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The energy harnessing from flow-induced vibrations (FIV) by an oscillating foil placed tandemly behind a circular cylinder (which serves as a vortex generator) is investigated. The foil is submerged in the wake produced by the fixed cylinder and could oscillate in the direction perpendicular to the incoming flow with single-degree freedom. The spacing ratio ranges from 1.0 to 5.0. The oncoming fluid velocity is U = 1–10 m/s, corresponding to the reduced velocity Ur = 3.81–38.08 and the Reynolds number Re = 9.58 × 103–9.58 × 104. Four harnessing damping ratios (ζharness = 0.0054–0.0216) are used to simulate the energy conversion conditions. The main conclusions are: (1) The optimal oscillation pattern related to the highest harnessed energy emerges as the spacing ratio close to 1.0. (2) The airflow energy converted by the foil is positively correlated with the harnessing damping ratio because the amplitude responses are similar at various harnessing damping ratios. A high velocity yields the highest harnessed power. (3) The harnessing efficiency of the foil could reach 48.89%, which is much more than that of an isolated flapping foil. [ABSTRACT FROM AUTHOR]

Flow around a cylinder is an important research problem in fluid mechanics. However, studies on traveling wave wall (TWW) flow control for the wake generated by the flow around a 3D cylinder remain limited. This study performs large eddy simulation (LES) to investigate the wake characteristics of flow around a three-dimensional (3D) circular cylinder with the TWW. The influence of the main control parameters of TWW on the aerodynamic forces and wake of the 3D cylinder is analyzed. The wake characteristics of the 3D cylinder at Re = 4 × 104 are obtained, and the traveling wave propagating downstream is achieved on the rear surface of the 3D cylinder using dynamic mesh. The control effects of wave amplitude, number of waves, and wave velocity of the traveling wave on the aerodynamic forces of the cylinder are analyzed, and the optimal control parameter combination is determined. The results demonstrate that the TWW control method completely eliminates the spanwise 3D flow characteristics of the cylinder when the amplitude ratio, number of waves, and velocity ratio are 0.02, 4, and 1.5, respectively. Moreover, it effectively suppresses the flow separation on the cylinder surface, eliminates the wake vortex, and suppresses vortex-induced vibration (VIV). [ABSTRACT FROM AUTHOR]

Through direct numerical simulations, the transition from pure mode A to mode B in the near wake of a circular cylinder is studied with no effect of vortex dislocations. The Reynolds number is computed from 100 to 330 with the computational spanwise length of 4 diameters. In the present part, fluid forces are analyzed. The results show that mode swapping still exists in the range of Reynolds numbers from 230 to 240. In this range, fluid forces with low and high levels occur intermittently. Moreover, when the critical Reynolds number of 193 ( ± 0.5) is exceeded, with the increase of the Reynolds number, the vortex-shedding frequency gradually shifts from a single low frequency, high and low frequency coexistence to a single high frequency. [ABSTRACT FROM AUTHOR]

The unsteady aerodynamic characteristics and interference effects of floating offshore wind turbine (FOWT) are mainly affected by the pitch motion of the ocean platform. Based on computational fluid dynamics (CFD) and advanced overset grid technology, the aerodynamic performance and wake characteristics of a fully configured wind turbine with rotating blades, nacelle , and tower are studied in this paper. The effects of the amplitude and frequency of pitch motion on the wind turbine aerodynamic loads and flow field are investigated herein in detail. The power and thrust between numerical simulation and experiment are compared. The results show that the grid and simulation parameters used in this study can accurately capture the aerodynamic characteristics and the flow field around wind turbines. The influence of the pitch amplitude and frequency on the performance of wind turbines is discussed. The complex flow interaction between the tip vortex, tower shedding vortex, and the turbulent wake was observed. The present results indicate that the pitch motion amplitude and frequency have a great influence on the power, thrust, and wake characteristics. [ABSTRACT FROM AUTHOR]

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]

Purpose: This study aims to investigate the wake behind an oscillating airfoil at a various angle of incidence and Reynolds number in a deep dynamic stall condition. Design/methodology/approach: NACA 0012 airfoil is allowed to undergo harmonic pitching motion about the quarter chord axis at Reynolds numbers of 0.5 * 105, 1.17 * 105, 1.7 * 105 and 2.12 * 105, and the reduced frequency of 0.1. The experiments are conducted at a set of mean and amplitude angle of attack that covered the angle of incidence from −5° to 25°. The wake rake is placed at a distance of one chord from the trailing edge of the airfoil. Findings: The hysteresis of the flow during the upstroke and the downstroke motion are captured. The huge growth in the velocity defect and the wake thickness beyond the angle of attack of 15° explicate the appearance of the strong unsteady effects on the wake. The results also show that at the reduced frequency of 0.1, the wake structure is of drag producing type due to the momentum deficit. Originality/value: Streamwise velocity profile and the turbulent intensity profiles are presented to show the effects of Reynolds number and angle of incidence on the wake behind the oscillating airfoil at the reduced frequency of 0.1, and in the intermediate range of Reynolds number is the novelty of the study. [ABSTRACT FROM AUTHOR]

