Showing papers on "Vortex shedding published in 2023"

Three-dimensional separation over unswept cantilevered wings at a moderate Reynolds number

Abstract: We conduct experiments on the structure of the 3D flow fields for unswept cantilevered wings at high angle of attack. Stall cell counter-rotating vortices on the suction surface form for a range of aspect ratios (AR), though AR changes the angle of attack at which the flow separates. Analysis of mean flow volume over the suction surface and near wake shows that the arch vortex forms in the wake and connects to the surface at the stall cell foci. Reynolds stress peaks indicate a strong reliance on the arch vortex location in the mean flow. Spectral analysis of the velocity field at select spanwise planes shows that shedding is coherent but intermittent, and varies strongly along the wingspan.

Vortex-induced vibrations of two tandem rigidly coupled circular cylinders with streamwise, transverse and rotational degrees of freedom

Abstract: This paper numerically investigates the effects of rigid coupling and the vibration degrees of freedom on the vortex-induced vibrations (VIV) of two tandem circular cylinders for a spacing ratio L/D = 4 at a Reynolds number of 150. Two (translational vibration) and three (translational and rotational vibration) degrees of freedom (RC-2DOF and RC-3DOF, respectively) cases are considered and compared with the case of two freely vibrating cylinders (FC). The dynamic response characteristics, flow structures and vortex dynamics are analyzed. The results show that the rigid coupling has a significant effect on VIV. Compared with FC, the maximum transverse amplitude of the downstream cylinder decreases by 30% for RC-2DOF and approximately 15% for RC-3DOF. The lock-in region narrows by 40% for RC-2DOF, while it widens by 80% for RC-3DOF. Reattachment and co-shedding regions are observed for FC and RC-3DOF, whereas only the co-shedding region exists for RC-2DOF. We further explore flow forces and fluid-structure-interaction mechanisms in the lock-in region for RC-3DOF. The rotation of the twin-cylinder system triggers some unique vibration characteristics, such as two transverse amplitude peaks of the upstream cylinder and a prominent streamwise amplitude at Vr = 7-9. The vortex-to-vortex and vortex-to-cylinder interactions are complicated and changeable with reduced velocity for RC-3DOF.

Effects of spacing ratio on vortex-induced vibration of twin tandem diamond cylinders in a steady flow

Abstract: Vortex-induced vibration of twin tandem square cylinders at an inclined angle of 45{degree sign} to the fluid, i.e., twin diamond cylinders of mass ratio m*=3, is numerically investigated at Reynolds number Re=100 and reduced velocity Ur=3~18. This paper focuses on the effects of cylinders' spacing ratio L* (= L/ B, where L is cylinders' center-to-center spacing, and B is the characteristic length) ranging from 2 to 6 on the oscillation responses of two-degree-of-freedom cylinders. The results indicate that the wake structure experiences two gap flow patterns, the reattachment and co-shedding regimes, and eight different wake modes. At a small spacing ( L*=2~3), the reattachment regime occurs for the lower or higher Ur with the approximate range of 3 and 16~18. Meanwhile, the reattachment regime mainly occurs for other ranges of Ur at L*=2~6. The more significant oscillation of each spacing appears in the cross-flow direction, and the maximum cross-flow amplitude of the upstream cylinder is smaller than that of the downstream cylinder. Additionally, although significant cross-flow oscillations occur at small spacings ( L*=2~3) with the Ur≈5~9 and 12~14, the intrinsic mechanisms are entirely different. As for the cross-flow oscillation characteristics of larger spacings ( L*=4~6), they are virtually similar.

Lagrangian particle tracking velocimetry investigation of vortex shedding topology for oscillating heavy spherical pendulums underwater

Abstract: Abstract The vortex shedding topology of a heavy pendulum oscillating in a dense fluid is investigated using time-resolved three-dimensional particle tracking velocimetry (tr-3-D-PTV). A series of experiments with eight different solid to fluid mass ratios $m^*$ in the range $[1.14, 14.95]$ and corresponding Reynolds numbers of up to $Re \sim O(10^4)$ was conducted. The period of oscillation depends heavily on $m^*$. The relation between amplitude decay and oscillation frequency is non-monotonic, having a damping optimum at $m^* \approx 2.50$. Moreover, a novel digital object tracking (DOT) method using vorticity-magnitude iso-surfaces is implemented to analyse vortical structures. A similar vortex shedding topology is observed for various mass ratios $m^*$. Our observations show that first, a vortex ring in the pendulum's wake is formed. Soon after, the initial ring breaks down to two clearly distinguishable structures of similar size. One of the two vortices remains on the circular path of the pendulum, while the other detaches, propagates downwards, and eventually dissipates. The time when the first vortex is shed, and its initial propagation velocity, depend on $m^*$ and the momentum imparted by the spherical bob. The findings further show good agreement between the experimentally determined vortex shedding frequency and the theoretical vortex shedding time scale based on the Strouhal number.

