Wake Patterns Research Articles

Numerical investigation of synthetic jet effects on vortex-induced vibrations in tandem circular cylinders

ABSTRACT The influence of symmetric synthetic jets, on the upstream cylinder's (Cylinder 1's) leeward side on VIVs of tandem circular cylinders, was numerically investigated at Re = 150, m* = 2.0. The amplitude responses, frequency responses, hydrodynamic forces and wake patterns were analysed in the reduced velocity U* range of 3.0–14.0. The results indicated that synthetic jets with sufficient momentum coefficients (C u = 2.0 and 4.0) could suppress transverse oscillations of both cylinders, resulting in a symmetric +2S wake pattern. While the standard deviations of lift coefficients for Cylinder 1 decreased accordingly, the time-averaged drag coefficients of both cylinders didn't always follow suit. Complex wake patterns, including 2S + 2S, 2P + 2S, 2P + 2P, 2(P + S) + 2P and 2(P + S) + 2(P + S), were observed when synthetic jets can't dominate Cylinder 1's vortex shedding. Galloping responses of both cylinders can also be initiated.

Turbulent flows past two elliptical cylinders in tandem arrangement: wake dynamics and aerodynamic characteristics

Abstract. This study investigates the aerodynamic phenomena of drafting, a common technique observed in various racing sports, by analyzing the wake dynamics behind two 2D elliptic cylinders placed in tandem. The research explores how the separation distance and the azimuthal angle between the cylinders affect turbulent wake patterns and aerodynamic coefficients. A numerical study was conducted using a RANS-based simulation, with findings validated through physical experiments in a wind tunnel. The study identifies six distinct wake dynamics: Wake states without vortex shedding, Single bluff body wake, Suppressed vortex shedding, Coupled vortex shedding, Anti-phase vortex shedding, and In-phase vortex shedding. These dynamics vary significantly with changes in separation ratio and angle of attack, highlighting the critical influence of these parameters on the wake behavior and aerodynamic performance. The results are presented in a phase diagram, illustrating the evolution of wake patterns across different conditions. This research fills existing knowledge gaps by providing a detailed analysis of the wake dynamics behind elliptic cylinders at high Reynolds numbers, with potential applications in improving aerodynamic strategies in various engineering and sporting contexts.

Numerical simulations for the flow-induced vibrations of tandem dual flexible circular cylinders

The three-dimensional large-eddy-simulation coupled with a mode superposition method was applied to numerically simulate the flow-induced vibrations (FIVs) of tandem dual flexible cylinders at Re = 1000 with three different spacing ratios (Sx/D = 2.5, 3.5, and 5, Sx is center-to-center spacing for tandem cylinders, and D is the diameter of the cylinder), corresponding to the reattachment flow, transition flow, and co-shedding flow regimes in stationary tandem cylinders, respectively. The effects of Sx/D on structural vibrations, flow fields, distributions of the surface pressures, and energy properties were investigated to reveal the mechanism for the FIV. Increasing Sx/D weakens the influence of the upstream cylinder on maximum response amplitudes and lock-in region for downstream cylinder. The wake patterns for tandem flexible cylinders are more complex compared to stationary or vibrated rigid tandem cylinders. The shielding effect reduces surface pressure on the downstream cylinder significantly when its vibrations are smaller, leading to a decrease in mean power as well. Furthermore, different mechanisms contribute to amplified FIV in downstream cylinders depending on Sx/D: when Sx/D = 2.5, the upstream vortices collide with the downstream cylinder's front surface and merge with the vortices generated by the downstream cylinder, increasing negative pressure on both front and rear surfaces of the downstream cylinders and promoting FIV; however when Sx/D =3.5 and 5, a binary vortex street forms behind the downstream cylinder without obvious negative pressures on its front surface, the dominant causes of FIV are primarily attributed to interactions among upstream and downstream vortices.

