Vortex Shedding Flow Research Articles

Feature-based manifold modeling for the quasiperiodic wake dynamics of a pair of side-by-side cylinders

We propose a feature-based manifold modeling (FeMM) framework for the quasiperiodic wake dynamics of a pair of side-by-side cylinders. The key enabler is to embed the most parsimonious mean-field manifold based on the extracted features, such as force coefficients and probing data from experiments and numerical simulations. The manifold model is then identified under the mean-field constraints of the model structure, ensuring human-interpretability. The FeMM method is demonstrated with a two-dimensional incompressible flow crossing a pair of side-by-side cylinders, exhibiting a flip-flopping wake in quasiperiodic behavior. The transient and post-transient dynamics are characterized by two coupled oscillators associated with vortex shedding and gap flow oscillations. Dynamic mode decomposition analysis reveals significant modal interactions between these two flow mechanisms, posing a serious challenge to projection-based modeling approaches, such as the Galerkin projection method. Nevertheless, the FeMM approach, based on force measurements, yields an interpretable model that accounts for the mechanisms underlying the quasiperiodic dynamics, demonstrating its applicability to higher-order dynamics with multiple scales and invariant sets. This approach is expected to have broad applicability in dynamic modeling and state estimation in various real-world scenarios.

Influence of surface roughness on vibration response characteristics of flexible structures on offshore floating platforms

The expanding interest in floating offshore renewable energy technologies, including floating wind turbines and solar platforms, is attributed to their multifarious benefits. One pivotal factor influencing the power generation efficiency of these technologies is the vibration response of the flexible structures connected to the platforms, such as mooring systems, dynamic power cables, and flexible pipelines. In this research, a numerical investigation is conducted on the response characteristics of vortex-induced vibration (VIV) of cylinders with varying surface roughness in oscillatory flow. The unsteady Reynolds-averaged Navier-Stokes (URANS) equations and dynamic equation are used for fluid-structure interaction. The results reveal that for rough cylinders, multiple frequencies are involved in the vibration. For most simulated cases, the vibration frequency in the cross-flow (CF) direction is an integer multiple of the frequency of the oscillatory flow and associated with the quantity of shedding vortex pairs during a single period of the oscillatory flow. For rough cylinders, the CF vibration frequency differs by one from the number of shedding vortex pairs within one period of the oscillatory flow. With the increase of surface roughness, the maximum vibration amplitude shows an increasing trend (12%), then a decreasing trend (21%). Furthermore, the vortex shedding flow pattern presents C + S mode or S mode. This study offers valuable insights for analyzing the vibration response characteristics of authentic flexible structures.

Fluid structure interaction problem for flow past three unequal sized square cylinders at different Reynolds numbers

The flow past three square cylinders of unequal size placed in an inline arrangement is studied using the lattice Boltzmann method at different Reynolds numbers [Re = (u∞ d)/ν] within the range of Re = 120, 150, 160, 175, and 200 for various gap spacings (g = s/d), ranging from 1 to 6. This study focused on the symmetric examination of flow behavior for various gap spacing within the three unequal-sized square cylinders. The main objective of this study was to investigate the effects of Reynolds numbers and gap spacing for flow structure mechanism and vortex shedding suppression in between the gap and down-stream position of all three cylinders. Results are obtained in terms of vorticity contours visualization, drag and lift coefficients, Strouhal number, and physical parameters. In vorticity contour visualization, different flow behaviors are observed, known as flow regimes, and are named according to their characteristics, and they are (i) steady flow regime, (ii) shear layer reattachment flow regime (SLR), (iii) fully developed vortex shedding flow regime, (iv) two-row fully developed vortex shedding flow regime, and (v) fully developed irregular vortex shedding flow regime. The present study also includes a discussion on aerodynamic forces, namely the mean drag coefficient (Cdmean), root mean square of the lift coefficient (Clrms), and Strouhal numbers (St) for three cylinders with sizes d = 20, d1 = 15, and d2 = 10, respectively. The maximum value of Cdmean for the first cylinder (C1) is obtained at (Re, g) = (200, 3) that is, 1.5156, where the existing flow regime is the SLR flow regime, while for C2 and C3, the maximum Cdmean values are examined at critical flow behaviors, where the existing flow regime is a fully developed irregular vortex shedding flow regime. Negative values of Cdmean are also examined for cylinders C2 and C3 at some combinations of (Re, g), attributed to the effect of thrust. Furthermore, it is noticed that the values of Strouhal number are increased with an increment in values of gap spacing. The highest value of the Strouhal number for all three cylinders is observed for C1 at (Re, g) = (120, 5), reaching 0.1556 along with a two-row fully developed flow regime. Furthermore, it is investigated from the present problem that the position of unequal sized square cylinders strongly influenced the flow structure mechanism. The information found and discussed in this study could be effective for structure designing arrangement in the case of three square cylinders of unequal size placed in a horizontal arrangement.