Passive Control of Ultra-Span Twin-Box Girder Suspension Bridges under Vortex-Induced Vibration Using Tuned Mass Dampers: A Sensitivity Analysis

Abstract: Suspension bridges’ in-plane extended configuration makes them vulnerable to wind-induced vibrations. Vortex shedding is a kind of aerodynamic phenomenon causing a bridge to vibrate in vertical and torsional modes. Vortex-induced vibrations disturb the bridge’s serviceability limit, which is not favorable, and in the long run, they can cause fatigue damage. In this condition, vibration control strategies seem to be essential. In this paper, the performance of a tuned mass damper (TMD) is investigated under the torsional vortex phenomenon for an ultra-span streamlined twin-box girder suspension bridge. In this regard, the sensitivity of TMD parameters was addressed according to the torsional responses of the suspension bridge, and the reached appropriate ranges are compared with the outputs provided by genetic algorithm. The results indicated that the installation of three TMDs could control all the vulnerable modes and reduce the torsional rotation by up to 34%.

Dynamic kirigami structures for wake flow control behind a circular cylinder

Abstract: A flow passing through a bluff body can produce Karman shedding vortex streets in its wake flow, resulting in strong unsteady loading and vibration. Existing passive control methods can disturb the wake flow, but are usually effective only under certain conditions and cannot adapt to changing environments due to their fixed topographies. Kirigami structures (the art of paper cutting) demonstrate programable out-of-plane buckling deformation under simple force actuations. By stretching and relaxing these kirigami sheets, an array of tilted surface elements can be easily activated and deactivated on the surface of a bluff body. For the first time, kirigami structures are used to achieve dynamic passive flow control. The control performance on the wake flow of a cylinder is validated in a wind tunnel using particle image velocimetry. Activated kirigami structures can push the shedding vortices further downstream from the cylinder by about four times of the uncontrolled one and reduce peak values of the turbulent intensity and Reynolds shear stress by 70% and 50%, respectively. The control performance is largely dependent on the height and shape of the kirigami structures.

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

Abstract: 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.

Control of Turbulent Flow over a Circular Cylinder Using Tabs

Abstract: In this study, we investigate tabs applied to turbulent flow over a circular cylinder for the reductions of the mean drag and lift fluctuations. Tabs are small and thin passive devices attached to the upper and lower surfaces of a circular cylinder near the flow separation. The Reynolds number considered is Re= 3900, based on the free-stream velocity and cylinder diameter. Large eddy simulations are performed using a dynamic global subgrid-scale eddy-viscosity model. A parametric study is carried out to find the optimal tab configuration for minimizing the mean drag and lift fluctuations. Parameters considered are the height (ly) and width (lz) of the tabs, and spanwise spacing (λz) between them. With the optimal parameters, the spanwise coherence of the vortex shedding behind the cylinder is effectively disrupted, resulting in three-dimensional vortical structures varying in the spanwise direction. As a result, the strength of the vortex shedding in the wake is successfully weakened, and the mean drag and lift fluctuations are significantly reduced by 14% and 95%, respectively, with the optimal tab configuration of ly/d=0.2, lz/d=0.3, and λz/d=4, where d is the cylinder diameter.

Relationship between vortex shedding noise and remotely-sensed surface pressure fluctuations of a structured porous-coated cylinder

Abstract: Tonal noise suppression of a cylinder placed in uniform flow has been achieved, to some extent, by coating it with a structured porous material as a form of passive flow and noise control. A previously studied structured porous-coated cylinder is investigated in an anechoic wind tunnel to determine the relationship between the far-field vortex shedding noise and the pressure recorded on the outer porous surface. To date, no experimental studies have been conducted on the surface pressure of any type of porous-coated cylinder. Acoustic measurements are obtained using an equispaced microphone arc array and simultaneously unsteady surface pressure fluctuations are obtained around the cylinder mid-span circumference using remote-sensing techniques. By obtaining simultaneous time-dependent signals, more light is shed on the underlying noise-reduction mechanism of the structured porous-coated cylinder. In this paper, strong relationships between surface pressures and acoustic signals are revealed at the vortex shedding frequency. A spatio-temporal relationship between surface pressure and vortex shedding phenomena is also presented that helps explain the role of the structured porous media in passive flow and noise control.

