Large Pressure Fluctuations Research Articles

Dual characters of interaction between particles and fluid

The unique characteristics of gas-solids two-phase flow and fluidization in terms of the flow structures and the apparent behavior of particles and fluid-particle interactions are closely linked to physical properties of the particles, operating conditions and bed configurations. Fluidized beds behave quite differently when solid properties, gas velocities or vessel geometries are varied. An understanding of hydrodynamic changes and how they, in turn, influence the transfer and reaction characteristics of chemical and thermal operations by variations in gas-solid contact, residence time, solid circulation and mixing and gas distribution is very important for the proper design and scale-up of fluidized bed reactors. In this paper, rather than attempting a comprehensive survey, we concentrate on examining some important positive and negative impacts of particle sizes, bubbles, clusters and column walls on the physical and chemical aspects of chemical reactor performance from the engineering application point of view with the aim of forming an adequate concept for guiding the design of multiphase fluidized bed chemical reactors. One unique phenomenon associated with particle size is that fluidized bed behavior does not always vary monotonically with changing the average particle size. Different behaviors of particles with difference sizes can be well understood by analyzing the relationship between particle size and various forces. For both fine and coarse particles, too narrow a distribution is generally not favorable for smooth fluidization. A too wide size distribution, on the other hand, may lead to particle segregation and high particle elutriation. Good fluidization performance can be established with a proper size distribution in which inter-particle cohesive forces are reduced by the lubricating effect of fine particles on coarse particles for Type A, B and D particles or by the spacing effect of coarse particles or aggregates for Type C powders. Much emphasis has been paid to the negative impacts of bubbles, such as gas bypassing through bubbles, poor bubble-to-dense phase heat & mass transfer, bubble-induced large pressure fluctuations, process instabilities, catalyst attrition and equipment erosion, and high entrainment of particles induced by erupting bubbles at the bed surface. However, it should be noted that bubble motion and gas circulation through bubbles, together with the motion of particles in bubble wakes and clouds, contribute to good gas and solids mixing. The formation of clusters can be attributed to the movement of trailing particles into the low-pressure wake region of leading particles or clusters. On one hand, the existence of down-flowing clusters induces strong solid back-mixing and non-uniform radial distributions of particle velocities and holdups, which is undesirable for chemical reactions. On the other hand, the formation of clusters creates high solids holdups in the riser by inducing internal solids circulations, which are usually beneficial for increasing concentrations of solid catalysts or solid reactants. Wall effects have widely been blamed for complicating the scale-up and design of fluidized-bed reactors. The decrease in wall friction with increasing the column diameter can significantly change the flow patterns and other important characteristics even under identical operating conditions with the same gas and particles. However, internals, which can be considered as a special wall, have been used to improve the fluidized bed reactor performance. Generally, desirable and undesirable dual characteristics of interaction between particles and fluid are one of the important natures of multiphase flow. It is shown that there exists a critical balance between those positive and negative impacts. Good fluidization quality can always be achieved with a proper choice of right combinations of particle size and size distribution, bubble size and wall design to alleviate the negative impacts.

Intraocular Pressure Fluctuations in Response to the Water-Drinking Provocative Test in Patients Using Latanoprost Versus Unoprostone

Impairment of outflow facility in glaucoma causes large intraocular pressure (IOP) fluctuations that have been shown to be a risk factor for disease progression. The water-drinking provocative test (WDT) has been proposed as an indirect measurement of outflow facility to compare intraocular pressure responses of glaucoma eyes to different drugs. This study was a double-masked, randomized, parallel-group clinical trial comparing the IOP fluctuations in response to the WDT in patients using latanoprost versus unoprostone. After completing a wash-out of ocular hypotensive medications, patients with primary openangle glaucoma or ocular hypertension were randomized to receive either latanoprost (N = 40) or unoprostone (N = 42). IOP was measured before treatment and at 8 weeks after treatment (baseline IOP for WDT), followed by the WDT. IOP fluctuations and maximum IOP after water ingestion were compared between the two groups. Analysis of covariance was used to adjust for the effects of baseline IOP and treatment efficacy. The mean percentage reduction of IOP was 27% in patients using latanoprost, as compared to 13% in patients using unoprostone (p < 0.001). Patients on treatment with latanoprost had significantly less IOP fluctuations in response to the WDT, compared to patients using unoprostone. From an overall baseline IOP of 20.0 mmHg and an overall treatment efficacy of 20%, the mean ± standard error of the mean (SEM) of the IOP fluctuation during the WDT was 5.3 ± 0.4 mmHg in the unoprostone group, and 3.6 ± 0.4 mmHg in the latanoprost group (p = 0.005, ANCOVA). This could represent an additional benefit of latanoprost over unoprostone in controlling the intraocular pressure of glaucomatous patients.

