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• Bridge piers group induced flow by three square-blocks has been investigated in this study. • A comprehensive turbulent structures analysis has been provided for the bridge-piers group induced vortices, in particular in the gap between each pier. • A full map of velocity in 3D (including in streamwise, lateral and vertical directions) and Reynolds shear stress have been provided and fully analysed. • This bridge-piers group study will fill the research gap and lead to detailed understanding on this complicated study. This paper aims to investigate the mean velocities and turbulence characteristics, which include Reynolds shear stresses (RSS) and turbulent intensities, in order to study temporal and spatial flow patterns around a complex three square-piers group. Measurements have been performed by acoustic Doppler velocimeter (ADV) to identify turbulence flow structures alteration and vortices formation in longitudinal, transverse, and vertical directions. In total, 164 locations around the bridge piers have been measured, while each location consists of no lesser than 15 vertical depth ADV data point measurements. To investigate the flow turbulence, critical locations have been observed from high vortex-influenced regions. The impacted mean velocity profiles have been compared to that at the unaffected region to study the flow alteration caused by piers group. The result reveals that the log-law layer near wall has been enhanced by the installation of pier. Furthermore, as identified from the most critical locations of turbulent structures across different measurement rows, the accelerated flow will result in lower observed turbulent structures. Although flow after each pier has been influenced by strong reversal velocities, vortices have managed to form at near wake. Analysis has been divided into inner and outer flow regions (IFR and OFR) based on flow depth to enhance the understanding of how bridge piers contribute to the development of flow turbulence. [ABSTRACT FROM AUTHOR]

Highlight • Sudden contraction into the inlets of two parallel jets was investigated with PIV. • Velocity vectors evolve with a position in the contraction. • The size and shape and position of vortices change with Reynolds number. • Turbulence started at the fast and slow flow partitions and intensifies with Reynolds number. Abstract The inlets to parallel rectangular jets are unique and a critical component of its design. A sudden contraction (contraction ratio = 4.2) of a vertically upward entry flow into two inlets has been investigated experimentally. Changes in velocity profile close to the inlets were measured, while the corner vortices, fluid separation and the development of axial and lateral turbulent intensities were observed from the result of particle image velocimetry analysis. The velocity profiles in the fluid adjacent to the inlets are independent of the Reynolds number, generally the same for similar geometries, concave but changes rapidly before finally adapting to the positions of the inlets. The radii of the corner vortices decreased and became elliptical with an increase in Re. Also, the detachment length shrunk by 60% when the Re was doubled. Lateral and axial turbulence increased and more fluid flowed parallel to the wall as Re increased. The trend and magnitude of the lateral to axial velocity agree with those of similar works. [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]

Highlights • Experimental characterization of a rectangular orifice synthetic jet has been done. • The flow physics and vortex behavior have been studied through LIF and Hot Film measurements. • The emphasis of this study is to explore conditions that promote vortex bifurcation. • We find that the vortex bifurcation is observed during axial switching of vortex rings. • Two primary reasons behind vortex bifurcation have been suggested. Abstract Synthetic jet actuators possess a continuous jet like behavior in its far region and have found wide-scale engineering applications since it allows momentum transport to the flow system without any net mass transfer across the flow boundaries. The case of a non-axisymmetric synthetic jet is particularly significant since it is affected by the differential shear layer at the orifice exit, that depends on its aspect ratio. However, despite exhaustive research on both continuous and synthetic jets, very few studies have experimentally investigated the case of rectangular orifice synthetic jets, focusing on the effect of aspect ratio of the orifice as well as the actuation frequency upon the vortex behavior and the flow physics. In particular, the intriguing phenomenon of vortex bifurcation has mostly been reported only for an individual vortex or for a plain jet. Yet, in a train of vortex rings, such as that obtained in a synthetic jet, the occurrence of vortex bifurcation can be expected, although the flow physics in the wake of individual vortex rings is significantly different. The present study experimentally investigates a rectangular orifice synthetic jet at different orifice aspect ratios and actuation frequencies, focusing on exploring the conditions at which vortex bifurcation occurs, through LIF imaging and Hot-film measurements. The primary objective of these experiments is to provide a qualitative physical insight into the synthetic jet ejected from a rectangular orifice (through LIF imaging), as well as to quantitatively explore the experimental conditions that promote different flow structures (through velocity time trace, time-averaged velocity profiles and power spectral density measurements), particularly the bifurcation of vortex rings. Our experiments indicate that the phenomenon of vortex bifurcation is observed during the axial switching of vortex rings, but only in a narrow range of experimental conditions. Further, the velocity measurements have ascertained that the two prominent reasons behind this bifurcation process are a large disparity in the velocities of the vortex core and the center of vortex ring, as well as the time lag in which the separation distance between the counter-rotating vortices decrease gradually to zero. [ABSTRACT FROM AUTHOR]

