Showing papers on "Pulsatile flow published in 2007"

Physiologic Pulsatile Flow Bioreactor Conditioning of Poly(ethylene glycol)-based Tissue Engineered Vascular Grafts

Abstract: Mechanical conditioning represents a potential means to enhance the biochemical and biomechanical properties of tissue engineered vascular grafts (TEVGs). A pulsatile flow bioreactor was developed to allow shear and pulsatile stimulation of TEVGs. Physiological 120 mmHg/80 mmHg peak-to-trough pressure waveforms can be produced at both fetal and adult heart rates. Flow rates of 2 mL/sec, representative of flow through small diameter blood vessels, can be generated, resulting in a mean wall shear stress of approximately 6 dynes/cm(2) within the 3 mm ID constructs. When combined with non-thrombogenic poly(ethylene glycol) (PEG)-based hydrogels, which have tunable mechanical properties and tailorable biofunctionality, the bioreactor represents a flexible platform for exploring the impact of controlled biochemical and biomechanical stimuli on vascular graft cells. In the present study, the utility of this combined approach for improving TEVG outcome was investigated by encapsulating 10T-1/2 mouse smooth muscle progenitor cells within PEG-based hydrogels containing an adhesive ligand (RGDS) and a collagenase degradable sequence (LGPA). Constructs subjected to 7 weeks of biomechanical conditioning had significantly higher collagen levels and improved moduli relative to those grown under static conditions.

Direct numerical simulation of stenotic flows, Part 1: Steady flow

Abstract: Direct numerical simulations (DNS) of steady and pulsatile flow through 75% (by area reduction) stenosed tubes have been performed, with the motivation of understanding the biofluid dynamics of actual stenosed arteries. The spectral-element method, providing geometric flexibility and high-order spectral accuracy, was employed for the simulations. The steady flow results are examined here while the pulsatile flow analysis is dealt with in Part 2 of this study. At inlet Reynolds numbers of 500 and 1000, DNS predict a laminar flow field downstream of an axisymmetric stenosis and comparison to previous experiments show good agreement in the immediate post-stenotic region. The introduction of a geometric perturbation within the current model, in the form of a stenosis eccentricity that was 5% of the main vessel diameter at the throat, resulted in breaking of the symmetry of the post-stenotic flow field by causing the jet to deflect towards the side of the eccentricity and, at a high enough Reynolds number of 1000, jet breakdown occurred in the downstream region. The flow transitioned to turbulence about five diameters away from the stenosis, with velocity spectra taking on a broadband nature, acquiring a -5/3 slope that is typical of turbulent flows. Transition was accomplished by the breaking up of streamwise, hairpin vortices into a localized turbulent spot, reminiscent of the turbulent puff observed in pipe flow transition, within which r.m.s. velocity and turbulent energy levels were highest. Turbulent fluctuations and energy levels rapidly decayed beyond this region and flow relaminarized. The acceleration of the fluid through the stenosis resulted in wall shear stress (WSS) magnitudes that exceeded upstream levels by more than a factor of 30 but low WSS levels accompanied the flow separation zones that formed immediately downstream of the stenosis. Transition to turbulence in the case of the eccentric stenosis was found to be manifested as large temporal and spatial gradients of shear stress, with significant axial and circumferential variations in instantaneous WSS.

Hemodynamic and exercise performance with pulsatile and continuous-flow left ventricular assist devices.

Abstract: Background— Continuous-flow rotary pumps with axial design are increasingly used for left ventricular assist support. The efficacy of this design compared with pulsatile, volume displacement pumps, with respect to characteristics of left ventricular unloading, and exercise performance remains largely unstudied. Methods and Results— Thirty-four patients undergoing implantation with a pulsatile, volume displacement pump operating in a full-to-empty cycle (HeartMate XVE; Thoratec Inc, Pleasanton, Calif; n=16) or continuous-flow rotary pump with an axial design operating at a fixed rotor speed (HeartMate II; Thoratec Inc; n=18) were evaluated with right heart catheterization and echocardiography preoperatively and at 3 months postoperatively and cardiopulmonary exercise testing 3 months postoperatively. Support with either the XVE or II resulted in significant ( P 2 −XVE: 46.8±10.2 versus II: 49.1±13.6). Echocardiography at 3 months demonstrated a significantly ( P Conclusions— The HeartMate XVE or II provided equivalent degrees of hemodynamic support and exercise capacity. The XVE was associated with greater left ventricular volume unloading. Characteristics of left ventricular pressure and volume unloading between these pump designs and mode of operation do not influence early exercise performance.

