Artykuły w czasopismach: „Pulsatile flow”

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Pier, Benoît, i Peter J. Schmid. "Linear and nonlinear dynamics of pulsatile channel flow". Journal of Fluid Mechanics 815 (21.02.2017): 435–80. http://dx.doi.org/10.1017/jfm.2017.58.

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The dynamics of small-amplitude perturbations, as well as the regime of fully developed nonlinear propagating waves, is investigated for pulsatile channel flows. The time-periodic base flows are known analytically and completely determined by the Reynolds number $Re$ (based on the mean flow rate), the Womersley number $Wo$ (a dimensionless expression of the frequency) and the flow-rate waveform. This paper considers pulsatile flows with a single oscillating component and hence only three non-dimensional control parameters are present. Linear stability characteristics are obtained both by Floquet analyses and by linearized direct numerical simulations. In particular, the long-term growth or decay rates and the intracyclic modulation amplitudes are systematically computed. At large frequencies (mainly $Wo\geqslant 14$), increasing the amplitude of the oscillating component is found to have a stabilizing effect, while it is destabilizing at lower frequencies; strongest destabilization is found for $Wo\simeq 7$. Whether stable or unstable, perturbations may undergo large-amplitude intracyclic modulations; these intracyclic modulation amplitudes reach huge values at low pulsation frequencies. For linearly unstable configurations, the resulting saturated fully developed finite-amplitude solutions are computed by direct numerical simulations of the complete Navier–Stokes equations. Essentially two types of nonlinear dynamics have been identified: ‘cruising’ regimes for which nonlinearities are sustained throughout the entire pulsation cycle and which may be interpreted as modulated Tollmien–Schlichting waves, and ‘ballistic’ regimes that are propelled into a nonlinear phase before subsiding again to small amplitudes within every pulsation cycle. Cruising regimes are found to prevail for weak base-flow pulsation amplitudes, while ballistic regimes are selected at larger pulsation amplitudes; at larger pulsation frequencies, however, the ballistic regime may be bypassed due to the stabilizing effect of the base-flow pulsating component. By investigating extended regions of a multi-dimensional parameter space and considering both two-dimensional and three-dimensional perturbations, the linear and nonlinear dynamics are systematically explored and characterized.

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Feldmann, Daniel, Daniel Morón i Marc Avila. "Spatiotemporal Intermittency in Pulsatile Pipe Flow". Entropy 23, nr 1 (30.12.2020): 46. http://dx.doi.org/10.3390/e23010046.

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Despite its importance in cardiovascular diseases and engineering applications, turbulence in pulsatile pipe flow remains little comprehended. Important advances have been made in the recent years in understanding the transition to turbulence in such flows, but the question remains of how turbulence behaves once triggered. In this paper, we explore the spatiotemporal intermittency of turbulence in pulsatile pipe flows at fixed Reynolds and Womersley numbers (Re=2400, Wo=8) and different pulsation amplitudes. Direct numerical simulations (DNS) were performed according to two strategies. First, we performed DNS starting from a statistically steady pipe flow. Second, we performed DNS starting from the laminar Sexl–Womersley flow and disturbed with the optimal helical perturbation according to a non-modal stability analysis. Our results show that the optimal perturbation is unable to sustain turbulence after the first pulsation period. Spatiotemporally intermittent turbulence only survives for multiple periods if puffs are triggered. We find that puffs in pulsatile pipe flow do not only take advantage of the self-sustaining lift-up mechanism, but also of the intermittent stability of the mean velocity profile.

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Hindman, Bradley J., Franklin Dexter, Tom Smith i Johann Cutkomp. "Pulsatile Versus Nonpulsatile Flow". Anesthesiology 82, nr 1 (1.01.1995): 241–50. http://dx.doi.org/10.1097/00000542-199501000-00029.

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Background Although pulsatile and nonpulsatile cardiopulmonary bypass (CPB) do not differentially affect cerebral blood flow (CBF) or metabolism during hypothermia, studies suggest pulsatile CPB may result in greater CBF than nonpulsatile CPB under normothermic conditions. Consequently, nonpulsatile flow may contribute to poorer neurologic outcome observed in some studies of normothermic CPB. This study compared CBF and cerebral metabolic rate for oxygen (CMRO2) between pulsatile and nonpulsatile CPB at 37 degrees C. Methods In experiment A, 16 anesthetized New Zealand white rabbits were randomized to one of two pulsatile CPB groups based on pump systolic ejection period (100 and 140 ms, respectively). Each animal was perfused at 37 degrees C for 30 min at each of two pulse rates (150 and 250 pulse/min, respectively). This scheme created four different arterial pressure waveforms. At the end of each perfusion period, arterial pressure waveform, arterial and cerebral venous oxygen content, CBF (microspheres), and CMRO2 (Fick) were measured. In experiment B, 22 rabbits were randomized to pulsatile (100-ms ejection period, 250 pulse/min) or nonpulsatile CPB at 37 degrees C. At 30 and 60 min of CPB, physiologic measurements were made as before. Results In experiment A, CBF and CMRO2 were independent of ejection period and pulse rate. Thus, all four waveforms were physiologically equivalent. In experiment B, CBF did not differ between pulsatile and nonpulsatile CPB (72 +/- 6 vs. 77 +/- 9 ml.100 g-1.min-1, respectively (median +/- quartile deviation)). CMRO2 did not differ between pulsatile and nonpulsatile CPB (4.7 +/- 0.5 vs. 4.1 +/- 0.6 ml O2.100 g-1.min-1, respectively) and decreased slightly (0.4 +/- 0.4 ml O2.100 g-1.min-1) between measurements. Conclusions During CPB in rabbits at 37 degrees C, neither CBF nor CMRO2 is affected by arterial pulsation. The absence of pulsation per se is not responsible for the small decreases in CMRO2 observed during CPB.

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Kobayashi, N., S. Miyachi, T. Okamoto, K. Hattori, T. Kojima, K. Hattori, K. Nakai, S. Qian, H. Takeda i J. Yoshida. "Computer Simulation of Flow Dynamics in an Intracranial Aneurysm". Interventional Neuroradiology 10, nr 1_suppl (marzec 2004): 155–60. http://dx.doi.org/10.1177/15910199040100s127.