Effect of Wavy Leading Edges on Airfoil Trailing-Edge Bluntness Noise

Abstract: Among the several noise-generation mechanisms of airfoil self-noise, trailing-edge bluntness noise is an important noise source, which is caused by the vortex shedding at blunt trailing edges. A numerical study on airfoil trailing-edge bluntness noise control using the bio-inspired wavy leading edges is presented in this paper. The high-fidelity, improved, delayed, detached eddy simulation (IDDES) method was used to calculate the flow field, and the acoustic analogy method was used for noise prediction. For both the blunt-trailing-edge airfoils, a baseline airfoil with a straight leading edge and a bio-inspired airfoil with a wavy leading edge were used in this study. The chord-based Reynolds number was 400,000, and there was no angle of attack. The numerical results show that the trailing-edge bluntness noise of the baseline airfoil was significantly reduced by the wavy leading edges. The sound pressure level reduction was about 3.7 dB at the characteristic frequency, and the maximum sound pressure level reduction was as high as 35 dB. The trailing-edge bluntness noise was decreased at all directional angles. The maximum overall sound pressure level reduction was 6.3 dB at 0°. In addition, by analyzing the pressure fluctuations, wake characteristics, turbulent vortex structures and spanwise correlation and coherence of the flow field, the noise-reduction mechanisms of the bio-inspired airfoil are deeply revealed.

Modal analysis for three-dimensional instability coupling mechanisms in turbulent wake flows over an airfoil

Abstract: We investigate the instability coupling mechanisms between the shear layer and the wake for turbulent separated flows over a NACA 0012 airfoil at Reynolds number of $23, 000$ with the use of spectral proper orthogonal decomposition and the bispectral mode decomposition. The datasets are obtained from the large-eddy simulations in two configurations: two-dimensional (2D) and three-dimensional (3D) with spanwise periodicity. The 2D flow is characterized by the periodic vortex shedding from the shear layer and the convection of these vortices into the wake, resulting in a frequency spectrum that peaks at the natural shedding frequency and its harmonics. On the contrary, the 3D flow exhibits distinctive frequency contents for the shear layer and the wake with the presence of dominant frequency associated with the coupled coherent structures. Spectral proper orthogonal decomposition (SPOD) is used to examine the spatial and temporal coherent fluctuation for both the flows. The SPOD is also performed with a weighting matrix that highlights the shear layer where certain frequencies higher than dominant shedding frequency are promoted which marks the the shear layer physics in the 3D flow. The coupled coherent structures in the 2D and 3D flows have different self-driving mechanisms. The coupling mechanism in the 2D flow is observed to be governed largely by the interactions between the fundamental shedding frequency and its harmonics. For the 3D flow the self-driving mechanism that supports the emergence of coupled coherent structures is mainly attributed to mean flow interaction. We believe the identification of active paths of triadic interactions can serve as a building block toward the design of active flow control.

Coherent Structures and Pressure Fluctuations over an Airfoil Using Time-Resolved Measurements

Abstract: This study focused on the unsteady behaviors of large-scale vortical structures and wall pressure fluctuations and their coupling mechanism on a low-Reynolds-number airfoil. Three incidence angles were chosen for comparison: [Formula: see text], and [Formula: see text] (representing two flow regimes, namely, the separation bubble formation and separation without subsequent reattachment). Simultaneous measurements of velocity fields and wall-pressure fluctuations were performed using a high-resolution time-resolved particle image velocimetry operating at 2 kHz and an array of flush-mounted microphones sampled at 10 kHz. The results provided a detailed description of the shear layer transition and the coherent structures development on the airfoil. The roll-up process, pairing process, spatial-temporal growth of the vortical structure, and vortex break-up were clarified through a long-period analysis. Spectral analysis demonstrated that vortical structures at the fundamental frequency were spatially and temporally localized and that their subharmonics were significantly enhanced. Frequency–wavenumber spectra revealed that the energy was dominantly concentrated along the convective ridge. With increasing incidence angle, the energy level of the inclined ridge increased, with a lower extension in the frequency domain and a higher dispersion in the wavenumber domain. The conditional averaging and quadrant analysis results demonstrated that strong events in the near-wall region were dominated by the sweep motion and ejection motion, which enhanced the interaction between the inner and outer layers. This also indicated the existence of a strong relationship between the wall-pressure peaks and turbulence production mechanisms.