We report an experimental study about the control of the turbulent wake of a square cylinder. The control device consists in a couple of compliant flaps implemented on the sides of the square cylinder. Both flaps are hinged so that they can freely rotate around their leading edge. Particular emphasis is given to the analysis of the wake through the investigation of both mean and turbulent velocity fields. The contribution to drag reduction of each of these physical parameters is evaluated from experimental data. The main conclusions are twofold: i. the control essentially modifies the turbulent velocity field leading to a reduction of the lateral flow entrainment, ii. this change is due to the synchronization of the vortex shedding on each side of the square cylinder. [ABSTRACT FROM AUTHOR]

In this paper, the effects of regular waves propagating against the current on the wake of a wide bathymetric obstacle are experimentally investigated. For that purpose, a square cylinder of height representing 1 / 8 t h of the water column, mounted perpendicular to the flow, is used. Both Reynolds and Froude numbers are close to the ones encountered at sea and swell properties (amplitude, frequency and wavelength) which are typical conditions in the English Channel. To study this complex hydrodynamic interaction, PIV measurements are used to capture the velocity field around and in the wake of the cylinder. The wake of this wide cylinder has been extensively studied and is characterised by its direction towards the water surface and by energetic and wide vortices arising from the sheared layer at a Strouhal number of approximatively 0.06, after a merging of smaller eddies. In this work, the focus is on how surface gravity waves, propagating against the current, modify the generation of these turbulent structures. The results show that the cylinder wake is strongly modified in the presence of waves, even if, on average, the main wake structure is globally conserved. The waves act both on the vortex generation and the wake development with modifications in terms of turbulent energy distribution and level, wake recovery and ascent rate. The vortex shedding becomes highly energetic and well-organised from the cylinder position. Its return period is extremely regular and corresponds to half of the wave frequency which is really close to the natural vortex shedding frequency. That results in a doubling of the statistical length scale of the vortices. • Swell induced-motion effects on high and isolated bathymetry roughness. • Bottom mounted cylinder wake characterisation in presence of waves opposing current. • Vortex generation and dynamic study. • Energy transfer between wave and vortex shedding frequency in the near-wake. [ABSTRACT FROM AUTHOR]

The effects of two different pitching frequencies (that is, Strouhal number, St) on the wake structure generated by two foils of aspect ratio 1.0 are examined numerically at a Reynolds number of 10,000. Strouhal numbers of 0.5 and 0.2 were studied, the first corresponding approximately to the peak in efficiency and the second corresponding to the point where the thrust is equal to the drag (the free-swimming condition). The two foils have either a square trailing edge or a convex trailing edge that mimics the shape of the caudal fin exhibited by certain species of fish. In previous works, the convex trailing edge panel was found to have higher thrust and efficiency compared with the square panel trailing edge. Here, these differences are related to their characteristic vortex formation and detachment processes leading to differences in wake coherence and extension. The wake of the square panel at St = 0.2 transitions slowly from a reverse von Kármán street (2S) pattern to a paired (2P) system as the wake develops downstream, whereas at St = 0.5, the wake almost immediately takes on a 2P form with an attendant split in the wake structure. For the convex panel, the transition from a 2S to a 2P structure at St = 0.2 is slower than that seen for the square panel, and for St = 0.5, the wake undergoes an abrupt transition leading to two distinct vortex streets that evolve at a considerably slower rate than seen for the square panel. [ABSTRACT FROM AUTHOR]

An emerging rectangular obstacle placed in a laminar boundary layer developing under a free-surface generates three vortical structures: a horseshoe vortex (HSV) in front of the obstacle, a wake downstream and two lateral recirculation zones at its sides. The present work investigates, through PIV measurements, the effect of the obstacle elongation (length over width ratio L ∕ W) on the HSV, which is partly indirect through the modification of the two other vortical structures. Horizontal velocity fields in the near-bottom region show that an increase of the obstacle elongation leads to a higher adverse pressure gradient in front of the obstacle, and in consequence, to the longitudinal extension of the HSV. This modification of geometry, in turn, impacts the vortex dynamics of the HSV. On top of that, maps of spectra and oscillation direction obtained from velocity fields indicate that each of the three structures (HSV, wake and lateral recirculation zones) exhibits a proper oscillation frequency. As the oscillation associated to the wake is energetically dominant and is strong enough to travel upstream, it impacts the HSV dynamics for sufficiently short obstacles. [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]

The application of an unsteady viscous flow around a bluff body has covered a wide range of scientific fields. This paper reviews the wakes of different cross-sectional bluff bodies, including circular, square, triangular and rectangular cylinders. The two most prominent parameters (e.g., Reynolds number and cylinder geometry) that can significantly alter the wake are taken into account. The dependence on the two parameters of Strouhal number, vorticity, circulation, and efflux angle of vortices in the wake of bluff bodies is investigated. The detailed features of the wakes for different bluff-body shapes at different Reynolds are identified and summarized. • Wakes of different cross-sectional bluff bodies are investigated. • The focus is given on the dependence of wake on Reynolds number and cylinder geometry. • The detailed features of the wakes are identified and summarized. [ABSTRACT FROM AUTHOR]

