Periodic Flow Structures Research Articles

Periodic flow structures in vertical gas-particle flows

Abstract Simulations of kinetic theory based two-fluid model for gas-particle flows in vertical riser segments have been performed. In two-dimensional (2D) simulations of flows in a vertical channel segment and in three-dimensional (3D) simulations of flows in a vertical cylindrical riser segment, periodic boundary conditions were employed in the axial direction. These were supplemented by simpler quasi-one-dimensional (1D) simulations of channel flow and quasi-2D simulations in cylindrical geometry, where it was postulated that the flow variables did not change axially. Both 2D and quasi-1D simulations of channel flow were found to manifest in a robust manner quasi-periodic solutions consisting of laterally traveling waves. Similarly, quasi-2D and 3D simulations of cylindrical riser flow manifested spinning flow states, which in the case of high aspect ratios showed signs of transition to swirling flow states. The period of the oscillating/spinning flow could be captured by a rather simple correlation, whose structure suggests a microscopic origin for the flow structure. The centrifugal force associated with the spin appears to be sufficient to impart an influence on the extent of radial segregation of particles.

Flow Visualization and Frequency Characteristics of Velocity Fluctuations of Complex Turbulent Flow in a Short Elbow Piping Under High Reynolds Number Condition

Flow visualization was performed on a single short elbow piping by means of two-dimensional particle image velocimetry. The piping was designed as a 1/7-scale model of a section of the cold-leg piping of a Japan sodium-cooled fast reactor. This study characterized the periodic motions and flow structures that appeared in and downstream of the elbow and potentially affected flow-induced vibrations. The flow field that related flow separation and frequency characteristics of the flow velocity fluctuation were explored for Reynolds number from 0.3 × 106 to 1.0 × 106, which belonged to the post-critical regime. Experimental results show that flow structures are not strongly dependent on Reynolds number in this range. Frequency analysis for the velocity fluctuation in terms of Strouhal number (St) reveals that there exist not only two kinds of vortices with different shedding periods, but also one periodic flow in the circumferential direction. In the flow separation region, vortices are periodically emitted with St ≈ 0.5, while those with about 1.0 are shed in a shear flow region located between the separation region and the pipe center. Moreover, a periodic motion with St ≈ 0.5 appeared in the circumferential direction in the vicinity near the separation region. These values of St were not strongly dependent on Reynolds number in this study.

PIV measurements of slow flow of a viscoelastic fluid within a porous medium

Particle image velocimetry (PIV) and pressure loss measurements were used to investigate slow flow through a square array of cylinders having a solid fraction of 10%. The test fluids were a Newtonian fluid and a Boger fluid, both of high viscosity such that the Reynolds number did not exceed 0.1. The pressure loss data reveal that the onset of elastic effects occurred at a Deborah number around 0.5 and that flow resistance was up to several times Newtonian values at Deborah numbers up to 3. PIV showed that the transverse velocity profiles for the Newtonian and non-Newtonian fluid were the same at Deborah numbers below onset. Above onset, the profiles became skewed, increasingly so as the Deborah number increased. In the wake regions between cylinders in a column, periodic flow structures formed in the spanwise direction. The structures were staggered from column to column, consistent with the skewing and were offset. These flow patterns are the result of an apparent elastic instability.

Packet waiting time for multiplexed periodic on/off streams in the presence of overbooking

We present simple approximation formulae for the distribution of the packet waiting time of multiplexed periodic traffic. The multiplexed streams may have different periods and packet sizes. We show by extensive simulations the accuracy of the proposed methods. They are simpler than other existing formulae which make them attractive for engineers, applicable in practice, and easy to implement in switching devices. Packetised speech traffic has a periodic flow structure. Many compression techniques preserve it during talk phases but suppress the generation of packets during silence phases. When such on/off streams are multiplexed, advantage can be taken of their reduced flow rates by overbooking the link bandwidth. We adapt the proposed formulae to cope with on/off traffic and overbooking and validate them by extensive simulations. They can be applied for admission control in networks carrying different types of real-time traffic.

Combustion Noise from Non-premixed and Lean-premixed Swirl Flames

The development of modern gas turbines and jet engines is focused on the reduction of pollutant emissions and, increasingly, on the reduction of overall noise emission, including combustion noise. This requires the minimization of the noise sources, namely noise from the turbulent flow, combustion noise and noise caused by periodic instabilities and fluctuations of the ignition zone. This has to be achieved under conservation of the benefits of swirl flames, e.g. high ignition stability and broad operation ranges. This paper is focused on the description and characterisation of fluctuations of the ignition zone due to different mixing and stabilization characteristics of noise from swirl flames. Another goal was the description of flame noise caused by the formation and burning of coherent, periodic flow structures, often detected in swirl flows and flames [1, 2, 3, 4]. The physical model offers the possibility to understand the formation mechanism of the coherent flow structures and their effect on the combustion of non-premixed and premixed swirl flames.

