Wake Length Research Articles

Characterizing the development of gravity-driven slug flows using high-speed imaging and PIV-PLIF techniques

Although developing gravity-driven slug flow frequently occurs in oil and gas, and energy systems, its development dynamics remain underexplored; this gap, in turn, has left the underlying relationships between flow evolution and transport phenomena in these applications inadequately characterized as well. The present study experimentally investigates the spatiotemporal-spectral development of gravity-driven air–water and CO2-water slug flows in a vertical 25.4 mm ID pipe. Enhanced flow visualization techniques, utilizing high-speed imaging and particle image velocimetry-planar laser induced fluorescence (PIV-PLIF), were employed to determine the behaviors of gas and liquid phases and interactions at four positions along the pipe axis. A machine vision-based algorithm was employed to extract slug unit cell characteristics and instantaneous void fraction signals, allowing for a comprehensive statistical analysis of gas phase behavior across the flow domain. A novel algorithm was also developed to preprocess raw PIV-PLIF images, facilitating phase discrimination and noise reduction before PIV cross-correlation analyses are conducted. The results showed a logarithmic growth in the lengths of Taylor bubbles, liquid slugs, and slug unit cells along the pipe, with liquid slugs constituting nearly 60 % of slug units downstream. Taylor bubble length distributions correlated well with log-normal fits, while liquid slug and unit cell lengths transitioned from log-normal patterns upstream to near-normal distributions downstream with broader and less peaked shapes. Taylor bubble velocities and appearance frequencies of the flow structures declined exponentially along the pipe, with Taylor bubble velocities showing narrower and more peaked near-normal distributions downstream. Instantaneous void fraction signals exhibited fewer, wider peaks and troughs with reduced small-amplitude oscillations downstream. The analysis of the signals indicated a complete bubbly-to-slug transition at Z/D=30. Gas-liquid phase interactions, classified as almost-zero, weak, and strong, impacted liquid phase velocity profiles and the behavior of Taylor bubbles, with minimum stable liquid slug lengths of 8–9 D and a wake length of 1.8 D observed. Empirical correlations were developed to represent the spatiotemporal-spectral aspects of flow development, with spectral parameters, particularly liquid slug frequency, identified as the most reliable indicators of the fully developed region, predicting entrance lengths of 114.0 D and 113.4 D for air–water and CO2-water, respectively. Gas density was found to strongly influence flow characteristics and transition, accelerating the approach to the fully developed region.

Flow and coherent structures generated by a circular array of rigid, emerged cylinders in a shallow channel

The study investigates the flow structure, dynamics of the large-scale coherent structures, drag forces and sediment entrainment mechanisms generated by a circular array of diameter D containing rigid emerged circular cylinders of diameter d placed in a smooth-bed channel of depth h under strong shallow flow conditions (D/h = 20, d/D = 0.0125 and 0.025). Eddy resolving simulations are conducted with different values of the solid volume fraction 0.025 ≤ SVF ≤ 0.1 and non-dimensional frontal area per unit volume (2.56 ≤ aD ≤ 10.2, where for the present configuration, a = SVF(1/d)4/ ${\rm \pi}$ ) and a fixed channel Reynolds number (Reh = 10 000). The flow conditions are such that a vortex street (VS)-type of shallow wake is expected to form for a solid cylinder of diameter D. Findings are compared with previous results obtained for cases with moderately shallow conditions 2.5 ≤ D/h ≤ 3.5 and with the limiting case of flow past a solid cylinder with D/h = 20. For moderately shallow conditions, the core of the main horseshoe vortex (HV) forming around the array occupies a small fraction of the water column and its coherence is the largest in front of the array. By contrast, for very shallow conditions, multiple HVs form around the array and their cores occupy a large fraction of the water column. Moreover, the coherence of most of these HVs peaks close to the sides of the array. Only for relatively large aD values, the main HV extends over the whole upstream face of the array, similar to the limiting case of a solid cylinder. For sufficiently high aD, a secondary instability is present inside the near wake that leads to the formation of parallel horizontal vortices in the vicinity of the wake roller vortices. The changes in the wake structure with decreasing SVF and aD are qualitatively similar to those observed for cases with moderately shallow flow conditions, with the antisymmetric wake shedding mode being suppressed for aD ≤ 2.5. However, the Strouhal number associated with the shedding of wake rollers, which is still close to 0.2 for a solid cylinder with D/h = 20, can be as high as 0.4 for aD = 2.5. The paper also discusses how the steady wake and total wake lengths and the strength of the bleeding flow vary with the SVF. Simulation results show that the capacity of the flow to entrain sediment inside and around the array peaks for SVF = 0.05 and aD = 5.2, with sediment entrainment monotonically increasing with the SVF outside the array and monotonically decreasing inside the array. Despite the differences in the flow structure next to and inside the array, the variation of the mean, time-averaged streamwise drag coefficient of the solid cylinders ${\bar{C}_d}$ with aD is close to that observed for arrays with moderately shallow flow conditions. The combined drag coefficient for the array decreases with increasing flow shallowness, with the decay being stronger for relatively large values of aD.

