Cylinder Vortex Research Articles

Numerical simulation of the influence of wave parameters on the horizontal cylinder in the nonlinear coupling of the wave–current

Considering that marine structures are frequently subjected to the combined effects of waves and currents, the wave–current coupling environment largely determines the structural load and the surrounding water–air mixed flow, which is a typical feature of offshore structures, such as ships and offshore platforms. This study focuses on the interaction between a horizontal cylinder and a free surface in a wave–current nonlinear coupled environment using numerical simulations. The numerical method is based on the incompressible Navier–Stokes equation, with the volume-of-fluid method used to capture the free surface. Based on the second-order Stokes wave theory, we studied the impact of wave height and steepness on the cylinder force, vorticity field, free-surface deformation, and spatial–temporal distribution characteristics of entrainment bubbles. The results showed that the wave height and wave steepness have opposite effects on the root mean square (RMS) value of the force and influence the amplitude and period of the force curve. The stretching of the negative vortex led to varying degrees of double-frequency oscillation modes in the force curves. The main sources of bubbles in the wake are the breaking of the free surface and the entrainment caused by the cylinder vortex, and the bubbles caused by the former account for the majority. Compared with the wave height, an increase in wave steepness can cause a more severe interface breaking, resulting in more entrainment bubbles.

Multi-fidelity, steady-state aeroelastic modelling of a 22-megawatt wind turbine

In this work we present multi-fidelity steady-state aeroelastic framework that leverages the state-of-the-art simulation tool HAWC2 for the structural model, and a variety of aerodynamic models, comprising of the low fidelity blade element momentum (BEM) method, the medium fidelity blade element vortex cylinder (BEVC) method and the coupled near wake and vortex cylinder method, and finally the high-fidelity CFD solver EllipSys3D. The aeroelastic framework is part of AESOpt, an aerostructural framework for design of wind turbine blades. The different aerodynamic models are applied to compute the aeroelastic steady state of the newly designed IEA 22 MW Reference Wind Turbine. The results show a very good agreement between the medium- and high-fidelity aerodynamic models with a maximum of 2.7% difference between the high-fidelity aeroelastic response and that of the lower fidelities.

A correction model for the effect of spanwise flow on the viscous force contribution in BEM and Lifting Line methods

Design optimization and aeroelastic load evaluation for wind turbines are still infeasible for CFD due to computational cost. These tasks are carried out using 3D engineering aerodynamic methods, where the local aerodynamic loads are obtained from tabulated 2D aerodynamic polars, which makes the models fast. With recent developments such as highly flexible blades, coned/prebent/swept blades, the relative inflow direction to the “inner” 2D airfoil section systems can have a significant component in the spanwise direction. The Crossflow Principle (CP) is used to treat the effects of this in a physically consistent manner. Cases dominated by pressure forces are described well with the CP method, but it is shown in the paper that CP fails to describe the part of the forces stemming from the friction forces correctly. It is shown in the paper that the power loss due to friction forces will be underestimated with the crossflow principle for rotors with significantly swept blades or other designs with a significant amount of local crossflow on the blades. The present work presents a simple model to correct the baseline crossflow principle method to take into account also the friction force part in a consistent manner. The model is validated with 3D CFD results on a 2D airfoil section. It is shown that the new model successfully corrects for the addition of the viscous forces due to spanwise flow component. The paper includes examples of the effect of using the model on rotor designs with different amounts of blade sweep simulated using a blade element momentum (BEM) and blade element vortex cylinder (BEVC) methods.

Modeling of Blockage and Wake Effect: Comparison with Field data

We apply engineering approaches for estimating the induction zone in front of a wind turbine and escalating to the global blockage at wind farm and cluster of farms scale as well accounting for wind farm wake. The methods include the vortex cylinder model and the self-similar model for blockage, and Jensen and Eddy-viscosity models for the wake estimation. To calibrate each model, Reynold Average Navier-Stokes with Actuator disc simulations are employed. Every model’s efficacy is assessed in comparison to the validation data of an operated wind farm in Scotland with more than 100 turbines of 10 MW each. We also examine the widely used process within the wind industry for transferring the application of calibrated wake and blockage models from one wind farm. According to our simulation analysis, blockage causes sideways acceleration and upstream slowdown. This also result in a gradient in the power produced by each turbine on the farm, which, if not properly accounted for, appears to be a wake model constraint. Furthermore, the response of different turbine type in another environment is not effectively predicted by the blockage and wake model, which is based on a particular turbine type and environment.

