Wake Transition Research Articles

The key to suppress vortex-induced vibration: Stability of the structural mode

The vortex-induced vibration (VIV) problem is common in nature and engineering fields. Current passive or active control methods try to suppress VIV by changing the flow field or improving the flow stability based on the recognition that VIV is caused by resonance. But is VIV really caused by resonance? Tuned mass damper (TMD) has been used in VIV or other vibration control for many years. However, is TMD simply an anti-resonance or energy dissipation device in VIV suppression? With the ARX(Autoregressive with exogenous input)-based reduced-order model (ROM) and the fluid–structure interaction numerical simulation, the physical mechanism of the VIV suppression with TMD in the lock-in region is investigated at a low Reynolds number (Re=60). According to the root loci, the introduction of TMD does not change the unstable wake mode, but effectively improves the stability of the structural mode. The results obtained by numerical simulation reveal that the violent vibration turns to a micro-amplitude response and the final response frequency is consistent with the vortex shedding frequency. Therefore, VIV is a stability problem rather than a dynamic response problem. Meanwhile, TMD suppresses VIV via structural mode stability enhancement rather than anti-resonance and energy dissipation. In addition, the study also found that when the TMD natural circular frequency is between 0.65 and 1.0, the vibration suppression effect is independent of the initial amplitude. This frequency range happens to be the region where the structural mode is tightly coupled with the wake mode.

Drag reduction of a generic transport vehicle model using a fluidic oscillator

Base blowing is performed using a sweeping jet actuator centrally located at the base of a square-back Ahmed body. Drag reduction along with a significant effect in terms of wake RSB mode suppression is reported for such fluid injection. All three scales of fluidic actuation exhibit similar wake bistability suppression characteristics. RSB mode suppression is also corroborated through independent force measurements. For all scales of actuators, the optimal blowing coefficient Cqopt is found to scale with bleed-to-base area ratio as (Sj/S)3/4, whereas the blowing coefficient corresponding to the minimum magnitude of wake asymmetry scales as Cqopt(Sj/S)5/12. The small-scale actuator also presents a more energy-efficient base blowing in terms of the amount of drag reduction achieved per energy consumed.

Relationship between sleep disorders and attention-deficit-hyperactivity disorder in children.

ObjectiveTo explore the correlation between sleep disorders and attention-deficit–hyperactivity disorder (ADHD) in children.MethodsWe studied 100 Chinese children (70 boys and 30 girls; mean age, 8.77 ± 2.39 years). Parents completed the Children's Sleep Disturbance Scale (SDSC) and the Swanson Nolan and Pelham Version IV Scale (SNAP-IV) questionnaires. SNAP-IV and SDSC scores were compared in children with and without sleep disorders and ADHD.ResultsThere were significant differences in SDSC scores, Arousal Disorder (AD) scores, and Sleep Breathing Disorder (SBD) scores between children with and without ADHD (P < 0.05). The sleep disorder group had higher SNAP-IV scores than the non-sleep disorder group (P < 0.05). Children with sleep disorders showed higher ADHD symptom values (inattention, hyperactivity/impulsivity, and oppositional defiance) than children without sleep disorders (P < 0.01). There was a moderate correlation between SDSC scores and SNAP-IV scores (r = 0.486, P < 0.05). Using SNAP-IV scores as the dependent variable, multiple linear regression analysis was applied, and a statistically significant effect of AD and Sleep–Wake Transition Disorder (SWTD) scores on SNAP-IV scores was found (P < 0.05). The area under the curve (95% CI) of the SDSC score for predicting sleep disorders with ADHD was 0.714 (0.606, 0.821; P = 0.0005).ConclusionChildren with ADHD are prone to sleep disorders. The higher the ADHD symptom score, the more sleeping problems. Sleep disorders can also cause or exacerbate ADHD symptoms, and the ADHD symptom score correlates with sleep disorder severity. We can reduce the severity of attention-deficit–hyperactivity in children with ADHD by improving their sleep with behavioral sleep interventions.

