Turbulent Motion Research Articles

Numerical experiments on scalar transport and mixing in turbulent boundary layers

In this work, we present numerical experiments aimed at dynamically establishing the separate role of the inner and outer cycles on the scalar transport in the configuration of a temporally evolving boundary layer. The experiments are based on the study of the evolution of passive scalars driven by velocity fields where inner and outer cycles are alternately suppressed. Two different approaches are implemented. In the first, the discrimination between inner and outer cycle activities is based on the scale dimension of the involved motions. The second instead, discriminates on the basis of the distance from the wall of the turbulent motions. The two approaches depict the same scenario. Both the inner and outer cycles appear to be autonomous and, in a sense, independent, since their dynamics remain qualitatively unaltered despite facing two different conditions. The outer cycle faces a free boundary at the top and simply rescales according to what is supplied by the inner cycle. The inner cycle, on the other hand, resides between the wall and the outer region. As a result, the reduction of the scalar fluxes in the outer region due to the suppression of the outer cycle causes a damping in the near-wall region activities.

Научно-технологические основы разработки месторождений подземных вод с принудительным самоизливом

Introduction. A two-well borehole system is proposed, providing for the extraction of groundwater by forced self-discharge. The essence is to create conditions for the emergence of a down hole energy source in the form of hydraulic shocks, alternating hydraulic impulses affecting aquifers and ensuring the rise of water to a height. Materials and methods of research. A block diagram of a two-well circulation system of wells consisting of an above-ground tank, an injection well with a ledge, a water-lifting well connected by an intermediate trunk. The objects of the study are the shape of the above–ground tank, its connection with the injection well, the location of the ledge and the conditions for conducting a water circulation channel from the ledge to the above-ground tank. Methods – calculation of the energy characteristics of a two-well system, experiments to study the mechanism of the origin of incomplete hydraulic shock, boundary values of the modes of water movement in the system and the coefficient of injection of rocks. They were carried out on the installation with the possibility of visual observation of the processes taking place in the system. Results. When an incomplete hydraulic shock occurs on the ledge of the injection well, the flow is divided into core and lateral components. The water flow from the tank into the well was in the range of 20–60 l/min. The origin of incomplete hydraulic shock occurred at a water flow rate of 20 l/min, with an increase in pressure up to 7.0 atmospheres. The boundary values of the laminar flow mode of water flow are 60 l/min. The height of the water rise (H) mainly depends on the depth of origin of incomplete hydraulic shock (H1). The ejection coefficient of rocks decreases with increasing flow rate of the working flow. Discussion. The results of the study are due to the design features of the elements of the two-well circulation system of wells. The operating conditions of the system are the type of communication ground tank – injection well – aquifer, the location of the ledge, the execution of the water circulation channel from the ledge to the entrance to the ground tank with one diameter, the preservation of the laminar flow regime of water, the exclusion of pressure loss and water leakage in the intermediate trunk. Conclusions. The conditions for the origin of a down hole energy source in the form of renewable hydraulic shocks, which have a hydro impulsive effect on aquifers, have been established. A block diagram of the injection and circulation system of wells for the extraction of groundwater by forced self-discharge has been formed. Resume. The validity of the idea of injection-forced self-discharge of groundwater is proved by the results of research. The results are applicable in the development of groundwater with low-pressure and non-pressure aquifers. The objects of future research are the optimal ratios of the diameter of the injection well and the passage section of the ledge, the expansion of the boundary values of the flow transition from laminar to turbulent motion, and an increase in the ejection coefficient.

