A Cartesian-to-Curvilinear Coordinate Transformation in Modified Ghost Fluid Method for Compressible Multi-Material Flows

Modified ghost fluid method (MGFM) provides us an effective manner to simulate compressible multi-material flows. In most cases, the applications are limited in relatively simple geometries described by Cartesian grids. In this paper, the MGFM treatment with the level set (LS) technique is extended to curvilinear coordinate systems. The chain rule of differentiation (applicable to general curvilinear coordinates) and the orthogonal transformation (applicable to orthogonal curvilinear coordinates) are utilized to deduce the Cartesian-to-curvilinear coordinate transformation, respectively. The relationship between these two transformations for the extension of the LS/MGFM algorithm is analyzed in theory. It is shown that these two transformations are equivalent for orthogonal curvilinear grids. The extension of the LS/MGFM algorithm using the chain rule has a wider range of applications, as there is essentially no requirement for the orthogonality of the grids. Several challenging problems in two- or three-dimensions are utilized to validate the developed algorithm in curvilinear coordinates. The results indicate that this algorithm enables a simple and effective implementation for simulating interface evolutions, as in Cartesian coordinate systems. It has the potential to be applied in more complex computational domains.     

A Hybrid Immersed Boundary-Lattice Boltzmann Method for Simulation of Viscoelastic Fluid Flows Interaction with Complex Boundaries

In this study, a numerical technique based on the Lattice Boltzmann method is presented to model viscoelastic fluid interaction with complex boundaries which are commonly seen in biological systems and industrial practices. In order to accomplish numerical simulation of viscoelastic fluid flows, the Newtonian part of the momentum equations is solved by the Lattice Boltzmann Method (LBM) and the divergence of the elastic tensor, which is solved by the finite difference method, is added as a force term to the governing equations. The fluid-structure interaction forces are implemented through the Immersed Boundary Method (IBM). The numerical approach is validated for Newtonian and viscoelastic fluid flows in a straight channel, a four-roll mill geometry as well as flow over a stationary and rotating circular cylinder. Then, a numerical simulation of Oldroyd-B fluid flow around a confined elliptical cylinder with different aspect ratios is carried out for the first time. Finally, the present numerical approach is used to simulate a biological problem which is the mucociliary transport process of human respiratory system. The present numerical results are compared with appropriate analytical, numerical and experimental results obtained from the literature.     

A Hermite WENO Method with Modified Ghost Fluid Method for Compressible Two-Medium Flow Problems

In this paper, we develop a novel approach by combining a new robust finite difference Hermite weighted essentially non-oscillatory (HWENO) method [51] with the modified ghost fluid method (MGFM) [25] to simulate the compressible two-medium flow problems. The main idea is that we first use the technique of the MGFM to transform a two-medium flow problem to two single-medium cases by defining the ghost fluids status based on the predicted interface status. Then the efficient and robust HWENO finite difference method is applied for solving the single-medium flow cases. By using immediate neighbor information to deal with both the solution and its derivatives, the fifth order finite difference HWENO scheme adopted in this paper is more compact and has higher resolution than the classical fifth order finite difference WENO scheme of Jiang and Shu [14]. Furthermore, by combining the HWENO scheme with the MGFM to simulate the two-medium flow problems, less ghost point information is needed than that in using the classical WENO scheme in order to obtain the same numerical accuracy. Various one-dimensional and two-dimensional two-medium flow problems are solved to illustrate the good performances of the proposed method.     

Adaptive Multi-Resolution Method for 3D Reactive Flows with Level Set Front Capturing

For compressible reactive flows with stiff source terms, a new block-based adaptive multi-resolution method coupled with the adaptive multi-resolution representation model for ZND detonation and a conservative front capturing method based on a level-set technique is presented. When simulating stiff reactive flows, underresolution in space and time can lead to incorrect propagation speeds of discontinuities, and numerical dissipation makes it impossible for traditional shock-capturing methods to locate the detonation front. To solve these challenges, the proposed method leverages an adaptive multi-resolution representation model to separate the scales of the reaction from those of fluid dynamics, achieving both high-resolution solutions and high efficiency. A level set technique is used to capture the detonation front sharply and reduce errors due to the inaccurate prediction of detonation speed. In order to ensure conservation, a conservative modified finite volume scheme is implemented, and the front transition fluxes are calculated by considering a Riemann problem. A series of numerical examples of stiff detonation simulations are performed to illustrate that the present method can acquire the correct propagation speed and accurately capture the sharp detonation front. Comparative numerical results also validate the approach’s benefits and excellent performance.     

