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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 • The incoming flow will enter both porous and non-porous regions. • The foam block causes a higher pressure drop than a solid block of the same size. • The effect of blockage on pressure drop is more dominant than pore density and inlet velocity. • The predicted friction factor is in agreement with the experimental data. Abstract This study experimentally investigates the effects of pore density, inlet velocity and blockage ratio on fluid flow behaviour and pressure drop in channels partially filled with a metal foam block. The fluid velocities in the free stream region, which is a clear (from foam) region on the top of foam block, are measured using Laser Doppler Anemometry (LDA) and hot-wire anemometry. The metal foam data are compared to those of solid blocks with the same size. For low blockage ratios, i.e. thin foam layers, the pressure drop caused by a solid block is higher than that of the foam when tested under identical conditions. Interestingly, nonetheless, beyond a threshold blockage ratio value, the pressure drop induced by the metal foam block exceeds that of the solid block of the same height tested at the same air flow rate. This behaviour is best described as the interplay between resistance caused by blockage versus that of the wake forms downstream and over the objects and additional frictional effects within the porous region and on the interface. Furthermore, a correlation, with ±16% deviation, is developed to predict the flow resistance caused by the solid and foam blocks across the partially filled channel. [ABSTRACT FROM AUTHOR]

• The flow field structure and heat flux distribution around an isolated cylinder and tandem cylinders at different angles of attack (AoA) and inflow conditions are investigated in a Mach 6 low-noise wind tunnel. • Downstream of the recirculation region, the heat flux distribution of the isolated cylinder model forms a parallel stripe structure, while in the tandem cylinder model, it resembles a "Λ-shaped" distribution. • With the decrease in AoA , the "Λ-shaped" heat flux structure in the wake of the downstream tandem cylinders gradually disappears, and the stripe structure near the second cylinder is in a "shuttle-shaped" distribution. • With the increase in the spacing between the two tandem cylinders, the spanwise width of the "shuttle-shaped" distribution increases. • The low-heat flux area behind the second cylinder initially increases with an increase in the spacing, but when the spacing is too large, a high-heat flux stripe begins to appear in the centre of the low-heat flux region. • Placing a third cylinder on the wake centreline of the tandem cylinder has a slight impact on the flow, but its effect is enhanced when the third cylinder is located outside the recirculation region of the tandem cylinder. The flow field structure and heat flux distribution around an isolated cylinder and tandem cylinders on a flat plate at different angles of attack (AoA) and inflow conditions are investigated in a Mach 6 low-noise wind tunnel, using nano-tracer-based planar laser scattering (NPLS) techniques and the temperature sensitive paint (TSP) technique. The results reveal that, first, at AoA = 10°, the downstream cylinder will cause a horseshoe vortex formed by the upstream cylinder, which originally propagates parallel to the downstream, to expand to both sides. Subsequently, when the number of cylinders increases, the spanwise width of the affected area is increased further. In addition, a low-heat flux region caused by the recirculation region appears behind the cylinder; downstream of the low-heat flux region, the heat flux distribution of the isolated cylinder model forms a parallel stripe structure, while in the tandem cylinder model, it resembles a "Λ-shaped" distribution. A second key finding is that, with the decrease in AoA , the region of influence of the cylinder decreases. Moreover, the "Λ-shaped" heat flux structure in the wake of the downstream tandem cylinders gradually disappears, and the stripe structure near the second cylinder is in a "shuttle-shaped" distribution. Notably, with the increase in the spacing between the two tandem cylinders, the spanwise width of the "shuttle-shaped" distribution increases. The low-heat flux area behind the second cylinder initially increases with an increase in the spacing, but when the spacing is too large, a high-heat flux stripe begins to appear in the centre of the low-heat flux region. In addition, placing a third cylinder on the wake centreline of the tandem cylinder has a slight impact on the flow, but its effect is enhanced when the third cylinder is located outside the recirculation region of the tandem cylinder. [ABSTRACT FROM AUTHOR]