Aerodynamic correlation and flow pattern of high-rise building with side ratio of 3:1 under twisted wind profile: A computational study

This study numerically investigates twisted-wind effect on aerodynamic correlation and flow pattern of high-rise building with 3:1 side ratio. Spatial-temporal correlation of aerodynamic forces and flow filed under conventional wind profile (CWP) and twisted wind profile (TWP) are comprehensively compared. Proper orthogonal decomposition (POD) is further employed to extract wake flow pattern and mode correlation. The results show that twisted-wind effect on building with long afterbody is unique. Specifically, compared with CWP, TWP augments mean wind loads in three perpendicular directions (e.g., CMx under TWP30 is increased by 60.56% compared to CWP). The original two vortex structures (i.e., leading-edge vortex shedding (LEVS) and trailing-edge vortex shedding (TEVS)) appearing in CWP transit to relatively weaker alternate-edge vortex shedding (AEVS) in TWP, thus, the fluctuating lateral base moment and torsional force are reduced by 30.60% and 28.24% respectively under TWP30 compared to CWP. Correlation of wind load components and correlation of layer force are significantly magnified because TWP intensifies vertical moment exchange and force coupling effect, this possibly results in stronger wind-induced responses. POD analysis reveals essential mechanism of twisted-wind effect and shows that flow pattern (i.e., energy proportion, dominant frequency, mode shape and mode correlation) is largely changed by TWP.

Characterization of bifurcated dual vortex streets in the wake of an oscillating foil

Wake evolution of an oscillating foil with combined heaving and pitching motion is evaluated numerically for a range of phase offsets ( $\phi$ ), chord-based Strouhal numbers ( $St_c$ ) and Reynolds numbers ( $Re$ ). The increase in $\phi$ from $90^\circ$ to $180^\circ$ at a given $St_c$ and $Re$ coincides with a transition of pitch- to heave-dominated kinematics that further reveals novel transitions in wake topology characterized by bifurcated vortex streets. At $Re= 1000$ , each of the dual streets constitutes a dipole-like paired configuration of counter-rotating coherent structures that resemble qualitatively the formation of $2P$ mode. A new mathematical relation between the relative circulation of coherent dipole-like paired structures and kinematic parameters is proposed, including heave-based ( $St_h$ ), pitch-based ( $St_{\theta }$ ) and combined motion ( $St_A$ ) Strouhal numbers, as well as $\phi$ . This model can predict accurately the wake transition towards $2P$ mode characterized by a bifurcation, at low $Re= 1000$ . At $Re= 4000$ , however, the relationship was inaccurate in predicting the wake transition. A shear splitting process is observed at $Re= 4000$ , which leads to the formation of reverse Bénard–von Kármán mode in conjunction with $2P$ mode. Increasing $\phi$ further depicts a consistent prolongation of the splitting process, which coincides with a unique transition in terms of absence and reappearance of bifurcated dipole-like pairs at $\phi = 120^\circ$ and $180^\circ$ , respectively. Changes in the spatial arrangement of $2P$ pairs observed consistently for oscillating foils with the combined motion constitute a novel wake transition that becomes more dominant at higher Reynolds numbers.

Active flow control of a wing section in stall flutter by dielectric barrier discharge plasma actuators

This paper investigates the potential of using an active flow control technique to modify stall flutter oscillations of a NACA (National Advisory Committee for Aeronautics) 0015 wing section. Wind tunnel experiments have been performed with a test-rig that provides the elastic degree of freedom in pitch. Measurements of the clean airfoil are taken at preset angles of θ0=6°−12°, and for Reynolds numbers of Rec=6.2×104−1.25×105, which reveal the dependency of the stall flutter oscillations to Rec and θ0. Then, flow control experiments are carried out at θ0=10° and Rec=1.04×105. Two dielectric barrier discharge plasma actuators have been employed simultaneously to exert dual-point excitation to the baseline flow. It is shown that during the upstroke half-cycle, plasma actuation postpones the dynamic stall of the airfoil and increases the maximum pitch angle of the stall flutter cycle. On the downstroke, dual-point excitation effectively improves the rate of pitching moment recovery and leads to flow reattachment at a larger pitch angle. Normalized excitation frequencies Fex=fex/fw (where fw is the wake mode frequency of the stalled airfoil) ranging from 0.1 up to 3 have been examined. Among the controlled cases, excitation with Fex=0.6 and Fex=2.2 provides the largest and smallest pitch amplitude, respectively, and the case of Fex=3 demonstrates the most impact on flow reattachment. Finally, it has been concluded that the employed control strategy is effectively capable of modifying the dynamic stall process and associated pitching moment. However, a more sophisticated control strategy would be required to significantly mitigate the stall flutter oscillations.