Cerebrospinal Fluid Flow in the Normal and Hydrocephalic Human Brain

Abstract: Advances in magnetic resonance (MR) imaging techniques enable the accurate measurements of cerebrospinal fluid (CSF) flow in the human brain. In addition, image reconstruction tools facilitate the collection of patient-specific brain geometry data such as the exact dimensions of the ventricular and subarachnoidal spaces (SAS) as well as the computer-aided reconstruction of the CSF-filled spaces. The solution of the conservation of CSF mass and momentum balances over a finite computational mesh obtained from the MR images predict the patients' CSF flow and pressure field. Advanced image reconstruction tools used in conjunction with first principles of fluid mechanics allow an accurate verification of the CSF flow patters for individual patients. This paper presents a detailed analysis of pulsatile CSF flow and pressure dynamics in a normal and hydrocephalic patient. Experimental CSF flow measurements and computational results of flow and pressure fields in the ventricular system, the SAS and brain parenchyma are presented. The pulsating CSF motion is explored in normal and pathological conditions of communicating hydrocephalus. This paper predicts small transmantle pressure differences between lateral ventricles and SASs (~10 Pa). The transmantle pressure between ventricles and SAS remains small even in the hydrocephalic patient (~30 Pa), but the ICP pulsatility increases by a factor of four. The computational fluid dynamics (CFD) results of the predicted CSF flow velocities are in good agreement with Cine MRI measurements. Differences between the predicted and observed CSF flow velocities in the prepontine area point towards complex brain-CSF interactions. The paper presents the complete computational model to predict the pulsatile CSF flow in the cranial cavity

Direct numerical simulation of stenotic flows, Part 2 : Pulsatile flow.

Abstract: Direct numerical simulations (DNS) of stenotic flows under conditions of steady inlet flow were discussed in Part 1 of this study. DNS of pulsatile flow through the 75% stenosed tube (by area) employed for the computations in Part 1 is examined here. Analogous to the steady flow results, DNS predicts a laminar post-stenotic flow field in the case of pulsatile flow through the axisymmetric stenosis model, in contrast to previous experiments, in which intermittent disturbed flow regions and turbulent breakdown were observed in the downstream region. The introduction of a stenosis eccentricity, that was 5% of the main vessel diameter at the throat, resulted in periodic, localized transition to turbulence. Analysis in this study indicates that the early and mid-acceleration phases of the time period cycle were relatively stable, with no turbulent activity in the post-stenotic region. However, towards the end of acceleration, the starting vortex, formed earlier as the fluid accelerated through the stenosis at the beginning of acceleration, started to break up into elongated streamwise structures. These streamwise vortices broke down at peak flow, forming a turbulent spot in the post-stenotic region. In the early part of deceleration there was intense turbulent activity within this spot. Past the mid-deceleration phase, through to minimum flow, the inlet flow lost its momentum and the flow field began to relaminarize. The start of acceleration in the following cycle saw a recurrence of the entire process of a starting structure undergoing turbulent breakdown and subsequent relaminarization of the post-stenotic flow field. Peak wall shear stress (WSS) levels occurred at the stenosis throat, with the rest of the vessel experiencing much lower levels. Turbulent breakdown at peak flow resulted in a sharp amplification of instantaneous WSS magnitudes across the region corresponding to the turbulent spot, accompanied by large axial and circumferential fluctuations, even while ensemble-averaged axial shear stresses remained mostly low and negative. WSS levels dropped rapidly after the mid-deceleration phase, when the relaminarization process took over, and were almost identical to laminar, axisymmetric shear levels through most of the acceleration phase.