Understanding intracranial aneurysm sounds via high-fidelity fluid-structure-interaction modelling

BackgroundSince the 1960s, the origins of intracranial aneurysm bruits and musical murmurs have been debated, with proposed mechanisms ranging from self-excitation (i.e., resonance) by stable pulsatile flow, to vibration caused by unstable (laminar vortex shedding or turbulent) flow. This knowledge gap has impeded the use of intracranial sounds a marker of aneurysm remodelling or rupture risk. New computational techniques now allow us to model these phenomena.MethodsWe performed high-fidelity fluid-structure interaction simulations capable of understanding the magnitude and mechanisms of such flow-induced vibrations, under pulsatile flow conditions. Six cases from a previous cohort were used.ResultsIn five cases, underlying flow instabilities present as broad-band, random vibrations, consistent with previously-described bruits, while the sac also exhibits resonance, rocking back and forth in different planes of motion, consistent with previously described musical murmurs. Both types of vibration have amplitudes in the range of 0.1 to 1 μm. The murmurs extend into diastole, after the underlying flow instability has dissipated, and do not exhibit the characteristic repeating frequency harmonics of previously hypothesized vortex-shedding mechanisms. The remaining case with stable pulsatile flow does not vibrate. Spectrograms of the simulated vibrations are consistent with previously reported microphone and Doppler ultrasound recordings.ConclusionsOur results provide a plausible explanation for distinct intracranial aneurysm sounds and characterize the mechanical environment of a vibrating aneurysm wall. Future work should aim to quantify the deleterious effects of these overlooked stimuli on the vascular wall, to determine which changes to the wall makeup are associated with vibration.

Features and mechanisms of asymmetric wake evolution downstream of two parallel circular cylinders

Uniform flow past side-by-side circular cylinders is a classical fluid dynamic model that triggers rich phenomena, from which asymmetric wakes usually emerge. Such asymmetry arising from a completely symmetric geometric setting is of theoretical interest when exploring the system bifurcations. Using direct numerical simulation, a detailed parametric map of the wakes behind two side-by-side circular cylinders is presented with several asymmetric wake patterns. These include asymmetric anti-phase (AAP), typical and special deflected (DF), and in-phase (IP) flows, for which AAP and special DF flows are discovered for the first time. Additionally, the IP flow is simulated by both two- and three-dimensional grids to explore the effect brought by three-dimensional vortical structures. The evolution of these asymmetric wakes is analyzed in different phases, with the aid of the wavelet transform, Hilbert–Huang transform, and dynamic mode decomposition, to reveal their temporal variations of developing features. Interestingly, although revealing with distinct fully developed flow fields, there are several common dynamics identified among these wake patterns: AP and IP vortex shedding, wake transition, and gap flow oscillation. The vicissitudes of dynamic flow evolution allow us to further differentiate several wake patterns and ultimately contribute to a deeper understanding of asymmetric flows.

