Vortex Shedding Process Research Articles

Energy concentration pipe based on passive jet control for enhancing flow induced vibration energy harvesting

Passive control is a classic method for enhancing the energy harvesting from flow-induced vibrations. This study introduces the Energy Concentration Pipe (ECP), a hollow pipe featuring a passive windward suction inlet and jet outlets, mounted on a circular cylinder to improve the performance of flow-induced vibration energy harvester. The effects of ECP configurations—single outlet, symmetrical dual outlet, and asymmetrical dual outlet—are investigated both experimentally and numerically. Experimental results demonstrate that the ECP significantly broadens the lock-in region and generates higher output voltage compared to conventional cylinders. Specifically, the lock-in region of the symmetrical dual outlet configuration increases by 108.33%, meanwhile the maximum output voltage improves by 12.69% over the baseline case. Numerical simulations further elucidate the mechanisms of the ECP, identifying the energy concentration phenomenon. An additional portion of incoming airflow, along with its energy, is channeled through the ECP and jetted into the wake vortex behind the cylinder, altering the wake vortex shedding process. The wake structure is characterized by large primary vortex pairs and smaller secondary vortices. The energy concentration also results in larger and stronger wake vortices, enhancing unsteadiness and broadening the lock-in region. Moreover, a transverse oscillation of vortices induces additional excitation on the cylinder due to pressure oscillation, leading to larger vibration amplitude and higher output voltage.

Control on Flow Separation over a Cylinder by a Ferrofluid Film Adsorbed by a Magnet

Flow separation can lead to increased resistance and vibration generation, which is a difficult problem that cannot be ignored in engineering. In this paper, we propose a method of controlling flow separation by adsorbing ferrofluid onto the surface of a magnetized cylinder, taking the common flow around a cylinder as an example. Parametric effects of the ferrofluid film, including its viscosity and thickness, on the flow behavior were investigated in terms of the vortex shedding process, velocity distribution, dominant frequency, pressure distribution, and the flow motion inside the ferrofluid film. The results indicate that the ferrofluid film can suppress the generation of flow separation and achieve effective control, which is mainly caused by wall slip and the internal movement of the ferrofluid film. Furthermore, the flow separation control effect of ferrofluid thin films with different parameters varies, with low-viscosity ferrofluid exhibiting a superior control effect.

Accuracy assessment of Beddoes-Leishman and IAG dynamic stall models for wind turbine applications

The study presents a systematic comparison between two of the most-credited dynamic stall models for wind turbine applications: the original Beddoes-Leishman (BL) model and the newly-developed IAG. The scope of such comparison, supported by experimental data, is to shed new light on the actual suitability of current dynamic stall models for their integration into modern wind turbine simulation codes, and on the best practices to calibrate them. Two different strategies are followed for the calibration of the BL model: 1) standard one, compliant with common practices found in the literature; 2) a physics-oriented one, focusing on the constants defining the dynamic stall onset as well as on the parameters governing the duration of the vortex shedding process. The IAG model, initially developed based on the first-order BL formulation and recently improved by reducing the number of constants and removing compressibility effects, is applied instead in its standard form only. The two models are compared across a range of oscillation mean angles, amplitudes, and reduced frequencies. Results demonstrate that the original BL model, although with a challenging calibration process, when properly tuned, can provide a very good description of aerodynamic unsteady loads. While showing consistent results, the IAG formulation appears to be more robust, as it employs fewer constants and extracts most of the needed information directly from the input polar data. The comparison between the calibrated BL and IAG models highlights critical modelling aspects, the computation of drag and determination of the stall onset above all, offering valuable insights for the future development of dynamic stall formulations.

Fluid-solid interaction simulations of an aeroelastic square prism in sinusoidal oscillatory flows

This study numerically investigates the aerodynamic and aeroelastic characteristics of a square prism (aeroelastic model) and wind field around it in sinusoidal oscillatory flows (SOFs). The reliability of the fluid-solid interaction (FSI) simulation is validated by a free vibration test and wind tunnel tests in smooth flow and SOF. The effects of the amplitude and frequency of SOFs are studied at the mean wind speed of vortex-induced resonance. The results show that increasing the amplitude and frequency of SOFs will amplify the root mean square (RMS) along-wind and across-wind base shear forces of the aeroelastic model but decrease the RMS across-wind displacement at the top of the aeroelastic model. The spectral analysis of the base shear forces indicates that the influence of vortex shedding on the across-wind base shear force is reduced by either increasing the amplitude or increasing the frequency of SOFs. The mean and instantaneous wind fields around the aeroelastic model in SOFs and smooth flow are compared, and the wake characteristics of the aeroelastic model in SOFs are analysed by dynamic mode decomposition. It is observed that when the frequency of SOFs is 1.5 times as large as the fundamental natural frequency of the aeroelastic model, the regular vortex shedding process is substantially affected.

