Search Results (65)

Local scour and vortex-induced vibrations around cylindrical structures in curved channels pose significant risks to the safety and stability of critical hydraulic infrastructure, such as bridge piers. To address these engineering challenges and elucidate the underlying flow mechanisms, this study conducts numerical simulations of flow past two side-by-side circular cylinders of equal diameter in a curved channel under subcritical conditions at Re = 3900, using the Realizable kε turbulence model. Spectral Proper Orthogonal Decomposition (SPOD) is introduced to quantitatively characterize the energy distribution and dominant coherent structures. Taking the spacing ratio L/D and the placement angle α as key design parameters, the flow field characteristics, modal energy distribution, and coherent structure evolution are systematically investigated for two side-by-side cylinders in three-dimensional straight and curved channels. The numerical results show that, in the straight channel, as L/D increases from 2 to 4, the flow field evolves from strong coupled interference to weak interaction. The vortex shedding frequency structure evolves from a single dominant frequency to a multi-frequency distribution with rich harmonic components, indicating a transition in wake dynamics from energy concentration to multimodal dispersion, accompanied by a significant improvement in flow stability. Under curved channel conditions, the results reveal an asymmetric flow field caused by pronounced energy concentration on the inner side of the channel. SPOD analysis further indicates that as the placement angle α increases from 30° to 90°, the modal energy distribution changes from concentrated to dispersed, the frequency spectrum broadens with enhanced harmonic components, and flow instability gradually intensifies. Overall, the spacing ratio L/D mainly governs the wake-interference pattern, whereas the placement angle α regulates the frequency structure and energy distribution. Among all the cases investigated, relatively favorable flow stability is achieved at L/D = 4 and α = 30°. The SPOD-derived modal energy distributions show that the streamwise fluctuation length of the dominant-mode energy is approximately 0.25 m at α = 30°, compared with 0.5 m at α = 90°, with the energy bandwidth nearly doubling. The combined CFD-SPOD approach effectively captures energy evolution and coherent structure characteristics of complex flows across spatial, temporal, and spectral dimensions. This enables a shift from conventional flow-field description to frequency-based mechanism analysis and provides a theoretical basis for structural layout optimization and scour protection in hydraulic engineering. Full article

This paper presents a dynamic model of a top-tensioned riser (TTR) subjected to combined vortex-induced vibration (VIV) and time-varying tension excitation. The model employs a van der Pol oscillator to simulate load excitation caused by vortex shedding and incorporates a tuned mass damper (TMD) to suppress nonlinear vibrations in the riser. The key contributions include, first, employing the Galerkin method to obtain a multi-mode approximate solution and analyzing it using single-mode approximate equations, and subsequently, applying a multi-scale approach to investigate the vibration reduction effect of the TMD under two typical resonance scenarios. By introducing a complex impedance term derived from the complex transfer function, the physical effect of the TMD is transformed into a frequency-dependent dynamic reaction force coupled to the riser’s equation of motion. The first involves 1:1 internal resonance between the structural frequency and vortex-induced frequency, while the second involves 1:2 parametric resonance between the structural frequency and the top tension frequency. Results indicate that when the structural frequency exhibits 1:2 parametric resonance with the top tension frequency, complex bifurcation behavior occurs, leading to large-amplitude structural responses. Findings demonstrate that TMDs effectively alter the system’s stability distribution and exhibit outstanding vibration-reduction efficiency under both typical resonance conditions. Full article

