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Kerry Hourigan

Monash University, Department of Mechanical and Aerospace Engineering, Faculty Member

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Fluid dynamics, bluff body wakes, transitions, flow-induced vibrations, aeronautics.
Phone: +61(0)399059624
Address: Department of Mechanical and Aerospace Engineering
Monash University
Wellington Road
Clayton 3800
Australia

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Papers by Kerry Hourigan

Flow around a tethered neutrally-buoyant sphere

The sphere is the generic geometry for three-dimensional bluff body flows. Here, the flow-induced... more The sphere is the generic geometry for three-dimensional bluff body flows. Here, the flow-induced vibrations of a tethered sphere is investigated for the special case of neutral buoyancy. A high order spectral element method is employed to predict the wake structures and oscillations of a sphere over a range of Reynolds numbers. The predicted transitions in the wake are related to those observed for the fixed sphere case. Experimental validation and the effect of slight buoyancy will also be presented at the conference.

Exotic wakes of an oscillating circular cylinder: how singles pair up

Journal of Fluid Mechanics

Fascinating wake vortex patterns emerge when a circular cylinder is forced to vibrate laterally t... more Fascinating wake vortex patterns emerge when a circular cylinder is forced to vibrate laterally to a uniform fluid flow, deviating from the well-known Kármán vortex street and first reported by Williamson & Roshko (J. Fluids Struct., vol. 2, 1988, pp. 355-381). The two rows of single vortices (2S mode) can suddenly transition to a row of paired vortices and a row of single vortices (P+S mode) as the forcing amplitude is increased. Further increase in amplitude finds another sudden jump back to the 2S mode. Through a series of elegant and carefully crafted numerical simulations, Matharu et al. (J. Fluid Mech., vol. 918, 2021, A42) determine that the transitions occur via bifurcations, but that underlying these observed 'jumps', a continuous evolution of the vortex street between the modes is seen along unstable branches connecting the two modes. As the Reynolds number decreases from 100, bistability and the P+S mode are eventually suppressed.

RESEARCH ARTICLE Power-Spectral density estimate of the Bloor-Gerrard instability in flows around circular cylinders

There have been differences in the literature concerning the power law relationship between the B... more There have been differences in the literature concerning the power law relationship between the Bloor-Gerrard instability frequency of the separated shear layer from the circular cylinder, the Bénard-von Kármán vortex shedding frequency and the Reynolds number. Most pre-vious experiments have shown a significant degree of scatter in the measurement of the development of the shear layer vortices. Shear layers are known to be sensitive to external influences, which can provide a by-pass transition to saturated growth, thereby camouflaging the fastest growing linear modes. Here, the spatial amplification rates of the shear layer instabilities are calculated using power-spectral density estimates, allowing the fastest growing modes rather than necessarily the largest structures to be determined. This method is found to be robust in deter-mining the fastest growing modes, producing results con-sistent with the low scatter results of previous experiments. 1

© 2004 Kluwer Academic Publishers. Printed in the Netherlands. 347 The Unsteady Wake of a Circular Cylinder near a

Abstract. The behaviour of the wake Strouhal number for flow past a cylinder close to a free surf... more Abstract. The behaviour of the wake Strouhal number for flow past a cylinder close to a free surface at both low and moderate Froude numbers is investigated numerically. For the low Froude number case (i.e., gravity-dominated), the results obtained are similar to those for flow past a cylinder close to an adjacent no-slip boundary. As the distance between the wall and the cylinder is reduced, the Strouhal number, as measured from the time varying lift, increases to a maximum at a gap ratio of 0.70. Further gap reduction leads to a rapid decrease in the Strouhal number, with shedding finally ceasing altogether at gap ratios below 0.16. The agreement between the results for a free surface and a no-slip boundary suggests that the mechanism behind the suppression of vortex shedding is common. For flow at a fixed gap ratio and a moderate Froude number, two distinctly different wake states are observed with the flow passing over the cylinder tending to switch from a state of attachment to...

Vortex-induced vibration of a transversely rotating sphere

Journal of Fluid Mechanics, 2018

The effects of transverse rotation on the vortex-induced vibration (VIV) of a sphere in a uniform... more The effects of transverse rotation on the vortex-induced vibration (VIV) of a sphere in a uniform flow are investigated numerically. The one degree-of-freedom sphere motion is constrained to the cross-stream direction, with the rotation axis orthogonal to flow and vibration directions. For the current simulations, the Reynolds number of the flow, $Re=UD/\unicode[STIX]{x1D708}$, and the mass ratio of the sphere, $m^{\ast }=\unicode[STIX]{x1D70C}_{s}/\unicode[STIX]{x1D70C}_{f}$, were fixed at 300 and 2.865, respectively, while the reduced velocity of the flow was varied over the range $3.5\leqslant U^{\ast }~(\equiv U/(f_{n}D))\leqslant 11$, where, $U$ is the upstream velocity of the flow, $D$ is the sphere diameter, $\unicode[STIX]{x1D708}$ is the fluid viscosity, $f_{n}$ is the system natural frequency and $\unicode[STIX]{x1D70C}_{s}$ and $\unicode[STIX]{x1D70C}_{f}$ are solid and fluid densities, respectively. The effect of sphere rotation on VIV was studied over a wide range of no...