Intraocular Pressure Fluctuations in Response to the Water-Drinking Provocative Test in Patients Using Latanoprost Versus Unoprostone

Impairment of outflow facility in glaucoma causes large intraocular pressure (IOP) fluctuations that have been shown to be a risk factor for disease progression. The water-drinking provocative test (WDT) has been proposed as an indirect measurement of outflow facility to compare intraocular pressure responses of glaucoma eyes to different drugs. This study was a double-masked, randomized, parallel-group clinical trial comparing the IOP fluctuations in response to the WDT in patients using latanoprost versus unoprostone. After completing a wash-out of ocular hypotensive medications, patients with primary openangle glaucoma or ocular hypertension were randomized to receive either latanoprost (N=40) or unoprostone (N=42). IOP was measured before treatment and at 8 weeks after treatment (baseline IOP for WDT), followed by the WDT. IOP fluctuations and maximum IOP after water ingestion were compared between the two groups. Analysis of covariance was used to adjust for the effects of baseline IOP and treatment efficacy. The mean percentage reduction of IOP was 27% in patients using latanoprost, as compared to 13% in patients using unoprostone (p<0.001). Patients on treatment with latanoprost had significantly less IOP fluctuations in response to the WDT, compared to patients using unoprostone. From an overall baseline IOP of 20.0 mmHg and an overall treatment efficacy of 20%, the mean+/-standard error of the mean (SEM) of the IOP fluctuation during the WDT was 5.3+/-0.4 mmHg in the unoprostone group, and 3.6+/-0.4 mmHg in the latanoprost group (p=0.005, ANCOVA). This could represent an additional benefit of latanoprost over unoprostone in controlling the intraocular pressure of glaucomatous patients.

Unsteady Boundary Layer Development Due to Wake Passing Effects on a Highly Loaded Linear Compressor Cascade

The effects of wake passing on boundary layer development on a highly loaded linear compressor cascade were investigated in detail on the suction side of a compressor blade. The experiments were performed in the High Speed Cascade Wind Tunnel of the Institut fuer Strahlantriebe at Mach and Reynolds numbers representative for real turbomachinery conditions. The experimental data were acquired using different measurement techniques, such as fast-response Kulite sensors, hot-film array and hot-wire measurements. The incoming wakes clearly influence the unsteady boundary layer development. Early forced transition in the boundary layer is followed in time by calmed regions. Large pressure fluctuations detectable in the ensemble averaged Kulite data reveal the existence of coherent structures in the boundary layer. Distinct velocity variations inside the boundary layer are amplified when approaching the blade surface. The time–mean momentum thickness values are reduced compared to the steady ones and therefore clarify the potential for a loss reduction due to wake passing effects.

微小気泡群の共振を用いた歯垢除去装置の開発(S25-3 気泡力学に関する基礎解析とその応用(3),S25 気泡力学に関する基礎解析とその応用)

The present study aims at developing another method instead of toothbrush for removing dental plaque on teeth. The method is based on large pressure fluctuations from a cluster of micro air-bubbles, which is excited by ultrasound. The excitation condition concerning size of micro bubbles, number of micro bubbles, size of cluster, etc. is derived by a theoretical analysis. Experiments on pressure fluctuation measurement and artificial dental plaque removal are also made.

