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The flow around roughened circular cylinders and the wake behind them were studied in a wind tunnel flow using the Particle Image Velocimetry (PIV) method. The Reynolds number ranged from 5140 to 11800, and the real diameter of the cylinders including surface roughness ranged from 15.06 to 17.34 mm. The flow properties and forces around the roughened cylinders was evaluated by means of Strouhal number and coefficient of drag. The wake was analysed by means of mean velocities along the cylinder centreline, its width and the Proper Orthogonal Decomposition (POD) method. The added surface roughness was found to decrease the wake length and increase the wake width at lower Reynolds numbers. At higher Reynolds numbers, the added surface roughness did not decrease the wake length, but did increase the wake width, although with less effect. The POD analysis showed changes in the higher modes of the flow. The kinetic energy of the first two modes covers up to 50% of the total kinetic energy; the first two modes in the case of the smooth cylinder have the lowest kinetic energy, whereas the first two modes in the case of the roughest cylinder have the highest kinetic energy. The similarity between the POD modes of the smooth and roughened cylinders might be due the fact that the actual Reynolds number range was below the transitional one. • Added surface roughness decreases the wake length and increases the wake width at lower Reynolds numbers. • Added surface roughness decreases the Strouhal numbers at lower Reynolds numbers. • The cylinder drag was increased at higher surface roughnesses. • The kinetic energy of the first two POD modes cover up to 50% of the total kinetic energy. • The absolute kinetic energy is increasing with increasing surface roughness. [ABSTRACT FROM AUTHOR]

Flows around the different head length diameter ratio (L E / D = 0.5, 1, and 2, where L E and D refer to the head length and mid-body diameter, respectively) of the teardrop-shaped slender body of revolution with a constant Reynolds number (Re D = 4.1 × 103) were studied using particle image velocimetry. The aim was to explore how the flow geometry affects the hydrodynamic characteristics. Through the time-averaged statistical characteristics of the wall and wake of slender body determined the dynamics performance. Using the power spectral density (PSD) function, proper orthogonal decomposition (POD), and the phase-averaged method, we explored the wake vortex structure, distribution, and strength. The wake of the teardrop-shaped slender body obeys the classical −2/3 decay law at 6 < x / D < 9 and evolves into a self-similar manner. The PSD and POD analyses imply that the POD Modes 1 and 2 are closely related to the large-scale vortex shedding, and the important frequencies of the POD Modes 1 and 2 coincide with the vortex shedding frequency. At the reconstructed flow field of the POD Modes 1 and 2, the analysis of the phase average indicates that the evolution process of a large-scale vortex should involve a periodic three-dimensional helical vortex structure. • We discuss the wake decay law and self-similar manner of the teardrop-shape slender body. • We introduce the evolution process of large-scale vortex of wake is a periodic three-dimensional helical vortex structure. • We identify a head length diameter ratio of teardrop-shape of slender body with good hydrodynamic properties. [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]

The influence of the velocity ratio (VR) between the jet and main flow on the wake structure and aerodynamic loss of the trailing edge jet is studied using particle image velocimetry and numerical simulations. Three different velocity ratios, namely, VR = 0.5, 1.0, 1.5, are chosen for this comparative study. The Reynolds number (Reh) based on the slot height (h) and the mainstream velocity (U0) are 3380. Results show that the influence of jet on wake structure is significant such that the wake region shrinks and the turbulent kinetic energy is enhanced as the velocity ratio increases. The distribution area of strong vorticity is enlarged with increasing velocity ratio. By using proper orthogonal decomposition and fast Fourier transfer analysis, the variation of velocity ratio demonstrates significant impact on vortex shedding and turbulent kinetic energy. The aerodynamic loss coefficient is nearly constant between VR = 0.5 and 1.0, but increases by 3.25 % as the velocity ratio increases from 1.0 to 1.5. [ABSTRACT FROM AUTHOR]