Vortex-induced vibrations of a sphere close to a free surface

Journal of Fluid Mechanics, 2018

Results are presented from an experimental investigation into the effects of proximity to a free ... more Results are presented from an experimental investigation into the effects of proximity to a free surface on vortex-induced vibration (VIV) experienced by fully and semi-submerged spheres that are free to oscillate in the cross-flow direction. The VIV response is studied over a wide range of reduced velocities: $3\leqslant U^{\ast }\leqslant 20$, covering the mode I, mode II and mode III resonant response branches and corresponding to the Reynolds number range of $5000\lesssim Re\lesssim 30\,000$. The normalised immersion depth of the sphere is varied in small increments over the range $0\leqslant h^{\ast }\leqslant 1$ for the fully submerged case and $0\leqslant h^{\ast }\leqslant -0.75$ for the semi-submerged case. It is found that for a fully submerged sphere, the vibration amplitude decreases monotonically and gradually as the immersion ratio is decreased progressively, with a greater influence on the mode II and III parts of the response curve. The synchronisation regime becomes...

An experimental investigation of flow-induced vibration of high-side-ratio rectangular cylinders

Journal of Fluids and Structures, 2019

An experimental investigation of flow-induced vibration of high-side-ratio rectangular cylinders.

Vortex-induced vibration of a rotating sphere

Journal of Fluid Mechanics, 2017

Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–s... more Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–structure interaction problems. Since vortex-induced vibration can lead to structural failure, numerous studies have focused on understanding the underlying principles of VIV and its suppression. This paper reports on an experimental investigation of the effect of imposed axial rotation on the dynamics of vortex-induced vibration of a sphere that is free to oscillate in the cross-flow direction, by employing simultaneous displacement and force measurements. The VIV response was investigated over a wide range of reduced velocities (i.e. velocity normalised by the natural frequency of the system): $3\leqslant U^{\ast }\leqslant 18$, corresponding to a Reynolds number range of $5000

Vortex wake and energy transitions of an oscillating cylinder at low Reynolds number

ANZIAM Journal, 2005

As a means of understanding the phenomenon of vortex-induced vibration, the forced oscillation of... more As a means of understanding the phenomenon of vortex-induced vibration, the forced oscillation of a cylinder in cross-flow has been examined. A lack of information regarding the connections between energy transfer and the wake mode of oscillating bodies in low Reynolds number flow prompted a two-dimensional numerical investigation at Re = 200. The region in which vortex shedding synchronized with the cylinder forcing frequency was defined and the energy transfer calculated. The mode of Kármán vortex shedding incorporated a gradual change from positive to negative energy transfer as the amplitude of the motion was increased. A mode of shedding was detected in the negative energy region in which a pair and a single vortex (P+S) were shed each motion cycle. Energy contours were established in the region of primary lock-in and the boundary of zero energy transfer was defined.

Fluid-Structure Interaction of a Rolling Cylinder with Offset Centre-of-Mass

Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2016

With the aim of understanding discrepancies between experimental observations and numerical simul... more With the aim of understanding discrepancies between experimental observations and numerical simulations for a cylinder rolling down an inclined plane, this study investigates the effect that offsetting the centre-of-mass from the cylinder centroid has on body forces, velocity and wake structures. The numerical cases considered focus on the same parameters as the referenced experiment: cylinder-to-fluid density ratio and wall inclination angle, for Reynolds numbers in a range around the critical value for the transition from stationary flow to periodic vortex shedding. The centre-of-mass is placed at a distance of up to 2 % of the diameter from the geometrical centre of the cylinder. It is found that the main features of the predicted wake flow are in good agreement with those observed experimentally. They include the inception of small-scale shear-layer vortices in the near wake, locked to the cylinder rotational frequency, as well as large-scale vortices further downstream. This is further confirmed through force and velocity histories, where two oscillations are found to operate at significantly different frequencies. While the amplitudes of the lift, drag and cylinder velocity oscillations see an increase with offset distance, the Strouhal numbers of the small-and large-scale structures remain unaffected and agree well with those measured in experiments at similar Reynolds numbers. Keywords Bluff-body wake • Fluid-structure interaction • Rolling cylinder •

Numerical Simulations of the Flow-Induced Vibrations of Tethered Bluff Bodies

5th International Symposium on Fluid Structure International, Aeroeslasticity, and Flow Induced Vibration and Noise, 2002

