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Flow structures around a wavy square cylinder with a perturbation wavelength of 5.6D are investigated using large eddy simulation at the Reynolds number of 23 500. The detailed force characteristics and wake flow structures of the wavy square cylinder are captured and compared with a square cylinder. Under the effect of wavy leading edge up to 27% and 98% reduction in the mean drag and the lift fluctuations are achieved, respectively. The 3D mean flow field implies a high shear flow between node and saddle positions, which is associated with the generation of additional streamwise and vertical vortex pairs. These vortex structures are responsible for the three-dimensionality of the wake flow behind the wavy square cylinder. The instantaneous flow patterns suggest that the staggered pattern of von Kármán vortex shedding is suppressed and replaced by symmetric vortex shedding into the near wake region of the wavy square cylinder. This symmetric vortex structure in the near wake plays a role in preventing the upper and lower shear layers from interacting with each other. The time–frequency analysis exhibits a reduction in the dominating vortex shedding frequency for the case of wavy square cylinder, indicating a relatively steady wake flow. This can be attributed to the vortex dislocation behind the wavy square cylinder. In the range of high frequencies, small streaks associated with small-scale fluctuations are enhanced by the wavy leading edge. The present study suggests that the perturbation wavelength obtained from the wavelength of Mode A instability at laminar flow regime can be served as a basis for flow control in turbulent flow regime. [ABSTRACT FROM AUTHOR]

The purpose of this study is to reduce the conveying velocity and power consumption in gas–solid two-phase flows. Four pieces of soft fins with different (non-uniform) lengths, which are fixed on the horizontal central plane of the inlet, are employed to self-excite the gas–solid flow. A horizontal acrylic pipe, approximately 5 m long and with an inside diameter of 80 mm, is selected as the test pipeline. The test solid materials are spherical polyethylene particles with average diameter and density of 2.3 mm and 978 kg/m3, respectively. The solid mass flow rate and average gas velocity are 0.10–0.47 kg/s and 10–17 m/s, respectively. Compared with non-fin and previous uniform soft fin gas–solid flows, the minimum gas velocity, pressure drop, additional pressure drop, and power consumption are reduced when a lower gas velocity is achieved with the use of non-uniform soft fins. The highest reduction rates in power consumption and minimum velocity are approximately 5.87% and 10.04%, respectively. The particle concentration in flows with non-uniform soft fins is higher than that in the non-fin flow in the upper part of the pipe of the acceleration region and lower than those in non-fin and uniform fin flows near the pipe bottom. By employing particle image velocimetry measurement, the time-averaged axial particle velocity and fluctuating particle velocity in all non-uniform fin flows are found to be higher than those in non-fin and uniform fin flows, especially near the upper part of the pipe. Unlabelled Image • The fins of non-uniform length were used to excite the gas-solid two-phase flow • Pressure drop, minimum velocity and particle velocity were measured • The lowest pressure drop and gas velocity were realized by the non-uniform fins • Particle velocity and fluctuating velocity increased by using the non-uniform fins [ABSTRACT FROM AUTHOR]

Abstract The effect of self-excited soft fins on a pneumatic conveying with different vertical curved 90° bends was experimentally studied to reduce pressure drop, power consumption and conveying air velocity. The distributions of particle velocity and particle fluctuating velocity near and in the first bend were measured based on the high-speed PIV. The polyethylene particles with a diameter of 2.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 14 m/s, and the stable mass flow rate was fixed at 0.45 kg/s. Compared to the non-fin pneumatic conveying, the pressure drop, the minimum pressure drop (MPD) velocity, power consumption, and additional pressure drop can be reduced by using soft fins for two bends of different radii (R/D = 3.2 and 4.4) in lower air velocity range. The maximum reduction rates of the MPD velocity and power consumption by using soft fins is about 8.2% and 8%, respectively. At the upstream of the bend, the particle velocity of the soft fins is higher than that of the non-fin. The effect of soft fins on the particle velocity and its fluctuating energy remains in the bend. The particle velocity and fluctuating energy of particle velocity of the soft fins are higher than that of the non-fin in the first vertical bend. At the downstream of the bend, the fluctuating energy of particle velocity of the soft fins is higher than that of non-fin in the large radius bendon the top part of the pipe. Graphical abstract Unlabelled Image Highlights • A pneumatic conveying with vertical bends by using soft fins was studied. • The pressure drop, additional pressure drop and particle velocity were measured. • The pressure drop and conveying velocity were reduced by fins. • Particle velocity of fins was higher than that of non-fin near and in the bend. • Effect of fins on the particle velocity and fluctuating energy remains in the bend. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes an application of a rotation swirler to swirling flow pneumatic conveying for reducing the conveying velocity and pressure drop decrease. A comparison with the swirling flow pneumatic conveying generated by a non-rotation swirler and axial flow pneumatic conveying was made, the total pressure drop decrease, minimum conveying velocity, energy consumption, and additional pressure drop decrease were evaluated experimentally. In addition, the cross-section profiles of the particle velocity and particle fluctuation velocity along a pipeline near the minimum pressure drop decrease were measured by the PIV (Particle Image Velocimetry) technique. The pipeline was a 5-m-long horizontal straight acrylic pipe with an inside diameter of 80 mm. Polyethylene particles of diameter of 2.3 mm were used as the conveying materials. The initial swirl number was varied from 0 to 0.66. The solid mass flow rates of 0.1, 0.28, and 0.47 kg/s were applied, and the superficial air velocity was varied from 9 to 16 m/s. It is revealed that the total pressure drop decrease and power consumption of the swirling flow using the rotation swirler exhibit the smallest values among all cases in the region of low air velocity, and are almost the same as the axial flow in the high air velocity region. The particle velocity and particle fluctuation velocity of the swirling flow originated by the rotation swirler are almost the same as the axial flow in the acceleration regime, but become larger than that of the axial flow near the bottom of the pipeline in the fully developed regime. Graphical abstract Unlabelled Image Highlights • A rotation swirler was applied to swirling flow pneumatic conveying. • Pressure drop, minimum velocity and particle velocity were measured. • The rotation swirler pneumatic conveying exhibits the lowest pressure drop • High particle velocity and fluctuation velocity were observed near the bottom of pipe. [ABSTRACT FROM AUTHOR]

