Search

This study proposes two crescent cooling holes based on favorable vortex structures to enhance the film cooling effectiveness. Large eddy simulations and laser-induced fluorescence were conducted. An up-convex crescent hole (UCC) was established first, which uses an anti-counter-rotating vortex pair (anti-CRVP) to prevent the coolant from lifting off the surface and another CRVP to achieve better cooling uniformity at the centerline. Then, a down-convex crescent hole (DCC) was further optimized based on the vortex of the UCC. The results show that the film cooling effectiveness of the DCC increases by 48 % at BR = 1.6 compared to the UCC. Apart from the time-averaged results, the instantaneous flow field of DCC was analyzed in detail, and the distribution and breakdown of hairpin vortexes highly affect the cooling effectiveness. Besides, the heat transfer coefficient and net heat flux reduction were analyzed to overall evaluate the film cooling performance of DCC. [ABSTRACT FROM AUTHOR]

The safe and stable operation of a hydrogen Knudsen compressor is essential for transporting hydrogen in microfluidic systems. This study uses proper orthogonal decomposition to identify the coherent structures within the hydrogen flow field during non-equilibrium evolution. A long short-term memory neural network is then used to create a multi-fidelity reduced-order model, connecting two-dimensional and three-dimensional data to uncover transient flow mechanisms and enable rapid flow field prediction. The results show that the coherent structures of hydrogen flow, representing the most energetic modes, retain 99% of the flow energy and significantly influence the evolution of Poiseuille and thermal transpiration flows during non-equilibrium processes. The multi-fidelity reduced-order model effectively captures hydrogen transient flow and instabilities at various stages, achieving a 99.4% reduction in computational time while maintaining a maximum relative error of 0.53%. This approach facilitates the rapid prediction and control of flow states during hydrogen transport. [Display omitted] • The coherent structures of hydrogen flow within microchannels are revealed. • The gradient distribution of coherent structures dominate hydrogen flow. • The coherent structures are consistent across different temperature rises. • A multi-fidelity reduced-order model for the Knudsen compressor is developed. • The prediction time for hydrogen flow within microchannels is reduced by 99.4%. [ABSTRACT FROM AUTHOR]

Magnetic holes at the ion-to-electron kinetic scale (KSMHs) are one of the extremely small intermittent structures generated in turbulent magnetized plasmas. In recent years, the explorations of KSMHs have made substantial strides, driven by the ultra-high-precision observational data gathered from the Magnetospheric Multiscale (MMS) mission. This review paper summarizes the up-to-date characteristics of the KSMHs observed in Earth's turbulent magnetosheath, as well as their potential impacts on space plasma. This review starts by introducing the fundamental properties of the KSMHs, including observational features, particle behaviors, scales, geometries, and distributions in terrestrial space. Researchers have discovered that KSMHs display a quasi-circular electron vortex-like structure attributed to electron diamagnetic drift. These electrons exhibit noticeable non-gyrotropy and undergo acceleration. The occurrence rate of KSMH in the Earth's magnetosheath is significantly greater than in the solar wind and magnetotail, suggesting the turbulent magnetosheath is a primary source region. Additionally, KSMHs have also been generated in turbulence simulations and successfully reproduced by the kinetic equilibrium models. Furthermore, KSMHs have demonstrated their ability to accelerate electrons by a novel non-adiabatic electron acceleration mechanism, serve as an additional avenue for energy dissipation during magnetic reconnection, and generate diverse wave phenomena, including whistler waves, electrostatic solitary waves, and electron cyclotron waves in space plasma. These results highlight the magnetic hole's impact such as wave-particle interaction, energy cascade/dissipation, and particle acceleration/heating in space plasma. We end this paper by summarizing these discoveries, discussing the generation mechanism, similar structures, and observations in the Earth's magnetotail and solar wind, and presenting a future extension perspective in this active field. [ABSTRACT FROM AUTHOR]

Matter entanglement is a common chaotic structure found in both quantum and classical systems. For classical turbulence, viscous vortices are like sinews in fluid flows, storing and dissipating energy and accommodating strain and stress throughout a complex vortex network. However, to explain how the statistical properties of turbulence arise from elemental vortical structures remains challenging. Here, we use the quantum vortex tangle as a skeleton to generate an instantaneous classical turbulent field with intertwined vortex tubes. Combining the quantum skeleton and tunable vortex thickness makes the synthetic turbulence satisfy key statistical laws, offering valuable insights for elucidating energy cascade and extreme events. By manipulating the elemental structures, we customize turbulence with desired statistical features. This bottom-up approach of designing turbulence provides a testbed for analyzing and modeling turbulence. [ABSTRACT FROM AUTHOR]

Deserts play an important role in the climate system, which is closely associated with the emission and transport of dust aerosols. Based on the intensive observation experiment in the Taklimakan Desert, the potential physical processes between the deep convective boundary layer (CBL) and dust emission are revealed in this study. Deep CBL enables the formation of clouds in the late afternoon, leading to significant cooling of surface. Large‐scale buoyant coherent structures thereby transform into the mechanical coherent structures confined near the surface. The responses promote the earlier occurrence of low‐level jet (LLJ) than in cloudless conditions, which allows the downward transport of LLJ momentum and substantially increases surface wind. Therefore, dust emission is initiated by strong wind at dusk and lasts for several hours. The results are useful to predict dust emissions and improve our understanding of distinctive boundary‐layer processes in desert regions. Plain Language Summary: Desert is a key component of regional/global climate system, which is closely associated with the emission and transport of dust aerosols. A better understanding of dust emission mechanisms and their interaction with atmospheric physical processes is of great significance for improving numerical weather and climate models. This study investigates possible physical mechanisms of the dust emission related to deep atmosphere boundary layer, based on the field experiment in the Taklimakan Desert. It is found that the formation of late‐afternoon clouds and the occurrence of wind maxima at 400–600 m are crucial during the processes. Initially, the formation of clouds is enabled by deep atmosphere boundary layer that developed in the late afternoon. After cloud formation, surface becomes cooling, and then the upper part of the boundary layer is gradually released from the surface frictional restraint, which promotes the development of wind maxima at 400–600 m. As the momentum of the wind maxima is transported downwards, surface wind substantially increases, which systematically blows dust particles up. Consequently, dust emission happens at dusk in desert regions. Key Points: Deep convective boundary layer enables the lifting condensation of moisture and the formation of boundary‐layer clouds in desert regionsThe emergence of late‐afternoon clouds strengthens the surface cooling and thus advances the development of low‐level jetDownward momentum transfer from low‐level jet to near‐surface wind is crucial to initiate dust emission at dusk [ABSTRACT FROM AUTHOR]