One of the most basic examples of fluid-structure interaction is provided by a tethered cylinder ... more One of the most basic examples of fluid-structure interaction is provided by a tethered cylinder or sphere in a fluid flow. The tendency of a tethered sphere to oscillate when excited by waves is a well-known phenomenon and it has only recently been found that the same system will act in a similar fashion when exposed to a uniform flow at moderate Reynolds numbers, with a transverse peak-to-peak amplitude of approximately two diameters over a wide range of velocities. The present paper presents results of DNS of the flow past a tethered cylinder. The coupled Navier-Stokes equations and the equations of motion of the cylinder are solved using a spectral element method. The fluid forces acting on the cylinder as well as the tension in the tether are computed and used to determine the resulting motion of the object. It is found that the mean amplitude response is greatest at high reduced velocities, i.e. when the cylinder is oscillating predominantly transverse to the fluid flow. Furthermore, the oscillation frequency is found to correspond to the vortex shedding frequency of a stationary cylinder, except at high reduced velocities. This is in contrast to a tethered sphere in which the oscillation frequency does not correspond to either the vortex shedding frequency or the natural frequency. Visualizations of the vortex structures in the wake reveal the mechanisms behind the motion of the cylinder, and suggest that the induced oscillations are highly significant in the prediction of cylinder response in a steady flow.

The Existence of Multiple Solutions for Rotating Cylinder Flows

A rotating circular cylinder can produce a substantial lift force, whilst at the same time reduci... more A rotating circular cylinder can produce a substantial lift force, whilst at the same time reducing drag to a minimal level. Amongst other things, they have been successfully used to replace sails for marine vessels, and have potential applications for MAVs. Recently, it has been shown that the flow past a rotating circular cylinder can admit multiple steady-state flow solutions, which, in turn, explains the seemingly sudden change in the lift curve as the rotation rate is increased. The current study extends that research to show that at higher Reynolds numbers, the possible flow states extend non-trivially, with the unique solution at lower rotation rates splitting into three remarkably different solution branches, with surprisingly different flow structures.

Flow dynamics and forces associated with a cylinder rolling along a wall

Physics of Fluids, 2006

The wake flow structures and the drag force for a cylinder rolling along a wall without slipping ... more The wake flow structures and the drag force for a cylinder rolling along a wall without slipping were calculated for the Reynolds number range 20<Re<200, covering the two-dimensional shedding regime. Time-dependent numerical computations show the wake undergoes a steady to periodic shedding transition between 85<Re<90. The Strouhal number varies only weakly at higher Reynolds number, and is a factor of 3–4 lower than for an isolated rotating or nonrotating body. Also, within this shedding regime, the wake is characterized by counter-rotating vortex pairs, which propagate away from the wall via mutual induction. These pairs are formed as compact vortex structures from the top separating shear layer induce secondary vorticity at the wall, which is pulled up from the boundary to form the semidiscrete flow structures. Over both the steady and unsteady regimes, the (time-mean) recirculation length and drag are quantified.

The shear-layer instability of a circular cylinder wake

Physics of Fluids, 2005

A reinterpretation is made of previously published data concerning the frequency of the instabili... more A reinterpretation is made of previously published data concerning the frequency of the instability waves in the separated shear layer from a circular cylinder for Reynolds numbers in the range 103–105. An accurate fit to the observed variation can be achieved using a piecewise fit based on theoretical and empirical arguments. A logical conclusion is that the ratio of the frequency of the instability waves to the Kármán vortex shedding frequency is indeed determined by the boundary-layer properties at separation, as suggested by Bloor.

Wake state and energy transitions of an oscillating cylinder at low Reynolds number

Physics of Fluids, 2006

This paper reports on an extensive parameter space study of two-dimensional simulations of a circ... more This paper reports on an extensive parameter space study of two-dimensional simulations of a circular cylinder forced to oscillate transverse to the free-stream. In particular, the extent of the primary synchronization region, and the wake modes and energy transfer between the body and the fluid are analyzed in some detail. The frequency range of the primary synchronization region is observed to be dependent on Reynolds number, as are the wake modes obtained. Energy transfer is primarily dependent on frequency at low amplitudes of oscillation, but primarily dependent on amplitude at high amplitudes of oscillation. However, the oscillation amplitude corresponding to zero energy transfer is found to be relatively insensitive to Reynolds number. It is also found that there is no discernible change to the wake structure when the energy transfer changes from positive to negative.

Longitudinal vortex structures in a cylinder wake

Physics of Fluids, 1994

This paper presents the velocity field of the longitudinal vortices found in the wake of a circul... more This paper presents the velocity field of the longitudinal vortices found in the wake of a circular cylinder, as measured using digital particle image velocimetry (PIV). Vorticity and circulation of the longitudinal vortices are presented, based on instantaneous velocity distributions in a transverse plane behind the cylinder.