In this paper, the wake modes behind a circular cylinder under streamwisely forcing oscillating motion are studied at Reynolds number Re = 360–460 which are observed by laser-induced fluorescence flow visualization technique. The forcing frequency f e ranges from 0 to 6.85 f s , where f s is the vortex shedding frequency behind a stationary cylinder, and the forcing amplitude A / d = 0.2 , 0.5 , 1.0 , where d is the cylinder diameter. Both time-invariant and linearly ramping f e are investigated. Following our previous modal notation, the following conclusions can be drawn: firstly, three rarely reported modes in numerical studies, C-I, C-II and S-III, are now confirmed in experiments (though with differentiable appearance in their far wake behaviour) at higher A and/or f e ranges and the envelope lines of S modes and C modes, yielded from a vortex circulation model, are shown to be dependent on the peak relative velocity of the free stream to the cylinder surface. So is the occurrence of the S-II mode. Secondly, near the demarcation of A modes and S-I mode, wake mode undergoes constant transition in a stochastic manner at fixed Re, A and f e . Thirdly, a typical hysteretic effect can be observed when the oscillation frequency of the cylinder ramps up and down in a linear way, and the extent of delay is dependent on the ramping rate k. Finally, mode switching during frequency ramping obeys a unidirectional order. During k < 0 (ramp-down), when S-I (Type-II) mode switches to A-IV mode, or A-IV to A-III, the flow structure downstream is affected by the upstream and the entire wake flow eventually switched, which is classified as slow switches. In contrast, during k > 0 (ramp-up), a clear and abrupt switch can be observed in the wake when A-IV or A-III switch to S-I (Type-II) modes, which are jump switches. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical investigations on the flow characteristics past a circular cylinder with a jet injection at the rear stagnation point are performed. Transient, incompressible, laminar, and isothermal flow governing equations are solved with finite volume method. Numerical simulations have been carried out at a Reynolds number of 150 with different injection ratios (IR) ranging from 0.5 to 7. Time evolutions of the coefficients of drag and lift, and streamline patterns are plotted. Three different flow pattern ranges are observed, namely wake dominant (IR=0-1.5), transition (IR=1.5-2.5) and jet dominant (IR > 2.5), and they are characterized by the combined effects of vortex shedding, undulation and jet dominant phenomena appearing in the flow downstream. It is also found that Cd decreases slightly with the injection ratio up to 1.5, after that it monotonically increases with the injection ratio. The similar incremental trend is observed in the Strouhal number variation with IR up to 1.5; then, it increases almost linearly till IR=4. When IR is greater than 4, there is a sudden drop in the Strouhal number value equal to zero and it remains constant after that for all the IR values considered in this study. The power spectral density of Cl indicates that the dominant frequency is present for the lower IR up to 4 and that no dominant frequency appears in the higher injection ratio range of 5-7 due to a complete suppression of the vortex shedding behind the cylinder by a dominant jet mechanism. [ABSTRACT FROM AUTHOR]

Near-wake characteristics of a low aspect ratio ( h / d = 4 ) cantilevered circular cylinder protruding a thin laminar boundary layer were investigated both numerically ( R e = 300 ) and experimentally ( R e = 10 , 400 ). Despite the substantial differences in the investigated Re , the wake dynamics show striking similarities and appear governed by similar instability mechanisms: (i) a Kármán-like vortex shedding instability, and (ii) a low-frequency instability related to the flow over the free end and near the cylinder-wall junction. Attention is drawn to the low-frequency instability, which comprises a significant portion of the kinetic energy content in the wake, and has not been reported in previous experimental or numerical investigations. It appears to be characteristic of intermediate aspect ratio cantilevered circular geometries and the boundary layer state, since the phenomenon is not observed for turbulent boundary layers of similar thickness. [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]

In this study, the wake characteristics of a sphere with a uniaxial through-hole (that is, vented sphere) and that of a sphere without a through-hole (that is, basic sphere) are investigated to clarify the effect of the uniaxial through-hole on the vortex shedding in the wake. A sphere with a diameter of 25.4 mm having a 6 mm circular uniaxial through-hole was fixed to the test section of the wind tunnel and arranged perpendicularly to the streamwise direction. Results show that the vented sphere stabilizes the vortex shedding. A dominant peak at f = 55 Hz is observed in the wake of the vented sphere, whereas a convex peak over a wide range of 40 < f < 70 Hz is observed in the wake of the basic sphere. In addition, the Reynolds stress in the wake of the vented sphere takes a constant value, whereas it attenuates as the flow moves toward the downstream direction for the basic sphere. The cause of these differences in the Reynolds stress is explained using the co-spectrum of the streamwise and spanwise velocity fluctuations. In the wake of the basic sphere, the negative co-spectrum peak over a wide range of 40 < f < 70 Hz attenuates as the flow moves toward the downstream direction. In contrast, a clear negative peak appears in the wake of the vented sphere at f = 55 Hz , indicating the persistence of the periodic structure of the vortex shedding as the flow moves toward the downstream direction. [Display omitted] • Investigation of the wake characteristics of the vented sphere. • A sphere with a uniaxial through-hole (vent) was placed in a uniform flow. • A dominant peak at 55 Hz is observed in the wake of the vented sphere. • Reynolds stress in the wake of the vented sphere takes a constant value. • Negative co-spectrum peak appears in the wake of vented sphere at 55 Hz. [ABSTRACT FROM AUTHOR]

