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This study experimentally investigates the influence of structural damping on the transverse flow-induced vibration (FIV) of an elastically mounted thin elliptical cylinder. The cylinder tested has an elliptical ratio of $\varepsilon = b/a = 5$ , where $a$ and $b$ are the streamwise and cross-flow dimensions, respectively, and a mass ratio (i.e. the total oscillating mass/the displaced fluid mass) of $17.4$. The FIV response was characterised over a reduced velocity range of $2.30 \leq U^* = U/(\,{{f_{{nw}}}} b) \leq 10.00$ (corresponding to a Reynolds number range of $300 \leq \textit {Re} =(U b)/\nu \leq 1300$) and a structural damping ratio range of $3.62\times 10^{-3}\leq \zeta \leq 1.87\times 10^{-1}$. Here, $U$ is the free stream velocity, ${{f_{{nw}}}}$ is the natural frequency of the system in quiescent fluid (water) and $\nu$ is the kinematic viscosity of the fluid. The FIV response was characterised by four wake–body synchronisation regimes (defined as the matching of the dominant fluid forcing and oscillation frequencies, and labelled regime I, regime II, regime III and the hyper branch) and a desynchronisation region, with the hyper branch representing a high amplitude regime not observed for a circular cylinder. Interestingly, the major vortex shedding mode was predominately two single opposite-signed vortices shed per body vibration cycle. Moreover, hydrogen-bubble-based flow visualisations revealed a secondary vortex street forming in the elongated shear layers associated with largest-scale vibration amplitudes ($A^* = A/b$ up to $7.7$) in the hyper branch and regime II. As the structural damping ratio was increased beyond $1.92 \times 10^{-2}$ , the hyper branch was found to be suppressed. The results have potential ramifications for the efficient extraction of energy from free-flowing water sources, which has become increasingly topical over the last decade. [ABSTRACT FROM AUTHOR]

Natural hydro environments such as rivers and ocean currents provide abundant and essentially free sources of kinetic energy. Although significant research has been devoted to harnessing this power through traditional turbine systems, there are also studies on the novel use of flow-induced vibration of a bluff body. The large oscillations of an elliptical cylinder resulting from the newly-discovered hyper branch have raised interesting questions about its potential as a source of effective renewable energy. This study investigates the power extraction characteristics of the hyper branch over a range of flow velocities. A factor-of-ten increase in the maximum time-mean power coefficient relative to an established flow-induced vibration system — the vortex induced vibration for aquatic clean energy converter is demonstrated. This suggests the possibility of a new and effective strategy to utilise flow-induced vibrations for energy generation and could help drive the commercial implementation and uptake of oscillating energy converters. [Display omitted] • Thin elliptical geometry is inherently aerodynamically unstable. • Elastically mounted thin elliptical cylinder yields large crossflow oscillations. • Electromagnetic damper attached to cylinder is used to simulate generator loads. • Elliptical cylinder yielded a maximum power coefficient of 3.31. • Factor-of-ten improvement in power coefficient compared to other similar devices. [ABSTRACT FROM AUTHOR]

The effect of angle of attack on the flow-induced vibration (FIV) response of an elastically mounted thin elliptical cylinder has been investigated by measuring the structural displacement and fluid forces acting on the body in water-channel experiments. Specifically, an elliptical cylinder with a cross-sectional elliptical ratio of ɛ = b / a = 5 was chosen due to the presence of a region of vibration response associated with the combined effect of vortex-induced vibration (VIV) and galloping, where large vibration amplitudes nearly eight times the cross-flow dimensions can be sustained. Here, a and b are the semi-minor axis (aligned with the streamwise direction) and the semi-major axis, respectively. The present experimental results demonstrated that the large vibration amplitudes (i.e. where the maximum observed value was approximately 6 b) generally decrease with the angle of attack, resulting in substantial reductions for α ≳ 2 ∘ (with α = 3. 5 0 ∘ corresponding to a ∼ 60% decrease in the maximum vibration amplitude). Particle image velocimetry (PIV) measurements revealed that the dominant vortex shedding mode consists of two single opposite-signed vortices shed per body vibration cycle. The presence of additional vorticity regions that were absent in the zero angle of attack case was also observed, including crescent-shaped wake structures and secondary inline vortices. This study shows the importance of maintaining axial symmetry in such an FIV system, and that the flow incidence angle is an essential consideration for efficient energy harvesting using this elliptical geometry. [ABSTRACT FROM AUTHOR]