The present work reports incompressible, viscous flow around a rigidly suspended stationary cube placed normal to the flow. The Reynolds number range covered in this work is more than two decades from Re  ∈ (500, 55,000). Measurements are conducted using particle image velocimetry and dye visualization is undertaken for better understanding of the flow. The flow is seen to be structureless at higher Reynolds number. Mean and rms velocities at different axial locations in the wake are examined along with the mean vorticity field. Two peaks are observed in the U rms profiles at different axial locations in the wake. V rms is single peaked at lower Re and has weak double peaks at higher Re . Behaviour of centerline velocity of the wake gives an insight about the recirculation length and shows its non-dependence on Re . Wake size has been determined and trajectory of the maximum vorticity in the wake is discussed. Drag coefficients are evaluated and compared with various correlations for sphere and cube. The modified wake survey method gives coefficients of drag between 0.63 and 0.89, which are close to the values for a cube given by Hölzer and Sommerfeld (2008) within the experimented Reynolds number range. Using the recirculation length, the data shows a dependence on Reynolds number till Re  ∼ 1200 beyond which it becomes independent of Re . Overall several similarities in the flow behaviour with respect to sphere is noted from the results. The paper is the first detailed study on a cube over such a wide range of Reynolds number. [ABSTRACT FROM AUTHOR]

An experimental study was undertaken to investigate the changes on the turbulent wake of a confined square cylinder, caused by the introduction of three multi-scale arrays of obstacles. The arrays were introduced upstream, downstream and around the square cylinder, using the same obstacles in all cases. The results show that changes on the confinement caused by the introduced elements produce an increase of the shedding frequencies and on the energy contribution of the leading modes obtained by a Proper Orthogonal Decomposition, POD. The results also show that different modifications of the wake properties are observed if the obstacles are located either upstream or downstream of the square cylinder. When the obstacles are mainly located upstream it is possible to observe an increase in the peak magnitude of Turbulent Kinetic Energy in the wake. These changes are associated to a redistribution of the energy contribution of the POD modes. For larger regions obstructed upstream, the leading modes reduce their energy contribution, while the higher order modes increase it. It is also observed that an increased reattachment length can be obtained by locating most of the array downstream of the square cylinder. All these observations are discussed in terms of the expected interactions of flow structures. From the results it is possible to conclude that the general spatial patterns of the POD modes are not changed importantly by the introduction of the arrangements compared with the unconfined case and previously reported confined cases with lower Reynolds number. The most relevant changes for the confined obstructed case analysed here can be found in the shedding frequency and the modal energy distribution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rotational Cylinder system complex wake feature distinctive patterns. • Particle Image Velocimetry captures flow structure. • Transition to turbulence events identified. Wake interactions from a three-cylinder system arranged in an intermediate-spacing setting in static and rotating conditions are investigated. The complex flows generated are relevant to fundamental fluid flow phenomena and engineering applications and merit investigation. In this study, experiments using hot-wire and two-dimensional (2D) Particle Image Velocimetry (PIV) are conducted in order to characterize the fluid flow at various Reynolds numbers and rotational speeds. The three-cylinder system setup is in equidistant triangular arrangement, each cylinder having diameter of 3.1 mm with a 9.7 mm circumscribed-circle diameter among them. To elucidate the cylinder system flow phenomena occurring during static and rotating conditions, the PIV measurement campaign was conducted in two stages. Firstly, a very close view of the flow was used to capture wake interaction between three cylinders in detail at a representative case of low Reynolds number, Re = 57, at three static orientations. Secondly, the experiment was conducted using a larger field of view to capture far field flow characteristics at increased Reynolds numbers, Re = 310 and 1250, for three static orientations and various rotating conditions. For static cases, the results show some flow phenomena that has been reported in several computational studies before, but new experimental evidence of early mixing and breakdown of the wake through a variety of flow patterns are revealed. With the addition of the new parameter of continuous rotation, the three-cylinder system further evolves into new scenarios, including cases with resembling single-cylinder wake patterns. Time-resolved characteristics of the flow are obtained from point-wise hot-wire anemometry and are presented as a function of cylinder system rotational speed, Re number, and non-dimensional frequency or Strouhal (St) number. [ABSTRACT FROM AUTHOR]

