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Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering. The wake formation and vortex shedding behind a variety of ocean structures such as spars, are subjected to fatigue failure limiting their life span. The additional influences due to ocean waves and currents further exacerbate these effects. In the present study, flow past an isolated circular cylindrical structure subjected to an oscillatory upstream are numerically investigated. These studies involve high resolution simulations over the low Reynolds number range (100-200). Although the practical range of interest is in high Reynolds number range of 10 3 - 10 5 , the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows. In the high Reynolds number range, statistical averaging tools in conjunction with suitable closure models would be necessary. The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder. A control algorithm was coupled to determine the quantum of actuation to the rotating elements. Although control of wake vortices was observed for harmonic in-let forcing, residual vortical structures were found to persist at higher amplitudes of oscillation. To study the efficacy of this control, numerical simulations were further extended, when the circular cylinder was flexibly mounted. The control of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.

This paper reports the experimental investigations of the wake of a rotating and translating sphere in the close proximity of a plane wall at moderate Reynolds numbers (Re = 450–2020, defined based on velocity of translation and diameter of the sphere) and for a non-dimensional rotation rate, Ω* (ratio of surface velocity to the translation velocity of the sphere), of 1. For the current gap (h)-to-diameter (d) ratio of the sphere (h/d = 1/16), at Re = 1000, hairpin vortices shed in the wake where looping of small-scale vortices increases with the increase in Re. At Re > 1000, the transition from hairpin type to shear layer vortex shedding occurs. At a relatively high Reynolds number (Re = 1130), the separated vortex sheets from both sides of the sphere exhibit Kelvin–Helmholtz instability, and the wake shows multiple vortical structures. At Re = 2020, a strong interaction of vortex rings in the wake of the sphere and boundary layer at plane wall results in the breakdown of larger vortical structures into smaller scales. The separation angle changes from the equatorial region to the pole of the sphere because of differential rotational speeds at other planes. At midplane, the separation angle increases with Re, and it decreases again when hairpin–shear layer vortex shedding transition occurs. The Strouhal number as a function of Re indicates the presence of dual frequency beyond this transition.

The fluid flow and heat transfer of a nanofluid past a circular cylinder in a rectangular duct under a strong transverse magnetic field is studied numerically using a quasitwo-dimensional model. Transition from laminar flow with separation to creeping laminar flow is determined as a function of Hartmann number and the volume fraction of nanoparticle, as are critical Hartmann number, and the heat transfer from the heated wall to the fluid. Downstream cross-stream mixing induced by the cylinder wake was found to increase heat transfer. The successive changes in the flow pattern are studied as a function of the Hartmann number. Suppression of vortex shedding occurs as the Hartmann number increases.

Vortex shedding phenomenon behind bluff bodies and its destructive unsteady wake can be controlled by employing active and passive flow control methods. In this quest, researchers employed experimental fluid dynamics (EFD), computational fluid dynamics (CFD) and an analytical approach to investigate such phenomena to reach a desired outcome. This study reviews the available literature on the flow control of vortex shedding behind bluff bodies and its destructive wake through the modification of the geometry of the bluff body. Various modifications on the bluff body geometries namely perforated bluff bodies, permeable and porous mesh, corner modification and wavy cylinder have been reviewed. The effectiveness of these methods has been discussed in terms of drag variation, wake structure modifications and Strouhal number alteration.

Flow over bluff bodies produces vortex shedding in their wake regions, leading to structural failure from the flow-induced forces. In this study, a passive flow control method was explored to suppress the vortex shedding from a circular cylinder that causes many problems in engineering applications. Perforated shrouds were used to control the vortex shedding of a circular cylinder at Reynolds number, Re = 200. The shrouds were of non-uniform and uniform holes with 67% porosity. The spacing gap ratio between the shroud and the cylinder was set at 1.2, 1.5, 2, and 2.2. The analysis was conducted using ANSYS Fluent using a viscous laminar model. The outcomes of the simulation of the base case were validated with existing studies. The drag coefficient, Cd, lift coefficient, Cl and the Strouhal number, St, as well as vorticity contours, velocity contours, and pressure contours were examined. Vortex shedding behind the shrouded cylinders was observed to be suppressed and delayed farther downstream with increasing gap ratio. The effect was significant for spacing ratio greater than 2.0. The effect of hole types: uniform and non-uniform holes, was also effective at these spacing ratios for the chosen Reynolds number of 200. Specifically, a spacing ratio of 1.2 enhanced further the vortex intensity and should be avoided.