Harnessing Instability Mechanisms in Airfoil Flow for Data-Driven Forecasting of Extreme Events

For certain Reynolds numbers, airfoils are subject to sporadic high-amplitude fluctuations in the aerodynamic forces. These extreme excursions may be seen as prototypical of the kind of unsteady and intermittent dynamics relevant to the flow around airfoils and wings in a variety of real-world applications. Here we investigate the instability mechanisms at the heart of these extreme events, and how they may be harnessed for efficient data-driven forecasting. Through a wavelet and spectral analysis of the pressure and vorticity, we find that the extreme events arise due to the instability of a specific frequency component distinct from the vortex shedding mode. During these events, this extreme event frequency draws energy from the energetically dominant vortex shedding flow and undergoes an abrupt transfer of energy from small to large scales. We propose a preprocessing algorithm to extract this extreme event frequency from the surface pressure data, which in conjunction with an extreme event-tailored loss function, allows us to avoid the commonly used long short-term memory architecture in favor of a simple feed-forward network—a significant reduction in cost over the previous state-of-the-art. Our model requires only three pressure sensors, and it is robust to their location—showing promise for the use of our model in dynamically varying applications. Finally, we show that relying solely on the statistics of the pressure and drag data for optimal sensor placement fails to improve model prediction over uniform or random sensor placement.

Numerical investigation of local scour around a vertically vibrating subsea pipeline under steady flow

Numerical simulations are conducted to investigate the local scour below a vertically vibrating subsea pipeline for gap ratios ranging from −0.3 to 0.1 and a wide range of reduced velocities ranging from 0.5 to 15. Positive and negative gap ratios represent the cases where the bottom of the pipeline is above and below the flat sand surface, respectively. The novelty of the work is the frequency analysis of the vibrating pipeline specially as a function of time as scour and erosion progression occurs. Both scour rate and the vibration amplitude are found to change suddenly at a critical time for reduced velocities inside the lock-in regime. The existence of the critical time is caused by the change of the response mode due to scour. Two types of critical time are found: Type 1 critical time where the vibration transitions from synchronisation to desynchronisation and Type 2 critical time where the flow transitions from steady flow to vortex shedding flow. The variations of the vibration amplitude with the reduced velocity for all the gap ratios follow the same trend and a distinct lock-in regime is found, but the width of the lock-in regime reduces with the reduction of the gap ratio. For all the studied gap ratios, the maximum scour depth consistently occurs at the reduced velocity of Ur = 5, which is the lower boundary of the lock-in regime.

A New Method of Identifying the Aerodynamic Dipole Sound Source in the Near Wall Flow

Consider that the sound dipole source in the flow field is composed of multiple micro-spherical oscillating sources. An aerodynamic sound source identification method is established by the relationship among the oscillating source, the radiated sound pressure, and the pressure gradient of flow in the near-wall flow field, and the formula for calculating the sound power of the sound dipole source in unsteady flow is derived. It shows that the power of sound dipole sources is proportional to the square of the oscillating force or pressure gradient. The combination of the formula and CFD method is further applied to the flow around the cylinder, which clearly presents the sound power and location characteristics of sound dipole sources. Further, the relationship between the sound source and the flow separation, or flow vortex shedding, is analyzed. The corresponding correlation analysis is also carried out, which indicates that the sound dipole source exists in a finite area of the attached wall. The front end of the area is at the separation point along the circumferential direction of the wall, and the end is at the location where the separation vortex completely falls off and a trailing vortex begins to form. In addition, the thickness of the area exists along the radial direction and gradually increases backward.

Energy harvesting of nanofluid-conveying axially moving cylindrical composite nanoshells of integrated CNT and piezoelectric layers with magnetorheological elastomer core under external fluid vortex-induced vibration

This paper deals with the renewable energy that can be harvested as electrical voltage due to the vibrations of an axially moving composite nanoshell conveying nanofluid. The significance of the study and its main innovation is the investigation of the possibility of yielding electrical energy caused by natural mechanical vibrations in a smart dynamic composite nanostructure, which can have potential applications in the nano industry. To this end, oscillations of nanocomposite interacting with the internal nanofluids are investigated under both non-periodic and the harmonic excitations of an external vortex fluid flow colliding with the outer layer, as a natural phenomenon. The considered composite nanoshell consists of three integrated layers of piezoelectric, magnetorheological elastomer, and carbon nanotube. As a theoretical methodology, using the Navier-Stokes equations and energy method, the governing coupled mechanical–electrical equations of dynamic composite nanoshells are derived based on the Euler-Bernoulli beam theory applying nonlocal-piezo-elasticity theory utilizing the van der Pol equation for the external vortex shedding flow. The vortex fluid flow leads to the appearance of the added mass and damping terms in the motion equations. The effects of velocities of moving nanocomposite, passing internal nanofluid, and external vortex fluid, the effects of nano-scale structure and aspect ratio, the effect of applied magnetic field intensity on the smart middle layer, the internal and external fluid densities effects (gas/liquid), and the effect of electrical resistive load on the frequency response and the output voltage amplitude of the dynamic nanosystem are also studied. As one of the significant obtained results, it is revealed that the output voltage due to the vortex-induced vibration (VIV) of the nanosystem will significantly increase as the densities of both internal fluid flow and external vortex fluid flow decrease. In addition, the extractable voltage can be increased or decreased by controlling the intensity of the magnetic field through its effect on the magnetorheological layer.

