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Wind energy in the atmospheric boundary layer serves as the primary source for energy absorption and structural load on wind turbines. However, the impact of turbulent coherent structures on the aerodynamic performance and wake characteristics of wind turbines has not been comprehensively evaluated. In this study, the proper orthogonal decomposition (POD) method is employed to assess the influence of turbulent coherent structures of varying scales on the aerodynamic performance and wake characteristics of wind turbines in the neutral atmospheric boundary layer. The results show that turbulent coherent structures are the main factor that determines the wind velocity fluctuation, aerodynamic performance and wake characteristics of wind turbine in the atmospheric boundary layer. When considering the 13th or lower order POD mode, the wind velocity fluctuation increases with the increase of energy content (more POD modes) of the turbulent coherent structures. When considering the first 19 POD modes, the dynamic loads and power of wind turbine fluctuate with high frequencies, the thrust fluctuates in an amplitude range between 2.4 % and 13.9 % around the mean value, and the power fluctuates from 4.5 % to 28.6 % of the mean value. When considering the first 40 POD modes, the average power generation of the wind turbine increases by 26 % compared to the case with no turbulent structures considered. The study of turbine wake shows that turbulent coherent structures can expand the wind turbine wake approximately to a width of 2.5 D and a height of 3 D (D is the diameter of the wind turbine), offset the wake approximately to 2 D , and move forward the position of the wake vortex beginning to dissipation approximately to 7 D behind the wind turbine. In addition, turbulent coherent structures can accelerate the wake velocity recovery by increasing the momentum exchange between the atmospheric boundary layer and wind turbine wake. • A POD-based inflow generation method is established for wind turbine simulation. • Study into the coherent structures with various energy contributions on wind turbine performance. • Obtained the most influence energy contribution of coherent structures in the ABL. • Unveil the influence of coherent structures on the expansion and deflection of wind turbine wakes. [ABSTRACT FROM AUTHOR]

A three‐state global estimator of marine surface layer atmospheric stratification is demonstrated using more than 600,000 Sentinel‐1 synthetic aperture radar wave mode images at incidence angle ≈36.8°. Stratification is quantified using a bulk Richardson number, Ri, derived from collocated ERA5 surface analyses. The three stratification states are defined as unstable: Ri < −0.012, near‐neutral: −0.012 < Ri < +0.001, and stable: Ri > +0.001. These boundaries are identified by the characteristic boundary layer coherent structures that form in these regimes and modulate the surface roughness imaged by the radar. An automated machine learning algorithm identifies the coherent structures impressed on the images. Data from 2016 to 2019 are used to examine spatial and temporal variation in these state estimates in terms of expected wind and thermal forcing. This new satellite‐based approach for detecting air‐sea stratification has implications for weather modeling and air‐sea flux products. Plain Language Summary: The air‐sea fluxes of momentum, heat, and water vapor are crucial climate data records because they represent lower boundary conditions on the atmospheric circulation and upper boundary conditions of ocean waves and currents. Global measurements of these fluxes using conventional fixed‐station systems are impractical making satellite observations attractive. It is very difficult to measure two crucial parameters: the temperature and humidity of the air near the sea surface. The study's motivation is that external information about the air‐sea stratification should improve the retrieval of these parameters. We demonstrate a global capability to classify air‐sea stratification based on variations in the sea surface texture of high‐resolution radar images. This is possible because the turbulent atmospheric boundary layer develops distinct circulations in different stratification regimes. Perturbation surface winds induced by these coherent structures induce a modulation of the ocean surface roughness that is resolved by orbiting radars. An automatic machine learning algorithm detects the characteristic structures, which we correlated to a standard global data assimilation surface analysis. An important research outcome is the largest all‐weather data set of boundary layer coherent structures and the conditions under which they exist. Examination of these data will advance our understanding of fundamental boundary layer processes. Key Points: Common atmospheric boundary layer coherent structures are classified using high‐resolution global satellite sea surface radar roughness dataCoherent structures partition into separate unstable, near‐neutral, and stable stratification regimesThree‐regime coherent structure and stratification classification can aid air‐sea flux estimation and atmosphere boundary layer parameterization [ABSTRACT FROM AUTHOR]

