Turbulence Levels Research Articles

The IXPE Science of Pulsars and Their Nebulae

Pulsars (PSRs) and Pulsar Wind Nebulae (PWNe) form some of the most interesting high-energy astrophysical systems. Their prominent synchrotron emission makes them ideal candidates for polarimetry. Here, after briefly summarizing the scientific rationale underpinning the importance of their polarimetric studies, we review the current status and achievements obtained by the IXPE mission. For the first time, we have space- and phase-resolved X-ray data that allow us to set constraints on the geometry and level of turbulence of the magnetic field which, in turn, can help us to better understand which acceleration and radiation model(s) might be at work in these systems. Interestingly, PWNe show a large variety in terms of polarization degree that might indicate key physical differences, still to be further investigated.

Optimizing thermal efficiency: Advancements in flat plate heat exchanger performance through baffle integration

Compact heat exchangers have been gaining popularity in many industrial applications. Various types of passive turbulising structures such as corrugations, protrusions and ribs are introduced in the flow path to increase the effective heat transfer area and the level of turbulence in the flow path. This study investigates the impact of introduction of baffles on the performance of PHEs in terms of flow characteristics, pressure drop, and heat transfer. Two distinct types of baffle structures, namely wedge and aerofoil configurations, were introduced at varying numbers - 1, 3, and 5. An extensive experimentation is conducted for a FPHE in a thermal power plant of 500 × 2 MW and various flow and thermal parameters are measured. Computational Fluid Dynamics is utilized in this study to find the optimum baffle configuration. A detailed validation study is executed to obtain the correct computational algorithm, that is the right mesh count, optimum turbulence model, and precise numerical algorithm by comparing the numerical results with the available experimental results. Wedge- type baffles create increased turbulence and pressure drop, while aerofoil-type baffles minimize stagnation and exhibit lower pressure drop. Both baffle configurations lead to a substantial increase in heat transfer, with the 5-wedge-baffle setup showing the highest up to a 55% enhancement of Nusselt number. The Performance Evaluation Criterion (PEC) of wedge and aerofoil type is about 1.24 to 1.3 and 1.22 to 1.24 to that of conventional one respectively.

Improved combustion stability of biogas at different CO2 concentrations using inhomogeneous partially premixed stratified flames

Biogas is a renewable and clean energy source that plays an important role in the current environment of low- carbon transition. If high-content CO2 in biogas can be separated, transformed, and utilized, it not only realizes high-value utilization of biogas but also promotes carbon reduction in the biogas field. To improve the combustion stability of biogas, an inhomogeneous, partially premixed stratified (IPPS) combustion model was adopted in this study. The thermal flame structure and stability were investigated for a wide range of mixture inhomogeneities, turbulence levels, CO2 concentrations, air-to-fuel velocity ratios, and combustion energies in a concentric flow slot burner (CFSB). A fine-wire thermocouple is used to resolve the thermal flame structure. The flame size was reduced by increasing the CO2 concentration and the flames became lighter blue. The flame temperature also decreased with increase in CO2 concentration. Flame stability was reduced by increasing the CO2 concentration. However, at a certain level of mixture inhomogeneity, the concentration of CO2 in the IPPS mode did not affect the stability. Accordingly, the IPPS mode of combustion should be suitable for the combustion and stabilization of biogas. This should support the design of highly stabilized biogas turbulent flames independent of CO2 concentration. The data show that the lower stability conditions are partially due to the change in fuel combustion energy, which is characterized by the Wobbe index (WI). In addition, at a certain level of mixture inhomogeneity, the effect of the WI on flame stability becomes dominant.

Population synthesis models indicate a need for early and ubiquitous disk substructures

