Wake Behavior Research Articles

A review of wind turbines in complex terrain

With global energy consumption increasing, a shift to renewable energy resources is critical for sustainable energy development. Wind energy has emerged as one of the most promising sustainable and renewable energy resources, with the ability to contribute to the world's energy demand significantly. Although wind is a renewable and generally inexpensive energy source, the amount of electricity available varies depending on wind conditions (e.g., speed and direction). As a result, it is critical to learn how to harvest wind energy more effectively and reliably. Furthermore, most newly constructed wind turbines were built onshore, demonstrating that the consequences of varied terrains are becoming increasingly crucial to the wind-energy field. As a result, before wind farm construction, the investment corporation must evaluate wind conditions and wind turbine wake behavior in the natural wind park site (which may include complex terrain). These investigations can be made using field measurements, laboratory experiments, or numerical simulations. In light of this trend, the present work contributes additional insights into the flow situation occurring around onshore wind turbines in the presence of different complex terrain types—Atmospheric Boundary Layer (ABL) wind profile and turbine wake recovery over hilly and forested terrains. On the contrary, there is still limited knowledge on some issues, such as how a wind farm operates in complex terrain.

Studying nocturnal activity of single Blue Tits Cyanistes caeruleus using motion-detecting IP Cams

The sleep–wake patterns and nocturnal activity of diurnal birds have been little studied. Here, we analysed, by chance, those behaviours for free-living Eurasian Blue Tits (Cyanistes caeruleus, Linnaeus, 1758) roosting in nest boxes equipped with motion sensor-triggered Internet Protocol (IP) cameras. By monitoring individual birds over six sessions for more than one hundred nights each session, we found repetitive activity patterns during the night as well as a correlation between daylength and roosting times. Using the same method, we found the roosting behaviour of Eurasian Blue Tits (Cyanistes caeruleus, Linnaeus, 1758) to be comparable to that of Great Tits (Parus major, Linnaeus, 1758) and Common Starlings (Sturnus vulgaris, Linnaeus, 1758) in nest boxes in the same structure. These data, which we showed can be collected relatively easily and inexpensively, suggest that in rural areas the sleep–wake behaviours of free-living birds are mainly determined by the circadian rhythm associated with natural light.

Parabrachial nucleus astrocytes regulate wakefulness and isoflurane anesthesia in mice.

Background: The parabrachial nucleus (PBN) is an important structure regulating the sleep-wake behavior and general anesthesia. Astrocytes in the central nervous system modulate neuronal activity and consequential behavior. However, the specific role of the parabrachial nucleus astrocytes in regulating the sleep-wake behavior and general anesthesia remains unclear. Methods: We used chemogenetic approach to activate or inhibit the activity of PBN astrocytes by injecting AAV-GFAabc1d-hM3Dq-eGFP or AAV-GFAabc1d-hM4Di-eGFP into the PBN. We investigated the effects of intraperitoneal injection of CNO or vehicle on the amount of wakefulness, NREM sleep and REM sleep in sleep-wake behavior, and on the time of loss of righting reflex, time of recovery of righting reflex, sensitivity to isoflurane, electroencephalogram (EEG) power spectrum and burst suppression ratio (BSR) in isoflurane anesthesia. Results: The activation of PBN astrocytes increased wakefulness amount for 4 h, while the inhibition of PBN astrocytes decreased total amount of wakefulness during the 3-hour post-injection period. Chemogenetic activation of PBN astrocytes decreased isoflurane sensitivity and shortened the emergence time from isoflurane-induced general anesthesia. Cortical EEG recordings revealed that PBN astrocyte activation decreased the EEG delta power and BSR during isoflurane anesthesia. Chemogenetic Inhibition of PBN astrocytes increased the EEG delta power and BSR during isoflurane anesthesia. Conclusion: PBN astrocytes are a key neural substrate regulating wakefulness and emergence from isoflurane anesthesia.

From ontogenesis to clinical practice: waking up to infant sleep.

This article describes the author's research journey exploring infant and toddler sleep. From polygraphic recording in hospital nurseries to using videosomnography in homes, the author traced the longitudinal development of infant/toddler nighttime sleep and waking behaviors. The home-based video observations led to a redefinition of a pediatric milestone; namely, "sleeping through the night," and provided a framework for assessing and treating infant/toddler nighttime sleep problems.

