Turbulent Transport by Coherent Structures in the Brazilian Pampa Region

Intermittent bursts of plasma density measured by Langmuir probes at the edge of TEXTOR tokamak are studied. These bursts appear as a result of turbulent plasma transport processes involving the formation and propagation of various coherent turbulent structures. Such processes impede controlled thermonuclear fusion: they degrade plasma confinement and entail increased heat load on the vacuum chamber walls and other components located near plasma; they also entail strong erosion of these components along with unwanted capture of tritium. Therefore, investigation of turbulent plasma transport processes and dynamics of coherent turbulent structures is one of the most important tasks to be solved for implementing controlled thermonuclear fusion. This is especially important in the context of elaborating and improving the methods for externally controlling the turbulent plasma transport processes. The electrode biasing method and a dynamic ergodic divertor are frequently used for externally influencing thermonuclear plasma and controlling turbulent transport processes. It should be noted that by studying the temporal characteristics of plasma density bursts together with their radial dependence it becomes possible to get better understanding of and deeper insight into the physical nature of turbulent plasma transport processes and dynamics of coherent turbulent structures. In this article, the temporal characteristics of plasma density bursts and their radial dependence are studied in two different modes: with electrode biasing and with a dynamic ergodic divertor. Conformable changes in the characteristics of intermittent bursts are observed in both cases. Namely, the average burst rate increases, and the average burst duration decreases in comparison with the ohmic regime. This is due to the fact that electrode biasing and certain regimes of the dynamic ergodic divertor cause changes in the radial electric field. This has a conformable effect on the dynamics of coherent turbulent structures and plasma transport processes through a shear poloidal flow, which emerges as a consequence of electric drift due to nonuniform radial electric field and toroidal magnetic field, which are perpendicular to each other. After detailed investigations and refinement, it should become possible to use certain regimes of the dynamic ergodic divertor as a means of contactless biasing for externally controlling the turbulent plasma transport in thermonuclear installations.

This numerical study presents a simple but extremely effective way to considerably enhance heat transport in turbulent wall-bounded multiphase flows, namely by using oleophilic walls. As a model system, we pick the Rayleigh–Bénard set-up, filled with an oil–water mixture. For oleophilic walls, using only $10\,\%$ volume fraction of oil in water, we observe a remarkable heat transport enhancement of more than $100\,\%$ as compared to the pure water case. In contrast, for oleophobic walls, the enhancement is only of about $20\,\%$ as compared to pure water. The physical explanation of the heat transport increment for oleophilic walls is that thermal plumes detach from the oil-rich boundary layer and carry the heat with them. In the bulk, the oil–water interface prevents the plumes from mixing with the turbulent water bulk and to diffuse their heat. To confirm this physical picture, we show that the minimum amount of oil necessary to achieve the maximum heat transport is set by the volume fraction of the thermal plumes. Our findings provide guidelines of how to optimize heat transport in wall-bounded thermal turbulence. Moreover, the physical insight of how coherent structures are coupled with one of the phases of a two-phase system has very general applicability for controlling transport properties in other turbulent wall-bounded multiphase flows.

On the turbulent energy transport related to the coherent structures in a planar jet

A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

To date, a growing body of literature has documented the existence and impacts of coherent structures known as large- and very-large-scale motions within wall-bounded turbulent flows under neutral and unstable thermal stratification. These coherent structures can account for a considerable fraction of the overall turbulent transport and have been found to modulate small-scale turbulent fluctuations near the wall. In the context of stably stratified flows, however, the examination of such coherent structures has garnered relatively little attention. Stable stratification limits vertical transport and turbulent mixing within flows, which makes it unclear the extent to which previous findings on coherent structures under unstable and neutral stratification are applicable to stably stratified flows. In this study, we investigate the existence and characteristics of coherent structures under stable stratification with a wide range of statistical and spectral analyses. Outer peaks in premultiplied spectrograms under weak stability indicate the presence of large-scale motions, but these peaks become weaker and eventually vanish with increasing stability. Quadrant analysis of turbulent transport efficiencies (the ratio of net fluxes to their respective downgradient components) demonstrates dependencies on both stability and height above ground, which is evidence of morphological differences in the coherent structures under increasing stability. Amplitude modulation by large-scale streamwise velocity was found to decrease with increasing gradient Richardson number, whereas modulation by large-scale vertical velocity was approximately zero across all stability ranges. For sufficiently stable stratification, large eddies are suppressed enough to limit any inner–outer scale interactions.