Context. Large millimeter surveys of star-forming regions enable the study of entire populations of planet-forming disks and reveal correlations between their observable properties. The ever-increasing number of these surveys has led to a flourishing of population study, a valuable tool and approach that is spreading in ever more fields. Population studies of disks have shown that the correlation between disk size and millimeter flux could be explained either through disks with strong substructure, or alternatively by the effects of radial inward drift of growing dust particles. Aims. This study aims to constrain the parameters and initial conditions of planet-forming disks and address the question of the need for the presence of substructures in disks and, if needed, their predicted characteristics, based on the large samples of disk sizes, millimeter fluxes, and spectral indices available. Methods. We performed a population synthesis of the continuum emission of disks, exploiting a two-population model (two-pop-py), considering the influence of viscous evolution, dust growth, fragmentation, and transport, varying the initial conditions of the disk and substructure to find the best match with the observed distributions. Disks both with and without substructure have been examined. We obtained the simulated population distribution for the disk sizes, millimeter fluxes, and spectral indices by post-processing the resulting disk profiles (surface density, maximum grain size, and disk temperature). Results. We show that the observed distributions of spectral indices, sizes, and luminosities together can be best reproduced by disks with significant substructure; namely, a perturbation that is strong enough to be able to trap particles, that is formed early in the evolution of the disk, and that is within 0.4 Myr. Agreement is reached by relatively high initial disk masses (10−2.3 M⋆ ⩽ Mdisk ⩽ 10−0.5 M⋆) and moderate levels of turbulence (10−3.5 ⩽ α ⩽ 10−2.5). Other disk parameters play a weaker role. Only opacities with a high absorption efficiency can reproduce the observed spectral indices. Conclusions. Disk population synthesis is a precious tool for investigating and constraining the parameters and initial conditions of planet-forming disks. The generally low observed spectral indices call for significant substructure, like that which planets in the mass range of Saturn to a few Jupiters would induce, to already be present before 0.4 Myr. Our results indicate that substructure, which so far has only been assessed in individual disks, is likely ubiquitous and extends to the whole population, and imply that most “smooth” disks hide unresolved substructure.

An improved delayed detached-eddy study on the aerodynamic braking technique based on blunting the streamlined section of the high-speed train

This paper utilizes the improved delayed detached-eddy simulation method to investigate an aerodynamic braking technique involving blunting the streamlined portion of a high-speed train (HST) at Re = 5.0 × 105. The accuracy of the numerical simulation method was validated through reduced-scale wind tunnel experiments at the same Reynolds number level. The study compares aerodynamic drag, pressure distribution, boundary layer, and wake flow characteristics between the original configuration and the braking configuration of the HST. Additionally, the impact of aerodynamic braking plates on the flow characteristics around the key components of the HST has also been discussed. The results indicate a significant increase in the pressure drag experienced by the HST with the application of aerodynamic braking plates to its streamlined sections, while there is a slight decrease in viscous drag. This leads to a remarkable 235.4% rise in the overall aerodynamic drag of the entire HST. The aerodynamic braking plates also have a substantial impact on the turbulent wake flow topology, significantly increasing turbulence levels in the near-wake region. Furthermore, the implementation of aerodynamic braking plates may affect pantograph current collection by significantly altering stream-wise and vertical velocity components, notably increasing velocity fluctuation around the contact position between the pantograph and power supply lines.

THE EVOLUTION OF CZECHIA’S SECURITY POLICY UNDER THE INFLUENCE OF THE RUSSIA-UKRAINE WAR

For now, almost all spheres of international life are in the process of transformation. A particularly high level of turbulence characterizes European security, which has faced numerous challenges over the past decade such as the migration crisis, terrorist acts, the COVID-19 pandemic, information threats, etc. Directly threatening the European Union’s eastern borders, Russia’s full-scale invasion has become one of the most crucial security challenges. In this regard, the EU and NATO member states have to review their approaches to ensuring security and stability. Due to Czechia’s membership in both of these blocs and its geographical proximity to a war zone, its security policy has undertaken changes at the current stage, which this article covered.This article defines the reasons for the evolution of the security policy of the Czech Republic. It is worth noting that due to its geographic proximity to the war zone, Czechia perceives Russia’s aggression as a direct threat to its national security. Apart from Russia’s full-scale invasion of Ukraine, the assessment of Russia’s perception of the country is influenced by the Russian military intelligence services’ involvement in the explosions at Czech weapons warehouses in 2014 and Russia’s espionage activities as well. Thus, the increasing regional tensions and the political crisis in international relations prompted the Czech Republic to revise its key security and defense files, according to which Russia and China have been defined as the main security threats.Moreover, the author concludes that support for Ukraine became part of Czechia’s security policy, where the country demonstrated its clear position and declared its intention to sign a security agreement with Ukraine. Also, it is necessary to commend the importance of Czechia’s artillery shells initiative to Ukraine. The Czech Republic has taken several practical steps to respond to security threats increasing its defense spending to 2% of GDP and investments in the defense industry, along with modernizing the military forces and diversifying energy resources in order to get rid of Russia’s dependence. Accordingly, a comprehensive approach to reacting to new security challenges and threats can minimize their impact on Czechia’s security in the future.