The unsteady wake of a rotating and translating sphere is experimentally investigated at Reynolds numbers (Re) ranging from 115 to 550 for two rotational speeds. Non-dimensional rotational speed, $$\Omega^{ * }$$ , is defined as the ratio of the maximum azimuthal speed of the sphere and the speed of translation. Motion of the sphere is generated using a specially designed mechanism in a channel, and its wake has been visualized using fluorescent dye. Unlike the steady flow past a stationary sphere where the limiting Reynolds number is 270, the flow becomes unsteady when sufficient rotation rate is introduced at much lower Reynolds number ( $${\text{Re}} = 115,\Omega^{ * } = 0.375$$ ). The transition from one-sided to double-sided hairpin vortex occurs at one critical Re for a given rotational speed, and it switches back to one-sided hairpin vortex shedding at another critical Re. Particle image velocimetry results demonstrate the vorticity distribution in the wake and the variation of the wake width with axial distance. At very low Re, velocity profiles in the wake are nearly self-similar. The strength of the vortex rings weakens in the far wake. The decay of centerline velocity in axial direction confirms the dissipation of the vorticity in the far wake. In this unsteady flow, the time-averaged value of the separation angle is higher than that of the steady flow on the advancing side due to enhanced adverse pressure gradient. The Strouhal number calculated from the dye visualization in this range of $$\Omega^{ * }$$ and Re varies from 0.175 to 0.25, a clear increasing trend both with increasing Re and $$\Omega^{ * }$$ . The coefficient of drag evaluated at the center plane remains constant at 0.94 for the present investigation up to $${\text{Re}} = 550$$ . The wavelength of the unstable hairpin vortices is found to scale inversely with the translational velocity of the sphere.

In this paper, the unsteady natural cavitation and ventilated cavitation are investigated with the large eddy simulation method. The predicted cavity morphology and evolution process before and after ventilation agree well with the available experimental data. The results indicate that in natural cavitation, the formation of a U-type cavity is closely related to the separation of a re-entrant jet and the effect of vortex evolution, while the local high pressure region plays an important role in shaping the appearance of the primary and secondary U-type cavity in the process of moving downstream. During ventilated cavitation, the deflation effect of the air drives the pair of vortices to shed periodically, accompanied by the downstream transportation of vorticity, and promotes the formation of arched air cavities. In the anticlockwise vortex shedding region, the vortex dilatation term has a concentrated distribution on the air-liquid interface which is due to the intense mixing of the phases. Further analysis demonstrates that, in the air cavity fluttering area, the magnitude of the velocity pulsation produces a significant downward tendency due to the gradual weakening of vortex intensity. Moreover, the interaction between vortices and turbulence shows a high degree of consistency in the wake flow.

Numerical simulations of the Vortex-Induced Vibration (VIV) about a large-scale flexible pipe subject to shear flow were carried out in this paper. Efficiency verification was performed firstly, validating that the proposed fluid–structure interaction solution strategy is competent in predicting the VIV response. Then, the VIV characteristics related to multi-mode and spanwise hybrid waveform about the flexible pipe attributed to shear flow were investigated. When inflow velocity rises, higher vibration modes are apt to be excited, and the spanwise waveform easily convertes from a standing-wave-dominated status to a hybrid standing–traveling wave status. The multi-mode or even multiple-dominant-mode is prone to occur, that is, the dominant mode is often followed by several apparent subordinate modes with considerable vibration energy. Hence, the shedding frequencies no longer obey Strouhal law, and vibration trajectories become intricate. According to the motion analysis concerning the coupled cross-flow and in-line vibrations, as well as the corresponding wake patterns, a tight coupling interaction exists between the structural deformation and the wake flow behind the flexible pipe. In addition, the evolution of the vortex tube along the pipe span and a strong 3D effect are observed due to the slenderness of the flexible pipe and the variability of the vortex shedding attributed to the shear flow.

