Undisturbed States Research Articles

Data-driven transient lift attenuation for extreme vortex gust–airfoil interactions

We present a data-driven feedforward control to attenuate large transient lift experienced by an airfoil disturbed by an extreme level of discrete vortex gust. The current analysis uses a nonlinear machine-learning technique to compress the high-dimensional flow dynamics onto a low-dimensional manifold. While the interaction dynamics between the airfoil and extreme vortex gust are parametrized by its size, gust ratio and position, the wake responses are well captured on this simple manifold. The effect of extreme vortex disturbance about the undisturbed baseline flows can be extracted in a physically interpretable manner. Furthermore, we call on phase-amplitude reduction to model and control the complex nonlinear extreme aerodynamic flows. The present phase-amplitude reduction model reveals the sensitivity of the dynamical system in terms of the phase shift and amplitude change induced by external forcing with respect to the baseline periodic orbit. By performing the phase-amplitude analysis for a latent dynamical model identified by sparse regression, the sensitivity functions of low-dimensionalized aerodynamic flows for both phase and amplitude are derived. With the phase and amplitude sensitivity functions, optimal forcing can be determined to quickly suppress the effect of extreme vortex gusts towards the undisturbed states in a low-order space. The present optimal flow modification built upon the machine-learned low-dimensional subspace quickly alleviates the impact of transient vortex gusts for a variety of extreme aerodynamic scenarios, providing a potential foundation for flight of small-scale air vehicles in adverse atmospheric conditions.

Stability of a photosurfactant-laden viscous liquid thread under illumination

This paper investigates the effects of a light-actuated photosurfactant on the canonical problem of the linear stability of a viscous thread surrounded by a dynamically passive fluid. A model consisting of the Navier–Stokes equations and a set of molar concentration equations is presented that capture light-induced switching between two stable surfactant isomer states, trans and cis. These two states display significantly different interfacial properties, allowing for some external control of the stability behaviour of the thread via incident light. Normal modes are used to generate a generalized eigenvalue problem for the growth rate which is solved with a hybrid analytical and numerical method. The results are validated with appropriate analytical solutions of increasing complexity, beginning with a solution to a clean interface, then analytical solutions for one insoluble surfactant, one soluble surfactant and a special case of two photosurfactants with a spatially uniform undisturbed state. Presenting each of these cases allows for a holistic discussion of the effect of surfactants in general on the stability of a liquid thread. Finally, the numerical solutions in the presence of two photosurfactants that display radially non-uniform undisturbed states are presented, and details of the impact of the illumination on the linear stability of the thread are discussed.

Influence of shearing strain rate on the mechanical behaviour of three structured clays

This paper presents the stress–strain behaviour of three structured clays subjected to a number of stepwise changes in strain rate during shearing in a triaxial apparatus. Undrained compression tests were performed on clay specimens in both undisturbed and reconstituted states. As a result, it has been possible to identify the influence of structure on the rate-dependent response of the clays. Step changes in applied shearing strain rates for all tests resulted in predominately isotache-type behaviour within the range of strains from very small strains to large strains at failure. At very small strains, evidence of a limiting elastic modulus was established for both the stiff sedimentary and artificially cemented soil, which is consistent with published literature on other structured geomaterials. The stress–strain behaviour of all three materials was affected by both the degradation of structure with increasing strain and applied strain rate. The influence of structure on the time-dependent behaviour of the clays is discussed by comparing the response of undisturbed and reconstituted specimens and following normalisation by the equivalent pressure. Results indicate that the rate sensitivity evolves with increasing strains and reaches a maximum at strain levels associated with peak shearing resistance.

The post-fire stability index; a new approach to monitoring post-fire recovery by satellite imagery

Ecological resilience is the capacity of a system to maintain function following disturbance. With the frequency and severity of wildfire activity increasing due to warmer and drier global climate conditions, there are increasing reports of declines in ecological resilience and ecosystems at risk of collapse due to post-fire recovery failure. Observational monitoring of post-fire recovery at the landscape scale is important to understand drivers, identify vulnerable ecosystems and prioritize management intervention to support resilience. Defining a suitably representative baseline condition to compare post-fire recovery states against can be challenging, particularly in broad-scale remote sensing approaches. Here, we introduce a new approach to monitoring post-fire recovery by satellite imagery, the post-fire stability index. The method is based on the concept that a disturbed system state will be reflected by increasing or decreasing rates of system change, while undisturbed or recovered system states are characterised by near-zero rates of change. This reflects the typical pattern of diminishing rate of change in post-fire recovery trajectories. We demonstrate strong performance and suitability of the post-fire stability index in comparison to alternative approaches through time series analyses, and independent validation from post-fire vegetation surveys taken at one-year post-fire. The post-fire stability index was consistently the best performing model across all field measures of vegetative response following the fire event. In ecosystems which exhibit post-fire resprouting, higher values in the post-fire stability index were associated with higher levels of field-based measures of foliage projective cover and canopy cover (+/− resprouting), and with lower levels of basal and no resprouting. The post-fire stability index provides a relatively simple and practical solution for consistent broad-scale monitoring post-fire recovery with satellite imagery, which together with standardised fire severity mapping, provide extensive opportunities for further fire and landscape ecology research.