On the origin of mode B instability of the wake of a square cylinder

Three-dimensional numerical simulations of flow passing a square cylinder are performed using a spectral element method. Reynolds numbers of 200 and 300, corresponding to modes A and B, respectively, are used to study the two- to three-dimensional transition of the wake flow. It is shown that the wake transition occurs in the early time prior to the onset of the vortex shedding event. For mode A, the spanwise instability grows exponentially downstream of the cylinder with constant wavelength in the early time dynamics and continues after the vortex separation. However, in the case of mode B, the exponential amplification reaches a saturation state before the vortex separation occurs. A splitting of the initially obtained wavelength of the spanwise instability occurs and is related to the splitting of spanwise vortical structures, which develops at the cores of the separation bubbles prior to the vortex shedding.

Large-eddy simulations and modal reconstruction of laminar transonic buffet

Transonic buffet refers to the self-sustained periodic motion of shock waves observed in transonic flows over wings and can limit the flight envelope of aircraft. Based on the boundary layer characteristics at the shock foot, buffet has been classified as laminar or turbulent and the mechanisms underlying the two have been proposed to be different (Dandois et al., J. Fluid Mech., vol. 18, 2018, pp. 156–178). The effect of various flow parameters (freestream Mach and Reynolds numbers and sweep and incidence angles) on laminar transonic buffet on an infinite wing (Dassault Aviation's supercritical V2C aerofoil) is reported here by performing large-eddy simulations (LES) for a wide range of parameters. A spectral proper orthogonal decomposition identified the presence of a low-frequency mode associated with buffet and high-frequency wake modes related to vortex shedding. A flow reconstruction based only on the former shows periodic boundary-layer separation and reattachment accompanying shock wave motion. A modal reconstruction based only on the wake mode suggests that the separation bubble breathing phenomenon reported by Dandois et al. is due to this mode. Together, these results indicate that the physical mechanisms governing laminar and turbulent buffet are the same. Buffet was also simulated at zero incidence. Shock waves appear on both aerofoil surfaces and oscillate out of phase with each other indicating the occurrence of a Type I buffet (Giannelis et al., Aerosp. Sci. Technol., vol. 18, 2018, pp. 89–101) on a supercritical aerofoil. These results suggest that the mechanisms underlying different buffet types are the same.

Initially generated pure mode A in the three-dimensional wake transition of a circular cylinder

The initially generated pure mode A, as a transitional stage, is investigated in the three-dimensional wake transition of a circular cylinder. Direct numerical simulations are carried out over a range of Reynolds numbers from 100 to 210. According to the different dynamic behaviors of hydrodynamic parameters and similar features in the spatiotemporal evolution of vorticity in the near wake, two stages are identified. The first, investigated here, is the initial generation of pure mode A at Reynolds numbers less than 195, while the second, already reported, is the full development of pure mode A at Reynolds numbers greater than 195. The relationship between the volume-RMS (root-mean-square) vorticity and Reynolds number indicates two critical Reynolds numbers, 145 and 195 (at most). The first critical Reynolds number denotes the initial appearance of three-dimensional instability. The second critical Reynolds number indicates the transition of pure mode A from the initially generated state to the fully developed state in the near wake. After the first critical Reynolds number, the evolution of the vorticity in the near wake and on the rear surface of the cylinder clearly shows that the appearance of pure mode A is a gradual process, rather than a sudden process accompanied by a jump in vortex shedding frequency. In particular, as the Reynolds number increases, the streamwise vorticity first appears on and near the cylinder surface, then in the shear layers, and finally in the shedding primary vortices, instead of appearing instantaneously in the shedding vortices after the instability of primary vortex cores.