Wavelets are employed to study atmospheric turbulent data of three wind components, temperature and passive scalars CO 2 and H 2 O. The multiresolution analysis (MRA) based on maximal overlap discrete wavelet transform (MODWT) is used to separate turbulent fluctuations from the mean flow. These turbulent fluctuations are further partitioned into small scales x ′ s and large scales x ′ L , and the fluxes are calculated by averaging over the given time interval. The large scales are responsible for much of the flux transport, while the small scales are fine scales consisting of non-transporting, nearly isotropic motions. The velocity spectrum for both small (non-coherent) and large scale (coherent) follow - 5 / 3 scaling, and the transfer efficiency R wa similarity laws are better satisfied for the large scales. The velocity probability distribution of partitioned signals shows a narrower distribution for small scales compared to large ones. However, the flatness factor indicates deviation from Gaussianity. The joint probability distribution for large scales is skewed, suggesting the dominance of ejections and sweeps. Despite their wave-like nature, the large scales are not linear waves as indicated by the phase spectrum. The large scales are subjected to the continuous wavelet transform (CWT) to detect and isolate the strong localized events. The Mexican Hat (MHAT) wavelet transform and zero-crossing method is used to estimate the duration, separation, and frequency of occurrence of the detected events. [ABSTRACT FROM AUTHOR]

The enstrophy convection velocity (denoted by u Ω) in the evolution equation of the boundary enstrophy is proportional to skin friction (denoted by τ) in an incompressible viscous flow, which can be determined globally as an optical flow problem from a time sequence of the boundary enstrophy fields. The correlation coefficient between | u Ω | and | τ | is evaluated in a turbulent channel flow at the friction Reynolds number Re τ = 180 , which is about 0.73 in the region of interest containing near-wall strong wall-normal velocity event associated with a sweep-ejection pair) and is approximately independent of the wall-normal coordinate in the viscous sublayer. This correlation coefficient is contributed mainly by strong near-wall coherent structures with high boundary enstrophy. The statistics of the difference between the normalized | u Ω | and | τ | are discussed. The statistical correlation between u Ω and τ elucidates the connection between near-wall flow structures and skin friction. [ABSTRACT FROM AUTHOR]

Time series of large samples of instantaneous velocity fields with a 7.26δ0.99×1.18δ0.99 size in a turbulent boundary layer have been obtained by experimental measurement using time-resolved particle image velocimetry (TRPIV) with a four-camera array. The resultant experimental Reynolds number (Reτ) is 1046. The continuous spatial wavelet transform has been employed to convert the streamwise velocity fluctuations into multi-scale components in the streamwise direction at all wall-normal positions. The distribution of turbulence kinetic energy across all spatial scales and all wall-normal positions are determined from the multi-scale wavelet coefficients. The continuous temporal wavelet transform has also been utilized to resolve the temporal series of longitudinal velocity fluctuations into multi-scale temporal components at all wall-normal positions. The use of the multi-scale wavelet coefficients delivered the turbulent kinetic energy distribution across all temporal scales and all wall-normal positions. Moreover, the distribution of multi-scale flatness factors with spatial scales and normal position, before and after removal of multi-scale coherent structures, has been calculated. The turbulent structures are detected by zero-crossing the wavelet coefficients. The conditional sampling scheme and spatial phase-locked method have been employed to establish typical multi-scale structures at different wall-normal positions. The universality of the small-scale structures has been confirmed where the associated vorticity is characterized by an alternating positive and negative quadrupole along the longitudinal direction and wall-normal direction, while the streamlines can be considered a dynamic system composed of a saddle point and focal points. The scaling exponent Ϛ(p), before and after the removal of the coherent structures, has been calculated, confirming that the multi-scale coherent structures are responsible for the anomalous scaling law. [ABSTRACT FROM AUTHOR]

We review and extend the theory of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence. The theory contains a solution to the 'dynamo problem', i.e., the problem of determining how a planetary or stellar body produces a global dipole magnetic field. We extend the theory to the case of ideal MHD turbulence with a mean magnetic field that is aligned with a rotation axis. The existing theory is also extended by developing the thermodynamics of ideal MHD turbulence based on entropy. A mathematical model is created by Fourier transforming the MHD equations and dynamical variables, resulting in a dynamical system consisting of the independent Fourier coefficients of the velocity and magnetic fields. This dynamical system has a large but finite-dimensional phase space in which the phase flow is divergenceless in the ideal case. There may be several constants of the motion, in addition to energy, which depend on the presence, or lack thereof, of a mean magnetic field or system rotation or both imposed on the magnetofluid; this leads to five different cases of MHD turbulence that must be considered. The constants of the motion (ideal invariants)—the most important being energy and magnetic helicity—are used to construct canonical probability densities and partition functions that enable ensemble predictions to be made. These predictions are compared with time averages from numerical simulations to test whether or not the system is ergodic. In the cases most pertinent to planets and stars, nonergodicity is observed at the largest length-scales and occurs when the components of the dipole field become quasi-stationary and dipole energy is directly proportional to magnetic helicity. This nonergodicity is evident in the thermodynamics, while dipole alignment with a rotation axis may be seen as the result of dynamical symmetry breaking, i.e., 'broken ergodicity'. The relevance of ideal theoretical results to real (forced, dissipative) MHD turbulence is shown through numerical simulation. Again, an important result is a statistical solution of the 'dynamo problem'. [ABSTRACT FROM AUTHOR]

The impact of particle addition jets on the flow field in natural gas pipelines was investigated, and the structural information of the flow field at different flow velocities in a symmetric jet flow was analyzed via numerical simulation. The results of coherent structures in the high-pressure natural gas pipeline reveal vortex structures of varying sizes both upstream and downstream of the jet flow. To determine the spatial distribution of the main vortex structures in the flow field, proper orthogonal decomposition (POD) mode analysis was performed on the unsteady numerical results. Moreover, the detailed spatial characteristics of the coherent vortex structures represented by each mode were obtained. The results indicate that the large-scale vortex structures within the pipeline are balanced and stable, with their energy increasing as the jet flow velocity increases. Additionally, higher-order modes exhibit significant shedding of small-scale vortex structures downstream of the jet flow. In this research, coherent structures present in symmetric particle addition jets are provided, offering theoretical support for future investigations on the distribution of particle image velocimetry (PIV) flowmeters. [ABSTRACT FROM AUTHOR]

Based on the large eddy simulation method, this study performed the threedimensional transient numerical analysis of the near-wall flow field of the spiral flow in a circular pipe and applied the sub-grid model of the kinetic energy transport. The low-speed bands, streamwise vortices and hairpin vortices of the spiral flow in the near-wall region of the circular pipe are determined using the Q criterion. The ejection and sweeping of coherent structures are identified using the velocity vector of the near-wall region; moreover, the two methods of creating the hairpin vortices are established by the image time series. The results demonstrate that the development directions of the near-wall bands, streamwise vortices and hairpin vortices of the spiral flow in the circular pipe develop along the path of the spiral line. The average spanwise period of the low-speed bands in the near-wall region is approximately 120 wall units, the length is more than 900 wall units and the height is not more than 40 wall units. The separation distance of the streamwise vortices is about 119 wall units. It has a certain angle with the wall (approximately 22°). The average burst period of a hairpin vortices is less than 0.015 s. [ABSTRACT FROM AUTHOR]