To further elucidate the mechanism of energy-conserving conveying in horizontal pneumatic conveying with the dune model, the high-speed particle image velocimetry is applied to measure particle fluctuation velocity near the minimum conveying velocity of the conventional pneumatic conveying. This study focuses on the effect of mounting dune models on the horizontal pneumatic conveying in terms of power spectrum, autocorrelation coefficients, two-point correlation coefficients, fluctuation intensity of particle velocity, skewness factor, and probability density function. It is found that the power spectrum peaks with the dune model are larger than those of the nondune system, suggesting the acceleration and suspending efficiency of the dune model, especially dune models mounted at the bottom of the pipe. Meanwhile, the profiles of particle fluctuation velocity intensity indicate that the large particle fluctuating energy is generated due to mounting the dune model so that the particles are more easily accelerated and suspended. This is one of the important reasons why the mounted dune model results in a low pressure drop and low minimum conveying velocity. Based on the distribution of skewness factor and probability density function, it is found that the particle fluctuation velocities of all cases follow the Gaussian distribution in the lower and middle parts of the pipe. The particle fluctuation velocities in the case of the dune models mounted at the bottom of the pipe obey the Gaussian-type fluctuation more. [ABSTRACT FROM AUTHOR]

Phase-average technique based on wavelet multi-resolution analysis and continuous wavelet transform are used to reveal the phase-averaged features of square cylinder wake measured by high-speed PIV. The one-dimensional orthogonal wavelet analysis is first applied to decompose the measured velocity fields into large-, intermediate- and small-scale structures. Then the phase information referenced with large- and intermediate-scale flow structures are clearly identified based on Morlet wavelet transform. Finally, the data ensembles are phase-sorted to give phase-averaged representations of measured flow field. The instantaneous multi-scale structures suggest that large-scale vortices are weakened and begin to transfer into intermediate-vortices at the downstream of separation region. The intermediate-scale vortex observed at the upper boundary of shear layer is considered to be associated with the secondary vortex movement. The phase-averaged intermediate-scale structures tend to convey downstream along streamwise direction, with the rotation sense varying from the first half period to the last half period. The peaks of phase-averaged large-scale Reynolds stress tend to move back and forth in the near-wake region. These findings suggest that the proposed phase-average technique is effective in revealing multi-scale fluid dynamics of wake flow structures. [ABSTRACT FROM AUTHOR]

The particle velocities are measured by the high-speed particle image velocimetry (PIV) in the acceleration and fully developed regimes of a horizontal pneumatic conveying. Based on the measured particle fluctuation velocities, continuous wavelet transform and one-dimensional orthogonal wavelet decomposition were applied to reveal particle dynamics in terms of time frequency analysis, the contribution from wavelet level to the particle fluctuation energy, spatial correlation and probability distribution of wavelet levels. The time frequency characteristics of particle fluctuation velocity suggest that the small-scale particle motions are suppressed and tend to transfer into large scale particle motions from acceleration regime to fully developed regime. In the near bottom part of pipe, the fluctuation energy of axial particle motion is mainly contributed from the wavelet levels of relatively low frequency, however, in the near top part of pipe, wavelet levels of relatively high frequency make comparable contribution to the axial particle fluctuation energy in the suspension flow regime, and this contribution decreases as particles are accelerated along the pipe. The low frequency wavelet levels exhibit large spatial correlation, and this spatial correlation increases as the particles flow from acceleration regime to fully developed regime. The skewness factor and kurtosis factor of wavelet level suggest that the deviation of Gaussian probability distribution is associated with the central frequency of wavelet level, and the deviation from Gaussian distribution is more evident as increasing central frequency. The higher wavelet levels can be linked to small sale particle motions, which lead to irregular particle fluctuation velocity. [ABSTRACT FROM AUTHOR]

Abstract: A passive flow control technique, which is to drill a horizontal hole going from the front surface to the rear surface or an inclined hole going from the front surface to the top surface inside a circular cylinder of an aspect ratio H/D = 1, is proposed to control the rear recirculation region and the vortices near the free end surface. Here, both the diameter D and height H of cylinder are 70 mm. PIV measurement was performed with Reynolds number of 8570 in a water tunnel. Furthermore, in order to consider the effect of the hole position of the front surface, the cylinder models having different hole height of front surface from wall were tested. It is found that the rear recirculation zones of the horizontal hole cylinders are smaller than that of the standard cylinder. The Reynolds shear stresses and the turbulent kinetic energy are evidently reduced by the flow issued from the horizontal hole. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone are broken down into several small-scale vortices with decreasing the height of the horizontal hole. Although the rear separation zone of the inclined hole cylinder increases, the recirculation region near the free end surface decreases. The areas of large Reynolds shear stress and high turbulent kinetic energy increase in the rear recirculation zone. However, with increasing the height of the inclined hole, the Reynolds shear stress decreases.Graphical Abstract: [ABSTRACT FROM AUTHOR]

To control the vortex structures and separation region of a low-aspect-ratio cylinder wake, a passive flow control method that is to drill a horizontal hole from the front side surface to the rear side surface was proposed for a short circular cylinder having an aspect ratio H/D  = 1 with height H and diameter D of 70 mm. The PIV measurements were performed at Reynolds number of 8570 in a circulation water tunnel in order to compare the flow characteristics between the controlling and no-controlling wakes. Furthermore, to study the position effect of the horizontal hole, three kinds of the hole positions having different height h from wall were tested. It was found that the jet flow from a horizontal hole leads to suppress the vortex formation and to reduce the rear separation region. The vorticity, Reynolds shear stresses and TKE were evidently suppressed by the jet flow of the horizontal hole in the rear recirculation zone, resulting in the reduction of the drag acted on the cylinder. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone were broken down into several small-scale vortices by the jet. As increasing the height of hole, the downwash flow is evidently suppressed and the upwash flow is increased. Spanwise vortex shedding is altered from symmetric to asymmetric arrangement. [ABSTRACT FROM AUTHOR]

In order to study the characters of particle fluctuation velocity at the minimum pressure drop (MPD) velocity in the acceleration and fully developed regimes of a horizontal pneumatic conveying, the particle fluctuation velocities measured by the high-speed particle image velocimetry (PIV) are first analyzed by time-averaged particle velocity, fluctuating energy, power spectrum and autocorrelation. Then the proper orthogonal decomposition (POD) and continuous wavelet transform are developed to reveal the particle fluctuation velocities in terms of the contributions to the particle fluctuation energy, time-frequency distribution, as well as probability density function from POD modes. The energy distributions of POD modes suggest that the first two POD modes are most energetic and dominate the particle motion, and the dominance increases from the acceleration regime to fully-developed regime. The time-frequency characteristics of POD modes reveal that small-scale particle fluctuations are suppressed in the fully-developed regime. It implies that the suppression of small-scale particle fluctuations may result in lower pressure drop at MPD velocity. The PDF distributions of POD modes exhibit that the particle fluctuation of first two POD modes follows the Gaussian distribution in the acceleration and fully developed regimes. However, the PDF distributions of POD modes gradually deviate from Gaussian distribution as increasing mode number. [ABSTRACT FROM AUTHOR]

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated. [ABSTRACT FROM PUBLISHER]

