Small-scale Coherent Structures Research Articles

Effects of Inflow Turbulent Coherent Structures on a Small-Scale Wind Turbine in the Atmospheric Boundary Layer of Long Corridor Terrains: An Example Study in the Hexi Corridor

Considering its Venturi effect, long corridor terrain is considered as one of the outstanding wind-energy resources, but has not been developed due to unclear inflow turbulence effects on wind turbines. To advance the wind-power technology in long corridors and reveal the physics behind it, we conducted a series of data-fusion analyses combining field measurements in the Hexi Corridor, Gansu, China, and numerical simulations and summarize the effort here. In this study, the classical spectrum for the atmospheric boundary layer for wind-energy research, the IEC Kaimal spectrum (also known as IECKwind, International Elector technical Commission 61400-1) cannot describe the wind profile in corridors. Here, we proposed a new spectrum to describe the wind in the Hexi Corridor by modifying the IEC Kaimal spectrum, and we call it the IECK-HX spectrum. The results demonstrate that it preferentially resides at lower frequencies (larger spatial scales) compared with IECKwind. Large-scale motions (LSMs) and very large-scale motions (VLSMs) coexist in the Hexi Corridor atmospheric boundary layer, which can be well described by the IECK-HX spectrum, and these two dominate the large fluctuations in the power, thrust, and blade root flapwise moment of wind turbines, while the high-frequency small-scale coherent structures mainly cause high-frequency fluctuations in wind-turbine load. Furthermore, IEC Kaimal spectrum can not describe the VLSMs, and the generated flow presents significant fluctuations and thereby reduces the power output in high wind-speed regions. This study provides fundamental support for wind-energy scientists and engineers who are interested in wind energy in corridor terrains. Published by the American Physical Society 2024

Multifield intermittency of dust storm turbulence in the atmospheric surface layer

Dust storms are typical dispersed two-phase atmospheric turbulence involving electrified charged dust particles. Previous observations have demonstrated that clean-air atmospheric turbulence is strongly intermittent. However, the intermittency of the wind velocity, concentration of dust particles with a diameter smaller than $10\ \mathrm {\mu }{\rm m}$ (PM10) and electric fields, known as multifield intermittency, has not been reported or characterized yet. Here, we quantify the small-scale multifield intermittency of dust storms using datasets obtained from the Qingtu Lake Observation Array and a wavelet-based data analysis technique. The results indicate that the probability density functions of the multifield increments are scale dependent, and the scaling exponents of the multifield structure functions exhibit anomalous scaling, suggesting that the multiple fields in dust storms are also highly intermittent. Specifically, the wind velocity during dust storms appears to be more intermittent as compared with clean-air conditions. Among the multiple fields, the small-scale intermittency is strongest for PM10 dust concentration, moderate for electric fields and weakest for wind velocity. Furthermore, the anomalous scaling of multiple fields is well described by the hierarchical structure theory of turbulence. It is theoretically predicted that the wind velocity displays a one-dimensional filamentary structure, while the PM10 dust concentration and electric fields display two-dimensional sheet-like structures. Finally, after removing the coherent components of the observed time series by the proposed wavelet conditioning statistics, Kolmogorov linear scaling is recovered for the multiple fields, suggesting that small-scale multifield intermittency is caused by the presence of small-scale coherent structures.

Effects of the airfoil probes on the aerodynamic performance of a transonic compressor cascade