Development of laminar-separating-flow perturbations on a wavy wing

The development of velocity perturbations at laminar boundary-layer separation from a wing with wavy surface has been examined. Experiments were carried out to identify flow features displayed by the spatially periodic flow structure. An analysis of the linear stability of measured velocity profiles is performed. As a result of the analysis, the influence of surface waviness on the frequency range and growth rates of instability waves was investigated, with a good agreement between calculated and experimental data.

Experimental and Computational Analysis of Periodic Flow Structure in Oscillatory Gas Flow Meters

AbstractThe oscillatory characteristics and dynamic structure of periodic flow in an oscillatory gas flow meter were studied experimentally and numerically. The flow oscillations were triggered by the Coanda effect and an universal correlation between Strouhal number and Reynolds number, Str = 1.09 × 10−3 for ReHD >800, was deduced based on spectral analysis of the pressure fluctuations in the flow channel. Numerical simulation indicated that the evolution of the flow patterns was classified into stages of induction and sustainable periodic oscillation. The transformation between the two stages was noticeably affected by the design of the feedback channels. The results further revealed that the development of the main vortex in the oscillating chamber and the small vortices at the entrance of the feedback channels concurrently modulate the mechanism of oscillation. The small vortices located at both entrances of the feedback channels play the role of a pair of modulating valves, which alternatively switch on and off the bypass flow through each feedback channel, thus reinforcing the periodic oscillation.

The Fluid Dynamical Properties of a Dilute Surfactant Solution around a Circular Cylinder

Influence of adding cationic surfactant (Ethoquard O12) with a consentration of 50 ppm on water flow around a cylinder in a range of Reynolds number Res between from 64 to 600 was investigated by using LDV and PIV. At Res =300, the flow with the surfactant formed a stagnant zone, which appeared periodically, in front of the cylinder expanding along the axial direction. The period of the appearance was about 120 [s]. The periodic flow structure was visualized by the measurements of velocity fluctuation. The development of the stagnant zone in front of the cylinder induced the expansion of a low speed flow area near the side of the cylinder.

Measurement of tonal-noise characteristics and periodic flow structure around NACA0018 airfoil

The characteristics of tonal noise and the variations of flow structure around NACA0018 airfoil in a uniform flow are studied by means of simultaneous measurement of noise and velocity field by particle-image velocimetry to understand the generation mechanism of tonal noise. Measurements are made on the noise characteristics, the phase-averaged velocity field with respect to the noise signal, and the cross-correlation contour of velocity fluctuations and noise signal. These experimental results indicate that the tonal noise is generated from the periodic vortex structure on the pressure surface of the airfoil near the trailing edge of the airfoil. It is found that the vortex structure is highly correlated with the noise signal, which indicates the presence of noise-source distribution on the pressure surface. The vorticity distribution on the pressure surface breaks down near the trailing edge of the airfoil and forms a staggered vortex street in the wake of the airfoil.

Kinematics and depth-integrated terms in surf zone waves from laboratory measurement