The effects of the coolant pulsation and the plasma aerodynamic actuation (PAA) on the film cooling are herein explored via large eddy simulations. The electrohydrodynamic force derived from the PAA was solved through the phenomenological plasma model. The Strouhal number of the sinusoidal coolant pulsation and the averaged pulsation blowing ratio were 0.25 and 1.0, respectively. Comprehensive analyses were carried out on the time-averaged flow fields, and the results reveal that the pulsed cooling jet might cause a deeper penetration into the crossflow, and this phenomenon could be remarkably mitigated by the downward force of the PAA. Comparing steady film cooling to pulsed film cooling revealed a modest 15.1% reduction in efficiency, while the application of the dielectric barrier discharge plasma actuator (DBDPA) substantially enhanced the pulsed film cooling efficiency by 42.1%. Moreover, the counter-rotating vortex pair (CRVP) was enlarged and lifted off from the wall more poorly due to the coolant pulsation, and the PAA weakened the detrimental lift-off effect and entrainment of the CRVP. Then, the spatial–temporal development of the coherent structures was figured out by the alterations in the centerline temperature, reflecting the formation of the intermittent coherent structures rather than hairpin vortices due to the coolant pulsation, and their size and upcast behaviors were reduced by the PAA; thus, the turbulent integration of the coolant with the crossflow was suppressed fundamentally. Finally, the three-dimensional streamlines confirmed that the coherent structure dynamic behaviors were significantly regulated by the PAA for alleviating the adverse influences of the coolant pulsation. In summary, the PAA can effectively improve the pulsed film cooling efficiency by controlling the spatial–temporal development of the dominant coherent structures. [ABSTRACT FROM AUTHOR]

Abstract Deserts play an important role in the climate system, which is closely associated with the emission and transport of dust aerosols. Based on the intensive observation experiment in the Taklimakan Desert, the potential physical processes between the deep convective boundary layer (CBL) and dust emission are revealed in this study. Deep CBL enables the formation of clouds in the late afternoon, leading to significant cooling of surface. Large‐scale buoyant coherent structures thereby transform into the mechanical coherent structures confined near the surface. The responses promote the earlier occurrence of low‐level jet (LLJ) than in cloudless conditions, which allows the downward transport of LLJ momentum and substantially increases surface wind. Therefore, dust emission is initiated by strong wind at dusk and lasts for several hours. The results are useful to predict dust emissions and improve our understanding of distinctive boundary‐layer processes in desert regions.

Based on the large eddy simulation method, this study performed the three-dimensional transient numerical analysis of the near-wall flow field of the spiral flow in a circular pipe and applied the sub-grid model of the kinetic energy transport. The low-speed bands, streamwise vortices and hairpin vortices of the spiral flow in the near-wall region of the circular pipe are determined using the Q criterion. The ejection and sweeping of coherent structures are identified using the velocity vector of the near-wall region; moreover, the two methods of creating the hairpin vortices are established by the image time series. The results demonstrate that the development directions of the near-wall bands, streamwise vortices and hairpin vortices of the spiral flow in the circular pipe develop along the path of the spiral line. The average spanwise period of the low-speed bands in the near-wall region is approximately 120 wall units, the length is more than 900 wall units and the height is not more than 40 wall units. The separation distance of the streamwise vortices is about 119 wall units. It has a certain angle with the wall (approximately 22°). The average burst period of a hairpin vortices is less than 0.015 s.

Combined with infrared thermography measurements, large eddy simulation was used to study the square-wave pulsating film cooling on turbine vane at the time-averaged blowing ratios of 0.5 and 1.5. On suction surface, the introduction of coolant pulsation results in the decrease of cooling effectiveness. On pressure surface, coolant pulsation at high blowing ratio can improve coolant coverage. Starting vortex is the typical structure in pulsating film cooling. However, on suction surface, due to fully-turbulent mainstream, plenty of small-scale vortices play a dominating role in flow fields, and the effect of starting vortex is unobvious. The statistic characteristics and power spectrum density (PSD) of velocity fluctuation were also analyzed in detail. The results show that the disturbance of coolant jet on suction-side mainstream boundary layer is very weak, However, on pressure side, coolant pulsation causes notable increase of PSD and turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

Coherent structure is an important phenomenon in the fluid field, and also exists in atmospheric boundary layer. For example, after the passage of a cold front in spring in northern China, there is a rather regular gusty wind with a period of approximately 3 min superimposed on the basic strong wind, and the gusty wind possesses a coherent structure: the vertical velocity is upward when horizontal velocity is in the valley phase, but downward when horizontal velocity is in the peak phase. The coherence makes transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence, which occurs in weakly stable, neutral and unstable stratification. However, research on the physical mechanism of this coherent structure is still lacking. This paper shows that the gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaks induced by quasi‐streamwise vortexes. The coherent structure can be obtained from the three‐dimensional Navier‒Stokes (N‒S) equations. The maximum intensities of vortices and streaks are located at 20.7% and 12.1% of the boundary layer heights respectively, which are consistent with the observed results at 200 and 120 m. Plain Language Summary: After the passage of a cold front in spring in northern China, there is a rather regular gusty wind superimposed on the basic strong wind, and its horizontal and vertical component are negatively correlated and make downward transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence. This paper points out the possible physical phenomenon underlying this negative coherence, gives a dynamic explanation of this phenomenon by the hydrodynamic stability method, and compares the solutions with the observation results. Key Points: The gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaksThe physical model of the gusty coherent structure can be obtained by the hydrodynamic stability analysis methodThe position of maximum intensity of vortexes and streaks obtained from the solution is consistent with the observation [ABSTRACT FROM AUTHOR]

Emergent vegetation with two stream-parallel interfaces leads to the enhanced reduction in contaminants retention time due to the exchange of cross-array vortex communication in vegetative patches. To investigate this phenomenon, in this study, we numerically simulate 2-dimensional (D) flow through an array of circular cylinders consisting of a channel in between. We use flow diagnostics to understand the complex interaction of wakes for various moderate Reynolds number (Re). The vortex identification techniques reveal that the vortex pairing mechanism is responsible for energetic vortical structures at higher Re. We then incorporate the velocity field into a Lagrangian particle tracking simulation, which includes advection and molecular diffusion. We observe that the transverse spreading of the tracer plume is enhanced with increasing Re. The tracer breakthrough curves (BTCs) obtained for a two-sided parallel flow interface show a significant increase in flushing relative to a one-sided parallel interface array at the higher Re. This suggests that two flow-parallel interface leverages additional vortex strength in reducing the residence time within the array of a circular cylinder. Finally, we upscale BTCs using a continuous-time random walk model parameterized with a particle tracking and numerical simulation. [ABSTRACT FROM AUTHOR]