Non-thermal broadening of coronal lines in a 3D MHD loop model

ABSTRACT Observed spectral profiles of emission lines from the corona are found to have widths exceeding the thermal line width. To investigate the physical mechanism, we run a 3D magnetohydrodynamics model of a single, straightened loop in which we partially resolve turbulent motions that form in response to the driving by self-consistently evolving magneto-convection in the photosphere. The convective motions shear and twist the magnetic field lines, leading to heating. From the model, we synthesize spectral profiles of emission lines forming at temperatures around and above 1 MK. The coronal heating process generates a range of velocity amplitudes and directions structured on a scale much smaller than the resolving power of current instruments, leading to a broadening of the spectral lines. Our model includes the mass exchange between corona and chromosphere, thus we also capture flows parallel to the loop axis. We find that the spectral lines show a non-thermal line broadening roughly consistent with observations for a viewing angle perpendicular to the axis. The broadening through field-parallel flows is comparable, although slightly smaller. The line broadening is independent of the instrument resolution for a perpendicular line of sight (LOS). We can connect the non-thermal line broadening to heating events and flows. While small-scale velocities along the LOS are mainly responsible for the broadening observed perpendicular to the loop, chromospheric evaporation is important for the line broadening observed along the loop. The model reproduces observed values for non-thermal line widths. In the model, these result from continuous driving by magnetoconvection, without imposing driving motions or starting from an already braided field.

Vertical distribution characteristics of the typhoon gust factor based on a 356-m high meteorological gradient tower

The characteristics of typhoon gust factors (GF) are important for the design of high-rise buildings in coastal areas. Most research on GF are based on observational data near the ground. This has made it difficult to meet the increasing demand for high-rise building designs in recent years. Thus, improving the knowledge of the vertical distribution of GF inside the typhoon is necessary and helpful for the design of high-rises and the prevention of gust disasters. In this study, the data from a 365-m high meteorological tower were used to explore the vertical distribution features of the gust factor within the atmospheric boundary layer (ABL) of typhoons in coastal region. The results indicate that: (1) The statistical probability distribution of GF during the typhoon exhibits a non-Gaussian distribution at all observational heights of the tower. While the generalized extreme value distribution (GEV) can well describe the properties of the probability distribution of GF at different heights. (2) The GF have a weak relationship with the average wind speed. However, GF have a strong relationship with turbulence intensity and increases linearly with the turbulence intensity. Notably, the results from the previous empirical model of the GF do not fully match the parameterised results during typhoons in this study. (3) In the typhoon boundary layer, the bulk wind shear plays a significant role in generating turbulent eddies with scales comparable to the observation height, especially at lower layer. Besides, the strong turbulent fluctuations at near-ground layers may be a result of the coupling of several elements. The turbulent momentum flux transported downward from the upper layers has a significant impact on the peak gust, and the downward transported energy may further enhance the turbulent motion in lower layers, which may lead to a higher gust factor near the surface.

Phenomenology of transition to quantum turbulence in flows of superfluid helium

Transition from laminar to turbulent states of classical viscous fluids is complex and incompletely understood. Transition to quantum turbulence (QT), by which we mean the turbulent motion of quantum fluids such as helium II, whose physical properties depend on quantum physics in some crucial respects, is naturally more complex. This increased complexity arises from superfluidity, quantization of circulation, and, at finite temperatures below the critical, the two-fluid behavior. Transition to QT could involve, as an initial step, the transition of the classical component, or the intrinsic or extrinsic nucleation of quantized vortices in the superfluid component, or a simultaneous occurrence of both scenarios—and the subsequent interconnected evolution. In spite of the multiplicity of scenarios, aspects of transition to QT can be understood at a phenomenological level on the basis of some general principles, and compared meaningfully with transition in classical flows.

Spontaneous Stochasticity Amplifies Even Thermal Noise to the Largest Scales of Turbulence in a Few Eddy Turnover Times.

How predictable are turbulent flows? Here, we use theoretical estimates and shell model simulations to argue that Eulerian spontaneous stochasticity, a manifestation of the nonuniqueness of the solutions to the Euler equation that is conjectured to occur in Navier-Stokes turbulence at high Reynolds numbers, leads to universal statistics at finite times, not just at infinite time as for standard chaos. These universal statistics are predictable, even though individual flow realizations are not. Any small-scale noise vanishing slowly enough with increasing Reynolds number can trigger spontaneous stochasticity, and here we show that thermal noise alone, in the absence of any larger disturbances, would suffice. If confirmed for Navier-Stokes turbulence, our findings would imply that intrinsic stochasticity of turbulent fluid motions at all scales can be triggered even by unavoidable molecular noise, with implications for modeling in engineering, climate, astrophysics, and cosmology.