In this paper, we numerically studied the effects of mass ratio (m ∗) on three identified responses of a transversely oscillating equilateral triangular prism in Chen et al. (J. Fluids Struct., vol. 97, 2020, 103099). Three angles of attack, i.e. α = 10 ° , 40° and 60°, were selected, with α = 0 ° being the configuration that one vertex faces upstream. Our results indicated that m ∗ exerts significant influences on both responses and wakes. At α = 10 ° , a higher m ∗ leads to a significant shrink of the large-amplitude vibration region while the largest amplitude occurs at a higher reduced velocity. At α = 40 ° , increasing m ∗ leads to the appearance of three distinct responses successively. At α = 60 ° , soft galloping dominates at m ∗ = 2 –10, while hard galloping where an initial disturbance is required occurs at m ∗ = 20. For the wake, only the 2S mode is observed at α = 10 ° owing to the small amplitude, while depending on the response, P+S, 2S, competition, and m S+ n S (m i S+ n i S) modes can dominate at α = 40 °. At α = 60 ° , 2S and m S+ n S (m i S+ n i S) modes occur. Furthermore, dynamic mode decomposition is applied to extract the vortex structures of different frequencies. Results indicate that von Karman and galloping modes coexist in the region where the galloping phenomenon appears. With increasing amplitude, the scale of the galloping mode increases, while the von Karman mode is almost unaffected. Flow physics for identified responses disclose that both vortex-induced vibration and galloping are closely related to the vortex formation process. The sustenance of the large amplitude in galloping is associated with the continuous shear layer reattachments since multiple vortices are shed from the prism. • Mass ratio has significant influences on both responses and wakes at different angles of attack. • At α = 40 ° , combined VIV and galloping occurs at m ∗ = 2 , separated galloping at m ∗ = 5 , and wavy galloping at m ∗ = 10 − 20. • At α = 60°, soft galloping dominates within m ∗ = 2–10, while hard galloping occurs at m ∗ = 20 when U ∗ > 14. 0. • DMD analysis showed that von Karman and galloping modes coexist in the region where galloping appears. [ABSTRACT FROM AUTHOR]

Visualization experiments for flow around a cylinder with parallel slit placed inside a circular pipe are carried out with water as the working medium. Two different color dyes are employed to visualize the complex vortex formation mechanism behind the bluff bodies. The objective of this study is to explore the potential of cylinder with parallel slit as an improved vortex generator for various practical applications. Wake parameters, such as Strouhal Number, wake width, and vortex formation length, are calculated from the recorded images. Three different slit widths with non-dimensional separation ratio of 0.1, 0.2, and 0.4 are chosen to study the effect on the vortex formation mechanism and the corresponding wake parameters. The Reynolds number range covered in this study is Re = 200-2300. Symmetric vortex formation from the outer surface is observed for lower slit widths, whereas for high slit widths, both in-phase and anti-phase shedding are observed. A separation bubble from each of the inner surface is formed which detaches from the bluff body to form a vortex at higher Reynolds numbers. The separation bubble is found to be both bi-stable and symmetric for lower slit widths. The interaction of the separation bubble with the outer vortices is seen to affect the strength of the primary/outer vortex. The slit width is seen to be an important parameter in determining the strength of the vortices. At higher slit width, the configuration behaves like a dual bluff body and strong crisscross vortex shedding is observed. Such novel kind of vortex formation mechanism can be utilized to improve the performance of numerous practical applications, such as flowmeters, heat exchangers, etc. Graphical abstract: [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]

The flow around four identical side-by-side circular cylinders placed normal to the oncoming flow is numerically simulated using the finite volume method (FVM) at a low Reynolds number of 100 based on cylinder diameter D and freestream velocity. How the wake structure, forces, and vortex shedding patterns are contingent on the spacing ratio g * (= g / D , where g is the gap spacing between the cylinders) is studied systemically when g * varies from 0.0 to 2.0. Based on the intrinsic features of the flow, four distinct flow regimes are identified in the range of g * examined. The total time-averaged drag force acting on the four cylinders escalates exponentially with a decrease in g * , as does the lift force, repulsive, on the outer cylinders. The lift forces of the inner cylinders are also repulsive but very weakly sensitive to g * . The Strouhal number is identical for each of the four cylinders in single body flow, different for the outer and inner cylinders in flip-flopping and quasi-interlocked flows, and again identical for interlocked flow. [ABSTRACT FROM AUTHOR]

