Pulsatile Flow Characteristics Research Articles

Secondary velocity and mixing characteristics of pulsatile flow in small-amplitude sinusoidal vessels

Numerical simulations on pulsatile flow within sinusoidally wavy shaped vessels were conducted for mean Reynolds number of 350, which is in the range of physiological flow rates. We considered the constant pitch of six tube diameters 6D and relatively small amplitudes from A = 0.1D to 0.5D for sinusoidal vessel geometry and investigated the secondary velocity profiles and particle mixing characteristics. The results showed that relatively small amplitude of wavy vessel geometry has significant influence on the nature of flow patterns and particle mixing characteristics. This implies that characterizing accurate geometry is essential for accurate prediction of in vivo hemodynamics and may motivate further study on any possibility of reflection of secondary flow on vascular remodeling and pathophysiology.

AN OVERVIEW ON PULSATILE FLOW DYNAMICS

Pulsatile flow dynamics in reference to the relevant experimental research on the manner between the time periods of 1997- 2015 is presented in this paper. The flow field under discussion is generated through a rigid circular cross-sectional pipe as an axial slightly- compressible and sinusoidal one in a controlled range of the oscillation parameters. Laminar and turbulent flow regimes are considered with a particular emphasis devoted to the transitional characteristics of laminar pulsatile flow into turbulence. The definitions of the flow characteristics, the methodology of the experimental studies and the results on the physical nature of the field are described. The departure of pulsatile flow from a steady one is presented in reference to the velocity field and frictional field analyse sunder the influence of oscillation frequency and amplitude. In this context, the critical magnitudes of Womersley number, time-averaged Reynolds number and oscillation amplitude to estimate the ranges of pulsatile flow field are proposed

Selective Internal Radiation Therapy: Quantifying Distal Penetration and Distribution of Resin and Glass Microspheres in a Surrogate Arterial Model

PurposeTo experimentally investigate the effects of microsphere density and diameter on distal penetration. Materials and MethodsA surrogate hepatic arterial system was developed to replicate the hemodynamics (pressures, flow rates, pulsatile flow characteristics) and anatomic geometry (vessel diameters) proximal and distal to the microsphere injection point. A planar tumor model, placed distal to the injection point, allowed visualization of deposited microspheres. Bland resin and glass microspheres, with physical characteristics approximating the characteristics of commercially available products, were injected into the surrogate system. Microsphere type, injection rate, systemic flow rate, and tumor model inclination were varied among tests (glass, n = 7; resin, n = 6) with replicates for 2 conditions. After injection, 254 micrographs were obtained at previously defined locations throughout the tumor model to document microsphere distributions. Average microsphere distributions and mass measurements of microspheres collected at the tumor outlet were analyzed to quantify distal penetration for each case. ResultsAcross all test conditions, average penetration depths of resin microspheres were higher compared with glass microspheres (45.1 cm ± 11.8 vs 22.3 cm ± 9.9). The analysis of variance indicated that the observed difference between microsphere type (glass vs resin) was significant (P = .005, df = 1,2). The observed distance means did not differ significantly across flow rate or inclination angle. ConclusionsPenetration depths of resin microspheres were significantly higher than penetration depths of glass microspheres in the surrogate hepatic arterial system.

The Role of Branch Vessels in Aortic Type B Dissection: An in vitro Study

In acute type B aortic dissection (ABAD) a patent false lumen portends a poor outcome. Patent branch vessels originating from the false lumen in a type B aortic dissection are assumed to contribute to persistent blood flow and patent false lumen. Therefore, the morphologic changes of the false lumen generated by different outflow rates in an in vitro model were investigated. An artificial dissection was created in two ex vivo porcine aortas. A thin cannula was placed in the false lumen, simulating a branch vessel originating from it. The aorta was positioned in a validated in vitro circulatory system with physiological pulsatile flow (1,500-2,700 mL/minute) and pressure characteristics (130/70 mm Hg). The cannula was attached to a small silicone tube with an adjustable valve mechanism. Three different valve settings were used for creating outflow from the false lumen (fully closed, opened at 50%, and fully opened at 100%). Measurements of lumen areas and flow rates were assessed with time-resolved magnetic resonance imaging. In order to study reproducibility, the experiment was performed twice in two different porcine aortas with a similar morphology. Increasing antegrade outflow through the branch vessel of the false lumen resulted in a significant (p < .01) increase of the mean false lumen area at the proximal and distal location in both models. The distal false lumen expanded up to 107% in the case of high outflow via the false lumen through the branch vessel. Increasing antegrade outflow through a branch vessel originating from the false lumen when no distal re-entry tear is present results in an expansion of the cross sectional false lumen area.