Long waves caused by storm surges or river floods can significantly impact marine and fluvial structures such as bridge piers. Apart from the forces that they generate on the structure, they also contribute to the formation of turbulent eddies downstream of the obstacle. This is relevant, as in this way they can affect both an erodible bottom and the ecosystem. The present study describes a medium-scale experiment, in which the propagation of two different long waves released on a steady current is investigated in the presence of a bottom-mounted rigid emergent cylinder. Velocity measurements were acquired by a Particle Image Velocimetry (PIV) system, providing instantaneous flow velocity vectors on selected 2D planes. For each experimental condition, the time-varying velocity field near the cylinder was examined in selected vertical and horizontal planes. First, we tested which analytical theory or approximated method can best represent the experimental waves. After this, we estimated the horizontal maps of velocity and vorticity downstream of the obstacle and finally processed the velocity signals by means of a wavelet-based technique, to derive the length scales of turbulent eddies. In such a way, we specifically derived how the spreading of coherent turbulent structures downstream of the cylinder depends on the features of the flume, cylinder, and wave. [ABSTRACT FROM AUTHOR]

Atmospheric turbulent velocity fluctuations are known to increase wind turbine structural loading and accelerate wake recovery, but the impact of vortical coherent structures in the atmosphere on wind turbines has not yet been evaluated. The current study uses flow imaging with natural snowfall with a field of view spanning the inflow and near wake. Vortical coherent structures with diameters of the order of 1m are identified and characterized in the flow approaching a 2.5 MW wind turbine in the region spanning the bottom blade tip elevation to hub height. Their impact on turbine structural loading, power generation and wake behaviour are evaluated. Long coherent structure packets (>~ 200 m) are shown to increase fluctuating stresses on the turbine support tower. Large inflow vortices interact with the turbine blades, leading to deviations from the expected power generation. The sign of these deviations is related to the rotation direction of the vortices, with rotation in the same direction as the circulation on the blades leading to periods of power surplus, and the opposite rotation causing power deficit. Periods of power deficit coincide with wake contraction events. These findings highlight the importance of considering coherent structure properties when making turbine design and siting decisions. [ABSTRACT FROM AUTHOR]

• Short roughness enhances the large- and very-large-scale motions in the outer layer. • Propagation of roughness-introduced disturbance forms a bottom-up scenario. • The modulation effect of roughness persists longer than traditionally believed. • Near-wall Reynolds shear stress is suppressed in the far-wake regime of roughness. In this study, the effect of cylindrical discrete roughness elements on multiscale turbulent motions in a canonical turbulent boundary layer was explored. Flow fields with a considerably large field of view in streamwise—wall-normal planes—were measured by two-dimensional particle image velocimetry. It was found that large-scale motions and very-large-scale motions in the outer layer of the middle-to-far-wake regime are enhanced due to the bottom-up process of roughness-introduced disturbance, where vortical structures shedding from the roughness elements lift up to higher flow layers as they convect downstream, thus promoting the cross-layer interaction. The modulation effect was found to extend to the lower bound of the outer region and persist longer than traditionally believed. An interesting finding is that in the near-wall region of the far-wake regime, small-to-moderate-scale components of Reynolds shear stress is suppressed, which may be associated with the inhibition of the generation of near-wall prograde vortical structures. This study offers a new perspective for turbulent control by passive discrete elements. [ABSTRACT FROM AUTHOR]

Temporal or spatial structures are readily extracted from complex data by modal decompositions like proper orthogonal decomposition (POD) or dynamic mode decomposition (DMD). Subspaces of such decompositions serve as reduced order models and define either spatial structures in time or temporal structures in space. On the contrary, convecting phenomena pose a major problem to those decompositions. A structure traveling with a certain group velocity will be perceived as a plethora of modes in time or space, respectively. This manifests itself for example in poorly decaying singular values when using a POD. The poor decay is counterintuitive, since a single structure is expected to be represented by a few modes. The intuition proves to be correct, and we show that in a properly chosen reference frame along the characteristics defined by the group velocity, a POD or DMD reduces moving structures to a few modes, as expected. Beyond serving as a reduced model, the resulting entity can be used to define a constant or minimally changing structure in turbulent flows. This can be interpreted as an empirical counterpart to exact coherent structures. We present the method and its application to a head vortex of a compressible starting jet. [ABSTRACT FROM AUTHOR]