Controlled pulsatile drug release from a ferrogel by a high-frequency magnetic field

Abstract: Stimuli-responsive controlled release drug from polymeric devices has been received great attention, as it provides advantages such as better delivery efficiency and site-specific therapy over the conventional routes of drug delivery. By making use of these advantages, a number of studies have been successfully proposed to integrate active drug molecules and host materials, to manipulate drug release desirably. Many previous studies have been reported in response to specific stimuli, such as temperature, 1-3 pH,4,5 electric field,6,7 mechanical signal, 8 and ultrasound. 9 A very recent achievement of a protein-containing hydrogel, reported by Ehrick et al., 10 addressed the stimuli-sensitive hydrogel that can be triggered as a result of the conformational change of the embedded calmodulin protein interacting to bioactive agents. Such a transfer from sensing actuation to mechanical action is novel, but it may be hard for it to reach high accuracy and sensitivity in therapeutic dosing as a burst-controlled drug delivery system. The use of a magnetic field to modulate drug release from polymeric matrices was previously developed. 11-13 Saslawski et al. reported the alginated microspheres for pulsed release of insulin by an oscillating magnetic field. 14 The release rate of insulin from alginate-strontium ferrite microspheres was enhanced in the absence of a magnetic field. Recently, a polyelectrolyte microcapsule embedded with Co/Au used external magnetic fields of 100 -300 Hz and 1200 Oe to increase its permeability to macromolecules. 15 In previous studies, a ferrogel with direct current (dc) magnetic-sensitive properties has been characterized and the amount of drug released from the ferrogel was effectively restricted while applying an external dc magnetic field. 16,17 So far, little investigation has been addressed on the controlled drug release under high-frequency magnetic fields (HFMF). A real-time burst release of drug needs a fast-responsive drug release system to “inject” a precise dose of drug when the body needs it and to “stop” or “slow” the release right after the injection; however, it is hard to easily achieve this with traditional stimuli-responsive hydrogels. A controllable pulsatile-type drug delivery device with repeatable dosing ability in therapeutically effective precision is clinically desirable. Therefore, in this communication, a high-frequency magnetic field (HFMF) triggered pulsatile drug delivery ferrogel with a mechanically reliable and flexible hybrid structure composed of gelatin and magnetic nanoparticles (nanomagnets) of 10-250 nm in diameter is reported. Furthermore, under cyclic exposures to the high-frequency magnetic stimuli, a highly controllable and repeatable burst release with desirable precision from the ferrogels is achieved. Figure 1a illustrates the SEM images of one of the ferrogels shaping in cylindrical geometry, namely 40nm@GE ferrogel, where 40nm@GE represents the ferrogels with gelatin crosslinked with genipin, reinforced by 2 wt % chitosan, and 3% Fe3O4 magnetic nanoparticles (nanomagnets) of average 40 nm in diameter. Chitosan provides several functions in the final hybrid structure, such as (1) matrix for drug encapsulation, (2) materials to provide deformation motion. and (3) binder to link the nanoparticles. As shown, the iron nanoparticles were embedded into the polymer matrix and the nanomagnets displayed a uniform spherical size and dispersed well in the ferrogel. TEM image, i.e., Figure 1b, exhibits that the gelatin * Corresponding authors. Telephone: +886-3-5731818. Fax:+886-35725490; E-mail: (S.-Y.C.) sanyuanchen@mail.nctu.edu.tw; (D.-M.L.) deanmo_liu@yahoo.ca. Figure 1. (a) SEM, (b) TEM, and (c) XPS analyses of ferrogels composed of iron oxide nanoparticles and a gelatin matrix. 6786 Macromolecules2007,40, 6786-6788

Frequency-dependent response of the vascular endothelium to pulsatile shear stress

Abstract: As a result of the complex blood flow patterns that occur in the arterial tree, certain regions of the vessel wall experience fluctuations in shear stress that are dominated by harmonic frequencies...

