Fluid structure interaction studies of human airways

Background: Fractional flow reserve (FFR) is used to evaluate the need for percutaneous coronary intervention (PCI) in cases of moderate stenosis of the coronary artery. Recently, diagnostic imaging support with computational fluid dynamics (CFD) analysis has been garnering attention. This study defines the relationship between FFR conducted for cardiac catheterization and CFD analyses conducted using coronary computed tomography (CT) for moderate stenosis, in addition to considering whether wall pressure (WP) and wall shear stress (WSS) can be used to evaluate ischemia.Methods and Results: Cases in which FFR was measured via coronary CT and cardiac catheterization was performed within 3 months were collected retrospectively. In the CFD analysis, WP and WSS were calculated and compared with FFR. Three groups were created to compare results of CFD analysis and FFR values according to the location of the stenosis: the right coronary artery, the left anterior descending artery, and the left circumflex artery. There was a correlation between FFR and WSS according to CFD analysis for moderate stenosis of the coronary artery, with a cut-off value for treatment able to be calculated.Conclusions: The results of this study suggest that ischemia can be evaluated by conducting CFD analysis (WSS) using coronary CT.

Atherosclerosis involves an inflammatory response due to plaque formation within the arteries, which can lead to ischemic stroke and heart disease. It is one of the leading causes of death worldwide, with various contributing factors such as hyperlipidemia, hypertension, obesity, diabetes, and smoking. Wall shear stress (WSS) is also known as a contributing factor of the formation of atherosclerotic plaques. Since the causes of atherosclerosis cannot be attributed to a single factor, clearly understanding the mechanisms and causes of its occurrence is crucial for preventing the disease and developing effective treatment strategies. To better understand atherosclerosis and define the correlation between various contributing factors, computational fluid dynamics (CFD) analysis is primarily used. CFD simulates WSS, the frictional force caused by blood flow on the vessel wall with various hemodynamic changes. Using apolipoprotein E knockout (ApoE-KO) mice subjected to partial ligation and a high-fat diet at 1-week, 2-week, and 4-week intervals as an atherosclerosis model, CFD analysis was conducted along with the reconstruction of carotid artery blood flow via magnetic resonance imaging (MRI) and compared to the inflammatory factors and pathological staining. In this experiment, a comparative analysis of the effects of high WSS and low WSS was conducted by comparing the standard deviation of time-averaged wall shear stress (TAWSS) at each point within the vessel wall. As a novel approach, the standard deviation of TAWSS within the vessel was analyzed with the staining results and pathological features. Since the onset of atherosclerosis cannot be explained by a single factor, the aim was to find the correlation between the thickness of atherosclerotic plaques and inflammatory factors through standard deviation analysis. As a result, the gap between low WSS and high WSS widened as the interval between weeks in the atherosclerosis mouse model increased. This finding not only linked the occurrence of atherosclerosis to WSS differences but also provided a connection to the causes of vulnerable plaques.

In this study a one-way coupled fluid-structure interaction (FSI) between ocean waves and a simplified offshore platform deck structure was modeled. The FSI model consists of a Volume of Fluid (VOF) based hydrodynamics model, a structural model and an interface to synchronize data between these two. A Computational Fluid Dynamics (CFD) analysis was used to capture the breaking wave and impact behavior of the fluid on the structure using commercially available software STAR-CCM+. A 3D Finite Element (FE) model of the platform deck developed in ABAQUS was used to determine the deflection of the structure due to hydrodynamic loads. Nonlinear material behavior was used for all structural parts in the FE model. Transient dynamic structural analysis and CFD analysis were coupled by transferring the CFD-predicted pressure distribution to the structural part in each time step using the co-simulation capabilities of STAR-CCM+ and ABAQUS. The one-way FSI model was applied to investigate the possible physical causes of observed wave damage of an offshore platform deck during a hurricane. It was demonstrated that with proper physical conditions/configurations, the FSI model could reproduce a structural deformation comparable to field measurement and provide valuable insight for forensic analysis.