A wake is induced in turbine passages due to the interaction of turbine stators and rotors. Each stage of the turbine blade has different geometric parameters, namely, the turning angle, leading edge radius, and solidity, which all vary. Comparison of flow and thermal characteristics between the first and second-stage blades was carried out. The flow and thermal characteristics of the first-stage blade endwall were investigated for various Strouhal numbers to determine the wake effect. Numerical simulations and experiments were performed to compare the flow and thermal characteristics of the different stage blades. Experiments were performed using a five-bladed linear cascade with moving cylindrical rods simulating the wake effect. The effective area and strength of secondary vortices differed in each stage of blade, which resulted in different local heat transfer distributions. In addition, the local heat transfer characteristics changed depending on the Strouhal number. The ‘without wake’ case ( St = 0) showed non-uniform heat transfer distribution on the endwall with the occurrence of horseshoe, passage and corner vortices. The ‘with wake’ effect cases ( St ≠ 0) showed a more uniform heat transfer distribution on the endwall. The wake effect disturbs the occurrence of secondary vortices. With an increase in the Strouhal number, the endwall is exposed to higher thermal load. [ABSTRACT FROM AUTHOR]

The effect of varying the roof-angle of the tail of a generic high-speed train on the unsteady wake structure and corresponding slipstream is investigated in a 1/10th scale wind-tunnel experiment. Insight into the slipstream and unsteady flow features are gained from 4-hole dynamic pressure probe measurements, surface-flow visualisations, measurements from a two-dimensional array of total pressure probes, and frequency analysis. The results show that increasing tail roof angle leads to a transition from an unsteady wake with a pair of streamwise vortices that exhibit sinusoidal, antisymmetric motion to an unsteady wake dominated by large-scale separation with vortex shedding from the sides of the train. It is the interaction of the streamwise vortices with vortex shedding from the sides that results in the widest unsteady wake, and consequently, the largest slipstream velocities. [ABSTRACT FROM AUTHOR]

Wakes of towed and self-propelled bodies in stratified fluids are significantly different from non-stratified wakes. Long time effects of stratification on the development of the wakes of bluff bodies moving at constant speed are well known. In this experimental study we demonstrate how buoyancy affects the initial growth of vortices developing in the wake of a hydrofoil accelerating from rest. Particle image velocimetry measurements were applied to characterize the wake evolution behind a NACA 0015 hydrofoil accelerating in water and for low Reynolds number and relatively strong stable stratified fluid ( Re = 5000, Fr ∼ O (1)). The analysis of velocity and vorticity fields, following vortex identification and an estimate of the circulation, reveal that the vortices in the stratified fluid case are stretched along the streamwise direction in the near wake. The momentum thickness profiles show lower momentum thickness values for the stratified late wake compared to the non-stratified wake, implying that the drag on an accelerating hydrofoil in a stratified medium is reduced at the acceleration stage. The findings may improve our ability to predict drag due to maneuvering of micro-air/water-vehicles in stratified conditions. [ABSTRACT FROM AUTHOR]

Flow structures and downstream wake behind two identical tandem flat plates and a square cylinder has been studied at high Reynolds number and similarities and dissimilarities in coherent and incoherent structures have been identified. It was found that the square cylinder produces higher coherent turbulence kinetic energy (TKE) with peaks up to seven times higher than the flat plates in tandem. Also, turbulence kinetic energy production contours revealed that while the turbulent peak occurs mainly at the edges of the normal flat plates in tandem, it develops at centreline in the wake of the square cylinder. Interestingly, it was observed that the square cylinder exhibits two alternative shedding frequencies along centreline in the wake region. [ABSTRACT FROM AUTHOR]