Kinematic signatures of planet–disk interactions in vertical shear instability-turbulent protoplanetary disks

Context. Planets are thought to form inside weakly ionized regions of protoplanetary disks, where turbulence creates ideal conditions for solid growth. However, the nature of this turbulence is still uncertain. In fast cooling parts of this zone the vertical shear instability (VSI) can operate, inducing a low level of gas turbulence and large-scale gas motions. Resolving the kinematic signatures of active VSI could reveal the origin of turbulence in planet-forming disk regions. However, an exploration of kinematic signatures of the interplay between VSI and forming planets is needed for a correct interpretation of radio interferometric observations. A robust detection of VSI would lead the way to a deeper understanding of the impact of gas turbulence on planet formation. Aims. The objective of this study is to explore the effect of VSI on the disk substructures triggered by an embedded fairly massive planet. We focus on the impact of this interplay on CO kinematic observations with the ALMA interferometer. Methods. We conducted global 3D hydrodynamical simulations of VSI-unstable disks with and without embedded massive planets, exploring Saturn- and Jupiter-mass cases. We studied the effect of planets on the VSI gas dynamics, and made a comparison with viscous disks. Post-processing the simulations with a radiative transfer code, we examined the kinematic signatures expected in CO molecular line emission, varying disk inclination. Further, we simulated deep ALMA high-resolution observations of our synthetic images, to test the observability of VSI and planetary signatures. Results. The embedded planet produces a damping of the VSI along a radial region, most effective at the disk midplane. For the Saturn case, the VSI modes are distorted by the planet’s spirals producing mixed kinematic signatures. For the Jupiter case, the planet’s influence dominates the overall disk gas kinematics. Conclusions. The presence of massive planets embedded in the disk can weaken the VSI large-scale gas flows, limiting its observability in CO kinematic observations with ALMA.

An investigation of the effect of stratification stability and saltation sand flux on the anisotropy of atmospheric surface layer wall turbulence

In the atmospheric surface layer (ASL), the anisotropic characteristics of turbulence become more complex due to the strong influence of heat flux and particle motion. In this paper, the effects of stratification stability and total saltation sand flux on turbulent anisotropy of ASL wall turbulence are systematically analyzed by using high-frequency 3D velocity, temperature, and saltation sand flux measured in the field. Our results show that the temperature gradient intensifies the spanwise turbulent motion and enhances the vertical motion asymmetry. Although the buoyancy-driven turbulence enhances the spanwise variation, the momentum transfer mode between the streamwise–vertical and spanwise–vertical does not change. With the increase in instability, the large-scale motions (LSM) of the spanwise velocity may be destroyed and more spanwise small-scale motions (SSM) formed. In addition, saltation increases the sharpness, randomness, and extreme frequency of spanwise velocity distribution, but does not change the momentum transfer mode. Saltation may enlarge the fluctuation range and scale of the spanwise velocity, but may not be the main factor affecting the anisotropy of SSM. These results can deepen people's understanding of wind-blown sand movement and turbulence properties in ASL, and provides empirical data and insights that can significantly contribute to the development and refinement of turbulence models, particularly those that need to account for the complex interplay between stratification stability, saltation sand flux, and turbulence anisotropy.

Mathematical modeling of the melt flow in the cone-ring channel of the extruder matrix

The peculiarity of the work is to develop a mathematical model of the melt flow in the cone-annular channel of the extruder die, which will allow you to choose the optimal geometric shape of the annular channel, as well as the angular rotation speed of the extruder screw. The object of the study is the cone-ring channel of the extruder matrix. To solve the three-dimensional problem of turbulent and laminar unsteady motion, the FlowVision software package was used, which allows solving partial differential equations using the finite volume method. The correct choice of the geometric shape of the annular gap made it possible to change the molecular structure of the product by creating a maximum pressure value. The optimally selected angular rotation speed of the extruder screw made it possible to carry out deep melting of the product due to the conversion of the mechanical energy of the screw into thermal energy. In the process of designing the forming of the cone-ring channel, the three volumes of the changing geometry of the stamp were selected, at the borders of the transition of which extreme points were observed. Deviations between the analyzed and experimental values of velocity and pressure were found in the limits of 9–12 % and 17–22 %, respectively. Based on the analysis of the obtained results of modeling, the optimal geometric shape of the annular gap and the recommended angular rotation speed of the extruder screw were revealed. The mathematical model proposed formed the basis of the developed design of the six-zone extruder. In the extruder, the geometric characteristics of the screw (variable pitch of the turns) were substantiated and selected, which provide the maximum effect of dissipation, i.e. autogenous mode of operation