Numerical study of pulsatile flow in a constricted channel

Pulsatile flow in a planar channel with a one-sided semicircular constriction has been simulated using direct numerical simulation and large-eddy simulation. This configuration is intended as a simple model for studying blood flow in a constricted artery. Simulations have been carried out over a range of Reynolds numbers (based on channel height and peak bulk velocity) from 750 to 2000 and a fixed non-dimensional pulsation frequency of 0.024. The results indicate that despite the simplicity of the chosen geometry, the simulated flow exhibits a number of features that have been observed in previous experiments carried out in more realistic configurations. It is found that over the entire Reynolds number range studied here, the flow downstream of the constriction is dominated by the complex dynamics associated with two shear-layers, one of which separates from the lip of the constriction and other from the opposite wall. Computed statistics indicate that for Reynolds numbers higher than about 1000, the flow transitions to turbulence downstream of the region where the separated shear layers first reattach to the channel walls. Large fluctuations in wall pressure and shear stress have also been associated with this reattachment phenomenon. Frequency spectra corresponding to velocity and pressure fluctuations have been analysed in detail and these indicate the presence of a characteristic shear-layer frequency which increases monotonically with Reynolds number. For Reynolds numbers greater than 1000, this frequency is found to be associated with the periodic formation of vortex structures in the shear-layers and the impact of this characteristic shear-layer frequency on the dynamics of the flow is described in detail.

An Experimental Study on the Fluid Forces Induced by Rotor‐Stator Interaction in a Centrifugal Pump

The pressure fluctuations and the radial fluid forces acting on the impeller, the pressures in the volute, as well as the vibration of the shaft in a centrifugal pump were measured simultaneously, and their relationship was investigated. Experiments were done for various diffuser vanes, flow rates, and rotating speeds. It was demonstrated that both the blade‐pressure fluctuations and the volute static pressures are nonuniform circumferentially (not axisymmetrical) under off‐design operating conditions and that the two have a strong relationship. At high flow rates, the blade pressure fluctuations, induced by rotor‐stator interactions, are large in areas where the volute static pressure is low. The traveling directions of the rotating pressure waves, the whirling directions of the radial fluid forces, and the most predominant frequency components of both the fluctuations and the forces are discussed, and an equation for predicting them is introduced. It was also noted that large alternating fluid forces are not necessarily associated with large pressure fluctuations. Furthermore, when measuring the radial fluid forces in the rotating frame, other frequency components, in addition to those related to the products of the diffuser vane number and the rotating frequency, may occur due to the circumferential unevenness of the pressure fluctuations on the impeller. These components are predictable.

Effects of One Blade Oscillation on the Characteristics of Shock Wave Motion in the Flow Passage of Transonic Compressor Cascade.

As for transonic compressor cascades, the movement of shock waves in the flow passage affects the aerodynamic characteristics, that is, cascade flutter characteristics, because it introduces large pressure fluctuations on the blade surfaces. Therefore, it is necessary to clarify the characteristics of shock wave movement in the flow field. However, there are few studies about the characteristics of shock wave movement in the transonic compressor cascade oscillating in the pitching mode. To make sure the characteristics of shock wave movement in the flow passage of transonic compressor cascade, we constructed a high-speed photographing system combined with a Shlieren optical unit. The photographing concept is based on the fact that the shock wave movement following the blade oscillation is periodic. From the pictures taken with this system, the characteristics of shock wave movement in the flow passages of the transonic linear compressor cascade are analyzed in the range from 0Hz to 300Hz of one blade oscillation frequency.

Electrostatic turbulence and transport with stochastic magnetic field lines

Electrostatic turbulence, associated convective transport and poloidal plasma rotation at the edge of a tokamak plasma are strongly modified in a layer of stochastic magnetic field lines. Experimental observations during ergodic divertor operation show a decrease of density fluctuations but there is no evidence of a change of the turbulent cross field diffusivity. In this paper, the underlying mechanisms are studied using three-dimensional numerical simulations of flux driven resistive ballooning turbulence. Three key elements are found to determine the characteristics of transport due to fluctuations in the stochastic layer: first, long-lived stationary eddies appear due to modified equilibrium pressure and potential. Second, large scale pressure fluctuations decrease but small scale velocity fluctuations tend to increase. Third, the poloidal plasma rotation is suppressed and zonal flows are reduced by an anomalous friction due to stochasticity. In total, the level of convective flux associated with fluctuations is not quenched by the magnetic field perturbation. The impact of stochastic field lines on electrostatic turbulence is of interest for many experimental situations such as the stochastic boundary of stellarators and transport in the vicinity of the separatrix of standard tokamak divertors.