Rapid dynamic aeroelastic response analysis of the highly flexible wing with distributed propellers influence

AbstractA rapid nonlinear aeroelastic framework for the analysis of the highly flexible wing with distributed propellers is presented, validated and applied to investigate the propeller effects on the wing dynamic response and aeroelastic stability. In the framework, nonlinear beam elements based on the co-rotational method are applied for the large-deformation wing structure, and an efficient cylinder coordinate generation method is proposed for attached propellers at different position. By taking advantage of the relatively slow dynamics of the high-aspect-ratio wing, propeller wake is modeled as a quasi-steady skewed vortex cylinder with no updating process to reduce the high computational cost. Axial and tangential induced velocities are derived and included in the unsteady vortex lattice method. For the numerical cases explored, results indicate that large deformation causes thrust to produce wing negative torsion which limits the displacement oscillation, and slipstream mainly increases the response values. In addition, an improvement of flutter boundary is found with the increase of propeller thrust while slipstream brings a destabilising effect as a result of the increment of dynamic pressure and local lift. The great potential of distributed propellers in gust alleviation and flutter suppression of such aircraft is pointed out and the method as well as conclusions in this paper can provide further guidance.

Effects of Diameter Parameters on Gas Flow Field Characteristics in Cyclones: An Experimental Investigation

The flow field characteristic is crucial for the separation process of cyclones, which includes time–mean and dynamic characteristics. The structural parameters of the cyclone have an important influence on the internal flow field characteristics, among which the cylinder diameter and vortex finder diameter are important structural parameters. This experimental study aimed to assess the effects of diameter parameters on the flow field characteristics of cyclones, especially the dynamic characteristics, which have received less attention in the literature. A hot wire anemometer (HWA) was employed in measuring the instantaneous tangential velocities in cyclones with different cylinder and vortex finder diameters. Time and frequency domain analyses of the measured data revealed that the diameter parameters of cyclones affected not only the distributions of the time–mean and instantaneous tangent velocities but also the intensity and dominant frequency of the instantaneous tangential velocity fluctuations. First, the maximum tangential velocity in the cyclone increased slightly when the cylinder diameter was increased and decreased significantly when the vortex finder diameter was increased. Second, the tangential velocity fluctuation intensity characterized by the standard deviation (Sd) increased on the same dimensionless axial section when the cylinder diameter was increased and the vortex finder diameter was decreased. It was also found that the increases in cylinder diameter and vortex finder diameter led to the dominant frequencies in the cyclone being reduced. Based on the results of this study, the dominant frequency calculation model for cyclones was improved. The conclusions presented in this study may provide valuable insights into the dynamic characteristics of flow fields in cyclones for future improvements to separation performance.

A review on complex turbulent dynamic phenomenon of natural vortex length in cyclone separator

Abstract The natural vortex length is a complex turbulent dynamic phenomenon of cyclone separator, which can provide reference for the height design. It is calculated by the axial distance between the vortex end and the bottom of vortex finder. At present, scholars mainly attribute the influencing factors of natural vortex length to cylinder diameter, inlet area and vortex finder diameter, ignoring the influence of other structural parameters and operating parameters, so the accuracy and applicability of empirical formula are poor. This study described the mechanism of the vortex end and analyzed the PVC phenomenon. Then, an example was provided to illustrate the limitations and shortcomings of empirical formula. In addition, the influences of some geometric parameters and operating parameters on natural vortex length were summarized. Therefore, this work could provide an important reference for design optimization of cyclone separator height.

Numerical simulation of flow past an 8:1 oscillating rectangular cylinder at Re = 22 000