Flow around an impulsively arrested circular cylinder

Physics of Fluids, 2007

The vortex dynamics of the flow around a suddenly arrested translating circular cylinder is inves... more The vortex dynamics of the flow around a suddenly arrested translating circular cylinder is investigated by direct numerical simulation and water tank experiments. In the numerical study, a method of visualization of streaklines in simulated-particle tracking computations is proposed, which is based on a variable-variance two-dimensional Gaussian-weighted summation of particles in the vicinity of each interpolation point, and for which a close similarity with physical dye visualizations is found. This technique is used to identify the trajectory of both the wake vortices, as well as the secondary vortices induced as the original wake convects over the arrested cylinder. Observations show that, in a fashion similar to the flow past an arresting sphere, each wake vortex induces a counter-rotating vortex pair, which subsequently self-propels over a range of sometimes surprising trajectories as the Reynolds number and cylinder translation distance are varied. At low Reynolds numbers and...

Experimental investigation of vortex shedding from a plate: effect of external velocity perturbation

Journal of Wind Engineering and Industrial Aerodynamics, 1993

Vortex shedding behind a plate with a square trailing edge was investigated in a water tunnel. Bo... more Vortex shedding behind a plate with a square trailing edge was investigated in a water tunnel. Both the case where the shedding occurs naturally and the case where the shedding is "forced" by an applied transverse velocity field were studied. The perturbation velocity at the trailing edge of the plate was calibrated by accurately measuring the velocity at this point using laser doppler anemometry. The spanwise cross-correlation of the fluctuating velocity was determined based on velocity measurements taken at two different spanwise separations; hot film sensors were used to measure the velocities. It was found that high spanwise correlation of the vortex shedding could be induced by applying a low level perturbation at the natural shedding frequency. It was found that there was a threshold below which the vortex shedding was not well correlated. To prove that the correlations were due to an ordering of the vortices as they were shed, rather than being mainly due to the perturbation field, validation tests were conducted. The phenomenon was also studied by flow visualization using hydrogen bubbles illuminated by a laser. In natural shedding, spanwise waviness of the vortices could be seen clearly in the cross-stream view. This explains the limited spanwise correlation length for this case. On the application of a low-level perturbation this waviness disappeared, corresponding to the high degree of spanwise correlation measured by the hot films.

Sound generated in laminar flow past a two-dimensional rectangular cylinder

Journal of Sound and Vibration, 2006

The far-field sound generated from low Mach number flow past a two-dimensional rectangular cylind... more The far-field sound generated from low Mach number flow past a two-dimensional rectangular cylinder is studied by using a two-step aeroacoustic prediction method. In the first step, the incompressible Navier-Stokes equations are solved numerically. This allows the time-dependent acoustic source to be determined from Powell's vortex sound theory. Using this information, in the second step, the inhomogeneous wave equation is solved numerically to predict the time-evolving acoustic field. This study considers the effects of the Reynolds and Mach number on the sound generation and propagation characteristics. Results show that acoustic wave generation can be associated with the shedding of vortices at both the leading and trailing edges of the cylinder. In particular, an attempt is made to quantify the individual contributions, showing that the trailing-edge region is a considerably stronger source. Similar to the case for a circular cylinder, the predicted sound field has a dipolar far-field directivity with the lift dipole dominating. However, the drag dipole becomes relatively more important as the Reynolds number is increased. Overall, the relative amplitude of the far-field acoustic signal increases substantially with Reynolds number. In addition, as the Reynolds number is increased, the far-field pressure signal contains significant harmonic content, unlike the situation at the lowest Reynolds number investigated. A harmonic decomposition in terms of polar angle allows the multipole content of the signal to be quantified. Results showed that the acoustic field is dominated by the lift forcing which is predominantly dipolar, at least up to a Mach number (Ma) of 0.2. While this is also true for the drag forcing for low Mach numbers, we found that for Ma40:1, the quadrupole term is of a magnitude comparable to the drag dipole. By taking into account the Doppler effect through a spatial transformation of the predicted acoustic solution, the dipolar field becomes skewed towards the upstream direction as the Mach number is increased. Various difficulties associated with direct acoustic computations, such as convergence problems associated with initial transients and grid stretching, and the introduction of errors from under-resolving the acoustic source, and treatment of the slowly decaying wake, are discussed. The methods used to overcome these problems are reported.

Particle image velocimetry and visualization of natural and forced flow around rectangular cylinders

Journal of Fluid Mechanics, 2003

Particle image velocimetry (PIV) measurements and flow visualization in a water tunnel show that ... more Particle image velocimetry (PIV) measurements and flow visualization in a water tunnel show that vortex shedding at the leading and trailing edges of rectangular cylinders can be simultaneously phase-locked to transverse velocity perturbations when the applied perturbation Stp is close to an impinging leading-edge vortex/trailing-edge vortex shedding (ILEV/TEVS) frequency. The transverse perturbations, analogous to β-mode duct acoustic resonances, are generated through harmonic oscillations of the sidewalls. When this occurs, the leading-edge vortices are found always to pass the trailing edge at the same phase in the perturbation cycle regardless of the chord-to-thickness (c/t) ratio. Applying perturbations at an Stp not equal to the natural global frequency also results in phase-locked vortex shedding from the leading edge, and a near wake with a frequency equal to the perturbation frequency. This is consistent with previous experimental findings. However, vortex shedding at the t...