This paper presents the experimental study on the flow field and mixing enhancement in a novel micromixer with actuation. Its demonstrated that asymmetric vortex can be generated in the mixer and enhance the mixing performance greatly. The details of the vortex are displayed clearly by tracing the fluorescent dye. The strength of the vortex can be controlled by adjusting the applied voltage. The numerical simulation by Ansys Fluent shows that the streamlines under actuation is in accordance with experimental results. The large-scale vortex generated under actuation can be applied for flow control and operation as well as mixing enhancement in microfluidics. [ABSTRACT FROM AUTHOR]

An experimental investigation is conducted to study the effect of middle length and inclination angle of an S-shaped channel on adiabatic two-phase flow patterns. To study this effect, three channel lengths of 0.25, 0.50, and 1.00m, and four bends at the angles of 22.5°, 45°, 67.5° and 90° were used along the test section which is 35.83m long. The channel cross section dimensions were 0.05×0.10m and the test fluids were air and water. The observed flow patterns in 12 cases were studied. Each flow pattern was identified by visual observation and recorded with video camera for further analysis of the collected data points. The momentum transfer phenomenon between the phases for passing the upward middle section along the S-shaped channel was closely investigated. Five flow patterns of vortex plug, vortex slug, vortex wavy-annular, churn and mist packet annular were observed. The vortex type flow patterns were recorded with high speed video camera. The physical mechanisms and momentum transfer related to formation of these flow patterns were explained in detail. Flow pattern maps were obtained and then compared for different middle lengths and inclination angles to perform a parametric study on the S-shaped channel. Moreover, effect of upstream/downstream of the S-shaped channel on the maps and transition lines were discussed. It was revealed that upstream effects reflect the interactions and momentum transfer between the phases. Therefore, flow pattern types mainly depend on upstream behavior. On the other hand, the amount of backward flow resulting in the establishment of countercurrent flow and creation of vortexes along middle section is determined by downstream effects. Increasing the middle length or inclination angle causes the transition to vortex plug flow pattern to occur at higher liquid velocities. It also causes the area with churn flow pattern on the map to expand ending in smaller vortex slug flow pattern area. However, increasing middle length or decreasing inclination angle, ends in smaller area of mist packet annular area. It is also observed that for higher middle lengths, increasing the inclination angle shifts the transition lines more remarkably. A dimensionless analysis of obtained flow pattern maps is also carried out. It is shown that effect of geometry changes along S-shaped pipeline on two-phase flow pattern transitions is determined by either inclination angle or elevation difference between two horizontal lines. The explained flow mechanisms, obtained flow pattern maps and the transition lines can be used to predict the particular flow pattern that will be established for a given mass flux, inclined length and angle along pipelines. [ABSTRACT FROM AUTHOR]

• Vortex intensity decreases with an increase in kinematic viscosity of the liquid. • Transition to two-phase flow observed after vortex formation within drain pipe. • Several flow regimes observed into the drainpipe at the end of draining. • The collision of the bubble is decreasing with increase in liquid viscosity. • The time of draining is increasing with an increase in kinematic viscosity of liquid. The present study investigates the influence of liquid properties and port position on draining characteristics from a square tank. Four different liquids, water, kerosene, glycerol 20%, and glycerol 60% are used. Three different port positions of a drainpipe of 9 mm id are used. Bubbly, slug, churn, elongated slug, and annular flow are observed in the drain port. Slug and bubbles never collide and rarely distorted for viscous dominated flow, whereas they collide and distort in inertia-dominated flow like draining of water, kerosene, glycerol 20% and glycerol 60%. Empirical correlations are proposed, which shows the time of draining depends on kinematic viscosity of the liquid and delays by the eccentric position of the port. Liquid height in tank found to be varied in quadratic nature with time. [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]

• The lift force on single bubbles rising in a linear shear flow is investigated. • The linear shear flow is generated by a specially designed water channel. • For small bubbles, the lift coefficient C L matches data in the literature. • Present data of C L are effectively estimated by the newly derived model. The lift force acting on single bubbles rising in a downward linear shear flow is measured experimentally. The flow is generated by a 90° sharply bent rectangular water channel, with uniformly distributed vanes installed directly after the corner, and the linear velocity profile is validated by particle image velocimetry. In the flow, single contaminated bubbles with a volume equivalent diameter d ranging from 2 mm to 20 mm are tested, corresponding to a Reynolds number Re based on d and relative velocity of the bubble in the quasi-steady state from 440 to 7200. The experimental results of the lift coefficient C L at d < 8 mm (Re < 2100) are consistent with both the data and correlation in the literature. In the case of d > 8 mm, the results of C L exhibit similar tendencies and magnitudes to those of Li et al. (2016a). Based on the ideal modeling of the vortex detachment, the wake-induced contribution of C L is estimated. Good agreement is achieved between the newly derived model and experimental results. [ABSTRACT FROM AUTHOR]