Activation of the ERK1/2 Cascade via Pulsatile Interstitial Fluid Flow Promotes Cardiac Tissue Assembly

Abstract: Deciphering the cellular signals leading to cardiac muscle assembly is a major challenge in ex vivo tissue regeneration. For the first time, we demonstrate that pulsatile interstitial fluid flow in three-dimensional neonatal cardiac cell constructs can activate ERK1/2 sixfold, as compared to static-cultivated constructs. Activation of ERK1/2 was attained under physiological shear stress conditions, without activating the p38 cell death signal above its basic level. Activation of the ERK1/2 signaling cascade induced synthesis of high levels of contractile and cell-cell contact proteins by the cardiomyocytes, while its inhibition diminished the inducing effects of pulsatile flow. The pulsed medium-induced cardiac cell constructs showed improved cellularity and viability, while the regenerated cardiac tissue demonstrated some ultra-structural features of the adult myocardium. The cardiomyocytes were elongated and aligned into myofibers with defined Z-lines and multiple high-ordered sarcomeres. Numerous intercalated disks were positioned between adjacent cardiomyocytes, and deposits of collagen fibers surrounded the myofibrils. The regenerated cardiac tissue exhibited high density of connexin 43, a major protein involved in electrical cellular connections. Our research thus demonstrates that by judiciously applying fluid shear stress, cell signaling cascades can be augmented with subsequent profound effects on cardiac tissue regeneration.

A hybrid one-dimensional/Womersley model of pulsatile blood flow in the entire coronary arterial tree

Abstract: Using a frequency-domain Womersley-type model, we previously simulated pulsatile blood flow throughout the coronary arterial tree. Although this model represents a good approximation for the smaller vessels, it does not take into account the nonlinear convective energy losses in larger vessels. Here, using Womersley's theory, we present a hybrid model that considers the nonlinear effects for the larger epicardial arteries while simulating the distal vessels (down to the 1st capillary segments) with the use of Womersley's Theory. The main trunk and primary branches were discretized and modeled with one-dimensional Navier-Stokes equations, while the smaller-diameter vessels were treated as Womersley-type vessels. Energy losses associated with vessel bifurcations were incorporated in the present analysis. The formulation enables prediction of impedance and pressure and pulsatile flow distribution throughout the entire coronary arterial tree down to the first capillary segments in the arrested, vasodilated state. We found that the nonlinear convective term is negligible and the loss of energy at a bifurcation is small in the larger epicardial vessels of an arrested heart. Furthermore, we found that the flow waves along the trunk or at the primary branches tend to scale (normalized with respect to their mean values) to a single curve, except for a small phase angle difference. Finally, the model predictions for the inlet pressure and flow waves are in excellent agreement with previously published experimental results. This hybrid one-dimensional/Womersley model is an efficient approach that captures the essence of the hemodynamics of a complex large-scale vascular network. The present model has numerous applications to understanding the dynamics of coronary circulation.

Disturbance of macro- and microcirculation: relations with pulse pressure and cardiac organ damage

Abstract: Whereas large arteries dampen oscillations resulting from intermittent ventricular ejection, small arteries steadily deliver optimal blood flow to various organs as the heart. The transition from pulsatile to steady pressure is influenced by several factors as wave travel, damping, and reflections, which are mainly determined by the impedance mismatch between large vessels and arteriolar bifurcations. The mechanism(s) behind the dampening of pressure wave in the periphery and the links between central and peripheral pulsatile pressure (PP) may determine cardiac damage. Active pathways participate to pulse widening and changes in pulse amplitude in microvessels. Steady and cyclic stresses operate through different transduction mechanisms, the former being focal adhesion kinase and the latter being free radicals and oxidative stress. Independently of mechanics, calcifications and attachment molecules contribute to enhance vessel wall stiffness through changes in collagen cross-links, proteoglycans, integrins, and fibronectin. Enhanced PP transmission may thus occur and precipitate organ damage at each time that autoregulatory mechanisms, normally protecting the heart from vascular injury, are blunted. Such circumstances, observed in old subjects with systolic hypertension and/or Type 2 diabetes mellitus, particularly under high-sodium diet, cause cardiac damage and explain why increased PP and arterial stiffness are significant predictors of morbidity and mortality in the elderly.

