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The influence of barchan dune interaction upon unsteady flow separation and wake dynamics around the fixed-bed downstream barchan dune (DBD) model are experimentally investigated at a Reynolds number of 2640 based on the tomographic particle image velocimetry (PIV) system. The time-averaged statistics including the mean velocities, recirculation area, vortex spatial topology, Reynolds stress, and turbulent kinetic energy were used to characterize the flow field and large-scale anisotropy. It was found that arch-shaped vortex "chains" with strong spanwise coherence shedding from isolated barchan crestline populate the whole wake region, while elongated rod-shaped vortex structures with strong streamwise coherence induced by the up-downwash flow around the DBD were found to fill the whole measurement range, which is closely related to "sheltering" effect on the incoming flow acting at DBD due to the presence of upstream barchan dune (UBD). Additionally, in order to study the complex dynamic features of these predominated vortex structure transformations, time-resolved planar particle image velocimetry was applied. This technique allows for providing complementary insights into the temporal behavior of the unsteady coherent flow structures populating the wake field in different experimental configurations. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions including vortex pairing, merging, and breaking up can all be analyzed by dividing into certain scales with ease in a combination wavelet and Lagrangian framework. [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]

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]

This paper proposes vehicle door mirrors with a tip shape and ditch to reduce the aerodynamic drag. The mean drag coefficients Cd of various mirror models were first measured using load cells within a 103–105 Reynolds number range in a wind tunnel. The Cd of controlled mirrors with different ditch widths remained constant at approximately 0.75 and was lower than that of conventional mirrors. Using a large eddy simulation (LES), the 3D flow structures around modified and conventional mirrors were numerically analyzed at a Reynolds number of 2.8 × 105. Based on a three-dimensional orthogonal wavelet multi-resolution technique, the instantaneous three-dimensional vorticity and velocity were decomposed into three wavelet levels or scales: large scale with a 62 mm central scale, intermediate scale with a 29 mm central scale, and small scale with a 16 mm central scale. This indicated that the length of the region of the vorticity iso-surfaces in an improved door mirror model as the tip ditch decreases. Because the tip ditch produced the more streamwise vortices, the large-scale spanwise vortices were suppressed. The streamwise vortices and spanwise vortices of the small- and intermediate-scale structures increased, which is mainly caused by the ditch in the tip. [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]