Variation Of Wall Shear Stress Research Articles

Abstract 12166: Biomechanical Impact of Valve Sparing Procedure Onto the Native Aorta, a Multi-Patient in silico Study

Introduction: Ascending thoracic aortic aneurysms (TAA) provide a nidus for dissection, which can be prevented by surgical prosthetic graft replacement. Grafts eliminate risk in resected regions, but are stiffer than native aortic tissue and can thus alter downstream hemodynamics. This study used computational modeling of fluid structure interaction (FSI) to study biomechanical impact of proximal aortic graft replacement on the descending aorta. Methods: TAA pts undergoing cardiac MRI (CMR) prior to and after valve sparing proximal aortic graft replacement (VSR) surgery were studied via FSI. Patient-specific anatomy was reconstructed by segmentation of 3D MR angiography. 4D flow was used to prescribe velocity at the inlet of the model and to define three-elements Windkessel parameters used to impose outlet boundary conditions. Results: FSI models were constructed in 5 male patients (61±10.6 years old) who underwent CMR pre (1.7±1.5 weeks) and post (28.5±2.3 weeks) proximal aortic graft replacement. Qualitative and quantitative comparison of numerical results with 4D-flow data confirmed FSI model’s ability to reproduce patients’ fluid-dynamics (mean RRMSE 0.244±0.059). Regarding post-operative hemodynamics, a post-operative oscillatory shear index (OSI) increment (Pt1: p-value < 0.05) and systolic peak wall shear stress (WSS) variation (Pt1: p-value <0.001, Pt3: <0.01, Pt5: <0.0001) were observed, particularly in proximal descending aorta ( Fig 1 ). Furthermore, from 9 to 63% post-operative distensibility increment was detected. Finally, a mean 4% principal strain intensification was noticed at graft’s suture site if compared to the native portion of the ascending aorta ( Fig 1 ). Conclusion: Computational FSI modeling analysis demonstrates VSR technique to alter downstream biomechanics, including wall shear stress variation and increased strain immediately distal to grafted territories.

Patient Specific Modelling of Blood Flow in Coronary ‎Artery

The performance of the heart is considerably affected by the blocks formed because of the deposition of ‎plaque inside the coronary artery. The blocks (stenosis) either in coronary artery or elsewhere force the ‎heart to work harder for pumping the oxygenated blood to the heart muscles and blood vessels. This ‎study analyses the flow through the stenosed coronary arteries via numerical modelling by using ANSYS ‎FLUENT software. Three real cases with different asymmetric stenosis levels (i.e., block level 33%, 66% ‎& 85%) are analysed by considering blood as a non-Newtonian fluid, and blood flow as pulsatile in ‎nature. As the flow regime falls in transition to turbulent region, the transition Shear Stress Transport ‎‎(SST) k-ω turbulence model is used to take care of the changeover stage from laminar to turbulent flow ‎and vice versa. The results show large variation both in Wall Shear Stress (WSS) and pressure drop near ‎the stenosis. Pressure drop becomes more significant at severe degrees of stenosis (66% and 85%) ‎compared to the mild case (33%). The study throws light on the critical distribution of shear stress and ‎pressure drop along the artery wall, which are considered as indicators of the commencement of heart ‎disease and further growth of stenosis. An indicator, viz., Fractional Flow Reserve (FFR), which relates ‎the percentage of stenosis to the pressure variations, can be used as an index to diagnose the severity of ‎stenosis. All the three cases with different stenotic levels were analysed under hyperaemic conditions and ‎found that even 45% stenosis case can go near to critical at hyperaemic flow conditions. The effect of ‎severity due to vessel constriction can be estimated by comparing the simulated pressure drop and WSS ‎before and after the stenosis, with the ones for a healthy artery. The present study developed a ‎methodology to calculate FFR value for unknown percentage of stenosis based on the simulated results ‎obtained from 33%, 66% and 85% stenosis. Thus, criticality of a patient with certain percentage stenosis ‎can also be evaluated. This simulation technique can be recommended as a non-invasive diagnostic tool ‎for the early detection of atherosclerosis‎‎.