FLAIR-Hyperintense Lesions in Anti-MOG-Associated Encephalitis with Seizures: A Case Report

Introduction: Myelin oligodendrocytic glycoprotein antibody-associated disease (MOGAD) is an immune-mediated demyelinating disorder of the central nervous system. We described, a case of encephalitis associated with anti-MOG antibodies with hyperintense lesions on FLAIR and seizures (FLAMES). Case Report: We report a case of a 5-year-old girl, with no significant personal or family history, who entered the emergency room with paroxysmal events and anarthria, in a febrile context. On neurological examination she presented right sided facial paresis and pyramidal signs on the right. Lumbar puncture (LP), showed pleocytosis, hyperproteinorrhaquia and the absence of oligoclonal bands. Electroencephalogram revealed encephalopathy and slow paroxysmal left frontal activity. The brain MRI detected hyperintense lesions, however, with an area of left frontal cortico-subcortical hyperintense signal on T2/FLAIR and diffusion restriction. Anti-MOG antibody was detected. These findings pointed to the diagnosis of FLAMES. She completed 7 days of methylprednisolone (30mg/kg/day), with a good response. On the day of discharge, a repeat EEG was conducted, demonstrating significant improvement compared to the initial one, with slightly slow wake and sleep patterns for her age, and no evidence of epileptic activity. She was assessed approximately four months post-hospitalisation and was found to be asymptomatic, with no focal neurological deficits. A further EEG was performed, which showed a structurally age-appropriate wake and sleep pattern with no abnormal graphoelements. Conclusion: We highlight a rare clinical-radiological entity, which is still poorly described, particularly in the paediatric population, known as FLAMES, with a favourable response to steroids. Early recognition and timely diagnosis of his condition improve clinical and therapeutic management and minimize neurological damage. Immediate initiation of appropriate treatment is important for achieving a favorable outcome.

Coupling of Slow Oscillations in the Prefrontal and Motor Cortex Predicts Onset of Spindle Trains and Persistent Memory Reactivations.

Sleep is known to drive the consolidation of motor memories. During nonrapid eye movement (NREM) sleep, the close temporal proximity between slow oscillations (SOs) and spindles ("nesting" of SO-spindles) is known to be essential for consolidation, likely because it is closely associated with the reactivation of awake task activity. Interestingly, recent work has found that spindles can occur in temporal clusters or "trains." However, it remains unclear how spindle trains are related to the nesting phenomenon. Here, we hypothesized that spindle trains are more likely when SOs co-occur in the prefrontal and motor cortex. We conducted simultaneous neural recordings in the medial prefrontal cortex (mPFC) and primary motor cortex (M1) of male rats training on the reach-to-grasp motor task. We found that intracortically recorded M1 spindles are organized into distinct temporal clusters. Notably, the occurrence of temporally precise SOs between mPFC and M1 was a strong predictor of spindle trains. Moreover, reactivation of awake task patterns is much more persistent during spindle trains in comparison with that during isolated spindles. Together, our work suggests that the precise coupling of SOs across mPFC and M1 may be a potential driver of spindle trains and persistent reactivation of motor memory during NREM sleep.

Transverse vortex-induced vibration of a sphere with the application of Lorentz force at low Reynolds number

In this study, the transverse vortex-induced vibration (VIV) of an elastically mounted sphere with the application of a streamwise Lorentz force is investigated through direct numerical simulation. The research parameter range is 300 ≤ Re ≤ 1100 and −0.8 ≤ N ≤ 1, where Re is the Reynolds number and N is the interaction parameter of the Lorentz force. The dependence of sphere responses, forces, and wake structures on Re and N is illustrated in detail. Within this range, two oscillation patterns are identified: VIV and desynchronization. Three wake patterns are identified: two-sided hairpin vortex emerges in the VIV region, while one-sided hairpin vortex and double-threaded wake structures are observed in the desynchronization region. The evolution of these wake patterns is related to the motion of the rear stagnation point (RSP) and separation line (SL) on the sphere surface. A large positive or negative Lorentz force suppresses the motion of RSP and SL, leading to the one-sided hairpin vortex or double-threaded wake structures replacing the two-sided hairpin vortex. Finally, the oscillation patterns are summarized on a map of amplitude response contours in the Re-N space.