In‐stream surface storage zones (SSZs) caused by lateral recirculation areas play a significant role in the transport and fate of contaminants in rivers. Lateral recirculating areas have long residence times that favor nutrient uptake, accumulation of pollutants, and interactions with reactive sediments. In watersheds affected by acid‐mine drainage, SSZs have profound effects on biogeochemical processes, controlling the local concentration and distribution of toxic elements along the channel. Despite the importance of turbulent flow dynamics on these processes, limited work has been carried out to analyze mass transport in natural SSZs with complex geometries. In this investigation we study a SSZ in the Lluta River, located in a high‐altitude environment in northern Chile, by coupling field measurements and 3‐D numerical simulations to understand the transport mechanisms with the main channel. We measure the velocity field using an acoustic Doppler velocimeter (ADV) and large‐scale particle image velocimetry (LSPIV), extracting the bathymetry from digital image processing. Using these data, we perform detached‐eddy simulations (DES) to analyze the mean flow, turbulence statistics, and the dynamics of large‐scale coherent structures. From this detailed description of the turbulent flow, we study the mass exchange and the time evolution of the mean concentration of a passive scalar in the SSZ by testing three upscaled models: a classical linear transport model, a two‐storage formulation, and a fractional transport model. The analysis integrates temporal and spatial scales to provide a new perspective on the turbulent flow in SSZs and their effects on global mass transport in rivers. Key Points: Large‐scale turbulent coherent structures are the fundamental mechanisms of transport between surface storage zones (lateral recirculation areas) and the main channel in riversA combination of field measurements and 3‐D numerical simulations provides a better understanding of the flow field and transport in the recirculation zoneThree global mass transport models are used to study the exchange processes and represent the temporal evolution of solute concentration [ABSTRACT FROM AUTHOR]

Sediment suspension and transport under plunging regular waves was investigated in a laboratory surf zone using the volumetric three-component velocimetry technique. The two-phase flow measurements captured the motions of sediment particles simultaneously with the three-component, three-dimensional velocity fields of turbulent coherent structures (large eddies) induced by plunging breakers. Sediment particles were separated from fluid tracers based on a combination of particle spot size and brightness in the two-phase flow images. The interactions between the large eddies and bottom sediment were investigated in the outer surf zone. The measured data showed that breaker vortices impinging on the bottom was the primary mechanism that lifted sediment particles into suspension. High suspended sediment concentrations were found in the wall-jet region where the impinging flow was deflected outward and upward. Sediment particles were also trapped by counter-rotating vortices behind the down flow. Suspended sediment concentrations were significantly lower in the impingement zone where the fluid velocities were downward, even though the turbulent kinetic energy in the down flow was very high. Suspended sediment concentration was well correlated with vertical velocity and apparent shear stresses in the deflected flow, and with vorticity magnitude in the counter-rotating vortices. A linear relationship was found between net sediment flux and net turbulent kinetic energy flux over one wave cycle. It was found that a strong deflected flow in front of the impingement zone enhanced onshore sediment transport compared to a more symmetrical flow pattern, while counter-rotating vortices kept sediment particles in suspension for transport offshore after flow reversal. Onshore sediment transport was observed in less than 20% of the breaking waves. In most wave cycles, net sediment flux was directed offshore due to advection by predominantly offshore flow velocities. [ABSTRACT FROM AUTHOR]

This paper deals with an experiment effort on regularizing the near-wall flow-field of a zero-pressure-gradient turbulent boundary layer via passive discrete roughness elements. The major concept is to introduce quasi-steady wake of spanwise-aligned roughness elements into the near-wall region of wall turbulence, so that the mean spanwise spacing of low-speed streaks in the near wall region can be modulated. [ABSTRACT FROM AUTHOR]