Introduction: We examined the viability of pairing wire-obtained flow pressure data with computational fluid dynamic (CFD) analysis for accurate patient-specific 3D reconstruction of flow through cerebral vessels—an approach that provides comprehensive velocity data and other flow parameters to assist the preventative diagnosis of cerebral stenosis. Methods: Two physical phantom models of a cerebral stenosis—differing in regard to vascular morphology at and around the stenosis—were prepared, filled with physiological saline and connected individually to a flow pump (Vascular Simulations LLC Left Heart Replicator) applying pulsatile flow. Pressure measurements were taken upstream of the stenosis by microcatheter-guided wires to define an inlet boundary condition for CFD analysis. Pressure and velocity at 2 cm proximal to stenosis, the stenosis inlet, the stenosis outlet, and 2 cm distal to stenosis were measured to validate the CFD simulated flow. Results: Excellent agreement was observed between CFD and wire-measured time-dependent flow pressure, with the expected pulsatile behavior observed at each location. Between the two methods, differences in maximum systolic pressures ranged between |0.73±0.72|% and |3.90±0.74|%. Differences in minimum diastolic pressures ranged between |1.79±1.33|% and |9.08±1.42|%. Velocity data from wire measurements lacked pulsatile behavior and showed differences ranging between |2.93±5.20|% and |120.10±2.91|% from CFD-obtained measurements, which did show correct pulsatile behavior. Larger discrepancies were strongly associated with areas of higher CFD predicted velocities (r=0.86). Conclusion: Guidewire flow measurements provided CFD analysis with accurate inlet boundary conditions and validated CFD simulation results. CFD analysis allowed for detailed visualization of key flow parameters that may be difficult to physically measure—such as velocity—that are relevant for understanding risk factors associated with cerebral stenosis on a case-by-case basis.

Using Computational Fluid Dynamics Analysis to Characterize Local Hemodynamic Features of Middle Cerebral Artery Aneurysm Rupture Points

High-Resolution Modeling of Aqueous Humor Dynamics in the Conventional Outflow Pathway of a Normal Human Donor Eye

The actual underlying mechanisms of acute type A aortic dissection (AAAD) are not well understood. The present study aimed to elucidate the mechanism of AAAD using computational fluid dynamics (CFD) analysis. We performed CFD analysis using patient-specific computed tomography imaging in 3 healthy control cases and 3 patients with AAAD. From computed tomography images, we made a healthy control model or pre-dissection model for CFD analysis. Pulsatile cardiac flow during one cardiac cycle was simulated, and a three-dimensional flow streamline was visualized to evaluate flow velocity, wall shear stress and oscillatory shear index (OSI). In healthy controls, the transvalvular aortic flow was parallel to the ascending aorta. There was no spotty high OSI area at the ascending aorta. In pre-dissection patients, accelerated transvalvular aortic flow was towards the posterolateral ascending aorta. The vortex flow was observed on the side of the lesser curvature in mid-systole and expanded throughout the entire ascending aorta during diastole. Systolic wall shear stress was high due to the accelerated aortic blood flow on the side of the greater curvature of the ascending aorta. On the side of the lesser curvature, high OSI areas were observed around the vortex flow. In all pre-dissection cases, a spotty high OSI area was in close proximity to the actual primary entry site of the future AAAD. The pre-onset high OSI area with vortex flow is closely associated with the future primary entry site. Therefore, we can elucidate the mechanism of AAAD with CFD analysis.

Introduction: Inflow blockage of aneurysms treated with flow diverter stents is dependent on the deployed braid angle (ϕ)—the angle between the yarns which comprise the stent braids and the longitudinal axis of the braid. We utilized computational fluid dynamic (CFD) analysis to examine the effects of braid angle on intra-aneurysmal hemodynamic parameters associated with long-term treatment success. Methods: Two flow diverter stents measuring 3.5 mm in diameter, each containing 88 strands with ϕ = 10° and ϕ = 35° were constructed and deployed into a patient model of an internal carotid artery saccular aneurysm containing pulsatile blood flow. Intra-aneurysmal flow velocity, pulsatility index (PI), turbulence, vorticity, and wall shear stress (WSS) parameters were obtained and compared for the two braid angles. Results: Reducing ϕ from 35° to 10° resulted in -2.59%, -1.92%, and -1.41% changes in systolic bulk velocity, and -1.79%, +0.57%, and -1.65% changes in PI at the aneurysm neck, body, and dome respectively. Vorticity changes were insignificant. High turbulence extending through the aneurysm neck peaked approximately 0.04 s later in systole, and was located closer to the parent vessel outlet when ϕ = 10°. Elevated intra-aneurysmal WSS was concentrated at the neck and body on the side of the parent vessel outlet for both ϕ = 10° and ϕ = 35°. Regions of WSS in excess of 10 Pa stretched higher into the aneurysm body when ϕ = 35°. Conclusion: Stenting at a lower ϕ resulted in higher blockage of flow and better reduction of WSS for initiating and facilitating long term intra-aneurysmal thrombosis. CFD analysis incorporating precise reconstruction of the stent deployment conformation into patient models can provide detailed visualization of flow parameters for assessing potential treatment efficacy throughout the diagnostic and treatment phases of patient care.