Flow features and vortex shedding in the wake zone behind V-shaped bluff bodies are experimentally investigated. These experiments were done for a single V-shaped body at various Reynolds number between 4.84 x 104 - 5.91 x 104, span angles between 45°-90°, and angles of attack between 0 and 15o . Experiments were also made for two V-shaped bluff bodies arranged in tandem and in the same vertical plane with different spaces between them. With the aid of a water tunnel, the phenomena of vortex shedding and flow recirculation behind the V-shaped bluff body were illustrated. Results show that the increase of Reynolds number monotonically reduces the length of the recirculation zone. A similar flow structure of the flow exists among near wake flows of V shaped bodies with different span angles. Increasing the span angle, the size of the recirculation zone increases. The variation in attack angle changes slightly the size of the recirculation zone and shifts the recirculation zone vertically. For the two bodies arranged in tandem, the size of the recirculation zone increases as increasing the spacing between the two V-shaped bodies. For the two bodies in the same vertical plane, decreasing the spacing between the two bodies yields to earlier mixing between the flows in the two wakes.

The wake of a single-box girder in sinusoidal oscillating flow is studied by three-dimensional large-eddy simulations (3D LES). The validation of numerical simulation is verified by active-controlled wind tunnel experiments. The mean and fluctuating velocity, as well as the vortex shedding distance in the sinusoidal oscillating flow are discussed. The fluctuating velocity near the wake increases with the increasing oscillation amplitude, however it is distribution period independent of oscillation amplitude. The vortex shedding distance decreases as the oscillation frequency increases, meanwhile, the oscillation amplitude has no effect on it.

With the increasing demand of higher travelling speed, a new streamlined high-speed maglev train has been designed to reach a speed of 600 km/h. To better capture the flow field structures around the maglev train, an improved delayed detached eddy simulation (IDDES) is adopted to model the turbulence. Results show that the new maglev train has good aerodynamic load performance such as small drag coefficient contributing to energy conservation. The main frequencies of aerodynamic forces for each car have a scattered distribution. There are two pairs of counter-rotating large vortices in the non-streamlined part of the train that make the boundary layer thicker. Many high-intensity vortices are distributed in the narrow space between skirt plates or train floor and track. In the gap between the train floor and track (except near the tail car nose), the main frequency of vortex shedding remains constant and its strength increases exponentially in the streamwise direction. In the wake, the counter-rotating vortices gradually expand and reproduce some small vortices that move downward. The vortex has quite random and complex frequencydomain distribution characteristics in the wake. The maximum time-averaged velocity of the slipstream occurs near the nose of the head car, based on which, the track-side safety domain is divided.

This study demonstrates an active flow control for deflecting a direction of wake vortex structures behind a NACA0012 airfoil using an active morphing flap. Two-dimensional direct numerical simulations are performed for flows at the chord Reynolds number of 10,000, and the vortex pattern in the controlled and noncontrolled wakes as well as the effect of an actuation frequency on the control ability are rigorously investigated. It is found that there is an optimum actuation-frequency regime at around F + = 2.00 which is normalized by the chord length and freestream velocity. The wake vortex pattern of the well-controlled case is classified as the 2P wake pattern according to the Williamson’s categorization [1] [2], where the forced oscillation frequency corresponds to the natural vortex shedding frequency without control. The present classification of wake vortex patterns and finding of the optimum frequency regime in the wake deflection control can lead to a more robust design suitable for vortex-induced-vibration (VIV) related engineering systems.

Many numerical and experimental researches have been done for the unsteady flow around a flapping airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, and MAV''s lift and thrust generation for the flow over a flapping airfoil. Although many researches have been studied to analyze the flow for the variation of frequency and amplitude, much research is still needed. In this study, thrust generation, wake structures, and acoustic wave propagation characteristics were studied for a flapping airfoil. The governing equation was the Navier-Stokes equation with LES turbulence model, and the Optimized High-Order Compact (OHOC) scheme and 4th order Runge-Kutta method were used. The Mach number and Reynolds number were 0.3 and 1000, respectively. The analysis of mean lift and drag coefficients showed the thrust generation characteristics according to the phase angle and the reduced frequency. The wake vortex shedding and propagation characteristics of acoustic waves could be simulated using the high order method.