This study focuses on the multi-scale turbulent structures of triangle cylinder wake flow measured by high-speed particle image velocimetry (PIV). The measured turbulent structures were spatially extracted into large-, intermediate-, relatively small-, and small-scale components using two-dimensional orthogonal wavelet analysis. Then the features of multi-scale turbulent structures were analyzed in terms of two-point auto correlation, power spectra, turbulent kinetic energy and Reynolds shear stress. It is found that the large-scale structure attributed to coherent motions in the wake flow makes largest contribution to the total fluctuating energy and maximum Reynolds shear stress, accounting for 84% and 92% respectively. The intermediate scale structures, which are unidentifiable by PIV measurement, are found to shed into the separated shear layer due to Kelvin-Helmholtz instability, and the central frequency of intermediate scale vortex is approximately twice of Kármán vortex shedding frequency. Besides, two symmetric regions of intensive velocity fluctuations having relatively small-scale are observed near the cylinder surface and the boundary of separation region, and they are not dominant. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) turbulent structure was simulated by large eddy simulation (LES), and then the numerical result was validated by PIV experiment. In order to give a detailed description of dune wake flow, the instantaneous velocity, vorticity, and pressure were decomposed into the large-, intermediate- and relatively small-scale components by 3D wavelet multi-resolution technique. To get a further understanding of coherent structure, the decomposed wavelet components were employed to calculate Q-criterion. It was found that the rollers and horse-shoe structures in the separation bubble were mainly contributed from large-scale structures and it made the most significance to the vorticity concentration. The observations of intermediate-scale horse-shoe structures indicated that the coherent structure was the combined effect of large- and intermediate-scale structures. Besides, from the visualization of 3D streamlines and pressure iso-surfaces, the separation bubble and pressure distribution are found to be dominated by large-scale structure. [ABSTRACT FROM AUTHOR]

The effect of using the soft fins on a horizontal pneumatic conveying of granular particles in various curved 90° bends was studied in this paper, in order to reduce pressure drop and conveying air velocity. Experimental measurements were performed in terms of the pressure drop, conveying air velocity, power consumption and additional pressure drop. The distributions of particle velocity near and in the curved 90° bend were measured by high-speed PIV. The test pipeline consisted of a 4.5 m-long horizontal straight acrylic tube, a curved 90° acrylic bend and a 1.5 m-long horizontal straight acrylic tube, having an inside diameter of 80 mm. The polyethylene particles with diameter of 2.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 14 m/s, and the solid mass flow rate was fixed at 0.45 kg/s. Comparing with the dilute phase pneumatic conveying, the pressure drop, the minimum pressure drop (MPD) velocity, power consumption and additional pressure drop can be reduced by using soft fins for various bends in lower air velocity range. The reduction becomes more evident with increasing the radius ratio of bend. The maximum reduction rates of the MPD velocity and power consumption by using soft fins is about 8.2% and 11.7%, respectively. At the upstream of bend, the particle velocity of the soft fins is evidently higher than that of the dilute phase in the bottom part of pipe for all bends. The effect of soft fins on the particle velocity and its fluctuating energy still remains in the bend and the downstream of bend, and the fluctuating energy of particle velocity gradually decreases through the bend. At the downstream of bend, the fluctuating energy of particle velocity decreases with increasing the radius ratio of bend. [ABSTRACT FROM AUTHOR]

The three-dimensional dune wake flow is evaluated by large eddy simulation at a Reynolds number of 5,534, and validated by time-averaged results of particle image velocimetry measurement at a central streamwise plane. To reveal multi-scale characteristics of vortical structure, the instantaneous velocity, vorticity, and pressure were decomposed into the large-, intermediate- and small-scale components by three-dimensional wavelet multi-resolution technique, the scale of each wavelet component is quantified by two-point autocorrelation function with the central scale of 43, 20 and 9.6 mm, respectively. It is found that large-scale structure determines the formation of separation bubble and makes the most significance to the vorticity. Some intermediate-scale and small-scale streamwise vortices cause the upwelling of vortical structures, and they tend to be more active at the downstream. By visualization of pressure distribution, we can find that pressure distribution is mainly characterized by large-scale structure, and the distribution of small-scale structure can be interfered as a reason why large-scale vortical structure breaks into small vortices at the near outlet region. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Most measurements of flow field caused by a rotating maple seed were performed in a vertical wind tunnel limited to fixing the vertical axis to simulate the stable falling spin period. In the nature, however, the free-falling motion of maple seeds induces airflow, which makes the seeds spin and generates lift to reduce falling speed. In this study, the flow structures around free-falling Japanese maple seeds in spin are measured using particle image velocimetry. In general, the free-falling spin flight of a seed is divided into three stages: transition period I, transition period II, and the stable period. In transition period II, it can be observed that the sizes of wingtip and root vortices increase with decreasing coning angle of the seed wing and result in an increase in seed rotation speed and decrease in free-falling speed. We also find that the flow structures induced by the free-falling motion of the maple seed in the stable period are different from those that suspend the rotation seed in a vertical wind tunnel. The leading-edge vortex (LEV) and trailing-edge vortex (TEV) are observed above the front of the leading edge and at the top back of the trailing edge, respectively. The position of the LEV core is located above the front of the leading edge, and it gradually approaches the leading edge of the seed along the spanwise direction, toward the wing tip. As the seed enters the stable period, the root vortex shrinks, and the tip vortex grows. [ABSTRACT FROM AUTHOR]

In order to reduce pressure drop and conveying air velocity, the paper studied the effect of the dune model and soft fins in horizontal pneumatic conveying involving a 90° bend. Experimental measurements were performed in terms of the pressure drop, conveying air velocity and additional pressure drop. The distributions of particle velocity near and in the curved 90° bend were measured by high-speed PIV. The test pipeline consisted of a 4.5m-long horizontal straight acrylic tube, a curved 90° acrylic bend (mean radius of curvature 250mm) and a 1.5m-long horizontal straight acrylic tube, having an inside diameter of 80mm. The polyethylene particles with diameter of 2.3mm were used as conveying materials. The superficial air velocity was varied from 10 to 14m/s, and the solid mass flow rate was fixed at 0.45kg/s. Comparing with the conventional pneumatic conveying, the pressure drop, the minimum pressure drop (MPD) velocity and additional pressure drop can be reduced by using soft fins in lower air velocity range. The reduction rates of the MPD velocity by using soft fins and dune model were about 4.1% and 8.3%, respectively. At the upstream of bend and in the bend, the particle velocity of using the dune model is evidently higher than that of the conventional pneumatic conveying and using soft fins. However, the effect of soft fins and dune model on the particle velocity is maintained downstream of the bend. [ABSTRACT FROM AUTHOR]