Design, modeling and fabrication of a constant flow pneumatic micropump

Abstract: This paper characterizes a bi-directional pneumatic diaphragm micropump and presents a model for performance of an integrated fluidic capacitor. The fluidic capacitor is used to convert pulsatile flow into a nearly continuous flow stream. The pump was fabricated in acrylic using a CNC mill. The stroke volume of the pump is ~1 µL. The pump is self-priming, bubble tolerant and insensitive to changes in head pressure and pneumatic pressure within its operating range. The pump achieves a maximum flow rate of 5 mL min−1 against zero head pressure. With pneumatic pressure set to 40 kPa, the pump can provide flow at 2.6 mL min−1 against a head pressure of 25 kPa. A nonlinear model for the capacitor was developed and compared with experimental results. The ratio of the time constant of the capacitor to the cycle time of the pump is shown to be an accurate indicator of capacitor performance and a useful design tool.

Effects of different types of fluid shear stress on endothelial cell proliferation and survival

Abstract: We attempted to clarify the effect of different types of shear stress on endothelial cell (EC) proliferation and survival. Bovine aortic ECs were subjected to either steady laminar, 1 Hz pulsatile, or 1 Hz to and fro shear at 14 dyne/cm(2). % of BrdU positive EC was 14.3 +/- 1.6% in steady, 21.5 +/- 3.2% in pulsatile, and 11.4 +/- 2.4% in to and fro after 4 h, respectively (P < 0.05). Pulsatile shear compared with static control. Rapamycin reduced BrdU incorporation in all shear regimens (P < 0.001). However, it was still higher in EC exposed to pulsatile shear than the other regimens (P < 0.005). PD98059 completely abolished the increased BrdU incorporation in all shear regimens, including pulsatile shear. Pulsatile shear had significantly elevated ERK1/2 phosphorylation at 5 min compared with steady (P < 0.05) and to and fro shear (P < 0.01) while there was no significant difference in pp70(S6k) phosphorylation between any shear regimen. The ratio of apoptotic cells in serum deprived EC in the presence of steady laminar, pulsatile and to and fro shear for 4 h were 2.7 +/- 0.78%, 2.7 +/- 0.42%, and 2.9 +/- 0.62%, respectively while after the addition of serum for 4 h, it was 4.3 +/- 0.73%. All shear regimens phosphorylated AKT in a time-dependent manner with no significant difference between regimens. Our results demonstrate that different types of shear stress regimens have different effects on EC and may account for the variable response of EC to hemodynamics in the circulation.

In-vivo flow simulation in coronary arteries based on computed tomography datasets: feasibility and initial results

Abstract: The purpose of this paper was to non-invasively assess hemodynamic parameters such as mass flow, wall shear stress (WSS), and wall pressure with computational fluid dynamics (CFD) in coronary arteries using patient-specific data from computed tomography (CT) angiography. Five patients (two without atherosclerosis, three with atherosclerosis) underwent retrospectively electrocardiogram (ECG) gated 16-detector row CT using ECG-pulsing and geometric models of coronary arteries were reconstructed for CFD analysis. Blood flow was considered laminar, incompressible, Newtonian, and pulsatile. The mass flow, WSS, and wall pressure were quantified and flow patterns were visualized. The wall pressure continuously decreased towards distal segments and showed pressure drops in stenotic segments. In coronary segments without atherosclerotic wall changes, WSS remained low, even during phases of high flow velocity, whereas in atherosclerotic vessels, the WSS was elevated already at low flow velocities. Stenoses and post-stenotic dilatations led to flow acceleration and rapid deceleration, respectively, including a distortion of flow. Areas of high WSS and high flow velocities were found adjacent to plaques, with values correlating with the degree of stenosis. CFD provided detailed mass flow measurements. CFD analysis is feasible in normal and atherosclerotic coronary arteries and provides the rationale for further investigation of the links between hemodynamic parameters and the significance of coronary stenoses.