Increasingly slender bridge decks are prone to wind-induced damage, where the complex interactions between the incoming wind, deck, and adjacent wake flows play a deciding role. However, the unsteady wake dynamics at small but realistic angles of attack and their compact reduced-order representation remain insufficiently understood. The unsteady wakes subject to angle of attack from 3° to 5° are investigated via Koopman analysis with the Dynamic Mode Decomposition (DMD), aiming to construct accurate reduced-order models for largely repeated canonical cases, while preserving physical and phenomenological fidelity. Instantaneous velocity and vorticity fields reveal a clear separation-reattachment cycle: leading edge separation bubbles form and migrate upstream at drag peaks, then collapse and reattach at drag valleys. Shear layers roll up into dual vortices that pair, merge with Kelvin–Helmholtz-type shear-layer instabilities, and alternately shed from the deck’s upper and lower surfaces, driving oscillatory wake deflection and attendant drag and lift fluctuations. DMD identifies four dominant modes that together account for over 90% of the turbulent kinetic energy: time averaged base flow, the fundamental vortex shedding mode, and two higher frequency shear-layer modes. Adequate truncation reduces data dimensionality by an order of magnitude while keeping the normalized error below 6%. The results demonstrate that a DMD-based reduced-order model built on Unsteady Reynolds Averaged Navier–Stokes (URANS) data can faithfully preserve both large-scale separation topology and fine-scale vortical structures across small angles of attack, providing a compact and accurate representation of bridge-deck wakes for repeated canonical configurations. Full article

This study numerically investigates laminar flow around two prismatic bodies, specifically square cylinders, arranged in tandem. The analysis covered gap ratios ($L/D=2$–7) and Reynolds numbers ($Re$ = 100–200), focusing on quantifying the aerodynamic characteristics and examining the wake flow structures within the established interference regimes. The time-averaged and unsteady parameters, including the drag and lift coefficients, RMS lift, vortex formation length, Strouhal number, recirculation length, wake width, and pressure distribution, were evaluated for both cylinders. A consistent critical spacing of $L/D\approx 4.5$ was observed across all Reynolds numbers, coinciding with the minimum Strouhal number, a sharp increase in unsteady lift, and divergence in wake width between cylinders. Notably, in the range $4.5\lesssim L/D\lesssim 6.5$ at higher $Re$, the DC exhibited a mean drag exceeding that of an isolated cylinder, attributed to base-pressure reduction and accelerated inflow from the upstream wake. A critical spacing in the co-shedding regime produced strong drag amplification on the DC, attaining an overall maximum value of 50.41% at $Re=200$ and $L/D=6.0$. To note, unlike mean drag, mean lift is found to be zero in all interference cases for both cylinders, irrespective of spacing ratio and Re, owing to the symmetry of the time-averaged pressure distribution on either side of the cylinders. Spectral and phase analyses reveal a transition from broadband, desynchronised oscillations to a frequency-locked state, with the phase angle between the cylinders reducing sharply to $\Delta \varphi \approx 0^{\circ }$ at the critical spacing. This indicates complete in-phase synchronisation or symmetry of the vortex-shedding process between the cylinders at the critical spacing. This confirmed the hydrodynamic transition between the coupled and independent shedding modes of the cylinders. The recirculation lengths for the DC reduce to as low as $0.6D$ in the co-shedding regime, highlighting rapid wake recovery. The research presented here offers new insights into force modulation, the evolution of wake structures, and the sensitivity to the Re that occurs when laminar flow occurs between two tandem square cylinders. These findings can be utilised to develop methods for controlling VIV and designing thermal-fluid systems. Full article

In the literature, there is a scarcity of studies examining the combined effects of vortex-induced vibrations (VIV) and surface roughness on a bluff body. This paper contributes to the limited studies and literature on VIV by highlighting the pronounced influence of roughness on the vortex formation modes of a circular cylinder forced to oscillate with respect to the freestream. The numerical approach utilizes a purely Lagrangian description through the discrete vortex method with a roughness model. Recent results obtained by our research group have shown that a two-dimensional roughness model is more sensitive than a simple turbulence model in capturing nonlinear multi-physics phenomena with a variety of applications in different engineering areas. In particular, the control of drag force and vortex shedding frequency can be studied based on the expected physics of viscous flow. In the present paper, the dimensionless oscillation amplitude is fixed at A/D = 0.13 (D is the outer cylinder diameter), and the cylinder forcing frequency varies in the range of 0.04 ≤ f_o ≤ 0.80 at a high Reynolds number value of Re = 1.0 × 10⁵. Three relative roughness sizes are chosen, i.e., ε/D = 0.001, 0.0045, and 0.007 (ε is the average roughness). The test cases without roughness effects are compared to experimental visualizations to capture two basic anti-symmetrical modes, namely the A-I and A-IV modes, the symmetric S-I (Type-I) mode, and the Chaotic mode categorized as C-I. Our strategy to identify these wake modes verifies the synchronization between the vortex shedding frequency f_CD, interpreted from temporal history of the drag force on an oscillating cylinder, and the body forcing frequency. In the test cases using the roughness model, it is possible to identify a desynchronization between the frequencies f₀ and f_CD as well as significant variations in the drag force. The roughness effect also provokes a regime of vortex formation, here classified as “A-IV mode with coalescence”. Full article