Abstract: The particle fluctuation velocities of a horizontal self-excited gas–solid two-phase pipe flow with soft fins near MPD (minimum pressure drop) air velocity are first measured by high-speed PIV in the acceleration and fully-developed regimes. Then orthogonal wavelet multi-resolution analysis and power spectrum are used to reveal multi-scale characteristics of particle fluctuation velocity. It is observed that the pronounced peaks of the spectra of axial and vertical fluctuation velocities appear in the range of low frequency near the bottom of pipe. These peaks of spectra become larger and their frequencies decrease by using fins. In the range of low frequencies (3–25Hz), the wavelet components of the fluctuating energy of axial particle velocity make the main contribution accounting for 87% and 93% respectively for non-fin and using fins near the bottom of pipe. In the range of relatively high frequency (50–400Hz), however, the wavelet components of using fins, accounting for about 49%, become smaller than that of non-fin, accounting for about 72%, in the suspension flow regime near the top of pipe. The skewness factor of axial particle fluctuation velocity indicates that the wavelet components follow the Gaussian probability distribution as the central frequency decreases. [Copyright &y& Elsevier]

This paper examines the three-dimensional (3D) wake flow structures induced by a wall-mounted short circular cylinder. The structures are controlled by the flow issuing from a horizontal hole (HH) drilled from the front to the rear of the cylinder. The 3D velocity fields are instantaneously measured at a Reynolds number of 10,720 using high-resolution tomographic particle image velocimetry (Tomo-PIV) in a water tunnel. The 3D vorticity fields, Q criterion, and characteristics of arch-type and tip vortices are compared between the HH cylinders and a standard cylinder based on the measured instantaneous 3D velocity fields. A 3D W-type arch vortex appears behind the short HH cylinders. Compared with the standard cylinder, the 3D W-type arch vortex and large-scale vortices break down more rapidly in the wake of the HH cylinders. The flow from the HH may reduce the rear recirculation region to 15%, effectively controlling the wake structures of short cylinders. A 3D wavelet multiresolution analysis is used to decompose the 3D velocity fields given by the Tomo-PIV data. The large-scale streamwise vortices in the HH cylinder wakes are smaller than those in the standard cylinder wake, and this becomes more evident as the hole height increases. The intermediate-scale vortices are also controlled by the flow issuing from the hole. • The short cylinder wake is controlled by the flow issuing from a horizontal hole in the cylinder. • 3D velocity fields are measured by high-resolution tomographic particle image velocimetry. • 3D wavelet multiresolution analysis are applied to decompose the 3D velocity fields. • A 3D W-type arch vortex appears behind the controlled short cylinders. • The large-scale streamwise vortex and intermediate-scale are controlled. [ABSTRACT FROM AUTHOR]

Abstract: In order to reveal the mechanism of the steady transport at the low conveying velocity by using soft fins, the high-speed particle image velocimetry (PIV) is used to measure and analyze particle fluctuation velocity near air conveying velocity of the minimum pressure drop (MPD) in a horizontal self-excited pneumatic conveying. The study focuses on the effect of the different fin's lengths on the horizontal pneumatic conveying in terms of the time-averaged particle concentration and velocity, power spectrum, auto-correlation coefficients, two-point correlation coefficients, fluctuation intensity of particle velocity, skewness factor and probability density function. It is found that the power spectra peaks of fins, especially the longer fins, are larger than that of non-fin even at lower air velocity, suggesting the accelerating efficiency of fins' vibration. Meanwhile, the profiles of particle fluctuation velocity intensity indicate that the fins' oscillation generates large particle fluctuating energy even at lower air velocity so that the particles are more easily accelerated and suspended. This is one of the important reasons why the fins' oscillation results in the low pressure drop and low MPD air velocity. From the distribution of the skewness factor and the probability density function, it is found that the particle fluctuation velocities of all cases follow the Gaussian distribution in the lower and middle parts of pipe, and departure from the Gaussian distribution in the upper part of the pipe. The particle fluctuation velocity of the most efficient Fin320 more obeys the Gaussian-type fluctuation. [Copyright &y& Elsevier]

Abstract: A horizontal pneumatic conveying by using oscillating soft fins to touching particle streams in the inlet of the gas–particle mixture is experimentally studied, to reduce power consumption and conveying velocity. The effect of different fin’s lengths and fin’s arrangement on a pneumatic conveying is evaluated in terms of the pressure drop, conveying velocity, power consumption coefficient, additional pressure drop, particle concentration, particle velocity and intensity of particle fluctuation velocity. Because the oscillating fins directly touch particle streams, the pressure drop, the MPD (minimum pressure drop) and critical velocities, power consumption coefficient and additional pressure drop can be largely reduced comparing to the conventional pneumatic conveying. It is found that the pressure drop, power consumption coefficient and additional pressure drop are reduced and are almost independent on the fins’ length and fins’ arrangement (horizontal or vertical). The maximum reduction rates of the MPD velocity, critical velocity and power consumption coefficient by using soft fins are about 15%, 13% and 23%, respectively. The length of fins effects on the MPD and critical velocities, but the difference of MPD velocity between the vertical and horizontal fins is very small. In the acceleration regime, particle velocity and particle fluctuation energy of fins are larger than those of the no-fin even though the conveying velocities of fins are lower than those of non-fin. It indicates that the oscillating fins play an important role to accelerate particles in the acceleration region for realizing low conveying velocity and steady pneumatic conveying. [Copyright &y& Elsevier]

Abstract: In order to reveal the reduction mechanism of conveying velocity and pressure drops in a horizontal self-excited pneumatic conveying by using soft fins, the distributions of particle velocity and concentration were measured by the high-speed particle image velocimetry (PIV) and image process at the minimum air velocity. The study focuses on the effect of the different fin’s lengths on the horizontal pneumatic conveying in terms of frequency features of fin’s oscillation, time-averaged particle concentration and velocity, fluctuation intensity of particle velocity and power spectrum of particle fluctuating velocity. It is found that the particle concentration of the fins is lower than that of non-fin near the bottom and is higher than that of non-fin near the top of pipe even at low air velocity. In the acceleration regime, the normalized particle velocity of the long fins is higher than that of non-fin, implying the efficiency of the long fin on reducing pressure drop and conveying velocity. For conveying particles having the relatively large terminal velocity, however, there is little effect of fins on particle velocity. The fluctuating energy of particle velocity of fins are larger than that of non-fin near the bottom of pipe, and the long fins exhibit the largest value in the top part of pipe in the fully developed regime. Based on analyzing the power spectrum of particle fluctuation velocity, it is indicated that the fins generate the large peaks of power spectrum in the low frequency range, which is related to the reduction of pressure drop. [Copyright &y& Elsevier]