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio $H/W=1.11$ in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number $2.1\times 10^5$ explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance $C$ (taken at mid-distance from the front and rear axles of the body), pitch angle $\alpha$ , and yaw angle $\beta$ (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces $(\beta,C)$ and $(\beta,\alpha )$ are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in $(\beta,C)$ and $(\beta,\alpha )$ parametric spaces can be unified at large yaw in the $(\beta,C_r)$ space, where the rear clearance $C_r$ separating the lower edge of the base from the ground is a simple function of the pitch $\alpha$ and the clearance $C$ . The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake $z$ -instability (Grandemange et al., Phys. Fluids, vol. 25, 2013a, pp. 95–103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map $(\beta,\alpha )$ defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.

Flow-induced vibrations of a D-section prism at a low Reynolds number

This paper presents the response and the wake modes of a freely vibrating D-section prism with varying angles of attack ( $\alpha = 0^\circ \text {--}180^\circ$ ) and reduced velocity ( $U^* = 2\text {--}20$ ) by a numerical investigation. The Reynolds number, based on the effective diameter, is fixed at 100. The vibration of the prism is allowed only in the transverse direction. We found six types of response with increasing angle of attack: typical vortex-induced vibration (VIV) at $\alpha = 0^\circ \text {--}35^\circ$ ; extended VIV at $\alpha = 40^\circ \text {--}65^\circ$ ; combined VIV and galloping at $\alpha = 70^\circ \text {--}80^\circ$ ; narrowed VIV at $\alpha = 85^\circ \text {--}150^\circ$ ; transition response, from narrowed VIV to pure galloping, at $\alpha = 155^\circ \text {--}160^\circ$ ; and pure galloping at $\alpha = 165^\circ \text {--}180^\circ$ . The typical and narrowed VIVs are characterized by linearly increasing normalized vibration frequency with increasing $U^*$ , which is attributed to the stationary separation points of the boundary layer. On the other hand, in the extended VIV, the vortex shedding frequency matches the natural frequency in a large $U^*$ range with increasing $\alpha$ generally. The galloping is characterized by monotonically increasing amplitude with enlarging $U^*$ , with the largest amplitude being $A^* = 3.2$ . For the combined VIV and galloping, the vibration amplitude is marginal in the VIV branch while it significantly increases with $U^*$ in the galloping branch. In the transition from narrowed VIV to pure galloping, the vibration frequency shows a galloping-like feature, but the amplitude does not monotonically increase with increasing $U^*$ . Moreover, a partition of the wake modes in the $U^*$ – $\alpha$ parametric plane is presented, and the flow physics is elucidated through time variations of the displacement, drag and lift coefficients and vortex dynamics. The angle-of-attack range of galloping is largely predicted by performing a quasi-steady analysis of the galloping instability. Finally, the effects of $m^*$ and ${\textit {Re}}$ , the roles of afterbody and the roles of separation point in determining vibration responses and vortex shedding frequency are further discussed.

Wake modes of a freely vibrating square cylinder

The cross-stream flow-induced vibrations of a square cylinder of mass ratio, m*=10, are studied numerically at a fixed Reynolds number, Re, of 250. The reduced speed, U*, is varied from 1 to 10 independent of Re. The flow-induced vibrations of a square cylinder have been previously investigated either by decoupling U* from Re or by coupling U* with Re. While most of the studies available in the literature follow the former approach, those dealing with m*=10 do not provide a detailed account of the branches of dynamic response, hysteresis, and wake patterns. The current effort aims at contributing to these research gaps. The vibrations are purely vortex-induced, and the dynamic response within synchronization is found to be composed of an initial branch, its extension or extended initial branch, and lower branch. For a square-section oscillator, the extended initial branch is resolved for the first time. The most noteworthy outcome of this work is perhaps the resolution of asymmetric as well as one-sided wake modes at certain reduced speeds. The one-sided shedding occurs either from the top or bottom surface of the oscillator. At U*=4.6, the non-zero mean lift changes sign in successive oscillation cycles, indicating that the solutions are bistable. The resolved asymmetric and one-sided modes are associated with positive and negatives values of mean lift, respectively. A very interesting result of this study is the mismatch of wake modes obtained at non-hysteretic U*=4.6 using forward and backward computations.

