The role of the separation point in streamwise vortex-induced vibrations

Structures in crossflow are susceptible to vortex-induced vibrations (VIV) when the vortex-shedding becomes synchronised with the structural vibration. Strategies to control VIV often include modifying the separation point on the cylinder surface, such as adding helical strakes, although there remains disagreement regarding the mechanism by which these work. We explore the role of the separation point on VIV acting in the streamwise (drag) direction, by performing high-speed Particle-Image Velocimetry (PIV) measurements of the wake and the structural displacement of a range of cylinders with different cross-sectional shapes, including circular (no fixed separation points), equilateral triangles (fixed separation points) and elliptical cylinders (which act as an intermediate case). None of the non-circular cylinders are found to exhibit VIV, despite having approximately the same experimental conditions (mass ratio, structural damping, Reynolds number range, etc.) as the circular cylinders, which undergo VIV. The phase-averaged PIV measurements of the near wake of the circular cylinder are used to calculate the separation angle throughout the shedding cycle for different wake modes, and it is shown that all the wake modes that are associated with VIV require a periodic movement of the separation point. In contrast, the variation in the separation angle was negligible for the von Kármán vortex street observed behind near-stationary circular cylinders and for all non-circular cylinders. The experiments illustrate the great sensitivity of the wake mode and streamwise VIV to modifications of the separation point and demonstrate that even a moderately elliptical cylinder (major to minor axis ratio of 1.54) is sufficient to completely suppress VIV

Dynamics and excess temperature of a plume throughout its life cycle

Measurements of the velocity field associated with plumes rising through a viscous fluid are performed using stereoscopic Particle-Image Velocimetry in the Rayleigh number range 4.4 × 105 − 6.4 × 105. The experimental model is analogous to a mantle plume rising from the core-mantle boundary to the base of the lithosphere. The behaviour of the plume is studied throughout its life cycle, which is broken up into four stages; the Formation Stage, when the plume forms; the Rising Stage, when the plume rises through the fluid; the Spreading Stage, when the plume reaches the surface and spreads; and finally the Declining Stage, when the heat source has been removed and the plume weakens. The latter three stages are examined in terms of the Finite-Time Lyapunov Exponent fields and the advection of passive tracers throughout the flow. The temperature at the heater and near the fluid surface are measured using thermocouples to infer how the presence of a mantle plume would produce excess temperature near the lithosphere throughout the various stages of its life cycle. In all experiments a time lag is observed between the removal of the heat source and the decline in the excess temperature near the surface, which is proportional to the rise time. A simple analytical model is presented, which suggests that under mantle conditions (i.e. negligible thermal diffusion), the relationship between the time lag and the rise time is robust and independent of the Rayleigh number; however, the constant of proportionality is closer to unity in the absence of diffusion. Once the heat source is removed, the excess temperature near the surface declines exponentially at a rate that is inversely proportional to the rise time. The implications of this result are discussed in terms of the decline in volcanism in the Louisville hotspot chain over the past 20 Ma. The rise velocity of material in the plume is examined; the rise velocity is found to vary significantly with the plume height in a manner that is inconsistent with many of the common semi-analytical models of thermal plumes in the literature. It is also argued that this height-dependency will cause estimates of the rise velocity based on the decay series of Uranium isotopes to significantly underestimate the true value

Shear-thinning mediation of elasto-inertial Taylor–Couette flow

We study the shear-thinning mediation of elasto-inertial transitions in Taylor–Couette flow of viscoelastic polymer solutions. Two types of high molecular weight polymers are used at various concentrations and in water–glycerol solvents of various viscosities. This allows us to access a wide range of elastic numbers and effective shear-thinning indices. Conservative ramp-up (slow acceleration of the inner cylinder and subsequent increase in Reynolds number) and steady-state (constant rotation speed) experiments are performed, in which the flow is monitored continuously using flow visualisation. Depending on the shear-thinning and elastic properties of the working fluid, very different behaviours are observed. In almost constant-viscosity fluids (Boger fluids), or shear-thinning fluids with significant elasticity, the flow transitions from purely azimuthal Couette flow (CF) to a highly chaotic flow state referred to as elasto-inertial turbulence (EIT) via Taylor vortex flow (TVF) and elasto-inertial rotating spiral waves (RSW). When the degree of shear-thinning is increased and elasticity reduced, elastic waves or EIT may fade to a wavy Taylor vortex flow (WTVF) with increasing inertia. Significant shear-thinning leads to a delay in the onset of EIT. Remarkably, in some highly shear-thinning cases, even with a significant elasticity, elastic flow features (EIT, RSW) are completely suppressed, and the flow exhibits a ‘Newtonian-like’ transition sequence (CF–TVF–WTVF). Shear-thinning acts to modify, delay, or even completely suppress elasto-inertial behaviours (RSW, EIT), that would otherwise have existed in the absence of shear-thinning. It is, thus, possible to induce various hydrodynamic regimes by tuning the relative degrees of shear-thinning, elasticity and inertia