A comprehensive investigation is undertaken to discern the structure of momentum flux, turbulent kinetic energy, and scalar fluxes like heat, CO 2 , and H 2 O in the atmospheric surface layer (ASL) at the Thumba Equatorial Rocket Launching Station—a coastal station on the west coast of southern peninsular India. The vertical transport, transfer efficiency, and dissimilarity between flux transport are studied as a function of stability using data collected over 1 year. The transfer efficiency for heat fluxes and momentum exhibits a strong dependence on stability (ζ ). However, the transfer efficiency of passive scalars CO 2 and H 2 O displays no apparent dependence on ζ . The correlation between fluxes and squared coherence estimates is evaluated to study the dissimilarity between flux transport. The correlation is strongest among momentum and heat fluxes and between CO 2 and H 2 O fluxes and shows a dependence on the prevailing stability conditions. However, the influence of stability is not evident for the various other combinations. The momentum and heat flux transport is dissimilar for unstable conditions, and it becomes similar during the transition from unstable to near-neutral conditions. The quadrant analysis is employed to study the contribution of different fluid motions to the aforementioned turbulent fluxes. Except for CO 2 and H 2 O fluxes, where all the quadrants have an equal contribution, ejections and sweeps are the dominating contributors for momentum and heat fluxes. The stability conditions greatly determine the ejection-sweep balance for heat flux, while some changes in duration and impact fraction are also detectable for momentum flux. Furthermore, contour maps of joint-probability function (JPDF) of vertical velocity fluctuations ( w ′ ) with streamwise velocity fluctuation ( u ′ ), temperature fluctuation ( T ′ ), and scalar fluctuations, respectively, are also presented. The dominance of the ejection and sweep cycles for turbulent fluxes provide evidence for the presence and importance of coherent structures in ASL. [ABSTRACT FROM AUTHOR]

Copyright of Cuadernos de Investigación Geográfica is the property of Universidad de la Rioja, Servicio de Publicaciones and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to study the evolution principle of the coherent structure in the low flow rate runaway condition, the pump turbine of a certain pumped storage power plant was employed. The transient dynamic stress of the runner was numerically simulated and examined in this study in order to analyze the coherent structure of the vortex and the stability of the grid connection during the transition process. Based on the realizable k-ε turbulent model, the unsteady flow of the whole pump turbine channels was calculated. The results show that the flow in the runner channels presents with a turbulence state, and with many different scales vortices. These vortex structures are mainly distributed in the inlet region of the blade, the area of the blade trailing edge and the middle section of the runner channels. These vortex structures affect the distribution of the blade pressure load. Moreover, the vortex structure at the inlet of the runner depends on the change in the attack angle. In the flow region formed at the outlet of the blade near the suction surface and the runner cone, the blade has a limited effect on the fluid; thus, the vortex structure depends on the Coriolis force and the centrifugal force joint action. The evolution of these vortex structures will have a greater impact on the grid connection of the power station. During the operation of the power station, it is necessary to reduce the time of transitional conditions so that the power station can operate efficiently. [ABSTRACT FROM AUTHOR]

The tubeless vortex reducer was used to reduce the pressure drop in a cavity with radial inflow. In previous studies, the mechanism of vortex breakdown in tubeless vortex reducers was not clear, and there was a lack of direct guidance for engineering design. In this paper, we studied the vortex and pressure in the cavity in detail by large-eddy simulation (LES) and experiments. A mathematical model, which has high applicability and compatibility, was established to guide the pre-design of tubeless vortex reducers. The LES results showed that the high-speed and large-scale vortices broke rapidly at the nozzle inlet and reached the minimum size at the nozzle outlet. Furthermore, the enhancement of vortex in the downstream cavity was restrained. When the rotating speed was relatively low, strong negative vortices formed and the pressure drop increased in the downstream cavity. The experiments showed that the nozzle performance directly affected the flow characteristics and pressure drop in the cavity. Compared with the simple cavity, the tubeless vortex reducer generated lower pressure drop. The average relative error between the mathematical model and the experimental results was only 9.3%.

We continue our study of the transition of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence from non-equilibrium initial conditions to equilibrium using long-time numerical simulations on a 128 3 periodic grid. A Fourier spectral transform method is used to numerically integrate the dynamical equations forward in time. The six runs that previously went to near equilibrium are here extended into equilibrium. As before, we neglect dissipation as we are primarily concerned with behavior at the largest scale where this behavior has been shown to be essentially the same for ideal and real (forced and dissipative) MHD turbulence. These six runs have various combinations of imposed rotation and mean magnetic field and represent the five cases of ideal, homogeneous, incompressible, and MHD turbulence: Case I (Run 1), with no rotation or mean field; Case II (Runs 2a and 2b), where only rotation is imposed; Case III (Run 3), which has only a mean magnetic field; Case IV (Run 4), where rotation vector and mean magnetic field direction are aligned; and Case V (Run 5), which has non-aligned rotation vector and mean field directions. Statistical mechanics predicts that dynamic Fourier coefficients are zero-mean random variables, but largest-scale coherent magnetic structures emerge and manifest themselves as Fourier coefficients with very large, quasi-steady, mean values compared to their standard deviations, i.e., there is 'broken ergodicity.' These magnetic coherent structures appeared in all cases during transition to near equilibrium. Here, we report that, as the runs were continued, these coherent structures remained quasi-steady and energetic only in Cases I and II, while Case IV maintained its coherent structure but at comparatively low energy. The coherent structures that appeared in transition in Cases III and V were seen to collapse as their associated runs extended into equilibrium. The creation of largest-scale, coherent magnetic structure appears to be a dynamo process inherent in ideal MHD turbulence, particularly in Cases I and II, i.e., those cases most pertinent to planets and stars. Furthermore, the statistical theory of ideal MHD turbulence has proven to apply at the largest scale, even when dissipation and forcing are included. This, along with the discovery and explanation of dynamically broken ergodicity, is essentially a solution to the 'dynamo problem'. [ABSTRACT FROM AUTHOR]

The dissimilarity between the turbulent transport of momentum and heat under unstable conditions and its physical mechanisms are investigated in this study, based on the multiple‐level turbulence observation from the Tianjin 255‐m meteorological tower. The transport dissimilarity is observed from the surface layer to the lower part of mixed layer as atmospheric instability increases. Although the transport dissimilarity is accompanied by the development of plumes and thermals under unstable conditions, plumes and thermals can produce intense transport both of momentum and heat simultaneously. It is convective circulations induced by vigorous thermals that cause transport dissimilarity. The horizontal divergence generated by convective circulations imposes a dominant large‐scale reduction in the along‐wind velocity component near the surface, which is related to increased counter gradient transport of momentum, while the temporal variations in temperature mainly reflect the role of plumes and thermals and thus the transport of heat is predominantly down‐gradient. This difference in respective physical processes subsequently leads to dissimilar transport between momentum and heat under unstable conditions. Therefore, it is of great interest to represent the influence of convective circulations on the momentum‐flux estimation in future investigations, aiming to improve the boundary‐layer parameterization schemes for mesoscale numerical weather models. Plain Language Summary: The turbulent transport of momentum and scalars (e.g., heat, water vapor, carbon dioxide) between the land surface and the atmosphere is crucial for the prediction of weather and climate. It has been considered that turbulence transports momentum and scalars similarly in atmospheric numerical models. However, an increasing number of studies have shown that there is often the dissimilarity between the turbulent transport of momentum and scalars in the actual atmosphere. Specially, the transport dissimilarity between the momentum and heat is more significant with increasing atmospheric instability, and it coincides with the appearance of plumes and thermals (i.e., a kind of turbulent coherent structures). Until now, how the coherent structures physically cause the transport dissimilarity is still debated. Using the multiple‐level turbulence observation from the Tianjin 255‐m meteorological tower, it is clarified that it is not the presence of plumes and thermals that directly causes the transport dissimilarity, but other physical processes relevant to them, which is proved to be convective circulations in this study. The horizontal divergence generated by convective circulations affects the wind field in the surface layer, and then modifies the transport of momentum, leading to the dissimilar transport between momentum and heat under unstable conditions. Key Points: Transport dissimilarity between momentum and heat exists from the surface layer to the lower part of mixed layer under unstable conditionsPlumes and thermals induce intense transport both of momentum and heatTransport dissimilarity links to the horizontal divergence caused by convective circulations as the atmospheric instability increases [ABSTRACT FROM AUTHOR]

