External Pulsation Research Articles

Non-Invasive Pulsatile Shear Stress Modifies Endothelial Activation; A Narrative Review.

The monolayer of cells that line both the heart and the entire vasculature is the endothelial cell (EC). These cells respond to external and internal signals, producing a wide array of primary or secondary messengers involved in coagulation, vascular tone, inflammation, and cell-to-cell signaling. Endothelial cell activation is the process by which EC changes from a quiescent cell phenotype, which maintains cellular integrity, antithrombotic, and anti-inflammatory properties, to a phenotype that is prothrombotic, pro-inflammatory, and permeable, in addition to repair and leukocyte trafficking at the site of injury or infection. Pathological activation of EC leads to increased vascular permeability, thrombosis, and an uncontrolled inflammatory response that leads to endothelial dysfunction. This pathological activation can be observed during ischemia reperfusion injury (IRI) and sepsis. Shear stress (SS) and pulsatile shear stress (PSS) are produced by mechanical frictional forces of blood flow and contraction of the heart, respectively, and are well-known mechanical signals that affect EC function, morphology, and gene expression. PSS promotes EC homeostasis and cardiovascular health. The archetype of inducing PSS is exercise (i.e., jogging, which introduces pulsations to the body as a function of the foot striking the pavement), or mechanical devices which induce external pulsations to the body (Enhanced External Pulsation (EECP), Whole-body vibration (WBV), and Whole-body periodic acceleration (WBPA aka pGz)). The purpose of this narrative review is to focus on the aforementioned noninvasive methods to increase PSS, review how each of these modify specific diseases that have been shown to induce endothelial activation and microcirculatory dysfunction (Ischemia reperfusion injury-myocardial infarction and cardiac arrest and resuscitation), sepsis, and lipopolysaccharide-induced sepsis syndrome (LPS)), and review current evidence and insight into how each may modify endothelial activation and how these may be beneficial in the acute and chronic setting of endothelial activation and microvascular dysfunction.

Numerical study on heat transfer enhancement by viscoelastic fluid pulsating laminar flow in rectangular microchannel heat sinks

Efficient heat transfer technique at microscale is a long-term demand for development of microelectronic device. Simple-structured microchannel heat sinks (MHSs) have the advantages of easier fabrication processes and profound applicability, which are however limited by the laminar flow with lower heat transfer efficiency. Against this backdrop, a new type of heat transfer enhancement (HTE) techniques by viscoelastic fluid pulsating laminar flow (VPL flow) is proposed and evaluated from different aspects in this research. First of all, the effects of VPL flow on the thermal performance in a rectangular microchannel heat sink (RMHS) are numerically studied with the focus on the parametric effects on HTE. It indicates that the overall heat transfer performance can be significantly augmented by VPL flow and possesses an optimal enhancement under cases of exerted pulsation frequencies closed to the characteristic frequency of viscoelastic fluid flow. The characteristic frequency is determined from the Poiseuille start-up flow of the viscoelastic fluid flow and can be summarized by a power-law function with the relationship of Reynold number (Re) and Elasticity number (El). Moreover, the optimal HTE of RMHS increases with external pulsation amplitude and El but decreases with Re, which can also be summed up with them in a power-law function. Then, by analyzing of flow performance, the reversal flow and the resonance effects by VPL flow are regarded as the reasons for the HTE, which results from a balance between relaxation time of viscoelastic microstructures deformation and external pulsation period. Finally, the law of HTE by VPL flow is extended and verified in a carefully designed leaf-type microchannel heat sink (LMHS), suggesting a promising micro-scale heat transfer enhancement technology by VPL flow which however still needs further investigations on the optimal operating conditions in the future.