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Using a supercomputer, the authors studied the effect of vessel wall pulsation on flow dynamics with a three-dimensional model simulating both a rigid and pulsatile style. The design of the aneurysm models was set with a 5 mm dome diameter and a 1 or 3 mm orifice size to simulate a carotid-ophthalmic aneurysm. Flow dynamics were analyzed according to flow pattern, wall pressure and wall shear stress. The flow pattern in the aneurysm sac showed the great difference between rigid and pulsatile models particularly in the small-neck aneurysm model. The arterial wall tended to be exposed to a higher pressure peak in the pulsatile model than in the rigid one, especially at its bifurcation and curved regions. Sites of shear stress peak were found on the aneurysmal dome as well as at the distal end of the orifice in both rigid and pulsatile models. The effects of vessel-wall pulsation should be considered whenever evaluating conditions in and around an aneurysm.

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Grossi, Eugene A., i F. Gregory Baumann. "Pulsatile Flow". Annals of Thoracic Surgery 40, nr 6 (grudzień 1985): 638. http://dx.doi.org/10.1016/s0003-4975(10)60376-1.

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Minh, Chau Nguyen, Hassan Peerhossaini i Mojtaba Jarrahi. "Phototactic microswimmers in pulsatile flow: Toward a novel harvesting method". Biomicrofluidics 16, nr 5 (wrzesień 2022): 054103. http://dx.doi.org/10.1063/5.0097580.

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Phototactic behavior is coupled with pulsatile flow features to reveal the advantages of pulsation for separating motile algae cells in a double Y-microchannel. The underlying mechanism is as follows: during half of the pulsation cycle, when the flow rate is low, the phototactic microswimmers are mainly redirected by the external stimulation (light); while, during the rest of the cycle, the flow effects become dominant and the microswimmers are driven toward the desired outlet. The results show that in the absence of light source, the pulsatile flow has no advantage over the steady flow for separation, and the microswimmers have no preference between the exit channels; the separation index (SI) is around 50%. However, when the light is on, SI increases to 65% and 75% in the steady and pulsatile flows, respectively. Although the experiments are conducted on the well-known model alga, Chlamydomonas reinhardtii, a numerical simulation based on a simple model demonstrates that the idea can be extended to other active particles stimulated by an attractive or repulsive external field. Thus, the potential applications can go beyond algae harvesting to the control and enhancement of separation processes without using any mechanical component or chemical substance.

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LIAO, WEI, T. S. LEE i H. T. LOW. "NUMERICAL STUDY OF PHYSIOLOGICAL TURBULENT FLOWS THROUGH STENOSED ARTERIES". International Journal of Modern Physics C 14, nr 05 (czerwiec 2003): 635–59. http://dx.doi.org/10.1142/s0129183103004838.

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A detailed analysis on the characteristics of transitional turbulent flow over a bell-shape stenosis for a physiological pulsatile flow is presented. The comparison of the numerical solutions to three types of pulsatile flows, including a physiological flow, an equivalent pulsatile flow and a simple pulsatile flow, are made in this work. Then the effects of the Reynolds number, Womersley number and constriction ratio of stenosis on the pulsatile turbulent flow fields for the physiological flow are considered. The comparison of the three pulsatile flows shows that the flow characteristics cannot be properly estimated if an equivalent or simple pulsatile inflow is used instead of actual physiological one in the study of the pulsatile flows through arterial stenosis. The equivalent or simple pulsatile inflow can lead to higher disturbance intensity in the vicinity of the stenosis than the physiological inflow. For a physiological flow, the recirculation zones with high disturbance intensity occur mainly in the distal of the stenosis. The larger Reynolds number and severer constriction ratio may result in more complex flow field and cause some important flow variables to increase dramatically near stenosis. The higher Womersley number leads to a larger phase lag between the imposed flow rate changes and the final converged flow field in one cycle. The turbulence intensity decreases with the increase of Womersley number for the same Reynolds number.

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Valencia, Alvaro. "Pulsating Flow in a Channel With a Backward-Facing Step". Applied Mechanics Reviews 50, nr 11S (1.11.1997): S232—S236. http://dx.doi.org/10.1115/1.3101841.

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The incompressible laminar flow in a channel with a backward-facing step is studied for steady cases and for pulsating inlet flow conditions. For steady flows, the influrnce of the inlet velocity profile, the height of the step, and the Reynolds number on the reattachment length is investigated. A parabolic entrance profile was used for pulsating flow. It was found with amplitude of oscillation of one by Re = 100 that the primary vortex breakdown through one pulsatile cycle and the wall shear stress in the separation zone varied markedly with pulsating inlet flow.

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Mathie, RT, JB Desai i KM Taylor. "The effect of normothermic cardiopulmonary bypass on hepatic blood flow in the dog". Perfusion 1, nr 4 (październik 1986): 245–53. http://dx.doi.org/10.1177/026765918600100403.

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Hepatic blood flow was investigated in two groups of eight anaesthetized dogs during and after one hour of either pulsatile or non-pulsatile cardiopulmonary bypass (CPB). Mean perfusion pressure was maintained at 60 mmHg. Hepatic arterial (HA) and portal venous (PV) blood flows were measured using electromagnetic flow probes, and hepatic O 2 consumption determined. The results demonstrate that: (a) pulsatile CPB reduces peripheral vascular resistance during and after perfusion, and more effectively preserves pump flow rate and cardiac output than non-pulsatile CPB; (b) total liver blood flow is sustained more effectively by pulsatile CPB than by non-pulsatile CPB due to relative preservation of both HA and PV flows; (c) hepatic O2 consumption is only marginally better preserved during and after pulsatile CPB than with non-pulsatile perfusion. We conclude that: (a) pulsatile CPB tends to maintain hepatic blood flow through a relative reduction in HA vascular resistance and an improvement in PV flow produced passively by a greater pump flow rate; (b) pulsatile CPB less effectively benefits hepatic O2 consumption because of poor O2 uptake from the hepatic PV blood supply.