The airfoil probe is typical instrumentation for measuring total pressure/temperature in the research fields of turbomachinery. Besides causing measurement error itself, it also generates extra flow loss inevitably and further affects the measuring accuracy. This paper utilizes the Reynolds-averaged Navier Stokes (RANS) and high-fidelity methods to understand the effects of airfoil probes on the aerodynamic performance of a compressor cascade. With the inclusion of airfoil probes, the load capacity and the shock waves decline, to some extent, mainly because of the streamwise vortex. The shedding vortex induced by the probes triggers an earlier and non-separated flow transition, which contributes to a suppression of the separation bubble on the suction surface and a reduction of the profile loss. However, through the quantitative traceability analysis of the flow loss, the extra loss is generated and redistributed to the overall flow field. The separated flow forms at the junction of the end wall, blade, and probe's pipes, due to the high adverse pressure gradient aggravated by shock waves. It is one of the reasons why the measuring accuracy of airfoil probes is quite sensitive to the Mach numbers. The Delayed-Detached Eddy Simulation (DDES) shows superiority in capturing small-scale coherent vortex structures. The shedding streamwise vortex generated by the probe's pipe is distinguished for the first time. Its transport progression to the wake region is revealed basically, which has been masked previously by the conventional RANS method. In addition, due to the supplement of the streamwise vortex generated by probes, the wake vortices maintain a higher shedding frequency. This work offers a glimpse of the complex fluid mechanisms in a transonic regime, which suggests carefully considering the size of instruments in the design and measurement process.

Influence of Synthetic Jets on Multiscale Features in Wall-Bounded Turbulence

This experimental research focuses on the impacts of submerged synthetic jets on a fully-developed turbulent boundary layer (TBL) under a drag reduction working case. Two-dimensional velocity vectors in the flow field are captured with the aid of a particle image velocimetry (PIV) system. Proper orthogonal decomposition (POD) analyses provide evidence that synthetic jets notably attenuate the induction effect of prograde vortex on the low-speed fluid in large-scale fluctuation velocity field, thereby weakening the bursting process of near-wall turbulent events. Furthermore, the introduced perturbance redistributes the turbulent kinetic energy (TKE) and concentrates the TKE onto small-scale coherent structures. Modal time coefficients in various orders of POD are divided into components of multiple frequency bands by virtue of complementary ensemble empirical mode decomposition (CEEMD). It is found that the turbulence signals are shifted from low-frequency to high-frequency bands thanks to synthetic jets, thus revealing the relationship between scales and frequency bands. One further method of scale decomposition is proposed, that is, the large-scale fluctuating flow field will be obtained after removing the high-frequency noise data with the help of continuous mean square error (CMSE) criterion.

Characteristics and mechanisms of air-core vortex meandering in a free-surface intake flow

The air-core vortex generated at hydraulic intakes is usually associated with efficiency reduction and performance deterioration of the hydraulic machinery. Numerical simulations of the air-core vortex in a horizontal intake system are performed for various intake submergence depths and flow rates. Our study focus on the characteristics and mechanisms of the air-core vortex meandering phenomenon. Statistical analyses of the meandering coordinates of the vortex core are carried out, which characterize the meandering scope, dispersion radii and frequencies of the meandering coordinates of the air-core vortex. Parametric studies of the effects of the flow parameters on the air-core vortex meandering indicate that the dispersion radii increase as the intake flow rate increases or the intake submergence depth decreases, and the meandering frequencies decrease with increasing flow rate. Snapshot proper orthogonal decomposition (POD) analyses of the velocity and vorticity fields reveal that the large-scale energetic coherent structures in the first two modes cause streamwise and spanwise displacement of the air-core vortex, and the small-scale coherent structures with low energy fractions in the third to sixth modes contribute to elliptical deformation of the vortex core. Multiple relationships among the temporal frequencies of the decomposed POD modes and the meandering coordinates are discovered, which further proves that the large-scale energetic coherent structure in the turbulent flow field is the intrinsic reason for the meandering phenomenon of the air-core vortex.

Identification of coherent structures in space plasmas: the magnetic helicity–PVI method

Context. Plasma turbulence can be viewed as a magnetic landscape populated by large- and small-scale coherent structures. In this complex network, large helical magnetic tubes might be separated by small-scale magnetic reconnection events (current sheets). However, the identification of these magnetic structures in a continuous stream of data has always been a challenging task. Aims. Here, we present a method that is able to characterize both the large- and small-scale structures of the turbulent solar wind, based on the combined use of a filtered magnetic helicity (Hm) and the partial variance of increments (PVI). Methods. This simple, single-spacecraft technique was first validated via direct numerical simulations of plasma turbulence and then applied to data from the Parker Solar Probe mission. Results. This novel analysis, combining Hm and PVI methods, reveals that a large number of flux tubes populate the solar wind and continuously merge in contact regions where magnetic reconnection and particle acceleration may occur.