Kinematics of nominally periodic surf zone waves have been measured in the laboratory using LDA (laser Doppler anemometry), above trough level as well as below, for weakly plunging breakers transforming into bores in shallower water. The aim was to determine, through phase- or ensemble-averaging, periodic flow structures in a two-dimensional vertical plane, from large-scale down to small-scale vortical structures. Coherent multiple vortical structures were evident at the initiation of breaking, becoming elongated along the surface during bore propagation. The initial region is likely to become more extensive as waves become more strongly plunging and could explain the difference in turbulence characteristics between plunging and spilling breakers observed elsewhere. Comparison of vorticity magnitudes with hydraulic-jump measurements showed some similarities during the initial stages of breaking, but these quickly grew less as breaking progressed into shallower water. Period-averaged kinematics and vorticity were also obtained showing shoreward mass transport above trough level and undertow below, with a thick layer of vorticity at trough level and a thin layer of vorticity of opposite rotation at the bed. There were also concentrated regions of mean vorticity near the end of the plunging region. Residual turbulence of relatively high frequency was presented as Reynolds stresses, showing marked anisotrophy. Dynamic pressure (pressure minus its hydrostatic component) was determined from the kinematics. The magnitudes of different effects were evaluated through the depth-integrated Reynolds-averaged Navier–Stokes (RANS) equations, which may be reduced to nine terms (the standard inviscid terms of the shallow-water equations conserving mass and momentum with hydrostatic pressure, and six additional terms), assuming that the complex, often aerated, free surface is treated as a simple interface. All terms were evaluated, assuming that a space/time transformation was justified with a slowly varying phase speed, and the net balance was always small in relation to the maxima of the larger terms. Terms due to dynamic pressure and vertical dispersion (due to the vertical variation of velocity) were as significant as the three terms in the inviscid shallow-water equations; terms involving residual turbulence were insignificant. The r.m.s. (root mean square) variation of each along the slope is highly irregular, with the inertia term due to (Eulerian) acceleration always greatest. This is consistent with complex, though repetitive, coherent structures. Modelling the flow with the shallow-water equations, using the surface elevation variation at the break point as input, nevertheless gave a good prediction of the wave height variation up the slope.

Development characteristics of fluctuating velocity field of drag-reducing surfactant solution flow in a duct

Development behavior of the fluctuating velocity of surfactant solution in a duct has been studied experimentally. The concentration of surfactants was kept constant at 1,000 ppm, mean velocity at 0.78 m/s and fluid temperature at 15 °C. Using laser Doppler velocimetry, the fluctuating streamwise velocity distributions at six cross sections, which ranged from 14 to 112 times of hydraulic diameter of the duct, were measured. From the results, the fluctuating structures of surfactant solution flow are observed to have structures different from that of turbulent water flow in the developing field. The wavelet analysis reveals that the high-level fluctuation of surfactant solution flow is characterized by periodicity rather than irregularity around the position where the fluctuation intensity takes a peak value and that the period and the scale of periodic flow structures are related to the relaxation times of the fluid. This indicates that the high-level fluctuation is deeply related to the elastic instability and has a different generation mechanism from that of turbulence observed in a Newtonian turbulent flow.

디지털 홀로그래픽 간섭계와 PIV를 이용한 Hele-Shaw Cell 내부 열유동 해석

Yariations of temperature and velocity fields in a Hele-Shaw convection cell (HSC) were investigated using a holographic interferometry and 2-D PlY system with varying Rayleigh number. To measure quasivariation of temperature field, two different measurement methods of holographic interferometry; double-exposure method and real-time method, were employed. In the double-exposure method, unwanted waves were eliminated effectively using a digital image processing technique. The reconstructed images are clear, but transient flow cannot be reconstructed clearly. On the other hand, transient convective flow can be reconstructed well using the real-time method. However, the fringe patterns reconstructed by the real-time method contain more noises, compared with the double-exposure method. Experimental results show a steady flow pattern at low Rayleigh numbers and a time-dependent periodic flow structure at high Rayleigh numbers. The periodic flow pattern at high Rayleigh numbers obtained by the real-time holographic interferometer method is in a good agreement with the PlY results.<br/>

Application of holographic interferometry and 2D PIV for HSC convective flow diagnostics

Variations of temperature and velocity fields in a Hele–Shaw convection cell (HSC) were investigated using a holographic interferometry and 2D PIV system with varying Rayleigh number. To measure quasi-steady variation of the temperature field, two different measurement methods of holographic interferometry—the double-exposure method and the real-time method—were employed. In the double-exposure method, unwanted waves were eliminated effectively using a digital image processing technique. The reconstructed images are clear, but transient flow cannot be reconstructed clearly. On the other hand, transient convective flow can be reconstructed well using the real-time method. However, the fringe patterns reconstructed by the real-time method contain more noise than the double-exposure method. Experimental results show a steady flow pattern at low Rayleigh numbers and a time-dependent periodic flow structure at high Rayleigh numbers. The periodic flow pattern at high Rayleigh numbers obtained by the real-time holographic interferometer method is in good agreement with the PIV results.

