Dynamics Of Coherent Structures Research Articles

Spontaneous formation of coherent structures by an intense laser pulse interacting with overdense plasma

The formation and the dynamics of coherent magnetic field structures in the context of laser plasma interaction has attracted considerable attention. In the literature the formation of these structures has, however, mostly been reported in the wake of a laser pulse propagating in an underdense plasma medium (Bulanov et al., Phys. Rev. Lett., vol. 76, 1996, pp. 3562–3565; Nakamura & Mima Phys. Rev. Lett., vol. 100, 2008, 205006; Bulanov et al., Plasma Phys. Rep., vol. 31, no. 5, 2005, pp. 369–381; Naumova et al., Phys. Plasmas, vol. 8, no. 9, 2001, pp. 4149–4155; Nakamura et al., Phys. Rev. Lett., vol. 105, no. 13, 2010, 135002). The study here focuses on the formation of coherent structures by an intense laser pulse when it interacts with an overdense plasma medium. The laser in this case gets reflected and partially dumps its energy to the lighter electrons species. Particle-in-cell simulation studies have been carried out in two dimensions to show that the energetic electrons (generated at the critical layer and having relativistic energies), together with the background plasma electrons often self-organize to form distinct electron current vortices. These electron vortices have associated magnetic fields with monopolar or dipolar symmetries depending on the rotation profile of the electron current. The formation, stability and dynamics of these structures in the context of overdense plasma is of special importance as they provide a possibility of energy transport into those regions of plasma which are inaccessible to lasers. For such applications, higher energy content (involvement of relativistic electrons in their formation) of these structures is desirable. It is shown that their salient propagation characteristics even at relativistic energies follow the rules of electron magnetohydrodynamics (EMHD) (Isichenko & Marnachev, Sov. Phys. JETP, vol. 66, 1987, p. 702; Biskamp et al., Phys. Rev. Lett., vol. 76, 1996, p. 1264) (Generalized - EMHD Yadav et al., Phys. Plasmas, vol. 15, no. 6, 2008, 062308; Yadav et al., Phys. Plasmas, vol. 16, no. 4, 2009, 040701) for homogeneous (inhomogeneous) plasma medium, respectively.

Near- and far-field structure of shallow mixing layers between parallel streams

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Direct numerical simulation of turbulent flow through a ribbed square duct

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The impact of atmospheric boundary layer, opening configuration and presence of animals on the ventilation of a cattle barn

Naturally ventilated livestock buildings (NVLB) represent one of the most significant sources of ammonia emissions. However, even the dispersion of passive gas in an NVLB is still not well understood. In this paper, we present a detailed investigation of passive pollutant dispersion in a model of a cattle barn using the wind tunnel experiment method. We simulated the pollution of the barn by a ground-level planar source. We used the time-resolved particle image velocimetry (TR-PIV) and the fast flame ionisation detector (FFID) to study the flow and dispersion processes at high spatial and temporal resolution. We employed the Proper Orthogonal Decomposition (POD) and Oscillating Patterns Decomposition (OPD) methods to detect the coherent structures of the flow. The results show that the type of atmospheric boundary layer (ABL) and sidewall opening height have a significant impact on the pollutant dispersion in the barn, while the presence of animals and doors openings are insignificant under conditions of winds perpendicular to the sidewall openings. We found that the dynamic coherent structures, developed by the Kelvin-Helmholtz instability, contribute to the pollutant transport in the barn. We demonstrate that in any of the studied cases the pollutant was not well mixed within the barn and that a significant underestimation (up to by a factor 3) of the barn ventilation might be obtained using, e.g. tracer gas method.

Dynamics of coherent structures in turbulent square duct flow

The underlying flow dynamics in a turbulent flow in a periodic square duct is investigated by using the snapshot Proper Orthogonal Decomposition (POD) technique. In this study, the friction Reynolds number based on the duct width is fixed at 300. The coherent structures are identified through the spatial and temporal analysis of POD modes. Analysis of two sets of POD data is performed. In obtaining the first set, POD is performed on the combined fluctuating velocity vector, while for the second set, only the fluctuating velocity along the y- or z-direction is used. It was found that the first two most energetic spatial POD modes are the streamwise-independent or non-propagating roll modes. The third and fourth most energetic modes are observed to be streamwise-dependent, propagating modes. The spatio-temporal analysis of POD modes confirms the presence of traveling waves in the square duct, and its average speed is also calculated. The POD of the second dataset showed only propagating modes, and no non-propagating modes were found. These propagating modes are also rotationally symmetric. It has been shown that there is an energy exchange between non-propagating modes and propagating modes. The flow dynamics of the first four reconstructed POD modes portray the self-sustaining turbulence mechanism in a square duct. The structures obtained from the first POD dataset reconstruction of 10% energy show well organized hairpin vortices. Furthermore, it is found that the energy content of 35% gives detailed information on the coherent structures aligned along the wavy streamwise direction.