We review and extend the theory of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence. The theory contains a solution to the ‘dynamo problem’, i.e., the problem of determining how a planetary or stellar body produces a global dipole magnetic field. We extend the theory to the case of ideal MHD turbulence with a mean magnetic field that is aligned with a rotation axis. The existing theory is also extended by developing the thermodynamics of ideal MHD turbulence based on entropy. A mathematical model is created by Fourier transforming the MHD equations and dynamical variables, resulting in a dynamical system consisting of the independent Fourier coefficients of the velocity and magnetic fields. This dynamical system has a large but finite-dimensional phase space in which the phase flow is divergenceless in the ideal case. There may be several constants of the motion, in addition to energy, which depend on the presence, or lack thereof, of a mean magnetic field or system rotation or both imposed on the magnetofluid; this leads to five different cases of MHD turbulence that must be considered. The constants of the motion (ideal invariants)—the most important being energy and magnetic helicity—are used to construct canonical probability densities and partition functions that enable ensemble predictions to be made. These predictions are compared with time averages from numerical simulations to test whether or not the system is ergodic. In the cases most pertinent to planets and stars, nonergodicity is observed at the largest length-scales and occurs when the components of the dipole field become quasi-stationary and dipole energy is directly proportional to magnetic helicity. This nonergodicity is evident in the thermodynamics, while dipole alignment with a rotation axis may be seen as the result of dynamical symmetry breaking, i.e., ‘broken ergodicity’. The relevance of ideal theoretical results to real (forced, dissipative) MHD turbulence is shown through numerical simulation. Again, an important result is a statistical solution of the ‘dynamo problem’.

This study identifies the coherent structure and determines its energy contribution in the upstream near-wake behind a long circular cylinder using proper orthogonal decomposition of data that is measured using particle image velocimetry. Members of the harmonic-frequency family (if they exist), which are large-scale, quasiperiodic motions, are firstly identified in the Fourier power spectrum of each mode coefficient. The other large-scaled, organized frequencies of the coherent structure in each mode coefficient are next identified by means of a condition of their peak magnitudes greater than the smallest magnitude of the identified harmonic-frequency family. The kinetic energy that is contributed by the coherent structure is then calculated. Calculation of the kinetic energy of the coherent structure in the first 39 modes, which contribute 80.04% of the total kinetic energy for all eigenmodes, is performed. An estimated 29% of the total kinetic energy is contributed by the coherent structure.

We consider the nonlinear interaction system of waves to identify discrete clusters of resonant triads, which are classified on the basis of the resonance condition. This study is conducted to investigate the coherent structure of incompressible fluid flow in the turbulent boundary layer. The discrete wave turbulence is characterised by weakly nonlinear interaction modes for amplitude Tollmien Schlichting in a single-mode approximation. Within the framework of multiple-scale analysis, the coherent part of the amplitude equation is defined in the case of multiple three-wave resonance. The resonance condition is defined from the dispersion relation of these amplitudes, which are determined from solving the spectral problem of Orr–Sommerfeld equation by the Chebyshev collocation method. The spectral characteristics of these amplitudes are investigated to define the condition of multiple three-wave resonance. This condition is also defined for a resonant cluster made out of triads sharing a common mode, where all triads satisfy the resonance condition. The coherent amplitudes are represented dynamically by an autonomous system of ordinary differential equations. Non-integrable system of a single triad and a cluster of triads is noted, where the interaction coefficients in the given system do not have the same complex phase. The obtained dynamical system admits a number of invariants, similar to the classical Manley–Rowe invariants but of a different nature. One of these invariants is called the energy invariant manifold that represents the sum of modules square amplitudes of the dynamical system, this invariant is normalised to be defined on the unit sphere. Therefore, Birkhoff–Khinchin theory is applied to calculate the time average of square harmonic and sub harmonic amplitudes. Moreover, this paper is also focused on studying the numerical solutions of both simple and complex structure of the dynamical system by using Runge–Kutta method with random initial conditions. The solution of the dynamical system is examined at different signs of the weight factors, where the bounded solutions of this system are found at both positive and negative signs. However, in another study of a dynamical system, an explosive instability is noted at a negative sign for only one of the weight factors, where all study cases are related to the choice of wave vectors. The random initial conditions are applied to both simple and complex dynamical system to study the behaviour of system solutions. The coupling different triads within the dynamical system lead to chaotic turbulence regime. [ABSTRACT FROM AUTHOR]

To explore the statistical characteristics of the coherent structure in typhoon field and its evolution in different parts of typhoon spatial structure, the present study uses spectral analysis of wavelet coefficients to comprehensively analyze the statistical characteristics and the evolution of turbulent coherent structures in strong Typhoon Hagupit (0814). The results show that for Typhoon Hagupit (0814), the coherent structures of the fluctuating wind speed of downwind and cross wind present multi-scale distribution. The mean principal periods of the coherent structures in downwind, cross wind and vertical wind are 70.2 s, 50.3 s and 22.8 s, respectively, and their mean principal scales are 54.4 s, 38.9 s and 18.2 s, respectively. The mean period scale ratios of the coherent structures in the three wind directions are 1.28, 1.27 and 1.29, respectively. The mean principal scales of the coherent structures in downwind, cross wind and vertical wind at the front outer vortex are 68.8 s, 41.7 s and 14.4 s, respectively, and the corresponding values at the front eyewall are 69.9 s, 32.5 s and 19.5 s, respectively. The mean principal scales of the coherent structures in the three wind directions at the wind eye and the back eyewall are 83.5 s, 50.3 s, and 15.3 s, and 55.1 s, 42.3 s and 11.2 s, and the corresponding values at the back outer vortex are 53.6 s, 39.1 s, and 10.9 s, respectively. Overall, the principal scale of coherent structures in the wind eye is the largest. [ABSTRACT FROM AUTHOR]

To accurately calculate the turbulent exchange coefficient, the contribution of multi-scale turbulent transportation needs to be considered, especially in the complex terrain of the coastal area. In September 2019, a comprehensive observation experiment on the offshore atmospheric boundary layer was carried out at the Yangmeikeng Ecological Monitoring Station and Sai Chung Gulf. Through scale decomposition, it is shown that the turbulent motion in the atmospheric boundary layer in the coastal area is affected by the underlying surface, such as that of the coastal land or the sea-land boundary. This is the main reason behind the phenomenon whereby different scales make different contributions to momentum flux. Different multi-scale characteristics of turbulent structures on the underlying surface affect the drag coefficient. Through wavelet transform and finite element method, the characteristics of the multi-scale flow structures produced by the complicated offshore terrain are analysed. It is found that large-scale flow structures enhance the pulsating intensity at the small scale, but the large-scale coherence characteristics are different from those at the small scale. In summary, in comparing these three sites, the flux exchange on the roof is greatest, followed by that on the tower. In the Gulf, the flux exchange is mainly dependent on small-scale structures, which are linked with the smallest values. [ABSTRACT FROM AUTHOR]