Analysis of Steady and Physiological Pulsatile Flow Characteristics in an Artery with Various Percentages of Restrictions

Atherosclerosis, a progressive process, silently and slowly restricts the blood flow through artery. The flow characteristics of blood in the artery are important factors in the progression and formation of atherosclerosis. A numerical simulation has been carried out for analysis of steady and physiological pulsatile blood flow characteristics in a modeled artery for various percentage of restrictions in the lumen area of artery. The continuity and momentum equations have been solved by finite volume method considering blood as Newtonian fluid and stenosis as bell shaped in nature. The flow characteristics such as time averaged wall pressure, stream line contour and time averaged wall shear stress have been analyzed for different percentage of restriction and compared for both type of flows. The flow characteristics significantly vary with the percentage of restriction. The post-stenotic regions are occupied by the vortex rings for higher percentage of restriction. The maximum time averaged pressure drop and the peak time averaged wall shear stress are found to be located at the stenosis throat for all type of flows and restrictions. The study shows that steady flow characteristics are dominated by physiological pulsatile flow characteristics from the perspective of atherosclerosis. The time averaged wall pressure drop, the size of recirculation zone and the peak wall shear stress increase with increase in percentage of restriction. Thus chances of progression and formation of atherosclerosis increase with increase in percentage of restriction.

Characteristics of Pulsatile Blood Flow Through 3-D Geometry of Arterial Stenosis

A numerical simulation is carried out to demonstrate the significant changes of flow behaviour for two different severities of arterial stenosis. Two stenosis levels of 65% and 85% are considered by area. The blood is considered as flowing fluid and assumed to be incompressible, homogeneous and Newtonian, while artery is assumed to be a rigid wall. The transient analysis is performed using ANSYS-14.5. The flow pattern, wall shear stress (WSS), pressure contours, and Centre-line velocity distribution are observed at early-systole, peak-systole and diastole for better understanding of arterial disease. Wall Share Stress distribution shows that as severity increases, sharing of flow also increases for all cases. Thus maximum stress is exerted in throat region at peak systole. The pressure distribution demonstrates that at all cases 85% stenotic artery creates more force than 65% stenotic artery at their pre-stenotic region. Interestingly, a recirculation region is visible at the post stenotic region in 85% stenotic artery for all cases and recirculation region increases with the decrease of the inlet flow velocity. Analysis indicates that the significant flow changes happen in the post stenotic region.

Hemodynamics of the aortic valve and root: implications for surgery.

Hemodynamics of the aortic valve and root: implications for surgery.

The effects of pulsatile flow characteristics on hemolysis, transfusion requirement, and hemostasis in open heart surgeries

Background: The aim of this study was to investigate the effects of pulsatile and nonpulsatile perfusion on hemolysis parameters and protection of blood components. Methods: This randomized, controlled and prospectively designed study included two independent groups, consisting of 27 nonpulsatile (NP) and 24 pulsatile (P) cases who had undergone open heart surgical procedures. Demographic and hemodynamic data were recorded. The effects of perfusion types were evaluated in terms of free hemoglobin (free Hb), haptoglobin (Hp), lactate dehydrogenase (LDH), potassium (K+), platelet number (plt) and urine Hb. These levels were measured in preoperative period, 30 and 60 minutes after initiation of cardiopulmonary bypass (CPB), two and 24 hours postoperatively. Also, total chest drainage and transfusion requirement were assessed. Results: Demographic and hemodynamic measurements did not differ between the groups. The mean arterial pressure of the NP group was statistically higher at the 30th minute. Free Hb, Hp, LDH and K+ levels did not differ in both groups in all time points. The urine Hb levels were significantly higher in the P group in the 30th minute. There was no difference regarding total chest drainage amounts and number of transfused erythrocyte cell packages. Conclusion: A clinically nonsignificant hemolytic effect of pulsatile perfusion can be outlined, although it is mentioned only in the urine Hb measurements on the laboratory basis.