Enhanced homeostatic sleep response and decreased neurodegenerative proteins in cereblon knock-out mice

Energy homeostasis and sleep have a bidirectional relationship. Cereblon (CRBN) regulates energy levels by ubiquitinating the AMP-activated protein kinase(AMPK), an energy sensor. However, whether CRBN participates in sleep is unclear. Here, we examine sleep–wake patterns in Crbn+/+ and Crbn−/− mice during 24-h baseline, 6-h sleep deprivation (SD), and following 6-h recovery sleep (RS). At baseline, overall sleep patterns are similar between genotypes. However, SD decreases CRBN expression in Crbn+/+ mice and increases phospho-Tau, phospho-α-synuclein, DNAJA1 (DJ2), and DNAJB1 (DJ1) in both genotypes, with Crbn−/− mice showing a lesser extent of increase in p-Tau and p-α-synuclein and a higher level of heat shock protein 70 (HSP70), DJ2, and DJ1. During RS, Crbn−/− mice show increased slow-wave activity in the low-delta range (0.5–2.5 Hz), suggesting higher homeostatic sleep propensity associated with AMPK hyperactivation. By illuminating the role of CRBN in regulating sleep–wake behaviors through AMPK, we suggest CRBN as a potential therapeutic target for managing sleep disorders and preventing neurodegeneration.

Investigation on the unsteady aerodynamic performance of the Drela AG 08 and Boeing B29 airfoils subjected to heaving motion

This article aims to investigate the unsteady flow characteristics of Drela AG08 and Boeing B29 airfoils when subjected to heaving motion. The influence of heave amplitude, heave frequency in terms of instantaneous force coefficients of the airfoil is investigated at Re = 5 × 104. Furthermore, the flow field investigations were carried out to analyze the Leading Edge Vortex (LEV) formation and its convection airfoil’s suction side. From the findings, a variation in the oscillation amplitude of force coefficients was observed and a non-sinusoidal behavior was also noticed. Additionally, a remarkable change in the instantaneous force coefficients (both qualitatively and quantitatively) is also observed with respect to the slight change in aforementioned parameters. A transition in the wake pattern from drag inducing to thrust producing is observed under certain conditions. It is observed that the flow separates at the leading edge by forming LEV on both airfoil’s suction side and pressure side. It is also noticed that whenever the flow departs the trailing edge, these vortices convect downstream of the airfoil and form alternating pattern of coherent vortices. With the increase in number of cycles, these vortices interfere with the previously shed vortices and affects the airfoil’s aerodynamic characteristics. From the investigations, it is observed that, by controlling the system parameters, the adverse effects of the LEV can be minimized.

Aerodynamic analysis of a small-scale Savonius wind turbine incorporated with wind lenses: A CFD approach for wake pattern and power prediction

Aerodynamic analysis of a small-scale Savonius wind turbine incorporated with wind lenses: A CFD approach for wake pattern and power prediction

Experimental observation of parabolic wakes in thin plates

Wakes are medium perturbations created by a moving object, such as wave patterns behind boats, or wingtip vortices following an aircraft. Here, we report about an experimental study of an uncharted form of parabolic wakes occurring in media with the group velocity twice larger than the phase velocity, as opposed to the conventional case of Kelvin wakes. They are formed by moving a laser spot on a thin plate, which excites a unique wake pattern made of confocal parabolas, due to the quadratic dispersion of the zero-order flexural Lamb mode. If the spatial dimensions are rescaled by the perturbation velocity and material constant, we obtain a single universal wake with constant parabolic focal lengths. We demonstrate an evanescent regime above the critical frequency where the wave components oscillate exclusively in the direction parallel to the perturbation path, with an opening angle of 90∘. We define a dimensionless number, analogous to Froude and Mach numbers, which determines whether or not the complete parabolic wake pattern will be excited by the moving source. Finally, we generalize the physics of wake shapes to arbitrary dispersion relations. Published by the American Physical Society 2024

From Breastfeeding to Support in Mothers' Feeding Choices: A Key Role in the Prevention of Postpartum Depression?