It is generally recognized that large-scale turbulent coherent structures play an important role in the transport of sediment and contaminants in rivers. They are also believed to be related to the origin and development of a variety of fluvial bed and plan forms. While intensive laboratory and field research has been devoted in recent years to the study of large-scale vertical coherent structures, no such efforts have yet been directed to the study of large-scale horizontal coherent structures. This paper is intended as a contribution to address the existing lack of information on the latter structures. Its objective is to report an application of continuous wavelet transform (CWT) to the detection and establishment of the length and time scales of the largest coherent structures existing in a shallow open channel flow (width to depth ratio equal to 25), focusing primarily on the horizontal structures. The analysis is based on measurements of instantaneous flow velocity previously carried out in a 21 m long, 1 m wide flume. These include 306 single point velocity measurements collected throughout the flow field at a constant distance from the bed surface, the duration of each measurement being 120 s; and 20 min long measurements carried out at selected locations. The velocity was measured with the aid of a 2D Micro Acoustic Doppler Velocity meter. Large-scale horizontal coherent structures could be identified in all of the velocity records, and appeared as quasi-cyclic, sustained features in the flow. The intervals of time where such structures could not be detected were invariably short in comparison to the measurement time. CWT was found to be particularly well suited to determine the average time and length scales of the structures, two quantities of special significance in river morphodynamics. The average time scale of the large-scale horizontal structures for the investigated flow was found to be equal to 22.3 s, which implies a length scale of five times the flow width. Individual horizontal coherent structures with characteristic times approximately twice larger than the value of average time scale could be identified in the flow. However, these were infrequent occurrences in the flow. [ABSTRACT FROM AUTHOR]

In the present paper, we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure- gradient turbulent boundary layer. An array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky structures. With both qualitative visualization and quantitative measurement, we found that the regularization only occurs in the region below the height of the roughness elements. Statistical analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements; however, the latter's effect is in competition with the intrinsic streak generation mechanisms of smooth wall turbulence. Below the top of the roughness elements, local streamwise turbulent fluctuation intensity can be reduced by about 10%. We used POD analysis to depict such regularization effect in terms of near-wall structure modulation. We further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence, there is no streak-regularization effect in the buffer region, so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce. [ABSTRACT FROM AUTHOR]

ABSTRACT: Measurements of the wall pressure have been carried out by high-resolution pressure transducers in a wind tunnel and a flume respectively. The characteristics of time scales of the wall pressure signal can be obtained by the non-traditional wavelet analysis with the maximum energy criterion for identifying burst events in wall turbulence. [Copyright &y& Elsevier]

Measurements of the wall pressure have been carried out by high-resolution pressure transducers in a wind tunnel and a flume respectively. The characteristics of time scales of the wall pressure signal can be obtained by the non-traditional wavelet analysis with the maximum energy criterion for identifying burst events in wall turbulence. [ABSTRACT FROM AUTHOR]

The numerical investigation is conducted to probe the heat transfer performance and flow behavior in a channel having miniature cuboid vortex generators (MCVGs) at Reynolds number 7490. The MCVGs are immersed inside the viscous sublayer, the transition area and the logarithmic law area. The effects of MCVGs in different areas on turbulent coherent structures are deeply probed. The overall efficiency of heat transfer enhancement is verified with the combined performance parameter, which takes into account skin friction coefficient and Nusselt number. It reveals that combined performance parameter of the cases that MCVGs are located in transition or logarithmic law area, are greater than 1. The combined performance parameter of the case that MCVGs is located in the viscous sublayer, is less than 1. Furthermore, the maximum combined performance parameter with MCVGs is achieved in the transition area. Besides, compared with the rectangular channel having no MCVGs, the friction coefficient decreases up to 8.05%, the Nusselt number increases up to 5.17%. Simultaneously, the combined performance parameter increases up to 8.15%. • The MCVGs are located in the viscous sub-layer, the transition area and the logarithmic law area. • The MCVGs immersed in various areas of turbulent boundary layer can reduce the flow drag. • Among all the cases, most of velocity profiles in the viscous sub-layer are suppressed. • Setting MCVGs in the transition area can lead to a big difference in the flow characteristic and heat transfer performance. • The newly-defined flow drag includes the flow drag caused by MCVGs. [ABSTRACT FROM AUTHOR]