Robust model predictive control for hypersonic vehicle with state‐dependent input constraints and parameter uncertainty

AbstractIn this article, a robust model predictive control strategy based on sum‐of‐squares (SOS‐RMPC) is proposed for hypersonic vehicle with state‐dependent input constraints and uncertain parameters. Based on feedback linearization model of hypersonic vehicle, the polytopic linear parameter varying error model with bounded disturbance is established for reference trajectory tracking. The real limits of the actual control inputs are transformed into the constraints of the virtual inputs equivalently with the state‐dependent nonlinear functions. Further, the multivariate linear fitting is used to approximate the state‐dependent input constraints into polynomials. A weighted composite virtual control is formulated as the combination of unconstrained control and auxiliary control. SOS technique is introduced to cast the polynomial constraints into the convex matrix SOS conditions via linear matrix inequality. The virtual control law can be obtained by solving a SOS‐RMPC convex optimization with infinite horizon. The invariant set is designed for the undisturbed error states and the norm‐bounding theory is applied to ensure that the predictive error states with disturbance can remain in the same invariant set. The real control law is obtained by inverse control. The simulation verifies the performance of the designed controller.

Press Disturbance Alters Community Structure and Assembly Mechanisms of Bacterial Taxa and Functional Genes in Mesocosm-Scale Bioreactors.

Press disturbances are of interest in microbial ecology, as they can drive microbial communities to alternative stable states. However, the effect of press disturbances in community assembly mechanisms, particularly with regard to taxa and functional genes at different levels of abundance (i.e., common and rare), remains largely unknown. Here, we tested the effect of a continuous alteration in substrate feeding scheme on the structure, function, and assembly of bacterial communities. Two sets of replicate 5-liter sequencing batch reactors were operated at two different organic carbon loads for a period of 74 days, following 53 days of acclimation after inoculation with sludge from a full-scale treatment plant. Temporal dynamics of community taxonomic and functional gene structure were derived from metagenomics and 16S rRNA gene metabarcoding data. Disturbed reactors exhibited different community function, structure, and assembly compared to undisturbed reactors. Bacterial taxa and functional genes showed dissimilar α-diversity and community assembly patterns. Deterministic assembly mechanisms were generally stronger in disturbed reactors and in common fractions compared to rare ones. Function quickly recovered after the disturbance was removed, but community structure did not. Our results highlight that functional gene data from metagenomics can indicate patterns of community assembly that differ from those obtained from taxon data. This study reveals how a joint evaluation of assembly mechanisms and community structure of bacterial taxa and functional genes as well as ecosystem function can unravel the response of complex microbial systems to a press disturbance.IMPORTANCE Ecosystem management must be viewed in the context of increasing frequencies and magnitudes of various disturbances that occur at different scales. This work provides a glimpse of the changes in assembly mechanisms found in microbial communities exposed to sustained changes in their environment. These mechanisms, deterministic or stochastic, can cause communities to reach a similar or variable composition and function. For a comprehensive view, we use a joint evaluation of temporal dynamics in assembly mechanisms and community structure for both bacterial taxa and their functional genes at different abundance levels, in both disturbed and undisturbed states. We further reverted the disturbance state to contrast recovery of function with community structure. Our findings are relevant, as very few studies have employed such an approach, while there is a need to assess the relative importance of assembly mechanisms for microbial communities across different spatial and temporal scales, environmental gradients, and types of disturbance.