Hydrodynamic and Flow Field Characteristics of Water Jet Propulsion under Mooring Conditions

The acceleration characteristics of a water jet-propelled ship during startup are related to its performance under mooring conditions. Water jet propulsion cavitation during startup increases the vibration and noise of the whole ship. Therefore, accurately predicting and analyzing the performance, hydrodynamics and flow field characteristics of water jet-propelled ships under mooring conditions can help elucidate the startup characteristics of the ships and optimize their acceleration strategies. In this study, the hydrodynamic and flow field characteristics of water jet propulsion and water jet propulsion ships under mooring conditions were studied using three-dimensional numerical modeling. First, the hydrodynamic performance of the water jet propeller was analyzed, and the relevant flow field law was derived. Then, the hydrodynamic performance, internal and external flow field characteristics, pulsation pressure and flow rate at the nozzle, and pulsation pressure at the monitoring points around the impeller of the water jet propulsion ship model were analyzed under mooring conditions. We obtained the open-water law for the water jet propeller and the hydrodynamic force and flow field law for a two-pump water jet propulsion ship. The ship model developed in this study provides a good theoretical foundation for further research on water jet propulsion.

Experimental Study on Unsteady Cavitating Flow and Its Instability in Liquid Rocket Engine Inducer

To study instability in the unsteady cavitating flow in a liquid rocket engine inducer, visualization experiments of non-cavitating and cavitating flows inside a model inducer were carried out at different flow conditions. Visual experiments were carried out to capture the evolution of non-cavitating and cavitating flows in a three-bladed inducer by using a high-speed camera. The external characteristic performance, cavitation performance, and pressure pulsation were analyzed based on the observation of non-cavitation and cavitation development and their instabilities. Under non-cavitation conditions, the change of flow rate has a significant impact on the pressure pulsation characteristics in the inducer. The occurrence of cavitation aggravated the instability of the flow and caused the intensity of pressure pulsation at each measuring point to increase. This cavitation structure has strong instability, and the tail region is often accompanied by shedding cavitation clouds perpendicular to the blade surface.

Experimental Study on Operational Stability of Centrifugal Pumps of Varying Impeller Types Based on External Characteristic, Pressure Pulsation and Vibration Characteristic Tests

This study performed external characteristics, dynamic pressure, and vibration tests on the closed impeller model, semi-open type impeller model, and open impeller model in order to study the influence of the impeller structure on the operational stability of the pump. According to the research findings, the external characteristics of the three impellers enjoyed favorable stability with flat curves under low flow rates. As the flow increases, the stability of the external characteristics weakens, and the poorest stability can be observed in the open impeller, while the enclosed impeller has the best stability under a large flow rate. In addition, intense pressure fluctuations arise near the casing tongue due to the dynamic and static interference between the blade and the casing tongue, causing a large amplitude of pressure pulsation near the casing tongue and the pump outlet. It can be seen that the stability of pressure pulsation of the closed impeller in the casing tongue area was the best, whereas the stability of pressure pulsation of the open impeller was the poorest. The minimum casing vibration can be found in the closed impeller, followed by the semi-open impeller, and the open impeller, with the maximum vibration. In addition, the optimum stability of rotor vibration can be observed in the closed impeller, semi-open impeller, and open impeller with similar stability of rotor vibration.

Flow characterization in the uphill region of pulsed oblique round jet

An experimental study to investigate the effect of jet pulsations on the wall jet development in the uphill region of an obliquely inclined round water jet has been performed using particle image velocimetry technique. The study has been performed at a constant nozzle to target wall distance, L/D = 4 (D is the diameter of the nozzle) by varying the jet impingement angle θ (=60o, 45o, and 30o), Reynolds numbers (ReD = 1900 and 3280; based on nozzle diameter and average nozzle exit velocity Uavg), and Strouhal number (0 ≤ St ≤ 0.9; St=fDUavg, where f is the frequency of external pulsation). It is observed that the pulsations have no significant effect on the jet in the free jet region when the target plate is kept at a distance less than the potential core length (the potential core extends up to 4D–6D from the nozzle exit toward the impinging plate), and the jet impingement region extends up to 1D from the plate. The location of the stagnation point is observed to depend on all three parameters: the jet pulsation, the Reynolds number, and the jet impingement angle. An increase in Reynolds number creates an adverse pressure gradient toward the downstream direction in the uphill region, resulting in an intrusion of ambient fluid toward the wall jet. The distance between the geometric center and the stagnation point is observed to be minimum for St = 0.44 at both the Reynolds numbers. The wall jet that develops in the uphill region exhibits a maximum velocity decay rate and a jet half width growth rate corresponding to St = 0.44. These parameters are also observed to increase with the increase in the Reynolds number and decrease in the jet impingement angle. The velocity fields reconstructed using proper orthogonal decomposition reveal the dominant modes in the upstream location for St = 0.44 than the other pulsations. Furthermore, we observed that the jet after impingement deviates entirely in the downhill region for θ=30o irrespective of the jet pulsation, suggesting a non-dependence of the critical angle of inclination on jet pulsations for L/D = 4.