The organized motion of characterized turbulent flow at low Reynolds number in a straight square duct

We study the self-sustaining turbulent flow at minimal frictional Reynolds number in a straight square duct using direct numerical simulation. The flow in the square duct is maintained through a constant pressure gradient in the stream-wise direction. The computational investigation of the flow phenomenon for minimal, marginal and fully turbulent flow is studied using organized motion and dynamics of coherent structures. The central theme of the present work is to demonstrate the organized motion of the flow regimes below the frictional Reynolds number of the fully turbulent flow. The controlled direct numerical simulation study and vortex-detection technique unveil hairpin vortices in fully and minimal turbulent flow in the square duct for the first time. Bursting of streaks is detected with variable interval time average, and the evolution of hairpin vortices is also addressed. Turbulent intensities and energy spectra are reported and compared between the minimal, marginal and fully turbulent flow. The minimal turbulence in a square duct indicates transitional turbulent flow characteristics like lesser occurrence of ejection and sweep, intermittent bursting events, and a steeper variation of energy spectrum as compared to fully turbulent energy spectrum distribution.

Влияние электродной поляризации и динамического эргодического дивертора на характеристики прерывистых всплесков плотности плазмы в токамаке

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.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

In this paper, we study the shape and dynamics of helical coherent structures found in the flow field of an annular swirling jet undergoing vortex breakdown. The flow field is studied by means of time-resolved tomographic particle image velocimetry measurements. The obtained flow fields are analysed using both classic and spectral proper orthogonal decomposition. Despite the simple geometrical set-up of the annular jet, the flow field is very complex. Two distinct large-scale helical flow structures are identified: a single and a double helix, both co-rotating with the swirl direction, and it is revealed that these structures are not higher harmonics of each other. The structures have a relatively low energy content which makes it hard to separate them from other dynamics of the flow field, notably turbulent motions. Because of this, classic proper orthogonal decomposition fails to identify both structures properly. Spectral proper orthogonal decomposition, on the other hand, allows them to be identified accurately when the filter size is set at around eight times the precession period. The precession frequencies of the single and double helices correspond to Strouhal numbers of 0.273 and , respectively. A global stability analysis of the mean flow field shows that these structures correspond to two separate global modes. The precessing frequencies obtained by the stability analysis and the related spatial structures match very well with the experimental observations. The current work extends our knowledge on turbulent vortex breakdown and on mean field global stability theory in general. It leads to the following conclusions. Firstly, single- and double-helix vortex breakdown are both manifestations of global modes. Previous studies have shown that both and modes can coexist in swirling jets. However, the mode has been identified as a second harmonic of the first mode, while this study identifies both as two independent global modes. Secondly, this work shows that the simultaneous occurrence of multiple helical global modes is possible within a turbulent flow and their shapes and frequencies are very well predicted by mean field stability analysis. The latter finding is of general interest as it applies to a wide class of fluid problems dominated by multiple oscillatory structures.

Experimental and Numerical Investigation of Wake Interactions of Marine Hydrokinetic Turbines

To study the performance and environmental impacts of marine hydrokinetic (MHK) turbine arrays, we carry out an investigation based on laboratory experiments and numerical models able to resolve the dynamics of turbulent wake interactions and their effects on the river bed. We investigate a scaled Sabella D10 mounted on a mobile bed for a single and two aligned turbines, measuring the flow velocity, the rotor angular velocity, and the scour on the sediment bed. Numerical simulations are performed using a detached-eddy simulation (DES) turbulence model coupled with the blade-element momentum (BEM) approach, which can capture the mean flow and resolve the dynamics of turbulent coherent structures in the wakes. The simulations show a good agreement on the velocity statistics obtained experimentally. Power and thrust coefficients for the downstream turbine show an average decrease and a larger variability due to the turbulent intensity produced by the upstream turbine, as compared to the single turbine case. Results of this investigation also provide a framework to assess the predictive capabilities, scope, and applicability of computational models parameterizing the turbines using BEM, for testing different turbine designs and siting strategies within the MHK array.