In recent years, reports have been made that predict the state of aneurysm by performing computational fluid dynamics (CFD) analysis using cerebral aneurysm blood flow analysis software. However, analysis results are not constant and there are various opinions, and it is conceivable that the image quality and analysis conditions of medical images used for CFD analysis are not standardized. In this study, CFD analysis of cerebral aneurysm phantom was performed, and image quality and analysis conditions were standardized. Then, we investigated whether increase of cerebral aneurysm and prediction of rupture are possible. From this study, stable results of wall shear stress analysis were obtained under slice thickness 1.0 mm or less, CT value 400 HU or more, reconstruction function as soft part function, image SD under 10 HU. In addition, as the size of the cerebral aneurysm became larger, wall shear stress tended to be lower, and the same tendency was shown also in clinical cases. Although CFD analysis using software dedicated to blood flow analysis did not reach the prediction of rupture, it can be evaluated based on the quantitative values for the aneurysm increase at the preceding stage and plays an important role in prediction there is a possibility.

In recent years, computational simulations have emerged as valuable tools for the evaluation of atherosclerosis progression in coronary anatomies, although only a few studies have utilized more realistic Fluid-Structure Interaction (FSI) simulations. This work aims to compare the results of Computational Fluid Dynamics (CFD), Structural Finite Element Analysis (structural FEA) and FSI simulations in order to assess differences in plaque progression indices estimation. We performed structural FEA, CFD and FSI on five patient-specific epicardial coronary anatomies using the commercial software LS-Dyna. To account for the vessel pre-stress, the zero-pressure configuration was calculated for each anatomy with an inverse elastostatic algorithm. CFD, structural FEA and FSI simulations were performed applying boundary conditions based on physiological values. The comparison between structural FEA and FSI showed similar stress distribution and vessel expansions, with differences found only in the distal parts of the coronaries, where pressure reduction due to pressure loss affects the vessel walls. The elastic walls of the coronaries impact blood flow, resulting in a more disturbed flow. However, time averaged wall shear stress (TAWSS) and oscillatory shear index (OSI) distributions are similar across each coronary between CFD and FSI; TAWSS is slightly higher in CFD while OSI peaks are higher in FSI. In conclusion, given the significantly higher computational costs of FSI, we believe that CFD and structural FEA offer a more practical and cost-effective approach, providing results comparable to those of FSI, making them preferable options.

Arteriovenous fistulae are created surgically to provide adequate access for dialysis patients suffering from end-stage renal disease. It has long been hypothesized that the rapid blood vessel remodeling occurring after fistula creation is in part a process to restore the mechanical stresses to some preferred level, i.e., mechanical homeostasis. The current study presents fluid-structure interaction (FSI) simulations of a patient-specific model of a mature arteriovenous fistula reconstructed from 3D ultrasound scans. The FSI results are compared with previously published data of the same model but with rigid walls. Ultrasound-derived wall motion measurements are also used to validate the FSI simulations of the wall motion. Very large time-averaged shear stresses, 10-15 Pa, are calculated at the fistula anastomosis in the FSI simulations, values which are much larger than what is typically thought to be the normal homeostatic shear stress in the peripheral vasculature. Although this result is systematically lower by as much as 50% compared to the analogous rigid-walled simulations, the inclusion of distensible vessel walls in hemodynamic simulations does not reduce the high anastomotic shear stresses to "normal" values. Therefore, rigid-walled analyses may be acceptable for identifying high shear regions of arteriovenous fistulae.

A computational fluid dynamic (CFD) analysis is pre sented to describe local flow dynamics in both 3-D spatial and 4-D spatial and temporal domains from reconstructions of intravascular ultrasound (IVUS) and bi-plane angiographic fusion images. A left anterior descending (LAD) coronary artery segment geometry was accurately reconstructed and subsequently its motion was incorporated into the CFD model. The results indicate that the incorporation of motion had appreciable effects on blood flow patterns. The velocity profiles in the region of a stenosis and the circumferential distribution of the axial wall shear stress (WSS) patterns in the vessel are altered with the wall motion introduced in the simulation. The time-averaged axial WSS between simulations of steady flow and unsteady flow without arterial motion were comparable (-0.3 to 13.7 Pa in unsteady flow versus -0.2 to 10.1 Pa in steady flow) while the magnitudes decreased when motion was introduced (0.3-4.5 Pa). The arterial wall motion affects the time-mean WSS and the oscillatory shear index in the coronary vessel fluid dynamics and may provide more realistic predictions on the progression of atherosclerotic disease.