To investigate the influence of structure’s oscillatory motion on flow, the present study employs the arbitrary Lagrangian–Eulerian method in k–ω shear stress transport (SST) turbulence model to simulate the flow past an oscillating rectangular cylinder at Re= 22 000. The cylinder undergoes reciprocating sinusoidal motion at a specified frequency f e, and the frequency ratio fr (defined as the ratio of cylinder oscillation frequency f e to the stationary cylinder vortex shedding frequency f 0), ranges from 0 to 4. The results demonstrate that, in the synchronization region (0.8 ⩽ fr⩽ 1.2), the drag coefficient shows the most notable variation and reaches its maxima at fr= 1.1 and the root mean square (r.m.s.) of the lift coefficient is proportional to the square of fr (, R 2 = 0.99). Moreover, the present study compares the similarities and differences of vortex shedding morphology between stationary and oscillating cylinders. With the increase of fr, the wake vortex gradually transforms from a single-row arrangement on the wake centerline to a parallel double-row arrangement, with the main vortex modes in the wake region observed as ‘2S’, ‘P + S’, ‘2P’ and ‘C’. Furthermore, spectral analysis, including amplitude spectrum analysis and wavelet analysis, in addition to probability density function statistical methods, are employed to comprehensively understand the velocity characteristics of the wake region. The results indicate that the oscillation of the cylinder reduces the correlation of the wake velocity. Those are beneficial in understanding the interaction between turbulence and structural fluid-induced motion.

Numerical study of aerodynamic characteristics and particle motion in square cylinder vortex flow with different void ratios

In actual production practice, blunt block structures usually exist in multiple forms simultaneously, such as buildings, bridges, and offshore platforms. Flow around multi-cylinder structures is often accompanied by complex phenomena, including separation and reattachment of shear layers, vortex-structure interactions, and the evolution of wake vortex streets. These phenomena can worsen fatigue damage or local accumulation of harmful substances on bluff block structures. The main purpose of this study is to analyze the complex flow phenomena of square cylinders with different void ratios and the distribution of particulate matter in the flow. The results show that the drag forces applied to the square cylinder under different void conditions will be very different, the flow will also vary greatly, and the location of particle deposition will also change significantly with the change of void ratio conditions.

The impact of cylinder vortex stabilizer on fluctuating turbulence characteristics of a cyclone separator based on Large Eddy Simulation

The main purpose of present study is to comprehensively clarify the impact of cylinder vortex stabilizer on fluctuating turbulence structure of a Stairmand cyclone separator on basis of Large Eddy Simulation. The cylinder vortex stabilizer is easy and could be applied to any existing cyclone model without any major replacement. This novel modification in cyclone body is considered to alleviate the negative effect of entrainment of particles from the ash hopper and swing of the vortex end in swirling flow. The numerical simulations were conducted based on Stairmand cyclone separator and three new models with variation of vortex stabilizer length and diameter. The results showed that the cylinder vortex stabilizer could enhance flow instability and improve fluctuating turbulence structure to some extent. It is confirmed that cylinder vortex stabilizer could significantly reduce the tangential velocity in the inner quasi-forced vortex region of the cyclones. Comparing with Stairmand cyclone, the swirling first and second peak frequency of cyclone model with vortex stabilizer (Length L/D: 6.5, diameter d/D: 0.12) have been confirmed to get considerable reduction of 11.54% and 10.86%, respectively. This modified cyclone model is comparatively better for enhancement of flow stability, providing about 18.4% maximum reduction of normalized flow angle, 24.8% of rotational kinetic energy in dust collector and 14.2% in the main body of cyclone.

Entrainment model for wind turbine wakes considering blockage effect

The entrainment model applied firstly by P. Luzzatto-Fegiz can predict the streamwise velocity of the far-wake more accurately than the traditional models (e.g., Jensen model, Frandsen model). The current model, however, neglects the wind speed drop approaching the rotor caused by the blockage effect, the phenomenon of which cannot be considered in the velocity recovery of the far-wake region, further affecting the precision of wake prediction. Hence, this paper proposes an entrainment model considering the blockage effect. We adopt several numerical simulation results to analyze the performance with the vortex cylinder model for the rotor vicinity. The initial entrainment parameter Einitial calculates, according to the formula of the breakdown position of tip vortices proposed by J.N. Sørensen. The Enear-wake can obtain with the length of the near-wake region, defined by P. E. J. Vermeulen. The linearized entrainment-parameterized expression we develop combines the linearization hypothesis with the above two positions and their entrainment values. The proposed model consists of two main components: the modified entrainment model and the vortex cylinder model, validated against the advanced Gaussian model of Bastankhah and Porté-Agel (BPA) and several LES cases. The results demonstrate that the proposed model significantly improves the streamwise wake distribution.