Ahmed body is a simplified vehicle model which retains important features of real ground vehicles. Present study aims to investigate effects of rear slant angles (α = 25°, 30° and 35°) on time-averaged and instantaneous flow characteristics downstream of the Ahmed body because angle of the rear slanted surface considerably affects the flow characteristics. The particle image velocimetry (PIV) technique is employed to measure the flow field in the symmetry plane z = 0 downstream of the Ahmed body. Demonstration and detailed discussions of the flow features are provided using time-averaged velocity vectors, streamlines, vorticity and TKE contours. Three critical flow points are revealed in the wake downstream of the Ahmed body. Namely, they are focus point 1 (F1), focus point 2 (F2) and saddle point (S). Variations in slant angle α cause shift in the locations of these critical flow points. Effects of change of slant angle α on each critical flow point vary according to their locations in the wake. Instantaneous vorticity contours and spectral analysis of the velocity field are also provided. As a result of the spectral analysis, a single dominant frequency location is only detected for α = 25°, on the other hand two dominant frequency locations are detected for α = 30° and 35° in the wake. Mean stream-wise and vertical velocity components are compared with available data in the literature. Although there are some discrepancies between the present and previous results at certain locations in the flow field, general agreement between these results is reasonably good when taking the difference between Reynolds numbers of the two studies into account. [ABSTRACT FROM AUTHOR]

Abstract: A new simple optical method for visualizing flow structures generated in a rotating flow is presented. The method consists of planar PTV with sheet illumination slightly offset to the rotation axis and a geometric reconstruction of velocity vector field under the assumption of azimuthal periodicity. Two types of columnar flows intrinsically associated with rotating flows are investigated with the method. The two types of flow are a local columnar wake induced behind a moving sphere, and an asymmetrically precessing Taylor Column created below a spinning disk. By means of this new technique we obtain the first quantitative visualization of the columnar wake and the three-dimensional structure of the Taylor Column. [Copyright &y& Elsevier]

Abstract: Flow characteristics around a sphere located over a smooth flat plate were experimentally investigated using dye visualization and PIV technique. The sphere was embedded in a turbulent boundary layer with a thickness of 63mm which was larger than the sphere diameter of D =42.5mm. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from the point of flow physics for different sphere locations in the range of 0⩽ G/D ⩽1.5 where G was the space between the bottom point of the sphere and the flat plate surface. Reynolds numbers with a range of 2500⩽Re⩽10000 based on the free-stream velocity while the velocity distributions over the plate surface are the developed turbulent boundary layer condition attained by using a tripwire. Distributions of velocity fluctuations, patterns of sectional streamlines, vorticity contours, velocity fields, turbulent kinetic energy and corresponding Reynolds stress correlations are obtained using PIV data. It was found that a jet-like flow stimulated the flow entrainment between the core and wake regions as a function of the sphere locations. The gap ratio has a strong influence on the flow structure of the wake-boundary layer interaction and the variation of the reattachment location of the separated flow from the plate surface. The time-averaged flow patterns yield asymmetric structures downstream of the sphere due to the effect of the boundary layer flow distribution. [Copyright &y& Elsevier]

Abstract: The separated and reattaching turbulent flow over a finite blunt plate with the chord-to-thickness ratio c/d =9.0 was experimentally studied; the Reynolds number based on the plate’s thickness (d) was . Wall-pressure fluctuations on the plate surface were taken into extensive consideration, which are closely related to unsteady flow behaviors buried in the separated and reattaching flow and the unsteady wake. Toward this end, synchronized measurements of wall-pressure fluctuations and velocity fluctuations were performed by using a microphone array and a split-fiber film probe, respectively. Characteristics of the separated and reattaching flow were discussed in terms of time-averaged streamwise velocity and its fluctuation intensity, reverse-flow intermittency, wall-pressure fluctuation coefficient, cross-correlation and coherence of wall pressure and velocity. The results showed that the peaked wall-pressure fluctuation coefficient (x/d =4.25) appears 1d behind the time-mean reattachment point (x/d =3.25), which is due to the intensified impingement fluid induced by the growing large-scale vortical structures in the reattachment zone. Two characteristic frequencies fd/U 0 =0.118 and 0.162 were determined, which correspond to the shedding large-scale vortical structures and the unsteady wake, respectively. Cross-correlation of the wall-pressure field demonstrated rapid decay of the large-scale vortical structures beyond the reattachment zone (x/d >4.75), which might be due to strong interference with the unsteady wake at higher frequency fd/U 0 =0.162. The wall-pressure auto-spectra showed that wall-pressure fluctuations near the trailing edge are overwhelmingly dominated by the unsteady wake at fd/U 0 =0.162. Coherence of the wall-pressure field and the streamwise velocity at x/d ⩾7.75 elucidated that unsteady behaviors of the redeveloping boundary layer at both frequencies fd/U 0 =0.118 and 0.162 play important role in wall-pressure fluctuations in large upstream area of the surface. [Copyright &y& Elsevier]