Temporal and spatial evolution of cavitating bubbles and coherent structures in a confined submerged jet shear layer

PurposeThe study aims to display the bubbles' evolution in the shear layer and their relationship with the pressure fluctuations. Furthermore, the coherent structures of the first six modes are extracted, in order to provide insight into their temporal and spatial evolution and determine the relationship between cavitating bubbles and coherent structures.Design/methodology/approachIn the present study, numerical simulations of submerged jet cavitating flow were carried out at a cavitation inception condition inside an axisymmetric cavity using the large eddy simulation (LES) turbulence model and the Schnerr–Sauer (S–S) cavitation model. Based on snapshots produced by the numerical simulation, dynamic mode decomposition (DMD) was performed to extract the three-dimensional coherent structures of the first six modes in the shear layer.FindingsThe cavitating bubbles in the shear layer are deformed to elongated ellipsoid shapes by shear forces. The significant pressure fluctuations are induced by the collapse of the biggest bubble in the group. The first mode illustrates the mean characteristics of the flow field. The flow in the peripheral region of the shear layer is mainly dominated by large-scale coherent structures revealed by the second and third modes, while different small-scale coherent structures are contained in the central region. The cavitating bubbles are associated with small size coherent structures as the sixth or higher modes.Practical implicationsThis work demonstrates the feasibility of LES for high Reynolds number shear layer flow. The dynamic mode decomposition method is a novel method to extract coherent structures and obtain their dynamic information that will help us to optimize and control the flow.Originality/value(1) This paper first displays the three-dimensional coherent structures and their characteristics in the shear layer of confined jet flow. (2) The relationship of bubbles shape and pressure fluctuations is illustrated. (3) The visualization of coherent structures benefits the understanding of the mixing process and cavitation inception in jet shear layers.

Turbulence cascade model for viscous vortex ring-tube reconnection

The reconnection of a vortex ring and a vortex tube in a viscous fluid with the effects of two vortex core sizes (σ0=0.12r0 and 0.24r0, where r0 are initial ring radius) and three initial flow configurations (left-offset, center, and right-offset) at Reynolds number (ReΓ) of 10 000 was investigated using a high-order vortex-in-cell method combined with a large-eddy simulation model. For the left-offset case, a large part of the ring, slipping over the tube, associates with a small part of the tube to establish a new vortex ring, whereas the rest of the tube is reconnected by another part of the ring. For the center case, half of the ring joins with a part of the tube to construct an elliptical vortex ring while the rest connects because of viscosity. The reconnected ring and tube become more stable and are like the initial ones in the ultimate stage. For the right-offset case, both the ring and tube's reconnection occurs, and the reconnected elliptical vortex ring is rapidly distorted. The proportion of reconnected ring increases, and then this ring section loses its integrity, decaying into a complex cluster of various-scales vortex structures in different shapes. At σ0=0.12r0, the secondary vortex structures surrounding the tube and ring appear in three cases, while they are only observed for the center case at σ0=0.24r0. For three flow configurations and two vortex core sizes, after the reconnection, the energy cascade of the flow approaches a k−5/3 slope of Kolmogorov's similarity hypotheses and a k−3 slope in the ranges of wavenumbers (k) from 3 to 10 and from 10 to 40, respectively. The highest population of small-scale coherent vortex structures is observed for the right-offset, followed by the center and left-offset. In addition, a larger number of these structures was observed for a smaller core size. This validates that the mixing performance is the best at a small vortex core and in the right-offset configuration.