Pressure Gradients in the Regenerator and Overall Pulse-Tube Refrigerator Performance

Significant pressure drops in the regenerator are typical in pulse-tube cryocoolers, with significant impact on performance. Irreversibilities due to viscous friction obviously lower efficiency, but in the pulse tube, this is not necessarily the most crucial issue. Indeed, by virtue of having only one driven element (the compressor), the pulse tube is a rather inflexible device from a design standpoint. Pressure and velocity amplitudes and phases determine energy fluxes. Impedances ultimately determine how large these fluxes are, hence how good a refrigerator a given design will produce. Impedance values are determined by the volume distribution, the orifice resistance, and the effect of viscous friction in the regenerator. The focus is on friction, which is difficult to deal with, especially if the device includes a bypass. An asymptotically consistent analysis has been developed, in which the regenerator is represented as an arbitrary porous medium. In contrast with most models, the analysis initially assumes arbitrary large pressure gradients and, of course, arbitrarily large temporal pressure fluctuations. The model thus obtained shows that when pressure differences due to viscous friction are comparable with the amplitude of temporal variations, viscous irreversibilities are much larger than the thermal ones. The regenerator formulation is then incorporated within a small-amplitude, harmonic model of the overall device, including the bypass, if any. For simple assumptions with respect to the temperature profile along the regenerator, such as linear and exponential profiles, closed-form solutions are obtained. Finally, the results are analyzed and their relevance is discussed.

Powder flow down a vertical pipe: the effect of air flow

The dynamics of dry granular flows down a vertical glass pipe of small diameter have been studied experimentally. Simultaneous measurements of pressure profiles, air and grain flow rates and volume fractions of particles have been realized together with spatio-temporal diagrams of the grain distribution down the tube. At large grain flow rates, one observes a stationary flow characterized by high particle velocities, low particle fractions and a downflow of air resulting in an underpressure in the upper part of the pipe. A simple model assuming a free fall of the particles slowed down by air friction and taking into account finite particle fraction effects through Richardson–Zaki's law has been developed: it reproduces pressure and particle fraction variations with distance and estimates friction forces with the wall. At lower flow rates, sequences of high-density plugs separated by low-density bubbles moving down at a constant velocity are observed. The pressure is larger than outside the tube and its gradient reflects closely the weight of the grains. Writing mass and momentum conservation equations for the air and for the grains allows one to estimate the wall friction, which is less than 10% of the weight for grains with a clean smooth surface but up to 30% for grains with a rougher surface. At lower flow rates, oscillating-wave regimes resulting in large pressure fluctuations are observed and their frequency is predicted.

On the mechanism of the bursting phenomena of propeller tip vortex cavitation

The bursting phenomenon of tip vortex cavitation of a propeller sometimes causes severe high-frequency vibration, but its mechanism has not yet been elucidated. In this study, we carried out model experiments by changing the propellers, wake distributions, thrust coefficients, and cavitation numbers parametrically, examined the bursting phenomenon with a high-speed video camera, and measured the pressure fluctuations caused by the phenomenon. We also measured flow distribution around the tip vortex. As a result, we found that in the bursting phenomenon, large pressure fluctuations occurred twice, and that they strongly depended on the wake distribution. Two means were suggested to suppress the bursting phenomenon, other than changing the wake distribution: stabilizing tip vortex cavitation or reducing the cavity volume. Numerical fluid simulations around a propeller in noncavitating, unsteady conditions were also conducted, and the strength of the tip vortex along the circumference and its derivative were examined. As a result, the phenomena were parameterized by the time derivative of the strength of the tip vortex, and if it was higher than a threshold value, the tip vortex cavitation burst. Therefore, it is possible to predict the occurrence of the bursting phenomenon by numerical analysis.