Flow around a tethered neutrally-buoyant sphere

The sphere is the generic geometry for three-dimensional bluff body flows. Here, the flow-induced... more The sphere is the generic geometry for three-dimensional bluff body flows. Here, the flow-induced vibrations of a tethered sphere is investigated for the special case of neutral buoyancy. A high order spectral element method is employed to predict the wake structures and oscillations of a sphere over a range of Reynolds numbers. The predicted transitions in the wake are related to those observed for the fixed sphere case. Experimental validation and the effect of slight buoyancy will also be presented at the conference.

Exotic wakes of an oscillating circular cylinder: how singles pair up

Journal of Fluid Mechanics

Fascinating wake vortex patterns emerge when a circular cylinder is forced to vibrate laterally t... more Fascinating wake vortex patterns emerge when a circular cylinder is forced to vibrate laterally to a uniform fluid flow, deviating from the well-known Kármán vortex street and first reported by Williamson & Roshko (J. Fluids Struct., vol. 2, 1988, pp. 355-381). The two rows of single vortices (2S mode) can suddenly transition to a row of paired vortices and a row of single vortices (P+S mode) as the forcing amplitude is increased. Further increase in amplitude finds another sudden jump back to the 2S mode. Through a series of elegant and carefully crafted numerical simulations, Matharu et al. (J. Fluid Mech., vol. 918, 2021, A42) determine that the transitions occur via bifurcations, but that underlying these observed 'jumps', a continuous evolution of the vortex street between the modes is seen along unstable branches connecting the two modes. As the Reynolds number decreases from 100, bistability and the P+S mode are eventually suppressed.

RESEARCH ARTICLE Power-Spectral density estimate of the Bloor-Gerrard instability in flows around circular cylinders

There have been differences in the literature concerning the power law relationship between the B... more There have been differences in the literature concerning the power law relationship between the Bloor-Gerrard instability frequency of the separated shear layer from the circular cylinder, the Bénard-von Kármán vortex shedding frequency and the Reynolds number. Most pre-vious experiments have shown a significant degree of scatter in the measurement of the development of the shear layer vortices. Shear layers are known to be sensitive to external influences, which can provide a by-pass transition to saturated growth, thereby camouflaging the fastest growing linear modes. Here, the spatial amplification rates of the shear layer instabilities are calculated using power-spectral density estimates, allowing the fastest growing modes rather than necessarily the largest structures to be determined. This method is found to be robust in deter-mining the fastest growing modes, producing results con-sistent with the low scatter results of previous experiments. 1

© 2004 Kluwer Academic Publishers. Printed in the Netherlands. 347 The Unsteady Wake of a Circular Cylinder near a

Abstract. The behaviour of the wake Strouhal number for flow past a cylinder close to a free surf... more Abstract. The behaviour of the wake Strouhal number for flow past a cylinder close to a free surface at both low and moderate Froude numbers is investigated numerically. For the low Froude number case (i.e., gravity-dominated), the results obtained are similar to those for flow past a cylinder close to an adjacent no-slip boundary. As the distance between the wall and the cylinder is reduced, the Strouhal number, as measured from the time varying lift, increases to a maximum at a gap ratio of 0.70. Further gap reduction leads to a rapid decrease in the Strouhal number, with shedding finally ceasing altogether at gap ratios below 0.16. The agreement between the results for a free surface and a no-slip boundary suggests that the mechanism behind the suppression of vortex shedding is common. For flow at a fixed gap ratio and a moderate Froude number, two distinctly different wake states are observed with the flow passing over the cylinder tending to switch from a state of attachment to...

Vortex-induced vibration of a transversely rotating sphere

Journal of Fluid Mechanics, 2018

The effects of transverse rotation on the vortex-induced vibration (VIV) of a sphere in a uniform... more The effects of transverse rotation on the vortex-induced vibration (VIV) of a sphere in a uniform flow are investigated numerically. The one degree-of-freedom sphere motion is constrained to the cross-stream direction, with the rotation axis orthogonal to flow and vibration directions. For the current simulations, the Reynolds number of the flow, $Re=UD/\unicode[STIX]{x1D708}$, and the mass ratio of the sphere, $m^{\ast }=\unicode[STIX]{x1D70C}_{s}/\unicode[STIX]{x1D70C}_{f}$, were fixed at 300 and 2.865, respectively, while the reduced velocity of the flow was varied over the range $3.5\leqslant U^{\ast }~(\equiv U/(f_{n}D))\leqslant 11$, where, $U$ is the upstream velocity of the flow, $D$ is the sphere diameter, $\unicode[STIX]{x1D708}$ is the fluid viscosity, $f_{n}$ is the system natural frequency and $\unicode[STIX]{x1D70C}_{s}$ and $\unicode[STIX]{x1D70C}_{f}$ are solid and fluid densities, respectively. The effect of sphere rotation on VIV was studied over a wide range of no...