New Actuator Disk Model for the Analysis of Wind Turbines Wake Interaction with the Ground

Wake models based on Actuator Disk theory are usually applied to optimize the wind farm layouts and improve their overall efficiency and expected AEP. Despite the effectiveness of the existing models, most Actuator Disk approaches are based on the flow axisymmetric assumption, without considering the ground effect on the wake behavior. However, it has been shown that the mast’s height, or distance from the wind turbine to the ground, has an influence on the wake expansion on both hub’s side and at downstream of the wind turbine. Therefore, in this study, a hybrid CFD-BEM-Actuator Disk approach is developed to address the lack of the existing models. In the proposed model, the 3D wind rotor is modeled by a set of blade elements. Then, the local lift and drag forces acting on each blade element are calculated using BEM theory and incorporated into the momentum equation. This BEM-AD model is implemented in a User Defined Function (UDF) that is loaded into the CFD software. Thereby, ground effects are considered to be a wall boundary and defining a wind boundary layer profile at the inlet boundary, which describes the Atmospheric Boundary Layer (ABL). For the validation of this new Actuator Disk model, an enhanced experimental study is conducted at the Dynfluid Laboratory wind tunnel (ENSAM School Paris Tech). The Particle Image Velocimetry (PIV) measurements are used for the experimental wake explorations applied to a miniature two-bladed wind turbine. The wake developments are analyzed at two different hub heights ratio, h/D = 0.7 and 1.0 (where h is the hub height, and D is the wind rotor diameter). The analysis of the outcomes showed that the numerical simulations are in good correlation with the experimental measurements of the ENSAM wind tunnel. The numerical results show that for h/D=0.7, the upper half of the rotor operates within the boundary layer whereas the lower tip vortices are mainly developed in the horizontal direction with lower intensity compared to the upper tip vortices. This effect was not observed for the case h/D=1.0 where the rotor operates outside of the boundary layer; however, the wake centerline is upward deflected at about 0.3D. The main conclusion is that a distance above 7D must be observed between wind turbines to optimize the wind farm performance and over 1D hub height be required to limit the influence of the ground boundary layer effect.

Effect of Open Area Ratio on Vortex Behavior of Turbulent Grid Wake

Effect of Open Area Ratio on Vortex Behavior of Turbulent Grid Wake

Caffeine Delays Ethanol-Induced Sedation in Drosophila.

Caffeine and ethanol are among the most widely available and commonly consumed psychoactive substances. Both interact with adenosine receptor-mediated signaling which regulates numerous neurological processes including sleep and waking behaviors. In mammals, caffeine is an adenosine receptor antagonist and thus acts as a stimulant. Conversely, ethanol is a sedative because it promotes GABAergic neurotransmission, inhibits glutamatergic neurotransmission, and increases the amount of adenosine in the brain. Despite seemingly overlapping interactions, not much is known about the effect of caffeine on ethanol-induced sedation in Drosophila. In this study, using Drosophila melanogaster as a model, we show that caffeine supplementation in food delays the onset of ethanol-induced sedation in males and females of different strains. The resistance to sedation reverses upon caffeine withdrawal. Heterozygous adenosine receptor mutant flies are resistant to sedation. These findings suggest that caffeine and adenosine receptors modulate the sedative effects of ethanol in Drosophila.

The impact of sleep behaviours, chronotype and time of match on the internal and external outcomes of a tennis match

Tennis match performance is often evaluated by a player's internal (heart rate) and external (match analytics) outcomes. Numerous factors could influence these outcomes, including the time of day, which may be advantageous or not depending on a player's chronotype. This study aimed to determine the influence time of day, chronotype and sleep–wake behaviour (SWB) had on internal and external outcomes of tennis matches. Twelve state-level male tennis players, with a mean age of 28 ± 7, stature of 183 ± 7 and body mass of 86.6 ± 17.4 wore an actigraph device and completed a sleep diary for the study duration. Based on rankings, players were paired against the same opponent and completed three tennis matches at 8:00 am, 2:00 pm and 8:00 pm, separated by a minimum of 48 h. Fatigue and chronotype, measured by the Fatigue Severity Scale, Morningness Eveningness Questionnaire and midsleep time, were assessed before matches; motivation, measured by the Intrinsic Motivation Inventory, was assessed after matches. During matches, players wore global positioning systems and heart rate monitors; the match analytics and players’ rate of perceived exertion were recorded. Increased unforced errors and decreased winners and forced errors were observed in the evening matches. Decreased total distance and rate of perceived exertion during the second set were found for the evening compared to morning and afternoon matches. Chronotype and sleep–wake behaviour had trivial to weak associations with internal and external outcomes. These results indicate that players and coaches should consider the match times when training for future tournaments.