• The turbulent flow was obtained with three kinds of flow mixing devices in lower plenum. • Turbulent coherent structures were extracted to study the turbulent mixing mechanism of flow mixing devices. • High-precision matched-index-of-refraction technique is employed to measure the turbulent flow in a scaled-down reactor pressure vessel. Turbulent flow mixing in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is important for design of flow mixing devices. In this study, turbulent flow in a scaled-down RPV model with three designs of mixing devices, including vortex suppression plate, flow skirt and semi-spherical perforated drum, was measured by time-resolved particle image velocimetry (TR-PIV) with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. Turbulent coherent structures were studied based on the experimental data. The time-averaged flow structures in lower plenum include flow separation at lower support plate, fluid bifurcation at the flow mixing devices, horizontal jets at holes of flow skirt, flow turning horizontally below the flow mixing devices and flow turning upward through the flow mixing devices. The strong turbulent mixing induced by flow mixing devices improves Reynolds stresses in lower plenum and introduces large spatial–temporal length scales in lower plenum. [ABSTRACT FROM AUTHOR]

By studying the flow-field structures, turbulent kinetic energy (TKE), flow angle distribution, and pump performance of centrifugal pump impellers under different flow conditions, the influence mechanisms of the blade leading-edge geometry (elliptical, arc-shaped and circular) on energy loss in centrifugal pump impellers were studied. At large flow rate condition, the negative velocity gradient at the inlet caused by the squeezing effect is the main factor causing energy loss. The blunter the blade inlet shape, the greater the squeezing effect, and the closer the negative velocity gradient region is to the inlet, resulting in greater energy loss. For the low flow rate conditions, the large-scale vortex structures at the channel outlet are the flow structures that contributed most to the energy loss. The effect of the blade leading-edge geometry on the fluid increases gradually as it moves from inlet to outlet, and the sharper the blade inlet shape, the slower the fluid deviates from the blade in its subsequent development, suppressing the early formation of large-scale vortex structures, making their strength more dispersed at the channel outlet, and closer to the suction side, a lower vortex intensity, resulting in smaller energy loss of the impeller. • The influence mechanism of the impeller blade leading-edge geometry on centrifugal pump performance was revealed. • The squeezing effect at the channel inlet at large flow rates was the main cause of low pump efficiency. • The energy loss at low flow rates is mostly driven by the vortex structure at the outlet of the flow channel. • The influence of blade leading edge geometry on flow field gradually increases as the fluid moves from inlet to outlet. • The stronger the outlet vortex structure, the more severe the "jet-wake" phenomenon. [ABSTRACT FROM AUTHOR]

The effects of the coolant pulsation and the plasma aerodynamic actuation (PAA) on the film cooling are herein explored via large eddy simulations. The electrohydrodynamic force derived from the PAA was solved through the phenomenological plasma model. The Strouhal number of the sinusoidal coolant pulsation and the averaged pulsation blowing ratio were 0.25 and 1.0, respectively. Comprehensive analyses were carried out on the time-averaged flow fields, and the results reveal that the pulsed cooling jet might cause a deeper penetration into the crossflow, and this phenomenon could be remarkably mitigated by the downward force of the PAA. Comparing steady film cooling to pulsed film cooling revealed a modest 15.1% reduction in efficiency, while the application of the dielectric barrier discharge plasma actuator (DBDPA) substantially enhanced the pulsed film cooling efficiency by 42.1%. Moreover, the counter-rotating vortex pair (CRVP) was enlarged and lifted off from the wall more poorly due to the coolant pulsation, and the PAA weakened the detrimental lift-off effect and entrainment of the CRVP. Then, the spatial–temporal development of the coherent structures was figured out by the alterations in the centerline temperature, reflecting the formation of the intermittent coherent structures rather than hairpin vortices due to the coolant pulsation, and their size and upcast behaviors were reduced by the PAA; thus, the turbulent integration of the coolant with the crossflow was suppressed fundamentally. Finally, the three-dimensional streamlines confirmed that the coherent structure dynamic behaviors were significantly regulated by the PAA for alleviating the adverse influences of the coolant pulsation. In summary, the PAA can effectively improve the pulsed film cooling efficiency by controlling the spatial–temporal development of the dominant coherent structures.

Abstract We propose criteria of tracking vortex surfaces in complex flows based on the vortex-surface field (VSF). The criteria characterize the accuracy and Lagrangian tracking performance of the numerical VSF solution, and determine the time period when the vortex surface tracking is satisfactory. Moreover, we develop a turbulent-like flow combining large-scale coherent structures in the Taylor–Green flow and small-scale turbulent structures in homogeneous isotropic turbulence (HIT). From tracking of vortex surfaces during the effective tracking period, we find that the imposed HIT disturbance significantly wrinkles vortex surfaces. Subsequently, the wrinkled vortex tube with large vorticity magnitude tends to be further twisted, contributing to energy cascade, while the wrinkling is mitigated in the region with small vorticity magnitude.

The biological surface structure comprising fish scales and a mucous membrane exhibits good turbulent drag reduction ability. Based on this structure, a bionic frictional drag reduction model composed of a grooved structure and mucous membrane was established herein, and its efficacy in reducing the resistance of a turbulent boundary layer was analyzed. Accordingly, the drag reduction performance of the bionic structure was investigated through large eddy simulations. The results revealed that the mucous membrane was evenly distributed on the groove wall through secretion, and effectively improved the drag reduction rate of the groove wall. The bionic grooves and mucous membrane structure successfully inhibited the turbulent kinetic energy, turbulence intensity, and Reynolds stress. The grooved structure improved the shape of the Λ vortex structure and the mucous membrane reduced the number of three-dimensional (3D) vortex structures. Furthermore, the streak structure near the bionic structure wall was reduced and its shape was regularized, which intuitively demonstrates the turbulence suppression ability of the proposed bionic structure. This paper presents the results of a hydrodynamic analysis of the frictional drag reduction characteristics of a bionic structure consisting of grooves and viscous membranes acting on the turbulent boundary layer of a wall.

Experiments and numerical simulations are performed to study the cold flow field characteristics in a two-stage axial swirl combustion chamber. Large eddy simulation with dynamic turbulent kinetic energy sub-grid scale model is used to calculate the flow field, and particle image velocimetry is used to measure the turbulent flow field. The calculated results are found to be in a good agreement with the experiment results. Due to the shear layers and the density difference between CO2 and air flows, the central recirculation zone appears. With the increase of the ratio of flow velocity between inner and outer tubes, the central recirculation zone shrinks gradually, while the length and range of the angular recirculation zone increase continuously. The vortex structure develops in the axial and radial directions, and the vortex breakdown mostly occurs in the upstream regions. However, outside the central recirculation zone, only one shear layer is observed, and its vortex structure is almost extended up to the exit of the combustion chamber. In addition, precession vortex core is not seen in all the conditions.