Near-surface turbulent dissipation at a laboratory-scale confluence: implications on gas transfer

AbstractRiver confluences contribute to the outflux of saturated dissolved gases in the water resulting from high dam discharges. This process is related to gas transfer across the water–air interface, which is primarily controlled by turbulent dissipation near the water surface. However, the near-surface turbulence dissipation is rarely reported in confluence hydrodynamics studies. This study conducted experiments with different discharge ratios to investigate near-surface turbulent motions at a laboratory-scale confluence. The higher dissipation rate $$\varepsilon H/U_{m}^{3}$$ ε H / U m 3 of near-surface turbulence was mainly located inside the interfacial shear layer between the two incoming streams (~ 10–4) and the bank separation zone (10–4–10–3) where high shear was found in the mean flow. By contrast, the dissipation rates were much lower inside the incoming flows and outside the two regions of high shear (~ 10–5). The magnitudes of the dissipation rate inside the shear layer were comparable in experiments where the mixing interface was in the Kelvin–Helmholtz mode or in the wake mode. The dissipation rate was found to increase away from the free surface outside the shear layer, while it was more uniformly distributed over the depth inside the layer possibly due to the presence of strongly-coherent, vertically-orientated vortices. In the far field, the mean shear within the shear layer was largely weakened. Nonetheless, the effects of flow separation persisted and laterally expanded to occupy the entire cross section. The dissipation rate $$\varepsilon H/U_{m}^{3}$$ ε H / U m 3 of the confluent flow was more than 10–4 even at a distance of 10 times the channel width in the post-confluence channel.

An analysis of drag reduction using spanwise forcing on rough walls

Spanwise opposed wall-jet forcing has been shown to reduce the skin-friction drag of wall-bounded turbulent flows by suppressing the near-wall turbulent motion (Yao et al., 2018). In the present work, the response of this drag reduction mechanism to the presence of surface roughness is studied. To this end, direct numerical simulations of flow in smooth and rough plane channels at a matched friction Reynolds number (Reτ=180) are carried out. The roughness is generated by distributing discrete roughness elements at two different values of frontal solidity, namely 0.021 and 0.045. The roughness elements height is k+=18 (＋indicates viscous scaling). By varying the forcing amplitude A+, it is shown that, when other forcing parameters are fixed, maximum drag reduction in a rough channel is achieved at a considerably larger A+ than in a smooth channel. Notably, the strength of the wall-jet at which the maximum drag reduction is achieved, is comparable for the smooth and both rough cases. Additionally, it is observed that the maximum drag reduction values attained in rough channels are smaller compared to those observed in smooth channels, for both lower and higher frontal solidity cases — 2.4% and 2%, respectively. The reduced drag reduction potential originates from two observations; firstly, drag on roughness elements, which is dominated by pressure drag, is not reduced by the method in question. Secondly, the drag on channel floor is reduced to a lesser extent when roughness elements are present. The latter can be attributed to the observation that, while in all cases the forcing suppresses streamwise random turbulent fluctuations and shear stress, this suppression is less pronounced when roughness elements are present.