Buzz characteristics and separation bubble dynamics in supersonic intake

The flow field in an axisymmetric supersonic mixed-compression inlet is numerically solved to understand the separation bubble dynamics at different freestream Mach numbers of M∞ = 2.0, 3.0, 5.0, and throttling ratios of ψ = 0.515, 0.40, and 0.35. The two-dimensional compressible Reynolds Averaged Navier Stokes equations were solved in a finite volume framework. The computational results were validated with the wind tunnel experimental data for steady as well as unsteady operating conditions. Throttling conditions are simulated by varying the exit area of the model in the form of plug inserts. The analysis of the flow features in the complete computation domain indicates the formation of multiple separation bubbles near the exit plug as well as near the isolator entrance of the inlet. The frequency of unsteadiness (f) associated with the ramp surface pressure decreases with a decrease in throttling ratio ψ. The possibility of a common origin that is regardless of any buzz criteria is analyzed by considering separation and reattachment zones in the flow field in terms of wall shear stress variation. Inside the supersonic intake, the multiple shock waves interact with the wall boundary layer that can lead to a large separation bubble. This bubble is always unstable inside the intake flow passage and can generate the source of instability. The propagation of a pressure disturbance is analyzed by the temporal variation of the pressure on the ramp wall. The cross-correlation coefficient Cij(t) of the pressure signal on the ramp surface suggests the upstream propagation of the disturbance generated from the separation zone near the exit plug. The numerical Schlieren provides further information on the formation and propagation of the separation bubble near the exit plug.

Optical Coherence Tomography-Based Patient-Specific Residual Multi-Thrombus Coronary Plaque Models With Fluid-Structure Interaction for Better Treatment Decisions: A Biomechanical Modeling Case Study.

Intracoronary thrombus from plaque erosion could cause fatal acute coronary syndrome (ACS). A conservative antithrombotic therapy has been proposed to treat ACS patients in lieu of stenting. It is speculated that the residual thrombus after aspiration thrombectomy would influence the prognosis of this treatment. However, biomechanical mechanisms affecting intracoronary thrombus remodeling and clinical outcome remain largely unknown. in vivo optical coherence tomography (OCT) data of a coronary plaque with two residual thrombi after antithrombotic therapy were acquired from an ACS patient with consent obtained. Three OCT-based fluid-structure interaction (FSI) models with different thrombus volumes, fluid-only, and structure-only models were constructed to simulate and compare the biomechanical interplay among blood flow, residual thrombus, and vessel wall mimicking different clinical situations. Our results showed that residual thrombus would decrease coronary volumetric flow rate by 9.3%, but elevate wall shear stress (WSS) by 29.4% and 75.5% at thrombi 1 and 2, respectively. WSS variations in a cardiac cycle from structure-only model were 12.1% and 13.5% higher at the two thrombus surfaces than those from FSI model. Intracoronary thrombi were subjected to compressive forces indicated by negative thrombus stress. Tandem intracoronary thrombus might influence coronary hemodynamics and solid mechanics differently. Computational modeling could be used to quantify biomechanical conditions under which patients could receive patient-specific treatment plan with optimized outcome after antithrombotic therapy. More patient studies with follow-up data are needed to continue the investigation and better understand mechanisms governing thrombus remodeling process.

Numerical Investigation on the effect of Blood pressure on Wall Shear Stress and Vorticity

Pulsatile blood flow through the human carotid artery is studied using Computational Fluid Dynamics (CFD) to investigate the effect of blood rheology on the hemodynamic parameters. The carotid artery model used is segmented and reconstructed from the Magnetic Resonance Images (MRI) of a specific patient. The results of a non-Newtonian (Carreau-Yasuda) model and a Newtonian model are studied and compared. The results are represented for each peak systole where it is observed that there is significant variation in the spatial parameters between the two models considered in the study. Comparison of local shear stress magnitude in different branches namely Common Carotid Artery (CCA), Internal Carotid Artery (ICA) and External Carotid Artery (ECA) show that the shear thinning property of blood influences the Wall Shear Stress (WSS) variation. This is observed in branches where there is reduction in diameter and where the diameter reduces due to plaque deposition and also in the region where there is flow recirculation like carotid sinus

Identification of intra-individual variation in intracranial arterial flow by MRI and the effect on computed hemodynamic descriptors.