Characteristics of the wake of an inclined prolate spheroid in uniform shear flow

Flow around an inclined 5:2 prolate spheroid with the incidence angle α = 45° is numerically investigated in a uniform shear flow. The Reynolds number based on the inflow center velocity Uc and the volume-equivalent sphere diameter De of the spheroid are considered at Re = 480, 600, 700, and 750. The non-dimensional shear rate K is ranged from 0 to 0.1. Five qualitatively different wake modes are observed, including a new mode characterized by multi-periodic shedding of hairpin vortices with regular rotation of the separation region. In general, the wake transition is suppressed with increasing shear rate. At high shear rates, the flow even reverts from unsteady to steady state at Re = 480, which we attributed to the reduction of the local Reynolds number at the leading-edge side of the spheroid. The time-averaged drag/lift coefficients and the Strouhal number increase with increasing the shear rate and the Reynolds number (except for K = 0). Finally, the effect of a sign-change of the incidence angle of the prolate spheroid on wake evolution is investigated. A physical exploration of the effect of the sign of the incidence angle and the amount of inlet shear is provided to give deeper insight into the physical mechanisms acting in the wake behind inclined non-axisymmetric bluff bodies in a shear flow.

Scaling laws for drag-to-thrust transition and propulsive performance in pitching flexible plates

The propulsion of a pitching flexible plate in a uniform flow is investigated numerically. The effects of bending stiffness ( $K$ ), pitching amplitude ( $A_L$ ) and frequency ( $St$ ) on the wake patterns, thrust generations and propulsive performances of the fluid–plate system are analysed. Four typical wake patterns, i.e. von Kármán, reversed von Kármán, deflected and chaotic wakes, emerge from various kinematics, and the $St-A_L$ wake maps are given for various $K$ . The drag-to-thrust transitions (DTT) and the wake transitions (WT) between the von Kármán and reversed von Kármán wakes are examined. Results indicate that the WT and DTT boundaries can be scaled by the chord-averaged distance of travel, $\mathcal {L}$ , which leads to $\mathcal {L}\times St \approx 1$ and $\mathcal {L}\times St \approx 1.2$ , respectively. Further, the resonance mechanism for the performance enhancement is revealed and confirmed in a wide range of parameters. The dimensionless average speed of plate, $\mathcal {U^*}\left (=\mathcal {L}\times St\right )$ , is adopted merely to characterize the propulsive performances. For the first time, the $\mathcal {U^*}$ -based scaling laws for the thrust and power are revealed in pitching rigid and flexible plates for various $A_L$ and $St$ . This study may deepen our understanding of biological swimming and flying, and provide a guide for bionic design.

Transition induced by wall-normal vibration in flow around a flat plate with roughness

The effects of wall-normal vibration on bypass transition in a boundary-layer flow over a flat plate with roughness elements in the form of circular cylinders are investigated using direct numerical simulations (DNS), linear Floquet analyses and dynamic mode decompositions (DMD). The vibration of the plate strengthens the streamwise vorticity, consequently enhancing the velocity streaks and reducing the critical Reynolds number for transition. A map is constructed to identify the coupling effect of the vibration amplitude and Reynolds number on transition. Among all investigated combinations of height and diameter of roughness, the critical Reynolds numbers at different vibration amplitude $A$ can be unified by a scaling function of $(1-10A)$ . Two instability modes are identified in the vibration-induced transition process: a wake mode immediately downstream of the roughness, related to the inviscid Kelvin–Helmholtz instability of the wall-normal shear in the wake; and a streak mode, in response to the spanwise shear of the streaky flow, occurring further downstream. The first one results in the generation of central hairpin vortices which then feed the second one. Further development of the streak instability leads to two arrays of hairpin vortices. Results from linear Floquet analyses and DMD further confirm the two modes of instability observed in DNS. A quantitative study suggests that the amplification of vibration-induced disturbance by the base shear dominates the production of streamwise vorticity and subsequently the hairpin vortices.