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Jasikova, Darina, Michal Kotek, Frantisek Pochyly i Vaclav Kopecky. "Flow field velocity measurement of liquid interaction with rigid and flexible wall". EPJ Web of Conferences 213 (2019): 02031. http://dx.doi.org/10.1051/epjconf/201921302031.

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The motivation of this research was to determine the flow interactions on the pulsation and to express the influence on the flow character in the rigid and flexible tube. The character of Newtonian liquid was measured with the Particle Image Velocimetry method (PIV). Here, we used glass tube and Tygon tube for our comparison. We build the circuit equipped with membrane pump for generating pulsatile flow. The results were analysed over the pulse period sampled in 10 time steps. The fluid flow varied from Re 560 to Re 8800. The velocity profiles uncovered backward revers flows closed to the wall. These structures are prevailing close to flexible wall. The effect of interaction between pulsatile liquid flow and flexible wall was experimentally proved.

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Yeh, Chih-Kuang, i Pai-Chi Li. "Doppler Angle Estimation of Pulsatile Flows Using AR Modeling". Ultrasonic Imaging 24, nr 2 (kwiecień 2002): 65–80. http://dx.doi.org/10.1177/016173460202400201.

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In quantitative ultrasonic flow measurements, the beam-to-flow angle (i.e., Doppler angle) is an important parameter. An autoregressive (AR) spectral analysis technique in combination with the Doppler spectrum broadening effect was previously proposed to estimate the Doppler angle. Since only a limited number of flow samples are used, real-time two-dimensional Doppler angle estimation is possible. The method was validated for laminar flows with constant velocities. In clinical applications, the flow pulsation needs to be considered. For pulsatile flows, the flow velocity is time-varying and the accuracy of Doppler angle estimation may be affected. In this paper, the AR method using only a limited number of flow samples was applied to Doppler angle estimation of pulsatile flows. The flow samples were properly selected to derive the AR coefficients and then more samples were extrapolated based on the AR model. The proposed method was verified by both simulations and in vitro experiments. A wide range of Doppler angles (from 30° to 78°) and different flow rates were considered. The experimental data for the Doppler angle showed that the AR method using eight flow samples had an average estimation error of 3.50° compared to an average error of 7.08° for the Fast Fourier Transform (FFT) method using 64 flow samples. Results indicated that the AR method not only provided accurate Doppler angle estimates, but also outperformed the conventional FFT method in pulsatile flows. This is because the short data acquisition time is less affected by the temporal velocity changes. It is concluded that real-time two-dimensional estimation of the Doppler angle is possible using the AR method in the presence of pulsatile flows. In addition, Doppler angle estimation with turbulent flows is also discussed. Results show that both the AR and FFT methods are not adequate due to the spectral broadening effects from the turbulence.

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Xu, Duo, i Marc Avila. "The effect of pulsation frequency on transition in pulsatile pipe flow". Journal of Fluid Mechanics 857 (30.10.2018): 937–51. http://dx.doi.org/10.1017/jfm.2018.789.

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Pulsatile flows are common in nature and in applications, but their stability and transition to turbulence are still poorly understood. Even in the simple case of pipe flow subject to harmonic pulsation, there is no consensus among experimental studies on whether pulsation delays or enhances transition. We here report direct numerical simulations of pulsatile pipe flow at low pulsation amplitude$A\leqslant 0.4$. We use a spatially localized impulsive disturbance to generate a single turbulent puff and track its dynamics as it travels downstream. The computed relaminarization statistics are in quantitative agreement with the experiments of Xuet al. (J. Fluid Mech., vol. 831, 2017, pp. 418–432) and support the conclusion that increasing the pulsation amplitude and lowering the frequency enhance the stability of the flow. In the high-frequency regime, the behaviour of steady pipe flow is recovered. In addition, we show that, when the pipe length does not permit the observation of a full cycle, a reduction of the transition threshold is observed. We obtain an equation quantifying this effect and compare it favourably with the measurements of Stettler & Hussain (J. Fluid Mech., vol. 170, 1986, pp. 169–197). Our results resolve previous discrepancies, which are due to different pipe lengths, perturbation methods and criteria chosen to quantify transition in experiments.

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Hoseinzadeh, S., P. S. Heyns i H. Kariman. "Numerical investigation of heat transfer of laminar and turbulent pulsating Al2O3/water nanofluid flow". International Journal of Numerical Methods for Heat & Fluid Flow 30, nr 3 (20.09.2019): 1149–66. http://dx.doi.org/10.1108/hff-06-2019-0485.

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Purpose The purpose of this paper is to investigate the heat transfer of laminar and turbulent pulsating Al203/water nanofluid flow in a two-dimensional channel. In the laminar flow range, with increasing Reynolds number (Re), the velocity gradient is increased. Also, the Nusselt number (Nu) is increased, which causes increase in the overall heat transfer rate. Additionally, in the change of flow regime from laminar to turbulent, average thermal flux and pulsation range are increased. Also, the effect of different percentage of Al2O3/water nanofluid is investigated. The results show that the addition of nanofluids improve thermal performance in channel, but the using of nanofluid causes a pressure drop in the channel. Design/methodology/approach The pulsatile flow and heat transfer in a two-dimensional channel were investigated. Findings The numerical results show that the Al2O3/Water nanofluid has a significant effect on the thermal properties of the different flows (laminar and turbulent) and the average thermal flux and pulsation ranges are increased in the change of flow regime from laminar to turbulent. Also, the addition of nanofluid improves thermal performance in channels. Originality/value The originality of this work lies in proposing a numerical analysis of heat transfer of pulsating Al2O3/Water nanofluid flow -with different percentages- in the two-dimensional channel while the flow regime change from laminar to turbulent.

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Yada, Isao, i Yoshinori Mitamura. "Pulsatile flow versus nonpulsatile flow". Journal of Artificial Organs 2, nr 1 (marzec 1999): 1–2. http://dx.doi.org/10.1007/bf01235516.

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Liu, H., i T. Yamaguchi. "Waveform Dependence of Pulsatile Flow in a Stenosed Channel". Journal of Biomechanical Engineering 123, nr 1 (16.10.2000): 88–96. http://dx.doi.org/10.1115/1.1339818.