The postpartum period represents a critical phase of profound transition for women. This timeframe encompasses the physical recuperation associated with childbirth, the intricate psychosocial adjustments inherent in assuming the role of motherhood and also important alterations in steroid and peptide hormones. Hence, as women navigate the reconfiguration of relationships and strive to address the diverse needs of their infants and family members, they concurrently grapple with dramatic transformations which are characteristic of the postpartum phase. In fact, relevant prevalence ranges are reported for maternity blues, a mild condition characterized by self-limited and transient depressive symptoms, but also a well-established risk factor for more serious postpartum mood disorders, such as depression (PPD), with an incidence of 10-15%. Unlike in the US, at the European level, there are no concrete recommendations for the routine integration of the assessment of the mother's emotional state by healthcare professionals, with a considerable risk of underdiagnosing or undertreating these conditions. In this regard, there is a growing body of scientific evidence on the important role of breastfeeding in reducing the risk of PPD and also of the importance of mothers' compliance with this practice. Indeed, sucking the baby regulates the circadian rhythm of the HPA axis and, together with the action of prolactin, the stress response is decreased. In addition, other positive consequences of breastfeeding, which are inversely correlated with the onset of PPD, include the regulation of sleep and waking patterns for mother and baby, the improvement of the mother's self-efficacy and her emotional involvement. It should also be considered that the request for support for breastfeeding can often conceal a request for support for motherhood itself and for the mother's emotional well-being. It therefore emerges that the personnel involved in primary pediatric care to provide adequate support in the transition to motherhood must support mothers in their breastfeeding choices, whether breastfeeding or formula feeding, so that each choice is made conscientiously and serenely. Therefore, neonatal feeding assumes a decisive role, since if, on the one hand, it regulates specific neurohormonal pathways that are protective for maternal emotional well-being (breastfeeding), on the other hand, support in mothers' breastfeeding choices, even in the case of formula feeding, means validating their being mothers in the absence of judgement and counteracting any feelings of inadequacy, conditions that are inversely correlated to DPP.

Numerical investigation of power-law flow past two side-by-side identical circular cylinders

The non-Newtonian flow past multiple cylinders is widely encountered in engineering applications, such as slurry transport, petroleum drilling, and heat transmission systems using hot kerosene. However, the wake characteristics of non-Newtonian flow past multiple cylinders are far from well understood. This paper reports the numerical results of power-law flow past two side-by-side identical circular cylinders with a various gap ratio (G/D = 1.1–6.0) and a power-law index (n = 0.8–1.5) at a fixed Reynolds number (Re = 100) based on the incoming uniform flow velocity. Six wake patterns are identified, including the single bluff-body regime, deflected regime, in-phase regime, anti-phase regime, and two subclasses of flip-flopping regime (FF1 and FF2 regimes). The hydrodynamic coefficients of two cylinders are sensitive to both the gap ratio and the power-law index. The wake structure evolution is closely related to the wake patterns, and six modes of wake evolution are accordingly observed. Since the apparent viscosity of power-law fluid changes with the shear rate, the distribution of local Reynolds number (ReL) around the cylinder surface varies with the wake pattern. As it goes outward along the normal direction from the cylinder surface, the ReL shows a trend of increasing and then decreasing when n < 1, while the opposite trend is observed when n > 1.