Flash floods cause damage to engineering structures and buildings mainly due to their huge impact force. Scientific investigation of the impact process of flash floods has significant theoretical and engineering implications in ensuring the safety of flood-resistant structures and human life. In this experimental study, the impact pressure and flow velocity in the flow field were measured synchronously using the impact detection system and a particle image velocimetry system in a water channel and the effects of the turbulence structure on the impact process of flash flood were investigated. As shown by the experimental measurements, the large-scale coherent structure in the flow field reached six times the boundary layer thickness. The turbulence and impact force energy spectra were very similar in the low-frequency, large-scale wave band, indicating that the large-scale turbulence structure dominated the impact process. Both the mean and maximum impact pressures increased with the turbulence kinetic energy. An equation for impact force characterization that considers the effects of turbulence and thus was more accurate was given, providing a theoretical basis for the protection of engineering structures from the damage by flash floods.

The tubeless vortex reducer was used to reduce the pressure drop in a cavity with radial inflow. In previous studies, the mechanism of vortex breakdown in tubeless vortex reducers was not clear, and there was a lack of direct guidance for engineering design. In this paper, we studied the vortex and pressure in the cavity in detail by large-eddy simulation (LES) and experiments. A mathematical model, which has high applicability and compatibility, was established to guide the pre-design of tubeless vortex reducers. The LES results showed that the high-speed and large-scale vortices broke rapidly at the nozzle inlet and reached the minimum size at the nozzle outlet. Furthermore, the enhancement of vortex in the downstream cavity was restrained. When the rotating speed was relatively low, strong negative vortices formed and the pressure drop increased in the downstream cavity. The experiments showed that the nozzle performance directly affected the flow characteristics and pressure drop in the cavity. Compared with the simple cavity, the tubeless vortex reducer generated lower pressure drop. The average relative error between the mathematical model and the experimental results was only 9.3%. [ABSTRACT FROM AUTHOR]

The clustering algorithm based on mutual K-nearest neighbors (MKNN) is presented to identify coherent structures in complicated fluid flows, in order to analyze the mass mixing and transport. First, both trajectory similarity and spatial proximity are used to describe and measure the coherence between particles. These two identification criteria are frame-invariant since they are derived from the relative distances of particles. Then, the concept of mutual K-nearest neighbors is introduced further, and particles with the same cluster label are identified as coherent structures after the initialization and merging process of clusters, while incoherent regions consist of incoherent particles, which cannot form a mutual K-nearest neighbors relationship with other particles. Finally, the MKNN-based clustering algorithm is applied to three examples, realizing the identification and tracking of coherent structures. The identification results show that the MKNN-based clustering algorithm is robust to parameter K, and a higher threshold λ of cluster quantity will be helpful to identify the finer structures in flows. Moreover, spatial proximity performs better in vortex identification, and trajectory similarity is more suitable for elongated structures (jets) identification. Importantly, the method presented analyzes the evolutions of vortices in detail, including the generation, stretching, and merging processes. In summary, the MKNN-based clustering algorithm takes particle trajectories as input data, analyzes the evolution of relative distances between particles quantitatively, and carries out clustering analysis on particles according to trajectory similarity and spatial proximity. The combination of the MKNN-based clustering algorithm and frame-invariant identification criteria shows great potential in coherent structure identification of complicated fluid flows. [ABSTRACT FROM AUTHOR]

In order to explore the formation mechanism of the nonstationary wind speed in the eye wall regions and eye region of a typhoon, taking the measured data of No. 0814 strong typhoon Hagupit as the research object, we identify and extract the coherent structure of the measured wind field by discrete wavelet transform combined with hypothesis testing, and analyze the influence of the coherent structure on the nonstationary and non-Gaussian wind speed and study the influence of the coherent structure on the wind turbulence characteristics. The results show that for the 0814 typhoon Hagupit process, the coherent structure is the main reason that leads to the nonstationary and non-Gaussian fluctuating winds in the typhoon field, and with the coherent structures being exacted from the original winds, the nonstationary and non-Gaussian characteristics of the nonstationary wind samples are significantly weakened. The average contribution of the coherent structure to the turbulence parameters ranges from 6. 42% to 32. 40%. The duration of the coherent structure is about 10. 60% of the sample duration, however, its contribution to turbulent kinetic energy is up to 40. 50%. The coherent structure has the characteristics of short duration and high energy content, which is the cause of the abnormally large turbulence parameter value. This study can provide a reference for the refined wind resistance design of structures under the action of nonstationary wind. [ABSTRACT FROM AUTHOR]

The influence of drag-reducing superhydrophobic (SHPo) surface on turbulent boundary layer (TBL) is investigated. A large area of the SHPo surface (about 10δ99 in the streamwise and 5δ99 in the spanwise) is fabricated to fully evolve the coherent structures in the TBL. A comparative experiment is carried out by time-resolved particle image velocimetry on a smooth surface and the SHPo surface at Reτ = 528. Velocity profiles with high spatial resolution are obtained by the single-pixel resolution ensemble correlation method. The reduction of the streamwise velocity gradient is observed in the near-wall region of y < 0.05δ99 on the SHPo surface. By comparing the turbulence statistics, it is discovered that the Reynolds shear stress is reduced by 15.7%, and the turbulent kinetic energy is reduced by 12.3% on the SHPo surface. The coherent structures are investigated by the snapshot proper orthogonal decomposition (POD) and conditional average method. The intensity of Q2/Q4 events on the SHPo surface has declined by 16.1% and 12.7%, respectively. The number of clockwise spanwise vortices is substantially reduced by 47%. Through spatial two-point correlation analysis, the streamwise and wall-normal direction scales of the coherent structures on the SHPo surface are suppressed. [ABSTRACT FROM AUTHOR]

The fluid stress or flow-induced vibration of annular gap flow always has some influence on the stable working conditions of a hydraulic machine. A time-averaged analysis of flow may not have to explicitly acknowledge these factors. Accordingly, a finite-axial-length annular gap was measured via particle image velocimetry (PIV), with inner boundary motion and a stable outer boundary. As a statistic result regarding the fluid stress, the Reynolds stresses soared in the first region, were sustained in the middle region, but decreased at last. The flow had a higher convective transportation intensity in the radial direction than in other directions. Flow diagnostics were also performed by proper orthogonal decomposition (POD). As a result, the coherent structures were found. Then, the power spectrum density (PSD) functions were also calculated for finding the flow-induced vibration characteristics; the functions had high amplitude in the low-frequency domain and low amplitude in the high-frequency domain, with an order of magnitude between the two amplitudes of 10−1 to 10−2. In addition, the frequency was higher at a smaller gap width in the middle-frequency domain, but the condition was the opposite in the high-frequency domain. In conclusion, the fluid stresses were changeable and uneven along the flow direction, and flow-induced vibration obviously existed. Remarkably, the turbulence characteristics of the annular gap flow were not "laminar approximating," while the diameter ratio of the gap was 0.6 to 0.8. [ABSTRACT FROM AUTHOR]