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Blood flow in arteries often shows a rich variety of vortical flows, which are dominated by the complex geometry of blood vessels, the dynamic pulsation of blood flow, and the complicated boundary conditions. With a two-dimensional model of unsteady flow in a stenosed channel, the pulsatile influence on such vortical fluid dynamics has been numerically studied in terms of waveform dependence on physiological pulsation. Results are presented for unsteady flows downstream of the stenosed portion with variation in the waveforms of systole and diastole. Overall, a train of propagating vortex waves is observed for all the cases, but it shows great sensitivity to the waveforms. The generation and development of the vortex waves may be linked to the presence of an adverse pressure gradient within a specific interval between two points of inflection of the systolic waveform. The adverse pressure gradient consists of a global pressure gradient that is found to be closely related to the dynamics of the pulsation, and a local pressure gradient, which is observed to be dominated by the nonlinear vortex dynamics.

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HUNG, TIN-KAN, i TOMMY M. C. TSAI. "NONLINEAR PULSATILE FLOWS IN RIGID AND DISTENSIBLE ARTERIES". Journal of Mechanics in Medicine and Biology 04, nr 04 (grudzień 2004): 419–34. http://dx.doi.org/10.1142/s0219519404001168.

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Blood flow in distensible arteries is nonlinear and time-dependent. The radial motion of the wall alters the dimension and geometry of the flow field. The nonlinear pulsating flow processes are successfully computed by mapping the wavy flow field to a fixed domain and by casting the geometric, kinematic and dynamic parameters of the flow into a dimensionless form of the Navier–Stokes equations. The complexity of the equations is compensated by a significant advantage in finite difference solutions. It is accomplished by using a fixed regular mesh network to model time-dependent irregular meshes in the physical domain for boundary layer development and vortices in pulsatile flows.

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Faryami, Ahmad, Adam Menkara, Daniel Viar i Carolyn A. Harris. "Testing and validation of reciprocating positive displacement pump for benchtop pulsating flow model of cerebrospinal fluid production and other physiologic systems". PLOS ONE 17, nr 5 (12.05.2022): e0262372. http://dx.doi.org/10.1371/journal.pone.0262372.

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Background The flow of physiologic fluids through organs and organs systems is an integral component of their function. The complex fluid dynamics in many organ systems are still not completely understood, and in-vivo measurements of flow rates and pressure provide a testament to the complexity of each flow system. Variability in in-vivo measurements and the lack of control over flow characteristics leave a lot to be desired for testing and evaluation of current modes of treatments as well as future innovations. In-vitro models are particularly ideal for studying neurological conditions such as hydrocephalus due to their complex pathophysiology and interactions with therapeutic measures. The following aims to present the reciprocating positive displacement pump, capable of inducing pulsating flow of a defined volume at a controlled beat rate and amplitude. While the other fluidic applications of the pump are currently under investigation, this study was focused on simulating the pulsating cerebrospinal fluid production across profiles with varying parameters. Methods Pumps were manufactured using 3D printed and injection molded parts. The pumps were powered by an Arduino-based board and proprietary software that controls the linear motion of the pumps to achieve the specified output rate at the desired pulsation rate and amplitude. A range of 0.01 mlmin to 0.7 mlmin was tested to evaluate the versatility of the pumps. The accuracy and precision of the pumps’ output were evaluated by obtaining a total of 150 one-minute weight measurements of degassed deionized water per output rate across 15 pump channels. In addition, nine experiments were performed to evaluate the pumps’ control over pulsation rate and amplitude. Results Volumetric analysis of a total of 1200 readings determined that the pumps achieved the target output volume rate with a mean absolute error of -0.001034283 mlmin across the specified domain. It was also determined that the pumps can maintain pulsatile flow at a user-specified beat rate and amplitude. Conclusion The validation of this reciprocating positive displacement pump system allows for the future validation of novel designs to components used to treat hydrocephalus and other physiologic models involving pulsatile flow. Based on the promising results of these experiments at simulating pulsatile CSF flow, a benchtop model of human CSF production and distribution could be achieved through the incorporation of a chamber system and a compliance component.

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Xu, Duo, Sascha Warnecke, Baofang Song, Xingyu Ma i Björn Hof. "Transition to turbulence in pulsating pipe flow". Journal of Fluid Mechanics 831 (13.10.2017): 418–32. http://dx.doi.org/10.1017/jfm.2017.620.

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Fluid flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of flow through a straight pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, $\unicode[STIX]{x1D6FC}$), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low $\unicode[STIX]{x1D6FC}$. In the present study we apply recent advances in the understanding of transition in steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence (‘puffs’) analogous to steady pipe flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile flow can be separated into three regimes. At small Womersley numbers the dynamics is dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi-steady flow assumption only taking puff decay rates into account. The transition point predicted in the zero $\unicode[STIX]{x1D6FC}$ limit equals to the critical point for steady pipe flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand, puff lifetimes are identical to those in steady pipe flow and hence the transition threshold appears to be unaffected by flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing $\unicode[STIX]{x1D6FC}$) from the decay dominated (quasi-steady) threshold to the steady pipe flow level.

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Xu, Duo, Atul Varshney, Xingyu Ma, Baofang Song, Michael Riedl, Marc Avila i Björn Hof. "Nonlinear hydrodynamic instability and turbulence in pulsatile flow". Proceedings of the National Academy of Sciences 117, nr 21 (11.05.2020): 11233–39. http://dx.doi.org/10.1073/pnas.1913716117.

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Pulsating flows through tubular geometries are laminar provided that velocities are moderate. This in particular is also believed to apply to cardiovascular flows where inertial forces are typically too low to sustain turbulence. On the other hand, flow instabilities and fluctuating shear stresses are held responsible for a variety of cardiovascular diseases. Here we report a nonlinear instability mechanism for pulsating pipe flow that gives rise to bursts of turbulence at low flow rates. Geometrical distortions of small, yet finite, amplitude are found to excite a state consisting of helical vortices during flow deceleration. The resulting flow pattern grows rapidly in magnitude, breaks down into turbulence, and eventually returns to laminar when the flow accelerates. This scenario causes shear stress fluctuations and flow reversal during each pulsation cycle. Such unsteady conditions can adversely affect blood vessels and have been shown to promote inflammation and dysfunction of the shear stress-sensitive endothelial cell layer.

