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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]

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]

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]

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]

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]

To reveal the physical mechanisms of particle transport in the pneumatic and hydraulic transports, this study experimentally investigated the dynamics of a free-falling finite-size sphere associated with its collision with a wall and the surrounding flow field in a boundary layer flow. The simultaneous motion of finite-size sphere and its inducing flow field near the wall were measured by using a time-resolved particle image velocimetry. The results revealed that the free-falling velocity of sphere increases with Reynolds number before the first impaction, and then is almost unrelated to Reynolds number. The effect of gravity on sphere motion becomes weak with increasing Reynolds number. As the sphere approaches to the wall, the vortex appeared at the front of sphere moves to left side near the wall and reduces the sphere falling speed. After the sphere collided on the wall, the upward motion of sphere induces another vortex pair, which generates strong rotation of sphere. [Display omitted] • Dynamics analysis of a free-falling sphere associated with its collision to a wall is performed. • The flow field induced by sphere are measured by using particle image velocimetry. • The effect of gravity on sphere motion becomes weak as increasing Reynolds number. • The vortex appears at the front of sphere near the wall and reduces the sphere falling speed. [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]

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]

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]

• Front inclined hole is used for passive flow control of a wall-mounted cube wake. • Jet injected from hole suppress separation flow above free-end significantly. • Flow structures and statistic characteristics in near-wake are influenced. • Wake vortical shedding becomes more regular and is accelerated near the free-end. • POD analysis indicates that alternating spanwise vortices are suppressed. To control the wake flow around a wall-mounted cube, passive control is performed by drilling a front inclined hole (FIH) from the front surface to the free end of the cube. To study the control features and mechanism, controlled and uncontrolled cubes are experimentally investigated. A circulation water tunnel is used to perform PIV measurements, where the Reynolds number is Re D = 11550 based on the cube side length of D = 70 mm, and the thickness of the boundary layer on the tunnel bottom is δ/D = 0.25. To study the effect of the hole position, three FIHs with inlet heights of 20, 35, or 50 mm are investigated. The time-averaged flow fields show that the velocity of the FIH jet increases with the FIH inlet height. Comparing with the flow structures around standard cube, it is found that the reverse zone and shear layer above free-end are obviously suppressed due to the FIH jet. In addition, the scale of the reverse zone and arch-type vortex in the near-wake is reduced by the FIH jets. Because the FIH jets disrupt the development of wake vortices, the Reynolds shear stress and turbulent kinetic energy in the near-wake decrease. The control effect is most apparent for the cube with an FIH at a spanwise height of 50 mm (the FIH50 cube). In this case the effect of the free-end downwash flow on the wake is weaker, and the shedding frequency of the wake vortices near the free-end increases. Proper orthogonal decomposition analysis indicates that the FIH50 suppresses the alternately arranged flow features and enhances the symmetrically arranged features. [ABSTRACT FROM AUTHOR]

To control the wake flow around a wall-mounted cube, a passive control method is proposed by drilling a horizontal hole from the front surface to rear surfaces of the cube. The controlled and uncontrolled wakes are measured by using particle image velocimetry (PIV). The cube model has a side length of D = 70 mm, and the diameter of horizontal control hole (HCH) is d = 10 mm. Reynolds number based on side length of cube is R e D = 11,550. Furthermore, to study the positional effect of the HCH, three kinds of holes with different heights h are used. The time-averaged statistical characteristics show that the jet generated from the HCH reduces the scale of the arch-type vortex, turbulence intensity, Reynolds shear stress and turbulent kinetic energy in the wake, and enhances the recovery of streamwise velocity. Among three HCHs, the HCH of h = 35 mm has the optimal control effectiveness. The analysis of vortex dynamics indicates that the HCH jet effectively suppresses the periodic flow structures and accelerates the wake vortex shedding. Meanwhile, the HCH jet exhibits different effect on the spanwise vortex at its different heights. The POD analysis implies that under the influence of the HCH, the alternant spanwise vortices are suppressed, but symmetric vortices are relatively enhanced. • >A passive flow control method using a horizontal hole is proposed. • The scale of the arch-type vortex was reduced. • Reynolds shear stresses and turbulent kinetic energy were reduced. • The symmetric vortices were relatively enhanced. • The periodic structure and the wake vortex shedding were effectively suppressed. [ABSTRACT FROM AUTHOR]