A comprehensive two-dimensional numerical investigation has been undertaken to calculate the energetic cost of propulsion and the various flow transitions of a fish-like body undulation mechanism based on a National Advisory Committee for Aeronautics 0012 hydrofoil. This covers a wide range of Strouhal (0 ≤ S t ≤ 1.4) and Reynolds (500 ≤ R e ≤ 5000) numbers from simulations based on a level-set function immersed-interface method. It is found that the time-averaged thrust coefficient displays a quadratic relationship with increasing St, and increases significantly with Re. Additionally, the time-averaged input power coefficient exhibits a cubic dependence with increasing St but is independent of Re. Both St dependences agree with those previously observed experimentally and numerically for an oscillating foil; however, for similar ranges of governing parameters, comparisons suggest that the body undulation mechanism possesses a higher propulsive efficiency. The S t ∝ R e − 0.19 scaling for the drag-to-thrust transition is consistent with that found for a wide variety of fish and birds in nature. Interestingly, for cases with an undulation wave-speed below the free-stream speed, the time-averaged drag coefficient is found to be higher than that of a stationary hydrofoil at the same Re. Furthermore, the time-averaged input power coefficient is negative, indicating the potential for the undulation mechanism to extract energy from the free-stream. Eight different wake patterns/transitions are documented for the parameter space; these have been assembled into a wake-regime parameter-space map. The present findings should aid in predicting and understanding different hydrodynamic forces and wake patterns for undulating kinematics. [ABSTRACT FROM AUTHOR]

In published literature of the vortex-induced vibration of a sphere, limited attention has been devoted to spheres having three degrees of freedom (DOF) motion and the effect on the vibration response. In this study, the vortex-induced vibration response of a 3-DOF elastically-mounted sphere was examined numerically. The response of a sphere allowed to oscillate in all three spatial directions (3-DOF motion) was compared with the response of a sphere whose motion was restricted to only the transverse direction (1-DOF). Simulations were conducted over the reduced velocity range U ∗ ≡ U ∕ (D f n) ∈ [ 3. 5 , 16 ] , using a sphere of mass ratio m ∗ ≡ ρ s ∕ ρ = 3 , at a fixed Reynolds number of 2000, where U is the free-stream velocity of the flow, f n is the natural frequency of the system, ρ s and ρ are the solid and fluid densities, respectively. When the sphere was allowed 3-DOF movement, it was initially excited to vibrate along a linear path in the transverse direction, synchronized with the two-sided shedding of vortex loops behind the sphere. As the simulation time advanced, the sphere trajectory eventually converted into a circular orbit with a spiralling wake behind it. The transition time between these two modes was found to increase with decreasing Reynolds number. The vibration amplitude was significantly smaller when the sphere motion was in all three spatial directions than when it was restricted only to the transverse direction. The maximum vibration amplitudes were approximately 0.6 and 0.8 diameters, for 3-DOF and 1-DOF motion, respectively. In the synchronization regime, the maximum time-averaged drag coefficient was approximately 100% greater than that of a stationary sphere, when the sphere had 3-DOF, similar to previous observations for a tethered sphere. However, the maximum change to the time-averaged drag coefficient was only 76% when the motion was restricted to the transverse direction. [ABSTRACT FROM AUTHOR]

Full three-dimensional numerical simulations are employed to investigate the flows over inclined trapezoidal low-aspect-ratio plates at low Reynolds numbers, aiming to understand the unsteadiness induced by the interaction between the trailing vortical wake structures originating from the swept edges, and those from the leading and trailing edges. The flows past eighteen different plate geometries in three broad sets are simulated to study the influence of aspect ratio, taper angle and angle of attack on the wake vortices and the force coefficients. Both taper ratio and angle of attack of plates with the same area are found to have a broadly predictable influence on the wake stability and asymptotic forces. Smaller taper ratios result in lower maximum lift, while an increase in the angle of attack results in a reduction in the differences in maximum lift. Two distinct modes of periodic unsteady flow with significant differences in frequency are observed. The corresponding vortex-shedding mechanisms are analysed with the aid of Q-criterion isosurfaces and streamlines. A low wake frequency is observed at small taper angles when there is relative independence between the von Kármán vortices originating from the leading and trailing edges, and weak swept-edge vortices. The dominant Strouhal number in this state is approximately 0.09. When the taper angle or angle of attack increases, the flows over the swept edges form stronger trailing vortex structures which interact strongly with the leading-edge vortices, combining to produce a regular stream of vortex loops shed into the wake. In this regime, the dominant Strouhal number increases to approximately 0.14-0.18. Higher Reynolds numbers and/or angles of attack result in a loss of centre plane reflection symmetry in the wake. The aerodynamic forces have been quantified as a function of the problem parameters and plate geometry. [ABSTRACT FROM AUTHOR]