Research on Aerodynamic Performance of Asynchronous-Hybrid Dual-Rotor Vertical-Axis Wind Turbines

This study analyzes the performance degradation of traditional hybrid wind turbines under high blade-tip-speed ratio conditions and proposes solutions through two-dimensional Computational Fluid Dynamics (CFD) simulations. It also introduces the design of two innovative asynchronous-hybrid dual-rotor wind turbines. The results indicate a remarkable 98.5% enhancement in torque performance at low blade-tip-speed ratios with the hybrid wind turbine model. However, as the blade-tip-speed ratio increases, it leads to negative torque generation within the inner rotor of the conventional design, resulting in a reduction of the power coefficient by up to 13.1%. The introduction of the new wind turbine design addresses this challenge by eliminating negative torque at high blade-tip-speed ratios through adjustments in the inner rotor’s operating range. This modification not only rectifies the negative torque issue but also enhances the performance of the outer rotor in the leeward region, consequently boosting the overall power coefficient. Moreover, the optimized inner rotor configuration effectively disrupts and shortens the wake length by 16.7%, with this effect intensifying as the rotational speed increases. This optimization is pivotal for enhancing the efficiency of multi-machine operations within wind farms.

Inflammatory pain in mice induces light cycle-dependent effects on sleep architecture.

As a syndrome, chronic pain comprises physical, emotional, and cognitive symptoms such as disability, negative affect, feelings of stress, and fatigue. A rodent model of long-term inflammatory pain, induced by complete Freund's adjuvant (CFA) injection, has previously been shown to cause anhedonia and dysregulated naturalistic behaviors, in a manner similar to animal models of stress. We examined whether this extended to alterations in circadian rhythms and sleep, such as those induced by chronic social defeat stress, using actigraphy and wireless EEG. CFA-induced inflammatory pain profoundly altered sleep architecture in male and female mice. Injection of the hind paw, whether with CFA or saline, reduced some measures of circadian rhythmicity such as variance, period, and amplitude. CFA increased sleep duration primarily in the dark phase, while sleep bout length was decreased in the light and increased in the dark phase. Additionally, CFA reduced wake bout length, especially during the dark phase. Increases in REM and SWS duration and bouts were most significant in the dark phase, regardless of whether CFA had been injected at its onset or 12 hours prior. Taken together, these results indicate that inflammatory pain acutely promotes but also fragments sleep.