Aerodynamic actions on three two-dimensional rectangular cylinders were studied in steady and accelerating wind tunnel experiments. The aim of these experiments was to determine how body aerodynamics is modified during flow acceleration. The acceleration rate studied roughly scales to that experienced during a strong thunderstorm downburst. As such, conclusions are drawn about wind loading during these events. Cylinders with side ratios of B/D = 1/3, 1 and 3 were tested. Results show that when accelerating flow from initially quiescent conditions, the onset of wake instability is delayed to higher Reynolds numbers than expected in steady flow. Following this, 5 or 6 vortex shedding cycles are required for most aerodynamic characteristics to collapse onto expected steady flow values. During this period, drag and lift force coefficients are generally less than in steady flow conditions. Cases that accelerated flow from an initial Reynolds number of 9,300 also led to modified aerodynamic loads, but no such modification occurred when initial Reynolds numbers were larger. This suggests that initial flow conditions, as well as body side ratio, play an important role in defining how flow acceleration modifies aerodynamic loads. Despite the existence of aerodynamic modifications throughout these experiments, pressures or forces greater than experienced during steady flow conditions at the target velocity (as observed by others in impulsively started flow tests) were not observed. As such, when considering the practical application of this work, for the body configurations tested, the level of flow acceleration experienced during strong thunderstorm downbursts are not expected to lead to larger wind loads than in steady flow conditions.

Two-dimensional simulations of heated circular and square cylinders are performed in order to validate a recently introduced entropic thermal lattice Boltzmann model (Frapolli et al. in Phys Rev E 90:043306, 2014), and the new boundary conditions for complex walls and multi-speed lattices. Validation is carried out by comparing simulation results with other reference simulations and experiments. For the hydrodynamic field, the wake structure was analyzed by means of the wake recirculation length for both steady and unsteady two-dimensional regimes, by comparing the Strouhal number of the vortex shedding associated with the first instability of the hydrodynamic field, and qualitatively by showing the streamlines for different Reynolds numbers. For the thermal part of the simulations, isotherms and mean Nusselt number are qualitatively and quantitatively compared with the literature results. All the computed quantities are found to agree excellently with the reference data available in the literature. This opens door for accurate simulations in complex geometries in thermal and compressible flow regimes with multi-speed lattices., Journal of Statistical Physics, 161 (6)

We investigate the large-scale signature of the random switches between two mirrored turbulent wake states of flat-backed bodies. A direct numerical simulation (DNS) of the flow around an Ahmed body at a Reynolds number ( $Re$ ) of 10 000 is considered. Using proper orthogonal decomposition (POD), we identify the most energetic modes of the velocity field and build a low-dimensional model based on the first six fluctuating velocity modes capturing the characteristics of the flow dynamics during and between switches. In the absence of noise, the model produces random switches with characteristic time scales in agreement with the simulation and experiments. This chaotic model suggests that random switches are triggered by the increase of the vortex shedding activity. However, the addition of noise results in a better agreement in the temporal spectra of the coefficients between the model and the simulation.

Despite their simplicity, photovoltaic (PV) modules are often arranged in structures that can be affected by severe and complex wind loads: in this context, the wind flow and the dynamic excitation induced by vortex shedding can introduce unexpected aeroelastic responses. This work introduces a novel wind tunnel application of experimental techniques to address this issue by the use of flow visualisation and video postprocessing, through the optical flow algorithm. Numerical simulations based on unsteady Reynolds-averaged Navier–Stokes (RANS) models are performed and compared against the experimental wind tunnel tests on a PV panel that was also instrumented with pressure taps. A setup with a 65∘ tilt angle was examined because, based on preliminary analyses, it was considered interesting for the free flow–wake transition associated with the dynamic response of the PV panel. The comparison of the experimental and numerical average wind fields supported that the proposed optical flow method was appropriate for characterising the wake of the panel, because there was enough seeding to perform the video postprocessing. Experiments and numerical predictions were compared as regards the average pressure distribution on the panel surfaces, and the average percentage was in the error of 7%, this supports that the URANS method was capable of reproducing the average behaviour of the panel, as well as for the selected configuration, which is particularly challenging. Furthermore, the simulated and measured power spectral densities of the wind speed were compared, and this resulted in the numerical model quite faithfully reproducing the frequency of the peak at 5 m/s, while the error was in the order of 20% for the 10 m/s case, this supports that, despite the URANS approach being affected by well-known critical points regarding the simulation of instantaneous quantities, it can be employed to elaborate information that can be particularly useful for the structural design of the panel. This kind of result can be considered as a first step, obtained with simplified and affordable methods, towards a characterisation of the dynamic behaviour of a PV panel in a real-world setup.

