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1. Wavelength-induced shedding frequency modulation of seal whisker inspired cylinders

   https://doi.org/10.1088/1748-3190/ad2b04  

   Dunt, Trevor K.; Heck, Kirby S.; Lyons, Kathleen; Murphy, Christin T.; Bayoán Cal, Raúl; Franck, Jennifer A. (March 2024, Bioinspiration & Biomimetics)

   Abstract The spanwise undulated cylinder geometry inspired by seal whiskers has been shown to alter shedding frequency and reduce fluid forces significantly compared to smooth cylindrical geometry. Prior research has parameterized the whisker-inspired geometry and demonstrated the relevance of geometric variations on force reduction properties. Among the geometric parameters, undulation wavelength was identified as a significant contributor to forcing changes. To analyze the effect of undulation wavelength, a thorough investigation isolating changes in wavelength is performed to expand upon previous research that parameterized whisker-inspired geometry and the relevance of geometric variations on the force reduction properties. A set of five whisker-inspired models of varying wavelength are computationally simulated at a Reynolds number of 250 and compared with an equivalent aspect ratio smooth elliptical cylinder. Above a critical non-dimensional value, the undulation wavelength reduces the amplitude and frequency of vortex shedding accompanied by a reduction in oscillating lift force. Frequency shedding is tied to the creation of wavelength-dependent vortex structures which vary across the whisker span. These vortices produce distinct shedding modes in which the frequency and phase of downstream structures interact to decrease the oscillating lift forces on the whisker model with particular effectiveness around the wavelength values typically found in nature. The culmination of these location-based modes produces a complex and spanwise-dependent lift frequency spectra at those wavelengths exhibiting maximum force reduction. Understanding the mechanisms of unsteady force reduction and the relationship between undulation wavelength and frequency spectra is critical for the application of this geometry to vibration tuning and passive flow control for vortex-induced vibration (VIV) reduction. 

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2. 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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3. Effect of Reynolds Number on Traveling Wave Flow Control

   https://doi.org/10.2514/6.2023-2137  

   Ogunka, Uchenna E.; Akbarzadeh, Amir; Borazjani, Iman (January 2023, AIAA SCITECH 2023 Forum)

   Large-eddy simulations (LES) of the fluid flow over a NACA0018 airfoil at AOA =20 degrees angle of attack are performed to investigate the effect of surface morphing oscillations on the aerodynamic performance of the airfoil over a wide range of Reynolds numbers (Re = 5,000 to 500,000). These oscillations are in the form of low amplitude backward (opposite to the airfoil's forward motion) traveling wave actuations on the upper surface of the airfoil. The sharp interface curvilinear immersed boundary (CURVIB) method is used to handle the moving surface of the airfoil. The nondimensional amplitude is a*=0.001 (a*=a/L; a: amplitude, L: chord length of the airfoil) and reduced frequency (f*= fL/U; f is the frequency and U is the freestream velocity) is chosen to match the leading edge vortex shedding frequency. The results of the simulations at the post-stall angle of attack (AOA =20 degrees) show that the lift coefficient increases more than 20% and the drag coefficient decreases more than 40% within the Reynolds number range of Re = 50,000-500,000 for traveling wave actuation of amplitude, a*=0.001, and frequency, f*=8. However, the lift and drag coefficients of the actuated airfoil were similar to the baseline airfoil for Re = 5,000. 

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4. 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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5. Flow topology changes with bubbly flow around a circular cylinder

   https://doi.org/10.1016/j.ijmultiphaseflow.2024.104917  

   Thacher, Eric; Van_Ruymbeke, Bruno; Gabillet, Céline; Jacques, Nicolas; Mäkiharju, Simo A (September 2024, International Journal of Multiphase Flow)

   Vortex induced vibration (VIV) experienced during flow past a cylinder can reduce equipment performance and in some cases lead to failure. Previous studies have shown that the injection of bubbles in the flow over a cylinder typically leads to a monotonic increase in shedding frequency with void fraction, however, a satisfactory explanation for this phenomenon has not been proposed. Unexplained scatter in the data exists, including that the increase in shedding frequency is not universal. More research is needed to characterize the influence of bubbles on the wake structure, and subsequent shift in shedding frequency. To this aim, the effect of bubbles on the structure of the wake and VIV was examined over two values of Reynolds number, 𝑅𝑒𝐷 = 100, 000 and 160,000. Time-resolved particle image velocimetry (TR-PIV), proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) of the wake structures, vibration of the cylinder, and bubble image velocimetry (BIV) were used to assess the flow topology changes under the influence of gas injection. Using SPOD/POD analysis in the near wake, it was found that the primary Karman shedding frequency decreased with the injection of gas, from a Strouhal number of St = 0.2 to St = 0.17−0.18; the width of the spectral peak was found to increase with void fraction. Notably, the vibration of the cylinder at the primary Karman shedding frequency was suppressed following the injection of gas, even at spanwiseaveraged volumetric qualities below 0.01%. This suppression occurred regardless of if gas was concentrated locally near the centerline of the channel, or along the span. BIV data suggests that gas accumulation in the near wake, driven by the high velocity vertical motion of gas, serves to uncouple the cylinder motion from the formation of the vortex street downstream while promoting faster wake recovery. 

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