Previous studies reported that the hydrodynamic propulsion of the water strider also results from transferring momentum to the underlying fluid through hemispherical dipolar vortices shed by its driving legs. However, there are no accuracy experimental measurements of these vortical structures to prove the mechanics of vortical propulsion. Here, we reveal the vortical structures by reporting the simultaneous measurements of the water strider's motion and the fluid velocity field with the high-speed PIV, and proposing a new method of calculating the vortex kinetic energy and vortex momentum. We found that the asymmetrical vortical structure in each dipolar vortex, generated by one driving stroke, propels the water strider forward, and the outer elliptic vortex is weaker than the inner circular vortex. The movement of the dipolar vortex is divided into two stages: (1) translating backward and (2) return curving. In this way, the water strider obtains the maximum velocity with minimal consumption of energy. The fluid vortical momentum, generated by the driving stroke, accounts for about 64-90% of the water strider's momentum. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: In order to reveal the mechanism of the low pressure drop at low air velocity in a horizontal excited gas–solid two-phase pipe flow of using soft fins, the distributions of particle velocity and concentration are measured by the high-speed particle image velocimetry (PIV) and image process. The test pipeline consists of a horizontal smooth acrylic tube with an inside diameter of 80mm and a length of about 5m. The polyethylene particles of density 978kg/m3 and 952kg/m3 with diameters of 2.3 and 3.3mm are used as test materials. The distributions of the time-averaged particle velocity and concentration are measured at the minimum air velocity and the solid mass flow rates of 0.45kg/s and 0.40kg/s in the acceleration and fully developed regimes. It is found that the particle concentration of using fins exhibits almost same profile as that of non-fin although the air velocities of using fins are lower than that of non-fin. In the acceleration regime, there is little difference of the normalized axial particle velocity up /Ua between short fins and non-fin. up /Ua of the long fins having low air velocity, however, is slightly higher than that of non-fin. In the fully developed regime, even though up /Ua of using long fins is obviously lower than that of other fins and non-fin in the lower part of pipeline, the steady gas–solid two-phase flow can be observed. It suggests that the long fins easily accelerate the particles and result in low pressure drop and low air velocity. For the relatively large particles, the effect of fins on up /Ua is smaller than that for the small particles. Due to the effect of fins’ oscillation, the vertical particle velocity decreases in the low part of pipeline, resulting in the particle dispersion and the reduction of particles–wall collision. [Copyright &y& Elsevier]

Abstract: Multi-scale characteristics of particle fluctuation velocity at the MPD (minimum pressure drop) air velocity in the acceleration and fully-developed regimes are experimentally investigated. The high-speed particle image velocimetry (PIV) is first used to measure the fluctuating particle velocities at the MPD air velocity in a horizontal pneumatic conveying. Then the fluctuating particle velocities in the acceleration and fully-developed regimes are analyzed by the continuous wavelet transform and orthogonal wavelet multi-resolution technique. It is found that the wavelet components of low frequency dominate the fluctuating energy of axial particle velocity in the bottom part of the pipe, and contribute about 70% and 84% to the acceleration and fully-developed regimes, respectively. The more contribution to the fluctuating energy of vertical particle velocity, however, comes from the wavelet components of high frequency, accounting for about 82%. The low frequency component of auto-correlation suggests a large-scale particle flow in both acceleration regime and fully-developed regime. The low frequency wavelet component also makes a large contribution to spatial correlation of axial particle velocity, which increases from the acceleration regime to fully-developed regime. The PDF of low frequency components are asymmetrical near the bottom of the pipe, but the PDF of the relatively high frequency exhibits symmetry and the spread of the PDF evidently shrinks for increasing frequencies, implying a reducing fluctuation of high frequency component. [Copyright &y& Elsevier]

Abstract: The high-speed particle image velocimetry (PIV) is employed to measure the distribution of particle fluctuation velocity near the minimum conveying velocity in the acceleration and fully developed regimes in a horizontal pneumatic conveying. The fluctuation characteristics of particle velocity are examined in terms of time-averaged particle velocity and concentration, power spectrum, auto-correlation coefficients, two-point correlation coefficients, intensity of particle fluctuation velocity, skewness factor and probability density function. It is revealed that the profile of the time-averaged particle velocity depends on the conveying velocity and solids loading ratio in the bottom part of the pipe in the fully developed regime. A quasi-periodical particle flow pattern with the lower frequency appears in this region and dominates the pressure drop of the pneumatic conveying. The intensities of particle fluctuation velocity of the sliding strand flows in the bottom part of pipe are smaller that of the suspension flows in the top part of pipe, revealing that the low intensity of particle fluctuation velocity can result in the low conveying pressure drop. The particle fluctuation velocity of the low or minimum conveying velocity follows the Gaussian probability distribution in the fully developed regime. [Copyright &y& Elsevier]

Abstract: In order to reduce power consumption and conveying velocity, a new pneumatic conveying system where soft fins are mounted in a horizontal plane through the pipe axis in the front of inlet is proposed in this paper. The experimental study focuses on the effect of the different fin''s lengths on the horizontal pneumatic conveying system in terms of frequency characteristics of fin''s oscillation, pressure drop, conveying velocity, particle flow pattern, power consumption and additional pressure drop. The test pipeline consisted of a horizontal smooth acrylic tube with an inside diameter of 80mm and a length of about 5m. The polyethylene spherical particles of density 978kg/m3 and 952kg/m3 with diameters of 2.3mm and 3.3mm are used as conveying materials. The mean air velocity is varied from 10 to 17m/s, and the solid mass flow rate is from 0.20 to 0.45kg/s. Comparing with the conventional pneumatic conveying, the pressure drop, minimum and critical velocities, power consumption and additional pressure drop can be reduced by using soft fins in lower air velocity range. The efficiency of fin is quite evident with increasing the length of fin. The maximum reduction rates of the minimum velocity and power consumption by using soft fins are about 14.8% and 25.5%, respectively. By analyzing the frequency characteristics of fin''s oscillation, Strouhal number of more efficient fins is about S t =0.65 in the range of low air velocity. [Copyright &y& Elsevier]

High-speed particle image velocimetry (PIV) is first used to measure two components of the fluctuating particle velocities for different particle sizes and solid mass flow rates at low air velocity in a horizontal pipe. Then, the continuous wavelet transform and orthogonal wavelet multi-resolution techniques are employed to analyze and decompose the fluctuating particle velocities to provide both quantitative and qualitative information on the particle fluctuation velocity of various frequencies. It is revealed that the fluctuating energy of axial particle velocity is mainly contributed from the wavelet components of low frequency, accounting for about 84%, near the bottom part of the pipe cross-section. However, the contribution to the fluctuating energy of vertical particle velocity accounts for about 82% from the wavelet components of high frequency. The auto-correlation analysis suggests a quasi-periodical large-scale axial particle fluctuating velocity. On the other hand, the spatial correlation analysis indicates that the low-frequency components of the axial particle velocity exhibit a large correlation near the bottom part of the pipe cross-section. From the probability density function (PDF) distribution, it is found that the low-frequency components of the axial particle velocity exhibit larger fluctuation, and this fluctuation reduces as the frequencies increase near the bottom part of the pipe cross-section. Near the top part of the pipe cross-section, however, a larger fluctuating axial particle velocity appears in the high-frequency range. [ABSTRACT FROM AUTHOR]