Shortcomings of the dissipation rate for understanding the turbulent environment of plankton—And a potential solution

AbstractFundamental marine ecosystem dynamics such as mating, predation, and infection require individual plankton to move relative to one another. Ambient turbulence is often invoked as a mechanism to facilitate such interactions. The local intensity of turbulence is quantified as the dissipation rate of turbulent kinetic energy. While the dissipation rate is central to understanding large‐scale fluxes of heat, salt, and nutrients in the ocean, we show that it can be a poor descriptor of the turbulent environment experienced by individual plankton. A dissipation rate is a single integrated quantity representing all the complex motions in a turbulent region; the instantaneous turbulent environment of plankton may bear little resemblance to that predicted by the dissipation rate or quantities derived from it. Most importantly, the statistics (probabilities) of the relative motions of plankton in turbulence cannot be recovered from the dissipation rate or its spectrum: the probabilities of the plankton experiencing any given turbulent relative velocity are lost in the calculation. This presents a fundamental barrier to our understanding of the effects of ambient turbulence on planktonic ecosystem dynamics in the ocean. Rather than relying on dissipation rates, we show that quantifying the probability distributions of the microscale turbulent motions can provide much richer insights into the turbulent environment of individual plankton. Expanding such statistical analyses, and improving our understanding of the Lagrangian properties of ocean turbulence as experienced by plankton in the ocean will lead to significant increases in our ability to understand and quantify the effects of turbulence on plankton.

Resolvent-based motion-to-wake modelling of wind turbine wakes under dynamic rotor motion

We propose a linearized deterministic model for predicting coherent structures in the wake of a floating offshore wind turbine subject to platform motions. The model's motion-to-wake predictive capability is achieved through two building blocks: a motion-to-forcing (M2F) part and a forcing-to-wake (F2W) part. The M2F model provides a unified framework to parameterize the effects of arbitrary floating wind turbine motions as unsteady loads of a fixed actuator disk, requiring only the radial distribution of the aerodynamics force coefficient on the blade as input. The F2W model is derived based on a bi-global resolvent model obtained from the linearized Navier–Stokes equations, using the time-averaged wake of a fixed wind turbine as input. In addition to its capability of predicting sensitive frequency ranges, the model excels linear stability analysis by providing spatial modes of the wake response in a motion-specific and phase-resolved manner. The model successfully predicts the wake pulsing mode induced by surge, as well as the similarity and difference of the wake meandering modes caused by sway and yaw. Large-eddy simulations under different inflow turbulence intensities (TIs) and length scales are further conducted to analyse the wake meandering triggered by the simultaneous excitation of free-stream turbulence and sway motion. The results show distinct frequency signatures for the wake dynamics induced by ambient turbulence and sway motion. The inflow TI is found to have a stabilizing effect on the wake, reducing the motion-induced wake responses. Such a stabilizing effect is captured satisfactorily with the proposed model, provided that the effective viscosity is calibrated properly using the data from the fixed turbine wake under the corresponding turbulent inflow.

Analysis of Radon Near-Surface Measurements, Using Co-Located Ozone Data, Radio-Sounding Vertical Profiles, Sensible Heat Flux and Back-Trajectory Calculation

Simultaneous and co-located observations of near-surface Radon-222, ozone and meteorological parameters in a central Italy observation site operated by the University of L’Aquila (Italy), are used to study the physical drivers of the radon abundance during night-time hours. The knowledge of the potential temperature vertical gradient in the surface layer of nocturnal thermal inversion is made possible using co-located radio-sounding vertical profiles of pressure and temperature, thus making possible to indirectly infer the local surface flux of atmospheric radon (16 ± 6 mBq m−2 s−1). The dynamical removal due to turbulent convective motions is found to be the dominant controlling process, determining large differences in the near-surface radon abundance between stable and unstable conditions of the nocturnal Planetary Boundary Layer (PBL). Usual unstable PBL conditions during daytime hours induce an effective dynamical vertical dilution of surface radon, which rapidly reaches a quasi-steady-state abundance during mid-day and afternoon hours, with very low concentration values (5.1 ± 2.0 Bq m−3). Using back-trajectory reanalyses, estimates of local radon fluxes and vertical mixing efficiencies inside the PBL along the air mass latitudinal-longitudinal path and finally the irreversible radon loss due to radioactive decay, we have explored the fraction of daytime radon attributable to long-range advection in the continental near-mountain measurement site of L’Aquila (44 ± 18%).