The flow-excited acoustic resonance of aerodynamically isolated side-by-side cylinders is experimentally investigated to identify the influence of the resonant acoustic field on the dynamics of vortex shedding in the wake of the cylinders. Self-excitation of the acoustic cross-modes inside the duct housing the cylinders is achieved when the vortex shedding from the cylinders occurs at locations away from the acoustic particle velocity node of an acoustic cross-mode, with acoustic pressure amplitude dependence on the cylinders’ proximity to the acoustic particle velocity antinode of the excited resonance mode. Phase-locked particle image velocimetry measurements indicate that the acoustic resonance excitation synchronizes the isolated wakes of the cylinders according to their relative phase with the acoustic particle velocity at the cylinder axis plane. Notably, such effect is absent for cylinders with wakes shed at the acoustic particle velocity nodes of a resonant mode. Investigation of the phase between cylinders’ wakes during resonance excitation for individual instants shows that the synchronization is not a perfect lock-in, but a statistical tendency of the wakes to shed vortices at a preferred phase, suggesting that the strong acoustic field engages in constructive or destructive interference with the shed vortices according to their phase. The results presented in this paper prove that the effectiveness of cylinder removal from certain locations in a tube bundle as a method to suppress resonance excitation may be reduced as other cylinders at large spacing may act in sync to excite acoustic cross-modes.

Pumping ventilation is a special single-sided wind driven building ventilation induced by periodic vortex shedding at the building wake. This work aimed to investigate the oscillating frequency at the building facade opening and ventilation rate of pumping ventilation. The effects of opening separations on different floors of a three-story reduce-scaled building with two openings were considered using boundary-layer wind tunnel experiments. The wind velocity of the surrounding flow field and the center of the opening were measured. The ventilation rate was obtained by tracer gas method with constant and continuous injection rate. The results showed that the oscillating frequency was independent of the opening separation except on the third floor probably because of the disturbance of the rooftop shedding frequency. The oscillating frequency on the first floor was the lowest due to the resistance of the ground to the vortex shedding. Pumping ventilation indicated that its ventilation rate is greater than that of general ventilation across the single opening with the same total opening area. The promotion of ventilation rate was up to about 123% on the first floor, about 65% on the second floor and about 44% on the third floor. Expansion of opening separations could boost the ventilation rate. Meanwhile, the fluctuation of opening center wind velocity and indoor species concentration were not positively correlated with the ventilation rate of pumping ventilation. Conclusions of this research could provide some useful reference to the design of natural ventilation for buildings.

The effect of actuation frequency, using moving surface actuation, is investigated for a square cylinder bluff body wake. Pressure sensor data are used to optimize actuation characteristics through the implementation of an NSGA-II evolutionary algorithm. Velocity field data are obtained using Particle Image Velocimetry (PIV) for baseline and optimized actuation cases. A Proper Orthogonal Decomposition (POD) analysis shows that the vortex shedding frequency shifts between frequencies associated with the actuation, moving between regions of lock-on and quasi-periodicity. Additionally, the POD shows that the energy contained in the coherent shedding motion is reduced through actuation, while the total energy in the velocity field stays relatively constant. A reconstruction of the first 10 POD modes indicates that the coherent contribution to the Reynolds stresses significantly decreases compared to the non-actuated case. The mechanism for drag reduction is investigated using the shed circulation flux and Kochin’s drag formulation model. The drag obtained using PIV measurements and Kochin’s formulation is consistent with trends observed for the base pressure as a function of actuation frequency.

Vortex induced vibrations (VIV) of a circular cylinder have been investigated experimentally using a cyberphysical apparatus with m∗ = 8, ζ = 0.005, and Re = 4000. This study considers the application of proper orthogonal decomposition (POD) and spectral POD (SPOD) analysis to the wake dynamics of the low-mass-ratio VIV of a circular cylinder in the lower branch at U∗ = 7.5. SPOD has been previously shown to better separate frequency-centered modal dynamics, compared to POD. Coherent POD and SPOD modes were compared and the newly separated third SPOD mode pair was found to have a periodicity characteristic of vortex shedding and a peak in the temporal coefficient spectra at St = f D/U∞ = 0.2248. The literature has identified that the wake dynamics within the lower branch are synchronized to the cylinder motion; however the present study suggests that some hidden dynamics persist at the Strouhal frequency. Low order models based on the first eight POD and SPOD modes were compared, and it was found that the filtering operation in SPOD removes the uncorrelated stochastic energy component of the POD modes while producing a comparable representation of the coherent deterministic part of the wake dynamics. Using SPOD to separate the distinct frequency-centered dynamics into unique, interpretable mode pairs will simplify future efforts to develop sparse dynamical models of the flow.