Measurement of pulsatile total blood flow in the human and rat retina with ultrahigh speed spectral/Fourier domain OCT

We present an approach to measure pulsatile total retinal arterial blood flow in humans and rats using ultrahigh speed Doppler OCT. The axial blood velocity is measured in an en face plane by raster scanning and the flow is calculated by integrating over the vessel area, without the need to measure the Doppler angle. By measuring flow at the central retinal artery, the scan area can be very small. Combined with ultrahigh speed, this approach enables high volume acquisition rates necessary for pulsatile total flow measurement without modification in the OCT system optics. A spectral domain OCT system at 840nm with an axial scan rate of 244kHz was used for this study. At 244kHz the nominal axial velocity range that could be measured without phase wrapping was ±37.7mm/s. By repeatedly scanning a small area centered at the central retinal artery with high volume acquisition rates, pulsatile flow characteristics, such as systolic, diastolic, and mean total flow values, were measured. Real-time Doppler C-scan preview is proposed as a guidance tool to enable quick and easy alignment necessary for large scale studies. Data processing for flow calculation can be entirely automatic using this approach because of the simple and robust algorithm. Due to the rapid volume acquisition rate and the fact that the measurement is independent of Doppler angle, this approach is inherently less sensitive to involuntary eye motion. This method should be useful for investigation of small animal models of ocular diseases as well as total blood flow measurements in human patients in the clinic.

Mathematical Modeling of Pulsatile Blood Flow and Heat Transfer Characteristics in Stenosed Artery

Mathematical Modeling of Pulsatile Blood Flow and Heat Transfer Characteristics in Stenosed Artery

A NUMERICAL STUDY OF PULSATILE POWER LAW FLUID FLOW IN A POROUS CHANNEL

The pulsatile flow characteristics of a power law fluid in a channel filled with a homogeneous porous medium are investigated by employing the Darcy–Brinkman–Forchheimer model. Finite element method in conjunction with β-family of time discretization schemes for parabolic equation have been used to numerically solve the model for analyzing the flow. Influence of various parameters, such as power law index (n), Darcy number (Da*), Forchheimer coefficient (Γ), pulsatile amplitude parameter (A), and Womerseley parameter (α), on the flow properties have been analyzed. Increasing Γ or decreasing Da* leads to decrease in velocities and shear stress for all values of n.

Computational Fluid Dynamics Simulations of Intracranial Aneurysms at Varying Heart Rates: A “Patient-Specific” Study

Rupture of an intracranial aneurysm (IA) is frequently associated with intense physical exertion and/or emotional excitement, events that are typically also accompanied by sudden significant changes in both heart rate and blood pressure. Very few experimental studies of aneurysm hemodynamics have examined the impact on hemodynamic parameters in and around an aneurysm resulting from changes in heart rate. In order to further understanding these changes, as they relate to hemodynamic features that may contribute to rupture of an IA, we examined the characteristics of pulsatile flow in and around two "patient-specific" intracranial aneurysms at three different cardiac frequencies. Three dimensional X-ray angiographic data (3D-DSA) were used to reconstruct accurate and patient-specific aneurysm geometries. Then, computational fluid dynamics techniques were utilized to analyze the characteristics of blood flow in and around the two aneurysms. Physiologically realistic flow conditions, as measured by transcranial Doppler ultrasound, were used in the simulations. Our results showed that there were significant changes in the overall flow patterns (e.g., vortex formation and translation) associated with the changes of heart rates. In both aneurysms, the calculated wall shear stress exhibited substantial increases with an increase in heart rate. Our results suggest that the changes in local hemodynamic forces associated with variations in heart rate are dependent not only on the heart rate but also on the aneurysm geometry. This thus precludes applying our observations about the impact of variations in cardiac rate to aneurysms in general.

A New System to Analyze Pulsatile Flow Characteristics in Elastic Tubes for Hemodynamic Applications

In this research, we present a new designed experimental setup for study of characteristics of pulsatile flow in elastic tubes, aiming to simulate arterial blood flow. This system includes four major components: (1) a pulsatile pump producing original arterial flow, (2) an elastic element to simulate coupling of the heart with the arterial system, (3) an elastic tube with mechanical characteristics of the arterial wall and assembly of pressure transducers to monitor inlet and outlet pulsatile pressures and the resultant pulsatile pressure gradient and (4) a resistant element to simulate peripheral resistant distal the artery. The system is capable of performing under different mechanical conditions, including tubes with different elastic moduli and fluids with different viscosities. Experimental results showed a precise ability of producing original blood flow waves and recording pressure pulses and pressure gradient waves under different mechanical conditions. The resultant pressure pulses were compatible with the diastolic-systolic pressure pulses of typical arteries. The system showed an accurate sensitivity to variations of fluid viscosity and elasticity of tube wall. Experimental results showed that stiffening of the wall resulted in decrease of mean pressure gradient pulse. Results also showed that an elevated fluid viscosity led to a higher mean value of pressure gradient and less fluctuation of pressure gradient pulse. Results are in good agreement with theoretical considerations of higher energy dissipation and consequent pressure drop by increased fluid viscosity. The designed experimental setup might be used in evaluation of hemodynamic parameters in patho- physiological situations such as stenotic arteries and age related stiffening.