This paper presents the results of an in-depth study of the flow past a streamwise oscillating cylinder, examining the impact of varying the amplitude and frequency of the oscillation, and the Reynolds number of the incoming flow. These findings are presented in a framework that shows that the relationship between the frequency of vortex shedding ${f}_{s} $ and the amplitude of oscillation ${A}^{\ast } $ is governed by two primary factors: the first is a reduction of ${f}_{s} $ proportional to a series in ${A}^{\ast 2} $ over a wide range of driving frequencies and Reynolds numbers; the second is nonlinear synchronization when this adjusted ${f}_{s} $ is in the vicinity of $N= {(1- {f}_{s} / {f}_{d} )}^{- 1} $, where $N$ is an integer. Typically, the influence of higher-order terms is small, and truncation to the first term of the series (${A}^{\ast 2} $) well represents the overall trend of vortex shedding frequency as a function of amplitude. However, discontinuous steps are overlaid on this trend due to the nonlinear synchronization. When ${f}_{s} $ is normalized by the Strouhal frequency ${f}_{St} $ (the frequency of vortex shedding from an unperturbed cylinder), the rate at which ${f}_{s} / {f}_{St} $ decreases with amplitude, at least for ${f}_{d} / {f}_{St} = 1$, shows a linear dependence on the Reynolds number. For a fixed $\mathit{Re}= 175$, the truncated series shows that the rate of decrease of ${f}_{s} / {f}_{St} $ with amplitude varies as ${(2- {f}_{d} / {f}_{St} )}^{- 1/ 2} $ for $1\leqslant {f}_{d} / {f}_{St} \leqslant 2$, but is essentially independent of ${f}_{d} / {f}_{St} $ for ${f}_{d} / {f}_{St} \lt 1$. These trends of the rate of decrease of ${f}_{s} $ with respect to amplitude are also used to predict the amplitudes of oscillation around which synchronization occurs. These predicted amplitudes are shown to fall in regions of the parameter space where synchronized modes occur. Further, for the case of varying ${f}_{d} / {f}_{St} $, a very reasonable prediction of the amplitude of oscillation required for the onset of synchronization to the mode where ${f}_{s} = 0. 5{f}_{d} $ is given. In a similar manner, amplitudes at which ${f}_{s} = 0$ are calculated, predicting where the natural vortex shedding is completely supplanted by the forcing. These amplitudes are found to coincide approximately with those at which the onset of a symmetric vortex shedding mode is observed. This result is interpreted as meaning that the symmetric shedding mode occurs when the dynamics crosses over from being dominated by the vortex shedding to being dominated by the forcing. [ABSTRACT FROM PUBLISHER]