The appearance of a ground surface can play an important role in the flow structures for the flows past a flat plate. We conduct two-dimensional numerical simulations on viscous flows past a flat plate inclined at an angle of attack of $$20^\circ $$ with ground effects using a finite-volume method. Results show that the effects on the separated flow from the ground are highly dependent on the gap ( G) between the plate and the ground. As the gap decreases, the strength of vortices generated from the trailing edge is restrained, which is consistent with experimental observations. Further decrease in the gap even eliminates the vortex shedding and yields a steady flow. It is also found that the flow between the gap can either be accelerated at large gap ratios ( $${G/L >1}$$ , G is the gap, L is the plate length), or be decelerated at small gap ratios ( $${G/L <1}$$ ). Furthermore, the numerical results show that the wake flow behind the plate can significantly change the distribution of surface shear stress on the ground. Specifically, the mean shear stress on the ground in the downstream region at a gap ratio $$G/L = 2.0$$ is one order of magnitude larger than that at a small gap ratio $$G/L = 0.3$$ , and the length of the downstream region where the shear stress can be effectively changed is much larger than the plate length, which provides a guideline to manipulate the ground wall surface shear stress using an inclined plate in the vicinity of the wall. [ABSTRACT FROM AUTHOR]

A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400 , 000 , with an angle of attack of A o A = 5 ∘ . The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Re τ ≃ 373 and R e θ = 2 , 800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Re τ ≈ 346 and R e θ = 818 while β changes from β ≈ 0.0 to β ≈ − 0.25 . The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers. [ABSTRACT FROM AUTHOR]

Sound radiation from a rectangular cylinder, the cross-sectional aspect ratio (AR) of which varied from 0.3 to 4, was studied experimentally at Reynolds numbers of 7.5 x 10³ to 1.4 x 105 and Mach numbers below 0.16. In addition to measurements of the lift-dipole sound, detailed flow fields around the rectangular cylinder were examined by means of PIV to better understand the dependence of sound radiation on the AR and Reynolds number as well as the vortex-cylinder interaction associated with sound generation. The mean square sound pressure was proportional to U6∞St², with almost the same factor of proportionality for aspect ratios larger than 0.5 in the Reynolds number range examined in spite of the fact that the spanwise correlation length and magnitude of the lift fluctuations were highly dependent on the AR. When vortex shedding at the trailing-edge was locked in phase with the leading-edge-generated vortices, which occurred for AR > 3, the shed vortices were quite two-dimensional, and correspondingly, the radiated sound exhibited a very sharp spectral peak like a line spectrum; the SPL was as strong as that for AR = 0.75, for which the maximum SPL was observed. Reynolds number effects on both SPL and the Strouhal number were weak except around the aspect ratio where reattachment of the separated leading-edge shear layer occurred. [ABSTRACT FROM AUTHOR]

This paper aims to provide a comprehensive review of the literature and to impart a perspective on the current status and future directions for fundamentals of flow around and heat transfer from a square cylinder with modified corners. The focus is on how rounded, chamfered, recessed, and cut corners of a square cylinder impact the wake structure, flow instability, fluid forces, vortex dynamics, wake width, vortex formation length, boundary layer thickness, displacement thickness, transition to turbulence, drag crisis, and heat transfer. A circular cylinder undergoes drag crisis flow while a square cylinder does not. Sufficiently rounded corners can rejuvenate the drag crisis flow. The corner modifications encompassing heat transfer performance from low to high can be sorted out as sharp, cut, rounded, and chamfered, given the same scale of modifications. Based on the review and analysis of data in the literature, the optimal corner modifications are suggested for reduced forces and enhanced heat transfer. The research gap in the literature is identified and put forward for future investigations. [Display omitted] • Flow around and heat transfer from a square cylinder with modified corners are reviewed. • Focus is on how rounded, chamfered, recessed, and cut corners impact the wake structure. • Flow instability, fluid forces, transition, drag crisis, and heat transfer are summarized. • No drag crisis flow for square cylinder, but sufficiently rounded corners can rejuvenate it. • Optimal corner modifications are suggested for reduced forces and enhanced heat transfer. [ABSTRACT FROM AUTHOR]

Drilling a vertical cavity from the bottom to the free-end surface has been a passive method of controlling wake flow around a wall-mounted square cylinder. The mean flow field around a wall-mounted finite-height square cylinder with a vertical rectangular cavity was studied experimentally in a water tunnel to explore the effects of inclined angle α. Measurements were conducted using an acoustic Doppler velocimeter at the Reynolds number of Re h = 17,525, in which the incidence angles of the cylinder were α = 0, 10°, 30°, and 45°. Results depict the presence of a recirculation zone in the cavity and a near-wake zone. As α increases from 0 to 10°, the mean wake behind the cylinder shifts in the opposite position, and a single peak is observed. When α changes from 10° to 45°, the mean wake regains its symmetry, and two peaks are observed at the wake. Self-similarity in terms of the streamwise velocities and Reynolds shear stress can be achieved by properly selecting the velocity and length scales irrespective of the angle of incidence. For the wake flow with a higher value of α , the length of the mean recirculation zone, momentum exchange, and turbulence kinetic energy increase, whereas the mean and variance values of the vortex shedding frequency reach a smaller one. In this study, the cavity flow and wake flow are also observed in terms of velocity distribution, turbulent kinetic energy budget, and turbulent events. • The effect of incidence angle on flow field around a square cylinder with a passive vertical suction was explored. • The differences between cavity flow in suction area and wake flow are analyzed. • The critical incidence angle for the structure is proposed based on the results of flow characteristic. [ABSTRACT FROM AUTHOR]