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Fu, Yang, Yimin Hu, Feng Huang i Maoying Zhou. "The Impact of Pulsatile Flow on Suspension Force for Hydrodynamically Levitated Blood Pump". Journal of Healthcare Engineering 2019 (3.06.2019): 1–6. http://dx.doi.org/10.1155/2019/8065920.

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Hydrodynamically levitated rotary blood pumps (RBPs) with noncontact bearing are effective to enhance the blood compatibility. The spiral groove bearing (SGB) is one of the key components which offer the suspension force to the RBP. Current studies focus on the suspension performance of the SGB under continuous flow condition. However, the RBP shows pulsatile characteristics in the actual clinical application, which may affect the suspension performance of the SGB. In this paper, the impact of pulsatile flow upon the suspension force from the SGB is studied. A model of the SGB with a groove formed of wedge-shaped spirals is built. Then, the CFD calculation of the hydrodynamic force offered by designed SGB under simulated pulsatile flow is introduced to obtain the pulsatile performance of the suspension force. The proposed method was validated by experiments measuring the hydrodynamic force with different bearing gaps. The results show that the suspension force of the SGB under pulsate flow is the same as under steady flow with equivalent effective pressure. This paper provides a method for suspension performance test of the SGB.

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Cullen, John P., Shariq Sayeed, Rebecca S. Sawai, Nicholas G. Theodorakis, Paul A. Cahill, James V. Sitzmann i Eileen M. Redmond. "Pulsatile Flow–Induced Angiogenesis". Arteriosclerosis, Thrombosis, and Vascular Biology 22, nr 10 (październik 2002): 1610–16. http://dx.doi.org/10.1161/01.atv.0000034470.37007.58.

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HINDMAN, B. J., F. DEXTER, T. SMITH i P. CUTKOMP. "Pulsatile Versus Nonpulsatile Flow". Survey of Anesthesiology 39, nr 6 (grudzień 1995): 352. http://dx.doi.org/10.1097/00132586-199512000-00015.

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Wright, Gordon, i Anthony Furness. "What Is Pulsatile Flow?" Annals of Thoracic Surgery 39, nr 5 (maj 1985): 401–2. http://dx.doi.org/10.1016/s0003-4975(10)61943-1.

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JAMES, C. B. "Pulsatile ocular blood flow". British Journal of Ophthalmology 82, nr 7 (1.07.1998): 720–21. http://dx.doi.org/10.1136/bjo.82.7.720.

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Zamir,, M., i RS Budwig,. "Physics of Pulsatile Flow". Applied Mechanics Reviews 55, nr 2 (1.03.2002): B35. http://dx.doi.org/10.1115/1.1451229.

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Harrison, S. T. L., i M. R. Mackley. "A pulsatile flow bioreactor". Chemical Engineering Science 47, nr 2 (luty 1992): 490–93. http://dx.doi.org/10.1016/0009-2509(92)80039-f.

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Sohma, A., K. Ohga, T. Oka, Y. Oda, T. Itoh i T. Morimoto. "Quantitative analysis of the effect of pulsatile flow on vascular resistance". Perfusion 4, nr 3 (lipiec 1989): 213–21. http://dx.doi.org/10.1177/026765918900400307.

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'Pulsatile flow' has been reported to reduce vascular resistance. In this study, the effect of pulsatile flow was assessed quantitatively, using perfusion of canine hindlimb. The perfusion circuit consisted of roller-type pulsatile pump (Cobe Inc., Stockert pump) and bubble oxygenator (Shiley Inc., S-070/s). Each flow curve was quantified with the mean flow rate (F) and pulse power index (PPI). PPI is derived by Fourier transformation of the flow curve and represents the degree of 'pulsation'. Vascular resistance was determined during perfusion with varied flow rate and PPI. The regression formula between vascular resistance (VR) and two parameters was obtained as follows: VR(F,PPI) = 41600x(F-1.37) -0.95+913 - PPIx{(3.99x(F-1.33)-040_0.69} where VR is measured in mmHg. min-1.kg.ml-1; F represents ml.min-1.kg -1 (range from 2.8 to 17.1); and PPI is dimensionless (range from 2.8 to 215.7). Using this formula, vascular resistance at a fixed flow rate and wave form can be predicted. When the flow rate is 6.27 ml.min-1.kg-1 and the PPI is 1466, (measured values under perfusion with own beating heart) the vascular resistance perfused by own beating heart is obtained. The results indicate that the pulsation of own beating heart contributes to a reduction in vascular resistance to 80%. It is also shown that the value of PPI which is necessary to reduce the vascular resistance to 80% is more than 1300 under the normal flow rate range.

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Nakano, Toshihide, Ryuji Tominaga, Ichiro Nagano, Hayato Okabe i Hisataka Yasui. "Pulsatile flow enhances endothelium-derived nitric oxide release in the peripheral vasculature". American Journal of Physiology-Heart and Circulatory Physiology 278, nr 4 (1.04.2000): H1098—H1104. http://dx.doi.org/10.1152/ajpheart.2000.278.4.h1098.

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The effects of pulsatility in blood flow on endothelium-derived nitric oxide (EDNO) release in the peripheral vasculature were investigated. The basal and flow-stimulated EDNO release were compared between pulsatile and nonpulsatile systemic flows before and after the administration of NO synthase inhibitor NG-monomethyl-l-arginine (l-NMMA). Peripheral vascular resistance (PVR) was significantly lower in pulsatile flow than in nonpulsatile flow, but this difference disappeared after l-NMMA. The percent increase in PVR by l-NMMA was significantly larger in pulsatile flow. In reactive hyperemia in the hindlimb, the peak flow did not differ; however, both the repayment flow and the duration were significantly larger in pulsatile flow. Percent changes of these parameters by l-NMMA were significantly larger in pulsatile flow. These data indicated that pulsatility significantly enhances the basal and flow-stimulated EDNO release in the peripheral vasculature under in vivo conditions. We also studied the involvement of the Ca2+-dependent and Ca2+-independent pathways in flow-induced vasodilation using calmodulin inhibitor calmidazolium and tyrosine kinase inhibitor erbstatin A. PVR was significantly elevated by erbstatin A but not by calmidazolium, suggesting that flow-induced vasodilation was largely caused by tyrosine kinase inhibitor-sensitive activation of NO synthase.