We continue our study of the transition of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence from non-equilibrium initial conditions to equilibrium using long-time numerical simulations on a 1283 periodic grid. A Fourier spectral transform method is used to numerically integrate the dynamical equations forward in time. The six runs that previously went to near equilibrium are here extended into equilibrium. As before, we neglect dissipation as we are primarily concerned with behavior at the largest scale where this behavior has been shown to be essentially the same for ideal and real (forced and dissipative) MHD turbulence. These six runs have various combinations of imposed rotation and mean magnetic field and represent the five cases of ideal, homogeneous, incompressible, and MHD turbulence: Case I (Run 1), with no rotation or mean field; Case II (Runs 2a and 2b), where only rotation is imposed; Case III (Run 3), which has only a mean magnetic field; Case IV (Run 4), where rotation vector and mean magnetic field direction are aligned; and Case V (Run 5), which has non-aligned rotation vector and mean field directions. Statistical mechanics predicts that dynamic Fourier coefficients are zero-mean random variables, but largest-scale coherent magnetic structures emerge and manifest themselves as Fourier coefficients with very large, quasi-steady, mean values compared to their standard deviations, i.e., there is ‘broken ergodicity.’ These magnetic coherent structures appeared in all cases during transition to near equilibrium. Here, we report that, as the runs were continued, these coherent structures remained quasi-steady and energetic only in Cases I and II, while Case IV maintained its coherent structure but at comparatively low energy. The coherent structures that appeared in transition in Cases III and V were seen to collapse as their associated runs extended into equilibrium. The creation of largest-scale, coherent magnetic structure appears to be a dynamo process inherent in ideal MHD turbulence, particularly in Cases I and II, i.e., those cases most pertinent to planets and stars. Furthermore, the statistical theory of ideal MHD turbulence has proven to apply at the largest scale, even when dissipation and forcing are included. This, along with the discovery and explanation of dynamically broken ergodicity, is essentially a solution to the ‘dynamo problem’.

The coherent structure and energy distribution of turbulence are major concerns in the research of turbulence dynamics. The present work conducts field measurements of atmospheric surface layer (ASL) turbulent motions on the windward slope centerline of an individual barchan dune located in Minqin, China. On the basis of comparing basic turbulence statistics (including the mean wind velocity, streamline angle, turbulence intensity and Reynolds stress) on the dune windward slope with existing results to confirm the reliability of our experimental methods and observation data, the present work analyzes the variation patterns of turbulent coherent structures and energy distributions along the windward slope. The results reveal that low-frequency (large-scale) turbulent motions increasingly dominate the airflow as turbulence evolves along the windward slope toward the slope crest, as reflected in the gradual increase in structure scales (the coherence scale and the largest energy-containing structure scale) along the slope and the more significant energy contribution of low-frequency coherent structures along the slope. The variation in turbulent kinetic energy distribution between frequencies (scales) makes the self-organizing state of turbulence change significantly along the dune windward slope; although the second-order streamwise structure functions still satisfy the Kolmogorov's two-thirds law in the inertial subregion of turbulence, the reduced energy-containing region causes the natural logarithmic scaling law of second-order structure functions to gradually to become insignificant along the windward slope, implying that the effects of favorable pressure gradient and airflow acceleration diminish the suitability of the attached eddy hypothesis as the streamwise position approaches the slope crest. The present work is the first to analyze the coherent structures and energy distribution characteristics of ASL turbulence on the windward slope of a natural barchan dune, which may provide assistance in gaining comprehensive insights into turbulent motions over dune terrains. • The scale of turbulent coherent structures increases from the dune toe to the crest. • The energy fraction of low-frequency structures increases toward the slope crest. • The log-linear law of 2nd-order structure functions gets unclear nearing the crest. [ABSTRACT FROM AUTHOR]

The review is devoted to the intermittency of atmospheric turbulence of different types (Kolmogorov and coherent) in the mountain boundary layer. The world scientific literature on the intermittency of turbulence is briefly reviewed for a better understanding of the place of each intermittency type. In view of the different interpretations of the coherent turbulence concept available in the literature, here, we supplement our earlier reviews of works on coherent turbulence and coherent structures, where the ways of generation of a coherent turbulence and its key properties were summarized and the differences and relationships between Kolmogorov and coherent turbulence were considered. The current concepts of the turbulence structure are discussed. Thus, we previously (2008–2019) showed that atmospheric turbulence could be considered an incoherent mixture of several coherent structures. A conjecture was suggested by Hopf in 1948 about the finite dimensionality of attractors in the phase space of solutions of the Navier–Stokes equations. In 1991 and 1992, a physical interpretation was suggested by Monin and Yaglom, where a turbulence structure was represented as a spatiotemporal chaos of a finite number of interacting coherent structures. The comparison between these representations has shown that our results actually prove the Hopf hypothesis in the interpretation by Monin and Yaglom, and that the turbulence "chaos" is largely deterministic. For the review, we use data on the intermittency of different turbulence types which are the result of long-term experimental studies of turbulence by acoustic and optical methods in high-mountain astronomical observatories. The lifetimes of Kolmogorov and coherent turbulence are derived from optical and meteorological measurements. The intermittency of turbulence is shown to characterize the local structure of turbulence above an observatory site, which has allowed us to formulate practical recommendations about the best observation conditions in astronomical observatories. [ABSTRACT FROM AUTHOR]

A series of high resolution simulations on film cooling with varying blowing ratios and inclination angles are carried out by using in-house code based on hybrid thermal lattice Boltzmann method. Three blowing ratios ranging from 0.2-0.8 and four inclination angles from 15° to 60° are chosen for the simulations. The evolutionary mechanism of coherent structure in three domains of film-covering region is studied from the view of space and time. Besides, the influencing mechanism of blowing ratio and inclination angle on flow and heat transfer features of film cooling is uncovered. Results show that hairpin vortex, hairpin packet, and quasi-streamwise vortex appearing in rotating domain play a key role in heat transfer process of film cooling. The strong ejection, sweep and vortex rotation resulted from these vortices enhance the convective heat transfer. It is also found that the size of coherent structure varies significantly with blowing ratio and its integral form shows a strong dependence on inclination angle. Moreover, inclination angle of coolant jet has a significant impact on turbulence fluctuation intensity. The influence of blowing ratio on the attachment of coolant film and film-cooling performance is more obvious than that of inclination angle. It is believed that all of these are related closely to the variation of streamwise and wall-normal jet velocity in the case of various blowing ratios and inclination angles. [ABSTRACT FROM AUTHOR]

In order to optimize the use of compact porous media as flow and heat transfer devices, it is imperative to understand those coherent structures of the associated flow that generate and sustain turbulence. Given the deficiency of data regarding this area in the literature, this study has been carried out to fill this need. To this end, a series of particle image velocimetry measurements were conducted to capture a turbulent flow field bounded by a model permeable medium of 85% porosity. The bulk Reynolds numbers based on the bulk velocity through the entire flow domain and the depth of flow over the permeable boundary are approximately 5.0 × 103 and 2.0 × 104. By applying velocity gradient eigenanalysis, quadrant decomposition, multi-point correlations, and proper orthogonal decomposition, requisite information about the coherent structures of the flow field is extracted. The results indicate the existence of spatial structures whose order, size, and orientation are dependent on the Reynolds number and location along the permeable boundary. While the largest scales are marked by sweeps, ejections, and high vortex activity, there is evidence of inward and outward interactive events at the upstream portions of the permeable boundary layer flow. This work helps to clarify some observations made on turbulent flow over the compact permeable boundary. [ABSTRACT FROM AUTHOR]