Abstract: The proper orthogonal decomposition (POD) analysis of the wall-pressure fluctuations below the constrained wake of a two-dimensional square cylinder in proximity to a plane wall was made on two systems, i.e., G/D =0.25 and 0.5, which corresponds to the wakes with and without suppression of the vortex shedding, respectively. Here, G is the gap distance and D is the width of the square cylinder. Synchronized measurements of wall-pressure fluctuations were made using a microphone array. For the system G/D =0.5, the first two energetic modes contribute 34.7% and 23.4% to the total fluctuation energy, respectively; however, the fluctuation energy corresponding to the third mode are relatively small and less than 10%. This sharp variation in eigenvalue is due to the presence and dominance of the Karman-like vortex shedding. However, for the system G/D =0.25, the considerable reduction in the eigenvalues of the first several modes is due to the suppression of the Karman-like vortex shedding. The spatial wavy pattern of the first several energetic eigenmodes was shown to be a good reflection of convective vortices superimposed in the wakes. The spectra of the POD coefficients determined the frequency of the dominant structures. Based on the coherence of the POD coefficients, an effective method of determining the number of POD modes for reconstruction of the low-order wall-pressure field was proposed. Accordingly, the low-order wall-pressure fluctuations in the systems G/D =0.5 and 0.25 were reconstructed by using the first four and five POD modes, respectively. The coherence and cross-correlation analysis of the reconstructed wall-pressure fluctuations, which excluded the influence of the small-scale structures and background ‘noise’, gave an insight view of the footprints of the dominant flow structures, which otherwise could not be effectively captured by using the original wall-pressure fluctuations. [Copyright &y& Elsevier]

Abstract: This paper presents the results of an experimental study of the near-wake of a NACA 0012 airfoil placed upstream of a 90° bend section using hot-wire anemometry. The bend has a cross-section of 0.457m×0.457m with a mean radius-to-height ratio of 1.17, and the airfoil is of chord length 0.15m. Among the features studied are the effects of changes in the orientation of the airfoil with respect to the oncoming flow and the effects of the freestream velocity, namely, 10m/s, 15m/s and 20m/s corresponding to chord Reynolds numbers of 1, 1.5 and 2×105, respectively. Emphasis is placed on the boundary layer on the upper surface of the airfoil and the wake up to one chord length downstream. The parameters of interest are the mean and turbulence quantities and their variations in the streamwise, normal and spanwise directions. Results from the investigations indicated the sensitivity of the wake characteristics to the boundary layers on the airfoil. Measurements of mean velocity and streamwise turbulence intensity taken in the spanwise direction within the wake revealed wave-like variations which are linked to the presence of vortical structures in the wake. [ABSTRACT FROM AUTHOR]