Vortex-induced vibrations of a sphere close to a free surface

Journal of Fluid Mechanics, 2018

Results are presented from an experimental investigation into the effects of proximity to a free ... more Results are presented from an experimental investigation into the effects of proximity to a free surface on vortex-induced vibration (VIV) experienced by fully and semi-submerged spheres that are free to oscillate in the cross-flow direction. The VIV response is studied over a wide range of reduced velocities: $3\leqslant U^{\ast }\leqslant 20$, covering the mode I, mode II and mode III resonant response branches and corresponding to the Reynolds number range of $5000\lesssim Re\lesssim 30\,000$. The normalised immersion depth of the sphere is varied in small increments over the range $0\leqslant h^{\ast }\leqslant 1$ for the fully submerged case and $0\leqslant h^{\ast }\leqslant -0.75$ for the semi-submerged case. It is found that for a fully submerged sphere, the vibration amplitude decreases monotonically and gradually as the immersion ratio is decreased progressively, with a greater influence on the mode II and III parts of the response curve. The synchronisation regime becomes...

An experimental investigation of flow-induced vibration of high-side-ratio rectangular cylinders

Journal of Fluids and Structures, 2019

An experimental investigation of flow-induced vibration of high-side-ratio rectangular cylinders.

Vortex-induced vibration of a rotating sphere

Journal of Fluid Mechanics, 2017

Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–s... more Vortex-induced vibration (VIV) of a sphere represents one of the most generic fundamental fluid–structure interaction problems. Since vortex-induced vibration can lead to structural failure, numerous studies have focused on understanding the underlying principles of VIV and its suppression. This paper reports on an experimental investigation of the effect of imposed axial rotation on the dynamics of vortex-induced vibration of a sphere that is free to oscillate in the cross-flow direction, by employing simultaneous displacement and force measurements. The VIV response was investigated over a wide range of reduced velocities (i.e. velocity normalised by the natural frequency of the system): $3\leqslant U^{\ast }\leqslant 18$, corresponding to a Reynolds number range of $5000

Vortex wake and energy transitions of an oscillating cylinder at low Reynolds number

ANZIAM Journal, 2005

As a means of understanding the phenomenon of vortex-induced vibration, the forced oscillation of... more As a means of understanding the phenomenon of vortex-induced vibration, the forced oscillation of a cylinder in cross-flow has been examined. A lack of information regarding the connections between energy transfer and the wake mode of oscillating bodies in low Reynolds number flow prompted a two-dimensional numerical investigation at Re = 200. The region in which vortex shedding synchronized with the cylinder forcing frequency was defined and the energy transfer calculated. The mode of Kármán vortex shedding incorporated a gradual change from positive to negative energy transfer as the amplitude of the motion was increased. A mode of shedding was detected in the negative energy region in which a pair and a single vortex (P+S) were shed each motion cycle. Energy contours were established in the region of primary lock-in and the boundary of zero energy transfer was defined.

Fluid-Structure Interaction of a Rolling Cylinder with Offset Centre-of-Mass

Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2016

With the aim of understanding discrepancies between experimental observations and numerical simul... more With the aim of understanding discrepancies between experimental observations and numerical simulations for a cylinder rolling down an inclined plane, this study investigates the effect that offsetting the centre-of-mass from the cylinder centroid has on body forces, velocity and wake structures. The numerical cases considered focus on the same parameters as the referenced experiment: cylinder-to-fluid density ratio and wall inclination angle, for Reynolds numbers in a range around the critical value for the transition from stationary flow to periodic vortex shedding. The centre-of-mass is placed at a distance of up to 2 % of the diameter from the geometrical centre of the cylinder. It is found that the main features of the predicted wake flow are in good agreement with those observed experimentally. They include the inception of small-scale shear-layer vortices in the near wake, locked to the cylinder rotational frequency, as well as large-scale vortices further downstream. This is further confirmed through force and velocity histories, where two oscillations are found to operate at significantly different frequencies. While the amplitudes of the lift, drag and cylinder velocity oscillations see an increase with offset distance, the Strouhal numbers of the small-and large-scale structures remain unaffected and agree well with those measured in experiments at similar Reynolds numbers. Keywords Bluff-body wake • Fluid-structure interaction • Rolling cylinder •

Numerical Simulations of the Flow-Induced Vibrations of Tethered Bluff Bodies

5th International Symposium on Fluid Structure International, Aeroeslasticity, and Flow Induced Vibration and Noise, 2002