The complex three-dimensional (3D) flows around a short cylinder on the wall, acted by a flow exiting from a fore angled hole (FAH) that penetrates the cylinder from the fore to the top end, were experimentally investigated. The 3D instantaneous velocity distributions were measured based on tomographic particle image velocimetry (Tomo-PIV). The 3D flow structures behind the FAH and no-hole cylinders are compared in terms of the measured 3D velocity and vorticity distributions, Q-criterion, and tip vortices. A 3D W-type arch vortex, which shows a larger peak or convexity on the horizontal part center than for the no-hole cylinder, is found around the FAH short cylinder. The exiting flow from the FAH increases the separation zone, and FAH height effectively controls the eddy structures. The instantaneous 3D velocity data obtained from Tomo-PIV are decomposed using 3D wavelet multiresolution analysis and proper orthogonal decomposition. With the FAH cylinder, the large-scale streamwise wake vortices are smaller than with the no-hole cylinder, and this exhibits more evident as increasing the FAH height. The time-mean 3D large-scale arch vortical structures, however, become stronger. Proper orthogonal decomposition indicates that the energetic flow structures of the FAH cylinders are stronger than those of the no-hole cylinder. • D flow structures around a short cylinder are controlled by the issuing flow. • D velocity fields measured by Tomo-PIV are analyzed using 3D wavelet transform. • Issuing flow increases the rear separation zone and controls the vortical structures. • Large-scale streamwise wake vortices are reduced by the issuing flow. • Energetic flow structures become stronger with the action of the issuing flow. [ABSTRACT FROM AUTHOR]

The flow characteristics around a cactus-analogue grooved cylinder and a smooth cylinder were extensively compared at Re D ==5.4×10 4 based on the free-stream velocity and cylinder diameter ( D ). Wall-pressure fluctuations on the cylinder surfaces and the fluid flows around the cylinders were measured using a microphone and planar particle image velocimetry, respectively. The influence of the cactus-analogue grooved cavities on the wake characteristics was assessed in terms of the circumferential distributions of the wall-pressure fluctuation intensity, the wall-pressure spectra, and the auto-correlation coefficient of the wall-pressure fluctuations. The global distributions of the time-averaged velocity field, reverse-flow intermittency, streamwise velocity fluctuation intensity, Reynolds shear stress, and spatial velocity correlation were also considered. The circumferential distributions of the wall-pressure fluctuation intensity showed that the wall-pressure fluctuations on the cactus-analogue cylinder were remarkably reduced by around 50% at most stations. On the grooved cylinder, the magnitude of the wall-pressure spectra at the characteristic frequency of the shedding from the large-scale vortical structures was considerably reduced by around 7 dB. This convincingly demonstrated that the unsteady aerodynamic forces on the cactus-analogue cylinder were significantly weakened. In contrast, the auto-correlation analysis of the wall-pressure fluctuations showed that the consecutive motion of the shedding from the large-scale vortical structures close to the grooved cylinder was much more organized. Inspection of the time-averaged velocity field around the cylinders disclosed that the velocity of the near-surface fluid around the grooved cylinder was increased by the small recirculation flows immersed inside the cavities. Near the grooved cylinder, the trace of the peaked streamwise velocity fluctuation intensity was highly localized at θ = 105 ° while in the smooth cylinder a very long, slender peaked region began at θ = 60 ° and extended to the leeside of the cylinder. Finally, a snapshot proper orthogonal decomposition analysis was performed on the velocity fields. The correlation map of the first two POD coefficients convincingly indicated that the vortex shedding process in the grooved cylinder system was more coherent but weaker than that in the smooth cylinder system, which is further confirmed by the phase-dependent contour plots of the swirling strength and its standard deviation. [ABSTRACT FROM AUTHOR]

Abstract: A dual-step cylinder is comprised of two cylinders of different diameters. A large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). The present study investigates the effect of Reynolds number (Re D ) and L/D on dual step cylinder wake development for D/d=2, 0.2≤L/D≤3, and two Reynolds numbers, Re D =1050 and 2100. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from both the large and small diameter cylinders for 1≤L/D≤3 at Re D =2100 and 2≤L/D≤3 at Re D =1050. At these conditions, large cylinder vortices predominantly form vortex loops in the wake and small cylinder vortices form half-loop vortex connections. At lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. At these lower aspect ratios, the presence of the large cylinder induces periodic vortex dislocations. Increasing L/D increases the frequency of occurrence of vortex dislocations and decreases the dominant frequency in the large cylinder wake. The identified changes in wake topology are related to substantial variations in the location of boundary layer separation on the large cylinder, and, consequently, changes in the size of the vortex formation region. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder. [Copyright &y& Elsevier]