Taylor-Couette flow of polymer solutions with shear-thinning and viscoelastic rheology

We study Taylor-Couette flow of a glycerol-water mixture containing a wide range of concentration (0-2000 ppm) of xanthan gum, which induces both shear-thinning and viscoelasticity, in order to assess the effect of the changes in rheology on various flow instabilities. For each suspension, the Reynolds number (the ratio of inertial to viscous forces) is slowly increased to a peak value of around 1000, and the flow is monitored continuously using flow visualisation. Shear-thinning is found to suppress many elasticity-controlled instabilities that have been observed in previous studies of viscoelastic Taylor-Couette flow, such as diwhirls and disordered oscillations. The addition of polymers is found to reduce the critical Reynolds number for the formation of Taylor vortices, but delay the onset of wavy flow. However, in the viscoelastic regime (concentration), the flow becomes highly unsteady soon after the formation of Taylor vortices, with substantial changes in the waviness with Reynolds number, which are shown to be highly repeatable. Vortices are found to suddenly merge as the Reynolds number increases, with the number of mergers increasing with polymer concentration. These abrupt changes in wavelength are highly hysteretic and can occur in both steady and wavy regimes. Finally, the vortices in moderate and dense polymer solutions are shown to undergo a gradual drift in both their size and position, which appears to be closely linked to the splitting and merger of vortices

Vortex-Induced Vibrations of a Cylinder in the Streamwise Direction

Vortex-induced vibration (VIV) of a circular cylinder has been the focus of extensive research, as it can lead to fatigue damage in a wide range of industrial applications. When the forces induced by the periodic shedding of vortices from a structure in crossflow coincide with one of its natural frequencies, the structure can exhibit large amplitude vibrations. The majority of the work performed in this area has focused exclusively on transverse vibration, while relatively little is known about VIV acting in the streamwise (flow) direction, although this is known to have a strong effect on the overall response of structures with multiple degrees-of-freedom (DOFs). This work aims to characterise the behaviour of the wake and the structural response of a cylinder throughout the streamwise VIV response regime, which is crucial if the wealth of information on the transverse-only case is to be extended to the more practical and complex case of multi-DOF structures. Experiments were performed on a cylinder free to move in the streamwise direction for a range of reduced velocities in a closed-loop water tunnel. Particle-Image Velocimetry (PIV) was used to simultaneously measure the cylinder displacement and the velocity field in the wake, in the Reynolds number range 400 - 5500. The response regime was characterised by two branches, separated by a region of low amplitude vibration, as reported in the literature. Five distinct regions were identified, each of which was discussed in terms of the dominant wake mode, structural response characteristics, velocity profiles and estimates of the strength and trajectories of the shed vortices. In the first branch the wake was found to switch intermittently between the symmetric S-I mode (in which two vortices were shed simultaneously from either side of the cylinder) and the alternate A-II mode (which is similar to the von Karman vortex street observed behind stationary bodies). A criterion was developed which could determine which mode was dominant in a given instantaneous PIV field, and the effect of both modes on the cylinder response and wake characteristics was examined. Multi-modal behaviour was also observed in the second branch. At one value of reduced velocity, the wake could exhibit one of three modes; the A-II, the SA (similar to the A-II mode, with the vortices forming closer to the cylinder base) or the A-IV mode (which was characterised by the shedding of two pairs of counter-rotating vortices). Each mode was associated with a different cylinder response amplitude. The stability of the cylinder response while each mode dominated was examined using phase-portraits, which indicated that the system behaved as a hard oscillator. The forces acting on the cylinder were estimated using two methods, based on the measurements of the cylinder displacement signal and the flow field, respectively. The results found using both methods were in agreement, and the accuracy of the estimates was discussed. It was found that the amplitude of the unsteady drag force was very low between the two response branches, which was thought to be the cause of the reduction in the cylinder vibrations in this region. Finally, the effect of the various wake modes on the amplitude of the fluid forces throughout the response regime was examined. The results presented in this study provide a comprehensive description of the behaviour of the wake and the associated fluid forces throughout the streamwise response regime. The work reveals the inherent differences between the extensively studied case of transverse-only VIV and the streamwise-only case, which is crucial if the wealth of information available on transverse VIV is to be extended to the more practical multi-DOF case

Modulation of elasto-inertial transitions in Taylor–Couette flow by small particles