Hydrodynamic performance of pitching foils with two leading-edge protuberances

Inspired by the tubercles on humpback whales, the ‘tubercle-effect’ first identified by (Fish and Battle, 1995; Fish and Lauder, 2006) had been studied extensively to understand their influence on overall efficiency for airfoils and wings. Pitching and/or heaving motion of foils produces net thrust inspired from fin-based locomotion of aquatic animals, and is often being proposed as a greener means of propulsion having reduced acoustic signature. With this aim in mind, the present work investigates the effect of using tubercles on pitching motion of foils. The hydrodynamic characteristics of finite aspect ratio pitching foils having leading-edge modifications in the form of two protuberances are investigated using experiments. The thrust and efficiency with and without protuberances are investigated for pure pitching motions at reduced frequencies (1.8 – 3.2) and pitching amplitudes (7.5°- 20°) for Reynolds number 2 × 104. The experimental results are compared with inviscid linear theory calculations which over-predict the mean thrust, while the amplitude of the lift force and its variation with reduced frequency is similar. The performance improvements in terms of thrust and efficiency for the modified design are restricted to ≥15° pitching amplitudes at higher reduced frequencies. The wake vorticity patterns obtained using Computational Fluid Dynamics investigations show the influence of the leading-edge vortex, which gets modified when the protuberances are used. The transition from 2S wake to 2P wake with the increase in the amplitude of oscillation is captured. The reversal of von Kármán Vortex Street with the pitching frequency and thus the transition from drag to thrust generation is observed for both the baseline and modified foil designs. The results indicate that protuberances over the leading-edge of hydrofoils can influence the hydrodynamics during pitching motion and can be effectively employed for performance tailoring based on the geometry and motion characteristics. The simplified setup used here is also meant to serve as a basis towards future research including verification and validation work for numerical solvers.

Dynamics and Wake Interference Mechanism of Long Flexible Circular Cylinders in Side-by-Side Arrangements

The vortex-induced vibrations of two side-by-side flexible cylinders in a uniform flow were studied using a three-dimensional direct numerical simulation at Reynolds number Re = 350 with an aspect ratio of 100, and a center-to-center spacing ratio of 2.5. A mixture of standing-traveling wave pattern was induced in the in-line (IL) vibration, while the cross-flow (CF) vibration displayed a standing-wave characteristic. The ninth vibration mode prominently occurred in both IL and CF directions, along with competition between multiple modes. Proximity effects from the neighboring cylinder caused the primary frequency to be consistent between IL and CF vibrations for each cylinder, deviating from the IL to CF ratio of 2:1 in isolated cylinder conditions. Repulsive mean lift coefficients were observed in both stationary and vibrating conditions for the two cylinders due to asymmetrical vortex shedding in this small gap. Comparatively, lift and drag coefficients were notably increased in the vibrating condition, albeit with a lower vortex shedding frequency. Positive energy transfer was predominantly excited along the span via vortex shedding from the cylinder itself and the neighboring one, leading to increasing lower-mode vibration amplitudes. The flip-flopping (FF) wake pattern was excited in the stationary and vibrating cylinders, causing spanwise vortex dislocations and wake transition over time, with the FF pattern being more regular in the stationary cylinder case.

Connections between propulsive efficiency and wake structure via modal decomposition

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry measurements of the flow field downstream of the foil are presented for a Reynolds number of $Re=11\times 10^3$ and Strouhal numbers in the range $St=0.16\unicode{x2013}0.35$ . These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.

Effect of a fixed downstream cylinder on the flow-induced vibration of an elastically supported primary cylinder

This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless center-to-center spacing (L/D, L is the intermediate spacing and D is the cylinder diameter). The present two-dimensional (2D) simulations are carried out in the low Reynolds number (Re) regime. The primary focus of this study is to reveal the underlying flow physics behind the transition from vortex-induced vibration to galloping in the response of the primary cylinder due to the presence of another fixed downstream cylinder. Two distinct flow field regimes, namely, steady flow and alternate attachment regimes, are observed for different L/D and Re values. Depending on the evolution of the near-field flow structures, four different wake patterns, “2S,” “2P,” “2C,” and “aperiodic,” are observed. The corresponding vibration response of the upstream cylinder is characterized as interference galloping and extended vortex-induced vibration. As the L/D ratio increases, the lift enhancement due to flow-induced vibration is seen to be weakened. The detailed correlation between the force generation and the near-wake interactions is investigated. The present findings will augment our understanding of vibration reduction or flow-induced energy harvesting of tandem cylindrical structures.