To control the wake of flat plate, passive control is performed by drilling a hole from the surface. The differences of the turbulence wake between a flat plate and a perforated plate are investigated using particle image velocimetry (PIV). Through power spectral density (PSD) function, proper orthogonal decomposition (POD) and the phase-averaged method combined with a conditional detection by spatial multi-scale local average function, the vortex structure and distributions of the velocity statistics are explored in the wake. Compared with the results of the wake of flat plate, it can be found that the hole can greatly reduce the turbulence intensity of the wake, and the turbulence tends to be isotropic but not homogeneous behind a distance of 35 D from the perforated plate trailing edge. The perforation changes the initial boundary condition of the very near wake, and the analysis of phase average indicates that the perforation effectively breaks the large-scale vortex structures, suppresses the intensity peak of velocity statistics and weaken the alternant of vortex. The PSD and POD analysis imply that under the influence of the hole, the wake vortex shedding are suppressed, and the energy distribution is more uniform in each mode. The Taylor microscale λ is computed using the spatial correlation function f(r) by Eq. (3). The smaller Kolmogorov length scale is computed as η = (ν 3/ ε)1/4. • The differences of the turbulence wake between a flat plate and a perforated plate are investigated by PIV. • The perforation can greatly reduce the turbulence intensity of the wake, and make the turbulence tends to be isotropic. • The vortical structures and the distribution of velocity statistics in the wake are investigated. • The perforation effectively breaks the large-scale vortex structures, suppresses the intensity peak of velocity statistics. [ABSTRACT FROM AUTHOR]

Abstract: The effect of hydrofoil trailing edge shape on the wake dynamic and flow induced vibration is investigated at high Reynolds number, Re=0.5 × 106–2.9 x 106. Two NACA 0009 hydrofoils with blunt and oblique trailing edges are tested. The velocity field is surveyed with the help of Laser Doppler Velocimetry (LDV), and Particle-Image-Velocimetry, (PIV). Proper-Orthogonal-Decomposition (POD) is used to extract coherent structures from PIV data. Besides, flow induced vibration measurements and high-speed visualization are also performed. A significant reduction of vortex induced vibration is obtained with the oblique trailing edge, in accordance with former reports. High speed videos clearly demonstrate that for both tested hydrofoils, the alternate vortices detach from upper and lower corners of the trailing edge. Due to the oblique truncation, the lower detachment location is shifted upstream with respect to the upper one. Therefore, as the upper vortex rolls up, it coincides with the passage of the lower vortex, leading to their collision. This strong interaction leads to a redistribution of the vorticity, which no more concentrates within the core of Karman vortices. The analysis of the phase locked average of velocity profiles reveals that the oblique truncation leads to a thickening of the core of upper and lower vortices as well as a disorganization of the alternate shedding in the near wake, recovers downstream. We strongly believe that the collision between upper and lower vortices and the resulting vorticity redistribution is the main reason of the vibration reduction obtained with oblique trailing edge. This result paves the way for further optimization of the trailing edge shape. [Copyright &y& Elsevier]

Abstract: We have performed precise and systematic experiments with PIV in order to measure the velocity field in the wake of a solid sphere and of a disk in a water channel, in the range of intermediate Reynolds number in which stationary and oscillatory instabilities appear, including the hairpin shedding regime. From these experimental data, we study the modal decomposition of the streamwise vorticity in an unsteady case and we describe the full nonlinear evolution of the bifurcating branches. We compare these results with recent theoretical and numerical studies on instability in the vortex shedding process at these intermediate Reynolds numbers. [Copyright &y& Elsevier]

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]

The flow past a triangular cylinder is one of the fundamental flows and widely utilized in flame stabilization and heat transfer. In this study, the near wake and vortex characteristics of the flow past an equilateral triangular cylinder are experimentally measured by a high frequency particle image velocimetry (PIV) system at 3 kHz. The triangular cylinder is installed in a wind tunnel with Reynolds numbers ranging from 10,700 to 17,700. The Reynolds-averaged and phase-averaged methods are utilized to analyze the flow field. Based on the flow fields, the length of the vortex formation region is about 1.5 times of the length of the equilateral triangle side. The residence time of a vortex in the vortex formation region is equal to a vortex shedding period. The stream wise velocity of the vortex core center downstream the vortex formation is about 0.8 times of the freestream velocity, which is slightly larger than the value about 0.7 for the flow past a circular cylinder at the same Reynolds number. The maximum tangential velocity at the periphery of the vortex core maybe occurs slightly in advance of the vortex reaching the boundary of the vortex formation region. The normalized lengths of the recirculation zone of the triangular cylinder keep nearly unchanged and are about 1.55 to 1.9 times of those of the circular cylinder at the same Reynolds number. The normalized normal wise instead of stream wise turbulence intensity has stronger effects on the distribution of the normalized turbulent kinetic energy. [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]