Publisher's Note: “Dynamics of wall jet flow under external pulsation” [Phys. Fluids 33, 095103 (2021)

First Page

Dynamics of wall jet flow under external pulsation

The present study aims to identify the dominant coherent structures in the wall jet flow subjected to external pulsation at Reynolds number 2600 (based on average jet exit velocity and nozzle diameter). The forcing frequency is varied between St = 0 and 0.99 (St is the Strouhal number). Quadrant analysis is employed to identify the relative contribution of different quadrant motions to the total Reynolds shear stress. Unlike boundary layer flows and channel flows, two distinct regions (inner shear region and outer shear region) are observed in the wall jet flows, and the characteristics of different quadrant motions change in these regions. About 70% of the total shear stress is contributed from the first and fourth quadrants in the outer shear region. We observe that ejection motion is more energetic than sweep motion in the downstream direction, although less frequent. The ejection motion is observed to be more violent for St = 0.44 than for the other frequencies. A proper orthogonal decomposition (POD) analysis reveals while the modal structures exist in different regions of the wall for different jet pulsation; there are no dominant modes (30 modes are required to recover about 75% of the total energy), and the energy is fairly distributed over a large number of modes. However, the POD analyses are capable of capturing the response of the wall jet to different jet pulsations. The most dominant and strongest modal structures are found nearer to the impingement region of the wall when St = 0.44 and the jet tends to laminarize for St > 0.9.

Effect of self-excited instability on single liquid swirl injector flow characteristics

Owing to their simple structure and high performance, swirl injectors have been used as propellant injectors for rocket engines in various countries, starting from Russia. However, when a swirl injector is used in combustion, flow instability becomes important because of its relation to combustion instability. Because the external flow of a swirl injector is a thin liquid sheet, wave-forming self-excited instability could occur. In this study, the effect of self-excited instability on the flow characteristics of a swirl injector was investigated. Moreover, through the use of a mechanical pulsator, the effect of self-excited instability frequency on the dynamic characteristics of the swirl injector was examined. As a result, self-excited instability was determined to cause the periodic breakup of the spray sheet. In addition, large droplets generated immediately after breakup are confirmed to be periodically concentrated with a specific frequency equal to the self-excited instability frequency. With external pulsation, breakup length was observed to be shortened with pulsation and had minimum value when pulsation of frequency equal to the self-excited instability frequency was applied. Droplet characteristics with pulsation were also investigated, and external pulsation was determined to cause mean droplet diameter (D10) and Sauter mean diameter (SMD) to be smaller. When SMDs at pulsations of different frequencies were compared, the SMD at pulsation with self-excited instability frequency was observed to be larger because this frequency causes droplet distribution to be more non-uniform. Through this study, self-excited instability was confirmed to be able to change the static and dynamic characteristics of a swirl injector. This result can serve as a reminder of the importance of the self-excited instability of a sheet in the design of a swirl injector.