Effect of Chevron Angles on the Thermal Performance of Corrugated Plate Heat Exchanger (CPHEs)

Corrugated plate heat exchangers (CPHEs) possess a greater surface area for heat transfer and a heightened level of turbulence as a result of the corrugations. This study presents an experimental analysis comparing the thermal performance of a flat plate heat exchanger and a corrugated plate heat exchanger (CPHE) with varied corrugation angles. The analysis was conducted using water as the test fluid. The comprehensive testing has been carried out on CPHEs with different chevron angles (β) of 30°/30°, 45°/45°, and 60°/60°. Experimental data is collected under steady- state conditions, with a single phase (water-water) system, covering Reynolds numbers (Re) ranging from 900 to 1250. The test fluid's mass flow rate, ranging from 0.05 to 0.16 kilograms per second, is measured together with the accompanying steady state temperatures. Each plate was equipped with four thermocouple sensors to detect the bulk temperature of the cold and hot fluids at the intake and output. The performance was evaluated using both parallel and counterflow configurations. Experimental observations are utilized to estimate the temperature difference (∆T) between the inlet and output streams, the logarithmic mean temperature difference (LMTD), and the exergy (E). The measured ∆T and E values for corrugation angles (30°, 45°, and 60°) of CPHE were compared to those of flat plate heat exchangers. For corrugation angles of 30°, 45°, and 60°, the values of ∆T and E (efficiency) increase as the mass flow rate of the fluid increases. An increase in the corrugation angle leads to an increase in turbulence in the flow, which in turn enhances heat transmission. In addition, the thermal effectiveness (ε) was calculated using the NTU method and compared for all of the plates. Key Words: Corrugated plate heat exchanger, Thermal performance, Chevron angle, heat transfer coefficient.

The impact of V-flow on preoperative diagnosis of thyroid tumors: individually and as part of multimodal sonographic imaging.

V-flow is a dynamic ultrasound technique that visualizes perfusion patterns by displaying dynamic arrows that change in response to the flow of erythrocytes. Furthermore, it provides quantitative values for the maximum and mean velocity of blood flow as well as a percentage value for turbulence. The aim was to enhance the preoperative diagnostic accuracy of thyroid lesions by combining V-flow with established ultrasound modes.B-mode, CCDS, elastography, CEUS, and V-flow were performed on 101 patients. After the ultrasound examination, every nodule was confirmed as benign or malignant via histopathology. The Kruskal-Wallis test, ROC curve, and binary logistic regression were used for the statistical analysis.93 benign regressive thyroid nodules and 8 carcinomas were included in this study. The average mean velocity value for benign lesions was measured at 19.5 cm/s and at 10.7 cm/s for malignant lesions (p = 0.039). The average turbulence percentage was 26.1% for benign nodules and 46.7% for carcinomas (p = 0.016). Carcinomas exhibited a slower and more turbulent perfusion pattern compared to benign tumors. A V-flow-centered system achieves a sensitivity of 100.0% and a specificity of 84.9% in predicting malignancy. This system could have reduced the number of unnecessary thyroid surgeries for benign lesions in our patient group by 70%.The capillary perfusion of thyroid nodules represents a significant indicator of its status. By analyzing the velocity and turbulence level of microvascular blood flow, V-flow offers promising prospects for accurately distinguishing between benign and malignant thyroid lesions. When integrated into a comprehensive multimodal sonographic imaging approach, V-flow further enhances diagnostic accuracy. · V-flow allows for qualitative and quantitative analysis of microvascular perfusion. · Malignant tumors are associated with slower and more turbulent microvascular hemodynamics. · Combining V-flow with other ultrasound modes eases the diagnosis of thyroid carcinomas. · Brandenstein MK, Zhang L, Scharf G et al. The impact of V-flow on preoperative diagnosis of thyroid tumors: individually and as part of multimodal sonographic imaging. Fortschr Röntgenstr 2024; DOI 10.1055/a-2350-0107.