This paper studies the fluid–structure interaction of an elastically mounted square cross-section cylinder immersed in a free stream. The cross-section is mounted such that its sides are at 45° to the free stream direction, in a “diamond” configuration, and its motion is constrained to the transverse direction relative to the flow direction. Apart from the cross-section, this setup is the same as the majority of single-degree-of-freedom vortex-induced vibration studies of cylinders. Two-dimensional direct numerical simulations of this system have been performed. The Reynolds number based on the point-to-point distance of the cross-section has been fixed at Re=200). Simulations at this Reynolds number allow a direct comparison with previous results from circular cylinders, and therefore focus directly on the impact of the geometry. The sensitivity of the flow, and therefore the motion of the cylinder, to geometrical effects is considered. This is done by rounding the two side corners (those pointing across the flow) at a given radius. For well-rounded corners, the flow behaviour resembles that of a circular cylinder undergoing vortex-induced vibration. However, below a critical radius, the dynamics are considerably altered. Highly disordered and irregular wakes and body motions are observed, as well as a synchronized, periodic P+S wake mode (Williamson and Roshko, 1988), which consists of a pair of vortices on one side, and a single vortex on the other side, shed per oscillation cycle, which results in a non-zero mean lift force. A period-doubled version of this P+S wake is also presented. The spatial structure, and the spatio-temporal symmetries of each of these modes is reported. The results show that even though the spatio-temporal symmetry of the flow is unaffected by the geometry when the body is rigidly mounted (the flow always saturating to a Kármán vortex street) geometric features such as sharp corners can induce a number of spontaneous symmetry breaking bifurcations when the body is elastically mounted. Which of these various modes is observed is shown to be a function of both the corner radius and the spring stiffness, expressed through the reduced velocity. [ABSTRACT FROM AUTHOR]

The transient response of optimal linear perturbations of liquid metal flow under a strong axial magnetic field in an electrically insulated rectangular duct is considered. The focus is on the subcritical regime, below the onset of von Kármán vortex shedding, to determine the role of optimal disturbances in developing wake instabilities. In this configuration, the flow is quasi-two-dimensional and can be solved over a two-dimensional domain. Parameter ranges considered include Reynolds numbers 50≤Re<=2100, modified Hartmann numbers 50≤Ha[closed_star]<=500, and blockage ratios 0.1 <= β <= 0.4. In some instances, the optimal disturbances are found to generate energy growth of greater than four orders of magnitude. Variation in the wake recirculation length in the steady flow regime is determined as a function of Reynolds number, Hartman number, and blockage ratio, and a universal expression is proposed. For all β, the energy amplification of the disturbances is found to decrease significantly with increasing Hartmann number and the peak growth shifts towards smaller times. The optimal initial disturbances are consistently located in the vicinity of the boundary layer separation from the cylinder, and the structure of these disturbances is consistent for all Hartmann numbers and blockage ratios considered in this study. The time evolution of the optimal perturbations is presented, and is shown to correspond to sinuous oscillations of the shear layer downstream of the wake recirculation. The critical Reynolds number for the onset of growth at different Hartmann numbers and blockage ratios is determined. It is found that it increases rapidly with increasing Hartmann number and blockage ratio. For all β, the peak energy amplification grows exponentially with Re at low and high Hartmann numbers. Direct numerical simulation in which the inflow is perturbed by a random white noise confirms the predictions arising from the transient growth analysis: that is, the perturbation excites and feeds energy into the global mode. [ABSTRACT FROM AUTHOR]

Simulations of a cylinder undergoing externally controlled sinusoidal oscillations in the free stream direction have been performed. The frequency of oscillation was kept equal to the vortex shedding frequency from a fixed cylinder, while the amplitude of oscillation was varied, and the response of the flow measured. With varying amplitude, a rich series of dynamic responses was recorded. With increasing amplitude, these states included wakes similar to the Kármán vortex street, quasiperiodic oscillations interleaved with regions of synchronized periodicity (periodic on multiple oscillation cycles), a period-doubled state and chaotic oscillations. It is hypothesized that, for low to moderate amplitudes, the wake dynamics are controlled by vortex shedding at a global frequency, modified by the oscillation. This vortex shedding is frequency modulated by the driven oscillation and amplitude modulated by vortex interaction. Data are presented to support this hypothesis. [ABSTRACT FROM AUTHOR]