The paper presents a novel extension of the two-dimensional Vortex Particle Method which allows complex transient flow features computed by an original simulation to be recreated for use as inflow conditions in other simulations. This is facilitated by recording velocity time signals of the original simulation and computing time traces of vortex particles to be seeded into the secondary simulation near its upstream domain boundary. The proposed Flow Reproduction Method (FRM) thus allows us to re-create the flow field, without the need to simulate the underlying physics responsible for the flow features. A natural field of application is the re-creation of wakes from flows past bluff bodies of arbitrarily complex geometry, the resolution of which is computationally expensive. The recording is performed on a sampling system of the velocity field. Reproduction of the sampled simulation is then performed by inserting vortex particles in defined positions and time intervals into the secondary simulation. This proceeds in a smaller domain with the advantage of significantly reduced computational cost. In the simulations presented here, wake flows of upstream cylinders are reproduced. Convergence studies are performed to validate the FRM. The quality of flow reproduction is assessed and quantified. The computational efficiency of the reproduction simulation is enhanced additionally by using different adaptive numerical techniques. The method is then applied to fluid-structure interaction simulations of a wake buffeting problem. Good agreement is found between the dynamic response quantities when comparing original simulations and those with reproduced flows, whereby the latter are performed up to five times faster. [ABSTRACT FROM AUTHOR]

The unsteady wake of a high-speed train is investigated experimentally. From a practical point of view, the wake region is of considerable importance as it is where slipstream velocities—velocities induced by the vehicles movement through air—are largest. In turn, this can create a considerable risk for passengers and track-side workers as the train passes. The flow is quantified in a 1:10 scale wind-tunnel experiment using high-frequency 4-hole dynamic pressure cobra probes, surface-pressure measurements and flow visualisation. The dominant feature of the time-average wake topology consists of a clearly identifiable counter-rotating streamwise vortex pair. Although the wake structure and evolution should perhaps be considered as a whole, the near wake exhibits periodic unsteadiness, at a Strouhal number of 0.2, that could be attributed to periodic shedding from the sides and to a lesser extent the top surface. This periodicity feeds into the trailing vortices, consistent with lateral and vertical displacement of the cores as they advect downstream and thus affecting maximum slipstream velocities. [ABSTRACT FROM AUTHOR]

The effect of a wake-mounted splitter plate on the flow around a surface-mounted finite-height square prism was investigated experimentally in a low-speed wind tunnel. Measurements of the mean drag force and vortex shedding frequency were made at Re=7.4×10 4 for square prisms of aspect ratios AR=9, 7, 5 and 3. Measurements of the mean wake velocity field were made with a seven-hole pressure probe at Re=3.7×10 4 for square prisms of AR=9 and 5. The relative thickness of the boundary layer on the ground plane was δ / D =1.5–1.6 (where D is the side length of the prism). The splitter plates were mounted vertically from the ground plane on the wake centreline, with a negligible gap between the leading edge of the plate and rear of the prism. The splitter plate heights were always the same as the heights of prisms, while the splitter plate lengths ranged from L / D =1 to 7. Compared to previously published results for an “infinite” square prism, a splitter plate is less effective at drag reduction, but more effective at vortex shedding suppression, when used with a finite-height square prism. Significant reduction in drag was realized only for short prisms (of AR≤5) when long splitter plates (of L / D ≥5) were used. In contrast, a splitter plate of length L / D =3 was sufficient to suppress vortex shedding for all aspect ratios tested. Compared to previous results for finite-height circular cylinders, finite-height square prisms typically need longer splitter plates for vortex shedding suppression. The effect of the splitter plate on the mean wake was to narrow the wake width close to the ground plane, stretch and weaken the streamwise vortex structures, and increase the lateral entrainment of ambient fluid towards the wake centreline. The splitter plate has little effect on the mean downwash. Long splitter plates resulted in the formation of additional streamwise vortex structures in the upper part of the wake. [ABSTRACT FROM AUTHOR]

Flow regulation between two tandem circular cylinders is a key topic in a variety of engineering applications. The numerical study of laminar flow regimes over two tandem grooved cylinders is presented in this research. Both cylinders have an identical diameter D , and separated by a distance L = 4D. The upstream cylinder has a single square groove whereas the downstream cylinder has a smooth surface. The investigations were conducted at different Reynolds number, Re ≤ 200, and groove orientation (0°, 45°, and 90°) measured from the stagnation point. The effects of Reynolds number and groove position on the leading surface of the cylinder are investigated. While the lift coefficients of tandem grooved cylinders can be enhanced by up to 45% when compared to tandem smooth cylinders, the drag coefficients of tandem grooved cylinders are around 20% lower. Furthermore, considerable changes in the upstream wake is detected due to the presence of the groove on the upstream cylinder. It is also found that once the groove is set at 45° and 90° for the upstream cylinder, the downstream cylinder experiences a maximum and minimum peaks of pressure coefficient, respectively. • It is found that the imposed pressure on the grooved upstream cylinders is unsymmetrical. • Considerable changes in the upstream wake is detected due to the presence of the groove on the leading surface of the upstream cylinder. • While the lift coefficients of tandem grooved cylinders can be enhanced by up to 45% when compared to tandem smooth cylinders. • The drag coefficients of tandem grooved cylinders are around 20% lower than those of tandem plain cylinders. [ABSTRACT FROM AUTHOR]