Data-driven prediction and study of vortex induced vibrations by leveraging hydrodynamic coefficient databases learned from sparse sensors

Semi-empirical models are currently the state-of-the-art industry tool for flexible cylinder vortex induced vibrations (VIV) fatigue prediction. Accurate prediction of the structural response relies entirely on the accuracy of the acquired hydrodynamic coefficient database. The construction of systematic hydrodynamic coefficient databases from rigid cylinder forced vibration experiments can be time-consuming for simple cases and intractable for multi-parametric cases. An alternative approach has been implemented in this work to improve the flexible cylinder VIV prediction by machine-learning optimal parametric hydrodynamic databases using physical experimental measurements. The methodology is applied to a straight riser in uniform flow and extended to non-straight riser configurations and non-uniform incoming flow profiles. Moreover, database inference is extended to using direct sparse sensor measurements along the structure. Specifically, a 19-dimensional parametric hydrodynamic coefficient database is obtained for: (i) straight riser in uniform flow (using sparse strain measurements), (ii) straight riser in sheared flow, and (iii) catenary riser in uniform flow with a 60 deg incidence angle between the catenary plane and the incoming flow stream. The predicted amplitude and frequency responses, using the extracted databases, are compared with observed experimental results.

How should the lift and drag forces be calculated from 2-D airfoil data for dihedral or coned wind turbine blades?

Abstract. In the present work, a consistent method for calculating the lift and drag forces from the 2-D airfoil data for the dihedral or coned horizontal-axis wind turbines when using generalized lifting-line methods is described. The generalized lifting-line methods refer to the models that discretize the blade radially into sections and use 2-D airfoil data, for example, lifting-line (LL), actuator line (AL), blade element momentum (BEM) and blade element vortex cylinder (BEVC) methods. A consistent interpretation of classic unsteady 2-D thin airfoil theory results reveals that it is necessary to use both the relative flow information at one point on the chord and the chordwise gradient of the flow direction to correctly determine the 2-D aerodynamic force and moment. Equivalently, the magnitude of the force should be determined by the flow at the three-quarter-chord point, while the force direction should be determined by the flow at the quarter-chord point. However, this aspect is generally overlooked, and most implementations in generalized lifting-line methods use only the flow information at one calculation point per section for simplicity. This simplification will not change the performance prediction of planar rotors but will cause an error when applied to non-planar rotors. In this work this effect is investigated using the special case, where the wind turbine blade has only out-of-plane shapes (blade dihedral) and no in-plane shapes (blade sweep), operating under steady-state conditions with uniform inflow applied perpendicular to the rotor plane. The impact of the effect is investigated by comparing the predictions of the steady-state performance of non-planar rotors from the consistent approach of the LL method with the simplified one-point approaches. The results are verified using blade-geometry-resolving Reynolds-averaged Navier–Stokes (RANS) simulations. The numerical investigations confirmed that the full method complying with the thin airfoil theory is necessary to correctly determine the magnitude and direction of the sectional aerodynamic forces for non-planar rotors. The aerodynamic loads of upwind- and downwind-coned blades that are calculated using the LL method, the BEM method, the BEVC method and the AL method are compared for the simplified and the full method. Results using the full method, including different specific implementation schemes, are shown to agree significantly better with fully resolved RANS than the often used simplified one-point approaches.

Effect of yaw angle on vibration mode transition and wake structure of a near-wall flexible cylinder

The multi-mode transition and vortex structures in the vortex-induced vibration (VIV) of a near-wall flexible cylinder under different yaw angles are investigated through three-dimensional direct numerical simulation. Yaw angles α = 0°–60°, gap ratio G/D = 0.8, and Re = 500 are adopted. With the increase in α, the dominated vibration mode decreases from the 6th to 1st mode in the in-line (IL) direction and the 3rd to 2nd mode in the cross-flow (CF) direction. For the IL vibration, no mode transition occurs at α = 0°, whereas frequently mode transition is observed at α > 0°, due to the intermittent participation and spanwise competition of different modes, thus showing an intensified traveling-wave characteristic. For the CF vibration, mode transition is not excited at any α case even with spanwise mode competitions, due to the significant weight of the dominated mode, thus showing a strong standing-wave characteristic. The asymmetrical distributions of vibration displacements and force coefficients are established because of irregular energy transfer along the span. The spanwise vortex tubes at α = 0°–30° are separated into several cells associated with the dominated vibration mode, showing a locally parallel vortex shedding. However, positively yawed and negatively yawed vortex shedding are observed at α = 45° and 60°, respectively. The vortex strengths vary along the cylinder, where large-scale and small-scale vortices are observed at the CF anti-node and node planes, respectively. The independence principle is only valid at α < 15° for predicting the multi-mode vibrations and hydrodynamics, significantly reduced from that of α < 45° in the wall-free case or the mono-mode VIV case.