Application of singular spectrum analysis to nonstationary time series in flow-induced vibrations of a circular cylinder

In numerical simulations of flow-induced vibrations (FIV) of a circular cylinder, abundant time series data are available, including cylinder displacement and acting forces. Singular spectrum analysis (SSA) is employed to deal with nonstationary multi-component time series produced in two FIV cases with proper interpretation in physics. In the first case, the cylinder displacement time series is decomposed into two oscillatory components using SSA. The instantaneous frequencies of these two components are obtained by Hilbert transform (HT) and found to be in agreement with the wavelet transform of the cylinder displacement. In the second case, three oscillatory components are extracted from the cylinder displacement time series by SSA. The dominant component is characterized by steady oscillations at the vortex shedding frequency, which suggests a relatively steady vortex shedding process behind the rear cylinder. In contrast, the second component, which is closely associated with the alternate boundary layer separations from the front cylinder, features in the increasing amplitude with time. This implies that the unsteady flow field in the gap might be attributed to the nonstationary cylinder oscillations. This work demonstrates that SSA, in conjunction with HT, enables a comprehensive time-frequency analysis of nonstationary time series obtained in FIV.

Self-excited vortex-acoustic lock-in in a bluff body combustor

We investigate self-excited vortex-acoustic lock-in in a bluff body combustor. The bluff body not only anchors the flame but also sheds vortices. Mutual interaction between vortex shedding and the combustor acoustic field leads to lock-in. A lower-order model for vortex shedding is coupled to the acoustic equations to obtain a set of discrete dynamical maps, which are then solved numerically. Lock-in is identified when a definite phase relationship between vortex shedding and acoustic field remains unaltered with time. The common frequency during (phase) lock-in is either close to the natural acoustic or vortex shedding frequencies, accordingly termed A- or V-lock-in, respectively. Instability and amplitude suppression occur with most parts of the A- and V-lock-in regions, respectively. Furthermore, we relate the lock-in and pre-lock-in regimes observed in our previous experiments (Singh & Mariappan, Combust. Sci. Technol., 2019, pp. 1–29) to A- and V-lock-in phenomena, respectively. Among the two lock-in phenomena, combustion instability is favoured by $1:1$ A-lock-in, whose boundaries in the parameter space can be obtained from a forced response of the vortex shedding process. Finally, we discuss the bifurcations leading to lock-in.

Wake-induced vibration and heat transfer characteristics of three tandem semi-circular cylinders

This paper studies the wake-induced vibration (WIV) and heat convection of the downstream tandem bluff bodies under the influence of an upstream bluff body at Re = 200 and Pr = 0.7. For the reduced fluidic velocity (Ur) in the range of 1–16 and different bluff body diameter ratios (d/D), three wake interference patterns, namely, the quasi-co-shedding (QCS) pattern, the co-shedding (CS) pattern, and the coupling between QCS and CS (QCS-CS) are formed due to the interference between the shear layer and vortex shedding process. The occurrence conditions of the three patterns are closely related to Ur and d/D. The vibration response of the midstream bluff body is roughly consistent with that of a heat-isolated semi-circular cylinder (HISC) if d/D is small. By increasing d/D, the vibration amplitude of the midstream bluff body first increases, then decreases with Ur. The vibration amplitude of the downstream bluff body starts to increase from small d/D and then can maintain a high value with the increase of Ur, even at large d/D. Compared with the HISC, the tandem configurations can help reduce the drag force. Because of the wake interference and the ‘blocking effect’ from the upstream bluff body, although the heat convection of the midstream and downstream bluff bodies can be effectively strengthened through WIV, the time-averaged Nusselt numbers (NuA) of the midstream and downstream bluff bodies are still lower than that of the HISC. Besides, heat transfer efficiency has been found to correlate with the transverse vibration amplitude of bluff bodies. Moreover, it is revealed that there is a trade-off between the heat transfer efficiency of the equipment and its service lifespan because increasing the vibration amplitude enhances convection but also increases the risk of potential damage to the structural integrity.