Marine risers are susceptible to vortex-induced vibrations (VIV) in complex ocean current environments, posing significant risks to structural safety and fatigue life. This study, conducted on the Ansys Workbench platform, establishes a three-dimensional numerical model using bidirectional fluid–structure interaction (FSI) methods. Wet modal analysis is employed to extract the riser’s natural frequencies, followed by a systematic comparison of vibration responses under uniform flow and linear shear flow conditions. The findings indicate that as the vortex shedding frequency approaches the structural natural frequency, the system exhibits pronounced frequency lock-in. Spectral analysis confirms that VIV dominates the dynamic response. Notably, under initial conditions (uniform flow velocity = 0.5 m/s; shear flow velocity = 0.05 m/s, Gradient = 0.025), shear flow induces larger vibration amplitudes. However, as flow velocity increases, uniform flow surpasses shear flow in both amplitude (maximum 0.03 D) and frequency (maximum 0.02 D). Modal analysis demonstrates that uniform flow excites the fourth-order mode, whereas shear flow confines the system in the second-order mode. Additional controlled simulations highlight the critical influence of the shear flow’s initial velocity on vibration modes, providing a theoretical basis for VIV suppression. Full article

The paper describes in detail the creation of selected aerodynamic sound sources created by the centrifugal compressor of the turbocharger in operating modes. The description of the creation of aerodynamic sources focuses on the operation of the compressor in a stable area of the characteristic. The analysis is based on a detailed survey of selected aerodynamic sources, mainly vortex shedding, TCN, and buzz-saw phenomena, with a focus on the mechanism of the source and the possibility of identifying the source in the frequency spectrum. Based on the survey, the selected sound sources characterize the assumed frequency ranges at which the sources are estimated to originate. Additional source conditions identified in the survey can be used to develop a methodology for identifying aerodynamic sound sources. In the case of aerodynamic sources of a centrifugal compressor, it was necessary to develop an experimental numerical methodology for their identification with regard to the operating condition of the compressor. The result of the proposed procedure is an algorithm that will enable the identification of aerodynamic sound sources in the frequency spectrum with respect to the operating state of the compressor. Full article

Long-span cable-supported bridges, such as cable-stayed and suspension bridges, are highly sensitive to wind-induced effects due to their flexibility, low damping, and relatively light weight. Aerodynamic analysis is therefore essential in their design and safety assessment. This study examines the aeroelastic stability of the Oued Dib cable-stayed bridge in Mila, Algeria, with emphasis on vortex shedding, galloping, torsional divergence, and classical flutter. A finite element modal analysis was carried out on a three-dimensional model to identify natural frequencies and mode shapes. A two-dimensional deck section was then analyzed using Computational Fluid Dynamics (CFD) under a steady wind flow of U = 20 m/s and varying angles of attack (AoA) from −10° to +10°. The simulations employed a RANS k-ω SST turbulence model with a wall function of Y+ = 30. The results provided detailed airflow patterns around the deck and enabled the evaluation of static aerodynamic coefficients—drag (C_D), lift (C_L), and moment (C_M)—as functions of AoA. Finally, the bridge’s aeroelastic performance was assessed against the four instabilities. The findings indicate that the Oued Dib Bridge remains stable under the design wind conditions, although fatigue due to vortex shedding requires further consideration. Full article