Comparison of aerodynamic planform optimization of non-planar rotors using blade element momentum method and a vortex cylinder model

The present work compares non-planar rotors designed using the blade element momentum (BEM) method and a vortex cylinder model. In a previous work, it is shown that blade element theory coupled with the superposition of the vortex cylinder model (BEVC) is able to model the loads of non-planar rotors. The result predicted by the BEVC model is in significantly improved agreement with higher-fidelity models than the loads as predicted using the BEM method. In this work, the BEM method and the BEVC method are integrated into a gradient-based optimization framework for aerodynamic planform optimization, in which the analytical gradients are obtained using the algorithmic differentiation (AD) method. In the present study, the rotor is assumed to be stiff for all cases such that the pure aerodynamic effects are highlighted. Loads of the optimized non-planar rotors with different geometries under different constraints designed from both methods are calculated using the BEM method, the BEVC method and also the higher-fidelity lifting-line (LL) method. Within the constraints of the present work it was found that the advantage of the BEVC method is not significant when comparing the integrated aerodynamic loads: the non-planar rotor designed using the BEM method gives similar total thrust and power as the rotor designed using the BEVC method when the designs are evaluated with the higher-fidelity LL method. However, the results confirmed that the distributed aerodynamic loads of the non-planar rotors predicted by the BEVC method are in improved agreement with the LL method compared to the BEM method.

Three-dimensional direct numerical simulations of two interfering side-by-side circular cylinders at intermediate spacing ratios

Side-by-side arranged circular cylinders show fierce interferences when they are not far apart. In this paper, we present the vibration response and wake characteristics of two elastically mounted circular cylinders in a side-by-side arrangement through three-dimensional direct numerical simulations (3-D DNS). The Reynolds number is Re = 500 and two center-to-center spacings (s) of 2.0D and 2.5D are considered where D is the diameter of the two cylinders. We found that with the increasing reduced velocity, the responses of both s/D cases can be divided into three different regions, which are named desynchronization region (DS), initial branch (IB), and lower branch (LB), with the former being further divided into two subregions (DS-I and DS-II). The vibration, hydrodynamic forces, and spectral features in each region are discussed in detail. According to the characteristics of cylinder oscillation and vortex dynamics, six types of wake patterns are identified, with the deflected (DF) and flip-flopping (FF) patterns at s/D = 2.0 while in-phase (IP), anti-phase (AP), in-/anti-phase (I/AP), and hybrid (HB) patterns at both s/D = 2.0 and 2.5. Furthermore, we investigated the wake three-dimensionality through the evolution of vortical structures, and the statistics and development of the enstrophies. It was observed that the presence of the streamwise vortices modifies the strength and spanwise coherence of the spanwise vortices. Finally, flow physics for the interactions of the two cylinders at different regions is elucidated.

Three-dimensional flow past a circular cylinder in proximity to a stationary wall

In this paper, we present direct numerical simulations (DNS) of the three-dimensional flow around a circular cylinder near a stationary wall at the Reynolds number of 500. The gap (G) between the cylinder surface and the stationary wall varies in the range of 0–3.0D where D is the cylinder diameter. We observe that the proximity wall significantly affects the flow topology around the cylinder, which is characterized by the asymmetric wake, upward gap flow, and strong interactions of the cylinder vortices with the wall-generated boundary layer. Three flow regimes are identified with variations of the fluid forces and vortex dynamics. (i) Steady regime: at G/D < 0.3, vortex shedding is suppressed in the near wake, and scattered streamwise vortices form in the far wake where the upper side shear layer meets the stationary wall. No fluctuating forces act on the cylinder. (ii) Biased unsteady regime: at 0.3 ≤ G/D < 1.5, the gap flow deflects upward and the vortices shed from the gap side are significantly suppressed. The cylinder has enlarged fluctuating drag and lift forces with identical dominant frequencies. (iii) Parallel unsteady regime: at G/D ≥ 1.5, vortices shed from both sides of the cylinder are of almost identical intensities while the gap flow is approximately parallel to the wall. The vortex dynamics approaches that of an isolated circular cylinder in unconfined flows. Flow features of each regime are illuminated through the flow fields, vorticity contours, statistics of the enstrophies and gap flow, and pressure distribution on the cylinder surface. Finally, we investigate the critical gap ratio for the onset of the vortex-shedding of the upper shear layer and discuss the underlying physics. Moreover, we observe that the occurrence of the second harmonic lift and fundamental drag frequencies are resulted from the asymmetric wake because of the proximity of the stationary wall.