Numerical simulations of flow around dual tandem circular cylinders under a strong axial magnetic field

This work is dedicated to understand the magnetohydrodynamic (MHD) flow around two identical tandem circular cylinders confined in a duct under external axial magnetic field. The influences of magnetic field (represented by the Hartmann number Ha), inlet velocity of conducting fluid (represented by the Reynolds number Re), and the gap ratio (L/d, the ratio of the distance between the centers of two cylinders to the diameter of the circular cylinder) on the flow regimes, vortex shedding frequency, pressure coefficients, drag coefficients, and pressure drop are investigated. The simulations are conducted in the parameter ranges 2≤L/d≤8, 180≤Re≤1440, and 101≤Ha≤1818, respectively. Four flow modes are observed, namely, no vortex shedding, single body, reattachment regime, and vortex shedding (VS) modes, with various Ha/Re2 and L/d values. For L/d>5, the flow field presents the VS flow model, a vortex street with synchronous periodic shedding appears behind the two cylinders, and the vortex shedding frequency matches well with that of the single one. The pressure coefficient shows different tendencies because of the arrangement of two circular cylinders. It can be found that the mean drag coefficients for the upstream circular cylinder and the downstream circular cylinder vary with Re/Ha0.8. For Re=361, Ha=504 and Re=722, Ha=1212, the flow field remains almost unchanged, which implies the balance between inertial force and Lorentz force because the inertial force would promote the flow, while the electromagnetic force leads to the reverse effect. Furthermore, the effects of magnetic field and inlet velocity of metal fluid on pressure drop ΔP between inlet and outlet can be fitted as a linear relation ΔP∼ReHa.

Effects of surface roughness on two-degree-of-freedom vortex-induced vibration of a circular cylinder in oscillatory flow

The vortex-induced vibration (VIV) response performances of a two-degree-of-freedom circular cylinder with different surface roughnesses were numerically investigated in this work. Four typical roughnesses values of 1 × 10−4, 5 × 10−3, 1 × 10−2, and 2 × 10−2 were examined for the range of the reduced velocity (Vr = 1–20). The VIV response characteristics including the vibration amplitude, vibration frequency, VIV trajectory, vortex shedding flowing pattern, and hydrodynamic force for different rough cylinders were systematically compared. The numerical results showed that the VIV response of roughness cylinder experiences five evolutions as Vr increases, including five typical X–Y trajectories: centrosymmetry, “M” or “W,” “dough twists” or “candy,” “∞,” and asymmetrical ∞, exhibiting three vortex shedding flow pattern: S mode, C + S mode, and SS mode (a pair of symmetric vortices along the x-axis) under certain conditions. The VIV response was sensitive to the roughness in regime IV and regime V. Furthermore, the normalized vibration frequency of the rough cylinder was an integer multiple of the oscillatory flow frequency, except for the Vr ranged from 6 to 7. The vibration frequency in in-line (IL) direction was consistent with the frequency of the oscillatory flow, which was immune to the change of roughness. Additionally, when roughness was equal to 2 × 10−2, the reduction of maximum vibration amplitude in the cross-flow direction reached 31.2%, comparing with smooth cylinder, whereas the maximum vibration amplitude in the IL direction increased only by 6.2%.