Modeling the hydrodynamic wake of an offshore solar array in OpenFOAM

Offshore solar is seen as a promising technology for renewable energy generation. It can be particularly valuable when co-located within offshore wind farms, as these forms of energy generation are complementary. However, the environmental impact of offshore solar is not fully understood yet, and obtaining a better understanding of the possible impact is essential before this technology is applied at a large scale. An important aspect which is still unclear is how offshore solar affects the local hydrodynamics in the marine environment. This article describes the hydrodynamic wake generated by an offshore solar array, arising from the interaction between the array and a tidal current. A computational fluid dynamic (CFD) modeling approach was used, which applies numerical large eddy simulations (LES) in OpenFOAM. The simulations are verified using the numerical model TUDFLOW3D. The study quantifies the wake dimensions and puts them in perspective with the array size, orientation, and tidal current magnitude. The investigation reveals that wake width depends on array size and array orientation. When the array is aligned with the current, wake width is relatively confined and does not depend on the array size. When the array is rotated, the wake width experiences exponential growth, becoming approximately 30% wider than the array width. Wake length is influenced by factors such as horizontal array dimensions and current magnitude. The gaps in between the floaters decrease this dependency. Similarly, the wake depth showed similar dependencies, except for the current magnitude, and only affected the upper meters of the water column. Beneath the array, flow shedding effects occur, affecting a larger part of the water column than the wake. Flow shedding depends on floater size, gaps, and orientation.

Direct numerical simulation study on convective heat transfer of two tandem permeable square cylinders

This study focuses on two-dimensional heat transfer and unsteady flow past two tandem heated porous square cylinders using lattice Boltzmann method combined with block-structured topology-confined mesh refinement. The effects of the Reynolds number (30≤Re≤150), the Darcy number (10−5≤Da≤10−2), and spacing ratio (1.5≤L/D≤5, where L and D are distance of two adjacent cylinder centers and square cylinder length, respectively) are investigated. The intended analysis links hydrodynamic and heat transfer coefficients and wake structures in parameter space of Re−Da−L/D to fluid mechanics. For upstream cylinder, drag coefficients decrease with a reduction of Da and range of Re≥100, while wake length increases with an increment of L/D ratio at the same range of Re. Time-averaged normalized velocity increases at higher permeability levels. A significant augmentation in a time-averaged Nusselt number is reported for an increase in Da and full L/D range. For downstream cylinder, the interaction of fluid vortices in the gap between the cylinders affects the flow pattern, causing irregularities in the drag coefficient variation. The impacts of L/D on the wake length is more obvious than that of Da. Both the wake length and time-averaged Nusselt number values are proportional to an increase in L/D. Consequently, all the investigated results of the upstream cylinder are significantly altered from those of the downstream cylinder due to the shadowing effect of the upstream cylinder.

Study on the internal waves induced by a submerged body moving in a continuously stratified fluid

This paper studies the characteristics of internal waves induced by the motion of a submerged body through a combination of experimental and numerical methods. First, by deploying an array of conductivity probes in an experiment, the temporal evolution of the internal wave at the plane of the conductivity probe array is obtained and compared with numerical simulation results based on the Navier–Stokes equations, thereby validating the accuracy of the Computational Fluid Dynamics (CFD) model. Subsequently, utilizing CFD calculations, further exploration is conducted on the spatiotemporal distribution characteristics of the internal wave, encompassing its generation and evolution, velocity field distribution, wake angle, and wave profile features. Finally, the variation of the internal wave with the Froude number (Fr) is investigated, revealing that it varies apparently with Fr, including the wave component, wave mode, wake length, wake angle, wave amplitude, and so on. It is found that when Fr is large, the internal wave converges toward the centerline behind the body, until forming a line. The wave magnitude changes with Fr in four stages, i.e., increasing with Fr from zero in the first stage, reaching a peak and decreasing thereafter in the second stage, then changing little in the third stage, and finally increasing approximately linearly in the fourth stage.