A computationally efficient engineering aerodynamic model for non-planar wind turbine rotors

Abstract. In the present work, a computationally efficient engineering model for the aerodynamic load calculation of non-planar wind turbine rotors is proposed. The method is based on the vortex cylinder model and can be used in two ways: either used as a correction to the currently widely used blade element momentum (BEM) method or used as the main model, replacing the BEM method in the engineering modeling complex. The proposed method needs the same order of computational effort as the ordinary BEM method, which makes it ideal for time-domain aero-servo-elastic simulations. The results from the proposed method are compared with results from two higher-fidelity aerodynamic models: a lifting-line method and a Navier–Stokes solver. For planar rotors, the aerodynamic loads are identical to the current BEM model when the drag force is excluded during the calculation of the induced velocities. For non-planar rotors, the influence of the blade out-of-plane shape, measured by the difference of the load between the non-planar rotor and the planar rotor, is in very good agreement with higher-fidelity models. Meanwhile, the existing BEM methods, even with a correction of radial induction included, show relatively large deviations from the higher-fidelity method results.

Nonlinear Aeroelastic Analysis of High-Aspect-Ratio Wings with a Low-Order Propeller Model

A nonlinear aeroelastic analysis framework for high-aspect-ratio wings that includes the aerodynamic effects of propellers is described. The high computational cost required for modeling aerodynamic interaction between the wing and propeller wake is reduced by taking advantage of the relatively slow dynamics of the wing. Consequently, the propeller wake is modeled as a straight vortex cylinder that does not require a computationally expensive wake updating process. By leveraging the smallness of the propeller, an averaged vortex cylinder method is proposed that calculates the induced velocities of the propeller vortex cylinder efficiently without suffering from a numerical singularity and loss of accuracy. The induced velocities are considered in the wing aerodynamic force calculation using an unsteady vortex lattice method. An efficient propeller cylinder coordinate generation method modeling the propeller-attached wing by absolute nodal coordinate formulation with a multibody dynamic theory is also proposed. The developed framework is validated by comparison with other formulations. A static aeroelastic analysis on a low-speed high-aspect-ratio wing demonstrates that the propeller-induced axial velocity causes the deflection change of 5.4%. A nonlinear dynamic result shows that the propeller decreases the vibration amplitude by 6.7% because the propeller-induced axial velocity enhances the aerodynamic damping.

Flow-induced vibration of a rotating circular cylinder at high reduced velocities and high rotation rates

Flow-induced vibration (FIV) of a rotating circular cylinder with high rotation rates between 4.5 and 8, a low Reynolds number (Re) of 100 and a mass ratio of 11.5 is investigated by two-dimensional numerical simulations. The responses of the high-speed rotating cylinder are classified into two steady flow regimes (S1 and S2), five vortex-induced vibration (VIV) regimes (VIV-1S, VIV-2S, VIV-T+S, VIV-2P and VIV-2P*) and two impulsive galloping (IG) regimes (IG and IG*). All the regimes are mapped on a rotation rate versus reduced velocity plane. In VIV-T+S regime, a triplex and a single vortex are shed alternatively from the cylinder in two consecutive vibration periods. VIV-2P* regime has the same number of vortices shed in each vibration period as VIV-2P, but the amplitude of vibration in two consecutive periods are different from each other. In all the identified VIV regimes, shear layers are separated from only one side of the cylinder and form vortices, instead of both sides which is observed at low rotation rates. This makes the flow patterns of all the identified VIV regimes different from the ones observed at low rotation rates. Impulsive galloping (IG) is the repetitive occurrence of the galloping peak followed by a few periods of VIV. Impulsive galloping occurs periodically in the IG regime and randomly in the IG* regime. During each IG period, the vibration displacement reaches an impulsive peak where the amplitude of vibration is increased significantly, followed by periodic oscillations of VIV.