Both Correlation and Morphological Methods of Detecting a Specified Acoustic Signal Propagating Through the Atmosphere

Two methods of detecting quasiperiodic signals—the well-known method of calculating correlation coefficients and the method of a morphological analysis that makes it possible to detect signals with variable phases, which is important in isolating signals at long distances from their sources—are compared. The efficiency of a morphological analysis is in solving the problems of isolating quasiperiodic signals against the background of noise that is comparable to or exceeds (in amplitude) the signal under study. These two methods are compared using subwoofer signals recorded in August 2009 at the Scientific Station of the A.M. Obukhov Institute of Atmospheric Physics. The signal frequencies were chosen within a range of 45–130 Hz and the distance to the source was from 0 to 1255 m. Signals were recorded during night and morning hours, which made it possible to study the potentialities of these methods in isolating a desired signal under both disturbed and undisturbed states of the atmosphere (in the absence or presence of noise of different origins).

Asian elephants modulate their vocalizations when disturbed

When disturbed, animals use various modes of communication to alert conspecifics about the source of danger. Some species have evolved graded or continuous signals specific to the type of threats. African elephants, Loxodonta africana, are known to differentiate between threats from bees and humans by changing the energy concentrations of their alarm calls. However, the mechanism by which Asian elephants, Elephas maximus, use vocalizations to alert conspecifics about imminent danger remains poorly explored. To understand disturbance-induced communication in free-ranging Asian elephants, we compared two call types, ‘rumbles’ (low-frequency calls) and ‘trumpets’ (high-frequency calls), produced in disturbed (by humans or other animals) and undisturbed (social interaction) states. We then analysed acoustic characters for both call types: absolute frequency parameters including fundamental frequency (F0), mean, minimum, maximum and range; temporal parameters including call duration, time to minimum F0, time to maximum F0, peak time and minimum time; and filter-related parameters including mean, minimum and maximum of first (F1) and second (F2) formant locations. We found that under disturbed conditions, Asian elephants increased the duration of rumbles and decreased the duration of trumpets. Similarly, the mean F0 and mean positions of F1 and F2 of rumbles decreased compared with the undisturbed condition; among trumpets, no significant differences were observed in mean F0 or formant position in either F1 or F2 between the two contexts. We also found that the duration of rumbles was influenced by an interaction between group size and context: smaller groups produced longer rumbles when disturbed. These results suggest that when disturbed Asian elephants can modify vocal signals whose likely function could be to alert conspecifics about potential threats.

Study of the macroalgae and application of ecological evaluation index (EEI-c) in the coastal waters of Algeria

The diversity of macroalgae and evaluation of the Algerian coastal waters using EEI-c index were studied. Macroalgae were sampled at seven stations in summer 2019. Coverage data for the macroalgae at each site were analysed at species level. Eleven species have been identified and the results showed the prevalence of Cystoseira compressa and Ulva sp .. The least abundant species were Cladophora sp. and Corallina elongata . Classification of sites based on the cluster analysis shewed an agreement with the water degradation information. Based on the results, the EEI-c values were quite homogeneous over all the studied sites, and generally correspond to undisturbed states except for one site.

Study of the benthic macrofauna and application of AMBI index in the coastal waters of Algeria

The aim of this of study was to examine the state of the benthic macrofauna community at six different sites in coastal waters of Algeria. The diversity of benthic macrofauna was studied and the AZTI marine biotic index (AMBI) was applied. Sampling was carried out during March and April 2018. Thus, 31 species were recorded. Higher species richness (13 species) was recorded in two sites. The highest density was estimated at 56.6 in / 0.1 m². Taxonomic analysis has shown the prevalence of Gastopoda, Bivalvia and Scaphopoda. Sediment analysis showed a low concentration of organic matter in the six sites. The AMBI index used in this study has been used extensively in Europe as indicador of marine pollution, but rarely used in Algeria. The AMBI values ​​were quite homogeneous over all the sites, and generally correspond to undisturbed states except for one site, which is slightly disturbed.

Nesting box imager: Contact-free, real-time measurement of activity, surface body temperature, and respiratory rate applied to hibernating mouse models.

Noncontact methods to measure animal activity and physiology are necessary to monitor undisturbed states such as hibernation. Although some noncontact measurement systems are commercially available, they are often incompatible with realistic habitats, which feature freely moving animals in small, cluttered environments. A growing market of single-board computers, microcontrollers, and inexpensive sensors has made it possible to assemble bespoke integrated sensor systems at significantly lower price points. Herein, we describe a custom-built nesting box imager (NBI) that uses a single-board computer (Raspberry Pi) with a passive infrared (IR) motion sensor, silicon charge-coupled device (CCD), and IR camera CCD to monitor the activity, surface body temperature, and respiratory rate of the meadow jumping mouse during hibernation cycles. The data are logged up to 12 samples per minute and postprocessed using custom Matlab scripts. The entire unit can be built at a price point below US$400, which will be drastically reduced as IR (thermal) arrays are integrated into more consumer electronics and become less expensive.