In order to understand the wake characteristics for the circular cylinder with the dimpled structure, particle image velocimetry (PIV) and proper orthogonal decomposition (POD) method are used to measure and analyze the flow field velocity and the flow characteristic of the wake flow of the circular cylinder with the dimpled surface. This study focuses on the distribution of Reynolds stress and turbulent kinetic energy, the velocity profiles and recirculation zones, and the velocity fluctuating characteristics of flow field with POD technology. It is found that the equivalent high-intensity Reynolds stress and turbulent kinetic energy regions of the circular cylinder with the dimpled surface are smaller, and the peak values are lower, and the velocity gradient in the wake region of the circular cylinder with the dimpled surface is larger. Otherwise, the energy contained by the dominant modes of the smooth cylinder is larger than that contained by the dominant modes of the circular cylinders with the dimpled surface, which means the energy of the dimpled cylinder is more distributed. At the same time, it is observed that the dimpled structure will decrease the vortex shedding intensity, but may increase the vortex shedding frequency, and destroy the inherent flow mode of the flow field around the cylinder.

In the current research, an active-flow control scheme characterized by leeward slot jet over a circular cylinder was experimentally investigated to manipulate the vortex shedding from the ...

Fishes are talented swimmers. Depending on the propulsion mechanisms many fishes can use flapping tails and/or fins to generate thrust, which seems to be connected to the formation of a reverse von Kármán wake. In the present work, the flow past a 2D flapping foil is simulated by solving the incompressible Navier-Stokes equations in the open-source OpenFOAM platform. A systematic study by varying the oscillating frequency, peak-to-peak amplitude and Reynolds number has been performed to analyze the transition of vorticity types in the wake as well as drag-thrust transition. The overset grid method is used herein to allow the pitching foil to move without restrictions. Spatial convergence tests have been carried out with respect to grid resolution and the size of overset mesh domain. Numerical results are compared with available experimental data and discussed. The results show that the adopted methodology can be well applied to simulate large amplitude motions of the flapping foil. The transitions in the types of wake are consistent with the benchmark experimental data, and the drag-thrust transition of the pitching foil does not coincide with von Kármán (vK)-reverse von Kármán (reverse-vK) wake transition and it is highly dependent on the Reynolds number.

The vortex induced vibrations (VIVs) of a pivoted cylinder with finite height have been numerically investigated. A mathematical model is introduced and described, and the resulting equations are numerically solved for low Reynolds number Re = 100, 200 and several combinations of the governing parameters. Results on the solid body trajectories, the maximum amplitude of the oscillations, the hydrodynamic force coefficients, the wake structure, and details on the vortex shedding near the cylinder are presented and discussed. The numerical results compare reasonably well with the canonical system of VIV of two-degrees of freedom circular cylinder in the laminar regime. Also, qualitative similarities with closely related VIV systems at larger Re suggest interesting lines of future research. Analytical approximations for limiting cases are done and an excellent agreement with the numerical results is obtained. This work was partially financially supported by the “Ministerio de Economıa y Competitividad, Secretarıa de Estado de Investigacion, Desarrollo e Innovacion,” Spain (Ref. TRA2017-88508-R). D. Pastrana acknowledges the support of the CONACyT-SENER graduate fellowship program to study abroad 278102/439162. J. C. Cajas acknowledges the financial support of UNAM through the project PAPIIT-IA106120 and the access to supercomputing resources at Miztli: LANCAD-UNAM-DGTIC-379.

This study is inspired by the velvety structures on an owl's upper wing surface. Anechoic wind tunnel experiments were conducted to study the effect of the velvety structures on trailing edge noise as well as the boundary layer flow of a flat plate model. The tests were conducted in The Hong Kong University of Science and Technology low-speed wind tunnel, ultra-quiet noise injection test and evaluation device (UNITED). It was found that the trailing edge noise spectra are significantly modified by the velvety structures. In general, the velvety structures increase the low-frequency noise below a cross-over Strouhal number $St_c$ but reduce the spectral level at higher frequencies. The velvety surface also changes the boundary layer characteristics in terms of the boundary layer thickness, non-dimensional velocity distribution and turbulence distribution. Vortex shedding is suppressed by the velvety coating despite the blunt trailing edge. An analytic model is proposed for the trailing edge noise of a flat plate, including the effect of finite trailing edge thickness and velvety structures on the flat plate surface. The model uses the near wake distribution of the mean and fluctuating velocities in the streamwise direction as the input. The predictions, which require no empirical corrections, match well with the experiments for both the baseline and velvet-coated configurations. With a detailed non-dimensional analysis, this study proposes a potential aeroacoustic function of velvet structures, i.e. noise control through manipulation of boundary layer statistics.

Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over the pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U∞≈1–5 BL s−1) and in periodic vortex streets (U∞≈2–4 BL s−1). Two different cylinders (∼2 and ∼3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (∼2 cm diameter; U∞≈2 BL s−1). In laminar flows, knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U∞. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins, which resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending and ribbon and pectoral fin movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder, but when placed one cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

International audience; The development of Marine Renewable Energy based on tidal currents at a selected area needs to first analyze the flow characteristics and thus the bathymetry induced turbulence in this area. The present study aims to investigate turbulence characteristics around two wall mounted obstacles in tandem, which represent seabed obstacles in a high Reynolds number flow. 2D planar PIV measurements are performed in multiple planes to access the main flow organization. Based on successive analyses of mean flow characteristics as well as of the flow organization, it is demonstrated that the presence of an upstream cube clearly impacts the cylinder wake development by preventing the rising of the large scale flow structures observed for an isolated wall-mounted cylinder in the same flow conditions. The TKE budget analysis shows that the energy is mainly produced in the upstream cube shear layer, and TKE exchanges are very small in the far wake field, in comparison with a single cylinder flow configuration. A Fourier analysis shows that the cube–cylinder interaction generates a low frequency peak in the far wake field which persists for long downstream distances. This particular frequency is the signature of the cylinder wake coherent structures. A statistical analysis based on the two-point correlation velocity tensor confirms that energetic flow structures present in the far wake are mainly of 2D nature and develop horizontally. Elongated structures are identified in the wake and seem to correspond to hairpin-like structures. Instantaneous vortex structures are also characterized with local vortex identification swirling strength criterion, confirming previous FFT and statistical results. The impact of the upstream cube is strongest in the near wake in the symmetrical transverse plane and prevents the large scale flow structures from rising in the water column.

When an object is accelerated in a fluid, a primary vortex is formed through the roll-up of a shear layer. This primary vortex does not grow indefinitely and will reach a limiting size and strength. Additional vorticity beyond the critical limit will end up in a trailing shear layer and accumulate into secondary vortices. The secondary vortices are typically considerably smaller than the primary vortex. In this paper, we focus on the formation, shedding and trajectory of secondary vortices generated by a rotating rectangular plate in a quiescent fluid using time-resolved particle image velocimetry. The Reynolds number based on the maximum rotational velocity of the plate and the distance between the centre of rotation and the tip of the plate is varied from 840 to 11 150. At low , the shear layer is a continuous uninterrupted layer of vorticity that rolls up into a single coherent primary vortex. At , the shear layer becomes unstable and secondary vortices emerge and subsequently move away from the tip of the plate. For , secondary vortices are discretely released from the plate tip and are not generated from the stretching of an unstable shear layer. First, we demonstrate that the roll-up of the shear layer, the trajectory of the primary vortex and the path of secondary vortices can be predicted by a modified Kaden spiral for the entire range considered. Second, the timing of the secondary vortex shedding is analysed using the swirling strength criterion. The separation time of each secondary vortex is identified as a local maximum in the temporal evolution of the average swirling strength close to the plate tip. The time interval between the release of successive secondary vortices is not constant during the rotation but increases the more vortices have been shed. The shedding time interval also increases with decreasing Reynolds number. The increased time interval under both conditions is due to a reduced circulation feeding rate.

Cavitating flow in the wake of a wedge-shaped bluff body is examined to understand the role of the presence of high void-fraction regions in the near-wake region on the process of vortex formation and shedding. Previous studies have noted that developed cavitation forming in the wake of bluff bodies typically leads to an increase in the vortex shedding rate, peaking at a particular cavitation number. Further reduction in cavitation number leads to a return to lower shedding rates as the cavity grows into a super-cavity. The underlying flow processes that lead to this phenomenon are explored using traditional flow visualisation combined with time-resolved void-fraction flow fields based on X-ray densitometry. These measurements allow us to relate the compressibility of the near-wake bubbly flow to the underlying flow processes. Specifically, we use proper orthogonal decomposition (POD) of the void-fraction fields to show that the increased rate of vortex shedding is associated with a pulsating mode of the void-fraction flow field, compared with a sinusoidal variation corresponding to the lower void-fraction shedding processes similar to that of the non-cavitating wake. The pulsating mode becomes more pronounced when the wake void fraction increases with decreasing cavitation number, with the maximum shedding occurring near the point that the wake flow becomes locally supersonic. The important influence of flow compressibility on the wake dynamics is confirmed through the examination of the effect of non-condensable gas injection.