The role of dynamic capabilities in the development of organisational ambidexterity and its effect on timely responsiveness in emerging market high‐tech firms

Although organisational ambidexterity (hereafter referred to as ambidexterity) allows emerging market high‐tech firms (EMHTFs) to align themselves with existing markets and to explore and seize new market opportunities to retain their competitiveness in rapidly changing business environments, little is known about how ambidexterity is developed and the role that dynamic capabilities play in the development of ambidexterity. We examine how dynamic capabilities, specifically, sensing and reconfiguring capabilities, influence the development of ambidexterity and the effect of ambidexterity on timely responsiveness. We also investigate the moderating effects of firm characteristics (i.e. firm size, sector, age, and status) and environmental turbulence (i.e. market, competitive, and technological turbulence) on both relationships. We find that the application of sensing and reconfiguring capabilities enables EMHTFs to develop ambidexterity and that ambidexterity improves timely responsiveness. Additionally, the relationship between the use of sensing capability and ambidexterity and between ambidexterity and timely responsiveness, appear robust across varying levels of market, competitive, and technological turbulence, and regardless of firm size, sector, age, and status. Furthermore, while EMHTFs across different sectors, age groups, and status benefit from using the reconfiguring capability to develop ambidexterity, we find that large firms benefit more than SMEs as the resource availability advantage of large firms allow them to reconfigure resources to support ambidextrous orientations in various ways. Hence, large firms can develop greater ambidexterity than SMEs, whose scarce resources limit how they can reconfigure resources to support ambidextrous orientations.

PAM4-modulated 2-micron band for enhanced indoor optical communications: experimental evaluation

This research explores the application of PAM4 modulation to optical signals in the 2-μm wavelength band for indoor optical communication. Experiments conducted in a simulated atmospheric turbulence environment demonstrated a BER of 10−5 for the 2-μm laser carrier with PAM4, surpassing the forward error correction threshold. Power loss comparisons to a back-to-back setup showed minimal degradation under various turbulence levels, with losses of 1.80 dB, 1.91 dB, 2.09 dB, and 2.55 dB for non-turbulent, weak-turbulent, moderate-turbulent, and strong-turbulent conditions, respectively. The system's sensitivity was measured at -7.69 dBm, emphasizing its effectiveness in detecting low-power signals. OptiSystem simulations indicated that PAM4 necessitates a higher optical OSNR than NRZ, suggesting the necessity for sophisticated signal processing techniques for systems operating in the 2-μm band. The study's results contribute to the body of knowledge on high-bandwidth optical communication and suggest future research avenues aimed at enhancing system performance while maintaining cost-effectiveness.

An extensive analysis of SOL properties in high-δ plasmas in ASDEX Upgrade

A set of dedicated H-mode discharges with constant heating power combining Neutral Beam Injection and Electron Cyclotron Resonance Heating have been executed at the ASDEX Upgrade tokamak using a high triangularity magnetic geometry in order to investigate the impact of filamentary transport to divertor and non-divertor components. The evolution of upstream scrape-off layer (SOL) profiles have been correlated with dedicated separatrix quantities, mostly with the turbulence control parameter αt (Eich and Manz 2021 Nucl. Fusion 61 086016) describing the turbulence level at the separatrix. With increasing αt , a broadening of the upstream density profiles in the near-SOL together with the formation of a density shoulder in the far-SOL have been observed. This phenomenon is associated with an enhanced filamentary transport dominating the radial turbulent transport in the far-SOL and confirmed by means of the cooling water calorimetry on non-divertor components. The probe measurements conducted with the ball-pen probe-head mounted on the midplane manipulator and a retarding-field analyzer close to the limiter surface indicate that the key mechanism increasing the radial filamentary transport to the first wall is an increase of the particle flux Γr,fil , caused primarily by the packing fraction fPF,fil and the filament density ne,fil . At the same time, the electron temperature Te and ion temperature Ti measured close to the limiter surface show only small variations above αt > 0.5. Both the filamentary heat flux and the gross erosion derived from the first wall probe measurements reach a magnitude that should be considered in the design of future fusion reactors.