Hierarchical resampling for bagging in multistudy prediction with applications to human neurochemical sensing.

We propose the "study strap ensemble", which combines advantages of two common approaches to fitting prediction models when multiple training datasets ("studies") are available: pooling studies and fitting one model versus averaging predictions from multiple models each fit to individual studies. The study strap ensemble fits models to bootstrapped datasets, or "pseudo-studies." These are generated by resampling from multiple studies with a hierarchical resampling scheme that generalizes the randomized cluster bootstrap. The study strap is controlled by a tuning parameter that determines the proportion of observations to draw from each study. When the parameter is set to its lowest value, each pseudo-study is resampled from only a single study. When it is high, the study strap ignores the multi-study structure and generates pseudo-studies by merging the datasets and drawing observations like a standard bootstrap. We empirically show the optimal tuning value often lies in between, and prove that special cases of the study strap draw the merged dataset and the set of original studies as pseudo-studies. We extend the study strap approach with an ensemble weighting scheme that utilizes information in the distribution of the covariates of the test dataset. Our work is motivated by neuroscience experiments using real-time neurochemical sensing during awake behavior in humans. Current techniques to perform this kind of research require measurements from an electrode placed in the brain during awake neurosurgery and rely on prediction models to estimate neurotransmitter concentrations from the electrical measurements recorded by the electrode. These models are trained by combining multiple datasets that are collected in vitro under heterogeneous conditions in order to promote accuracy of the models when applied to data collected in the brain. A prevailing challenge is deciding how to combine studies or ensemble models trained on different studies to enhance model generalizability. Our methods produce marked improvements in simulations and in this application. All methods are available in the studyStrap CRAN package.

Temporal Associations between Actigraphy-Measured Daytime Movement Behaviors and Nap Sleep in Early Childhood.

The purpose of this micro-longitudinal study was to explore daily associations between daytime movement behaviors (sedentary time and physical activity) and nap sleep in young children. In 298 children (age = 51.0 ± 9.6 months, 43.6% female), wrist-based actigraphy (mean wear time = 10 days) assessed sedentary time, total physical activity, and provided an estimate of nap sleep duration and efficiency. Multilevel logistic and linear regression models were used to examine temporal within-person relations between wake behaviors and nap sleep, and adjusted for overnight sleep duration between days of interest, age, sex, and socioeconomic status. Movement behaviors were not related to the likelihood of next-day napping, but when children were less sedentary (OR = 0.96; p < 0.001) or more active (OR = 1.01; p = 0.001) in the morning, they were more likely to nap that same day. Movement behaviors were not associated with nap sleep duration or efficiency. Conversely, on days children napped, they were less sedentary (B = -2.09, p < 0.001) and more active (B = 25.8, p < 0.001) the following day. Though napping and movement behaviors had some reciprocal relations, effect sizes in the present study were small. Further studies should examine children with more diverse sleep health and from different childcare settings.

Conserved Population Dynamics in the Cerebro-Cerebellar System between Waking and Sleep.

Despite the importance of the cerebellum for motor learning, and the recognized role of sleep in motor memory consolidation, surprisingly little is known about neural activity in the sleeping cerebro-cerebellar system. Here, we used wireless recording from primary motor cortex (M1) and the cerebellum in three female monkeys to examine the relationship between patterns of single-unit spiking activity observed during waking behavior and in natural sleep. Across the population of recorded units, we observed similarities in the timing of firing relative to local field potential features associated with both movements during waking and up state during sleep. We also observed a consistent pattern of asymmetry in pairwise cross-correlograms, indicative of preserved sequential firing in both wake and sleep at low frequencies. Despite the overall similarity in population dynamics between wake and sleep, there was a global change in the timing of cerebellar activity relative to motor cortex, from contemporaneous in the awake state to motor cortex preceding the cerebellum in sleep. We speculate that similar population dynamics in waking and sleep may imply that cerebellar internal models are activated in both states, despite the absence of movement when asleep. Moreover, spindle frequency coherence between the cerebellum and motor cortex may provide a mechanism for cerebellar computations to influence sleep-dependent learning processes in the motor cortex.SIGNIFICANCE STATEMENT It is well known that sleep can lead to improved motor performance. One possibility is that off-line learning results from neural activity during sleep in brain areas responsible for the control of movement. In this study we show for the first time that neuronal patterns in the cerebro-cerebellar system are conserved during both movements and sleep up-states, albeit with a shift in the relative timing between areas. Additionally, we show the presence of simultaneous M1-cerebellar spike coherence at spindle frequencies associated with up-state replay and postulate that this is a mechanism whereby a cerebellar internal model can shape plasticity in neocortical circuits during sleep.