Vortex-induced vibrations in an active pitching flapping foil power generator with two degrees of freedom

The energy harvesting characteristics of actively pitching flapping foils under a two-degrees-of-freedom (2DOF) system were investigated through numerical simulations. At a Reynolds number of 1100, the effects of the pitching amplitude, reduced frequency, and structural parameters on the energy harvesting performance were compared with the traditional one-degree-of-freedom (1DOF) case. The optimal pitching amplitude (85°), reduced frequency (0.16), and structural parameters (bx*=0.5, kx*=0.7) of the streamwise vibrating flapping foil were determined. The additional velocity generated by streamwise vibrations increased the optimal reduced frequency and pitching amplitude over the traditional case. Streamwise vibrations accelerate the wake propulsion, and the wake vortevx spacing is about 0.8 times the chord length larger than that of the traditional case. Furthermore, the 2DOF case allows the vortex-shedding process of the flapping foil to participate in wake propulsion. The trajectory of the streamwise vibrating flapping foil was observed to be a figure “8” shape. The “8” shape gradually regularizes with an increased streamwise damping coefficient. There is an ideal parameter combination at the optimal reduced frequency that allows the flapping foil to reach the most unstable motion mode. The energy harvesting efficiency of the flapping foil can be increased by up to 25% due solely to vortex-induced vibrations of the 2DOF.

Simulation-based study of low-Reynolds-number flow around a ventilated cavity

Ventilated cavitating flows are investigated via direct numerical simulations, using a coupled level set and volume of fluid method to capture the interface between the air and water phases. A ventilated disk cavitator is used to create the cavity and is modelled by a sharp-interface immersed boundary method. The simulation data provide a comprehensive description of the two-phase flow and the air leakage and vortex shedding processes in the cavitating flow. The mean velocity of the air phase suggests the existence of three characteristic flow structures, namely the shear layer (SL), recirculating area (RA) and jet layer (JL). The turbulent kinetic energy (TKE) is concentrated in the JL in the closure region, and streamwise turbulent fluctuations dominate transverse fluctuations in both SL and JL. Budget analyses of the TKE show that the production term causes the TKE to increase in the SL due to the high velocity gradients, and decrease in the JL due to streamwise stretching effects. Air leakage and vortex shedding occur periodically in the closure region, and the one-to-one correspondence between these two processes is confirmed by the velocity and volume fluid spectra results, and the autocorrelation function of the air volume fraction. Moreover, the coherent flow structures are analysed using the spectral proper orthogonal decomposition method. We identify several fine coherent structures, including $SL_{KH}$ induced by the Kelvin–Helmholtz instability, $SL_{out}$ associated with large-scale vortex shedding, $SL_{in}$ associated with small-scale vortex shedding, and $SL_{r}$ associated with upstream turbulent convection. The present study complements previous research by providing detailed descriptions of the turbulent motions associated with the violent mixing of air and water, and the complex interactions between different characteristic structures in cavitating flows.

Large-scale sinusoidal gust effect on aerodynamic pressures and forces on a square cylinder

Aerodynamic pressures and forces on a two-dimensional square cylinder were experimentally investigated in large-scale sinusoidal gusts. The effects of streamwise and transverse gusts with a wide range of gust amplitudes were studied at a Reynolds number of Re = 2.1 × 104. To enhance our understanding of the large-scale sinusoidal gust effect, the pressure fields and global aerodynamic forces of the square cylinder in large-scale gusts were compared with those in smooth and grid-generated turbulent flows. This study shows that both large-scale streamwise and transverse gusts can promote early pressure recovery, resulting in larger mean pressures on the side and leeward surfaces. The root mean square (RMS) of the pressures on the side surface increased and decreased with an increase in the streamwise and transverse gust amplitudes, respectively. The mean drag on the square cylinder was reduced under the effects of large-scale streamwise and transverse gusts. The increase in the streamwise and transverse gust amplitudes significantly affected the RMS of the drag and lift on the square cylinder, respectively. A proper orthogonal decomposition (POD) analysis showed that the contribution of the first POD mode switched from fluctuating lift to drag when the streamwise gust amplitude was very high. The spectral analysis demonstrated that the alternating von Kármán vortex-shedding process still dominated the fluctuating lift under the effect of large-scale gusts. In addition, the spanwise correlations of drag were significantly strong under the effects of large-scale streamwise gusts. However, the spanwise correlations of lift were reduced under the effects of large-scale gusts with high gust amplitudes.

