/m2∗s), wall heat flux (5, 10, and 15kW/m2) and saturated temperature (300.15 and 303.15K) are investigated. The coupled level set and volume of fluid (CLSVOF) method and non-equilibrium phase model are implemented to capture the two-phase interface, and the lateral merger process. It is found that the whole transition process can be divided to three sub-stages: sliding, merger, and post-merger. The evaporation rate is much higher in the first two stages due to the boundary layer effects in. Both the mass flux and heat flux affect bubble growth. Specifically, the bubble growth rate increase with the increase of heat flux, or the decrease of mass flux.

Modelling of liquid sloshing using CLSVOF method and very large eddy simulation

Two interface capturing methods, the volume of fluid (VOF) and the coupled level set (LS) and VOF (CLSVOF), coupled with four turbulence modelling strategies, laminar flow assumption, RANS k–ε model, very large eddy simulation (VLES) and large eddy simulation (LES), are employed to simulate sloshing flows in a rectangular tank. The new VLES model is developed in this paper to overcome some of the apparent deficiencies of RANS and LES. The CLSVOF method, which combined the LS and VOF methods, was proposed by Sussman and Puckett. Two filling levels, 18.3% and 43.7%, are considered in the present analysis. Some complicated sloshing phenomena, including the plunging wave breaking, wave impact on the sidewall and the roof and the beating phenomenon can be observed in the present sloshing cases. The computational velocity fields, wave profiles and pressure data are compared with experimental results from Delorme and Souto–Iglesias. It must be emphasized that the analysis of the velocity amplitude and the free surface evolution are mainly focused on non-breaking wave and violent breaking wave, respectively. The specific objectives are: to evaluate the performance of the CLSVOF method and VLES model for modelling of liquid sloshing; (ii) to investigate the influence of interface capturing method and turbulence model on sloshing simulation results. This study will demonstrate that, contrary to the commonly held view, turbulence modelling and free surface tracking in violent and non–violent sloshing simulation play a dominant role and a secondary role, respectively.

Gas–liquid two-phase flow in serpentine microchannel with different wall wettability

Abstract Gas–liquid flow in serpentine microchannel with different surface properties exhibits drastically different flow behavior. With water and air as working fluids, the method of numerical simulation was adopted in this paper based on CLSVOF (coupled level set and volume of fluid method) multiphase model. After verifying the reasonability of the model through experiment, by changing wall properties and Re number ( Re Po number were discussed. Moreover, the difference of pressure drop between curve and straight microchannel was also calculated. Beyond that, the combined effect of curve channel and wall properties on flow resistance was analyzed. This paper finds that wall properties have great influence on gas–liquid flow in microchannels not only on flow regime but also flow characteristics. Meanwhile, the pressure drop in curve microchannels is larger than straight. It is more beneficial for fluid flowing when the straight part of microchannel is hydrophilic smooth wall and curve part is hydrophobic with large roughness.

Simulation of drop deformation and breakup in supersonic flow

The deformation and breakup process of a liquid drop in supersonic flow is numerically studied with a coupled Level Set and Volume of Fluid method as the interface tracking approach. The Navier–Stokes equations in the liquid phase are solved by an incompressible flow solver using a finite volume method, and the governing equations in the gas phase are solved by a compressible flow solver using a finite difference method. Proper boundary conditions are specified at the interface for both liquid and gas flow solvers in order to correctly capture the interaction between the liquid and gas flows. It is demonstrated that the simulation cost can be significantly reduced by reducing liquid/gas density ratio while keeping the same Weber number and Ohnesorge number. Drop breakup at different Weber numbers is simulated. Bag breakup, bag stamen breakup, and multimode breakup modes are reproduced by the present two-phase flow solver. The physical mechanism for drop breakup in supersonic flow is investigated, and Rayleigh–Taylor instability is found to determine the breakup morphology for the studied Weber number range.

