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1. The dynamics of coherent structures in a turbulent wake past a sphere at Re= 3700

   Zhang, Fengrui; Peet, Yulia T. (July 2022, Turbulence and shear flow phenomena)

   The turbulent wake flow past a sphere at ReD= 3700 is investigated via Direct Numerical Simulation (DNS). The characteristic motions in the wake flow, such as vortex shedding and bubble pumping are identified by the probes placed in the near wake with a dominating frequency of St= fu∞/D= 0.22 and 0.004, respectively. The modal analysis is conducted in the wake area using Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). The vortex shedding and bubble pumping motions are also captured by the modal analysis. The results from POD and DMD show comparable patterns of both characteristic motions. For the bubble pumping motion, the dominating frequency of the corresponding POD mode is St= 0.004, while the DMD mode that is directly related to the separation bubble has the frequency of St= 0.009. 

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2. Harvesting sustainable energy through vortex-induced vibrations of finite length cylinder

   https://doi.org/10.1063/5.0260317  

   Fershalov, Andrei; Elvin, Niell; Orlandini, Pieter; Avros, Ilya; Liu, Yang (April 2025, Physics of Fluids)

   Vortex-induced vibration (VIV) has emerged as a promising method for small-scale energy harvesting. This research explores the key parameters affecting VIV in a cylinder-cantilever beam system within a Reynolds number range of 400–7500. The investigation focused on identifying the airflow velocity thresholds that initiate vibrations, measuring peak vibration amplitudes, and determining the critical airflow velocities where vibrations are maximized. By systematically varying mass, stiffness, and cylinder diameter, we examined their distinct effects on system behavior. Key outcomes indicate that larger cylinder diameters lead to increased vibration amplitudes and broader operational bandwidths, while adding mass reduces the bandwidth. Higher stiffness boosts both the maximum amplitude and bandwidth, shifting these to higher airflow velocities. The lock-in regime was observed to initiate at a Strouhal number (St) between 0.175 and 0.197, with vibration cessation occurring at an approximately consistent Strouhal number for each cylinder diameter. The peak vibration amplitude occurred at St ≈ 0.16, with fluctuations of less than 5% across all models. Additionally, the wake structure behind the cylinder and its behavior across the vibration bandwidth were analyzed using flow visualization techniques. A hot-wire anemometer positioned downstream measured velocity fluctuations from vortex shedding. These findings offer practical insights for optimizing VIV-based energy harvesting, linking wake behavior to amplitude response and power output. This study contributes to the broader understanding of VIV energy harvesters and provides a foundation for validating numerical models and enhancing the efficiency of sustainable energy systems. 

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3. Spectral Analysis of a Cylinder Wake Interacting with Coherent Structures in a Turbulent Boundary Layer

   https://doi.org/10.2514/6.2024-0075  

   Torres_De_Jesus, Elizabeth; Waghmare, Palash; Saxton-Fox, Theresa (January 2024, American Institute of Aeronautics and Astronautics)

   The spectral and spatial behavior of the wake of a small cylinder immersed in a turbulent boundary layer at different wall-normal heights is studied and compared to a canonical turbulent boundary layer. Time-resolved particle image velocimetry measurements were taken downstream of the position where the cylinder is immersed. Measurements were also taken in of the unperturbed turbulent boundary layer in the same region without the cylinder for the same freestream velocity. The pre-multiplied energy spectra was computed for the seven cases and compared. Changes to the spectral content of the wake and of the boundary layer were observed for cases where the cylinder was nearer to the wall, while little interaction was observed for cases with the cylinder outside of the boundary layer thickness. Spectral proper orthogonal decomposition modes were calculated at wavelengths relevant to the wake vortex shedding and to the energetic turbulent structures and modifications to the modes were observed for cases with strong interaction. Vortex detection methods were used to visualize the wake and suggested that both a breakdown of periodicity of the vortex spacing and an overall spatial meandering of the wake may be responsible for the spectral modifications observed. 

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4. Fluid–structure–surface interaction of a flexibly mounted pitching and plunging flat plate in proximity to the free surface

   https://doi.org/10.1017/jfm.2024.172  

   Samsam-Khayani, Hadi; Seyed-Aghazadeh, Banafsheh (April 2024, Journal of Fluid Mechanics)

   This experimental study investigates the fluid–structure–surface interactions of a flexibly mounted rigid plate in axial flow, focusing on flow-induced vibration (FIV) response and vortex dynamics of the system within a reduced velocity range of$$U^*=0.29\unicode{x2013}8.73$$, corresponding to a Reynolds number range of$$Re=518\unicode{x2013}15\,331$$. The plate, with one and two degrees of freedom (DoFs) for pitching and plunging oscillations, is examined at various submerged heights near the free surface. Results show that the plate exhibits divergence instability at low reduced velocities in both 1DoF and 2DoF systems. As the flow velocity surpasses a critical reduced velocity, periodic limit-cycle oscillations (LCOs) occur, increasing in amplitude until a second critical reduced velocity is reached. Beyond this point, LCOs are suppressed, and the plate experiences an increased static divergence angle with further flow velocity increase. The proximity to the free surface significantly influences the FIV response, with decreasing submerged heights leading to reduced LCO amplitudes and a shift of instabilities to higher reduced velocities. Vortex dynamics are analysed using time-resolved volumetric particle tracking velocimetry and hydrogen bubble flow visualisation. The analysis reveals disruptions in the symmetric flow field near the free surface, causing elongation and fragmentation of vortices in the wake of the plate, as well as vortex coupling. Proper orthogonal decomposition (POD) identifies dominant coherent structures, including leading-edge and trailing-edge vortices, captured in the first and second paired modes. On the other hand, higher POD modes capture the interaction of vortices in the wake and near the free surface. 

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5. Verification of DES for Flow over Rigidly and Elastically-Mounted Circular Cylinders in Normal and Yawed Flow

   https://doi.org/10.1016/j.jfluidstructs.2020.102895  

   Wu, X.; Jafari, M.; Sarkar, P.; Sharma, A. (January 2020, Journal of fluids and structures)

   null (Ed.)

   A computational approach based on a k-ω delayed detached eddy simulation model for predicting aerodynamic loads on a smooth circular cylinder is verified against experiments. Comparisons with experiments are performed for flow over a rigidly mounted (static) cylinder and for an elastically-mounted rigid cylinder oscillating in the transverse direction due to vortex-induced vibration (VIV). For the static cases, measurement data from the literature is used to validate the predictions for normally incident flow. New experiments are conducted as a part of this study for yawed flow, where the cylinder axis is inclined with respect to the inflow velocity at the desired yaw angle, β = 30◦. Good agreement is observed between the predictions and measurements for mean and rms surface pressure. Three yawed flow cases (β = 15◦, 30◦, & 45◦) are simulated and the results are found to be independent of β (independence principle) when the flow speed normal to the cylinder axis is selected as the reference velocity scale. Dynamic (VIV) simulations for an elastically-mounted rigid cylinder are performed by coupling the flow solver with a solid dynamics solver where the cylinder motion is modeled as a mass–spring–damper system. The simulations accurately predict the displacement amplitude and unsteady loading over a wide range of reduced velocity, including the region where ‘‘lock-in’’ (synchronization) occurs. VIV simulations are performed at two yaw angles, β = 0◦ and 45◦ and the independence principle is found to be valid over the range of reduced velocities tested with a slightly higher discrepancy when the vortex shedding frequency is close to the natural frequency of the system. 

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