To determine the intra-individual flow variation in serially acquired studies, and the influence of this variation on subsequent hemodynamic simulations using the inlet flow as a boundary condition. Author: Kindly check and confirm whether the corresponding authors are correctly identified.Confirmed. This prospective study included 51 patients (37 females and 14 males) with unruptured intracranial aneurysms who have received more than three times follow-up of 2D phase-contrast MR. The flow and velocity parameters were extracted to calculate the reproducibility and variation. Patient-specific computational fluid dynamics simulations were performed using the measured flows. Intraclass correlation coefficients for mean and maximum velocity and flow parameters ranged from 0.77 to 0.90. A 10% CV of mean flow was identified. Variations of 10% in inlet flow resulted in hemodynamic changes including 41.41% of peak systolic wall shear stress; 39.13% of end-diastolic wall shear stress; 2.79% of low shear area at peak systole; 2.12% of low shear area at end diastole: 47.57% of time-averaged wall shear stress; and 0.17% of oscillatory shear index. This study identified 10% of intra-individual mean flow variation on phase-contrast MR. Intra-individual flow variation resulted in a non-negligible variation in wall shear stress, but relatively small variation in low shear area in hemodynamic calculations.

Analytical treatment of radiative Casson fluid over an isothermal inclined Riga surface with aspects of chemically reactive species

This article concerns an examination of thermal attributes generated in non-linearly varying viscosity-dependent viscoelastic fluid flowing over an isothermally inclined riga surface. The mathematical formulation is conceded by obliging conservation laws of momentum and energy in the form of PDE’s. Thermal aspects are highlighted in the attendance of radiative heat flux. Similarity variables are capitalized for the transmutation of PDE’s into ODE’s.Analytical solution of attained differential setup is attained by employing Laplace transform technique. The influence of flow concerning variables on related distributions is divulged in a graphical manner. The variation in quantities of engineering interest like wall shear stress, thermal and mass variation on the surface is also measured.It is found thatthe accelerating parameter, chemical reaction parameter, positive modifiedHartmannnumber, and radiation values improve skin friction while heat absorption parameter retards friction. Moreover, an increment in chemical reaction and heat absorption parameters causes a reduction in momentum distribution.

Bioconvection flow in accelerated couple stress nanoparticles with activation energy: bio-fuel applications

On the account of significance of bioconvection in biotechnology and several biological systems, valuable contributions have been performed by scientists in current decade. In current framework, a theoretical bioconvection model is constituted to examine the analyzed the thermally developed magnetized couple stress nanoparticles flow by involving narrative flow characteristics namely activation energy, chemical reaction and radiation features. The accelerated flow is organized on the periodically porous stretched configuration. The heat performances are evaluated via famous Buongiorno’s model which successfully reflects the important features of thermophoretic and Brownian motion. The composed fluid model is based on the governing equations of momentum, energy, nanoparticles concentration and motile microorganisms. The dimensionless problem has been solved analytically via homotopic procedure where the convergence of results is carefully examined. The interesting graphical description for the distribution of velocity, heat transfer of nanoparticles, concentration pattern and gyrotactic microorganism significance are presented with relevant physical significance. The variation in wall shear stress is also graphically underlined which shows an interesting periodic oscillation near the flow domain. The numerical interpretation for examining the heat mass and motile density transfer rate is presented in tubular form.

Wall Shear Stress Topological Skeleton Independently Predicts Long-Term Restenosis After Carotid Bifurcation Endarterectomy