The empirical mode decomposition (EMD) is used to study the scale properties of turbulent transport and coherent structures based on velocity and temperature time series in stably stratified turbulence. The analysis is focused on the scale properties of intermittency and coherent structures in different modes and the contributions of energy-contained coherent structures to turbulent scalar counter-gradient transport (CGT). It is inferred that the velocity intermittency is scattered to more modes with the development of the stratified flow, and the intermittency is enhanced by the vertical stratification, especially in small scales. The anisotropy of the field is presented due to different time scales of coherent structures of streamwise and vertical velocities. There is global counter-gradient heat transport close to the turbulence-generated grid, and there is local counter-gradient heat transport at certain modes in different positions. Coherent structures play a principal role in the turbulent vertical transport of temperature.

We investigated an alternative means for quantifying daytime ecosystem respiration from eddy-covariance data in three forests with different canopy architecture. Our hypothesis was that the turbulent transport by coherent structures is the main pathway for carrying detectable sub-canopy respiration signals through the canopy. The study extends previously published work by incorporating state-of-the-art wavelet decomposition techniques for the detection of coherent structures. Further, we investigated spatial and temporal variability of the respiration signal and coherent exchange at multiple heights, for three mature forest sites with varying canopy and terrain properties for one summer month. A connection between the coherent structures and identified sub-canopy respiration signal was clearly determined. Although not always visible in signals collected above the canopy, certain cases showed a clear link between conditionally sampled respiration events and coherent structures. The dominant time scales of the coherent structure ejection phase (20–30 s), relative timing of maximum coincidence between respiration events and the coherent structure ejection phase (at approximately −10 s from detection) and vertical transport upward through the canopy were shown to be consistent in time, across measurement heights and across the different forest sites. Best results were observed for an open canopy pine site. We conclude that the presented method is likely to be applicable at more open rather than dense (closed) canopies. The results provided a confirmation of the connection between below- and above-canopy scalar time series, and may help the development or refinement of direct methods for the determination of component fluxes from observations above the canopy.

The exchange of momentum, heat and trace gases between atmosphere and surface is mainly controlled by turbulent fluxes. Turbulent mixing is usually parametrized using Monin–Obukhov similarity theory (MOST), which was derived for steady turbulence over homogeneous and flat surfaces, but is nevertheless routinely applied to unsteady turbulence over non-homogeneous surfaces. We study four years of eddy-covariance measurements at a highly heterogeneous alpine valley site in Finse, Norway, to gain insights into the validity of MOST, the turbulent transport mechanisms and the contributing coherent structures. The site exhibits a bimodal topography-following flux footprint, with the two dominant wind sectors characterized by organized and strongly negative momentum flux, but different anisotropy and contributions of submeso-scale motions, leading to a failure of eddy-diffusivity closures and different transfer efficiencies for different scalars. The quadrant analysis of the momentum flux reveals that under stable conditions sweeps transport more momentum than the more frequently occurring ejections, while the opposite is observed under unstable stratification. From quadrant analysis, we derive the ratio of the amount of disorganized to organized structures, that we refer to as organization ratio (OR). We find an invertible relation between transfer efficiency and corresponding organization ratio with an algebraic sigmoid function. The organization ratio further explains the scatter around scaling functions used in MOST and thus indicates that coherent structures modify MOST. Our results highlight the critical role of coherent structures in turbulent transport in heterogeneous tundra environments and may help to find new parametrizations for numerical weather prediction or climate models.

Pressure transport in direct numerical simulations of turbulent natural convection in horizontal fluid layers

Abstract. We investigate the time intermittency of turbulent transport associated with the birth-death of self-organized coherent structures in the atmospheric boundary layer. We apply a threshold analysis on the increments of turbulent fluctuations to extract sequences of rapid acceleration events, which is a marker of the transition between self-organized structures. The inter-event time distributions show a power-law decay ψ(τ) ~ 1/τμ, with a strong dependence of the power-law index μ on the threshold. A recently developed method based on the application of event-driven walking rules to generate different diffusion processes is applied to the experimental event sequences. At variance with the power-law index μ estimated from the inter-event time distributions, the diffusion scaling H, defined by ⟨ X2⟩ ~ t2H, is independent from the threshold. From the analysis of the diffusion scaling it can also be inferred the presence of different kind of events, i.e. genuinely transition events and spurious events, which all contribute to the diffusion process but over different time scales. The great advantage of event-driven diffusion lies in the ability of separating different regimes of the scaling H. In fact, the greatest H, corresponding to the most anomalous diffusion process, emerges in the long time range, whereas the smallest H can be seen in the short time range if the time resolution of the data is sufficiently accurate. The estimated diffusion scaling is also robust under the change of the definition of turbulent fluctuations and, under the assumption of statistically independent events, it corresponds to a self-similar point process with a well-defined power-law index μD ~ 2.1, where D denotes that μD is derived from the diffusion scaling. We argue that this renewal point process can be associated to birth and death of coherent structures and to turbulent transport near the ground, where the contribution of turbulent coherent structures becomes dominant.