Modified Ghost Fluid Method with Axisymmetric Source Correction (MGFM/ASC)

In this work, we show that the modified ghost fluid method might sufferpressure mismatch at material interfaces and thus leads to inaccurate numerical results when directly applied to long term simulations of multi-medium flow problemswith an axisymmetric source term. We disclose the underlying reason and then develop a technique of linear distribution to take into account the effect of the axisymmetric source on the definition of ghost fluid states. In order to faithfully consider theeffect of the source term, the interfacial conditions related to derivatives are derivedand linear distributions of ghost fluid states are constructed based on a generalizedaxisymmetric multi-medium Riemann problem. Theoretical analysis and numericalresults show that the modified ghost fluid method with axisymmetric source correction (MGFM/ASC) can effectively eliminate the pressure error.     

Comparison of Lattice Boltzmann,Finite Element and Volume of Fluid Multicomponent Methods for Microfluidic Flow Problems and the Jetting of Microdroplets

We show that the lattice Boltzmann method (LBM) based color-gradient model with a central moments formulation (CG-CM) is capable of accurately simulating the droplet-on-demand inkjetting process on a micrometer length scale by comparing it to the Arbitrary Lagrangian Eulerian Finite Element Method (ALE-FEM). A full jetting cycle is simulated using both CG-CM and ALE-FEM and results are quantitatively compared by measuring the ejected ink velocity, volume and contraction rate. We also show that the individual relevant physical phenomena are accurately captured by considering three test-cases; droplet oscillation, ligament contraction and capillary rise. The first two cases test accuracy for a dynamic system where surface tension is the driving force and the third case is designed to test wetting boundary conditions. For the first two cases we also compare the CG-CM and ALE-FEM results to Volume of Fluid (VOF) simulations. Comparison of the three methods shows close agreement when compared to each other and analytical solutions, where available. Finally wedemonstrate that asymmetric jetting is achievable using 3D CG-CM simulations utilizing asymmetric wetting conditions inside the jet-nozzle. This allows for systematic investigation into the physics of asymmetric jetting, e.g. due to jet-nozzle manufacturing imperfections or due to other disturbances.     

A Novel Iterative Penalty Method to Enforce Boundary Conditions in Finite Volume POD-Galerkin Reduced Order Models for Fluid Dynamics Problems

A Finite-Volume based POD-Galerkin reduced order model is developed for fluid dynamics problems where the (time-dependent) boundary conditions are controlled using two different boundary control strategies: the lifting function method, whose aim is to obtain homogeneous basis functions for the reduced basis space and the penalty method where the boundary conditions are enforced in the reduced order model using a penalty factor. The penalty method is improved by using an iterative solver for the determination of the penalty factor rather than tuning the factor with a sensitivity analysis or numerical experimentation.The boundary control methods are compared and tested for two cases: the classical lid driven cavity benchmark problem and a Y-junction flow case with two inlet channels and one outlet channel. The results show that the boundaries of the reduced order model can be controlled with the boundary control methods and the same order of accuracy is achieved for the velocity and pressure fields. Finally, the reduced order models are 270-308 times faster than the full order models for the lid driven cavity test case and 13-24 times for the Y-junction test case.     

A Simple 3D Immersed Interface Method for Stokes Flow with Singular Forces on Staggered Grids

In this paper, a fairly simple 3D immersed interface method based on theCG-Uzawa type method and the level set representation of the interface is employedfor solving three-dimensional Stokes flow with singular forces along the interface. Themethod is to apply the Taylor's expansions only along the normal direction and incorporate the jump conditions up to the second normal derivatives into the finite difference schemes. A second order geometric iteration algorithm is employed for computing orthogonal projections on the surface with third-order accuracy. The Stokesequations are discretized involving the correction terms on staggered grids and thensolved by the conjugate gradient Uzawa type method. The major advantages of thepresent method are the special simplicity, the ability in handling the Dirichlet boundary conditions, and no need of the pressure boundary condition. The method canalso preserve the volume conservation and the discrete divergence free condition verywell. The numerical results show that the proposed method is second order accurateand efficient.     