Renal blood flow autoregulation in blood pressure control

Although the kidney strives to maintain its perfusion within tight boundaries, considerable blood flow fluctuations do occur. The reasons for this are the rather slow acting compensatory mechanisms of renal blood flow autoregulation, the effects of renal nerves, hormonal influences, etc. It seems that variations in renal perfusion can exert a major influence on renal excretory functions, on renin release and on blood pressure. The clinical importance of renal blood flow variability is not fully understood. In many situations, the absence of normal cardiovascular oscillations seems to be a risk factor. Large fluctuations in perfusion pressure to the kidney, however, in the long run, may induce target organ damage.

Finite Amplitude Standing Wave in an Axi-Symmetric Closed Duct with Cross-Sectional Area Change.

When an air-column in an axi-symmetric closed tube with constant cross-section is driven by a piston vibrating at resonant frequency at one end, finite amplitude standing wave is induced in the tube. The increase of oscillation amplitude in the tube brings about the nonlinear phenomena of wave motion, such as mean pressure distribution along the tube axis, acoustic streaming, and thermoacoustic effect. The large amplitude pressure fluctuation and the thermoacoustic effect generated by finite amplitude standing wave induced in the closed tube are principal physical phenomena in the engineering applications of the finite amplitude wave motion to acoustic compressor and thermoacoustic refrigerator. In order to develop these practical systems, it is essential to realize large amplitude oscillation in the tube. However, the increase of oscillation amplitude is limited by the acoustic saturation caused by nonlinear wave distortion which steepens wave front to shock wave. In order to avoid the acoustic saturation, axi-symmetric tubes with cross-sectional area contraction toward the closed end are used. Basic characteristics of wave motion in the tubes with area change have been discussed with linear acoustic theory. Finite amplitude wave motions generated in different shape tubes have been analyzed numerically and compared with experimental results. Fundamental knowledge and estimation for the development of the acoustic compressor have been given from the results obtained.

Non-Gaussian aspects of thermal and kinetic Sunyaev-Zel’dovich effects

We discuss non-Gaussian effects associated with the local large-scale structure contributions to the cosmic microwave background (CMB) anisotropies through the thermal Sunyaev-Zel'dovich (SZ) effect. The non-Gaussianities associated with the SZ effect arise from the existence of a significant four-point correlation function in large scale pressure fluctuations. Using the pressure trispectrum calculated under the recently popular halo model, we discuss the full covariance of the SZ thermal power spectrum. We use this full covariance matrix to study the astrophysical uses of the SZ effect and discuss the extent to which gas properties can be derived from a measurement of the SZ power spectrum. With the SZ thermal effect separated in temperature fluctuations using its frequency information, the kinetic SZ effect, also known as the Ostriker-Vishniac effect, is expected to dominate the CMB temperature fluctuations at small angular scales. This effect arises from the baryon modulation of the first order Doppler effect resulting from the relative motion of scatterers. The presence of the SZ kinetic effect can be determined through a cross-correlation between the SZ thermal and a CMB map at small scales. Since the SZ kinetic effect is second order, however, contributions to such a cross-correlation arise to lower order in the form of a three-point correlation function, or a bispectrum in Fourier space. We suggest an additional statistic that can be used to study the correlation between pressure traced by the SZ thermal effect and the baryons traced by the SZ kinetic effect involving the cross-power spectrum constructed through squared temperatures instead of the usual temperature itself. Through a signal-to-noise calculation, we show that future small angular scale multifrequency CMB experiments, sensitive to multipoles of a few thousand, will be able to measure the cross-correlation of SZ thermal and SZ kinetic effects through a temperature squared power spectrum.