Self-Excited Instability of Closed-Type Swirl Injector with Varying Conditions

The dynamic characteristic of an injector is important for improving engine characteristics and preventing instability. In this study, the experiments on the unstable flow of a closed-type swirl injector were conducted, and the dynamic characteristic analysis of the relationship between internal and external flows was discussed. Consequently, it was found that the unstable characteristics of internal and external flows were similar. Main geometric parameters affecting the injector self-excited instability were also investigated. The fluctuating amplitude was mainly related to the momentum of the flow. On the other hand, the instability frequency was related to the axial velocity inside the swirl injector. Experiments with external pulsation were carried out to investigate the effect of the instability frequency in real environments. It was observed that the mean breakup length reduced by 25%, and the fluctuation amplitude of the spray angle increased more than twice compared with the out of tuned case. The results of this study can be an inspiration to engine design, considering the dynamic characteristics of an injector.

Effect of pulsation on the near flow field of a submerged water jet

The current study investigates the effect of pulsation frequency on the near field characteristics of a submerged water jet using the technique of dye visualization. Flow visualization was performed in water over the range: Reynolds number 540–1540, Strouhal number 0.16–1.75, and at constant amplitude of pulsation of 18%. The results show that the mixing and entrainment process at lower Reynolds number occurs due to diffusion process owing to relatively stable shear layer for the case of a steady jet, whereas the external pulsation promotes an early instability in the shear layer where irregular structures promotes mixing between the jet and surrounding fluids. Images of streaklines show that initial mixing and entrainment processes in the potential core of the jet is due to the development of large vortical structures. While beyond the potential core, mixing and entrainment are governed by the small-scale structures. Further results show that the initiation and growth of vortices in the shear layer depends on the pulse frequency. For a given Reynolds number and amplitude, the number of vortical structures and their size changes with frequency. With an increase in the pulsation frequency, there is an increase in the spreading of the jet along with stretching of the vortical structures. An optimum pulsating frequency at which the effect of pulsation on the flow is maximum occurs at St = 0.44, independent of Reynolds number. These results should eventually lead to a better understanding of the physical phenomena responsible for enhanced mixing and entrainment processes in the presence of pulsating jets.

Effect of external pulsation on kinematics of fluid particles in the field of Lamb–Oseen vortex pair

The effect of external pulsation on a pair of stationary Lamb–Oseen vortices of equal strength has been analyzed to investigate kinematic behavior of a fluid particle. The assumption of vortices being treated stationary or fixed vortex filaments is valid in a reference frame attached to the vortex system with axes along and perpendicular to the line of their centers. Also, it is assumed that change in core shape and size is much small, with least possibility of core merger. In such situations, periodic particle paths are observed and superposition of pulsation becomes beneficial. In the present work, motion of a representative fluid particle is modeled as a non-linear dynamical system by varying both amplitude and frequency of external pulsation. Effect of external pulsation has been brought out with the help of quantification of deviation from periodic paths by using the concept of total average deviation. Results are presented in terms of particle paths, velocity phase plots, velocity signals and their spectra for varying amplitude and frequency of external pulsation.

Investigation of the effect of external periodic flow pulsation on a cylinder wake using linear stability analysis

The aim of this paper is to investigate the receptivity of cylinder wake to external periodic flow pulsation at low Reynolds number using linear stability analysis. The inlet flow pulsation appears as a forcing term in the linearised equation set. The full non-linear N-S equations as well as the linearised set for small perturbations around the time-averaged flow are solved using an in-house finite volume solver. The results are first validated against reference data for growth rate and frequency of the most unstable eigenmode for flow past a fixed cylinder with steady base flow at various Reynolds numbers. A special numerical technique is developed to separate the components of the solution in the wake that vary with the natural shedding frequency and the external pulsating frequency. The developed approach requires temporal integration over one period of vortex shedding and solution of a 4×4 linear system at every cell of the domain. The results show that both cross-stream and streamwise velocity components in the near cylinder region are strongly affected by flow pulsation, and its effect is spatially localised in the near wake. Increasing the pulsation frequency reduces the spatial extent within which pulsation plays an important role. A symmetric shedding pattern is established and at every period of external pulsation, two pairs of symmetric vortices are shed from the top and bottom of the cylinder. The width of the wake periodically widens and narrows, which is similar to “wake breathing” observed in a streamwise oscillating cylinder.