Comparison and Experimental Validation of Fluid Dynamic Numerical Models for a Clinical Ventricular Assist Device

With the recent advances in computer technology, computational fluid dynamics (CFDs) has become an important tool to design and improve blood‐contacting artificial organs, and to study the device‐induced blood damage. Commercial CFD software packages are readily available, and multiple CFD models are provided by CFD software developers. However, the best approach of using CFD effectively to characterize fluid flow and to predict blood damage in these medical devices remains debatable. This study aimed to compare these CFD models and provide useful information on the accuracy of each model in modeling blood flow in circulatory assist devices. The laminar and five turbulence models (Spalart‐Allmaras, k‐ε (k‐epsilon), k‐ω (k‐omega), SST [Menter's Shear Stress Transport], and Reynolds Stress) were implemented to predict blood flow in a clinically used circulatory assist device, the CentriMag centrifugal blood pump. In parallel, a transparent replica of the CentriMag pump was constructed and selected views of the flow fields were measured with digital particle image velocimetry (DPIV). CFD results were compared with the DPIV experimental results. Compared with the experiment, all the selected CFD models predicted the flow pattern fairly well except the area of the outlet. However, quantitatively, the laminar model results were the most deviated from the experimental data. On the other hand, k‐ε renormalization group theory models and Reynolds Stress model are the most accurate. In conclusion, for the circulatory assist devices, turbulence models provide more accurate results than the laminar model. Among the selected turbulence models, k‐ε and Reynolds Stress Method models are recommended.     

A Wavelet-Adaptive Method for Multiscale Simulation of Turbulent Flows in Flying Insects

We present a wavelet-based adaptive method for computing 3D multiscale flows in complex, time-dependent geometries, implemented on massively parallel computers. While our focus is on simulations of flapping insects, it can be used for other flow problems. We model the incompressible fluid with an artificial compressibility approach in order to avoid solving elliptical problems. No-slip and in/outflow boundary conditions are imposed using volume penalization. The governing equations are discretized on a locally uniform Cartesian grid with centered finite differences, and integrated in time with a Runge–Kutta scheme, both of 4th order. The domain is partitioned into cubic blocks with different resolution and, for each block, biorthogonal interpolating wavelets are used as refinement indicators and prediction operators. Thresholding the wavelet coefficients allows to generate dynamically evolving grids, and an adaption strategy tracks the solution in both space and scale. Blocks are distributed among MPI processes and the grid topology is encoded using a tree-like data structure. Analyzing the different physical and numerical parameters allows us to balance their errors and thus ensures optimal convergence while minimizing computational effort. Different validation tests score accuracy and performance of our new open source code, WABBIT. Flow simulations of flapping insects demonstrate its applicability to complex, bio-inspired problems.     

Development of Design Methods for a Centrifugal Blood Pump with a Fluid Dynamic Approach: Results in Hemolysis Tests

The purpose of this study was to examine the relationship between local flow conditions and the hemolysis level by integrating hemolysis tests, flow visualization, and computational fluid dynamics to establish practical design criteria for centrifugal blood pumps with lower levels of hemolysis. The Nikkiso centrifugal blood pump was used as a standard model, and pumps with different values of 3 geometrical parameters were tested. The studied parameters were the radial gap between the outer edge of the impeller vane and the casing wall, the position of the outlet port, and the discharge angle of the impeller vane. The effect of a narrow radial gap on hemolysis was consistent with no evidence that the outlet port position or the vane discharge angle affected blood trauma in so far as the Nikkiso centrifugal blood pump was concerned. The radial gap should be considered as a design parameter of a centrifugal blood pump to reduce blood trauma.     

A Non-Ersatz Material Approach for the Topology Optimization of Elastic Structures Based on Piecewise Constant Level Set Method

The topology optimization of a linearized elasticity system with the area(volume) constraint is investigated. A non-ersatz material approach is proposed. Byintroducing a fixed background domain, the linearized elasticity system is extendedinto the background domain by a characteristic function. The piecewise constant levelset (PCLS) method is applied to represent the original material region and the voidregion. A quadratic function of PCLS function is proposed to replace the characteristicfunction. The functional derivative of the objective functional with respect to PCLSfunction is derived, which is zero in the void region and nonzero in the original material region. A penalty gradient algorithm is proposed. Four numerical experiments of2D and 3D elastic structures with different boundary conditions are presented, illustrating the validity of the proposed algorithm.     

The Safety Assessment of Irrigation Fluid Management for Shoulder Arthroscopy and Its Effect on Postoperative Efficacy

Objective

Fluid extravasation is a potentially dangerous complication associated with shoulder arthroscopy. Most relevant studies have involved respiratory system, while the primary purpose was to reveal the effects of the fluid extravasation on cardiovascular system and postoperative function.