Enhanced Clearance In An Artificial Kidney Incorporating A Pulsatile Pump

Abstract: A continuous renal replacement therapy (CRRT) device is provided that weighs between 2 and 10 pounds. The CRRT device can be portable, mobile or completely worn on the person of the patient. Blood and dialysate are each pumped in a pulsed or pulsatile manner through a dialyzer such that a significant portion of the peak pulse of the blood flow coincides with a significant portion of a low pulse portion of the dialysate flow. An differential pressure between a dialysate inlet of the dialyzer and the blood inlet of the dialyzer periodically changes from a high differential pressure of between 70 and 120 mmHg for a first time period and a low differential pressure of between −10 and 10 mmHg for a first time period and a low differential pressure of between −10 and 10 mmHg for a second time period. The frequency of the high and low differential pressure cycle is between about 0.5 and 4 Hz.

Effects of pulsatile blood flow in large vessels on thermal dose distribution during thermal therapy.

Abstract: The aim of this study is to evaluate the effect of pulsatile blood flow in thermally significant blood vessels on the thermal lesion region during thermal therapy of tumor. A sinusoidally pulsatile velocity profile for blood flow was employed to simulate the cyclic effect of the heart beat on the blood flow. The evolution of temperature field was governed by the energy transport equation for blood flow together with Pennes' bioheat equation for perfused tissue encircling the blood vessel. The governing equations were numerically solved by a novel multi-block Chebyshev pseudospectral method and the accumulated thermal dose in tissue was computed. Numerical results show that pulsatile velocity profile, with various combinations of pulsatile amplitude and frequency, has little difference in effect on the thermal lesion region of tissue compared with uniform or parabolic velocity profile. However, some minor differences on the thermal lesion region of blood vessel is observed for middle-sized blood vessel. This consequence suggests that, in this kind of problem, we may as well do the simulation simply by a steady uniform velocity profile for blood flow.

Does cortisol acting via the type II glucocorticoid receptor mediate suppression of pulsatile luteinizing hormone secretion in response to psychosocial stress

Abstract: This study assessed the importance of cortisol in mediating inhibition of pulsatile LH secretion in sheep exposed to a psychosocial stress. First, we developed an acute psychosocial stress model that involves sequential layering of novel stressors over 3-4 h. This layered-stress paradigm robustly activated the hypothalamic-pituitary-adrenal axis and unambiguously inhibited pulsatile LH secretion. We next used this paradigm to test the hypothesis that cortisol, acting via the type II glucocorticoid receptor (GR), mediates stress-induced suppression of pulsatile LH secretion. Our approach was to determine whether an antagonist of the type II GR (RU486) reverses inhibition of LH pulsatility in response to the layered stress. We used two animal models to assess different aspects of LH pulse regulation. With the first model (ovariectomized ewe), LH pulse characteristics could vary as a function of both altered GnRH pulses and pituitary responsiveness to GnRH. In this case, antagonism of the type II GR did not prevent stress-induced inhibition of pulsatile LH secretion. With the second model (pituitary-clamped ovariectomized ewe), pulsatile GnRH input to the pituitary was fixed to enable assessment of stress effects specifically at the pituitary level. In this case, the layered stress inhibited pituitary responsiveness to GnRH and antagonism of the type II GR reversed the effect. Collectively, these findings indicate acute psychosocial stress inhibits pulsatile LH secretion, at least in part, by reducing pituitary responsiveness to GnRH. Cortisol, acting via the type II GR, is an obligatory mediator of this effect. However, under conditions in which GnRH input to the pituitary is not clamped, antagonism of the type II GR does not prevent stress-induced inhibition of LH pulsatility, implicating an additional pathway of suppression that is independent of cortisol acting via this receptor.