Fluid–structure interaction of turbulent pulsatile flow within a flexible wall axisymmetric aortic aneurysm model

Pulsatile turbulent flow characteristics in an axisymmetric aortic aneurysm (AA) model were analyzed numerically using a simulated physiological waveform. The transport equations were solved using the finite element formulation based on the Galerkin method of weighted residuals. A fully-coupled fluid–structure interaction (FSI) analysis was utilized in this work. We investigated the effects of turbulent flow characteristics on the distribution of wall stress and flow patterns in AA models. Wall stress distributions were calculated by computational solid stress (CSS) model, which ignores the effect of the blood flow, and the FSI model that takes into account flow and solid mechanics. Our results showed that peak wall stress and peak deformation were found to occur shortly after peak systolic flow in the FSI model and at the peak luminal pressure condition in the CSS model. Further, CSS model underestimated wall stress calculations when compared to the FSI model. There were also significant differences in the structure of flow fields between the flexible and rigid wall aneurysm models. Contour plots of kinetic energy dissipation and the application of the Kolmogorov microscale suggest that the conditions that result in red blood cell damage and platelet activation most likely occur in the near-wall region of AA during turbulent flow.

Analysis of pulsatile flow in the parallel-plate flow chamber with spatial shear stress gradient

The Parallel-Plate Flow Chamber (PPFC), of which the height is far smaller than its own length and width, is one of the main apparatus for the in-vitro study of the mechanical behavior of cultured vascular Endothelical Cells (ECs) exposed to fluid shear stress. The steady flow in different kinds of PPFC has been extensively investigated, whereas, the pulsatile flow in the PPFC has received little attention. In consideration of the characteristics of geometrical size and pulsatile flow in the PPFC, the 3-D pulsatile flow was decomposed into a 2-D pulsatile flow in the vertical plane, and an incompressible plane potential flow in the horizontal plane. A simple method was then proposed to analyze the pulsatile flow in the PPFC with spatial shear stress gradient. On the basis of the method, the pulsatile fluid shear stresses in several reported PPFCs with spatial shear stress gradients were calculated. The results were theoretically meaningful for applying the PPFCs in-vitro, to simulate the pulsatile fluid shear stress environment, to which cultured ECs were exposed.

Characteristics of Pulsatile Blood Flow Through the Curved Bileaflet Mechanical Heart Valve Installed in Two Different Types of Blood Vessels: Velocity and Pressure of Blood Flow

The aim of this study was to investigate the flow fields of blood flowing through the curved bileaflet mechanical heart valve. A numerical analysis was carried out with the fluid-structure interaction between the blood flow and the motion of leaflets in two different types of blood vessels (type A, with sinus blood vessel, and type B, without sinus blood vessel). When the leaflet was fully opened, a fluttering phenomenon was detected in association with the blood flow, and recirculation flows were observed in the sinus region of the blood vessel for type A. During the closing phase, regurgitation was formed between the ring and the edge of the each leaflet for both types. When the leaflet came into contact with the valve ring at the end of the closing phase, rebound of the leaflet occurred. In consideration of the entire domain, the pressure drop occurs mainly in the valve region. The present results showed tendencies similar to those obtained by previous experiments for blood flow and contribute to the development of the curved bileaflet mechanical heart valve prostheses.

Effect of Surrounding Tissue on Vessel Fluid and Solid Mechanics

There is no doubt that atherosclerosis is one of the most important health problems in the Western Societies. It is well accepted that atherosclerosis is associated with abnormal stress and strain conditions. A compelling observation is that the epicardial arteries develop atherosclerosis while the intramural arteries do not. Atherosclerotic changes involving the epicardial portion of the coronary artery stop where the artery penetrates the myocardium. The objective of the present study is to understand the fluid and solid mechanical differences between the two types of vessels. A finite element analysis was employed to investigate the effect of external tissue contraction on the characteristics of pulsatile blood flow and the vessel wall stress distribution. The sequential coupling of fluid-solid interaction (FSI) revealed that the changes of flow velocity and wall shear stress, in response to cyclical external loading, appear less important than the circumferential stress and strain reduction in the vessel wall under the proposed boundary conditions. These results have important implications since high stresses and strains can induce growth, remodeling, and atherosclerosis; and hence we speculate that a reduction of stress and strain may be atheroprotective. The importance of FSI in deformable vessels with pulsatile flow is discussed and the fluid and solid mechanics differences between epicardial and intramural vessels are highlighted.