Understanding patient characteristics and medication prescriptions in children with mental health and neurodevelopmental disorders referred to a sleep clinic-A quality improvement/quality assurance analysis.

Motivated by challenges faced in outpatient sleep services for mental health and neurodevelopmental disorders (MHNDD) during the COVID-19 clinical shutdown, a pan-Canadian/international working group of clinicians and social scientists developed a concept for capturing challenging sleep and wake behaviours already at the referral stage in the community setting. In a quality improvement/quality assurance (QIQA) project, a visual logic model was the framework for identifying the multiple causes and possible interventions for sleep disturbances. Intake forms informed clinicians about situational experiences, goals/concerns, in addition to the questions from the Sleep Disturbances Scale for Children (SDSC), the ADHD Rating Scale-IV and medication history. Descriptive statistics were used to describe the sample. 66% of the pilot study patients (n = 41) scored in the SDSC red domains (highest scoring) with highest sub-scores for insomnia (falling asleep 73%; staying asleep: 51%) and daytime somnolence (27%). A total of 90% of patients were taking at least one medication; 59% sleep initiation/sleep medications, 41% in combination with further non-stimulant medications, 9% with stimulants, 27% with antidepressants and 18% with antipsychotics. Polypharmacy was observed in 62% of all patients and in 73% of the ones medicated for sleep disturbances. Qualitative information supported individualisation of assessments. Our intake process enabled a comprehensive understanding of patients' sleep and wake profiles prior to assessment, at the referral stage. The high prevalence of insomnia in patients, combined with polypharmacy, requires special attention in the triaging process at the community level.

P115 Exploring the effectiveness of Fatigue Risk Management Systems

Abstract Introduction Fatigue Risk Management Systems (FRMS) are increasingly being used in a range of safety-critical industries to manage fatigue-related risk. These data-driven practices use a risk-based approach to managing fatigue, rather than solely relying on prescriptive hours of work limits. However, some organisations are reluctant to implement FRMS as evidence of effectiveness is unclear. A review was undertaken to evaluate the available evidence on FRMS effectiveness, and to provide evidence-based policy guidance for FRMS use and implementation. Methods Seven electronic databases (MEDLINE (Ovid), PsycINFO, IEEE Xplore Digital Library, Scopus, Web of Science, MESH Occupational Health, NIOSHTIC II) were searched for relevant peer reviewed and grey literature. Documents were included if they addressed fatigue management interventions. Screening was performed on a total of 2129 documents, with 231 included in the final review. Results Few (n = 5) documents evaluated FRMS as a whole. However, components of FRMS (e.g., performance monitoring, fatigue detection technology, prior sleep wake behaviour assessment) appeared to improve key safety and fatigue outcomes. Discussion The effectiveness of FRMS components suggests that FRMS as a whole is likely to improve organisational safety outcomes. Key implementation enablers included organisational and worker commitment, and safety culture. Where organisations have limited resources and/or immature safety cultures, FRMS may not be appropriate. As such, we propose that a hybrid or transitional model of fatigue management (including risk-based and prescriptive components) could be implemented – so organisations can increase operational flexibility while improving safety.

Amplitude-optimized Koopman-linear flow estimator for wind turbine wake dynamics: Approximation, prediction and reconstruction

The low-order modeling of wind turbine dynamic wake is a challenging problem, which requires approximating high-dimensional, nonlinear dynamic systems with a limited number of states and linear structures. The Koopman-linear flow estimator is regarded as a promoting method in this area to approximate the dynamic wake field from a few physical-measured states. This paper presents optimization methods for the Koopman-linear flow estimator to improve its generalization applicability and modeling accuracy in specific applications. Firstly, the flow estimator’s wake field prediction process is organized using Koopman modes and amplitudes; both are identified initially. Then, a optimization method is proposed to optimize the Koopman amplitudes for a given application scenario, which maintains a consistent form for uncontrolled and controlled systems based on the error-source analysis. After this, a sequential particle swarm optimization algorithm is adopted, which improves the computationally-intensive problem during sensor configuration optimization. The proposed algorithm quickly solves a feasible and optimized sensor configuration plan instead of the unavailable global-optimal one. The verification results show two conclusions: On the one hand, the nonlinearity during the yaw-induced wake deflection process is evident, which poses a significant challenge to the linear low-order approximation of the dynamic wake field. On the other hand, the proposed optimization methods highly improve the dynamic wake modeling accuracy under free-wake and yaw-controlled scenarios. Sparse sampling is necessary for dynamic wake behavior study in industrial. This paper solves the incomplete measurements and wake deflection nonlinearity problem caused by sparse sampling and promotes the generalization applicability for specific applications.