Abstract: The multi-scale structures of turbulent wakes generated by three kinds of bluff body, i.e. circular cylinder, square cylinder and compound of cylinder and square (CS) cylinders, have been experimentally investigated in this paper. Firstly, the instantaneous velocity fields and vorticity were measured by the high-speed PIV technique in a circulating water channel. The instantaneous streamlines and corresponding normalized vorticity contours are obtained at a Reynolds number of 5600. Then one- and two-dimensional wavelet multi-resolution technique was used to analyze the instantaneous velocities and vorticity measured by the high-speed PIV. The turbulence structures were separated into a number of subsets based on their central frequencies, which are linked with the turbulence scales. The instantaneous vorticity and Reynolds shear stresses of various scales were examined and compared between the three generators. It is found that the large-scale turbulent structure makes the largest contribution to the vorticity and Reynolds shear stresses for the three wake generators and exhibits a strong dependence upon the initial conditions or the wake generators. The large-scale vorticity and the sizes of vortex in the circular and square cylinders are larger than those in the CS cylinder wake. The contributions to the Reynolds shear stresses from the large-scale turbulent structures account for 90–96% to the measured maximum Reynolds shear stresses for the three wakes. However, the small-scale structures make less contribution to the vorticity and Reynolds shear stresses. [Copyright &y& Elsevier]

Abstract: The purpose of this study focuses on analyzing the particle velocity and concentration characteristics in a horizontal pneumatic conveying with dune model, so as to reveal the mechanism of the low conveying velocity and saving-energy conveying. The test pipeline consisted of a horizontal smooth acrylic tube with an inside diameter of 80mm and a length of about 5m. The polyethylene particles of density 978kg/m3 and 952kg/m3 with diameters of 2.3 and 3.3mm are used as conveying materials. High-speed PIV was first applied to measure the time-averaged particle velocity and was proven to be an efficient measurement technique in the pneumatic conveying. Then the particle velocity and concentration distributions of three locations were measured at mean air velocities of 12m/s and 13m/s and the solid mass flow rates of 0.45kg/s and 0.43kg/s. A comparison of the particle velocity and concentration profiles between dune model and non-dune model was performed. It is found that the particle concentration of using dune model becomes higher in the upper part of pipeline and becomes lower near the bottom of pipeline in the acceleration region. The particle velocities of using dune model are clearly higher than that of the conventional pneumatic conveying along pipeline and display a uniform profile at the downstream. It is also clear that the particles can be effectively accelerated by increasing air velocity and impacting the surface of dune model. The effect of dune model on the velocity profile of relatively small particles is larger than that of the larger particles and maintains to the downstream. [Copyright &y& Elsevier]

Abstract: In order to reduce power consumption and conveying velocity, a pneumatic conveying system where a dune model is mounted in a pipeline is proposed in this paper. The experimental study focuses on the effect of the mounted dune model in the horizontal pneumatic conveying system in terms of pressure drop, power consumption and conveying velocity. The test pipeline consisted of a horizontal smooth acrylic tube with an inside diameter of 80mm and a length of about 5m. Polyethylene spherical particles with a density of 952kg/m3 and diameters of 2.3 and 3.3mm are used as conveying materials. The mean air velocity is varied from 9 to 16m/s, and the solid mass flow rate is from 0.25 to 0.45kg/s. Firstly, the effect of the dune model location on pneumatic conveying is experimentally studied. It is found that in the lower air velocity range, the pressure drop of the pneumatic conveying with a mounted dune model is lower than that of a conventional pneumatic conveying system. A lower conveying velocity and energy-saving conveying can be realized by installing a dune model in the conveying pipe. Especially the case of fixing the dune model on the bottom of the pipe at the inlet of particle feed is more effective. The particle flow patterns also show that the dune model reduces the deposition of particles. Then, the effect of different surface materials of the dune model is examined. By using a surface material of the dune model with a large coefficient of restitution, the pressure drop of conveying large particles is the lowest. When conveying relatively small particles, however, the pressure drop becomes the lowest by a small coefficient of restitution. The maximum reduction rates of the minimum velocity and power consumption by the dune model are about 19% and 34%, respectively. [Copyright &y& Elsevier]

Abstract: The effect of initial conditions on a self-preserving plane wake has been investigated for two wake generators, i.e. a circular cylinder and a screen of 50% solidity. Measurements were made at (x is the streamwise distance downstream of the cylinder and θ is the momentum thickness of the wake)=580 for the circular cylinder and 830 for the screen using 8 X-wires in the plane of mean shear. A vector wavelet multi-resolution technique is applied for analysis of the hot-wire data. This technique decomposes turbulent structures into a number of components based on their central frequencies, which are linked with the turbulence scales. Sectional streamlines, vorticity contours and probability density functions of the fluctuating velocities at the same central frequency, i.e. the comparable scales of turbulent structures, are examined and compared between the two generators. Discernible differences are observed in the turbulent structures of large-down to intermediate-scales. The differences are further quantified in terms of contributions from the turbulent structures of different scales to the Reynolds stresses and vorticity variance. The vorticity behavior of the screen wake is different from that of the circular cylinder. The difference exhibits an extraordinary similarity to that in the near wake. [Copyright &y& Elsevier]

The flow feature of the jet issuing from the circular pipe with the rotating inclined section has been investigated by the methods of flow visualization and image processing. It has been found that the jet diffusion is affected by the inclined angle and the rotating speed. The coherent structure of the jet has also been studied by using wavelet multiresolution analysis. The multiscale turbulent structures were visualized and the core and edge of the vortex were identified at different broad scales. [ABSTRACT FROM AUTHOR]

Wavelet multiresolution analysis has been applied to digital images of ethanol sprays injected into air at atmospheric pressure and at room temperature. The imaging was performed with laser-sheet imaging using the 308 nm light from a XeCl excimer laser. Image capturing was performed with a CCD camera. The importance of different scales in the turbulent structure of the sprays was easily recognized using wavelet multiresolution analysis. It was observed that the intermediate and small scale levels contained information of such important phenomena as the spray break-up. This was not easily recognized in the original images, due to averaging over all scales. [ABSTRACT FROM AUTHOR]