• Flows and heat transfer from nine prisms in a square array is numerically investigated. • Five distinct flows are identified as the prism spacing-to-width ratio increases from 1.2 to 7.0. • The lateral-interaction-induced coshedding flow induces largest fluid forces and maximum heat transfer. • The maximum heat transfer is ascribed to the creeping flow generated around the center prism. Structures in an array with multiple rows and columns may undergo both inline and side-by-side interferences. A system of nine square prisms in a 3 × 3 square array is numerically investigated for pitch ratio L / D = 1.2 – 7.0 at a Reynolds number of 150, where L is the spacing between the centers of two adjacent prisms, and D is the side width of a square prism. The focus is given on investigating the effect of L / D on flow topology, fluid forces, heat transfer, vortex shedding, and recirculation bubbles. An increase in L / D from 1.2 to 7.0 leads to evolutions of five distinct flows: single bluff body flow (L / D < 1.6), reattachment flow (1.6 < L/D < 3.3), lateral-interaction-induced coshedding flow (3.3 < L/D < 4.1), mixed flow (4.1 < L/D < 4.6) and free coshedding flow (4.6 < L/D < 7.0). The reattachment flow corresponds to small fluid forces while the lateral-interaction-induced coshedding flow induces large fluid forces. The time-mean drag force coefficient of the center prism drastically increases from 0.09 to 1.0 as the flow evolves from the reattachment flow to the lateral-interaction-induced coshedding flow. The maximum heat transfer from the center prism, 10% higher than that from a standalone prism, occurs for the lateral-interaction-induced coshedding flow where the flow around the center prism resembles creeping flow, no recirculation bubble forming on the front or rear surface of the prism. The coherence between heat transfer and flow patterns is discussed, including the impacts of shear layer reattachment, flow recirculation, and vortex shedding on heat transfer. [Display omitted] [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]

• Both hydrodynamic instability and vortex merging are possible formation mechanisms for the secondary vortices. • The formation mechanisms are mapped out over a parameter space of Reynolds number (Re) and aspect ratio (AR). • By increasing Re and decreasing AR , the formation mechanism changes from hydrodynamic instability to vortex merging. • By increasing free-stream perturbation amplitude, the formation mechanism may change at a decreased Re. The transition from the primary (Kármán) and two-layered vortex streets to a secondary vortex street in the relatively far wake of a rectangular cylinder is examined through direct numerical simulations. The parameter space spans a range of the cross-sectional aspect ratios (AR ; the ratio between the streamwise and transverse lengths) of the cylinder between 0.01 and 0.5, and a range of the Reynolds numbers (Re) between 50 and 200. Unlike some previous studies which attributed the formation of the secondary vortices to either hydrodynamic instability of the mean flow or vortex merging, the present study shows that both mechanisms exist. Over the (AR , Re) parameter space, the two formation mechanisms are mapped out. For relatively small Re and relatively large AR , the secondary vortices emerge at the streamwise locations downstream of the annihilation of the two-layered vortices, such that the secondary vortices cannot be formed from the merging of the two-layered vortices and are thus formed from the hydrodynamic instability of the two bare shear layers. In contrast, for relatively large Re and relatively small AR , the secondary vortices emerge at the streamwise locations where the two-layered vortices still exist, such that the secondary vortices are formed from the merging of the two-layered vortices. In addition, the influence of free-stream perturbation on the formation mechanism is examined. An increase in the perturbation amplitude may result in a decreased critical Re for the change in the formation mechanism. The general conclusions are expected to be applicable to other bluff-body flows. [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]

Direct numerical simulation of two-dimensional flow past a normal flat plate having an aspect ratio of 0.125 has been carried out in the Reynolds number range of 30-175 to investigate the effect of various transitions of the wake with increasing Reynolds number reported previously on the mean and RMS flow field. The time-averaged and RMS flow field, particularly the far-wake, has been observed to be sensitive to various transitions. However, a marginal effect is observed on the mean integral parameters such as the drag coefficient and the Strouhal number. [ABSTRACT FROM AUTHOR]

Abstract: A prototype device for measuring the volumetric flow-rate by counting vortices has been designed and realized. It consists of a square-section pipe in which a two-dimensional bluff body and a strain gauge force sensor are placed. These two elements are separated from each other, unlike the majority of vortex apparatus currently available. The principle is based on the generation of a separated wake behind the bluff body. The volumetric flow-rate measurement is done by counting vortices using a flat plate placed in the wake and attached to the beam sensor. By optimizing the geometrical arrangement, the search for a significant signal has shown that it was possible to get a quasi-periodic signal, within a good range of flow rates so that its performances are well deduced. The repeatability of the value of the volume of fluid passed for every vortex shed is tested for a given flow and then the accuracy of the measuring device is determined. This quantity is the constant of the device and is called the digital volume (V p ). It has the dimension of a volume and varies with the confinement of the flow and with the Reynolds number. Therefore, a dimensionless quantity is introduced, the reduced digital volume (V r ) that takes into account the average speed in the contracted section downstream of the bluff body. The reduced digital volume is found to be independent of the confinement in a significant range of Reynolds numbers, which gives the device a good accuracy. [Copyright &y& Elsevier]