Wind Tunnel Study on Wake Instability of Twin H-Rotor Vertical-Axis Turbines

In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An analysis of the wake characteristics of twin turbines forms the basis of array optimisation. This study aimed to examine the instability characteristics of a twin-turbine wake with two rotational configurations. The dynamic evolution characteristics of coherent structures with different scales in the wake were analysed via wavelet analysis. The results show that an inverse energy cascade process occurs after the high-frequency small-scale coherent structures induced by rotation lose their coherence. This self-organising characteristic is more apparent in the quasi two-dimensional wake of a forward-moving counter-rotating turbine (Array 1) than in that of a backward-moving counter-rotating turbine (Array 2). With greater organisation and coherence, the wake of Array 1 exhibits low-frequency instability characteristics dominated by a large-scale coherent structure. In addition, the signals reconstructed using wavelet transform show that asymmetric modes exist between low-frequency large-scale coherent structures. The experimental results provide a new perspective on the instability mechanism of twin-turbine wakes, as well as important data for numerical modelling.

Proper Orthogonal Decomposition Analysis and Dispersion Characteristics of Resonant Acoustic Flow

The investigation on the flow field and mixing characteristics of resonant sound mixing is of great significance for the dispersion mixing of superfine materials. In order to simulate the flow field and dispersion characteristics of resonant acoustic mixing, a gas-liquid-solid three-phase flow model based on the coupled level-set and volume-of-fluid (CLSVOF) and discrete particle model (DPM) was established. The CLSVOF model solves the gas-liquid interface, and the DPM model tracks the particle position. Then, the particle image velocimetry (PIV) experiment was performed using a self-made resonance acoustic hybrid prototype under different oscillation accelerations, and the radial velocity distribution between the experiment and simulation was compared. Finally, the proper orthogonal decomposition (POD) is used to decompose the flow field under different oscillation accelerations and fill levels, and the energy distribution law and the energy structure of different scales are extracted. The results show that the energy of the instantaneous flow field of the resonant sound is mainly concentrated in the low-order mode, and a close relationship was revealed between the energy distribution law and dispersion behavior of particles. The larger the small-scale coherent structures distribute, the more energy it has and the more favorable it is for fast and uniform dispersion.

Magnetohydrodynamic and kinetic scale turbulence in the near-Earth space plasmas: a (short) biased review

The near-Earth space is a unique laboratory to explore turbulence and energy dissipation processes in magnetized plasmas thanks to the availability of high-quality data from various orbiting spacecraft, such as Wind, Stereo, Cluster, Themis, and the more recent one, the NASA Magnetospheric Multiscale (MMS) mission. In comparison with the solar wind, plasma turbulence in the magnetosheath remains far less explored, possibly because of the complexity of the magnetosheath dynamics that challenges any “realistic” theoretical modeling of turbulence in it. This complexity is due to different reasons such as the confinement of the magnetosheath plasma between two dynamical boundaries, namely the bow shock and the magnetopause; the high variability of the SW pressure that “shakes” and compresses continuously the magnetosheath plasma; and the presence of large-density fluctuations and temperature anisotropies that generate various instabilities and plasma modes. In this paper, we will review some results that we have obtained in recent years on plasma turbulence in the SW and the magnetosheath, both at the magnetohydrodynamics (MHD) and the sub-ion (kinetic) scales, using the state-of-the-art theoretical models and in situ spacecraft observations. We will focus on three major features of the plasma turbulence, namely its nature and scaling laws, the role of small-scale coherent structures in plasma heating, and the role of density fluctuations in enhancing the turbulent energy cascade rate. The latter is estimated using (analytical) exact laws derived for compressible MHD theories applied to in situ observations from the Cluster and Themis spacecraft. Finally, we will discuss some current trends in space plasmas turbulence research and future space missions dedicated to this topic that are currently being prepared within the community.