Wall Shear Stress (WSS) topological skeleton, composed by fixed points and the manifolds linking them, reflects the presence of blood flow features associated to adverse vascular response. However, the influence of WSS topological skeleton on vascular pathophysiology is still underexplored. This study aimed to identify direct associations between the WSS topological skeleton and markers of vascular disease from real-world clinical longitudinal data of long-term restenosis after carotid endarterectomy (CEA). Personalized computational hemodynamic simulations were performed on a cohort of 13 carotid models pre-CEA and at 1 month after CEA. At 60 months after CEA, intima-media thickness (IMT) was measured to detect long-term restenosis. The analysis of the WSS topological skeleton was carried out by applying a Eulerian method based on the WSS vector field divergence. To provide objective thresholds for WSS topological skeleton quantitative analysis, a computational hemodynamic dataset of 46 ostensibly healthy carotid bifurcation models was considered. CEA interventions did not completely restore physiological WSS topological skeleton features. Significant associations emerged between IMT at 60 months follow-up and the exposure to (1) high temporal variation of WSS contraction/expansion (R2 = 0.51, p < 0.05), and (2) high fixed point residence times, weighted by WSS contraction/expansion strength (R2 = 0.53, p < 0.05). These WSS topological skeleton features were statistically independent from the exposure to low WSS, a previously reported predictor of long-term restenosis, therefore representing different hemodynamic stimuli and potentially impacting differently the vascular response. This study confirms the direct association between WSS topological skeleton and markers of vascular disease, contributing to elucidate the mechanistic link between flow disturbances and clinical observations of vascular lesions.

Estimating Arterial Cyclic Strain from the Spacing of Endothelial Nuclei

BackgroundThe non-uniform distribution of atherosclerosis within the arterial system is widely attributed to variation in haemodynamic wall shear stress. It may also depend on variation in pressure-induced stresses and strains within the arterial wall; these have been less widely investigated, at least in part because of a lack of suitable techniques.ObjectivesHere we show that local arterial strain can be determined from impressions left by endothelial cells on the surface of vascular corrosion casts made at different pressures, even though only one pressure can be examined in each vessel. The pattern of pits in the cast caused by protruding endothelial nuclei was subject to “retro-deformation” to identify the pattern that would have occurred in the absence of applied stresses.MethodsRetaining the nearest-neighbour pairs found under this condition, changes in nearest-neighbour vectors were calculated for the pattern seen in the cast, and the ratio of mean changes at different pressures determined. This approach removes errors in simple nearest-neighbour analyses caused by the nearest neighbour changing as deformation occurs.ResultsThe accuracy, precision and robustness of the approach were validated using simulations. The method was implemented using confocal microscopy of casts of the rabbit aorta made at systolic and diastolic pressures; results agreed well with the ratio of the macroscopic dimensions of the casts.ConclusionsApplying the new technique to areas around arterial branches could support or refute the hypothesis that the development of atherosclerosis is influenced by mural strain, and the method may be applicable to other tissues.

Dynamic surface heat transfer and re-attachment flow measurement using luminescent molecular sensors

Luminescent molecular sensors, which can be used as Temperature Sensitive Paint (TSP) or Pressure Sensitive Paint (PSP) in respective temperature or pressure measurements, has been proved to be one promising surface quantity measurement technology in recent years. It is advantageous in the experiments with complicated surface and is able to give flow field information such as surface flux and wall shear-stress. The current study is focused on the dynamic heat transfer measurement of a backward-facing step model, using luminescent molecular sensors as temperature probe. The effects of flow separation and re-attachment after a back-step model were experimentally discussed in wind tunnel tests. The experimental system was consisted of a molecular sensor calibration system, a dynamic data recording system and a data processing system. It is found that the reattachment process will form a low temperature region, which then gives the clear temperature field of the flow. Dynamic temperature field data show a re-attachment position around x/h = 5.7, which agrees well with oil-flow measurements as well as previous experiments. The dynamic temperature fluctuation data is discussed with the vibrations of flow and transient heat transfer behaviors after the backward step, which then is used in the analysis of surface wall shear-stress variations. It is concluded that the current luminescent molecular sensor method is capable of quantitative measurement for surface heat transfer and fluid flows.