Does time of day and player chronotype impact tennis-specific skills and physical performance?

Tennis players’ success relies on tennis skills, such as groundstrokes and serves, and physical attributes, such as strength, speed and endurance. This study aimed to determine if players’ tennis skills and physical attributes are influenced by time of day, chronotype or sleep–wake behaviour (SWB). Twelve male tennis players (age (years): 28.17 ± 7.85) competing in state-level competitions wore a wrist-worn activity monitor (GT3X, Actigraph) and completed a modified version of the Consensus Sleep Diary to measure SWB. The Chalder Fatigue Scale and Morningness and Eveningness Questionnaire were used to measure players’ fatigue and preferred chronotype. Mid-sleep with a sleep correction was used to determine players’ current chronotype. After the baseline period, players were tested at 8:00 am, 2:00 pm and 8:00 pm, with the order of testing sessions randomised for each player. Testing sessions were separated by at least 48 hours. Players’ groundstrokes, serve speed, agility, overhead medicine ball throw and Hit and Turn Test performance were measured in each session. General linear modelling revealed that backhand consistency was less in the evening compared to the morning by 17% ( p = 0.020) and afternoon by 15% ( p = 0.040). Maximal service velocity was less in the evening compared to the afternoon by 10.5 km/h ( p = 0.041). Chronotype did not influence tennis skills or physical performance. Average and maximal backhand velocities were reduced for every hour that time at lights out, and sleep-onset time was postponed. Tennis skills, but not physical performance tests, were influenced by time of day and SWB.

Numerical Assessment of Vertical Axis Tidal Turbine Performances in Shallow Water

Although blessed with plenty of natural resources, Malaysia has now begun transitioning away from them towards renewable energy alternatives. This is evidenced by various initiatives and plans made towards reducing dependency on hydrocarbons, such as a pledge to “become a carbon neutral country by 2050 as outlined in 12th Malaysia Plan. Among the green energy that can be considered, explored, and focused on by Malaysia is marine renewable energy, such as tidal, since the country is surrounded by large oceans. Nonetheless, there is currently insufficient data regarding the behaviour of the flow when passing through the devices downstream, especially in shallow water applications. Thus, this paper aims to highlight on the turbulence wake characteristics, and also to analyse the behaviour of wake produced by the vertical axis tidal turbine (VATT) in harnessing tidal energy at shallow water application, specifically further downstream of the device. Comparison will be made against the horizontal axis tidal turbine (HATT) to highlight their differences. Numerical analysis using the CFD method is used to analyse the model of turbines, which is represented by cylindrical and disc geometries. The simulation is evaluated in terms of the wake characteristic for both single and array configurations of the turbine. From a previous study, it has been demonstrated that the wake distance has a significant influence on the performance of tidal devices on the subsequent rows. Hence, this statement is examined in this research work to verify its validity for vertical axis tidal turbines.

FACTORS ASSOCIATED WITH SLEEP QUALITY AND DURATION AMONG BEIRUT ARAB UNIVERSITY STUDENTS

Sleep is part of the everyday physiological rhythm that is vital for enhancing wellness and appropriate body functions. University students are vulnerable to sleep disturbance due to many factors that affect their sleep–wake behavior. No study has so far evaluated the association between sleep quality and duration and the health and nutritional status of Lebanese college students. Thus, the present study was designed to evaluate the sleep quality and duration of Beirut Arab University (BAU) students in North Lebanon and to examine associations with their nutritional status, sociodemographic, eating behaviors, lifestyles and health characteristics. To do so, a cross-sectional study was conducted among a sample of 288 students (168 males and 120 females) aged between 17 and 25 years who were registered in the Fall of 2018–2019 in Tripoli Campus, and randomly selected from the different faculties. Students completed a multi-component questionnaire. According to this study, more than half of BAU students had poor sleep quality (64.2%) and short sleep duration (71.5%). The multiple regression analysis revealed that employed students were 82% less likely to have poor sleep quality (ORadj: 0.181; 95% CI: 0.062–0.528) (P