Numerical analysis on hydrodynamic performance and wake recovery of helical-bladed hydrokinetic turbine: Impact of section inclination angle

In the context of exploiting renewable sources, cross-flow helical hydrokinetic turbine is conceded as one of the promising choices to unlock the potential in free-flowing water bodies. This study examines the impact of blade section inclination angle (−10° to +8°) and operational parameters, namely, tip speed ratio (0.6–1.4) and inflow velocity (1.0–2.5 m/s) on the performance as well as wake recovery of a 4-bladed rotor. The coefficients of power and section-averaged mean streamwise velocity deficit values are computed by solving the unsteady Reynolds-Averaged Navier-Stokes equations coupled with k-ε RNG turbulence model using numerical simulations to assess the performance and velocity recovery, respectively. The power coefficients are seen to be improved with an increase in negative section inclination angle and are found optimum in the range from −6° to −10° at a tip speed ratio of 1.0. The simulation results also reveal that the section inclination angle and tip speed ratio parameters significantly influence the velocity recovery. Furthermore, the performance decreases as the inflow velocity increases; however, the wake recovery is unaffected, with an identical wake length of 13 times the rotor diameter along the streamwise direction at a tip speed ratio of 1.0.

A multi-region analysis of unsteady Oseen's equation for accelerating flow past a circular cylinder

It is difficult to find a valid high-order (of Reynolds number) analytical approximation to the uniform viscous flow past an infinite circular cylinder by applying perturbation techniques. In this study, we show that by accelerating flow past a circular cylinder with taking unsteady Stokes' solution as an initial approximation, higher-order approximation solutions to unsteady Oseen's equation can be obtained by an iteration scheme of perturbation, which exactly satisfy the boundary conditions. As a matter of fact, the nonlinear (convective) term linearized by Oseen's approximation is overweighted especially near the body surface. To eliminate the overweight, a multi-region analysis is proposed in this study to improve analytical unsteady Oseen's solution so as to extend the valid range of Reynolds numbers. In other words, the flow region is hypothetically divided into several annular regions, and the linearized convective term in each region is modified by multiplying with a Carrier's coefficient c (0 < c ≤ 1). The results show that, when the accelerating parameter in the range of 0.5 ≤ a ≤ 4, the maximum effective Reynolds number Reeff of the fourth-order five-region solution is both much larger than that of Stokes' and Oseen's ones. For example, when a = 0.5, Reeff = 22.26 is roughly eight times that of Stokes' solution (Reeff = 2.67), and nine times that of Oseen's solution (Reeff = 2.41). Moreover, when the instantaneous Reynolds number Re(t) is smaller than the Reeff, the flow separation angle and the wake length are both consistent with the numerical results obtained by accurately solving the full Navier-Stokes equations. In addition, the flow properties, including the drag coefficients, streamline patterns, and the pressure coefficients, as well as the vorticity distributions also agree well with the numerical results. This study represents a significant extension of our previous study for unsteady Stokes' equation [Xu et al., Phys. Fluids 35, 033608 (2023)] to unsteady Oseen's equation and its generalization.

Quantifying wake dynamics subjected to stream vegetation patch elongation: The influence of patch-edge vortices

Vortices are generated across a wide range of scales due to the interaction between in-stream vegetation and surrounding flows, significantly influencing hydro-geomorphodynamics in earth surface water systems. Recent advance in vegetation patch hydrodynamics has revealed that the elongation of the middle channel patch can generate complex wake flow patterns by adjusting the bleed flow from the patch and triggering the patch-edge Kelvin–Helmholtz (KH) vortices. With a broader range of experimental configurations, this study reveals how the patch wake mixing is apparently strengthened by the presence of KH vortices, indicated by a larger steady wake velocity, a shorter steady wake length, and a damped energy of wake von Karman vortex. Furthermore, we quantify these characteristic metrics of patch wake behavior with and without the influence of KH vortices. Our findings provide insights into the role of vegetation-induced vortex interactions in regulating mixing processes, thereby promoting informed practices in environmental flows.