Methods

The clinical data of 92 patients was retrospective analyzed, in which 84 cases with rotator cuff injury, three cases with shoulder instability, three cases with fractures of the greater tuberosity of the humerus, and two cases with frozen shoulder. All the patients were undergoing shoulder arthroscopy. The relationship between the basic information of the patients and cardiac index (CI) or pulse pressure variation (PPV) were evaluated by linear regression analysis. The change of CI or PPV at different states were evaluated by the one-way analysis of variance. The liquid retention (TR) and postoperative clinical outcomes was analyzed using linear regression.

Results

The preoperative CI was affected by anesthesia status and body position, while PPV was not affected. Multivariate mixed-effects model analysis of CI found that there was a statistically significant difference in groups of older than 55 years old and those with obesity (BMI > 24). After the operation, the retention of irrigation fluid significantly influenced the circumference of the deltoid (P < 0.001 (95%CI: [0.30, 1.00])), but not on the circumference of the deltoid, neck, and arm. The multivariate analysis of the American Shoulder and Elbow Surgery (ASES) scores at 3 and 6 months after surgery showed that the fluid retention volume was correlated with the ASES score at 3 months after surgery, especially when the retention volume was greater than 2 L (P = 0.001 (95%). %CI: [−12.49, −3.22]).

Conclusion

The retention of irrigation fluid after shoulder arthroscopic surgery causes swelling of local limbs, and has an effect on peripheral blood vessels, which is mainly reflected in its influence on PPV and the postoperative function. Therefore, surgeons need to improve their surgical technique, shorten the operation time and reduce fluid retention.     

Development of a bariatric surgery specific risk assessment tool for perioperative myocardial infarction

BackgroundPerioperative myocardial infarction (PMI) is a feared complication after surgery. Bariatric surgery, due to its intraabdominal nature, is traditionally considered an intermediate risk procedure. However, there are limited data on MI rates and its predictors in patients undergoing bariatric surgery.ObjectivesTo enumerate the prevalence of PMI after bariatric surgery and develop a risk assessment tool.SettingBariatric surgery centers, United States.MethodsPatients undergoing bariatric surgery were identified from the MBSAQIP participant use file (PUF) 2016. Preoperative characteristics, which correlated with PMI were identified by multivariable regression analysis. PUF 2015 was used to validate the scoring tool developed from PUF 2016.ResultsWe identified 172,017 patients from PUF 2016. Event rate for MI within 30 days of the operation was .03%; with a mortality rate of 17.3% in patients with a PMI. Four variables correlated with PMI on regression, including history of a previous MI (odds ratio [OR] = 8.57, confidence interval [CI] = 3.4–21.0), preoperative renal insufficiency (OR = 3.83, CI = 1.2–11.4), hyperlipidemia (OR = 2.60, CI = 1.3–5.1), and age >50 (OR = 2.15, CI = 1.1–4.2). Each predicting variable was assigned a score and event rate for MI was assessed with increasing risk score in PUF 2015; the rate increased from 9.5 per 100,000 operations with a score of 0 to 3.2 per 100 with a score of 5.ConclusionThe prevalence of MI after bariatric surgery is lower than other intraabdominal surgeries. However, mortality with PMI is high. This scoring tool can be used by bariatric surgeons to identify patients who will benefit from focused perioperative cardiac workup.     

Development of Design Methods of a Centrifugal Blood Pump with In Vitro Tests, Flow Visualization, and Computational Fluid Dynamics: Results inHemolysis Tests

Abstract There are few established engineering guidelines aimed at reducing hemolysis for the design of centrifugal blood pumps. In this study, a fluid dynamic approach was applied to investigate hemolysis in centrifugal pumps. Three different strategies were integrated to examine the relationship between hemolysis and flow patterns. Hemolytic performances were evaluated in in vitro tests and compared with the flow patterns analyzed by flow visualization and computational fluid dynamic (CFD). Then our group tried to establish engineering guidelines to reduce hemolysis in the development of centrifugal blood pumps. The commercially available Nikkiso centrifugal blood pump (HPM-15) was used as a standard, and the dimensions of 2 types of gaps between the impeller and the casing, the axial and the radial gap, were varied. Four impellers with different vane outlet angles were also prepared and tested. Representative results of the hemolysis tests were as follows: The axial gaps of 0.5, 1.0, and 1.5 mm resulted in normalized index of hemolysis (NIH) values of 0.0028, 0.0013 and 0.0008 g/100 L, respectively. The radial gaps of 0.5 and 1.5 mm resulted in NIH values of 0.0012 and 0.0008 g/100 L, respectively. The backward type vane and the standard one resulted in NIH values of 0.0013 and 0.0002 g/100 L, respectively. These results revealed that small gaps led to more hemolysis and that the backward type vane caused more hemolysis. Therefore, the design parameters of centrifugal blood pumps could affect their hemolytic performances. In flow visualization tests, vortices around the impeller outer tip and tongue region were observed, and their patterns varied with the dimensions of the gaps. CFD analysis also predicted high shear stress consistent with the results of the hemolysis tests. Further investigation of the regional flow patterns is needed to discuss the cause of the hemolysis in centrifugal blood pumps.     