Characteristics of Turbulence Intermittency, Fine Structure, and Flux Correction in the Taklimakan Desert

Abstract The Taklimakan Desert is one of key climate regions in East Asia, both highly influencing and highly sensitive to local/regional climate change. Based on a comprehensive observation experiment from 1 to 31 May 2022 in the hinterland of the Taklimakan Desert, the characteristics and mechanisms of turbulence intermittency are investigated in this study, with the purpose to correct turbulent fluxes. Using an improved algorithm to decompose turbulence and submeso motions, two intermittency regimes are recognized in the Taklimakan Desert, namely, D and T intermittency and onD intermittency. The former occurs under strongly stable conditions, characterized by the coexistence of dynamic and thermodynamic turbulence intermittency. The latter occurs under strongly unstable conditions and represents only dynamic turbulence intermittency. Physically, the D and T intermittency regime is related to submeso waves, whereas the onD regime is caused by the horizontal convergence/divergence of convective circulations. With the influence of intermittency and submeso motions, the observed turbulent statistics deviate from reality, which would mask the similarity relationships. To overcome the problem, turbulent statistics are corrected by removing submeso components from original fluctuations. The effectiveness of this method is demonstrated based on the flux–gradient relationships. It is also suggested that, for a big dataset, the impact of onD intermittency can be simply corrected by a correction factor while that of D and T intermittency cannot. The results of this study are helpful to develop the parameterization of turbulent exchange processes in the Taklimakan Desert, which is significant to improve the accuracy of weather forecasting and climate prediction. Significance Statement The Taklimakan Desert plays an important role in the evolution of weather and climate in East Asia. With strong surface thermal forcing, turbulence often shows distinctive intermittency, which largely constrains the evaluation of land–atmosphere exchange in this key climate region. This study aims to understand the characteristics of turbulence intermittency and its physical mechanisms, and further to correct the influence of turbulence intermittency on turbulent fluxes in the Taklimakan Desert. This is significant because the results are helpful to improve the parameterization of subgrid processes in the key climate region for atmospheric models, which points the way toward enhancing the accuracy of weather forecasting and climate prediction.

Oceanic Frontal Turbulence

Abstract Upper-ocean turbulence results from a complex set of interactions between submesoscale turbulence and local boundary layer processes. The interaction between larger-scale currents and turbulent fluctuations is two-way: large-scale shearing motions generate turbulence, and the resulting coherent turbulent fluxes of momentum and buoyancy feed back onto the larger flow. Here we examine the evolution and role of turbulence in the intensification, instability, arrest, and decay (i.e., the life cycle) of a dense filament undergoing frontogenesis in the upper-ocean boundary layer, i.e., cold filament frontogenesis (CFF). This phenomenon is examined in large-eddy simulations (LES) with resolved turbulent motions in large horizontal domains using 109 grid points. The boundary layer turbulence is generated by surface buoyancy loss (cooling flux) and is allowed to freely interact with an initially imposed cold filament, and the evolution is followed through the frontal life cycle. Two control parameters are explored: the initial frontal strength M2 = ∂xb and the surface flux . The former is more consequent: initially weaker fronts sharpen more slowly and become arrested at a later time with a larger width. This reflects a competition between the frontogenetic rate induced by the secondary circulation associated with vertical momentum mixing by the turbulence and the instability rate for the along-filament shear flow. The frontal turbulence is energized by the shear production of the latter, is nonlocally transported away from the primary production zone at the filament centerline, and cascades to dissipation in a broad region surrounding the filament. The turbulent momentum fluxes arresting the frontogenesis are supported across a wide range of horizontal scales.

Does the magnetothermal instability survive whistler suppression of thermal conductivity in galaxy clusters?

The hot and dilute intracluster medium (ICM) plays a central role in many key processes that shape galaxy clusters. Nevertheless, the nature of plasma turbulence and particle transport in the ICM remain poorly understood, and quantifying the effect of kinetic plasma instabilities on the macroscopic dynamics represents an outstanding problem. Here we focus on the impact of whistler-wave suppression of the heat flux on the magnetothermal instability (MTI), which is expected to drive significant turbulent motions in the periphery of galaxy clusters. We perform small-scale Boussinesq simulations with a sub-grid closure for the thermal diffusivity in the regime of whistler-wave suppression. Our model is characterized by a single parameter that quantifies the collisionality of the ICM on the astrophysical scales of interest that we tune to explore a range appropriate for the periphery of galaxy clusters. We find that the MTI is qualitatively unchanged for weak whistler suppression. Conversely, with strong suppression the magnetic dynamo is interrupted and MTI turbulence dies out. In the astrophysically relevant limit, however, the MTI is likely to be supplemented by additional sources of turbulence. Investigating this scenario, we show that the inclusion of external forcing has a beneficial impact and revives even MTI simulations with strong whistler suppression. As a result, the plasma remains buoyantly unstable, with important consequences for turbulent mixing in the ICM.