On the dynamics and wakes of a freely settling Platonic polyhedron in a quiescent Newtonian fluid

We investigate systematically the free settling of a single Platonic polyhedron in an unbounded domain filled with an otherwise quiescent Newtonian fluid. We consider a particle–fluid density mimicking a rock in water. Five Platonic polyhedrons of increasing sphericity are studied for a range of Galileo numbers $10 \leqslant \mathcal {G}a \leqslant 300$ . We construct a regime map in the parameter space of Galileo number and particle volume fraction ( $\mathcal {G}a, \phi$ ), highlighting how the angularity of the Platonic polyhedron impacts its settling path and the onset of instabilities. We find that the initial angular position solely affects the transient settling process. All the Platonic polyhedrons maintain a stable settling angular position at low $\mathcal {G}a$ . Higher angularity leads to path unsteadiness at lower $\mathcal {G}a$ . Path instability progresses from steady vertical to unsteady vertical, followed by oblique settling observed for highly spherical particles, but helical settling (HS) for more angular particles. The particle autorotation is found to be the pivotal factor influencing path instability and the regime transition of angular particles. Beginning in the unsteady oblique and helical regimes, particle autorotation becomes more prevalent, escalating further in the chaotic regime as $\mathcal {G}a$ increases. The particle angular velocity vector is shown to be predominantly situated in the horizontal plane. A thorough force balance in the horizontal plane reveals that the Magnus force is the primary driving force of the HS regime. Additionally, we establish two new empirical correlations to predict the particle settling velocity and the disturbed wake length that solely require the physical properties of the system ( $\mathcal {G}a$ and $\phi$ ). Our numerical results suggest that an increase of the density ratio from $2$ to $3$ exerts only a marginal impact on the path instability of the most angular particle, the settling tetrahedron.

Wake characteristics behind a tidal turbine with surface waves in turbulent flow analyzed with large-eddy simulation

To design tidal stream turbine arrays, turbine wakes need to be fully characterized to assess the adequate row spacing considering environmental factors such as onset turbulence, velocity shear, and surface waves. The role of waves on wake development varies depending on their characteristics, such as wavelength and amplitude, and needs to be carefully understood. Here large-eddy simulations are performed to analyze the instantaneous and time-averaged wake characteristics developed downstream of a tidal stream turbine for four wave-current conditions ranging from nearly deep to intermediate waves and compared to current-only results. The tip-vortex convection near the free surface is highly influenced by the waves. During the period of increased surface elevation, there is an upwards motion of the vortices which have merged into a single vortical structure during the wave trough period. Downwards vertical transport of tip vortices occurs after the pass of the wave crest, enabling vertical entrainment of momentum into the turbine wake region. Disk-averaged velocity deficit recovery increases for wave-current cases compared to the current-alone condition, with all intermediate wave cases showing a very similar wake recovery rate throughout the wake length. The nearly deep-water waves lead to a slightly slower recovery rate up to 12 turbine diameters (D) downstream while, after this distance, the recovery rate is the highest among the simulated wave conditions. The integration of the mean kinetic energy (MKE) budget in the region up to 12D shows there is a similar distribution in the balance between terms irrespective of the wave-current case. However, in the far-wake region covering from 12D to 20D, the nearly deep waves have a larger contribution for MKE replenishment from the transverse convection and turbulence transport terms, balanced with their vertical counterparts and the streamwise convection term. Analysis of the wake self-similarity shows that a Gaussian model description holds for the recovery rate and lateral profile of the wake of a tidal turbine operating in coupled wave-current conditions, with a similar rate of velocity recovery after 12D irrespective of the wave characteristics. Lateral wake spreading is smaller for the shortest wavelength, which relates to an increased contribution of transverse fluxes in the MKE budget. Published by the American Physical Society 2024

Energetic Stratified Turbulence Generated by Kuroshio–Seamount Interactions in Tokara Strait

Abstract Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up- and downstream of the ∼10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates and diapycnal diffusivities above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time of N−1 ∼ 100 s and the 0.5 m s−1 Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10–100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Average ε vertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10–100 m vertical wavelengths comparable to water depth with spectral levels independent of ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.