AN UNSTEADY ANALYSIS OF NONLINEAR TWO‐LAYERED 2D MODEL OF PULSATILE FLOW THROUGH STENOSED ARTERIES

The effects of red cell concentration and peripheral layer viscosity on physiological characteristics of pulsatile flow in presence of mild stenosis are investigated. The flowing blood is represented by a two‐fluid model, consisting of a core region of suspension of all the erythrocytes assumed to be non‐Newtonian (inhomogeneous Newtonian) and a peripheral plasma layer free from cells of any kind as a Newtonian fluid. In the realm of the flow characteristics of blood the viscosity is taken to be a function of hematocrit in a manner that it varies radially only in the central core characterising its non‐Newtonian behaviour while it remains constant in the plasma region. The arterial wall motion and its effect on local fluid mechanics is also incorporated in the present theoretical study. Finite difference scheme has been used to solve the unsteady Navier‐Stokes equations in cylindrical coordinates assuming axial symmetry under laminar conditions, so that the problem effectively becomes two‐dimensional. The nonlinear terms appearing in the Navier‐Stokes equations governing blood flow are accounted for. Finally, the numerical illustration presented at the end of the paper provides an effective measure of the flux, the resistive impedance and the wall shear stress quantitatively in order to validate the applicability of the present model.

고주파 유압시스템의 주파수 특성과 축압기 효과의 실험적 연구

Characteristics of the high frequency pulsatile flow have been investigated experimentally to understand the flow phenomena in the hydraulic system. One axis fatigue test bed, which is widely used for automobile field test, is used. Four pressure transducers, an amplifier and a A/D convertor are used to obtain the high frequency pulsatile pressure waveform in hydraulic system. The characteristics of frequency are analyzed by power spectrum method. According to the variations of pump input pressure and actuator acceleration frequency, the pressure is measured with or without an accumulator. The amplitude of pressure with accumulator is very lower than those without accumulator due to absorbing function of accumulator. As the frequency of actuator acceleration is increased, the effect of accumulator become very important to decrease the amplitude of pulsatile pressure waveform with high frequencies.

Experimental flow studies in an elastic Y-model

To determine the causes and history of atherosclerosis it is necessary to understand the hemodynamic parameters of blood circulation. Hemodynamic parameters play an important role in the formation of atherosclerotic plaques, especially near bends and bifurcations where the flow separates from the wall. Here the flow is laminar and non-axial with eddies, secondary flow, flow separation and stagnation points. Stenoses are found predominantly in flow separation areas. Therefore, it is important to separately study the following flow parameters: steady and pulsatile flow, wall elasticity and non-Newtonian flow behavior of blood. A simplified silicon elastic y-model simulating the human carotid artery was used for the analysis of these parameters. This model can be used for numerical studies as well. Flow was visualized at steady flow using dyes and at pulsatile flow with a photoelastic apparatus and a birefringent solution. The local axial velocity at steady and pulsatile flow was determined with a one-component Laser-Doppler-Anemometer (LDA). Pulsatile flow was generated by a piston membrane pump. A glycerin-water solution was used to simulate the Newtonian flow behavior of blood. A DMSO-Separan water solution was used to simulate the non-Newtonian flow behavior. Pulsatile flow creates higher and lower shear rates so called oscillating shear rate compare to steady flow depending on the velocity amplitude. The non-Newtonian fluid showed a markedly different flow behavior than the Newtonian fluid especially in areas of flow separation. Shear gradients were calculated from these velocity measurements using a bicubic spline interpolation. Shear stresses were calculated from these velocity shear gradients and the viscosity of the non-Newtonian fluid at these shear gradients. At special areas, high shear stresses > 10 Pa were found. The elasticity of the model wall also influences the flow behavior. The measurements showed that the characteristics of pulsatile flow and the elasticity of the model wall should be observed concomitantly. This paper presents the steady and pulsatile flow with a Newtonian and non-Newtonian fluid in an elastic model.