Development of magnetic bearing system for a new third-generation blood pump

A magnetic bearing system is a crucial component in a third-generation blood pump, particularly when we consider aspects such as system durability and blood compatibility. Many factors such as efficiency, occupying volume, hemodynamic stability in the flow path, mechanical stability, and stiffness need to be considered for the use of a magnetic bearing system in a third-generation blood pump, and a number of studies have been conducted to develop novel magnetic bearing design for better handling of these factors. In this study, we developed and evaluated a new magnetic bearing system having a motor for a new third-generation blood pump. This magnetic bearing system consists of a magnetic levitation compartment and a brushless direct current (BLDC) motor compartment. The active-control degree of freedom is one; this control is used for controlling the levitation in the axial direction. The levitation in the radial direction has a passive magnetic levitation structure. In order to improve the system efficiency, we separated the magnetic circuit for axial levitation by using a magnetic circuit for motor drive. Each magnetic circuit in the bearing system was designed to have a minimum gap by placing mechanical parts, such as the impeller blades, outside the circuit. A custom-designed noncontact gap sensor was used for minimizing the system volume. We fabricated an experimental prototype of the proposed magnetic bearing system and evaluated its performance by a control system using the Matlab xPC Target system. The noncontact gap sensor was an eddy current gap sensor with an outer diameter of 2.38 mm, thickness of 0.88 mm, and resolution of 5 μm. The BLDC motor compartment was designed to have an outer diameter of 20 mm, length of 28.75 mm, and power of 4.5 W. It exhibited a torque of 8.6 mNm at 5000 rpm. The entire bearing system, including the motor and the sensor, had an outer diameter of 22 mm and a length of 97 mm. The prototype exhibited sufficient levitation performance in the stop state and the rotation state with a gap of 0.2 mm between the rotor and the stator. The system had a steady position error of 0.01 μm in the stop state and a position error of 0.02 μm at a rotational speed of 5000 rpm; the current consumption rates were 0.15 A and 0.17 A in the stop state and the rotation state, respectively. In summary, we developed and evaluated a unique magnetic bearing system with an integrated motor. We believe that our design will be an important basis for the further development of the design of an entire third-generation blood pump system.     

Simultaneous Hernofiltration/ Hernodialysis (HF/HD): Short- and Long-Term Tolerance. Introduction of a System for Automatic Fluid Replacement

Data are presented concerning our experiences with simultaneous hemofiltration and hemodialysis (HF/HD) in uremic patients. Ten randomly selected patients have been treated for two and a half to three yeare with either conventional hemodialysis (HD, 3×4 hour/week, 1.4 m² surface area Cuprophan dialyzer) or HF/HD (RP-6, 3×3 hours/week). No differences could be detected with regard to the blood chemistry or the blood pressure between the two groups of patients. During HF/HD the well-being of the patients was better than during HD, which can be attributed to a greater stability of the blood pressure and of the serum osmolarity and to a decrease of the intraocular pressure during treatment. An apparatus is described which is in use enabling the automatic replacement of the required substitution fluid. With regard to the performance and the tolerance, HF/HD is comparable to hemofiltration, but the treatment sessions are shorter.     

Method for biopsy of the wall of a thyroid cyst

Cystic thyroid lesions can arise from benign and malignant or potentially malignant thyroid tumors that have undergone cystic degeneration. In this paper we described a method for biopsing the wall of these cystic lesions to help determine the underlying pathologic abnormality that leads to cyst formation. Initial results have shown that most cysts arise from benign thyroid disease. Twenty five percent of the cysts, however, did arise from degeneration of thyroid adenomas.