A high-resolution Tomographic particle image velocimetry (Tomo-PIV), which consists of four high-resolution double-exposure CCD cameras, was used to measured three-dimensional (3D) flow structures behind a wall-mounted short cylinder of an aspect ratio 1 at Reynolds number 10,720 in a water tunnel. Based on the measured instantaneous 3D velocity distribution, 3D velocity fields, the vorticity, the Q criterion, the rear separation region and the characteristic of arch type vortex and tip vortices are first discussed. In order to evaluate 3D multi-scale wake flow structures and energetic flow structures, the 3D orthogonal wavelet multi-resolution technique and proper orthogonal decomposition (POD) are applied to analyze instantaneous 3D velocity fields of Tomo-PIV. A 3D W-type arch vortex behind the short cylinder is found, which is originated by the interaction between upwash and downwash flows. The head shape of arch vortex structure is associated with the aspect ratio of the cylinder. By using the 3D orthogonal wavelet multi-resolution analysis, the instantaneous arch vortex and streamwise vortices are extracted in the large-scale structures of the wavelet level 1. It also found that the intermediate-scale upwash flows play an important role in originating the head of W-type arch vortex. Strong small-scale structures appeared in the shear layer and were contained in the intermediate-scale structures. The POD analysis indicates that the first two modes account for 13.3% of the total turbulent fluctuating energy, and the first four POD modes exhibit several large-scale streamwise vortices. • 3D short cylinder wakes are investigated by Tomographic PIV. • 3D velocity fields are analyzed by 3D orthogonal wavelet transform. • A 3D W-type arch vortex behind the cylinder is first found. • The large-scale streamwise vortex and W-type arch vortex are extracted. • The intermediate-scale upwash vortices play an essential role in W-type arch structure. [ABSTRACT FROM AUTHOR]

To control the wake flow of a short cylinder, a passive flow control method, a front inclined hole in the cylinder was drilled from the front surface side to the free end surface. A cylinder of an aspect ratio H/D = 1 with a height H = 70 mm and a diameter D = 70 mm is used. A circulation water tunnel is used to perform PIV measurements at Reynolds number of 8,570 for comparing the flow structures between the no hole and hole wakes. In order to consider the influence of the hole position, the different cylinder models of the hole height at the front from the wall are tested. Comparing with the flow structures around standard cylinder, it is found that the separation zone near the free end surface decreases and the rear separation ron increases because of the blown flow from the inclined hole. The vorticity and Reynolds shear stresses are also suppressed in the rear recirculation zone. The instantaneous large-scale eddies in the rear separation zone are divided into several small-scale eddies as increasing the angle of the inclined hole. • A passive flow control method using a front inclined hole is proposed. • The rear separation ron was reduced. • The vorticity strength is decreased by the blowing flow. • Reynolds shear stresses and turbulent kinetic energy were reduced. • The large-scale vortex was broken down into several small-scale vortices. [ABSTRACT FROM AUTHOR]

This study is focused on reducing the pressure drop, conveying velocity, and power consumption in a horizontal gas–solid flow. Four types of lattice models, which are fixed in the particle inlet, are proposed to easily generate the oscillation of the wake flow to easily convey the gas–solid two-phase flow. A horizontal pipeline with an inner diameter of 80 mm and length of approximately 5 m was used. Polyethylene particles with an average diameter of 2.3 mm and density of 978 kg/m3 were employed as conveying solid materials. The experimental ranges of the average gas velocity and solid mass flow rate were 10–16 m/s and 0.10–0.47 kg/s, respectively. Compared to the nonlattice gas–solid flow, reductions in the total pressure drop, conveying gas velocity, power consumption, and additional pressure loss were achieved in the range of lower gas velocity when using the lattice model. The highest reduction rates of the minimum conveying velocity and additional pressure drop were approximately 5.12% and 15.2%, respectively. In the acceleration region, the particle concentration in the flows with the lattice model was larger than that in the nonlattice flow near the upper part of the pipe and lower than those in the nonlattice flows in the lower part of the pipe. The time-mean axial particle velocity and particle fluctuating velocity in all lattice model flows were higher than those in the nonlattice flow, according to particle image velocimetry measurements, particularly in the upper part of the pipe. [Display omitted] • The lattice models were applied to gas-solid two-phase flow. • Pressure drop, minimum velocity, and particle velocity were measured. • The pressure drop and minimum velocity are reduced by using lattice model. • High particle velocity and fluctuating velocity were observed in the upper part of pipe. [ABSTRACT FROM AUTHOR]

To induce large oscillations of air flow and suspend particles easily near the particle supply point in a low air velocity region of horizontal air-solid two-phase flow, a short cylinder model with a streamlined front side (refer to streamlined model) is proposed and is fixed on the bottom or top of the pipe in front of the particle feed. The streamlined front side of the model is designed to reduce the flow resistance, and the rectangular rear section of the model generates wake vortical flow so as to induce large airflow oscillation. The test tube consists of a horizontal pipe having an inner diameter of 80 mm and a length of 5 m. Spherical polyethylene particles with an average diameter of 2.3 mm and density of 978 kg/m3 were used as the test solid particles. Average air velocities and particle mass flow rates ranged from 9 to 16 m/s and 0.11 to 0.51 kg/s, respectively. Compared with the non-model air-solid two-phase flow, the streamlined model has reduced the pressure drop, critical and minimum conveying air velocities, power consumption, and additional pressure drop. This model achieved the highest reduction rate with a minimum air velocity of approximately 6% and an additional pressure loss of approximately 37.5%. According to particle image velocimetry measurement of air-solid two-phase flow, the time-averaged particle velocity and fluctuating particle velocity generated by the streamlined model are larger than those in the non-model flow, so even low air velocities can easily convey particles. [Display omitted] • A short cylinder model with a streamlined front side to induce vortical flow is proposed. • Pressure drop and minimum air velocity of air-solid two-phase flow are reduced. • The highest reduction rate of an additional pressure loss is approximately 37.5%. • The time-averaged and fluctuating particle velocities increase. [ABSTRACT FROM AUTHOR]

In the pneumatic conveying process, the particles move to the bend under the influence of inertia to form a particle rope, which will cause a serious collision between the pipe wall and the particles. In order to reduce energy consumption, three sizes of dune models were designed according to the bend structure and blockage ratio. The system pressure drops, particle flow pattern, particle velocity distribution, and pulsation velocity intensity are studied in this research, and the wavelet multi-resolution analysis is used to perform the particle velocity. The findings indicate that the dune model effectively reduces the ideal conveying gas velocity and pressure drop, and the maximum reduction rate of ideal conveying gas velocity is 10.26%. The dispersion of particles is fully in the vertical pipe in the condition of dune model and the axial velocity is larger than the condition of no model. Moreover, the large-scale low-frequency motion dominates the movement of particles in the pipe, while the appearance of small-scale high-frequency motion breaks the pattern of low-frequency motion. [Display omitted] • Dune model is installed in a 90° bend to reduce energy consumption and wear. • Increasing tail size of dune model facilitates particle dispersion in vertical tube. • The maximum reduction rates of optimal conveying gas velocity is 10.26%. • Large-scale low-frequency motion dominates the movement of particles in tube. [ABSTRACT FROM AUTHOR]