Modeling of non-Newtonian blood flow through a stenosed artery incorporating fluid-structure interaction

Abstract: We investigate fluid and structural responses to pulsatile non-Newtonian blood flow through a stenosed artery, using ANSYS. The artery was modeled as an axisymmetric stenosed vessel. The wall of the vessel was set to be isotropic and elastic. The blood behavior was described by the Power Law and the Carreau non-Newtonian models, respectively. When compared to the Newtonian flow models, the result from the Carreau model showed very little difference, in terms of velocity, pressure and wall shear stress, whereas the result from the Power Law model showed more significant vortices and smaller wall shear stresses. The highest stress concentration was also found at the throat of the stenosis.

Pulsatile Control System for a Rotary Blood Pump

Abstract: A system for controlling the speed of a rotary blood pump. The system oscillates the speed of the pump to produce pulsed pressure at the outlet of the pump. The speed is oscillated synchronously with the natural cardiac cycle. The natural cardiac cycle is determined using a pulsatility index calculated from the back EMF produced by the pump or an impedance sensor.

Pulsatile magneto-biofluid flow and mass transfer in a non-Darcian porous medium channel

Abstract: A numerical study of pulsatile flow and mass transfer of an electrically conducting Newtonian biofluid via a channel containing porous medium is considered. The conservation equations are transformed and solved under boundary conditions prescribed at both walls of the channel, using a finite element method with two-noded line elements. The influence of magnetic field on the flow is studied using the dimensionless hydromagnetic number, Nm, which defines the ratio of magnetic (Lorentz) retarding force to the viscous hydrodynamic force. A Darcian linear impedance for low Reynolds numbers is incorporated in the transformed momentum equation and a second order drag force term for inertial (Forchheimer) effects. Velocity and concentration profiles across the channel width are plotted for various values of the Reynolds number (Re), Darcy parameter (λ), Forchheimer parameter (Nf), hydro-magnetic number (Nm), Schmidt number (Sc) and also with dimensionless time (T). Profiles of velocity varying in space and time are also provided. The conduit considered is rigid with a pulsatile pressure applied via an appropriate pressure gradient term. Increasing the hydromagnetic number (Nm) from 1 to 15 considerably depresses biofluid velocity (U) indicating that a magnetic field can be used as a flow control mechanism in, for example, medical applications. A rise in Nf from 1 to 20 strongly retards the flow development and decreases the velocity, U, across the width of the channel. The effects of other parameters on the flowfield are also discussed at length. The flow model also has applications in the analysis of electrically conducting haemotological fluids flowing through filtration media, diffusion of drug species in pharmaceutical hydromechanics, and also in general fluid dynamics of pulsatile systems.

Instability modes and transition of pulsatile stenotic flow: pulse-period dependence

Abstract: The instability modes arising within simple non-reversing pulsatile flows in a circular tube with a smooth axisymmetric constriction are examined using global Floquet stability analysis and direct numerical simulation. The sectionally averaged pulsatile flow is represented with one harmonic component superimposed on a time-mean flow. We have previously identified a period-doubling global instability mechanism associated with alternating tilting of the vortex rings that are ejected out of the stenosis throat with each pulse. Here we show that while alternating tilting of vortex rings is the primary instability mode for comparatively larger reduced velocities associated with long pulse periods (or low Womersley numbers), for lower reduced velocities that are associated with shorter pulse periods the primary instability typically manifests as azimuthal waves (Widnall instability modes) of low wavenumber that grow on each vortex ring. Convective shear-layer instabilities are also supported by the types of flow considered. To provide an insight into the comparative role of these types of instability, which have still shorter temporal periods, we also introduce high-frequency low-amplitude perturbations to the base flows of the above global instabilities. For the range of parameters considered, we observe that the dominant features of the primary Floquet instability persist, but that the additional presence of the convective instability can have a destabilizing effect, especially for long pulse periods.