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Tutty, O. R. "Pulsatile Flow in a Constricted Channel". Journal of Biomechanical Engineering 114, nr 1 (1.02.1992): 50–54. http://dx.doi.org/10.1115/1.2895449.

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A nonuniform channel is used as a simple model of a constricted arterial vessel. Flow patterns have been calculated for pulsatile flow with both sinusoidal and nonsinusoidal flow rates for a range of Reynolds number, Re, and Strouhal number, St. The results show that even for relatively low frequency flows a strong vortex wave will be generated with a complex wall shear stress distribution and peak values much greater than those found in steady or unsteady parallel flow. The vortex wave increases in strength with increasing Re and St, with its total length and wavelength independent of Re but inversely proportional to St. The form of the imposed flow rate is found to have an important effect on the flow and the shear stress distribution.

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Runge, T. M., J. C. Briceño T., M. E. Sheller, C. E. Moritz, L. Sloan, F. O. Bohls i S. E. Ottmers. "Hemodialysis: Evidence of Enhanced Molecular Clearance and Ultrafiltration Volume by Using Pulsatile Flow". International Journal of Artificial Organs 16, nr 9 (wrzesień 1993): 645–52. http://dx.doi.org/10.1177/039139889301600904.

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We describe several in vitro experiments showing evidence that pulsatile flow hemodialysis enhances ultrafiltration volume and molecular clearance as compared with steady flow hemodialysis. A new pulsatile pump and a conventional roller pump were compared using different hollow fiber dialyzers and a simulated blood solution containing urea, aspartame and vitamin B-12 at different flow rates and configurations. Ultrafiltration volume and concentration of urea, aspartame and B-12 were measured and molecular clearance (K) calculated. Ultrafiltration volume markedly increased with pulsatile flow. After 10 min K for urea with pulsatile flow was higher in all experiments even when ultrafiltration was prevented. Clearance of aspartame and B-12 also increased with pulsatile flow. We propose three mechanisms by which pulsatile flow is more efficient than steady flow hemodialysis: greater fluid energy, avoidance of molecular channeling and avoidance of membrane layering. We hypothesize that using pulsatile flow in hemodialysis can significantly shorten the duration of dialysis sessions for most of the patients, and consequently reduce the duration of the procedure and its cost.

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Finol, E. A., K. Keyhani i C. H. Amon. "The Effect of Asymmetry in Abdominal Aortic Aneurysms Under Physiologically Realistic Pulsatile Flow Conditions". Journal of Biomechanical Engineering 125, nr 2 (1.04.2003): 207–17. http://dx.doi.org/10.1115/1.1543991.

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In the abdominal segment of the human aorta under a patient’s average resting conditions, pulsatile blood flow exhibits complex laminar patterns with secondary flows induced by adjacent branches and irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. This work examines the hemodynamics of pulsatile blood flow in hypothetical three-dimensional models of abdominal aortic aneurysms (AAAs). Numerical predictions of blood flow patterns and hemodynamic stresses in AAAs are performed in single-aneurysm, asymmetric, rigid wall models using the finite element method. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating flow-induced stresses at the aneurysm wall, specifically wall pressure and wall shear stress. Physiologically realistic abdominal aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50⩽Rem⩽300, corresponding to a range of peak Reynolds numbers 262.5⩽Repeak⩽1575. The vortex dynamics induced by pulsatile flow in AAAs is depicted by a sequence of four different flow phases in one period of the cardiac pulse. Peak wall shear stress and peak wall pressure are reported as a function of the time-average Reynolds number and aneurysm asymmetry. The effect of asymmetry in hypothetically shaped AAAs is to increase the maximum wall shear stress at peak flow and to induce the appearance of secondary flows in late diastole.

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Brindise, Melissa C., i Pavlos P. Vlachos. "Pulsatile pipe flow transition: Flow waveform effects". Physics of Fluids 30, nr 1 (styczeń 2018): 015111. http://dx.doi.org/10.1063/1.5021472.

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Ündar, A., T. Masai, S. Q. Yang, M. Mueller, M. McGarry, R. Inman, O. H. Frazier i C. D. Fraser. "PULSATILE FLOW AND REGIONAL CEREBRAL BLOOD FLOW". ASAIO Journal 45, nr 2 (marzec 1999): 158. http://dx.doi.org/10.1097/00002480-199903000-00154.

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HEWITT, RICHARD E., i PETER W. DUCK. "Pulsatile jets". Journal of Fluid Mechanics 670 (12.01.2011): 240–59. http://dx.doi.org/10.1017/s0022112010005227.

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We consider the evolution of high-Reynolds-number, planar, pulsatile jets in an incompressible viscous fluid. The source of the jet flow comprises a mean-flow component with a superposed temporally periodic pulsation, and we address the spatiotemporal evolution of the resulting system. The analysis is presented for both a free symmetric jet and a wall jet. In both cases, pulsation of the source flow leads to a downstream short-wave linear instability, which triggers a breakdown of the boundary-layer structure in the nonlinear regime. We extend the work of Riley, Sánchez-Sans & Watson (J. Fluid Mech., vol. 638, 2009, p. 161) to show that the linear instability takes the form of a wave that propagates with the underlying jet flow, and may be viewed as a (spatially growing) weakly non-parallel analogue of the (temporally growing) short-wave modes identified by Cowley, Hocking & Tutty (Phys. Fluids, vol. 28, 1985, p. 441). The nonlinear evolution of the instability leads to wave steepening, and ultimately a singular breakdown of the jet is obtained at a critical downstream position. We speculate that the form of the breakdown is associated with the formation of a ‘pseudo-shock’ in the jet, indicating a failure of the (long-length scale) boundary-layer scaling. The numerical results that we present disagree with the recent results of Riley et al. (2009) in the case of a free jet, together with other previously published works in this area.