Numerical study of viscoelastic flow around an oscillating circular cylinder

Abstract: The viscoelasticity-induced fluid-structure interaction studies have a significant influence on practical applications. To clarify the lock-in phenomenon and the wake topology of the vibrating cylinder placed in the viscoelastic flow, the Oldroyd-B fluid flows around an oscillating circular cylinder have been numerically investigated at $Re=10$ and $Re=60$, respectively. The governing equations are solved by the coupling of the SRCR approach and the DG method in framework of the high-order dual splitting scheme. Besides, the ALE formulation is implemented in the coupling procedure in order to account for the interaction between the fluid and the oscillating body in the flow field. With this, complex boundary movements can be tackled simply and efficiently. The force coefficients and the wake structures of vortex and stress are discussed in some detail. At $Re=10$, when the frequency of cylinder is small, it is obvious that the vortex shedding takes place in the wake. As the frequency increases, almost no obvious vortex shedding is observed. And the wake still oscillates at the same frequency of the cylinder for all cases, even for high $Wi$ numbers. However, different wake modes of vortex and stress are found for various frequencies at $Re=60$ and $Wi=0.1$. In the lock-in region, the 2S mode of wake type are observed. Beyond the lock-in region, the wake type is no longer 2S but the formation of vortex shedding and stress distribution in the far wake recovers to its natural mode. These numerical results open up a new field of study for viscoelastic fluids.

Active control of vortex shedding past finite cylinders under the effect of a free surface

Abstract: This paper presents the analysis of the active flow control promoted by low-aspect-ratio cylinders under the effect of a free surface at a low Froude number, modeled as a slip-allowing plane. To advance the literature in this merit, that is scarce compared with infinitely long and surface-mounted bodies, we carry out Detached-eddy simulations at Reynolds number of 1000 to investigate the active control provided by eight spinning rods surrounding a larger body. One of the ends of this system was immersed in the free stream, while the other was in contact with a free water surface. Our results reveal that when the rods spun with sufficiently large angular velocities, the (non-Kármán) vortex street was progressively organized and the part of the wake associated with the mechanism of vortex formation described by Gerrard [“The mechanics of the formation region of vortices behind bluff bodies,” J. Fluid Mech. 25, 401–413 (1966)] was eliminated. Nevertheless, tip-vortices prevailed throughout the examined range of spinning velocities. We also contrasted drag mitigation with power loss due to viscous traction and found that to reduce the mean drag on the system to a lower value than that of the bare cylinder necessarily required power expenditure. Steady reduction of mean drag and less significant mitigation of root mean square of lift and mean side force were verified to occur for the entire system and for the central body. However, the side force proved less affected by the wake-control mechanism. We demonstrate this to be associated with a novel ring-like vortex that prevailed throughout the simulations. Vortex dynamics and formation of these turbulent structures are explored.

Wake tail plane interactions for a tandem wing configuration in high-speed stall conditions

Abstract: Abstract In this work, wake-tail plane interactions are investigated for a tandem wing configuration in buffet conditions, consisting of two untapered and unswept wing segments, using hybrid Reynolds-Averaged Navier–Stokes / Large Eddy Simulations (RANS/LES) with the Automated Zonal Detached Eddy Simulation (AZDES) method. The buffet on the front wing and the development of its turbulent wake are characterized, including a spectral analysis of the fluctuations in the wake and a modal analysis of the flow. The impact of the wake on the aerodynamics and loads of the rear wing is then studied, with a spectral analysis of its lift and surface pressure oscillations. Finally, the influence of the position and the incidence angle of the rear wing is investigated. For the considered flow conditions, 2D buffet is present on the front wing. During the downstream movement of the shock, the amount of separation reaches its minimum and small vortices are present in the wake. During the upstream movement of the shock, the amount of separation is at its maximum and large turbulent structures are present accompanied by high fluctuation levels. A distinct peak in the corresponding spectra can be associated with vortex shedding behind the wing. The impingement of the wake leads to a strong variation of the loading of the rear wing. A low-frequent oscillation of the lift, attributed to the change of the intensity of the downwash generated by the front segment, can be distinguished from high-frequent fluctuations that are caused by the impingement of the wake’s turbulent structures.