High-order optimal mode decomposition analysis of the ground effect on flow past two tandem inclined plates

Two-dimensional flow past two tandem near-ground plates with inclination angles of 25° at the Reynolds number of 150 is numerically simulated via the high-order spectral element method. Plate-to-ground gap is varied from G = 0.2L to 1.6L with intervals of 0.2L at two representative inter-plate spacings (i.e., X = 2.5L and 6L). The ground effect on the fluid force, power spectral density, asymmetric gap flow, and wake structure of plates is systematically evaluated. Then, the high-order optimal mode decomposition (HOOMD) method is proposed to synchronously analyze the velocity and pressure fields. The results show that the fluid force and flow structure are closely dependent on G. The presence of the ground inhibits vortex shedding when G < 0.6L; as the gap increases from 0.6 L to 1.4 L, the fluctuating forces are continuously enhanced until the ground effect basically disappears at G > 1.4L. The ground effect exacerbates the asymmetry of the vortex structure near the upper and lower parts of the inclined plates, consequently changing the fluid force. The downstream plate is more sensitive to the ground effect because of impingement from the upward-biased jet flow generated in the narrow gap between the upstream plate and ground. The HOOMD method well captures the spatial morphology and temporal evolution features of different dominant modes at the transition or vortex shedding flow regime. Mode analysis affords a correspondence between the coherent vortex structure and fluid force of plates. Furthermore, the ground effect can simultaneously change the global mode energy and local pressure mode shape, subsequently influencing the fluid force. However, the global mode energy plays the determinant role in the variation of the fluid force of plates with the plate-to-ground distance herein.

Passive Flow Control around Two Side‐by‐Side Square Cylinders Using a Control Plate

This article is concerned with the numerical analysis of reduction of fluid forces (drag and lift) and suppression of vortex shedding around two side‐by‐side square cylinders using the lattice Boltzmann method. For this purpose, a passive flow control methodology is adopted in which a flat plate (termed as the control plate) is placed at the upstream position of cylinders. The gap ratio (g) between the cylinders and the plate is varied from 0 to 4 while the height of the control plate is varied from 0.2 to 0.8 at fixed Reynolds number of 150. Different flow patterns are found in this study depending on the gap ratios and different heights of the control plate. These flow patterns are single bluff body flow, flip‐flopping flow, and antiphase vortex shedding flow patterns. The narrow gap ratio (g = 0.5) is found to have more impact in terms of force reduction and flow control as compared to larger gap ratios. This article also shows that the fluid flow parameters such as the mean drag coefficient, the Strouhal number, and rms values of drag and lift coefficients of both cylinders reduce considerably due to the control plate as compared to without the control plate. In this regard, it is found that the mean drag coefficient of upper cylinder reduces upto 45.94%, the Strouhal number of both cylinders reduces upto 18.66%, while a maximum reduction of 83.47% and 95.39% is observed in root‐mean‐square values of drag and lift coefficients of upper cylinder, respectively. Another important finding of this article is that the control plate does not suppress forces and control flow at all gap ratios and heights of control plate considered in this article. In some cases, the flow‐induced forces were found to be increasing as compared to without the control plate, and this generally happened at higher gap ratios.

An LES study of the wake flow dynamics and heat transfer characteristics of two side-by-side finite wall-mounted square cylinders

This paper studies the wake flow dynamics and heat transfer characteristics of two side-by-side finite wall-mounted square cylinders through large-eddy simulations at Reynolds and Prandtl numbers of Re = 1.2 × 104 and Pr = 0.7. The influence of normalized center-to-center distances between cylinders (S/d, where d is the side of the cylindersʹ cross-section) is investigated for S/d = 2–5, while both cylinders pose an identical aspect ratio of AR = 7. First, the numerical approach is validated against the published experimental data. Then the instantaneous and time-mean wake flow structures, as well as the global quantities, turbulence statistics and heat transfer are studied. It is found that the biased/flip-flop flow at S/d = 2 turns into anti-phase coupled vortex shedding flow at S/d ≥ 3. This flow regime transition induces a relatively sharp change in drag coefficient and vortex shedding frequency, minimum values at S/d = 2 and maximum values at S/d = 3. The Nusselt number mildly changes at S/d ≥ 3. The turbulent deflected gap flow at S/d = 2 results in inequality of drag forces on and heat transfer from two cylinders. That is, the drag coefficient and Nusselt number are larger for the cylinder towards which the gap-flow is deflected. It is also shown that swinging the deflected gap flow at S/d = 2 is a more intense source of turbulence than the vortex shedding from lateral faces, albeit the Reynolds stress is magnified with increasing S/d. The flow interference is steadily reduced with S/d, almost faded at S/d = 5.