Route to transition in propulsive performance of oscillating foil.

Transition in the propulsive performance and vortex synchronization of an oscillating foil in a combined heaving and pitching motion is numerically investigated at a range of reduced frequencies (0.16 ≤f^{*}≤ 0.64), phase offsets (0^{∘} ≤ϕ≤ 315^{∘}), and Reynolds number (1000≤Re≤16000). Focusing on the common case of Re=1000, the drag to thrust transition is identified on a ϕ-f^{*} phase map. Here, the range of 90^{∘} ≤ϕ≤ 225^{∘} depicted a drag-dominated regime for increasing reduced frequency. However, thrust-dominated regimes were observed for ϕ< 90^{∘} and ϕ> 225^{∘}, where increasing the reduced frequency led to an increased thrust production. The isoline-depicting drag-thrust boundary was further observed to coincide with transitions in the characteristic near-wake modes with increasing reduced frequency, which ranged from 2P+2S to 2P and reverse von Kármán modes. However, evaluation of the wake with changing phase offsets at individual reduced frequencies only depicted effects on the spatial configuration of the vortex structures, while the number of vortices shed in one oscillation period was unchanged. The existence of similar wake modes with significantly different propulsive performance clearly suggests that transitions of the wake topology may not always be a reliable tool for understanding propulsive mechanisms of fish swimming or development of underwater propulsion systems. We further assessed a possible route to drag production via investigation into the mean velocity fields at increasing phase offset and at intermediate reduced frequencies ranging from 0.24 to 0.40. This revealed bifurcation of a velocity jet behind the foil on account of the wake topology and dynamics of shed vortex structures. The changes posed by increasing ϕ on wake structure interactions further hints at potential mechanisms that limit the achievement of optimum efficiency in underwater locomotion.

Topological bifurcations in the transition from two single vortices to a pair and a single vortex in the periodic wake behind an oscillating cylinder

We explore the two-dimensional flow past a cylinder undergoing forced transversal oscillations with the aim of elucidating the evolution of the topology of the wake under variation of the forcing amplitude at a Reynolds number of 100. In particular, we study the change from two single vortices (2S mode) to a pair and a single vortex ( $\text {P}+ \text {S}$ mode) being shed per period. Matharu et al. (J. Fluid Mech., vol. 918, 2021, p. A21) showed that a dynamical symmetry-breaking pitchfork bifurcation plays a key role in this transition. We show that in addition to this bifurcation, a number of topological bifurcations in the vorticity field occur, both on the symmetric 2S branch and on the asymmetric $\text {P}+ \text {S}$ branch. The topological bifurcations are cusp bifurcations where an extremum of vorticity is created or destroyed. To describe the effect of the topological bifurcations we introduce an extended symbolic classification of the wake modes to account for the spatial variations of the vortex patterns that occur in the transition process. We identify four amplitude values that define critical stages in the transition, and provide a complete qualitative picture of the transition from 2S to $\text {P}+ \text {S}$ mode. We confirm the robustness of the observed transition process by simulations at Reynolds number 80.

Toward a high performance and strong resilience wind energy harvester assembly utilizing flow-induced vibration: Role of hysteresis