Kinematics of a Fluid Particle due to Interaction of Fixed Inviscid Vortex Filaments in Presence of External Translation and Pulsation

Kinematics of a Fluid Particle due to Interaction of Fixed Inviscid Vortex Filaments in Presence of External Translation and Pulsation

Analysis of pulsating convection from two heat sources mounted with porous blocks

The present work details the numerical simulation of forced pulsating convective flow in a parallel-plate channel with two porous-block-attached strip heat sources at the bottom wall. The analysis is based on the use of unsteady Navier–Stokes equation in the fluid region, the transient Darcy–Brinkman–Forchheimer flow model in the porous region, and the energy equation on the thermal field. Both the mixing enhanced convection caused by porous block and the oscillatory enhanced convection caused by pulsation flow are examined. This study details the effects of variations in the Darcy number, pulsating frequency and amplitude, porous blockage ratio, to illustrate important fundamental and practical results. The results show that the periodic alteration in the structure of recirculation flows, caused by both porous block and flow pulsating, has a direct impact on the flow and thermal characteristics near the heat source region. In addition, it is shown that specific choices in certain governing parameters, such as the size or permeability of porous block, and the frequency and amplitude of external pulsation can produce profound enhanced heat transfer on the cooling of heat sources. The cycle-average local surface temperature distribution of heater for pulsating-flow case with porous block is more uniform than that for pure steady-flow case. The method combining flow pulsation with finite-size fiber-porous heat sink can be considered as an augment heat transfer tool for cooling electronic devices.

Resonant responses in periodic turbulent flows: computations using a k–ε eddy viscosity model

Periodically-oscillated pipe flows in which the bulk velocity is varied about a non-zero mean level (Ub = Ub0[1 + γ cos ωt]) are computed using a low-Reynolds-number k–ε turbulence model. Comparison is made with data for periodic pipe flow and it is shown that model is capable of resolving the principal features of the highly non-universal turbulence profiles that occur under conditions of harmonic forcing. There follows an examination of the frequency response of the phase-averaged turbulent kinetic energy, k, which is analysed in terms of its first harmonic variation (k(r, ωt) = k 0 + k 1 cos(ωt + ψ); k 0, k 1,ψ = f(r)). In confirmation of the stress-transport model results of Cotton and Guy [J. Hydraul. Res. 42 (2004) 293], it is found that the modulation of the turbulent kinetic energy, k 1/γ k0 first responds in a quasi-steady manner and then, with increasing frequency, exhibits resonant behaviour, which is itself succeeded by a frozen response at higher frequencies. The resonant condition occurs when the time scale of large-scale turbulence is an order of magnitude less than the period of the imposed oscillation. The paper concludes with a discussion of the parallels that may be drawn between the present results and the experimental study of Mizushina et al. [J. Chem. Engng. Jpn. 6 (1973) 487] in which the effect of external pulsation on the turbulence “bursting” process was investigated

Numerical simulation of the flow and heat transfer around a cylinder with a pulsating approaching flow at a low Reynolds number

Two-dimensional numerical simulations of flow and heat transfer around a cylinder at a Reynolds number Re= 100 have been performed in order to investigate the effect of imposed inlet velocity pulsation on the heat transfer and flow fields. First the code is validated against existing results from the literature and then several external frequencies are examined. The numerical results confirm the existence of a vortex shedding lock-on regime where the wake behaves in a very ordered manner (completely periodic). Outside the lock-on region the flow is quasiperiodic. The length and centre of the mean recirculating zone downstream of the cylinder are also affected by the external pulsation. Regarding heat transfer, the results indicate that by imposing an external velocity pulsation, the root mean square (r.m.s.) of the local Nusselt number Nu increases, but the mean value increases only in the area downstream of the separation point. The mechanism responsible for this is identified: hot fluid is engulfed by stronger vortices (compared with the steady approaching flow case) shed from the upper and lower side of the cylinder and returned close to the downstream stagnation point. This mechanism also explains the observed variation in Nu with time. In the front part of the cylinder, the Nu varies almost sinusoidally and closely follows the imposed external velocity pulsation. The results indicate also that there is a range of external frequencies where the time and spatially averaged Nu number is maximized.