A detailed numerical study of the separating and reattaching flow over a square leading-edge plate is presented, examining the instability modes governing transition from two- to three-dimensional flow. Under the influence of background noise, experiments show that the transition scenario typically is incompletely described by either global stability analysis or the transient growth of dominant optimal perturbation modes. Instead two-dimensional transition effectively can be triggered by the convective Kelvin–Helmholtz (KH) shear-layer instability; although it may be possible that this could be described alternatively in terms of higher-order optimal perturbation modes. At least in some experiments, observed transition occurs by either: (i) KH vortices shedding downstream directly and then almost immediately undergoing three-dimensional transition or (ii) at higher Reynolds numbers, larger vortical structures are shed that are also three-dimensionally unstable. These two paths lead to distinctly different three-dimensional arrangements of vortical flow structures. This paper focuses on the mechanisms underlying these three-dimensional transitions. Floquet analysis of weakly periodically forced flow, mimicking the observed two-dimensional quasi-periodic base flow, indicates that the two-dimensional vortex rollers shed from the recirculation region become globally three-dimensionally unstable at a Reynolds number of approximately 380. This transition Reynolds number and the predicted wavelength and flow symmetries match well with those of the experiments. The instability appears to be elliptical in nature with the perturbation field mainly restricted to the cores of the shed rollers and showing the spatial vorticity distribution expected for that instability type. Indeed an estimate of the theoretical predicted wavelength is also a good match to the prediction from Floquet analysis and theoretical estimates indicate the growth rate is positive. Fully three-dimensional simulations are also undertaken to explore the nonlinear development of the three-dimensional instability. These show the development of the characteristic upright hairpins observed in the experimental dye visualisations. The three-dimensional instability that manifests at lower Reynolds numbers is shown to be consistent with an elliptic instability of the KH shear-layer vortices in both symmetry and spanwise wavelength. [ABSTRACT FROM PUBLISHER]

Predictions from embedded-LES are presented of a model of a section of a double-stacked freight wagon subjected to different local loading configurations. In total, 15 different upstream ( G f r o n t ) and downstream ( G b a s e ) gap spacings were simulated to characterise the change in the flow topology. The mean flow fields indicate that the inter-wagon flow undergoes a significant topology change over the range of G f r o n t = 1.77 W – 3.23 W (W = wagon width). For G f r o n t ≤ 1.77 W , the mean recirculating flow in the gap covers its entire length. In contrast, for G f r o n t ≳ 3.23 W , the complete wake closure for the upstream wagon occurs enabling the upstream shear layers to impinge on the entire downstream surface, in turn, increasing the rate of change of drag force as G f r o n t is increased. The change in the wake shedding frequency, directly affecting the base pressure and consequently the drag, due to the variation in G f r o n t and G b a s e , is presented. [ABSTRACT FROM AUTHOR]

Abstract: The experimental research reported here employs particle image velocimetry to extend the study of , recording detailed vorticity fields in the near-wake of a circular cylinder undergoing combined translational and rotational oscillatory motions. The focus of the present study is to examine the effect of the ratio between the cross-stream translational and rotational velocities and frequencies on the synchronization of the near-wake structures for multiple phase differences between the two motions. The frequencies are fixed close to that of the natural frequency of vortex shedding. The results are presented for a fixed amplitude of rotational oscillation of 1rad and a range of ratios between the translational and rotational velocities and for a range of frequency ratios . In particular, it was found that varying the V R value changed the near-wake structure. The results show that at the lower value of , for all of the phase differences examined, the vortices are shed in a single-row 2S mode aligned in the medial plane with a slight offset from the centreline and also synchronized with the combined oscillatory motion. As V R increases the vortex shedding mode changes from a 2S single-row to a 2S double-row structure and eventually back to the single-row (at . Increasing V R further resulted in the loss of lock-on over the range of negative phase angles and a transition from the 2S to P+S mode for the in-phase case. There was transition back to the 2S wake mode with a further decrease in . For higher V R the range of desynchronization increased. In the second and third parts of this paper it is shown that the occurrence of unlocked wake flow as the phase angle is varied is greater when the frequency ratio between the imposed oscillatory motions and the natural vortex shedding frequency is higher than unity. The vortices are synchronized in the near-wake at values less than unity and unlocked when . In particular, the near-wake structures have also been shown to be synchronized for and unlocked for . [Copyright &y& Elsevier]

Abstract: A numerical investigation of the flow past a circular cylinder centred in a two-dimensional channel of varying width is presented. For low Reynolds numbers, the flow is steady. For higher Reynolds numbers, vortices begin to shed periodically from the cylinder. In general, the Strouhal frequency of the shedding vortices increases with blockage ratio. In addition, a two-dimensional instability of the periodic vortex shedding is found, both empirically and by means of a Floquet stability analysis. The instability leads to a beating behaviour in the lift and drag coefficients of the cylinder, which occurs at a Reynolds number higher than the critical Reynolds number for the three-dimensional mode A-type instability, but lower than a Reynolds number for any mode B-type instability. [Copyright &y& Elsevier]