Stereoscopic PIV measurements of flow behind an isolated low-speed axial-fan

The flow field behind a forward-swept axial-fan with five blades rotating in a water tank has been measured by a stereoscopic particle image velocimetry (SPIV) system based on the translation configuration. In this study, a three-dimensional calibration procedure was employed to compensate the distortion and refraction of particle images. Phase-averaged velocity fields show that the flow behind an axial-fan has a periodic flow structure with respect to the blade phase and the characteristic flow structure is shifted downstream in the succeeding phase. The flow speed of fan wake has a local maximum value in the radial distance of about y/ R=0.75. Strong counter-clockwise tip vortices are shed from the utmost trailing edges of the blade, while relatively weak clockwise trailing vortices exist in the hub region. The phase-averaged velocity and turbulence intensity of the out-of-plane velocity component have large values in the region near the tip vortices and they clearly show the evolution and dissipation of tip vortices and trailing vortices moving toward the tangential direction.

Quantitative analysis of flow inside the accumulator of a rotary compressor

The flow structure inside the accumulator of a rotary compressor was investigated experimentally using a hybrid particle tracking velocimetry (PTV) velocity field measurement technique. A proto-type accumulator was studied while operating under real working conditions. The flow inside the accumulator was qualitatively visualized using a high-speed camera. The period of one cycle of the rotary compressor was divided into 4 phases and the velocity fields inside the accumulator were measured using a phase-averaging technique. The flow inside the accumulator, especially in the region between the screen holder and tube holder, showed a pulsating periodic flow structure according to the rotational phase of rotary compressor.

Stereoscopic PIV 기법의 개발과 이를 이용한 축류 홴 후류의 유동해석

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Investigation of the Electroconvective Flow of a Weakly Conducting Liquid with Unipolar Injection Conductivity by the Finite-Element Method

The electroconvective flow of a weakly conducting liquid dielectric in a plane-parallel electrode system is investigated by the finite-element method of Galerkin in the vicinity of the crisis of equilibrium-stability loss. The threshold value of the strength of a homogeneous electric field (corresponding to the occurrence of stationary two-dimensional spatially periodic flow structures) is determined. The empirical relation for the extremum of the current function in the vicinity of the crisis is obtained. The distribution of the volume density of charges over the interelectrode gap in the ascending and descending convective flows is found.

Transcritical Patterns of Cavitating Flow and Trends of Acoustic Level

Hydroacoustics of the transcritical cavitating flows on a NACA16012 hydrofoil at a 2/5/8-degree angle of attack and axisymmetric bodies with hemispherical and 45-degree conical headforms were studied, and the process of cloud cavitation shedding was observed by means of high-speed cinegraphy. By expressing the cavitation noise with partial acoustic level, it is found that the development of cavitation noise varies correspondingly with cavitation patterns. The instability of cavitation is a result of cavity-flow interaction, and is mainly affected by the liquid flow rather than by the cavitation bubbles. A periodic flow structure with a large cavitation vortex is observed and found to be responsible for inducing the reentrant-jet and consequent cavitation shedding, and explains the mechanism of periodic cavitation shedding from a new viewpoint. New terms for the three stages, growing, hatching and breaking, are used to describe the process of cavity shedding.

Local convective heat transfer to submerged pulsating jets

An experimental investigation was performed to study the effect of flow pulsations on the local heat transfer characteristics of a planar air jet. A microsensor provided localized and essentially instantaneous heat flux and temperature signals. Heat transfer and thermal anemometry flow measurements were synchronized to associate influences of periodic flow structures and pulsations on heat transfer. Pulsation frequencies corresponded to Strouhal numbers below 0.106, and pulse amplitude was varied from 0 to 50% of the mean flow velocity. Nusselt numbers in the nozzle mid-plane increased by up to 12%, and by up to 80% at distances downstream.

Visualization of vortex shedding at the proximal side-to-end artery-graft anastomosis

The motion of guanine particles was recorded by video to visualize transitional flow phenomena in models of a proximal side-to-end anastomosis. Close examination of successive video fields revealed that above a critical Reynolds number, particles were periodically shed into the graft from a vortex situated near the anastomosis heel, and this disturbed the flow patterns in the graft causing vortex shedding to occur near to the toe of the anastomosis. The images clearly demonstrated that periodic flow structures propagated distally along the graft for over 15 tube diameters from the proximal anastomosis. The frequency of the vortex shedding was found to increase with Reynolds number. Under pulsatile conditions, the primary vortex at the heel of the anastomosis became unstable during the deceleration phase of the flow cycle and particles were shed downstream into the graft. Although it was possible briefly to observe the characteristic banded structure in the bypass graft, the flow patterns were highly three-dimensional and were quickly broken up by the accelerating flow. Dynamic flow visualization using guanine particles was found to be a complementary technique to particle tracer flow visualization and was highly effective in identifying transitional flow phenomena and the mass transport mechanisms associated with them.