Numerical analysis and insight of drop impacting dynamics upon a liquid film

A single liquid drop impinging onto very thin spherical films was observed with the aid of a high-speed digital camera at $$10^{4}$$ frames per second. The liquid sheet produced at a high impact velocity was mainly concerned. Also more features and insights of the sheet were numerically studied using the coupled level set and volume of fluid method, considering effects of four non-dimensional parameters: Reynolds number, Weber number, dimensionless film thickness and drop–film curvature ratio. Results reveal that splashing can be suppressed by decreasing Reynolds number and Weber number. An increase in both the dimensionless film thickness and the film–drop curvature ratio is adverse to splashing onset. In addition, these four parameters can also greatly affect the geometrical shape and diameter of the sheet. In the second part, the air gap generated in the sheet–film contact neck region was investigated. It was noted that gas and liquid vortices produced near the liquid film surface render the film liquid to sink to form the air gap. Meanwhile, liquid accumulates inside the liquid sheet base gradually. Analysis in the velocity field suggests that the very thin preexisting film should be responsible for the vortex generation and the curvature ratio has minor effects on it, which is confirmed quantitatively as well.

Large eddy simulation of liquid jet primary breakup in supersonic air crossflow

The paper proposed a two-phase flow Large Eddy Simulation (LES) algorithm for atomization in supersonic gas flow. A coupled Level Set and Volume of Fluid (VOF) interface tracking method is used. The supersonic gas flow is solved using a compressible flow solver, and the liquid phase is solved using an incompressible flow solver. Appropriate boundary conditions are specified on the interface for both solvers to correctly capture the interaction between the gas and liquid. The water jet primary breakup in a supersonic air crossflow is well predicted, with the shock waves ahead of the liquid column and drops/ligaments properly captured. Two kinds of vortices respectively relating to surface breakup and column breakup are observed. Surface waves on the windward side of the liquid jet are numerically reproduced, and the wavelength is well correlated to an effective Weber number which is determined by the gas flow behind the shock. As gas flow goes across the strong shock wave ahead of liquid column, the disintegrating aerodynamic force is weakened. Rayleigh–Taylor instability is found to be responsible for the development of surface waves which causes column breakup. It is demonstrated that LES can produce almost the same breakup morphology as an under-resolved DNS.

Numerical investigation of scale effect of various injection diameters on interaction in cold kerosene-fueled supersonic flow

The incident shock wave generally has a strong effect on the transversal injection field in cold kerosene-fueled supersonic flow, possibly due to its affecting the interaction between incoming flow and fuel through various operation conditions. This study is to address scale effect of various injection diameters on the interaction between incident shock wave and transversal cavity injection in a cold kerosene-fueled scramjet combustor. The injection diameters are separately specified as from 0.5 to 1.5mm in 0.5mm increments when other performance parameters, including the injection angle, velocity and pressure drop are all constant. A combined three dimensional Couple Level Set & Volume of Fluids (CLSVOF) approach with an improved K-H & R-T model is used to characterize penetration height, span expansion area, angle of shock wave and sauter mean diameter (SMD) distribution of the kerosene droplets with/without considering evaporation. Our results show that the injection orifice surely has a great scale effect on the transversal injection field in cold kerosene-fueled supersonic flows. Our findings show that the penetration depth, span angle and span expansion area of the transverse cavity jet are increased with the injection diameter, and that the kerosene droplets are more prone to breakup and atomization at the outlet of the combustor for the orifice diameter of 1.5mm. The calculation predictions are compared against the reported experimental measurements and literatures with good qualitative agreement. The simulation results obtained in this study can provide the evidences for better understanding the underlying mechanism of kerosene atomization in cold supersonic flow and scramjet design improvement.

Comparison of laminar model, RANS, LES and VLES for simulation of liquid sloshing

Liquid sloshing in storage tank is a fundamental problem of great engineering importance. Sloshing motion can be laminar or turbulent. However, the necessity for inclusion of turbulence in CFD simulation of sloshing flows has not yet been established. In this paper, three roll–induced sloshing cases are studied to assess the merits and shortcomings of the laminar model and three most–commonly used turbulence models (RANS k–ε, LES and Very LES). To overcome the deficiencies in the RANS and LES, the new Very LES (VLES) model, which combines the RANS k–ε and LES, is developed in this paper. The free surface profiles are reconstructed by a coupled Level–Set and Volume–of–Fluid (CLSVOF) method. To the authors’ knowledge, the comprehensive and systematical assessment of the effect of turbulence on sloshing simulation has not been reported in the literature. The numerical results are evaluated using experimental measurements from Delorme and Souto−Iglesias. The present study indicates that the inclusion of an appropriate turbulence model has a profound influence on the simulations of violent and non–violent sloshing flows. The VLES and LES models can provide accurate predictions of free surface profiles and impact pressures, whereas the laminar flow assumption and the RANS model cannot adequately capture the energy dissipation in the sloshing simulation and lead to the inaccurate flow predictions.