Roof-level large- and small-scale coherent structures in a street canyon flow

The characteristics of large- and small-scale turbulent motions at roof-level in a street canyon flow were experimentally investigated along with their spatio-temporal organization and their mutual interaction in this region. Quadrant analysis was conducted to identify sweep and ejection events, whilst a spanwise spatial filter was used to identify the large-scale motions in the flow. The present analysis was conducted for six configurations; three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3). The upstream roughness arrays consisted of three-dimensional cubes (plan area density, λp = 25%), 1h spaced two-dimensional bars (λp = 50%, corresponding to the skimming-flow regime) and 3h spaced two-dimensional bars (λp = 25%, corresponding to the wake-interference flow regime). It was found that the roughness configuration has a strong effect on the size of the quadrant events in the street canyon, with the wake-interference flow regimes producing significantly larger sweep and ejection events than the skimming-flow regimes. Upstream wake-interference flow regimes were found to have a significantly greater temporal correlation than in the skimming-flow regimes. Large-scale streamwise and spanwise velocity components were found to have large spatio-temporal correlation integral scales, in agreement with the large-scale structures existing in the overlying boundary layer. It was also confirmed that there is a coupling between large- and small-scales at roof-level of the street canyon. While sweep and ejection events were found to be due to the dynamical contribution of the small-scales, their occurrence is shown to be coupled with that of large-scale low- and high-momentum regions, respectively.

Heat transfer and flow field features between surface mounted trapezoidal-ribs

Present work is an experimental study of flow over array of trapezoidal-ribs transversely placed on the wider bottom surface of the rectangular passage. It is intended to study the profound impact of taper angle variation (0 to 20°) on the flow mechanism, and subsequently on heat transfer improvement. The local and augmentation heat transfer patterns have been investigated using liquid crystal thermography (LCT). Further, the aerodynamic characteristics, in a module between seventh and eight ribs, have been obtained using particle image velocimetry (PIV) for understanding the flow physics. Existence of large- and small-scale coherent structures within the detached shear layer zone have been confirmed and further explained by means of critical points. As compared to square rib, trapezoidal rib provides higher heat transfer rate just downstream of the rib, which is observed to be in line with the fluid flow results.

Direct numerical simulation of hypersonic transition induced by an isolated cylindrical roughness element

Hypersonic boundary layer transition induced by an isolated cylindrical roughness element is investigated using direct numerical simulation method based on a finite volume formulation. To simulate the transition procedure by resolving the generation and evolvement of small-scale coherent structures, and capture the shock wave at the same time, high-order minimum dispersion and controllable dissipation scheme is validated and then applied. The results are compared with the available measurements in the quiet wind tunnel, such as the dominated frequency and root mean square of pressure. The computational dominated frequency of 19.23 kHz is very close to the experimental one, 21 kHz. Also, the disturbances of the roughness are mostly generated by the “jet” just before the roughness, and then they travel and develop downstream with the shear layer and vortex shedding. The transition is mainly dominated by the instabilities of both the horseshoe vortex and the shear layer.

A note on the relationship between turbulent coherent structures and phase correlation.

Various definitions of coherent structures exist in turbulence research, but a common assumption is that coherent structures have correlated spectral phases. As a result, randomization of phases is believed, generally, to remove coherent structures from the measured data. Here, we reexamine these assumptions using atmospheric turbulence measurements. Small-scale coherent structures are detected in the usual way using the wavelet transform. A considerable percentage of the detected structures are not phase correlated, although some of them are clearly organized in space and time. At larger scales, structures have even higher degree of spatiotemporal coherence but are also associated with weak phase correlation. A series of specific examples are shown to demonstrate this. These results warn about the vague terminology and assumptions around coherent structures, particularly for complex real-world turbulence.

Dynamics of a two-dimensional flow subject to steady electromagnetic forces

A novel experimental setup is presented to study the dynamics of a two-dimensional (2D) flow formed of an electrolyte subject to steady electromagnetic forcing. A thin layer of potassium hydroxide is poured into a square-base container with a strong magnetic field (\(\vec{B}\)) achieved by permanent neodymium magnets inserted underneath the base. The set of electrodes of alternating polarity distributed along the perimeter of the container generates currents (\(\vec{j}\)) in opposite directions. Coherent primary vortices of scales about 2 cm are thus generated by the \(\vec{j} \times \vec{B}\) force. We also show, and for the first time, that fluid motion is caused by the magnetic field gradient where the amplitude of \(\vec{B}\) is equal to zero. It leads to the generation of jets with size about that of the container, that is, 25 cm. The interaction between these gradB jets and the edge vortices leads to a final flow dominated by large-scale vortices resulting from the inverse cascade process that destroys the small-scale coherent structures on one hand and on the other modifies the initial scale and direction of the gradB jets.