Highlights • The three-dimensional dune wake are investigated by PIV. • The large-scale structure is responsible for the formation of separation region. • The intermediate-scale structure is more active near the separation region. • The intermediate-scale structure makes more contribution to Reynolds stresses. • The small-scale vortical structures decay quickly at the downstream. Abstract The three-dimensional wake structures behind a barchan dune model are investigated by the high-speed particle image velocimetry (PIV) at the Reynolds number of 5530. The measured velocity fields are first evaluated by time-averaged flow patterns and power spectra analysis. Then the one-dimensional orthogonal wavelet analysis is applied to reveal multi-scale turbulent structures and their corresponding second order statistics in the dune wake. The instantaneous multi-scale structures suggest that the large-scale structure is responsible for the formation of separation region. The intermediate-scale structure tends to be more active at the boundary of separation region, which may be due to the secondary vortex movement caused by the interactions between the wake and main flow. The appearance of small-scale vortices at the downstream of separation region indicates the breakdown process of large-scale vortices. The Reynolds stress and kinetic energy distribution suggest that the large-scale structures dominate the generation of turbulence in the dune wake. The lower kinetic energy contained in intermediate- and small- scale structures is considered to be related to smaller-scale interactions that reflect energy cascading phenomenon. The Reynolds shear stresses distributions suggest that the intermediate-scale structures also make significance in the separated boundary layer. As for small-scale structure, it is worthy to note that a region of high Reynolds shear stresses with an opposite sign is observed around the dune crest. The cross-correlation function of multi-scale structures suggest the quasi-periodicity of large-scale structure. The intermediate-scale structures also show a certain periodicity, however, such structures tend to be more unstable and the strength of them reduces as they travel downstream. When decreasing to small-scale, the vortical structures decays quickly at the downstream with larger convective velocity. This may indicate the breakdown process of large-scale vortices and the quick dissipation of small-scale vortices. [ABSTRACT FROM AUTHOR]

In this study, multi-scale vortical structures and vortex dynamics around a fixed-bed barchan dune have been experimentally investigated based on the particle image velocimetry technique, wavelet transform, and the finite-time Lyapunov exponent method. It was found that the dynamic characteristics of a dune wake are predominated by large- and intermediate-scale coherent structures. Quadrant analysis of the Reynolds-stress distribution for the corresponding wavelet components revealed that ejection and sweep events are the main contributors to the whole field, while outward and inward interaction events just dominate the region near the dune crest. In addition, the process of ejection and sweeping motions associated with the turbulent bursting sequence can also be captured by applying proper orthogonal decomposition analysis of the decomposed velocity field for the different wavelet components. Finally, a continuous development process of the different wavelet scale structures in the shear layer was visualized in the Lagrangian framework. The small-scale waves grow exponentially and gradually develop into larger-scale vortices when convected downstream until the reattachment point, and larger-scale vortices break into the smaller ones. [ABSTRACT FROM AUTHOR]

In this study, multiscale flow features in a wall-mounted cube wake are investigated experimentally based on two-dimensional time-resolved particle image velocimetry measurements and wavelet transform. Moreover, the control mechanism of the horizontal control hole (HCH) on the cube wake is studied. The width of the cube model is D = 50 mm, and the corresponding Reynolds number is R e D = 7800. The flow control cases include nine kinds of HCHs with three different diameters and three different heights. The results show that the shear layer contains a continuous merging process of multiscale vortices, which leads to the momentum deficit. Particularly, the evolution of large-scale vortices causes exponential growth of momentum deficit. In the xy-plane, the large spanwise vortices cause fluctuations in the near wake (x/D ≲ 3), which are stronger than those in the shear layer but are not present in the xz-plane. The downstream wake is anisotropic due to its strong downwash flows and weak inward flow. The HCH issuing flow weakens the intermediate- and large-scale vortices in the shear layer and hinders the interacting shear flows in the wake, thus reducing the momentum deficit in the near wake. In the downstream wake, the effect of HCH is also anisotropic: in xz-plane, the momentum recovery is slow due to the decrease in the downwash flow by HCH; in the xy-plane, the momentum recovery is fast due to the increase in the range of the inward flow by HCH. [ABSTRACT FROM AUTHOR]

High-speed PIV is first used to measure the fluctuating particle velocities of a horizontal self-excited pneumatic conveying at low air velocity of minimum pressure drop in the acceleration and fully-developed regimes. Then, an orthogonal wavelet multi-resolution analysis and power spectrum are employed to reveal multi-scale characteristics of particle fluctuation velocity. It is found that the pronounced peaks of the spectra of axial and vertical fluctuation velocities appear in the low frequency range near the bottom of the pipe, and the peaks of the spectra become larger by using fins. In the range of low frequencies (3–25 Hz), the wavelet components of the fluctuating energy of axial particle velocity are the main contributions, accounting for 35% and 91% for non-fin and using fins, respectively, near the bottom of the pipe. However, in the range of relatively high frequencies (50–400 Hz), the wavelet components of using fins, accounting for approximately 57%, become smaller than that of non-fin, accounting for approximately 66%, in the fully-developed regime near the top of the pipe. The auto-correlation with fins displays large quasi-periodical waves in the low frequency regime as τ increases. [ABSTRACT FROM AUTHOR]

A preface for the 2006 issue of the "International Journal of Wavelets, Multiresolution and Information Processing" is presented.

The purpose of this study is to apply the oscillation flow induced by a combination of a circular cylinder and non-uniform soft fins to gas–solid two-phase flows to reduce the transport gas velocity and power consumption in a horizontal pipe. In order to evaluate the oscillation flow, several different sizes of square and circular cylinders are used to combine four pieces of soft fins with non-uniform lengths, which are mounted on the horizontal central plane in front of the particle supply. The test pipe comprises a horizontal acrylic pipe with a length of 5 m and an inside diameter of 80 mm. Spherical polyethylene particles with an average diameter of 2.3 mm and density of 978 kg/m3 were used as the test particles. The average gas velocity was 9–16 m/s, and the solid mass flow rate was 0.11–0.51 kg/s. It is found that the combination of a circular cylinder or square cylinder of L = 10 mm with non-uniform soft fins causes the lowest pressure drop and highest velocity fluctuation in the oscillation flow based on single-phase flow (gas only) measurement. Compared to conventional gas–solid two-phase flows, the reduction in the minimum transport velocity, pressure drop, power consumption, and additional pressure drop were obtained using a combination of a circular cylinder with non-uniform soft fins. This combination provides the highest reduction rates in the minimum conveying velocity and an additional pressure drop by approximately 10.0% and 34.1%, respectively. Based on particle image velocimetry measurements, the time-mean particle velocity and particle fluctuating velocity of a circular cylinder with non-uniform soft fins were higher than those of conventional flows near the bottom part of the pipe, thus easily accelerating and suspending particles near the pipe bottom, even at lower gas velocities. [Display omitted] • A combination of a circular cylinder and soft fins was used to induce the oscillation flow. • The oscillation flow was applied to gas-solid two-phase flow for energy-saving conveying. • The pressure drop, minimum velocity, and power consumption are reduced. • The particle velocity and fluctuating velocity were high near the bottom part of the pipe. [ABSTRACT FROM AUTHOR]