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He, Wen Bo, Yuan Wang i Xiao Yan Gong. "Influences of the Compliances and the Resistance on Pulsatile Flow Waveforms". Applied Mechanics and Materials 477-478 (grudzień 2013): 217–20. http://dx.doi.org/10.4028/www.scientific.net/amm.477-478.217.

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A pulsatile circulation system has been built to simulate aortic blood flow. The system has both upstream and downstream compliances and resistances to simulate the elasticity and resistances of blood circulation system. Influences of the compliances and resistances on the pulsatile pressure waveforms have been systematically studied using this system. We found that in absence of the compliances and the downstream resistance, it results in an oversize negative pulsation to the pressure waveforms. The downstream resistance hardly affects the structure of the pressure waveforms, whereas the mean amplitude of pressure increases along with the downstream resistance. The compliance can reduce the peak value of the pressure, which is unrelated to the location of the compliance but related to the elasticity in the system. With fixed upstream system elasticity, the pressure waveform is more stable.

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Hakim, T. S. "Flow-induced release of EDRF in the pulmonary vasculature: site of release and action". American Journal of Physiology-Heart and Circulatory Physiology 267, nr 1 (1.07.1994): H363—H369. http://dx.doi.org/10.1152/ajpheart.1994.267.1.h363.

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Pulsatile flow is thought to lower pulmonary vascular resistance by passive recruitment of capillaries and by active vasodilation. This study was undertaken to investigate the role of endothelium-derived relaxing factor (EDRF) during pulsatile flow in isolated canine left lower lobes pretreated with indomethacin. The lobes were perfused in situ with autologous blood (approximately 500 ml/min) using a nonpulsatile pump (Masterflex) or a pulsatile pump (Harvard). With the occlusion techniques, vascular resistance was partitioned into four segments: arterial (Ra), small arterial (R'a), small venous, and venous (Rv). Pulsatile flow (frequency = 70 min-1) did not lower total vascular resistance during baseline or during vasoconstriction. Distribution of vascular resistance among the four segments was not altered significantly by pulsatile flow during normoxia and angiotensin. In contrast, switching to pulsatile flow during hypoxia was associated with an increase in Ra and a decrease in R'a and Rv. N omega-nitro-L-arginine (L-NNA) had no effect on total or segmental resistance during baseline conditions but potentiated the hypoxic pressor response and prevented its recovery by 50%. In addition, the reduction in R'a by pulsatile flow was attenuated by L-NNA, suggesting that EDRF is released by pulsatile flow in this segment. We conclude that a shear stress-induced EDRF release from the small arteries is present in canine lungs and is experimentally demonstrable during pulsatile flow and hypoxia.

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Ryval, J., A. G. Straatman i D. A. Steinman. "Two-equation Turbulence Modeling of Pulsatile Flow in a Stenosed Tube". Journal of Biomechanical Engineering 126, nr 5 (1.10.2004): 625–35. http://dx.doi.org/10.1115/1.1798055.

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The study of pulsatile flow in stenosed vessels is of particular importance because of its significance in relation to blood flow in human pathophysiology. To date, however, there have been few comprehensive publications detailing systematic numerical simulations of turbulent pulsatile flow through stenotic tubes evaluated against comparable experiments. In this paper, two-equation turbulence modeling has been explored for sinusoidally pulsatile flow in 75% and 90% area reduction stenosed vessels, which undergoes a transition from laminar to turbulent flow as well as relaminarization. Wilcox’s standard k-ω model and a transitional variant of the same model are employed for the numerical simulations. Steady flow through the stenosed tubes was considered first to establish the grid resolution and the correct inlet conditions on the basis of comprehensive comparisons of the detailed velocity and turbulence fields to experimental data. Inlet conditions based on Womersley flow were imposed at the inlet for all pulsatile cases and the results were compared to experimental data from the literature. In general, the transitional version of the k-ω model is shown to give a better overall representation of both steady and pulsatile flow. The standard model consistently over predicts turbulence at and downstream of the stenosis, which leads to premature recovery of the flow. While the transitional model often under-predicts the magnitude of the turbulence, the trends are well-described and the velocity field is superior to that predicted using the standard model. On the basis of this study, there appears to be some promise for simulating physiological pulsatile flows using a relatively simple two-equation turbulence model.

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38

Shi, Z. D., S. H. Winoto i T. S. Lee. "Experimental Investigation of Pulsatile Flows in Tubes". Journal of Biomechanical Engineering 119, nr 2 (1.05.1997): 213–16. http://dx.doi.org/10.1115/1.2796082.

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Based on cam-piston-valve arrangement, a mechanical pulsatile flow generator is designed to investigate sinusoidal flow and other types of pulsatile flow in straight rigid tube. Measurement reveals the relation between pressure gradient and flow rate. Numerical simulation using the k-ε turbulence model are carried out to compare the pulsatile flow produced by the generator with a sinusoidal flow and a physiological flow in a rigid tube. The results show that the pulsatile flow generated has similar dynamic properties to the physiological flow. Hence, the present setup can be used for in-vitro investigation of biofluid phenomena.

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Yun, B. Min, L. P. Dasi, C. K. Aidun i A. P. Yoganathan. "Computational modelling of flow through prosthetic heart valves using the entropic lattice-Boltzmann method". Journal of Fluid Mechanics 743 (3.03.2014): 170–201. http://dx.doi.org/10.1017/jfm.2014.54.