Interactions within the microbiome alter microbial interactions with host chemical defences and affect disease in a marine holobiont

Our understanding of diseases has been transformed by the realisation that people are holobionts, comprised of a host and its associated microbiome(s). Disease can also have devastating effects on populations of marine organisms, including dominant habitat formers such as seaweed holobionts. However, we know very little about how interactions between microorganisms within microbiomes - of humans or marine organisms – affect host health and there is no underpinning theoretical framework for exploring this. We applied ecological models of succession to bacterial communities to understand how interactions within a seaweed microbiome affect the host. We observed succession of surface microbiomes on the red seaweed Delisea pulchra in situ, following a disturbance, with communities ‘recovering’ to resemble undisturbed states after only 12 days. Further, if this recovery was perturbed, a bleaching disease previously described for this seaweed developed. Early successional strains of bacteria protected the host from colonisation by a pathogenic, later successional strain. Host chemical defences also prevented disease, such that within-microbiome interactions were most important when the host’s chemical defences were inhibited. This is the first experimental evidence that interactions within microbiomes have important implications for host health and disease in a dominant marine habitat-forming organism.

Environmental conditions affecting re-release from particulate matter of 4-Nonylphenol into an aqueous medium.

4-nonylphenol is a persistent organic pollutant with endocrine-disrupting properties. A nonpolar product of microbial degradation derived from the surfactant nonylphenol polyethoxylate, 4-nonylphenol is capable of long-range transport attached to particulates. Bioactive concentrations of 4-nonylphenol have been found in the surface water, soils, snow, and particulate matter of the Eastern Sierra Nevada Mountains (USA) hundreds of miles from their origins. As a result of particulate deposition, seasonal and glacial snow pack concentrations measured 20 to 100 times higher than in surface waters. Batch desorption assays were run on particulate matter dosed with 4-nonylphenol. Desorption was measured in 63 to 500 μm particles under 2 different temperature conditions with varying fractions of organic carbon in turbulent or undisturbed states. Lower temperatures (4 °C) decreased the mean percentage of 4-nonylphenol released from particulates in disturbed and undisturbed conditions, whereas the mean percentage of 4-nonylphenol released at 20 °C was reduced by agitation. The effect of agitation at 4 °C was not practically or statistically significant. Particulates with a higher percentage of organic carbon (75%) released very little of the bound 4-nonylphenol (0.53%) compared with particulates containing 4 to 5% of organic carbon that released up to 13%. Larger particles released the least amount of 4-nonylphenol, whereas smaller particles released the most amount. Water and sediment samples taken from below the Palisades Glacier in the Sierra Nevada Mountains showed the greatest 4-nonylphenol concentrations directly below the glacier, implying that glacial particulates will release adsorbed 4-nonylphenol. Environ Toxicol Chem 2019;38:350-360. © 2018 SETAC.

On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability

Abstract Nonlinearity in the Northern Hemisphere’s wintertime atmospheric flow is investigated from both an intermediate-complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasigeostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel principal component (PC) space, consistent with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20 hPa) and troposphere (500 hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced, and undisturbed states of the winter polar vortex. In the troposphere, the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with a signature of the negative North Atlantic Oscillation (NAO). Over the North Atlantic–European sector, a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or −NAO) positions of the eddy-driven jet stream. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (a decrease) in the frequency of the polar high (low), consistent with an increase of zonal flow frequency.

Ambiguities in model-independent partial-wave analysis

Partial wave analysis is an important tool for analyzing large data sets in hadronic decays of light and heavy mesons. It commonly relies on the isobar model, which assumes multihadron final states originate from successive two-body decays of well-known undisturbed intermediate states. Recently, analyses of heavy-meson decays and diffractively produced states have attempted to overcome the strong model dependences of the isobar model. These analyses have overlooked that model-independent, or freed-isobar, partial-wave analysis can break the orthogonality of partial waves and introduce mathematical ambiguities into results. We show how these ambiguities arise and present general techniques for identifying their presence and for correcting for them. We demonstrate these techniques with specific examples in both heavy-meson decay and pion--proton scattering.