Hemodynamic factors, induced by pulsatile blood flow, play a crucial role in vascular health and diseases, such as the initiation and progression of atherosclerosis. Computational fluid dynamics, finite element analysis, and fluid-structure interaction simulations have been widely used to quantify detailed hemodynamic forces based on vascular images commonly obtained from computed tomography angiography, magnetic resonance imaging, ultrasound, and optical coherence tomography. In this review, we focus on methods for obtaining accurate hemodynamic factors that regulate the structure and function of vascular endothelial and smooth muscle cells. We describe the multiple steps and recent advances in a typical patient-specific simulation pipeline, including medical imaging, image processing, spatial discretization to generate computational mesh, setting up boundary conditions and solver parameters, visualization and extraction of hemodynamic factors, and statistical analysis. These steps have not been standardized and thus have unavoidable uncertainties that should be thoroughly evaluated. We also discuss the recent development of combining patient-specific models with machine-learning methods to obtain hemodynamic factors faster and cheaper than conventional methods. These critical advances widen the use of biomechanical simulation tools in the research and potential personalized care of vascular diseases.

Blood-contacting devices are vital in treating cardiovascular diseases but may trigger coagulation and thrombosis, requiring lifelong anticoagulation or repeated surgeries. Endothelial cells, which regulate vascular homeostasis, form a barrier between blood and tissue. Promoting stable endothelialization on device surfaces may reduce these risks. However, conventional devices often fail due to high wall shear stress (WSS), which damages the endothelium. This study numerically evaluates surface topographies to identify those that lower WSS and enhance cell stability. CFD simulations were used to estimate WSS on quadrangular and grooved patterns (10 × 4 mm) with depths of 1, 5, 12 mm, spaced 20 mm apart. Two flow conditions were considered: steady (0.4 m/s) and pulsatile with a sinusoidal profile. Blood was modeled as a Newtonian fluid (density: 1060 kg/m³, viscosity: 3.5·10⁻³ Pa·s). In-silico simulations assessed the impact of meso- and micro-topographies on WSS across various patterns. After an initial instability, WSS stabilized along the 10 mm section. The highest mean WSS (12 Pa) occurred at the shallowest depth, decreasing to 8 Pa and 2 Pa with greater depths. Notably, 8 Pa and 2 Pa fall within the physiological range favorable to cell survival. The comparison with grooved topographies, showed similar WSS at 1 and 5 mm depths, increasing to 5 Pa at 12 mm. Surface topography affects WSS and thereby endothelial growth. The square pattern best preserves tissue integrity by reducing WSS. Future work will explore additional patterns and flow conditions. Cellular experiments are planned to validate the numerical models.

Key Words: Fluid-Structure Coupled Analysis(유체-구조연계해석), Low-Pressure Turbine(저압터빈), SweptbackWing(후퇴익), NASA Rotor 37초록: 본논문에서는, 운전조건에서의저압최종단블레이드에대하여약결합유체-구조연계해석기법을이용한해석이수행되었다. 저압터빈최종단에대한해석이전에경계조건에대한검증을위하여15도후퇴익과NASARotor 37을대상으로예비해석이수행되었다. 각각의해석결과들을기존문헌들과비교하였다. 15도후퇴익에대한안정상태와불안정상태에서동적FSI해석이수행되었다. 해석결과변위가일정한안정상태에비하여고속에서는0.05 초만에발산하는결과를산출하였다. NASA Rotor 37의정상상태해석결과를바탕으로정적·동적FSI해석을수행하여뒷전의변형크기에대하여정적FSI 결과와동적FSI간의차이가있음을확인하였다. 저압최종단블레이드의형상측정및동특성시험을바탕으로해석모델을생성하였다. 이를바탕으로, 운전조건에서저압최종단블레이드에대한정적유체-구조연계해석을성공적으로수행하였다.Abstract: In this study, a loosely coupled fluid-structure interaction (FSI) analysis was conducted for a low-pressure(LP) final-stage rotor blade. Preliminary FSI analyses of a 15° sweptback wing and a NASA Rotor 37 compressorblade were performed for verifying the boundary conditions. The results were compared with the established literaturesfor each model. The FSI analysis of the 15° sweptback wing was carried out under both stable and unstableconditions. The excessive deformation of the wing was observed within 0.05 s under the unstable condition which ishigher than the divergence speed of a wing compared with the stable condition. On the basis of the results of asteady-state study, an unsteady state FSI analysis was conducted for a NASA Rotor 37. Different deformations wereobserved at trailing edge of the blade in the static FSI and dynamic FSI analysis. A 3D FE model of a LP rotor wasgenerated from the span-wise section data. In order to develop a reasonable model, an impact test was performed andcompared to the FE model. Using this FE model, the steady-state FSI analysis was performed successfully.