This study focuses on multi-scale particle dynamics of horizontal pneumatic conveying equipped with two types of soft fins at the air velocity of the minimum pressure drop. The particle velocity fields of non-fin and using soft fin's cases are measured by high-speed particle image velocimetry. Then the particle fluctuation velocities are decomposed into various scales based on one-dimensional orthogonal wavelet decomposition. It is found that the non-uniform case of Fin260 yields the highest axial- and vertical- particle velocities and fluctuation energies, giving rise to the lowest pressure drop and conveying air velocity. The overall relative energy distributions of wavelet components suggest that the particle fluctuation energies become more concentrated at wavelet levels 3 and 4 by using fins, especially for the case of non-uniform Fin260. The energy concentration on specific wavelet levels indicates a more organized particle motions under the effect of air turbulence induced by non-uniform soft fins. The correlation analysis suggests that the spatial correlation of wavelet component is associated with the contribution of wavelet component to the measured particle fluctuation energy. The oscillating soft fins modulate the air turbulence and promote the particle motions to be spatially correlated in a range of specific frequency. [Display omitted] • Multi-scale analysis of particle fluctuation velocity is performed • Particle fluctuation energies become concentrated at levels 3 and 4 by using fins • Multi-scale spatial correlation is associated with particle fluctuation energy • The oscillating soft fins promote the particle motions to be spatially correlated [ABSTRACT FROM AUTHOR]

A hybrid two-dimensional orthogonal wavelet multiresolution and proper orthogonal decomposition technique is developed to analyze the wake flow behind a semi cylinder. The two-dimensional orthogonal wavelet analysis is first applied to decompose the measured velocity fields into four wavelet components based on their spatial scales. Then the extracted multi-scale structures are further investigated based on proper orthogonal decomposition. The modal energy distributions of multi-scale flow structures suggest that the dominance of first two POD modes is reduced and the relative energy becomes more distributed as the flow structure changes from large-scale to small-scale. The first two POD modes of intermediate-scale structure also appear in a mode pair and they are related to secondary resonance accompanied with large-scale flow oscillation. The POD mode 3 of intermediate-scale structure suggest the existence of symmetric vortex shedding pattern. The first two POD modes of small-scale structure also display a symmetrically distributed vortex pair and the distance between adjacent vortices is about half of the case of mode 3 of intermediate-scale structure. The PSD distributions of POD modes of multi-scale structures suggest that the bandwidth of PSD becomes less concentrated as the flow structure varies from large-scale to small-scale. Distinctive flow patterns associated with first and second order harmonics of fundamental flow oscillation can be successfully extracted from the dominating POD modes of intermediate-scale and small-scale structures. The proposed hybrid technique provides a tool for relate the most energetic flow events over a period of time with multi-scale structures cascaded in the turbulence flow. • A hybrid 2D orthogonal wavelet multiresolution and proper orthogonal decomposition technique is developed. • The multi-scale structures of semi cylinder wake are decomposed by 2D orthogonal wavelet analysis. • The decomposed multi-scale structures are further analyzed by POD. • Distinctive flow patterns are extracted from the dominating POD modes of intermediate-scale and small-scale structures. [ABSTRACT FROM AUTHOR]

A comparative study on the flow structures behind square- and wavy-square were performed to understand the wake characteristics under spanwise periodic perturbation. The high-speed PIV measurements were carried out at Reynolds number of 4075. The time-averaged flow field suggested that wavy leading edge triggered the alternative secondary vortices along the span. The instantaneous flow field indicated that the regular asymmetric vortex shedding mode was changed to symmetric mode by wavy leading edge. It was found that periodical large-scale flows were suppressed, while the motions of small-scale vortical structures were enhanced by the wavy leading edge based on the continuous wavelet transform. The modal energy distribution of the dynamic mode decomposition suggested that the wavy leading edge weakened the dominance of large-scale vortical structure and promoted the transition from large-scale to small-scale vortical structure. The spatial patterns of representative dynamic modes DM 1 and DM 2 in streamwise plane indicated that the wavy leading edge changed the wake flow by the distortion of Karman vortex and its corresponding secondary vortical structure, which made the vortical structures more unstable and dissipates more quickly. Furthermore, the phase averaging process based on temporal coefficients of DM 1 and DM 2 revealed that the separated shear layer of wavy square cylinder extended to further downstream and the separated vortex was broken into several relatively small vortices during the shedding process. • A comparative study on the flow structures behind square- and wavy square-cylinders were performed. • Continuous wavelet transform and dynamic mode decomposition were used. • The oscillation of large-scale flow was suppressed and the small-scale structures were enhanced by the wavy leading edge. • The distortion and the breakdown process of shedding vortex were observed in the phase averaging process. [ABSTRACT FROM AUTHOR]