Effects of cavitation on the vortex shedding behind a truncated hydrofoil subjected to forced oscillation

Abstract Cavitating vortex shedding behind a truncated NACA 0009 hydrofoil subjected to forced oscillation has been numerically investigated using a one degree-of-freedom (1 DOF) model coupled with a homogeneous mixture cavitation model. The pitching motion of the hydrofoil has been prescribed using a sinusoidal function with a fixed amplitude and variable frequencies ranging from lock-off values below the vortex shedding frequency to values well above it and passing through the lock-in condition. The obtained results have permitted us to study the change in the dynamics and morphology of the shed-vortex structures with varying hydrofoil oscillation frequencies when cavitation takes place. More specifically, it has been found that the frequency regime of the lock-in condition is altered due to the occurrence and development of cavitation. Also, two distinct wake patterns have been observed for oscillating frequencies below and above the vortex shedding frequency. Furthermore, the phase angle between the applied torque and the hydrofoil surface acceleration has been estimated as a function of the excitation frequency and the cavitation number, which has helped to understand the effects of cavitation on the dynamic behaviour of the shed vortices.

Estimating the performance of wind turbines misaligned with respect to the wind direction using a simplified CFD based approach

Wind direction in atmospheric boundary layer changes constantly. As such, the power extracted from the wind turbines in a wind farm is subjected to change. This change in wind direction impacts the wake pattern which in turn affects the power output of the downstream wind turbines. Empirical wind farm models are useful in such cases to assess the overall performance of the wind farms. However, the results provided by these models can be made more useful if fluid-structure interaction (FSI) effects are also considered. In the present work, we begin by looking at the potential flow model and the issues associated with it and used a modified model with constant vorticity to analyze the wind power available to the downstream wind turbines in a wind farm misaligned with respect to wind direction for irrotational flow. A simplified FSI based approach is used to carry out the simulations without the need of modelling the complicated wind turbine geometry.

A numerical analysis of wind farm wake characteristics in the southern part of the North Sea

The rapid expansion of wind farms in the North Sea requires a better understanding of the wind turbines’ wake effects. In this study, the classification of wake patterns under different atmospheric boundary layer stability conditions is investigated. For this purpose, the Weather Research and Forecast model is utilized to calculate wind farm wake effects over the southern North Sea for a year-long period. The atmospheric stability is characterized by the value of Monin-Obukhov length, and seven different classes are considered. The results have shown that the predominance of the stability condition depends on seasonality. In autumn and winter months very unstable conditions prevail, while in spring and summer periods near-neutral and stable events occur more frequently. The different atmospheric stability conditions have distinct effects on the averaged wind-speed deficits. Specifically, in near-neutral and stable stratification, wakes propagate further downwind of the wind turbines affecting neighboring wind farms, while in the case of unstable conditions, these effects are weaker.

Wake Structures and Hydrodynamic Characteristics of Flows around Two Near-Wall Cylinders in Tandem and Parallel Arrangements

To clarify the hydrodynamic interference characteristics of flows around multiple cylinders under the wall effect, the two-dimensional (2D) flows around the near-wall single, two tandem and parallel cylinders are simulated under different gap ratios (0.15 ≤ G/D ≤ 3.0) and spacing ratios (1.5 ≤ T/D ≤ 4.0) at a Reynolds number of Re = 6300. We also examine the wake patterns, the force coefficients, and the vortex-shedding frequency with emphases on the wall effect and effects of the two-cylinder interference. A critical wall gap of G/D = 0.6 is identified in the single-cylinder case where the wall can exert significant influences. The two near-wall tandem cylinders exhibit three wake states: stretching mode, attachment mode, and impinging mode. The force coefficients on the upstream cylinder are significantly affected by the wall for G/D ≤ 0.6. The downstream cylinder is mainly influenced by the upstream cylinder. For G/D > 0.6, the force coefficients on the two cylinders exhibit a similar variation trend. In the parallel arrangement, the two cylinders exhibit four wake states in different G/D and T/D ranges: double stretching mode, hetero-vortex scale mode, unilateral vortex mode, and free vortex mode. Moreover, the two parallel cylinders in the hetero-vortex scale or free vortex mode have two states: synchronous in-phase state and synchronous out-of-phase state. The mean drag coefficients on the two cylinders decrease, while the mean lift coefficients exhibit opposite variation trends, as the T/D grows.