Search

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]

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 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]

In this study, tomographic particle image velocimetry (PIV) was used to measure three-dimensional (3D) flow structures behind a fixed-bed barchan dune model at a Reynolds number of 2528 in a circulation water tunnel. The topological evolution of the 3D instantaneous vortex structures and their dynamic characteristics in the dune wake were analyzed. 3D instantaneous arch-shaped vortex "chains" shedding from the barchan and typical quasi-streamwise vortex structures induced by the development of an internal boundary layer were found to be located both before and after the reattachment region, which differs a little from the results of previous studies. Nevertheless, both conditionally averaged and typical instantaneous 3D flow fields revealed that the arch-shaped vortex system dominates the barchan dune wake, and the inclination angle of these spanwise-oriented structures was found to change as it propagates downstream. Additionally, the dynamic characteristics of the dune wake were found to be similar to a flapping phenomenon such as oscillation of the recirculation in a backward-facing step flow. To provide complementary insight into this phenomenon, the separated and reattached shear flow were also examined using two-dimensional time-resolved planar PIV. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions can all be analyzed with ease in a Lagrangian framework. [ABSTRACT FROM AUTHOR]

To reveal the multiscale wake structures of a wall-mounted cube and the effects of a front inclined hole (FIH), a combination of wavelet multiresolution analysis and vortex identification techniques is applied to analyze time-resolved particle image velocimetry data. The cube model has a side length D = 50 mm and the Reynolds number ReD = 7800. The inlet of the FIH is located at a height 0.71D on the front surface of the cube, and the outlet is located at the center of the free end. Statistical analysis indicates that an FIH can reduce the recirculation zone around the cube, suppress downwash flow, and alter the development of free-end shear flow. The turbulence intensity and turbulent kinetic energy in the wake also decrease significantly in the presence of an FIH. The wavelet multiresolution analysis shows that the main structures of the flow field experience a transformation from small to intermediate and then large scales from the shear layer around the standard cube to its wake. However, in the presence of an FIH, the scale of the flow structure near the free end becomes smaller, the large-scale structure in the wake is weakened, and the transition from small to intermediate scales is no longer obvious. The wavelet vortex dynamics analysis reveals the evolutions of vortex structures with different scales. In the presence of an FIH, the trend of large- and intermediate-scale coherent structures moving to the bottom wall is weakened, while the streamwise extent of small-scale structures is greater. In the large-scale component of the wake, a transverse secondary vortex is observed, which is formed by the combined action of a shedding transverse vortex and downwash flow. [ABSTRACT FROM AUTHOR]

This paper describes an experimental investigation of flow control for the wake around a wall-mounted cube using horizontal control holes (HCHs) of different diameters drilled from the center of the front surface to the rear surface of the cube. The cube has side lengths of D = 50 mm, and HCHs with diameters of d = 10, 12, and 14 mm are considered. The instantaneous velocity fields are measured at a Reynolds number of 7800 based on time-resolved particle image velocimetry in a water tunnel. The HCHs suppress the recirculation zone, turbulence intensity, and Reynolds stress, and these control effects gradually increase with the hole diameter. The issuing flow from the large-diameter HCH completely obstructs the development of the downwash flow to the bottom wall and decomposes the near-wake arch-type vortex into a double arch-type structure. As the hole diameter increases, the dominant frequency of the spectrum in the HCH wake increases. Proper orthogonal decomposition analysis indicates that the large-scale spanwise vortices are suppressed by HCH. In the dynamic evolution process of the cube wake, the HCH issuing flow hinders the interaction of shear flow on both sides, and the issuing vortices attract the spanwise vortices and accelerate their shedding. [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 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]

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]

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]

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]

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: 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: 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: 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]

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]

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]

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]

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]

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]

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]

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]