• Water channel experiments were conducted on cylindrical square-backed bodies. • Two different nose configurations were tested. • Separation induced by the nose generated greater asymmetry in the wake. • Greater separation over the nose inhibited coherent vortex shedding. • The upstream, overbody and wake flows exhibited Reynolds number sensitivity. An experimental study was conducted on flow around two-dimensional, square-backed bluff bodies in the presence of a ground plane. Two frontal geometries were tested: one with significant leading-edge separation and one without. The extensive leading-edge separation was found to induce significant asymmetry in the time-averaged wake. One-sided shedding from the top trailing edge was found to occur in the absence of significant separation, but broad-spectrum turbulence was found to be responsible for much of the fluctuating energy. More pronounced upstream separation was found to further inhibit regular shedding of large-scale structures in the wake. Over the range considered, a degree of Reynolds number sensitivity was observed, with the boundary layer along the ground plane separating upstream of the body at low Reynolds numbers. The effect of increasing the ground clearance was also considered, which weakened the leading-edge separation whilst increasing the prominence of vortex shedding. [ABSTRACT FROM AUTHOR]

A new class of vibrations has been discovered where the vibrations are induced by forced sinusoidal rotary oscillation of a sphere. They have been termed Rotary-Induced Vibrations (RIV). RIV is observed only for selected forcing parameters, where the vibrations are locked to the forcing frequency. For these cases, the vibration amplitude increases monotonically with an increase in U ∗ , similar to a galloping response. However, unlike the galloping response which is typically observed for high flow velocities and is generated by lift caused by body asymmetry, RIV is observed for the entire fundamental synchronisation regime of a sphere. RIV is found to be driven by vortex shedding as well as the imposed rotary oscillation, which causes an oscillatory Magnus force. As is typical of forced oscillations, the band of forcing frequencies that leads to RIV becomes wider with forcing amplitude, and is centred on the resonant condition for which the forcing frequency matches the natural system frequency. At a reduced velocity of 20, which is approximately 3 times the resonant reduced velocity, the vibration amplitude was more than 50% greater than the unforced case. [ABSTRACT FROM AUTHOR]

The flow-induced deformation of a viscoelastic thin plate attached to the lee side of a circular cylinder subjected to laminar flow is numerically investigated. An in-house fluid–structure interaction solver couples the sharp-interface immersed boundary method for fluid dynamics with a finite-element method for structural dynamics. Extending published results on elastic materials, the Standard Linear Solid (SLS) model is used to represent viscoelasticity of the plate, governed by the following two parameters: (a) the ratio of the non-equilibrium to equilibrium Young's modulus (R) , and (b) the ratio of dimensionless material damping to the dimensionless non-equilibrium Young's modulus (τ). The focus of the present study is to examine the dynamic response of the viscoelastic plate at Reynolds number of R e = 100. The amplitude and the time to achieve a dynamic steady state of a plate with 0. 1 < R < 5 and 0. 1 < τ < 10 have been investigated. The plate attains a self-sustained time-periodic oscillation with a plateau amplitude for the range of R = [0.1–5] for all values of τ. The dimensionless tip displacement amplitude (A Y , t i p) is found to be a non-monotonic function of τ. When the forcing frequency (vortex shedding frequency) is lesser (greater) than the natural frequency (considering the equilibrium Young modulus) of the plate, A Y , t i p decreases (increases) asymptotically. The theoretical analysis of a simple spring–mass–dashpot model of SLS is undertaken to understand the effect of sinusoidal forcing on the plate dynamics. Analytical predictions show general agreement with the non-monotonic behaviour of the numerically computed A Y , t i p. The results suggest that careful tuning of the damping may be effectively employed to enhance power output for energy extraction applications or to suppress flow-induced vibration when it is detrimental to the structure. [ABSTRACT FROM AUTHOR]