The effect of spanwise wavelength of surface heterogeneity on turbulent secondary flows

We examine the behaviour of turbulent boundary layers over surfaces composed of spanwise-alternating smooth and rough strips, where the width of the strips $S$ varies such that $0.32\leqslant S/\overline{\unicode[STIX]{x1D6FF}}\leqslant 6.81$ , where $\overline{\unicode[STIX]{x1D6FF}}$ is the boundary-layer thickness averaged over one spanwise wavelength of the heterogeneity. The experiments are configured to examine the influences of spanwise variation in wall shear stress over a large $S/\overline{\unicode[STIX]{x1D6FF}}$ range. Hot-wire anemometry and particle image velocimetry (PIV) reveal that the half-wavelength $S/\overline{\unicode[STIX]{x1D6FF}}$ governs the diameter and strength of the resulting mean secondary flows and hence the observed isovels of the mean streamwise velocity. Three possible cases are observed: limiting cases (either $S/\overline{\unicode[STIX]{x1D6FF}}\ll 1$ or $S/\overline{\unicode[STIX]{x1D6FF}}\gg 1$ ), where the secondary flows are confined near the wall or near the roughness change, and intermediate cases ( $S/\overline{\unicode[STIX]{x1D6FF}}\approx 1$ ), where the secondary flows are space filling and at their strongest. These secondary flows, however, exhibit a time-dependent behaviour which might be masked by time averaging. Further analysis of the energy spectrogram and fluctuating flow fields obtained from PIV show that the secondary flows meander in a similar manner to that of large-scale structures occurring naturally in turbulence over smooth walls. The meandering of the secondary flows is a function of $S/\overline{\unicode[STIX]{x1D6FF}}$ and is most prominent when $S/\overline{\unicode[STIX]{x1D6FF}}\approx 1$ .

Shear stress rosettes capture the complex flow physics in diseased arteries

Wall shear stress (WSS) is an important parameter in arterial mechanobiology. Various flow metrics, such as time averaged WSS (TAWSS), oscillatory shear index (OSI), and transWSS, have been used to characterize and relate possible WSS variations in arterial diseases like aneurysms and atherosclerosis. We use a graphical representation of WSS using shear rosettes to map temporal changes in the flow dynamics during a cardiac cycle at any spatial location on the vessel surface. The presence of secondary flows and flow reversals can be interpreted directly from the shape of the shear rosette. The mean WSS is given by the rosette centroid, the OSI by the splay around the rosette origin, and the transWSS by its width. We define a new metric, anisotropy ratio (AR), based on the ratio of the length to width of the shear rosette, to capture flow bi-directionality. We characterized the flow physics in controls and patient specific geometries of the ascending aorta (AA) and internal carotid artery (ICA) that have fundamentally different flow dynamics due to differences in the Reynolds and Womersley numbers. The differences in the flow dynamics are well reflected in the shapes of the WSS rosettes and the corresponding flow metrics.

Analysis of local hemodynamics in complex aneurysms: an effect of the vessel arising from the dome or the neck

Assessment of rupture risk for intracranial aneurysms (IA) is a particular challenge in cases of so-called complex aneurysms due to their variable morphometric characteristics. Arterial branch arising from the dome or the neck of IA is one of the least explored features of complex aneurysms. The methods of computational fluid dynamics may be valuable to determine the influence of arterial branches of IA on local hemodynamics. To analyze local hemodynamics in IA with arterial branch arising from the cupola or the neck depending on the structure of the aneurysm and blood flow rate in the parent vessel. CT angiography data of 4 patients with IA were estimated in this study. Modifications of the baseline 3D models of the aneurysms resulted 12 patient-specific models included into analysis. Hemodynamic calculations were made by using of ANSYS Workbench 19 software package. Wall shear stress (WSS) was characterized by the most significant variability, especially in case of sidewall aneurysms. Small cross-sectional area of additional branch in relation to the neck of IA was not followed by considerable changes of blood flow patterns inside IA after «virtual» removal of the vessel. Otherwise, the intensity of flows was drastically reduced. Simulation of high inlet flows demonstrated substantial variation of WSS in the area of jet. Additional arterial branch arising from the dome or the neck of IA significantly influences local hemodynamics. This influence depends on the localization of IA in relation to the parent vessel and the diameter of additional arterial branch.