Wake flow control of a square cylinder via distributed jets over the rear porous surface

An active control method was used to attenuate the unsteady vortex shedding in the wake of a square cylinder. Distributed jets were applied to the rear porous surface at a Reynolds number of 1.07 × 104. The strength of the jets was measured using the dimensionless jet momentum coefficient CQ. The dynamic mode decomposition, instantaneous vorticity evolution, and time-averaged characteristics, e.g., turbulent kinetic energy, streamlines, velocity profiles and Reynolds stress distributions of the wake flow field were used to analyze the control effects. The results suggest that with increasing CQ, two jet vortices gradually form near the rear cylinder wall and compete with the vortex shedding. Consequently, the vortex shedding pattern became nearly symmetrical, whereas the jet vortices remained asymmetrical. The vortex shedding process was pushed downstream. The fluctuations in the wake, particularly near cylindrical walls, were significantly suppressed. Thus, this scheme was successful, indicating potential for drag reduction and energy efficiency. Better control effects were associated with higher CQ in the present study.

Turbulent flow around a short rectangular cylinder in uniform flow at moderate Reynolds numbers

The influence of Reynolds number (Re) on the wake characteristics, Kelvin-Helmholtz instabilities and von-Kármán vortex shedding process around a short rectangular cylinder with length to height ratio of 0.5 was studied using particle image velocimetry. The Reynolds numbers based on cylinder height and free-stream velocity ranged from 3000 to 21000. The flow characteristics were analyzed using the instantaneous and mean velocity fields, as well as the Reynolds stresses, while the unsteady dynamics were elucidated using frequency spectra, proper orthogonal decomposition (POD) and reverse flow area. The results demonstrate that the size of the wake vortex reduced but the magnitudes of the Reynolds stresses increased with increasing Re, until Reynolds number independence is reached at Re = 10000. Regardless of Reynolds number, spectral analyses of the velocity fluctuations, POD mode coefficients and fluctuating reverse flow area showed dominant peaks at the same fundamental vortex shedding frequency. Meanwhile, instantaneous flow visualization showed that the structural coherency of the large-scale vortices increased with increasing Reynolds number. Cross-correlation analysis also revealed that the odd and even POD mode coefficients of the first two mode pairs, respectively, acted to contract and expand the instantaneous reverse flow area at the higher Reynolds numbers.

Flow-Induced Forces for a Group of One Large and Several Small Structures in the Sheared Turbulent Flow

Evaluating the hydrodynamic force fluctuations acting on each structure in a group of subsea objects of different cross-section shapes, sizes and relative positions represents a challenge due to the sensitivity of the vortex shedding process, especially for a variety of sheared flows. The present study uses the numerical 2D computational fluid dynamics model to estimate the flow-induced forces on a group of small circular and D-shaped cylinders in the linear and parabolic sheared flow, which are placed in proximity to a larger structure of the squared cross-section. This allows us to evaluate loads, which are affected by the presence of subsea equipment located on the seabed. The average Reynolds number of the considered linear flow profile is 3900, while the parabolic flow profile has the maximum Reynolds number of 3900. The k-ω SST turbulence model is used for simulations. The work demonstrates the effect of the cross-sectional shape of smaller cylinders on hydrodynamic coefficients, explores the effect from the spacing in between the structures and highlights differences between loads in the linearly sheared and parabolic flow. The results obtained show that the presence of the squared cylinder notably influences the mean drag coefficient on the first cylinder, for both circular and D-shaped cylinders. The parabolic sheared flow profile in this series leads to the highest mean drag and the highest amplitudes of the fluctuating drag and lift coefficients.

Effect of Cross Thermal Buoyancy on Cu–H2O Nanofluid Flow Over Bluff Objects at Low Reynolds Numbers

Increasing the solid volume fraction (φ) in a nanofluid may trigger the vortex shedding around a bluff object even for a low Reynolds number (Re) steady regime. The cross thermal buoyancy may also trigger the vortex shedding around bluff objects at low Re. When the nanofluid flow is subjected to cross buoyancy, the initiation of the vortex shedding process around bluff objects could be accelerated. For a given range of Re, thermal buoyancy and solid volume fraction decide the characteristics of the flow. Both these two parameters can separately have a critical value at which the shedding process initiates. However, the presence of one parameter could affect the other significantly. In order to substantiate the above facts, a two-dimensional numerical simulation is performed to study the effect of cross thermal buoyancy on the free stream nanofluid (Cu–H2O) flow over two-dimensional square and circular cylinders. The initiation of the shedding process is observed for 10 ≤ Re ≤ 30 and 0% ≤ φ ≤ 10% through computation of the critical Richardson numbers for all the φ in the range. The relevant flow and thermal parameters are also computed to further establish the facts.