Influence of an upstream attached flat plate on the newtonian flow past a square cylinder

In this study, thelattice Boltzmann method is used to analyze the flow around a single square cylinder with a flat plate attached atfront side of the cylinder. In order to perceive the impact of the attached flat plate on flow characteristics of the square cylinder, the dimensionless plate lengths are variedfrom 0.1 to 6, while the Reynolds number is fixed at 150. Corresponding to all plate lengths the well-known von Karman vortex street is observed in the cylinder’s wake region. However, the increase in plate’s length results in reducing recirculation regions, enlarging the wake length and thus stabilizing the flow. The fluid forces and velocity field are significantly affected with increment in plate length. A maximum of 65%, 76%, 62% and 11% reduction is observed in the mean drag coefficient (CDm), root-mean-square values of drag coefficient (CDrms) and lift coefficient (CLrms) and amplitude of lift coefficient (CLAmp), respectively, as compared to single cylinder without a flat plate. Pressure is also found to be significantly reduced due to presence of plate at front surface of cylinder.

An integrate-and-fire mathematical model of sleep-wake neuronal networks in the developing mammal.

Sleep behavior is present in nearly all animals, and is a vital part of growth, development, and overall health. Infant mammals cycle randomly between short bouts of sleep and wake, and the lengths of these bouts both follow an exponential distribution. As mammals mature into adulthood, the mean sleep and wake bout lengths increase, and we also observe a change in the distribution of wake bout lengths from exponential to power law. Focusing on three regions of the brainstem that are involved in sleep-wake regulation, we develop a novel integrate-and-fire neuronal network model to expand upon previous mathematical models of sleep-wake regulation in mammals, focusing on rats. This model allows fine control over neuronal connectivity while simultaneously increasing the size and complexity of the modeled system to make it more representative of reality. We establish a relationship between neuronal network structure and function that could explain the different sleep-wake behaviors observed in rats as they progress through development. We explore the relationship between three different neuronal populations as well as the overall network behavior of the system. We find that increasing synaptic connectivity strength between the wake-promoting region and the wake-active region accounts for the observed changes in mammalian sleep-wake patterns. This dynamic neuronal connectivity is a possible mechanism that accurately accounts for sleep-wake pattern changes observed during mammalian development.

ACTIGRAPHIC MEASURES OF SLEEP AND METABOLIC SYNDROME AMONG OLDER ADULTS

Abstract Self-reported sleep disturbances are associated with obesity and diabetes, but little is known about links between objectively measured sleep and metabolic syndrome (MetS) in later life. We investigated associations between actigraphic sleep parameters and MetS in 406 participants (53.5% women, 20.9% Black) aged 72.8±10.1 years at baseline (range: 50-96) in the Baltimore Longitudinal Study of Aging. Participants completed wrist actigraphy (6.6±1.0 nights) and were classified as having MetS if they had ≥3 of the following: waist circumference ≥102 cm for men, ≥89 cm for women; high-density lipoproteins < 40 mg/dL in men, < 50 mg/dL in women; triglycerides >150 mg/dL; high blood pressure ≥130/85 mmHg; fasting blood glucose ≥100 mg/dL. Overall, 37 participants had MetS at baseline and 27 participants developed MetS over 2.3±2.2 years of follow-up. Cross-sectionally, compared to participants with total sleep time (TST) < 6 hours, those with 6-8 hours TST had lower odds of MetS (OR=0.28, 95%CI 0.13,0.61); higher sleep efficiency (SE) was associated with lower odds of MetS (OR=0.65, 95%CI 0.48,0.89) and longer wake bout length (WBL) was associated with higher odds of MetS (OR=1.47, 95%CI 1.07,2.01). Excluding 37 participants with MetS at baseline, TST >8 hours was associated with faster increases in the likelihood of developing MetS (OR=1.62, 95% CI 1.03, 2.55). Findings link intermediate TST and higher SE to lower odds of MetS, and longer WBL to higher odds of MetS in older adults cross-sectionally; TST >8 hours was associated with higher odds of incident MetS. Further studies are needed to understand mediators of sleep-MetS associations.