Çeşitli Boru Tiplerinde Çeper Kayma Gerilmesi Değişiminin Deneysel Olarak İncelenmesi ve Yapay Sinir Ağı ile Modellenmesi

Bu çalışmada, gelişen türbülanslı boru akışında çeper kayma gerilmesinin boru boyunca değişimi deneysel olarak elde edilmiştir. Bu amaçla değişik pürüzlülükte 5 adet boru (alüminyum, bakır, siyah, galvanizli ve PPRC “polypropylene random copolymer” ) seçilmiştir. Bu borulara hidrodinamik geçiş bölgesinde sık aralıklarla tam gelişmiş akış bölgesinde ise sadece giriş ve çıkışta olmak üzere çok sayıda basınç prizleri yerleştirilmiştir. Deneyler yapılırken debi hidrolik tank yardımıyla ölçülmüş ve bu debiye karşılık gelen statik basınç değerleri, sisteme kurulan sabit bir dijital kamera yardımıyla belirli zaman aralıklarında kaydedilmiştir. Kaydedilen bu görüntülerden yararlanarak bir görüntü işleme programı yardımı ile anlık basınç çalkantıları elde edilmiştir. Anlık basınç çalkantılarının ortalaması alınarak her bir basınç prizindeki zaman ortalamalı basınç değerleri hesaplanmıştır. Birbirine yakın basınç prizleri arasındaki basınç farkından yararlanarak çeper kayma gerilmeleri tespit edilmiştir. Her bir boru için değişik Reynold sayılarında kayma gerilmesinin boru boyunca değişimi grafik olarak sunulmuştur. Deney çalışmalarından elde edilen kayma gerilmeleri için yapay sinir ağı (YSA) kullanılarak tahminsel bir model elde edilmiştir. Elde edilen modelin geçerliliğini belirlemek için farklı hata analizleri yapılmıştır. YSA tarafından kayma gerilmesi değerleri için oluşturulan tahminsel modeller içerisinde en az hata oranı %10 değeri ile bakır boruya ait tahminsel modelidir. Sonuç olarak farklı pürüzlülüğe sahip borulardaki çeper kayma gerilmeleri belirlenmiş ve en fazla kayma gerilmesi değeri (20.13 Pa) PPRC boruda görülmüştür.

A pilot study of aortic hemodynamics before and after thoracic endovascular repair with a double-branched endograft

Branched endografts have been developed to treat complex pathology in the aortic arch and ascending aorta. This study aims to evaluate the haemodynamic performance of a double-branched thoracic endograft by detailed comparison of flow patterns and wall shear stress in the aorta and supra-aortic branches before and after stent-graft implantation. Pre- and post-intervention CT images were acquired from two patients who underwent thoracic endovascular aortic repair (TEVAR) with a double-branched endograft for thoracic aortic aneurysms. These images were used to reconstruct patient-specific models, which were analysed using computational fluid dynamics employing physiologically realistic boundary conditions. Our results showed that there was sufficient blood perfusion through the arch branches. The presence of inner tunnels caused flow derangement and asymmetric wall shear stress in the ascending aorta, where shear range index was up to 6 times higher than in the pre-intervention model. Wall shear stress in the aortic arch increased considerably after intervention as a result of accelerated flow. The maximum flow-induced displacement forces on the branched endografts were around 22 ​N for both patients, which was below the threshold for device migration. Results from this pilot study demonstrated that aortic flow patterns were significantly altered by the branched endograft which caused increased spatial variation of wall shear stress in the ascending aorta and the arch. Although no obvious adverse hemodynamic features were found immediately after intervention for the cases we analysed, follow-up studies will be needed to assess durability of the device.

A study on two-layered (K.L-Newtonian) model of blood flow in an artery with six types of mild stenoses