Particle clusters within inertial vortical flows in micro-cross-shaped channels

Inertial vortical flows can be used as a tool to capture and manipulate microparticles, vesicles, or cells. Current work follows our previous report [Zhang et al., Int. J. Multiphase Flow 150, 104030 (2022)] to study the flow of diluted particle suspension in micro-cross-shaped channels at 20 < Re < 500 by micro-laser-induced fluorescence and high-speed photography. Effects of inlet aspect ratio (α) and Reynolds numbers (Re) on flow regimes and particle capture were studied. Numerical simulation was adopted to reveal vortex breakdown dynamics associated with particle capture. For each α, as Re increases, segregated flows, steady engulfment flows, vortex shedding flows, and unsteady engulfment flows appear in turns. Experimental results demonstrate a flow-induced, Re and α-dependent particle cluster within steady engulfment and vortex shedding flows, and an increase in α decreases the onset Re of the cluster. With increasing Re, an interesting oscillation of the cluster is established, which triggers the escape of captured particles. Simulation results show that the oscillation frequencies of the cluster and fluid are comparable. Although isolated particles display brief recirculating paths under unsteady engulfment flows, the particle cluster disappears.

Vortex Shedding Dynamics Behind a Single Solar PV Panel Over a Range of Tilt Angles in Uniform Flow

Solar photovoltaic (PV) panels are very slender structures that can be equipped with a tracking system to adjust their orientation and maximise their energy yield. Theses slender structures are exposed to wind loads and their aerodynamic response can vary considerably depending on the wind speed and operating tilt angle (θ) that can be in the range of ±60∘. Large-eddy simulations are performed to unveil the governing mechanisms involved in the vortex shedding and mean flow separation around a solar PV panel. Our results show that three regimes can be distinguished: at θ=±10∘, leading-edge vortices are shed and convected along the panel’s surface without significant flow separation; at θ=±10∘−35∘, a low-frequency large-scale structure governs the vortex shedding with less-energetic tailing- and leading-edge vortices being shed at higher frequencies; and, at θ=±35∘−60∘, the flow on the suction side is fully separated by non-symmetric vortex shedding due to the proximity of the structure to the bottom ground. The highest Strouhal number is observed for θ=±35∘ at which the tilt moment coefficient is also maximum. Decreasing the distance to the ground slightly increased the Strouhal number for negative tilt angles whilst no changes were observed for positive inclinations.

Momentum and heat transfer characteristics of a rotating elliptical cylinder in vortex shedding flow of power-law fluids

A numerical analysis is carried out to comprehend the fluid flow and heat transfer phenomena of non-Newtonian power-law fluid flow around a rotating elliptic cylinder. The investigations are accomplished for parameters, namely, the aspect ratio of the cylinder, e=0.1; the rotational speed of the cylinder, 0.5≤α≤2.0; the Reynolds number, Re=100; the power-law index, 0.4≤n≤1.6; and the Prandtl number, 1≤Pr≤100. A detailed vorticity and isotherm patterns are presented to demonstrate the vortex shedding and heat transfer phenomenon around the cylinder. The results clearly show the strong dependency of fluid behaviors and rotational speed on flow and heat transfer phenomena. At a low rotational speed (α≤1.0), the standard vortex-shedding patterns appear. The behavior of the fluid mainly affects the size and strength of the vortices. At a higher rotational speed (α > 1.0), a hovering vortex (HV) appears for Newtonian and shear-thinning fluids. The shear-thinning tendency of the fluid encourages the formation of HV; however, the HV is not observed for shear-thickening fluids. Due to the rotational motion of the cylinder, the surface Nusselt number varies in a periodic manner with time/orientation. As expected, the Prandtl number (Pr) and shear-thinning (n<1.0) behavior of the fluid encourage the heat transfer from the cylinder. The rotational motion of the cylinder also favors the heat transfer phenomena. Finally, a correlation is presented for the time average surface Nusselt number (Nuavg) as a function of the Prandtl number (Pr), fluid behavior (n), and rotational speed (α).