Vegetation‐Generated Turbulence Does Not Impact the Erosion of Natural Cohesive Sediment

AbstractPrevious studies have demonstrated that vegetation‐generated turbulence can enhance erosion rate and reduce the velocity threshold for erosion of non‐cohesive sediment. This study considered whether vegetation‐generated turbulence had a similar influence on natural cohesive sediment. Cores were collected from a black mangrove forest with aboveground biomass and exposed to stepwise increases in velocity. Erosion was recorded through suspended sediment concentration. For the same velocity, cores with pneumatophores had elevated turbulent kinetic energy compared to bare cores without pneumatophores. However, the vegetation‐generated turbulence did not increase bed stress or the rate of resuspension, relative to bare cores. It was hypothesized that the short time‐scale fluctuations associated with vegetation‐generated turbulence were not of sufficient duration to break cohesion between grains, explaining why elevated levels of turbulence associated with the pneumatophores had no impact on the erosion threshold or rate.

Validating Volumetric Measurements Using Helium-Filled Soap Bubbles in a Turbulent Boundary Layer And Wake Of An Axisymmetric Body Of Revolution

This study focuses on the validation of a volumetric measurement technique to characterize the turbulent boundary layers (TBL) and wake of a slender axisymmetric body. Specifically, Lagrangian Particle Tracking (LPT) was employed to analyse a TBL subjected to an adverse pressure gradient (APG) and transverse curvature at the tail of an axisymmetric body, with a Reynolds number based on model length of 3.95×10 6 . For the LPT method, a seeding rake with 100 nozzles was used to disperse helium-filled soap bubbles (HFSB), which served as tracers. These tracers were illuminated by high-powered LED arrays, allowing their motion to be captured by three high-speed cameras at acquisition rates up to 20 kHz. A key aspect of this research was using shadowgraphy to measure the diameter of the bubbles exiting the nozzles and obtaining their statistical variations across the various nozzles in the seeding rake. The study identified that the diameter of bubbles averaged 0.45 mm, indicating a state of near-neutral buoyancy. The LPT results were then obtained using the shake-the-box (STB) tracking alogirthm, while ensemble bin-averaging of the scattered particle tracks was used to obtain turbulence statistics on a regular grid. To validate the LPT results, comparisons were made with time-resolved 2D Particle Image Velocimetry (TR-PIV) and highly resolved hot-wire anemometry. This comparative analysis revealed good agreement of the mean profiles across the methodologies, highlighting the robustness and accuracy of the LPT technique. While the turbulence levels in the outer layer were well captured by the LPT, some discrepancies were observed near the wall in both PIV and LPT data, indicating limitations in capturing small-scale phenomena. The implications of these findings, particularly in understanding the complex 3D flow effects of such TBLs and the resulting wake flow field are discussed.

KHz Rate Fs/Ps-CARS Thermometry For Supersonic H_2/Air Combustion Studies

Accurate flow field measurements are essential to probe the phenomena involved in aeronautical engines undergoing high temperature and high turbulences. Indeed, reliable experimental datasets are needed to compare and validate numerical models. Among other laser diagnostics, Coherent Anti-stokes Raman Scattering (CARS) is a well-established non-linear spectroscopic technique used to probe harsh environments, since it provides non-intrusive local point measurements of chemical composition and temperature. We report here hybrid fs/ps Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) thermometry campaign performed on N 2 in a supersonic H2 /air combustion. We used a mobile and compact laser architecture that shapes, synchronize and generates the laser beams necessary for CARS spectroscopy. The instrument was moved to a large-scale facility representative of a scramjet combustor (ONERA LAERTE/LAPCAT-II bench). H2 /air was injected during several burst at 0,12 and 0,15 equivalent ratios. Using kHz single-shot measurements combined with motorized translation stages, spatial and temporal resolution were obtained to probe the flow and the combustion at various positions upstream and downstream the hydrogen injectors. At these positions, horizontal and vertical temperature profiles were retrieved and compared with a numerical unsteady fluid dynamic simulation using delayed detached eddy simulations (DDES) developed at ONERA (CEDRE). The position of the combustion inside the combustion chamber could be identified through several horizontal and vertical profile temperature measurements. The signal-to-noise ratio of the detection was optimized by empirically adjusting the number of averaged laser shots based on the level of flow turbulence. For instance, far downstream the H2 injection, averaging over 10 to 100 pulses was possible as the heated air flow was steady. This was confirmed by the good agreement with numerical simulations although some distortion could still be observed and will be used to adjust the numerical simulation algorithm. However, averaging was no longer possible close to the injectors, where a highly turbulent H2 /air combustion was observed. Nevertheless, taking advantages of the high rate single-shot capacity of the spectroscopic technique, temperature could still be retrieved inside this area. These measurements provided a valuable experimental database for comparing and refining computational fluid dynamics modeling.