Flow control over a circular cylinder using vortex generators: Particle image velocimetry analysis and machine-learning-based prediction of flow characteristics

Controlling the flow around circular cylinders is crucial to mitigate vortex-induced vibrations and prevent structural damage in a range of applications, such as marine and offshore engineering, tall buildings, long-span bridges, transport ships, and heat exchangers. In this study, we aimed to control the turbulent flow structure around a circular cylinder by placing vortex generators (VGs). We examined the flow structure using particle image velocimetry (PIV). This enabled quantitative data acquisition, intuitive flow visualization, and drag coefficient determination from PIV data. We developed artificial neural network (ANN) models that successfully predict both mean and instantaneous flow characteristics for different scenarios. Our findings show that using VGs elongated the wake and increased vortex formation lengths while reducing velocity fluctuations and the drag coefficient. A minimum drag coefficient of 0.718 was achieved with VGs oriented at α = 60° & β = 60°, reducing the drag by 35.3% compared to the bare cylinder. The drag coefficient exhibited a substantial inverse correlation with both wake and vortex formation lengths. This study is significant for controlling flow structures, providing detailed insights into the near-wake region, and highlighting the potential applications of machine learning in fluid dynamics.

Effects of Patch Density and Incoming Sediment on Flow Characteristics and Bed Morphology

This study focuses on the effects of vegetation patch density, bed condition, and incoming sediment on flow structure and bed morphology within and around a patch. The variation in upstream adjustment velocity is not well defined for low-density vegetation patches but decreases with increasing patch density in high-density patches. The length of the upstream adjustment region is greater for high-density vegetation. Incoming sediment causes a reduction in both the steady wake velocity and the length of the steady wake at a low density. The length of the recovery region increases with density when vegetation is sparse, but remains constant in a dense patch. Additionally, the length of the recovery region decreases due to incoming sediment. Turbulent kinetic energy is not affected by the bedform and incoming sediment when reaching its first peak. However, the second maximum of the turbulent kinetic energy increases when the bed is movable. The evolution of bed morphology is closely related to the flow structure and the growth of the von Karman vortex street. Both the rising length and the adjusted length decrease with increasing patch density, while the incoming sediment causes an increase in the adjusted length. Behind the patch wake, the first minimum elevation, maximum elevation, and second minimum elevation decrease as the patch density increases. These values, in turn, increase with the sediment supply upstream of the flume.

Investigating the impact of various operating parameters on blade aeroelasticity and wake characteristics of large-scale wind turbines

The current study endeavors to investigate the impact of various operating parameters on the fluid-structure interaction characteristics of wind turbine blades and wake field characteristics within a wind farm. The observed underperformance of wind farms, attributed to downstream wind turbines being exposed to upstream wake effects, highlights the need for implementing active wake control (AWC). Consequently, in-depth research into the fluid-structure interaction characteristics of wind turbines and wake characteristics under AWC conditions becomes imperative. To address this, a novel aeroelastic analysis model based on geometrically exact beam theory, lifting line theory, and optimization is developed. Numerical simulations utilizing the IEA WIND 10 MW wind turbine, developed by the International Energy Agency, are employed to assess the fluid-structure interaction characteristics of the blades and wake characteristics under various wake control strategies. The results demonstrate that cone angle control in blade attitude alters the wake field width but may reduce velocity loss recovery rate, while pitch angle control significantly improves wake field length at the expense of increased response load. Yaw and tilt control redirect wake flow, with tilt control exhibiting lower thrust and structural response loads. The coupling of yaw and tilt control emerges as a promising approach with enhanced velocity loss recovery and reduced response loads. These findings contribute to understanding the effectiveness of wake control strategies in optimizing large-scale wind turbine performance.

Quantifying effect of overload-induced residual stress behind crack tip on fatigue crack growth

Separating and quantifying effect of overload-induced residual stress (RS) behind and ahead of crack tip on fatigue behavior are inevitable for random fatigue life prediction. In this study, fatigue crack growth measurements under constant stress intensity and different stress ratios for the same overload are performed together with analyzing crack compliance curves. The results show crack closure-depended RS contribution behind crack tip to fatigue retardation results from the variation of crack opening stress intensity which can be directly obtained in fatigue test. Crack closure influence is determined by both effective RS behind crack tip and contact length of crack wake.