Numerical computations of flow around three equilateral-triangular square cylinders with rounded corners using momentum exchange-based IB-LBM at low Reynolds numbers

In this study, the flow around three square cylinders with rounded corners in equilateral-triangular arrangements were investigated using the momentum exchange-based immersed boundary-lattice Boltzmann method (IB-LBM) at low Reynolds (Re) numbers. The influences of Re number and gap ratio δ to the flow characteristics, force coefficients, and Strouhal (St) number were examined by selecting one hundred different gap ratios δ and forty different Re numbers. The wake flow patterns are divided into six distinct categories according to the gap ratio, these are single bluff-body, deflected, flip-flopping, anti-phase, anti-phase & in-phase coexist, and fully developed in-phase flow patterns. In addition, the Re number was also found to have a significant impact on the vortex shedding frequencies and flow properties. The drag coefficients, lift coefficients, and St number were calculated to demonstrate the relationship between vortex shedding properties, gap ratio, and Re numbers. The results revealed that the wake flow pattern, flow characteristics, and force coefficients depended significantly on the gap ratio and Re numbers. In addition, the power spectral density of the upstream cylinder were significantly lower than those of downstream cylinders for low gap ratios, and the mutual interference between each cylinders vanished as the gap ratio increased.

Numerical study on flow-induced vibrations of elastically mounted rectangular cylinders without and with barrier walls

This work presents a two-dimensional (2D) numerical study on the flow-induced vibrations (FIV) of elastically mounted rectangular cylinders without and with barrier walls in a wide range of reduced velocities U*=1–200 and Scruton numbers Sc=0–64 at the Reynolds number of 1000. It reveals that a slender rectangular cylinder of aspect ratio 5:1 without barrier walls may experience vortex-induced vibration (VIV), while the one with barrier walls can have both VIV and soft galloping motions. The VIV motions of both cylinders occur at relatively small reduced velocities U*≤20, and their maximum amplitudes are gradually reduced with the increase in the Scruton number. The galloping motions of the cylinder with barrier walls take place at higher reduced velocities U*>30 with smaller frequencies and larger oscillation amplitudes. Quantitative analysis on the amplitude, displacement, and frequency of oscillation for both cylinders is carried out. Two phase diagrams of the vortex-shedding flow patterns are presented to illustrate the flow characteristics in VIV and galloping modes. Meanwhile, the dynamic mode decomposition analysis indicates the difference between the dominant mode of the dynamic flow field in the VIV and galloping motions. With the quasi-steady theory, it further shows that the occurrence of the soft galloping motion of the cylinder with barrier walls is caused by the negative slope of the lift coefficient at the angle of attack zero. These results may shed new light onto deeper understanding of the FIV phenomenon and provide some inspirations to engineering applications in the design of civil and offshore structures.

Mass transfer rate effects on the cavitating vortex shedding flow around a circular cylinder at a low Reynolds number

Cavitating vortex shedding is a flow phenomenon commonly encountered behind bluff bodies in hydraulic machinery and systems. The generated vapor bubbles significantly change the dynamic behaviors of the vortices. In addition, the collapses of the cavities become major sources of unwanted effects such as vibration, noise and material erosion. In this study, the numerical results of the cavitating vortex shedding behind a circular cylinder at a low Reynolds number have been compared and analyzed using different mass transfer rate values between the vapor and the liquid water phases. For that, the transport equation for the vapor volume fraction coupled with the source term based on the simplification of the Rayleigh-Plesset equation has been solved. Commonly, this source term that accounts for the mass transfer rate is a function of the pressure difference, the vapor volume fraction, and the empirical factors. To understand the effects of this rate, its has been gradually increased from its default value up to the infinite when the equilibrium assumption is satisfied. The current results show that as the rate increases, higher gradients of the vapor volume fraction and of the pressure near the interface between the vapor and the liquid phases are predicted. As a consequence, the frequency of the vortex shedding is also affected by the variation of the mass transfer rate.