Data-based approach for time-correlated closures of turbulence models.

Developed turbulent motion of fluid still lacks an analytical description despite more than a century of active research. Nowadays, phenomenological ideas are widely used in practical applications, such as small-scale closures for numerical simulations of turbulent flows. In the present paper, we use a shell model of turbulence to construct a closure intended to have a solid theoretical background and to capture intrinsic probabilistic features of turbulence. Shell models of turbulence are dynamical deterministic systems used to model energy cascade and other key aspects of the Navier-Stokes such as intermittency. We rescale the variables of the Sabra model in a way which leads to hidden symmetries and universal distributions. We then use such fine distributions to write closures, i.e., missing expressions for some of the Sabra variables. Our closures rely on approximating probability density functions using a Gaussian mixture model, which makes them probabilistic by nature and allows us to write time-correlated closures. We also provide a framework where other machine learning tools can be employed with reduced black-box aspects.

Fluid Mixing during Phase Separation in Crystallizing White Dwarfs

Accurate models of cooling white dwarfs must treat the energy released as their cores crystallize. This phase transition slows the cooling by releasing latent heat and also gravitational energy, which results from phase separation: liquid C is released from the solid C/O core, driving an outward carbon flux. The Gaia color–magnitude diagram provides striking confirmation of this theory by revealing a mass-dependent overdensity of white dwarfs, indicating slowed cooling at the expected location. However, the observed overdensity is enhanced relative to the models. Additionally, it is associated with increased magnetism, suggesting a link between crystallization and magnetic field generation. Recent works aimed at explaining an enhanced cooling delay and magnetic field generation employ a uniform mixing prescription that assumes large-scale turbulent motions; we show here that these calculations are not self-consistent. We also show that thermohaline mixing is most likely efficient enough to provide the required chemical redistribution during C/O phase separation, and that the resulting velocities and mixing lengths are much smaller than previous estimates. These reduced fluid motions cannot generate measurable magnetic fields, suggesting any link with crystallization needs to invoke a separate mechanism. Finally, this mixing alters the chemical profiles, which in turn affects the frequencies of the pulsation modes.

A numerical study on load effects of low-rise buildings in a wind-blown sand environment

The transportation process of wind-blown sand, represented by sandstorms, occurs frequently in arid and semi-arid regions. The combined action of wind and sand impact load in a wind-sand environment poses a serious threat to the safety of construction, transportation and other engineering structures. However, most of the current structural designs only considers the effect of wind, and there is little detailed research on the surface load distribution of structures caused by sand impact, which leads to uncertainty in the safety assessment of engineering structures in wind-sand areas. In this paper, low-rise buildings which are easily damaged in strong wind-sand environments are taken as the research object. A numerical model of coupling turbulent wind field and particle motion was then established, and the reliability of the numerical simulation results was verified through wind tunnel experiments. Finally, the wind-sand movement around the low-rise building model and the load distribution characteristics on the building surface are simulated under different wind speed and sand concentration conditions. The results show that the edges and corners of engineering structures are subject to more severe particle erosion-abrasion. As the sand concentration increases in the flow field, the wind speed and the wind load acting on the structures weakens. Whereas the sand impact load increases with the increment of wind speed and sand concentration. The increased effect of sand particles on the total load can be considered by amplifying the wind load shape coefficient on the windward side. This study is helpful to understand the mechanism of sand wind-induced damage on low-rise buildings, and provides basic tools and a theoretical basis for the wind and wind-blown sand resistance design and performance optimization of engineering structures.