Positive-pressure breathing as a protective technique during +Gz acceleration.

ArticlePositive-pressure breathing as a protective technique during +Gz acceleration.S J Shubrooks JrS J Shubrooks JrPublished Online:01 Aug 1973https://doi.org/10.1152/jappl.1973.35.2.294MoreSectionsPDF (1 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByOh G: The x, y and z of human physiological responses to acceleration17 November 2021 | Experimental Physiology, Vol. 106, No. 12Real-time assessment of global and regional lung ventilation in the anti-gravity straining maneuver using electrical impedance tomographyComputers in Biology and Medicine, Vol. 135Physical Condition Does Not Affect Gravity-Induced Loss of Consciousness during Human Centrifuge Training in Well-Experienced Young Aviators26 January 2016 | PLOS ONE, Vol. 11, No. 1Effects of inertial load and countermeasures on the distribution of pulmonary blood flowMyron A. Chornuk, Susan L. Bernard, John W. Burns, Robb W. Glenny, Don D. Sheriff, Scott E. Sinclair, Nayak L. Polissar, and Michael P. Hlastala1 August 2000 | Journal of Applied Physiology, Vol. 89, No. 2Effects of chest wall counterpressures on lung mechanics under high levels of CPAP in humansMaurice Beaumont, Damien Lejeune, Henri Marotte, Alain Harf, and Frédéric Lofaso1 August 1997 | Journal of Applied Physiology, Vol. 83, No. 2Cardiovascular responses to acceleration stress: a computer simulationProceedings of the IEEE, Vol. 76, No. 6Synchronized External Pulsation for Improved Tolerance to Acceleration Stress: Model Studies and Preliminary ExperimentsIEEE Transactions on Biomedical Engineering, Vol. BME-32, No. 2Review: Artifical ventilation with positive end-expiratory pressure (PEEP)European Journal of Intensive Care Medicine, Vol. 2, No. 2Positive-pressure breathing as a G-protection device: safety concernsSelf-protective anti-Gz straining maneuvers (AGSM) physiology More from this issue > Volume 35Issue 2August 1973Pages 294-8 https://doi.org/10.1152/jappl.1973.35.2.294PubMed4579120History Published online 1 August 1973 Published in print 1 August 1973 Metrics

Sequenced external counterpulsation and intraaortic balloon pumping in cardiogenic shock

Sequenced external counterpulsation, a noninvasive technique for ventricular assistance, was compared with intraaortic balloon pumping. With sequenced external counterpulsation external pressure is applied during ventricular diastole to cuffs located from distal to proximal sites on each limb. This procedure causes return of arterial blood to the central circulation which augments central aortic pressure and, possibly, coronary flow. The limb cuffs are rapidly deflated just before ventricular systole so that peripheral resistance is lessened, thereby decreasing left ventricular afterload. The cardiovascular effects of this method have been studied in anesthetized baboons with normal blood pressure and with cardiogenic shock, and in awake human beings with normal blood pressure. Intraaortic balloon pumping was studied in normal dogs and baboons and then in these same animals during cardiogenic shock. Sequenced pulsation caused cardiac output to increase by 25 percent in normal baboons, 17 percent in baboons with cardiogenic shock and 11 percent in normal human volunteers. Intraaortic balloon pumping caused cardiac output to increase by 4 percent in normal animals and 9 percent in animals with cardiogenic shock. Diastolic augmentation of the central aortic pressure pulse was achieved uniformly with both methods. Thus, sequenced external pulsation increased cardiac output to a greater degree than intraaortic balloon pumping, whereas diastolic pressure was augmented equivalently. External counterpulsation probably augments venous return in addition to its effects on the arterial system. Because the technique is totally noninvasive it may play a role in the therapy of patients with cardiogenic shock.

Phonocardiography and external pulsation tracings in infants with congenital heart disease.

Phonocardiography and external pulsation tracings in infants with congenital heart disease.