Laboratory investigation on the compressibility of Singapore marine clays

ABSTRACTThe compressibility characteristics of Singapore marine clay in reconstituted and undisturbed states were studied using oedometer, constant rate of strain, Rowe cell, and isotropic consolidation tests. The intrinsic compression curve of the reconstituted clay was found to be similar to that proposed earlier with some minor deviations at low vertical stresses of less than 100 kPa. The field and laboratory compression behaviors were found to be similar, hence the laboratory curve could be used as a reference for interpreting the field behavior. Factors affecting the measurements of compression index and yield stress were discussed. As the coefficient of lateral earth pressure at the top upper clay was close to 1, the compression curves of vertically and horizontally trimmed samples were almost similar. The yield stress was mainly controlled by the strain rate; i.e., the higher strain rate resulted in the higher yield stress. Constrained modulus as derived from cone penetration tests and flat dilatometer tests were also examined and compared with laboratory test results. The in situ tests showed the decrease in constrained modulus with depth and generally the dilatometer test was found to register a higher modulus value.

Empirical model of variations in the upper atmosphere continuum emission. 3. Emitting-layer altitude

The specific features and variations in the continuum emission volume intensity for different heliogeophysical conditions were revealed based on rocket and satellite measurements of this emission in the upper atmosphere in the visible spectral region. It was indicated that the main interval of the continuum emitting layer related to the NO + O process is located at an altitude of 80–110 km. If an emission is caused by the interaction between nitrogen oxide and ozone molecules in disturbed and undisturbed states, the emission emitting layer covers the entire middle atmosphere above the troposphere.

Transport properties in a Sb–Te binary topological-insulator system

Sb–Te layers having various compositions between Sb2Te3 and Sb2Te are grown using molecular beam epitaxy. The structural and electrical properties of the layers change gradually with composition but exhibit a discontinuity involving a bistability. The holes in the layers are generated by Sb bilayers intercalated between Sb2Te3 quintuple layers and their mobility is governed by the scattering from the parent acceptors. Magnetoresistance for compositions around SbTe is linear, for which the reduction of the parabolic component due to low mobility is crucial. Density functional calculations predict Sb2Te3 and SbTe to be topological insulators (TIs) resembling Bi2Se3 and Bi2Te3, respectively. The prefactor of the weak antilocalization effect is α =− 1 regardless of the composition. The Sb–Te system is thus a family of TIs possessing undisturbed surface states for which the location of the Dirac point with respect to the bulk band gap is adjustable.

Inter-phase slip velocity and turbulence characteristics of micro particles in an obstructed two-phase flow

A computational investigation has been made to study the effects of particle size on inter-phase slip velocity and flow turbulence in a solid–liquid two-phase flow through a rectangular duct. Finite volume method with an algebraic slip mixture model and renormalization k-\(\varepsilon \) model has been used in the simulation. Simulations have been made for three different sizes of particles to show their effects on mean and turbulent flow properties. The presence of obstruction changes the typical stratified distribution of micro particles in the stagnation and recirculation regions where stagnation region is characterized with high value of solid particles concentration and recirculation region is characterized with low value. The slip velocity between the particles and liquid phases has been observed more in the upstream compared to the downstream of the obstruction. The change in particles distributions and slip velocities caused by the presence of obstruction disappears at certain downstream distance of the obstruction and the flow properties regain their un-disturbed states. This settling distance depends upon the particle size. Particles enhance the flow turbulence and the effect in complex flow region has been observed more for large size particles. Even though Stokes number associated with the flow is small, the turbulence and slip velocity have been increased due to flow disturbance created by the obstruction.

From expert judgement to supervised classification: A new approach to assess ecological status in lowland streams

The EC Water Framework Directive (WFD) clearly states that undisturbed reference states of aquatic ecosystems should be used to set standards for restoration. Across Europe defining biological reference status and setting boundaries for ecological status classes continues to represent a major challenge. In the present study we investigate if a paradigm exists among experts that can guide the development of assessment systems based on the normative definitions of ecological status classes of the WFD. Our main questions were: 1) Will experts from species abundance data and typology descriptors independently arrive at similar assessments of ecological status, and 2) Can the expert interpretation of ecological status be transferred into a statistical model allowing for a standardization of assessments from plant assemblages in lowland streams? We used a large dataset covering 1244 randomly distributed stream sites in Denmark and asked a group of experts to independently classify the sites using the WFD's normative definitions of ecological status. According to the combined expert group, no Danish stream sites belonged to the undisturbed reference state. For the remaining ecological status classes we found good concordance in the classification made by the five experts. From this we infer that a common paradigm does exist, which may guide the development of assessment methods for aquatic plants in lowland streams. We also found that the common view of the experts could be transferred into a supervised classification model that can serve as a classification tool for aquatic plant assemblages in lowland streams. We conclude that the combined use of experts and advanced multivariate statistics can provide a useful approach in the development of systems for assessment of ecological status in water types, where a reference network cannot be established.