Abstract: The flow past a sphere rotating about an axis aligned with the streamwise direction is numerically investigated. The dynamics is governed by the incompressible Navier–Stokes equations and depends on two control parameters: the Reynolds number Re and rotation rate . The present investigation systematically covers the range and . First, the axisymmetric steady base flow (whether stable or not) is computed for all values of the control parameters. Then, after linearisation of the equations about the base flow, the growth rates and frequencies of the leading eigenmodes are obtained. Fully nonlinear direct numerical simulations yield the detailed flow fields and hydrodynamic forces acting on the sphere. Different wake modes (low-frequency periodic helical, quasi-periodic shedding and high-frequency periodic helical) are identified and their characteristic frequencies precisely determined. [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: Numerical investigation has been performed for flows past an inclined square cylinder in the range of Re≤300 to elucidate the characteristics of the laminar three-dimensional wake behind the cylinder depending upon Reynolds number and angle of incidence (θ). An immersed boundary method was used for implementation of the cylinder on a Cartesian grid system. Both Floquet stability analysis and full three-dimensional simulation were employed to detect the onset of the secondary instability leading to three-dimensional flow, and to provide quantitative flow data. It was revealed that mode A becomes more unstable for “symmetric” flow configuration, while mode C is dominant for asymmetric flows in the range of 10°≤θ≤25°. The most unstable three-dimensional modes predicted by the Floquet stability analysis were well confirmed by the full 3D simulations which were conducted at Re=150, 200, 250, and 300, with varying angle of incidence. The full 3D simulations also provided the key flow characteristics such as mean flow-induced force/moment coefficients and Strouhal number of vortex shedding. It was seen that they are sensitive to slight inclination of the square cylinder, and the Re effects are insignificant except for mean lift coefficient. Vortical structures in the wake, taken from the results of the Floquet stability analysis and the full 3D simulations, respectively, were visualized by Q-contours, revealing good agreement between the two. [Copyright &y& Elsevier]

Abstract: Experimental results on the unsteady behaviour of the wake of a modelled wind turbine in an atmospheric boundary layer (ABL) wind tunnel are presented. Tests were performed by modelling in the wind tunnel an ABL above a neutral moderately rough terrain at a geometric scale of 1:400, and a wind turbine with the help of the actuator disk concept. In order to characterise the meandering properties of its wake, velocity spectra and space–time correlations were measured through hot wire anemometry, both in the wake of the modelled wind turbine and in the wake of a solid disk. Comparing these two configurations allowed the examination of the differences between the random motion of the whole wind turbine wake characterising the meandering in the first case, and the periodic oscillations of the well-known vortex shedding, which appears behind a bluff-body, in the second case. Finally, the same experiments were performed in homogeneous and isotropic turbulent flows to exhibit the role of the large atmospheric turbulent scales in the meandering process. [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: 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: 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: A comparative study of the wake characteristics behind 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 and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. For the system G/D=0.5, a regular Karman-like vortex shedding at fD/U 0=0.13 was found by inspection of the streamwise velocity spectra. When the gap width is reduced to G/D=0.25, a low-speed separation bubble was formed in the region 4.25

Abstract: In this study, we control unsteady motion in the wake behind a sphere using a suboptimal feedback control method based on the sensing of surface pressure. The cost function to be reduced is the square of the difference between the potential-flow and real pressures at the sphere surface. The actuation velocity (blowing/suction) is obtained from the suboptimal feedback control procedure. The sensing and actuation on the whole surface of the sphere is considered. This is an ideal case but provides a clear understanding of the effect of suboptimal feedback control on the present flow. We choose four different Reynolds numbers, Re=100, 250, 300, and 425, covering four different flow regimes (steady axisymmetric, steady planar–symmetric, unsteady planar–symmetric, and unsteady asymmetric flows, respectively). With the present control, the vortex shedding disappears for Re=300 and 425 and the drag is significantly reduced for all the Reynolds numbers considered. [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]

A uniform flow past two unequal sized square cylinders arranged in a side-by-side pattern and at a Reynolds number of 50,000 has been investigated using large eddy simulation (LES) technique. The modelling of sub-grid scales of turbulence is done using the Smagorinsky model. The effect of the transverse gap ratio (T/D) on the flow characteristics has been studied. Numerical simulations are carried out for five different transverse gap ratios (T/D), namely 1.120, 1.250, 1.375, 1.750 and 2.500. Results in terms of the aerodynamic forces, Strouhal number, mean base pressure coefficient, streamlines, vorticity, surface pressure distribution, normal and shear stresses are presented. A shift in the stagnation point for the small square cylinder from the centre of its front face towards its gap side is seen at smaller T/D ratios. The presence of a jet-like flow seen in the gap side is more pronounced at T/D = 1.12. A biased gap side flow towards the near wake of the small square cylinder is seen at smaller T/D ratios. No interference effect is seen at T/D = 2.5. The flow behaviour is similar to that of the isolated square cylinder at this gap ratio. [ABSTRACT FROM AUTHOR]