Wind energy harvester utilizing flow-induced vibration (FIV-WEH) has attracted much attention because of its ability to capture low-speed wind energy. This study is motivated to design a reliable FIV-WEH assembly in a wind farm, and to understand the altered environmental flow and disturbed output performance through large-eddy simulations and wind tunnel experiments, for the sake of harnessing fluctuating near-ground wind energy efficiently. The results demonstrates that the proposed eco-friendly FIV-WEH assembly has less impact on the surroundings due to the fast recovery of turbulent wake. Moreover, it is equipped with remarkable hysteresis caused by the delay of wake transition from 2P to C mode, which can enhance the output power, effective wind band, and more importantly, resilience. The resilience guarantees that the FIV-WEH assembly can harvest the fluctuating wind resources optimally even under strong turbulence intensity. Specifically, compact FIV-WEH assembly (l ≤ 1.5, where l is dimensionless streamwise span) based on extended-body regime has a low cut-in speed (3 m/s), showing great potential to harvest breeze energy in a near-ground residential environment. Loose FIV-WEH assembly (1.5 < l < 5) based on reattachment regime has a large cut-in speed (9 m/s) and low span-sensitivity, which can be flexibly adjusted to extract the wind energy in spacious wind-abundant areas. Finally, two strategies developed by modifying surfaces of FIV-WEH assembly can further enhance the resilience and conversion efficiency by approximately 200%.

Coherence of unsteady wake of periodically plunging airfoil

We present an experimental investigation of the flow structure in the near wake of a NACA0012 airfoil plunging sinusoidally at a chord Reynolds number of Re = 20 000 and for a wide range of reduced frequency k and Strouhal number based on peak-to-peak amplitude St. Estimated mean thrust coefficients using the mean and fluctuating velocity fields confirm the St2 dependence as well as a significant effect of the reduced frequency for k ≤ 1. Generally, time-averaged flow quantities are better correlated with St than k in the range tested (k ≤ 3.14 and St ≤ 0.24). Analysis of the streamwise flow and cross-flow in the near wake using two-point cross-correlations and proper orthogonal decomposition reveals that the unsteady characteristics are even better correlated with St than the mean flow quantities. The percentage energy of the fundamental wake modes of the streamwise flow and the flapping mode of the cross-flow increases with increasing St, but at different rates in the drag-producing and thrust-producing wakes. There are similarities to the wake synchronisation behind oscillating bodies. The spanwise-averaged cross-correlation coefficient in the measurement domain grows linearly for small St (in drag-producing wakes), and is nearly constant at a high value for larger St (in thrust-producing wakes). Results show that the Strouhal number is the most important parameter that determines the degree of two-dimensionality of the wake, and suggest that spanwise vortices are quasi-two-dimensional for St ≥ 0.05 and x/c ≤ 4. The implications for experimental gust generators using oscillating airfoils are discussed.

Linking clinical complaints and objective measures of disrupted nighttime sleep in narcolepsy type 1.

Despite its high frequency in narcolepsy type 1(NT1), disrupted nocturnal sleep (DNS) remains understudied, and its determinants have been poorly assessed. We aimed to determine the clinical, polysomnographic (PSG), and biological variables associated with DNS in a large sample of patients with NT1, and to evaluate the effect of medication on DNS and its severity. Two hundred and forty-eight consecutive adult patients with NT1 (145 untreated, 103 treated) were included at the National Reference Center for Narcolepsy-France; 51 drug-free patients were reevaluated during treatment. DNS, assessed with the Narcolepsy Severity Scale (NSS), was categorized in four levels (absent, mild, moderate, severe). Clinical characteristics, validated questionnaires, PSG parameters (sleep fragmentation markers: sleep (SB) and wake bouts (WB), transitions), objective sleepiness, and orexin-A levels were assessed. In drug-free patients, DNS severity was associated with higher scores on NSS, higher sleepiness, anxiety/depressive symptoms, autonomic dysfunction, worse quality of life (QoL). Patients with moderate/severe DNS (59%) had increased sleep onset REM periods, lower sleep efficiency, longer wake after sleep onset, more N1, SB, WB, sleep instability, transitions. In treated patients, DNS was associated with the same clinical data, and antidepressant use; but only with longer REM sleep latency on PSG. During treatment, sleepiness, NSS scores, depressive symptoms decreased, as well as total sleep time, WB, SB, transitions. DNS improved in 55% of patients, without predictors except more baseline anxiety. DNS complaint is frequent in NT1, associated with disease severity based on NSS, several PSG parameters, and objective sleepiness in untreated and treated conditions. DNS improves with treatment. We advocate the systematic assessment of this symptom and its inclusion in NT1 management strategy.