One of the most basic examples of fluid-structure interaction is provided by a tethered cylinder ... more One of the most basic examples of fluid-structure interaction is provided by a tethered cylinder or sphere in a fluid flow. The tendency of a tethered sphere to oscillate when excited by waves is a well-known phenomenon and it has only recently been found that the same system will act in a similar fashion when exposed to a uniform flow at moderate Reynolds numbers, with a transverse peak-to-peak amplitude of approximately two diameters over a wide range of velocities. The present paper presents results of DNS of the flow past a tethered cylinder. The coupled Navier-Stokes equations and the equations of motion of the cylinder are solved using a spectral element method. The fluid forces acting on the cylinder as well as the tension in the tether are computed and used to determine the resulting motion of the object. It is found that the mean amplitude response is greatest at high reduced velocities, i.e. when the cylinder is oscillating predominantly transverse to the fluid flow. Furthermore, the oscillation frequency is found to correspond to the vortex shedding frequency of a stationary cylinder, except at high reduced velocities. This is in contrast to a tethered sphere in which the oscillation frequency does not correspond to either the vortex shedding frequency or the natural frequency. Visualizations of the vortex structures in the wake reveal the mechanisms behind the motion of the cylinder, and suggest that the induced oscillations are highly significant in the prediction of cylinder response in a steady flow.

The Existence of Multiple Solutions for Rotating Cylinder Flows

A rotating circular cylinder can produce a substantial lift force, whilst at the same time reduci... more A rotating circular cylinder can produce a substantial lift force, whilst at the same time reducing drag to a minimal level. Amongst other things, they have been successfully used to replace sails for marine vessels, and have potential applications for MAVs. Recently, it has been shown that the flow past a rotating circular cylinder can admit multiple steady-state flow solutions, which, in turn, explains the seemingly sudden change in the lift curve as the rotation rate is increased. The current study extends that research to show that at higher Reynolds numbers, the possible flow states extend non-trivially, with the unique solution at lower rotation rates splitting into three remarkably different solution branches, with surprisingly different flow structures.

Flow dynamics and forces associated with a cylinder rolling along a wall

Physics of Fluids, 2006

The wake flow structures and the drag force for a cylinder rolling along a wall without slipping ... more The wake flow structures and the drag force for a cylinder rolling along a wall without slipping were calculated for the Reynolds number range 20<Re<200, covering the two-dimensional shedding regime. Time-dependent numerical computations show the wake undergoes a steady to periodic shedding transition between 85<Re<90. The Strouhal number varies only weakly at higher Reynolds number, and is a factor of 3–4 lower than for an isolated rotating or nonrotating body. Also, within this shedding regime, the wake is characterized by counter-rotating vortex pairs, which propagate away from the wall via mutual induction. These pairs are formed as compact vortex structures from the top separating shear layer induce secondary vorticity at the wall, which is pulled up from the boundary to form the semidiscrete flow structures. Over both the steady and unsteady regimes, the (time-mean) recirculation length and drag are quantified.

The shear-layer instability of a circular cylinder wake

Physics of Fluids, 2005

A reinterpretation is made of previously published data concerning the frequency of the instabili... more A reinterpretation is made of previously published data concerning the frequency of the instability waves in the separated shear layer from a circular cylinder for Reynolds numbers in the range 103–105. An accurate fit to the observed variation can be achieved using a piecewise fit based on theoretical and empirical arguments. A logical conclusion is that the ratio of the frequency of the instability waves to the Kármán vortex shedding frequency is indeed determined by the boundary-layer properties at separation, as suggested by Bloor.

Wake state and energy transitions of an oscillating cylinder at low Reynolds number

Physics of Fluids, 2006

This paper reports on an extensive parameter space study of two-dimensional simulations of a circ... more This paper reports on an extensive parameter space study of two-dimensional simulations of a circular cylinder forced to oscillate transverse to the free-stream. In particular, the extent of the primary synchronization region, and the wake modes and energy transfer between the body and the fluid are analyzed in some detail. The frequency range of the primary synchronization region is observed to be dependent on Reynolds number, as are the wake modes obtained. Energy transfer is primarily dependent on frequency at low amplitudes of oscillation, but primarily dependent on amplitude at high amplitudes of oscillation. However, the oscillation amplitude corresponding to zero energy transfer is found to be relatively insensitive to Reynolds number. It is also found that there is no discernible change to the wake structure when the energy transfer changes from positive to negative.

Longitudinal vortex structures in a cylinder wake

Physics of Fluids, 1994

This paper presents the velocity field of the longitudinal vortices found in the wake of a circul... more This paper presents the velocity field of the longitudinal vortices found in the wake of a circular cylinder, as measured using digital particle image velocimetry (PIV). Vorticity and circulation of the longitudinal vortices are presented, based on instantaneous velocity distributions in a transverse plane behind the cylinder.