A spectral force representation and its physical implication for vortex shedding past a stationary or an oscillating circular cylinder at low Reynolds number

Vortex shedding is an ubiquitous phenomenon behind a bluff body (such as circular cylinder) and becomes more complicated when the body is also in oscillation. It is apparent that periodic behavior must be accompanied by the time-varying force, such as lift and drag (coefficients) with known distinguished cases (say, at Re=200) of low-frequency modulation (LFM), sub-harmonic synchronization (SHS), and normal harmonic synchronization (NHS). In a classical analysis, the force spectrum is often analyzed by the Fourier transform or some more recent methods, and typically, a quite complex frequency spectrum is obtained owing to the inherent nonlinearity in the flow system. In the present study, we extend the principal frequency analysis [Lu et al., “An EMD-based principal frequency analysis with applications to nonlinear mechanics,” Mech. Syst. Signal Process. 150, 107300 (2021)] to the principal spectrum analysis (PSA) with both its amplitude and phase in a composite functional form and provide a spectral representation (SR) of the force coefficients only in terms of the characteristic frequencies. In particular, we consider the unsteady laminar flow past a stationary circular cylinder or an oscillating circular cylinder (with frequency f0), while the resulting vortex shedding frequency is denoted by fVS. The spectral representation via the proposed PSA can reveal nonlinear interactions of the two characteristic frequencies (f0 and fVS) in influencing the force coefficients and distinguish direct and interactive modes in which f0 and fVS interact with each other. As a matter of fact, the successively shed vortices are not identical in the strength (amplitude) nor in the phase function. The spectral representation further enables us to identify complicated vorticity activity near around the bluff body: the periodicity of the strength of the shed vortices and the phase shift in the successive vortex shedding—all at the integer multiples of the greatest common-divisor (gcd) of the (two) characteristic frequencies. The gcd frequency of ⟨f0, fVS⟩ is identified as the genuine (slow, long-term) frequency of the entire vortex shedding process in contrast to the (fast, short-term) vortex shedding frequency. It turns out in this scheme of classification by the PSA-SR that all the distinguished types of the above-mentioned LFM, SHS, and NHS can be considered to be gcd-frequency synchronization.

Acoustic receptivity in the airfoil boundary layer: An experimental study in a closed wind tunnel

Airfoil trailing edge noise with a tonal frequency at a medium-Reynolds number (from 2 × 10 5 to 3 × 10 5 in this work) is related to periodic fluctuations in the airfoil boundary layer. Acoustic receptivity plays an important role, in that it constructs a feedback loop to induce ladder-structure phenomena and discrete peak frequencies. The present work is devoted to the experimental study of the acoustic receptivity in the airfoil boundary layer by employing a time-resolved particle image velocimetry method. The symmetric vortex shedding process is noticed, and a hysteresis phenomenon is discovered with the increasing and decreasing wind speed. The author applies the Hilbert transform to show a space-wavenumber spectrum of wall-normal velocity fluctuations to locate resonance points, where acoustic pressure resonates with fluctuations in the boundary layer. The results show that the acoustic reception can affect the local velocity to increase and decrease the wavenumber before and after reach points. The trailing edge noise impacts on the airfoil boundary layer to control the system states and follows the same acoustic feedback loop from Arbey and Bataille [“Noise generated by airfoil profiles placed in a uniform laminar flow,” J. Fluid Mech. 134, 33–47 (1983)].