Coherent structures detection within a dense Alpine forest

Turbulent heat flux and Reynolds stress fluctuations in fully developed pipe flow have been measured by employing the combined cold wire and V-shaped hot wire anemometry technique, and then conditionally sampled and averaged. Instantaneous turbulent heat transfer is strongly associated with and dominated by the coherent turbulent structures, and thus the inherent intermittency of turbulent heat transport process is the consequence of the intermittent coherent motion near the wall. Conditionally averaged patterns of velocity, temperature and relevant turbulent heat flux fluctuations are obtained by analyzing the amplitude, mean period and duration of coherent motions. These recognized patterns reveal how the turbulent heat transfer is affected by four distinct types of coherent motions originating in the wall region and extending through a greater of the pipe section.

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.

Terrestrial ecosystems are characterized by a wide range of canopy vegetation density, which is known to affect turbulent transport processes across the canopy–atmosphere interface. In the presence of a dense and horizontally homogeneous canopy, the canopy sublayer has been described as resembling a plane mixing layer. At the other extreme, where the canopy is essentially absent, the canopy sublayer is typically assumed to be similar to a turbulent boundary layer over a rough surface. However, it remains unclear how the canopy turbulence changes from boundary-layer-like to mixing-layer-like as the vegetation density increases. We use large-eddy simulation to study five different vegetation densities varying from an extremely sparse canopy to an extremely dense canopy. This investigation draws on the study of flow statistics as well as large-scale coherent turbulent structures within the canopy sublayer. The coherent structures are identified through the use of proper orthogonal decomposition. The results of skewness of velocity components and characteristic length scales suggest that, as the vegetation density increases, the canopy turbulence gradually undergoes a transition from resembling a rough-wall boundary layer to being similar to a mixing layer. As demonstrated by others, we found that the coherent structures within the canopy sublayer consist of a strong sweep/ejection motion framed by a counter-rotating vortex pair with elliptical cross-sections. As the canopy becomes denser, these important structures are shown here to be more elevated. Vegetation density does not appear to have a significant effect on the percentage of the total turbulent kinetic energy that is represented by the coherent structures.

<p>Last year the scientific community lost a great scientist; leader in environmental turbulence and planetary boundary layer research; recipient of the 2019 International Meteorological Organization (IMO) Prize and many other science awards; leader of numerous international research projects; outstanding mentor, and dear friend, Professor Sergej Zilitinkevich.</p><p>Among his numerous outstanding scientific achievements in the boundary layer theory, several theoretical results broadly used in the numerical weather prediction, climate, and air pollution modelling communities, in particular, should be mentioned:</p><ul><li>The Zilitinkevich formula for the depth of stably stratified PBLs is often called that depth scale by Sergej’s name, which indicates that his result is a truly classical one.</li> <li>The Zilitinkevich correction to the rate equation for the depth of a convectively mixed layer, and the resistance and the heat and mass transfer laws for geophysical turbulent flows are also widely known and used.</li> <li>The Zilitinkevich scale - a length scale of a rotational stratification turbulent mixing in stably stratified PBLs.</li> <li>Conceptual models of new types of atmospheric PBLs, i.e.,: <em>conventionally neutral</em> PBLs settled on the background of the strongly stable stratification typical of the free atmosphere are several times thinner than truly neutral PBLs settled in neutral stratification; and <em>long-lived stable</em> PBLs typical in winter time at high latitudes and affected by the stably stratified free atmosphere.</li> <li>Discovered and described by Zilitinkevich: the “weak turbulence regime,” typical of the free atmosphere, which determines the turbulent transport of energy and momentum and the diffusion of passive scalars.</li> <li>Non-local turbulent transport for BLM and the pollution dispersion aspects of the coherent structure of convective flows.</li> </ul><p>These results have paved the way towards improved theories and parametrizations of boundary layers in many NWP, climate, and ACT models worldwide.<br>Over the last few decades, Sergej Zilitinkevich was deeply concerned with general questions of the physical nature of geophysical (and astrophysical) turbulence. The classical view, pioneered by Kolmogorov, assumes a cascade process from large eddies towards small eddies and eventually to heat. This “chaos out of order” paradigm put forward for shear-generated non-stratified turbulence is shifted towards an “order out of chaos” paradigm more appropriate for real-world turbulence complicated by body forces, where small-scale motions can organize themselves and give rise to quasi-organized coherent structures at larger scales. Sergej made a remarkable contribution to this paradigm shift. He passionately addressed several fundamental issues, such as the: origin and transport properties of coherent motions, effect of buoyancy on turbulent transport, and maintenance of turbulence at strongly stable stratification. This promising and long-awaited scientific revolution in this area of research will allow for a better understanding of the nature of global pollution and climate change.<br>In this presentation we analyze the scientific legacy of Sergej Zilitinkevich for further developments in boundary layer research and modelling.</p>