WITHDRAWN: Gas–liquid two-phase flow in serpentine microchannel with different wall properties

Gas–liquid flow in serpentine micro channel with different surface properties exhibits drastically different flow behavior. With water and air as working fluids, the method of numerical simulation analysis is based on CLSVOF (coupled level set and volume of fluid method) multiphase model. After verifying the reasonability of the model through experiment, by changing wall properties and Re number, influences of contact angle and surface roughness on flow regime and Po number were discussed. Meanwhile, we compared gas–liquid two-phase flow pattern in serpentine microchannel with a combination of different number of hydrophobic and hydrophilic wall surfaces under several typical flow conditions, and in-depth analyzed the influence of curved channel on gas liquid phase distribution and interfacial fluctuation. Beyond that, the combined effect of curve channel and wall properties on flow resistance was analyzed. This paper finds that wall properties have great influence on gas–liquid flow in micro channels not only on flow regime but also flow characteristics. It is more beneficial for fluid flowing when the straight part of micro channel is hydrophilic smooth wall, and curve part is hydrophobic wall with larger roughness.

High fidelity simulation and analysis of liquid jet atomization in a gaseous crossflow at intermediate Weber numbers

Recent advances in numerical methods coupled with the substantial enhancements in computing power and the advent of high performance computing have presented first principle, high fidelity simulation as a viable tool in the prediction and analysis of spray atomization processes. The credibility and potential impact of such simulations, however, has been hampered by the relative absence of detailed validation against experimental evidence. The numerical stability and accuracy challenges arising from the need to simulate the high liquid-gas density ratio across the sharp interfaces encountered in these flows are key reasons for this. In this work we challenge this status quo by presenting a numerical model able to deal with these challenges, employing it in simulations of liquid jet in crossflow atomization and performing extensive validation of its results against a carefully executed experiment with detailed measurements in the atomization region. We then proceed to the detailed analysis of the flow physics. The computational model employs the coupled level set and volume of fluid approach to directly capture the spatiotemporal evolution of the liquid-gas interface and the sharp-interface ghost fluid method to stably handle high liquid-air density ratio. Adaptive mesh refinement and Lagrangian droplet models are shown to be viable options for computational cost reduction. Moreover, high performance computing is leveraged to manage the computational cost. The experiment selected for validation eliminates the impact of inlet liquid and gas turbulence and focuses on the impact of the crossflow aerodynamic forces on the atomization physics. Validation is demonstrated by comparing column surface wavelengths, deformation, breakup locations, column trajectories and droplet sizes, velocities, and mass rates for a range of intermediate Weber numbers. Analysis of the physics is performed in terms of the instability and breakup characteristics and the features of downstream flow recirculation, and vortex shedding. Formation of “Λ” shape windward column waves is observed and explained by the combined upward and lateral surface motion. The existence of Rayleigh-Taylor instability as the primary mechanism for the windward column waves is verified for this case by comparing wavelengths from the simulations to those predicted by linear stability analyses. Physical arguments are employed to postulate that the type of instability manifested may be related to conditions such as the gas Weber number and the inlet turbulence level. The decreased column wavelength with increasing Weber number is found to cause enhanced surface stripping and early depletion of liquid core at higher Weber number. A peculiar “three-streak-two-membrane” liquid structure is identified at the lowest Weber number and explained as the consequence of the symmetric recirculation zones behind the jet column. It is found that the vortical flow downstream of the liquid column resembles a von Karman vortex street and that the coupling between the gas flow and droplet transport is weak for the conditions explored.

Density-Scaled Balanced Continuum Surface Force Model with a Level Set Based Curvature Interpolation Technique

We examine the recently-proposed density-scaled balanced continuum surface force (CSF) model with a level set-based curvature interpolation technique. The density-scaled balanced CSF model is combined with a numerical framework which is based on the coupled level set and volume-of-fluid (CLSVOF) method, the tangent of hyperbola for interface capturing/weighted line interface calculation (THINC/WLIC) scheme, the constrained interpolation profile conservative semi-Langrangian with rational function (CIP-CSLR) and the volume/surface integrated average-based multi-moment method (VSIAM3). The present CSF model is examined for various bench mark problems such as drop–drop collisions and drop splashing. Comparisons of the present model results with experimental observations and those from the other CSF models show that the present CSF model can minimize spurious current and capture complicated fluid phenomena with minimizing floatsam.