Effects of streamwise aspect ratio on the spatio-temporal characteristics of flow around rectangular cylinders

Turbulent flows over rectangular cylinders with streamwise aspect ratios (AR) of 1, 2 and 4 were experimentally studied using time-resolved particle image velocimetry at a Reynolds number based on free-stream velocity and cylinder height of 16200. These aspect ratios represent cylinders for which the mean separated shear layers from the leading edges are shed directly into the wake region (AR1), reattach over the cylinder (AR4), and an intermediate case (AR2). The effects of aspect ratio on the transport phenomena, turbulence production and its budget are investigated. The spatio-temporal characteristics and the interactions between the small-scale Kelvin-Helmholtz (KH) vortices and the energetic large-scale von-Kármán (VK) vortices are examined using frequency spectra, while the dynamics of the coherent structures are elucidated using proper orthogonal decomposition (POD). The mean flow topology reveals a massive recirculation bubble in the wake of the AR2 cylinder, compared to AR1 and AR4. While a single dominant vortex shedding frequency is observed for AR1 and AR4, a dual vortex shedding instability is manifested for the AR2 cylinder, which is intimately connected with the intermittent reattachment and detachment of flow over the AR2 cylinder. The interactions between the KH and VK vortices are shown to vary non-linearly with aspect ratio, as the flow transitions from the fully detached case (AR1) to fully reattached case (AR4). The differences in the vortex shedding process are revealed in the dynamics of the first POD mode pair. For AR1 and AR4, the fundamental shedding frequency is dominant and the first POD mode pair contain similar energy content, suggesting regular vortex shedding patterns. For AR2, dual shedding frequencies are observed with significant cycle-to-cycle variation, suggesting irregular vortex shedding patterns.

Experimental investigation on cylinder noise and its reductions by identifying aerodynamic sound sources in flow fields

Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameter-based Reynolds numbers ranging from 3×104 to 1.3×105, covering the sub-critical, critical, and supercritical regimes. The force and noise measurement results show that dimple fabric simultaneously reduces noise and drag in the critical regime. The changes in flow structures were characterized by the Time-resolved Particle Image Velocimetry (TR-PIV) measurements. Based on the vortex sound theory, the flow analysis shows that the dominant sound sources are found to be concentrated near the cylinder surface, which is caused by the unsteady vortex motions near the separation locations during the process of vortex shedding. The cross-correlation between the synchronized TR-PIV and microphone measurements further supports the conclusions. Moreover, the cylinder noise reductions controlled by the dimples are directly associated with the reduced sound sources in the critical and supercritical regimes, corresponding to the reduced strength of the vortex shedding.

Large eddy simulation of flow around two side-by-side circular cylinders at Reynolds number 3900

This paper investigates the flow dynamics around two circular cylinders in a side-by-side arrangement with different spacing ratios (T/D, T is the center-to-center cylinder spacing and D is the diameter) under a subcritical Reynolds number condition (Re = 3900). A three-dimensional (3D) numerical model was developed with Large Eddy Simulation (LES) technique. The model was well validated against published data of flow around a single cylinder at Re = 3900. Numerical simulations were conducted for flow around two side-by-side circular cylinders with T/D = 1.2, 1.5, 1.75, 2, 2.5, 3, 3.5, and 4. Based on the LES results, three wake regimes were identified: single bluff body regime (T/D = 1.2), biased flow regime (T/D = 1.5–2), and parallel vortex streets regime (T/D = 2.5–4). In the single bluff body regime with T/D = 1.2, the stable deflection of gap flow is also observed which indicates that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flip-flopping phenomenon is found to be related to the merging tendency between gap-side vortices in narrow wake region and free-flow-side vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the in-phase mode and anti-phase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the anti-phase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flip-over transitions is closely related to the spacing ratios and is independent of the Reynolds number.

A CFD Study of Vortex-Induced Motions of a Semi-Submersible Floating Offshore Wind Turbine

Vortex-induced motion (VIM) is a critical issue for floating structures made of one or more columns, due to its significant impacts on their operational stability. Supported by column-type floating platforms, floating offshore wind turbines (FOWTs) may also experience large-amplitude VIM responses in current flow. Existing research on FOWTs has mostly focused on their wind/wave induced responses, yet less attention has been paid to their responses in current flow. In this paper, the VIM of the OC4 semi-submersible FOWT platform is studied numerically over a wide range of flow velocity. Three incidence angles, i.e., 0°, 90°, and 180°, are considered and the effect of current incidence on platform VIM is analysed. Results show that the so-called lock-in phenomenon is present and that a large transverse response amplitude of more than 0.3D persists until Vr = 30, with its maximum reaching over 0.8D at Vr = 8. Meanwhile, the transverse response amplitude for cases with the incidence angle of 180° is generally smaller, with a narrower lock-in regime, than those under the other two incidence scenarios. Flow field visualisation reveals that upstream vortices continuously interact with the downstream side column when the incidence angle turns to 180°, impacting the vortex shedding process and consequently fluid forces of the downstream column.