This study aims to investigate the spatiotemporal evolution of cavitating flow and the associated acoustic responses around a NACA0015 hydrofoil. A coupled fluid–acoustic interaction model is developed by integrating a nonlinear cavitation model with vortex–sound coupling theory. Numerical simulations are conducted within a computational domain established for the hydrofoil to capture the interactions between cavitation dynamics and acoustic radiation. The results indicate that the temporal variations in cavity evolution and pressure fluctuations agree well with experimental observations. The simulations predict a dominant pressure fluctuation frequency of 30.15 Hz, consistent with the cavitation shedding frequency, revealing that the evolution of leading-edge vortex structures governs the periodic variations in the lift-to-drag ratio. Cavitation significantly modifies the development of vortex structures, with vortex stretching effects mainly concentrated near cavitation regions. The dilation–contraction term is closely associated with cavity formation, while the pressure–torque tilting term predominantly affects cloud cavitation collapse. Dynamic mode decomposition (DMD) shows that the coherent structures of the leading modes exhibit morphological similarity to multiscale cavitation and vortex structures. Furthermore, hydrofoil cavitation noise consists mainly of loading noise and cavitation-induced pulsating radiation noise, with surface acoustic sources concentrated in cloud cavitation shedding regions. The dominant frequency of cavitation-induced radiation noise is highly consistent with experimental measurements. Full article

This study investigated the aerodynamic structures generated by transverse jet injection in supersonic flows around high-speed vehicles. The unsteady evolution of these structures was analyzed using an improved delayed detached Eddy simulation (IDDES) approach based on the Reynolds stress model (RSM). The simulations successfully reproduced experimentally observed shock systems and vortical structures. The time-averaged flow characteristics were compared with the experimental results, and good agreement was observed. The flow characteristics were analyzed, with particular emphasis on the formation of counter-rotating vortex pairs in the downstream region, as well as complex near-field phenomena, such as flow separation and shock wave/boundary layer interactions. Time-resolved spectral analysis at multiple monitoring locations revealed the presence of a global oscillation within the flow dynamics. Within these regions, pressure fluctuations in the recirculation zone lead to periodic oscillations of the upstream bow shock. This dynamic interaction modulates the instability of the windward shear layer and generates large-scale vortex structures. As these shed vortices convect downstream, they interact with the barrel shock, triggering significant oscillatory motion. To further characterize this behavior, dynamic mode decomposition (DMD) was applied to the pressure fluctuations. The analysis confirmed the presence of a coherent global oscillation mode, which was found to simultaneously govern the periodic motions of both the upstream bow shock and the barrel shock. Full article

Control valves are the main throttling resistance components in industries such as chemical engineering, nuclear power, aerospace, hydrogen energy, natural gas transportation, marine engineering, and energy systems. Flow-induced vibration fatigue failure is a common failure mode. To provide engineers and researchers with a reference for reliable design analysis of control valves and to predict and prevent potential failures, this article reviews and categorizes vibration-induced failure in control valves by integrating numerous engineering cases and research articles. The vibration failures of control valves are mainly divided into categories such as jet flow, vortex flow, cavitation, and acoustic cavity resonance. This paper reviews control valve vibration research from three aspects: theoretical models, numerical simulations, and experimental methods. It highlights the mechanisms by which internal unstable flow, jet flow, vortex shedding, cavitation, and acoustic resonance lead to vibration-induced fractures in valve components. Additionally, it examines the influence of valve geometry, component constraints, and damping on flow-induced valve failures and summarizes research on vibration and noise reduction in control valves. This paper aims to serve as a reference for the analysis of vibration-induced failures in control valves, helping identify failure mechanisms under different operating conditions and proposing effective solutions to enhance structural reliability and reduce the occurrence of vibration failures. Full article