This is a numerical investigation of the unsteady mixed convective flow over an isothermally heated circular cylinder. The flow is directed vertically downwards with buoyancy forces in the opposite direction. A parametric analysis was conducted at five Reynolds numbers Re = 10 , 20 , 30 , 40 , 100 , and for Richardson numbers between Ri = 0 and 0.5, to evaluate the influence of thermal buoyancy on the instantaneous flow behaviour, vortex-shedding characteristics, thermal fields, and mean heat-transfer rates. The simulations show that at the highest Reynolds number investigated, Re = 100 , the normal periodic von Kármán vortex street still characterises the flow as Richardson number is increased; however, the oscillation frequency decreases and the oscillation amplitude increases. For Re ≤ 40 , a new vortex street appears, named the " Buoyancy-Opposing Vortex Street ", due to thermal buoyancy modifying the von Kármán street. Interestingly, at very low Re = 20 and 10, the von Kármán street completely disappears in the heated wakes, whereas the "Buoyancy-Opposing Vortex Street" remains as the sole vortex street characterising the flow. • Unsteady mixed convective flow vertically downward directed to across a heated circular cylinder, is studied. • The analysis was conducted at Re = 10, 20, 30, 40, 100 and for Richardson number ranging between Ri = 0–0.5. • The aim is to evaluate the influence of the buoyancy effects on the vortex shedding attributes. • Unusually Kármán vortex street is generated for R e ⩽ 40 by heating the cylinder. • A new vortex street, namely the "Opposing-Buoyancy Vortex Street", is discovered by heating the Kármán vortex street. [ABSTRACT FROM AUTHOR]

This paper reports on an extensive numerical investigation of the effects of pivot location and mass ratio ( m ∗ = solid/fluid mass) on flow-induced vibration (FIV) of a foil undergoing fully passive two-degree-of-freedom (2-DOF) plunging and pitching motion in a two-dimensional free-stream flow. Here, the normalised pivot location is defined by x = x p ∕ c , with c the foil length and x p the distance to the foil leading edge. A comprehensive set of numerical simulations were conducted employing an Immersed Boundary Method at a Reynolds number of 400. By analysing the FIV dynamics for three selected mass ratios, m ∗ = 5 , 20 and 200, at two pivot locations, x = 0. 35 and 0.50, it is found that there are two types (type-I and type-II) of FIV responses, one is primarily a driven static instability while the other is strongly associated with vortex shedding. Interestingly, for x = 0. 50 , which is close to the mass centre, increasing the mass ratio can favour suppression of the chaotic response. Importantly, it is shown that there exists a critical mass ratio, above which the foil oscillations are suddenly suppressed. The findings indicate that the combined effects of eccentricity and mass ratio on the foil dynamics can be profound. [ABSTRACT FROM AUTHOR]

In this study, we perform a numerical investigation to better comprehend the domination and the effect of strong thermal buoyancy on hydrodynamic and thermal characteristics, vortex shedding in particular, of a horizontal circular heated cylinder subject to a vertically upward laminar flow stream. The Reynolds number is varied in the range 20 ≤ R e ≤ 150 while maintaining the Prandtl number constant at P r = 7.1. The effect of thermal buoyancy on the system is determined by varying the Richardson number up to R i = 5. The behaviour is determined by solving the unsteady laminar two-dimensional Navier–Stokes and standard energy equations numerically, utilising the spectral-element method with buoyancy considered using the Boussinesq approximation. Representative vorticity, streamline, and thermal patterns are presented, and the average Nusselt numbers are plotted as a function of the Richardson number for a set of Reynolds numbers to explain, in detail, the role of superimposed thermal buoyancy on the wake flow and rates of heat transfer. The predictions demonstrate that the wake flow exhibits unsteady periodic characteristics for the selected moderate Reynolds numbers, and as the buoyancy parameter increases, the fluid flow and temperature fields become less stable. Subsequently, it was observed that heat transfer augmented considerably, the vortex shedding stopped abruptly, the wake converted to steady twin vortices beyond certain critical Richardson numbers, and the Nusselt numbers suddenly reduced to minimum values. These critical values are determined to increase with the Reynolds number. The results also confirm that for increased heating, the recirculating flow in the cylinder near the wake disappears and flow separation occurs only at the backward stagnation point. [ABSTRACT FROM AUTHOR]

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes. • Strong wake counter flow is an essential feature of symmetric vortex shedding. • Wake counter flow depends on wall–vorticity interactions in the near wake. • The nonlinear vorticity production due to wake counter flow can be significant. • Vorticity generated at base wall can also be shed in to wake. • The differing mechanisms of vortex formation can interact in the near wake. [ABSTRACT FROM AUTHOR]