This paper addresses the challenges of pressure-based sensing using axisymmetric probes whose axes are at small angles to the mean flow. Mean pressure measurements around three yawed circular cylinders with aspect ratios of 28, 64, and 100 were made to determine the effect of changes in the yaw angle, γ, and freestream velocity on the average pressure coefficient, , and drag coefficient, . The existence of four distinct types of circumferential pressure distributions—subcritical, transitional, supercritical, and asymmetric—were confirmed, along with the appropriateness of scaling and on a streamwise Reynolds number, , based on the freestream velocity and the fluid path length along the cylinder in the streamwise direction. It was found that there was a distinct difference in the values of and at identical values for cylinders yawed between 5° and 30°, and for cylinders at greater than a 30° yaw. For γ &lt, 5°, there did not appear to be any large-scale vortices in the near wake, and and pNappeared to become independent of . Over the range of 5° ≤ γ ≤ 30°, there was a complex interplay of freestream speed, yaw angle, and aspect ratio that affected the formation and shedding of Kármán-like vortices.

Wake flows behind porous patches are complex and host several spatiotemporal features associated with multi-scale excitation. This is in stark contrast to flow past a square cylinder dominated by nonlinear energy cascade stemming from primary vortex shedding instability. In this study, we analyze wakes created by multi-scale patches containing three iterations while maintaining a constant plan porosity. The results are compared to flows obstructed by a square cylinder and a single-scale patch of uniformly distributed elements from the second iteration having the same footprint. Multi-scale porous patches show a protracted wake having a greater spanwise dimension compared to square cylinder. The characteristics are dependent on the arrangement of elements within the patch which affect the extent of bleed flow through the configuration. Besides, we use proper orthogonal decomposition and dynamic mode decomposition to elucidate instability mechanisms at different scales. Distributions of spatial modes reveal element-scale flow structures in the near-wake region with patch-scale turbulence further downstream. Our analysis confirms manipulation of characteristic scales and the associated high-energy events driven by the arrangement of elements within a given patch despite having the same plan porosity.

We numerically investigate the passive flight of a flexible$\unicode[STIX]{x039B}$-flyer in a vertically oscillating airflow with zero mean stream. The flexibility of the flyer is introduced by a torsion spring installed at the hinged joint. We study the effects of spring stiffness, density, resting angle and actuation efforts on the hovering performance. The results suggest that the occurrence of resonance in flexible flyers can result in significantly different performances in flexible and rigid flyers. It is found that flexibility can have two opposing effects, reducing or increasing the actuation efforts for hovering, depending on the range of driving frequency. This result is explained by the modulation of relative motion between the flyer and the imposed background flow due to the involvement of passive angular oscillation. The angular oscillation patterns, the wake symmetry properties and the postural stability behaviours under different driving conditions are also explored. Based on the findings of the present study, the ideal parameter values for stable hovering are suggested. The results of this study offer novel insight into the mechanism by which the flexibility of the flyer affects the passive hovering performance.

Wake patterns behind two freely vibrating circular cylinders in side-by-side arrangements are numerically investigated through two-dimensional computational fluid dynamics simulations. The two cylinders, with an equal diameter and a low mass ratio (the cylinder mass/the displaced fluid mass) of 2, can only oscillate in the transverse direction, and they are subject to the uniform laminar flows with the Reynolds number of 100. The center-to-center spacing over a cylinder diameter ratio (s/D) is varied from 2 to 5. For each s/D, the reduced velocity U r (defined by U ∞ ∕ f n D, where U ∞ is the free-stream velocity and f n is the cylinder natural frequency) is varied from 0 to 30. A small parametric increment of Δ U r = 0 . 1 and Δ s/ D = 0 . 1 is considered, providing a comprehensive analysis and insights into the wake patterns of the vortex shedding behind both cylinders. With more than 1500 new simulation cases, 8 distinctive wake patterns are identified through a map of U r and s/D, with several well-defined boundaries governing different flow patterns. Qualitatively distinctive wake characteristics are analyzed through the evaluations of vortex dynamics, hydrodynamic lift/drag forces and cylinder vibration responses.