NREM–REM alternation complicates transitions from napping to non-napping behavior in a three-state model of sleep–wake regulation

The temporal structure of human sleep changes across development as it consolidates from the polyphasic sleep of infants to the single nighttime sleep episode typical in adults. Experimental studies have shown that changes in the dynamics of sleep need may mediate this developmental transition in sleep patterning, however, it is unknown how sleep architecture interacts with these changes. We employ a physiologically-based mathematical model that generates wake, rapid eye movement (REM) and non-REM (NREM) sleep states to investigate how NREM–REM alternation affects the transition in sleep patterns as the dynamics of the homeostatic sleep drive are varied. To study the mechanisms producing these transitions, we analyze the bifurcations of numerically-computed circle maps that represent key dynamics of the full sleep–wake network model by tracking the evolution of sleep onsets across different circadian (∼ 24 h) phases. The maps are non-monotonic and discontinuous, being composed of branches that correspond to sleep–wake cycles containing distinct numbers of REM bouts. As the rates of accumulation and decay of the homeostatic sleep drive are varied, we identify the bifurcations that disrupt a period-adding-like behavior of sleep patterns in the transition between biphasic and monophasic sleep. These bifurcations include border collision and saddle–node bifurcations that initiate new sleep patterns, period-doubling bifurcations leading to higher-order patterns of NREM–REM alternation, and intervals of bistability of sleep patterns with different NREM–REM alternations. Furthermore, patterns of NREM–REM alternation exhibit variable behaviors in different regimes of constant sleep–wake patterns. Overall, the sequence of sleep–wake behaviors, and underlying bifurcations, in the transition from biphasic to monophasic sleep in this three-state model is more complex than behavior observed in models of sleep–wake regulation that do not consider the dynamics of NREM–REM alternation. These results suggest that interactions between the dynamics of the homeostatic sleep drive and the dynamics of NREM–REM alternation may contribute to the wide interindividual variation observed when young children transition from napping to non-napping behavior.

High-Fidelity Simulation Study of the Unsteady Flow Effects on High-Pressure Turbine Blade Performance

Abstract Unsteadiness, in the form of both broadband background disturbances and discrete coherent wakes, can have a strong effect on the performance of turbomachinery blades. The influence of the incoming flow has received much interest as it inevitably affects the blade boundary layers and develops as it passes through the machine. In the present work, we investigate the effect of unsteady flow on high-pressure turbines (HPTs), using high-fidelity datasets produced by wall resolved large-eddy simulation of an HPT stage. The effects of incident wakes from an upstream stator, compounded by the presence of freestream turbulence, on the downstream rotor are investigated. Based on analyzing cases with different turbulence intensities and length scales prescribed at the inlet, we show that changing the freestream turbulence characteristics has a direct effect on the unsteady behavior of the stator wakes. As a result, the performance of the rotor is also significantly affected. By detailing the influence of the wake–turbulence interaction, we aim to distinguish driving forces on rotor performance, be it changes in the incident wakes or direct influence from the freestream turbulence. Furthermore, the aerothermal behaviors of the rotor blades have been extensively investigated, showing that the blade boundary layers on the suction and pressure sides respond differently to external disturbances. The insights gained can provide designers with guidelines in understanding the unsteady flow effects of a given flow state, and how the unsteadiness present, either broadband or deterministic, will affect the performance of downstream blades.

Turbulent wakes in a non-uniformly stratified environment

This study explores the behavior of turbulent wakes generated by a sphere propagating with constant speed in a non-uniformly stratified fluid. The investigation is based on a series of high-resolution direct numerical simulations in which the background stratification is systematically varied. We consider one linear and three nonlinear density profiles and discover that even modest, spatially localized non-uniformities of stratification can profoundly influence the wake dynamics, structure, and evolution. The analysis of microstructure signatures shows that wakes in non-uniformly stratified fluids tend to be more spread horizontally, and internal waves are much stronger than in linear stratification. Simulations performed with Gaussian perturbations are characterized by a vertically asymmetric energy distribution, which is attributed to internal wave reflections from low-gradient regions. Using microstructure decay rates, we estimate the effective persistence period of wakes, showing that it substantially increases with the increasing Froude number. We also find that wakes persist much longer in high-gradient profiles, whereas weak local gradients can substantially reduce the wake longevity.