Time series prediction of aerodynamic noise is critical for oscillatory instabilities analyses in fluid systems. Due to the significant dynamical and non-stationary characteristics of aerodynamic noise, it is challenging to precisely predict its temporal behavior. Here, we propose a method combining variational mode decomposition (VMD) and echo state network (ESN) to accurately predict the time series of aerodynamic noise induced by flow around a cylinder. VMD adaptively decomposes the noise signal into multiple modes through a constrained variational optimization framework, effectively separating distinct frequency-scale features between vortex shedding and turbulent fluctuations. ESN then employs a randomly initialized reservoir to map each mode into a high-dimensional dynamical system, and learns their temporal evolution by leveraging the reservoir’s memory of past states to predict their future values. Aerodynamic noise data from cylinder flow at a Reynolds number of 90,000 is generated by numerical simulation and used for model validation. With a rolling prediction strategy, this VMD-ESN method achieves accurate prediction within 150 time steps with a root-mean-square-error of only 3.32 Pa, substantially reducing computational costs compared to conventional approaches. This work enables effective aerodynamic noise prediction and is valuable in fluid dynamics, aeroacoustics, and related areas. Full article

This study investigates the connection between coherent structures in the flow around a vehicle and the aerodynamic forces acting on its body. Dynamic Mode Decomposition (DMD) was applied to analyze the flow field of a squareback Ahmed body at $R{e}_H=7.7\times {10}^5$. DMD enabled the identification of coherent structures in the near and far wake by isolating their individual oscillation frequencies and spatial energy distributions. These structures were classified into three regimes based on their underlying mechanisms: symmetry breaking, bubble pumping, and large-scale vortex shedding in range of $St\le 0.2$. The energy contributions of these flow regimes were quantified across different regions of the flow field and compared to the aerodynamic forces on the body. Additionally, the linear correlation between pressure and velocity components was examined using Pearson correlation coefficients of DMD spectral amplitudes. The locations of maximum and minimum correlation values, as well as their relationship to energy contributions, were identified and analyzed in detail. Full article

This study assesses the wind resistance and vortex-induced vibration (VIV) risks of the Dongzhou River Bridge in China reconstruction during critical construction stages. Computational Fluid Dynamics (CFD) simulations analyzed wind effects when the twin main girders were maximally separated, revealing asymmetric vortex shedding patterns influenced by upstream–downstream aerodynamic interactions. The upstream girder’s wake generated complex flow fields, increasing turbulence on the downstream girder and indicating elevated VIV susceptibility. A 1:50 scale aeroelastic model validated these findings through wind tunnel tests, confirming that CFD-predicted critical VIV wind speeds aligned with experimental observations. Tests identified a distinct “jump-like” vibration mode at specific wind speeds (35–40 m/s full-scale equivalent), characterized by abrupt amplitude escalation rather than gradual growth—a signature of unstable VIV resonance. However, measured amplitudes remained below the 61.5 mm full-scale equivalent safety threshold, confirming that vibrations posed no critical risk. While aerodynamic coupling between girders requires monitoring during cantilever construction, the study concludes that existing control measures ensure safe construction and operation without structural modifications. These results provide actionable guidelines for wind risk mitigation through construction sequencing and real-time wind speed restrictions. Full article

In this paper, we present a linear stability analysis on flow-induced vibration of an elastically mounted cylinder subjected to forced rotation. Four series of cases, with different combinations of degrees of freedoms in oscillation and Reynolds number are investigated. For each series of cases, a wide range of reduced velocity at various rotation rates are considered. The variations of growth and frequency with reduced velocity for the leading modes are presented. Some phenomena observed in previous numerical studies are interpreted by using the results of linear stability analysis. The supressing of vortex shedding at moderate rotation rate is explained by the absence of unstable fluid mode. The amplitude enhancement in high range of rotaton rate is explained by the emergence of unstable elastic mode. The stability properties of the leading modes provide some new insight into the influences of forced rotation on flow-induced vibration. The results of the current study have important implications in the design of offshore structures and energy-havesting devices. Full article