Flow around an impulsively arrested circular cylinder

Physics of Fluids, 2007

The vortex dynamics of the flow around a suddenly arrested translating circular cylinder is inves... more The vortex dynamics of the flow around a suddenly arrested translating circular cylinder is investigated by direct numerical simulation and water tank experiments. In the numerical study, a method of visualization of streaklines in simulated-particle tracking computations is proposed, which is based on a variable-variance two-dimensional Gaussian-weighted summation of particles in the vicinity of each interpolation point, and for which a close similarity with physical dye visualizations is found. This technique is used to identify the trajectory of both the wake vortices, as well as the secondary vortices induced as the original wake convects over the arrested cylinder. Observations show that, in a fashion similar to the flow past an arresting sphere, each wake vortex induces a counter-rotating vortex pair, which subsequently self-propels over a range of sometimes surprising trajectories as the Reynolds number and cylinder translation distance are varied. At low Reynolds numbers and...

Experimental investigation of vortex shedding from a plate: effect of external velocity perturbation

Journal of Wind Engineering and Industrial Aerodynamics, 1993

Vortex shedding behind a plate with a square trailing edge was investigated in a water tunnel. Bo... more Vortex shedding behind a plate with a square trailing edge was investigated in a water tunnel. Both the case where the shedding occurs naturally and the case where the shedding is "forced" by an applied transverse velocity field were studied. The perturbation velocity at the trailing edge of the plate was calibrated by accurately measuring the velocity at this point using laser doppler anemometry. The spanwise cross-correlation of the fluctuating velocity was determined based on velocity measurements taken at two different spanwise separations; hot film sensors were used to measure the velocities. It was found that high spanwise correlation of the vortex shedding could be induced by applying a low level perturbation at the natural shedding frequency. It was found that there was a threshold below which the vortex shedding was not well correlated. To prove that the correlations were due to an ordering of the vortices as they were shed, rather than being mainly due to the perturbation field, validation tests were conducted. The phenomenon was also studied by flow visualization using hydrogen bubbles illuminated by a laser. In natural shedding, spanwise waviness of the vortices could be seen clearly in the cross-stream view. This explains the limited spanwise correlation length for this case. On the application of a low-level perturbation this waviness disappeared, corresponding to the high degree of spanwise correlation measured by the hot films.

Sound generated in laminar flow past a two-dimensional rectangular cylinder

Journal of Sound and Vibration, 2006

The far-field sound generated from low Mach number flow past a two-dimensional rectangular cylind... more The far-field sound generated from low Mach number flow past a two-dimensional rectangular cylinder is studied by using a two-step aeroacoustic prediction method. In the first step, the incompressible Navier-Stokes equations are solved numerically. This allows the time-dependent acoustic source to be determined from Powell's vortex sound theory. Using this information, in the second step, the inhomogeneous wave equation is solved numerically to predict the time-evolving acoustic field. This study considers the effects of the Reynolds and Mach number on the sound generation and propagation characteristics. Results show that acoustic wave generation can be associated with the shedding of vortices at both the leading and trailing edges of the cylinder. In particular, an attempt is made to quantify the individual contributions, showing that the trailing-edge region is a considerably stronger source. Similar to the case for a circular cylinder, the predicted sound field has a dipolar far-field directivity with the lift dipole dominating. However, the drag dipole becomes relatively more important as the Reynolds number is increased. Overall, the relative amplitude of the far-field acoustic signal increases substantially with Reynolds number. In addition, as the Reynolds number is increased, the far-field pressure signal contains significant harmonic content, unlike the situation at the lowest Reynolds number investigated. A harmonic decomposition in terms of polar angle allows the multipole content of the signal to be quantified. Results showed that the acoustic field is dominated by the lift forcing which is predominantly dipolar, at least up to a Mach number (Ma) of 0.2. While this is also true for the drag forcing for low Mach numbers, we found that for Ma40:1, the quadrupole term is of a magnitude comparable to the drag dipole. By taking into account the Doppler effect through a spatial transformation of the predicted acoustic solution, the dipolar field becomes skewed towards the upstream direction as the Mach number is increased. Various difficulties associated with direct acoustic computations, such as convergence problems associated with initial transients and grid stretching, and the introduction of errors from under-resolving the acoustic source, and treatment of the slowly decaying wake, are discussed. The methods used to overcome these problems are reported.

Particle image velocimetry and visualization of natural and forced flow around rectangular cylinders

Journal of Fluid Mechanics, 2003

Particle image velocimetry (PIV) measurements and flow visualization in a water tunnel show that ... more Particle image velocimetry (PIV) measurements and flow visualization in a water tunnel show that vortex shedding at the leading and trailing edges of rectangular cylinders can be simultaneously phase-locked to transverse velocity perturbations when the applied perturbation Stp is close to an impinging leading-edge vortex/trailing-edge vortex shedding (ILEV/TEVS) frequency. The transverse perturbations, analogous to β-mode duct acoustic resonances, are generated through harmonic oscillations of the sidewalls. When this occurs, the leading-edge vortices are found always to pass the trailing edge at the same phase in the perturbation cycle regardless of the chord-to-thickness (c/t) ratio. Applying perturbations at an Stp not equal to the natural global frequency also results in phase-locked vortex shedding from the leading edge, and a near wake with a frequency equal to the perturbation frequency. This is consistent with previous experimental findings. However, vortex shedding at the t...