Large Eddy Simulation of flame stabilisation dynamics and vortex control in a lifted H2/N2 jet flame

Flame stabilisation in (highly) preheated mixture is common in several industrial applications. When the reactants are injected separately in the device (usually at high-speed), the flame is lifted so that the fuel and oxidant first mix to give an ignitable mixture. If the temperature of the mixture is adequate, it auto-ignites stabilizing the flame. Here we focus on an academic lifted jet flame and Large Eddy Simulation (LES) is used to capture the flame and auto-ignition dynamics. Comparisons with experimental data show that LES simulates accurately high OH fluctuation levels at the stabilisation location. The vortex dynamics linked to these fluctuations is analyzed and it is found that small scale coherent structures play a vital role in the auto-ignition process. These structures are axial vorticity tubes (braids) and are located relatively far (in the radial direction) from the shear-layer. As a consequence, the lift-off height varies dramatically in time leading to OH fluctuations of the order of the mean OH concentration. This scenario is monitored in the compositional space highlighting the simultaneous evolution of OH, HO 2 and temperature. Further, different strategies for open-loop control of the flame lift-off height are tested. In order to anchor the flame at different positions downstream of the nozzle, the vortex dynamics in the shear-layer was modified. Promoting successively vortex ring and braids, the auto-ignition region was moved significantly. In particular, modified nozzle geometries impacted the formation of braids and ensured a good premixing very close to the nozzle. As a consequence, it was possible to reduce significantly the lift-off height and stabilise the flame few diameters downstream of the nozzle.

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

[1] With the help of a 2.5-D particle-in-cell simulation code, we investigate the physics of the acceleration of auroral electrons, through the interaction of an isolated Alfven wave packet with a plasma density cavity. The cavity is edged by density gradients perpendicular to the magnetic field. We show that a single passing of an isolated wave packet over a (infinite) cavity creates an electron beam. It triggers local current and beam-plasma instabilities and small-scale coherent electric structures. The energy flux of downgoing electrons is significantly increased, whereas upgoing electrons are also accelerated, even if no beam is formed. Accelerated electrons remain after the passage of the Alfvenic pulse, allowing the observation of energetic particles without any significant electromagnetic perturbation. The dependence of this process on the electron to ion mass ratio is consistent with its control by inertial effects.

Geometric properties of particle trajectories in turbulent flows

We study the statistics of curvature and torsion of Lagrangian trajectories from direct numerical simulations of homogeneous and isotropic turbulence (at e λ ≈280) in order to extract informations on the geometry of small-scale coherent structures in turbulent flows. We find that, as previously observed by Braun et al. (W. Braun, F. De Lillo, and B. Eckhardt, Geometry of particle paths in turbulent flows, J. Turbul. 7 (2006), p. 62) and Xu et al. (H. Xu, N.T. Ouellette, and E. Bodenschatz, Curvature of Lagrangian trajectories in turbulence, Phys. Rev. Lett. 98 (2007), p. 050201), the high curvature statistics is dominated by large-scale flow reversals where velocity magnitude assumes very low values. We show that flow-reversal events are characterized by very short correlation times. We introduce both time filtering and threshold in the minimum velocity amplitude in order to disentangle intense curvature events generated from genuine small-scale vortex structures from simple flow-reversal. We present for the first time measurements of torsion statistics in fully developed turbulent flows. By studying the joint statistics of curvature and torsion, we present further evidences that intense and persistent events are dominated by helical-type trajectories.

Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.