Particle suspensions in non-Newtonian liquid matrices are frequently encountered in nature and industrial applications. We here study the Taylor–Couette flow (TCF) of semidilute spherical particle suspensions (volume fraction ≤0.1 ) in viscoelastic, constant-viscosity liquids (Boger fluids). We describe the influence of particle load on various flow transitions encountered in TCF of such fluids, and on the nature of these transitions. Particle addition is found to delay the onset of first- and second-order transitions, thus stabilising laminar flows. It also renders them hysteretic, suggesting an effect on the nature of bifurcations. The transition to elasto-inertial turbulence (EIT) is shown to be delayed by the presence of particles, and the features of EIT altered, with preserved spatio-temporal large scales. These results imply that particle loading and viscoelasticity, which are known to destabilise the flow when considered separately, can on the other hand compete with one another and ultimately stabilise the flow when considered together

Experimental insights into elasto-inertial transitions in Taylor-Couette flows

Since the seminal work of Taylor in 1923, Taylor–Couette (TC) flow has served as a paradigm to study hydrodynamic instabilities and bifurcation phenomena. Transitions of Newtonian TC flows to inertial turbulence have been extensively studied and are well understood, while in the past few years, there has been an increasing interest in TC flows of complex, viscoelastic fluids. The transitions to elastic turbulence (ET) or elasto-inertial turbulence (EIT) have revealed fascinating dynamics and flow states; depending on the rheological properties of the fluids, a broad spectrum of transitions has been reported, including rotating standing waves, flame patterns (FP), and diwhirls (DW). The nature of these transitions and the relationship between ET and EIT are not fully understood. In this review, we discuss experimental efforts on TC flows of viscoelastic fluids. We outline the experimental methods employed and the non-dimensional parameters of interest, followed by an overview of inertia, elasticity and elasto-inertia-driven transitions to turbulence and their modulation through shear thinning or particle suspensions. The published experimental data are collated, and a map of flow transitions to EIT as a function of the key fluid parameters is provided, alongside perspectives for the future work. This article is part of the theme issue 'Taylor–Couette and related flows on the centennial of Taylor’s seminal Philosophical Transactions paper (part 1)'

Vortex merging and splitting: A route to elastoinertial turbulence in Taylor-Couette flow

We report experimental evidence of a new merge-split transition (MST) to elastoinertial turbulence (EIT) in Taylor-Couette flows of viscoelastic polymer solutions, caused by merging and splitting of base Taylor vortices when crossed by elastic axial waves (rotating standing waves, RSW). These vortex merging and splitting events are not due to transient behavior, finite aspect ratio, or shear-thinning behavior. They are random in nature and increase in frequency with Re; when superimposed on a RSW flow state they cause abrupt changes in the axial spatial wavelength, leading to the transition from a RSW to the EIT state. We thus identify MST as an inertial feature solely triggered by elasticity and independent of any shear-thinning behavior

Streamwise vortex-induced vibrations of cylinders with one and two degrees-of-freedom

Measurements are presented of the structural response and wake of a two degree-of-freedom (2-DOF) pivoted cylinder undergoing streamwise Vortex-Induced Vibrations (VIV), which were carried out using Particle-Image Velocimetry. The results are compared to those of previous studies performed in the same experimental facility examining a cylinder free to move only in the streamwise direction (1-DOF). The aim of this study is to examine to what extent the results of previous work on streamwise-only VIV can be extrapolated to the more practical, multi-DOF case. The response regimes measured for the 1- and 2-DOF cases are similar, containing two response branches separated by a low amplitude region. The first branch is characterised by negligible transverse motion and the appearance of both alternate and symmetric vortex-shedding. The two wake modes compete in an unsteady manner; however, the competition does not appear to have a significant effect on either the streamwise or transverse motion. Comparison of the phase-averaged vorticity fields acquired in the second response branch also indicates that the additional DOF does not alter the vortex-shedding process. However, the additional DOF affects the cylinder-wake system in other ways; for the 1-DOF case the second branch can appear in three different forms (each associated with a different wake mode), while for the 2-DOF case the second branch only exists in one form, and does not exhibit hysteresis. The cylinder follows a figure-of-eight trajectory throughout the lock-in range. The phase angle between the streamwise and transverse motion decreases linearly with reduced velocity. This work highlights the similarities and differences between the fluid-structure interaction and wake dynamics associated with 1- and 2-DOF cylinders throughout the streamwise response regime, which has not received attention to date

Fluid forces acting on a cylinder undergoing streamwise vortex-induced vibrations

This brief communication examines the fluid forces acting on a cylinder free to move in the streamwise direction throughout its response regime. The amplitude and phase of the unsteady drag coefficient are estimated from the displacement signals and a simple harmonic oscillator model. We examine the counter-intuitive reduction in vibration amplitude observed in streamwise vortex-induced vibrations (VIV) at resonance, which has remained one of the most poorly understood aspects of VIV. Our results show that it is not caused by a change in the phase of the fluid forcing with respect to the cylinder displacement, as suggested by previous researchers; instead, we show that there is a sudden decrease in the amplitude of the unsteady drag coefficient in this region. The possible cause of this result, relating to three-dimensionality in the wake, is briefly discussed