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AbstractPrevious clinical, in vitro experimental and in silico simulation studies have shown the complex dynamics of flow through prosthetic heart valves. In the case of bileaflet mechanical heart valves (BMHVs), complex flow phenomena are observed due to the presence of two rigid leaflets. A numerical method for this type of study must be able to accurately simulate pulsatile flow through BMHVs with the inclusion of leaflet motion and high-Reynolds-number flow modelling. Consequently, this study aims at validating a numerical method that captures the flow dynamics for pulsatile flow through a BMHV. A $23~ \mbox{mm}$ St. Jude Medical (SJM) Regent™ valve is selected for use in both the experiments and numerical simulations. The entropic lattice-Boltzmann method is used to simulate pulsatile flow through the valve with the inclusion of reverse leakage flow, while prescribing the flowrate and leaflet motion from experimental data. The numerical simulations are compared against experimental digital particle image velocimetry (DPIV) results from a previous study for validation. The numerical method is shown to match well with the experimental results quantitatively as well as qualitatively. Simulations are performed with efficient parallel processing at very high spatiotemporal resolution that can capture the finest details in the pulsatile BMHV flow field. This study validates the lattice-Boltzmann method as suitable for simulating pulsatile, high-Reynolds-number flows through prosthetic devices for use in future research.

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Qian, K. X., T. Jing, H. Y. Yuan, H. Wang, F. Q. Wang i P. Zeng. "R&D of a Pulsatile Rotary Heart Pump Imitating the Native Ventricle". Applied Mechanics and Materials 190-191 (lipiec 2012): 1234–37. http://dx.doi.org/10.4028/www.scientific.net/amm.190-191.1234.

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It is evident that a pulsatile flow is important for blood circulation because the flow pulsatility can reduce the resistance of peripheral vessels. It is difficult, however, to produce a pulsatile flow with an impeller pump, since blood damage will occur when a pulsatile flow is produced. Further investigation has revealed that the main factor for blood damage is turbulence shear, which tears the membranes of red blood cells, resulting in free release of haemoglobin into the plasma, and consequently lead to haemolysis. Therefore, the question for producing a pulsatile flow with low haemolysis becomes how to develop a pulsatile impeller pump with less turbulence? The authors have successively developed a pulsatile axial pump and a pulsatile centrifugal pump. In the pulsatile axial pump, the impeller reciprocates axially and rotates simultaneously. The reciprocation is driven by a pneumatic device and the rotation by a DC motor. For a pressure of 40mm Hg pulsatility, about 50mm axially reciprocation amplitude of the impeller is desirable. In order to reduce the axial amplitude, the pump inlet and the impeller both have cone-shaped heads, thus the gap between the impeller and the inlet pipe changes by only 2mm, that is, the impeller reciprocates up to 2mm, a pressure pulsatility of 40mmHg can be produced. As the impeller rotates with a constant speed, low turbulence in the pump can be expected. In the centrifugal pulsatile pump, the impeller changes its rotating speed periodically; the turbulence is reduced by designing an impeller with twisted vanes which enable the blood flow to change its direction rather than its magnitude during the periodic change of the rotating speed. In this way, a pulsatile flow is produced and the turbulence is minimized. Compared to the axial pulsatile pump, the centrifugal pulsatile pump needs only one driver and thus has more application possibilities. The centrifugal pulsatile pump has been used in animal experiments. The pump assisted the circulation of calves for several months without harm to the blood elements and the organ functions of the experimental animal. The experiments demonstrated that the pulsatile impeller is the most efficient pump for assisting heart recovery, because it can produce a pulsatile flow like a diaphragm pump and has no back flow as what occurs in a non-pulsatile rotary pump; the former reduces the circulatory resistance and the later increases the diastole pressure in aorta, and thus increase the perfusion of coronary arteries of the natural heart.

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Gourlay, T., i KM Taylor. "Pulsatile flow and membrane oxygenators". Perfusion 9, nr 3 (maj 1994): 189–96. http://dx.doi.org/10.1177/026765919400900306.

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Wang, I., A. Natalio, K. N. Litwak, S. Kihara, C. F. McTiernan, A. M. Feldman i K. L. Kormos. "PULSATILE VS CONTINUOUS FLOW LVAD". ASAIO Journal 48, nr 2 (marzec 2002): 150. http://dx.doi.org/10.1097/00002480-200203000-00101.

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Vajravelu, K., K. Ramesh, S. Sreenadh i P. V. Arunachalam. "Pulsatile flow between permeable beds". International Journal of Non-Linear Mechanics 38, nr 7 (październik 2003): 999–1005. http://dx.doi.org/10.1016/s0020-7462(02)00045-8.

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Ündar, Akif, Mohammed S. Koudieh i Charles D. Fraser. "Precise quantification of pulsatile flow". Annals of Thoracic Surgery 73, nr 4 (kwiecień 2002): 1358. http://dx.doi.org/10.1016/s0003-4975(02)03378-7.

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Chakravarty, S. "Pulsatile blood flow through arterioles". Rheologica Acta 26, nr 2 (marzec 1987): 200–207. http://dx.doi.org/10.1007/bf01331978.

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Dincau, Brian, Emilie Dressaire i Alban Sauret. "Pulsatile Flow in Microfluidic Systems". Small 16, nr 9 (28.10.2019): 1904032. http://dx.doi.org/10.1002/smll.201904032.

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de Graaf, R. G., i J. P. Groen. "MR angiography with pulsatile flow". Magnetic Resonance Imaging 10, nr 1 (styczeń 1992): 25–34. http://dx.doi.org/10.1016/0730-725x(92)90369-b.

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de Andrade Lima, L. R. P., i A. D. Rey. "Pulsatile flow of discotic mesophases". Chemical Engineering Science 60, nr 23 (grudzień 2005): 6622–36. http://dx.doi.org/10.1016/j.ces.2005.05.042.

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Waaben, J., K. Andersen i B. Husum. "Pulsatile Flow During Cardiopulmonary Bypass:Evaluation of a New Pulsatile Pump". Scandinavian Journal of Thoracic and Cardiovascular Surgery 19, nr 2 (styczeń 1985): 149–53. http://dx.doi.org/10.3109/14017438509102710.

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YAO, YOSHIZUMI. "Comparison of pulsatile flow and non-pulsatile flow in peripheral circulation.Using laser Doppler flowmeter." Japanese journal of extra-corporeal technology 19, nr 2 (1993): 41–42. http://dx.doi.org/10.7130/hokkaidoshakai.19.2_41.

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