Rough bed hydrodynamics under wave-current interactional flow field: double averaging approach

ABSTRACT The effect of surface wave on the double-averaged (DA) current-only flow properties is examined over two different types of rough beds comprising hemispherical elements with regular and staggered arrangements. For each case, the relative roughness spacing of p/r = 4 is maintained, where p = pitch distance and r = hemisphere height. The changes in turbulence characteristics against the water depth are analysed for superimposed surface waves propagating along the current with 1 and 2 Hz frequencies. The effect of regular and staggered roughness arrangements on different turbulence characteristics is explored. Results demonstrate that the DA stream-wise velocity decreases and increases at the outer region and within the roughness region, respectively, due to wave propagation. Owing to the surface wave, the DA intensity and form-induced intensity increase above the roughness layer and decrease within the bottom boundary layer. The values of form-induced intensities are smaller (greater) than the DA turbulence intensities at each considered flow depth within the roughness (outer) layer. For the entire flow depth, the values of DA Reynolds stress decrease due to wave interactions along a steady current. Again, the turbulence is observed to be close to isotropic for the current-only flow at the outer zone, while the flow transits to an anisotropic state with wave imposition on current. Further, the computed values of the turbulence indicator and dispersion functions are used for the identification of the level of turbulence and dispersion at each flow depth for the locations upstream and downstream of a hemisphere in the hemispherical bed.

Direct numerical simulation of transition under free-stream turbulence and the influence of large integral length scales

Under the action of free-stream turbulence (FST), elongated streamwise streaky structures are generated inside the boundary layer, and their amplitude and wavelength are crucial for the transition onset. While turbulence intensity is strongly correlated with the transitional Reynolds number, characteristic length scales of the FST are often considered to have a slight impact on the transition location. However, a recent experiment by Fransson and Shahinfar [J. Fluid Mech. 899, A23 (2020)] shows significant effects of FST scales. They found that, for higher free-stream turbulence levels and larger integral length scales, an increase in the length scale postpones transition, contrary to established literature. Here, by performing well-resolved numerical simulations, we aim at understanding why the FST integral length scale affects the transition location differently at low- and high turbulence levels. We found that the integral length scales in Fransson and Shahinfar's experiment are so large that the introduced wide streaks have substantially lower growth in the laminar region, upstream of the transition to turbulence, than the streaks induced by smaller integral length scales. The energy in the boundary layer subsequently propagate to smaller spanwise scales as a result of the nonlinear interaction. When the energy has reached smaller spanwise scales, larger amplitude streaks results in regions where the streak growth are larger. It takes longer for the energy from wider streaks to propagate to the spanwise scales associated with the breakdown to turbulence, than for those with smaller spanwise scales. Thus, there is a faster transition for FST with lower integral length scales in this case.

How should counselling psychologists investigate their practice in an era of volatility?

Content and Focus:Counselling psychologists offer their services to clients in organisational, political and economic contexts that are characterised by increasing levels of complexity, uncertainty and turbulence. Investigating the effectiveness of what we do remains critical to the future of the psychology professions, but our traditional approaches to scientific enquiry may not be adequate for the types of dilemma generated by complexity.Conclusions:This paper describes a framework, adapted from contemporary complexity theory, that can support us in thinking systematically and creatively about the range of investigative skills we might need if professional practice is to prove fit for purpose in the decades to come. It is proposed that complexity theory can assist us in responding creatively to the dilemmas of living and working in unprecedented times.

Exploration-exploitation model of moth-inspired olfactory navigation.