A mathematical model of the steady flow of blood in arteries with six different types of mild stenoses (symmetric, triangular, trapezoidal, elliptical, bell-shaped and composite) is analyzed. The model comprises a core (suspension of cells) surrounded by a layer of plasma near the wall. The rheology of blood in the core region is characterized by a K-L (non-Newtonian) fluid model and the peripheral plasma layer as a Newtonian fluid. An attempt has been made to obtain analytical expressions for the velocity of blood flow in the stenotic arterial region, radius of the plug-core region, the velocity of the fluid in the plug-flow region, flow flux, shear stress at wall and resistance to flow, using the suitable boundary conditions. The computational results were presented graphically for several values of the parameters involved in the present analysis, and also several comparisons have been made with the existing models to endorse the applicability of the present model. The study revealed that the velocity of blood decreases by increasing values of the yield stress of the blood and K-L fluid parameters and an opposite nature is observed for wall shear stress and resistive impedance to flow. It is found that the shape of the plug flow region is purely dependent on the shape of the stenosis present in the cardiovascular system. It is also recovered that the wall shear stress and flow resistance are significantly reduced for the two-layered model (K.L-Newtonian) than those of the one-layered model (K-L) irrespective of the type of geometries of stenosis. Also, the axial variation of wall shear stress and resistive impedance are lowest for the bell-shaped stenosis than those of the other stenoses considered in the present study, and they found to be highest for the trapezoidal stenosis. A comparison of the velocity profiles obtained from the present study with the experimental data is made, and a good agreement between them is found. Further, the relevant physiological applications of the present mathematical model have been analyzed.

Use of Distributed Threshold Fouling Model at Local Scale in Numerical Simulations

Numerical simulation is studied for shell and tube heat exchangers incorporating the usage of the local Ebert and Panchal threshold fouling model on the overall fouling prediction. The overall fouling rate obtained by averaging local fouling rates along the whole heat exchanger was found to be consistent with the fouling rate calculated with overall quantities, at least for the shell-and-tube heat exchanger tested, under the operating conditions studied (299.5 °C film temperature, 4.4 Pa wall shear stress) and for the period of linear fouling. The deposition rate, which is minimally affected by the spatial variations of the thermohydraulic properties, controls the fouling rate. Heterogeneities are caused by variations of the local wall shear stress which varies in a large proportion (2.5 to 11.7 Pa) but whose influence on the fouling rate is low. The wide range of local shear stress recorded is the consequence of the flow establishment in the tubes.

Atherosclerosis research: the impact of physiological parameters on vascular wall stress

In the clinic, atherosclerosis has been associated with abnormal physiological conditions, such as high cholesterol level, high blood pressure and diabetes, but the mechanisms by which they are linked have not yet been determined. In recent years, hemodynamic factors have been found to have a great influence on atherosclerosis. In particular, wall shear stress (WSS), an important factor in atherosclerosis from the development of early lesions to the rupture of later plaques, is now one of the most important risk indicators of atherosclerosis. Here, we used a numerical investigation to determine the impact of four physiological parameters on vascular wall stress (including WSS and wall pressure) in the right coronary arteries, in order to assess their influence on atherosclerosis risk. Each parameter was investigated both separately and in integrated models. The results indicate that the blood flow rate has the most significant influence on WSS and plays a decisive role in the variation of WSS under normal physiological conditions. Blood pressure had a minor influence on WSS under conditions of a flexible vessel wall, but this effect was lost in the absence of flexibility. Blood viscosity and vascular elasticity could also affect WSS directly, but in vivo their influences were negligible.

Three-dimensional distribution of wall shear stress and its gradient in red cell-resolved computational modeling of blood flow in invivo-like microvascular networks.

Using a high‐fidelity, 3D computational model of blood flow in microvascular networks, we provide the full 3D distribution of wall shear stress (WSS), and its gradient (WSSG), and quantify the influence of red blood cells (RBCs) on WSS and WSSG. The deformation and flow dynamics of the individual RBCs are accurately resolved in the model, while physiologically realistic microvascular networks comprised of multiple bifurcations, convergences, and tortuous vessels are considered. A strong heterogeneity in WSS and WSSG is predicted across the networks, with the highest WSS occurring in precapillary bifurcations and capillary vessels. 3D variations of WSS and WSSG are shown to occur due to both network morphology and the influence of RBCs. The RBCs increase the WSS by as much as three times compared to that when no RBCs are present, and the highest increase is observed in venules. WSSG also increases significantly, and high WSSGs occur over wider regions in the presence of RBCs. In most vessels, the circumferential component of WSSG is observed to be greater than the axial component in the presence of RBCs, while the opposite trend is observed when RBCs are not considered. These results underscore the important role of RBCs on WSS and WSSG that cannot be predicted by widely used 1D models of network blood flow. Furthermore, the subendothelium‐scale variations of WSS and WSSG predicted by the present model have implications in terms of endothelial cell functions in the microvasculature.