Abstract: Influence of wall proximity on characteristics of the wake behind a two-dimensional square cylinder was experimentally studied in the present work. A low-speed recirculation water channel was established for the experiment; the Reynolds number based on the free-stream velocity and cylinder width (D) was kept at ReD =2250. Four cases with different gap width, e.g., G/D =0.1, 0.2, 0.4 and 0.8, were chosen for comparison. Two experimental techniques, e.g., the standard PIV with high image-density CCD camera and TR-PIV with a high-speed camera were employed in measuring the wake field, enabling a comprehensive view of the time-averaged wake pattern at high spatial resolution and the instantaneous flow field at high temporal resolution, respectively. For the four cases, the difference in spatial characteristics of the wake in the vicinity of the plane wall was analyzed in terms of the time-averaged quantities measured by the standard PIV, e.g., the streamline pattern, the vector field, the streamwise velocity fluctuation intensity and the reverse-flow intermittency. The proper orthogonal decomposition (POD) method was extensively used to decompose the TR-PIV measurements, giving a close-up view of the energetic POD modes buried in the wake. The low-order flow model of the wake at G/D =0.8 and 0.4 was constructed by using the linear combination of the first two POD modes and the time-mean flow field, which reflected well the vortex shedding process in the sense of the phase-dependent patterns. The intermittent appearance of the weakly separated region near the wall was found at G/D =0.4. On going from G/D =0.8 to 0.4, the remarkable variation of the instantaneous wake in the longitudinal direction confirmed that the wall constraint stretches the vortices in the plane of the wall and transfers the energy to the longitudinal component at the expense of the lateral one. [Copyright &y& Elsevier]

Abstract: Experimental investigations were carried out for flow past a sphere in a closed circuit wind tunnel. The sphere was made of polymer material and had a diameter of 102mm. Freestream turbulence was generated by placing an orificed perforated plate at the entrance of the test section. The Reynolds number of the flow, based on the diameter of the sphere (d), was set at 3.3×104, 5×104 and 6.6×104 by varying the mean flow velocity. The sphere was placed at 20D (=7.5d) downstream from the entrance of the test section, where D =37.5mm is the size of the holes in the perforated plate. Velocity profile of the flow was measured using a hot wire probe at 10D (=3.75d), 20D (=7.5d) and 30D (=11.25d) downstream of the sphere (center). Experiments were carried out without and with the perforated plate in place in order to study the influence of freestream turbulence on parameters such as Kolmogorov length, dissipation rate, integral length, and the skewness and flatness factors. The presence of freestream turbulence was found to reduce the vortex shedding process behind the sphere, that is, large organized motions were suppressed by the freestream turbulence. [Copyright &y& Elsevier]

Abstract: The pinch-off of liquid drops from the end of a flat plate has been studied. Gas flows parallel to the plate with mean velocity v, and creates a wake at the plate end. The break-up process is found to be reproducible, with the number of satellites increasing with both increasing velocity and increasing mass density. The diameter of the main drop after detachment, d, at Weber numbers exceeding 5 is found to be given bywith C=(5.6±0.3)·10−5 for ethanol and C=(2.9±0.4)·10−5 for water. The diameter in quiescent gas, dv=0, is easily predicted, and Repl is the Reynolds number based on v and the thickness of the plate. Diameters at smaller Weber numbers are better predicted by a similar correlation in terms of the Weber number. Instantaneous acceleration and forces on a drop have been measured and analyzed. Hydrodynamic interaction between liquid and gas at high gas velocities at moments of strong interaction could be quantified with the usual expression for the drag force and a drag coefficient of about 1.3. [Copyright &y& Elsevier]

Flow structures and turbulence properties of the elevated jets in a crosswind are experimentally studied via the laser-light sheet smoke-pattern flow visualization technique and laser Doppler velocimeter. Smoke patterns, velocity vectors, and streamlines are superimposed to clarify and compare the flow structures at different jet-to-crosswind momentum flux ratios. Four flow structures with apparent differences are identified. They are downwash, crosswind-dominated, transitional, and jet-dominated flows. Large amount of jet fluids in the downwash and crosswind-dominated flows are entrained down to the tube wake because a clockwise-rotating vortical structure is formed in the near wake due to the combined effects of the jet shear and tube-tip downwash. In the transitional and jet-dominated flows, the up-flows induced by large up-shooting jet velocities overwhelm downwash effect so that most of the jet fluids go downstream rather than to the tube wake. The jet fluids of the transitional and jet-dominated flows thus can transport and disperse to far field. The iso-vorticity contours are superimposed to the streamlines patterns to compare the vorticity-concentrated locations with the features of the flow structures. Distributions of average velocity components and turbulence intensities in the near wake are presented and discussed to improve the understanding of the flow characteristics. [Copyright &y& Elsevier]