Squared temperature-temperature power spectrum as a probe of the CMB bispectrum

It is now well known that mode-coupling effects associated with certain secondary effects generate higher order correlations in cosmic microwave background (CMB) temperature anisotropies, beyond the two-point function. In order to extract such a non-Gaussian signal at the three-point level, we suggest a two-point statistic in the form of an angular power spectrum involving correlations between squared temperature and temperature anisotropies. This power spectrum contains compressed information from the bispectrum and can be easily measured in data with the same techniques that have been considered for the measurement of the usual temperature-temperature anisotropy power spectrum. We study the proposed power spectrum resulting from the non-Gaussian signal generated by correlations involved with gravitational lensing angular deflections in CMB and the Sunyaev-Zel’dovich (SZ) effect due to large scale pressure fluctuations. Using the Planck frequency cleaned CMB and SZ maps, the CMB2-SZ power spectrum provides a direct estimate of the cross-power between lensing angular deflections and the SZ effect. Through an optimal filter applied to the squared CMB map, the proposed statistic allows one to obtain all information from the lensing-SZ bispectrum. The observational measurement of the lensing-SZ cross-correlation is useful to understand the relation between large scale structure pressure and dark matter fluctuations.

Renal Blood Flow Dynamics and Arterial Pressure Lability in the Conscious Rat

It is not known whether renal blood flow (RBF) is still autoregulated when the kidney is exposed to large transient blood pressure (BP) fluctuations such as those occurring spontaneously in conscious sinoaortic baroreceptor-denervated (SAD) rats. In this study, BP and RBF were simultaneously recorded in 8 SAD rats (2 weeks before study) and 8 baroreceptor-intact rats during approximately 3 hours of spontaneous activity. The kidney used for RBF recordings was denervated to prevent the interference of changes in renal sympathetic tone with autoregulatory mechanisms. In intact rats, RBF variability (coefficient of variation 9.1+/-0.8%) was larger (P<0.02) than BP variability (5.9+/-0.2%). This was mainly because of slow changes in RBF that were unrelated to BP and also to a prominent oscillation of RBF of approximately 0.25-Hz frequency. Autoregulatory patterns were identified at frequencies <0.1 Hz and provided a modest attenuation of BP fluctuations. In SAD rats, RBF variability (12.4+/-1.6%) was lower (P<0.02) than BP variability (18.2+/-1.1%). Autoregulation powerfully attenuated BP changes <0.1 Hz (normalized transfer gain 0.21+/-0.02 in the 0.0015- to 0.01-Hz frequency range) but at the expense of an oscillation located at approximately 0.05 Hz that possibly reflected the operation of the tubuloglomerular feedback. Large transient hypertensive episodes were not translated into RBF changes in SAD rats. We conclude that autoregulatory mechanisms have an ample capacity to protect the kidney against spontaneous BP fluctuations in the conscious rat. This capacity is not fully used under normal conditions of low BP variability.

Editor's Choice

Editor's Choice

Ischemic optic neuropathy associated with idiopathic dilated episcleral veins

A man had acute, painful vision loss due to nonarteritic anterior ischemic optic neuropathy. Although he initially had a mildly elevated intraocular pressure, he later was found to be experiencing large fluctuations in intraocular pressure. He had dilated, tortuous episcleral veins in that eye, which were thought to be the cause of his elevated intraocular pressure.

Steady-unsteady measurements of the flow field downstream of a transonic high-pressure turbine stage

The three-dimensional flow field of a transonic high-pressure turbine stage has been studied experimentally in the von Karman Institute short duration compression tube facility. The test programme encompasses two Reynolds numbers ( Re2, c = 1.0 × 106, 1.26 × 106) and three different pressure ratios ( Po1/ Ps3 = 2.3, 3.1, 4.1). The steady flow angle, pressure and temperature measurements rely on five-hole pneumatic probes and micro-thermocouples. Fast response pressure and cold wire resistance thermometer probes measure the unsteady pressure and temperature. The experimental time-averaged spanwise distributions are compared with steady state three-dimensional Navier-Stokes stage simulations; zones of secondary flows are identified in the spanwise distributions. The stage exit pitchwise traverses show that the stator exit flow field affects the stage exit flow. Downstream of the stator vane, large time-resolved static pressure fluctuations are identified, attributed to the variation of the origin, the incidence and strength of the stator trailing edge shock. At the stage outlet, large total temperature fluctuations appear owing to the periodic passage of the tip leakage flow. For the high-pressure ratio, the rotor is choked and large pressure and temperature fluctuations are recorded owing to the rotating rotor trailing edge shock system. Zones of high mixing (secondary flows) tend to attenuate these fluctuations.