Navigation of male moths towards females during the mating search offers a unique perspective on the exploration-exploitation (EE) model in decision-making. This study uses the EE model to explain male moth pheromone-driven flight paths. Wind tunnel measurements and three-dimensional tracking using infrared cameras have been leveraged to gain insights into male moth behaviour. During the experiments in the wind tunnel, disturbance to the airflow has been added and the effect of increased fluctuations on moth flights has been analysed, in the context of the proposed EE model. The exploration and exploitation phases are separated using a genetic algorithm to the experimentally obtained dataset of moth three-dimensional trajectories. First, the exploration-to-exploitation rate (EER) increases with distance from the source of the female pheromone is demonstrated, which can be explained in the context of the EE model. Furthermore, our findings reveal a compelling relationship between EER and increased flow fluctuations near the pheromone source. Using an olfactory navigation simulation and our moth-inspired navigation model, the phenomenon where male moths exhibit an enhanced EER as turbulence levels increase is explained. This research extends our understanding of optimal navigation strategies based on general biological EE models and supports the development of bioinspired navigation algorithms.

Air-Film Coupling in Prefilming Airblast Atomisers and the Implications for Subsequent Atomisation

AbstractPrefilming airblast atomisers are commonly used in gas turbine combustion system fuel injectors. As the film propagates across the prefilmer it interacts with the high velocity gas stream above it. In this paper a numerical investigation into this interaction is presented. A Coupled Level Set & Volume of Fluid method is used to simulate the development of the film along the KIT-ITS planar prefilmer (Gepperth et al., in: 23rd European conference on liquid atomization and spray systems (ILASS-Europe 2010), Brno, Czech Republic, September, 2010). Initial results showed the importance of correctly specifying the contact angle as too high a value leads to the formation of rivulets instead of a continuous film. An analysis of the film and air showed two-way coupling. The presence of the film increases the growth rate of the gas phase boundary layer, and the strength and size of the turbulent structures within it. Surface waves form in the film, initially driven by the turbulent fluctuations, but developing into transverse waves. These waves are shown to be independent, stochastic events instead of a periodic wave system. At the trailing edge of the prefilmer the increased turbulence level in the air, the variations in the film thickness and the associated change in fuel mass flow and momentum will have large implications for the atomisation process and subsequent fuel spray. These will also impact simulation of the atomisation, as the boundary condition complexity is much greater than commonly used, and the variations will require larger domains and longer simulation times to obtain fully converged atomisation statistics.

Thermal conductivity with bells and whistlers: Suppression of the magnetothermal instability in galaxy clusters

In the hot and dilute intracluster medium (ICM) in galaxy clusters, kinetic plasma instabilities that are excited at the particle gyroradius may play an important role in the transport of heat and momentum, thus affecting the large-scale evolution of these systems. In this paper, we continue our investigation of the effect of whistler suppression of thermal conductivity on the magneto-thermal instability (MTI), which may be active in the periphery of galaxy clusters and may contribute to the observed levels of turbulence. We use a subgrid closure for the heat flux inspired from kinetic simulations and show that MTI turbulence with whistler suppression exhibits a critical transition as the suppression parameter is increased: for modest suppression of the conductivity, the turbulent velocities generated by the MTI decrease accordingly, in agreement with scaling laws found in previous studies of the MTI. However, for suppression above a critical threshold, the MTI loses its ability to maintain equipartition-level magnetic fields through a small-scale dynamo (SSD), and the system enters a ``death-spiral.'' We show that analogous levels of suppression of thermal conductivity with a simple model of flat uniform suppression would not inhibit the dynamo. We propose a model to explain this critical transition, and speculate that conditions in the hot ICM are such that in substantial portions of the galaxy cluster periphery the MTI might struggle to sustain its own dynamo. We then look at spatial correlations and energy transfers in spectral space and find that, with whistler suppression, most of the heat is transported along thin bundles of strong magnetic fields (the Autobahns of electrons), while high-beta regions are brought out of thermal equilibrium. We link this behavior to the intermittent nature of magnetic fields, and we observe an overall reduction of the efficiency of MTI turbulent driving at the largest turbulent scales. Finally, we show that external turbulence interferes with the MTI and leads to reduced levels of MTI turbulence. While individually both external turbulence and whistler suppression reduce MTI turbulence, we find that they can exhibit a complex interplay when acting in conjunction, with external turbulence boosting the whistler-suppressed thermal conductivity and even reviving a ``dead'' MTI. Our study illustrates how extending magnetohydrodynamics with a simple prescription for microscale plasma physics can lead to the formation of a complicated dynamical system and demonstrates that further work is needed in order to bridge the gap between micro- and macro scales in galaxy clusters.