On the Rayleigh-Taylor unstable dynamics of three-dimensional interfacial coherent structures with time-dependent acceleration

Rayleigh-Taylor instability (RTI) occurs in a range of natural and industrial processes. Whereas the majority of existing studies have considered constant acceleration, RTI is in many instances driven by variable acceleration. Here we focus on RTI driven by acceleration with a power-law time-dependence, and, by applying a group theoretic method, find solutions to this classical nonlinear boundary value problem. We deduce that the dynamics is dominated by the acceleration and that the solutions depend critically on the acceleration parameters for values of the acceleration exponent greater than (-2). We find that in the early-time dynamics, the RTI growth-rate is defined by modified Bessel functions. For the late-time dynamics, we link the interface dynamics with an interfacial shear function, find a continuous family of regular asymptotic solutions and identify invariant properties of nonlinear RTI. The essentially interfacial and multi-scale character of the dynamics is also demonstrated. The velocity field is potential in the bulk, and vortical structures may appear at the interface due to interfacial shear. The multi-scale character becomes clear from the invariance properties of the dynamics. We achieve good agreement with existing observations and elaborate new benchmarks for future research.

Effect of angle of attack on vortex dynamics in laminar separation bubbles

The dynamics of coherent structures that develop within laminar separation bubbles over a NACA 0018 airfoil at Rec = 100 000 and Angles of Attack (AOA) of 0°, 5°, 8°, and 10° are investigated experimentally using a high-speed flow visualization technique and vortex tracking via embedded microphones. The results identify vortex shedding within the separation bubble for all the cases investigated. The vortices form upstream of mean transition location and break down in the vicinity of mean reattachment, with irregular vortex merging events detected in the aft portion of the separation bubble. As the mean size of the separation bubble decreases with increasing angle of attack, vortex shedding characteristics also change appreciably, with shedding frequency increasing and characteristic streamwise wavelength decreasing. However, vortex roll up and salient aspects of vortex development take place within approximately the same regions downstream of separation location in terms of percentage of the bubble length. For all the cases examined, significant cycle-to-cycle variability is observed in salient vortex characteristics, being particularly pronounced in the aft portion of the bubble where vortex merging occurs and significant spanwise deformations of vortex filaments are expected. Merging of the shear layer vortices is shown to occur irregularly between the mean transition and reattachment location, with merging events separated by a relatively wide range of shedding cycles. As the angle of attack increases from AOA = 5° to 10°, the fraction of primary vortices involved in merging increases up to about 25%. However, enhancement in vortex merging is also noted at lower AOA where shedding occurs close to the trailing edge, which is speculated to be linked to upstream scattering of periodic pressure fluctuations induced at the trailing edge.

Explaining Homeopathy with Quantum Electrodynamics.

Every living organism is an open system operating far from thermodynamic equilibrium and exchanging energy, matter and information with an external environment. These exchanges are performed through non-linear interactions of billions of different biological components, at different levels, from the quantum to the macro-dimensional. The concept of quantum coherence is an inherent property of living cells, used for long-range interactions such as synchronization of cell division processes. There is support from recent advances in quantum biology, which demonstrate that coherence, as a state of order of matter coupled with electromagnetic (EM) fields, is one of the key quantum phenomena supporting life dynamics. Coherent phenomena are well explained by quantum field theory (QFT), a well-established theoretical framework in quantum physics. Water is essential for life, being the medium used by living organisms to carry out various biochemical reactions and playing a fundamental role in coherent phenomena. Quantum electrodynamics (QED), which is the relativistic QFT of electrodynamics, deals with the interactions between EM fields and matter. QED provides theoretical models and experimental frameworks for the emergence and dynamics of coherent structures, even in living organisms. This article provides a model of multi-level coherence for living organisms in which fractal phase oscillations of water are able to link and regulate a biochemical reaction. A mathematical approach, based on the eigenfunctions of Laplace operator in hyper-structures, is explored as a valuable framework to simulate and explain the oneness dynamics of multi-level coherence in life. The preparation process of a homeopathic medicine is analyzed according to QED principles, thus providing a scientific explanation for the theoretical model of "information transfer" from the substance to the water solution. A subsequent step explores the action of a homeopathic medicine in a living organism according to QED principles and the phase-space attractor's dynamics. According to the developed model, all levels of a living organism-organelles, cells, tissues, organs, organ systems, whole organism-are characterized by their own specific wave functions, whose phases are perfectly orchestrated in a multi-level coherence oneness. When this multi-level coherence is broken, a disease emerges. An example shows how a homeopathic medicine can bring back a patient from a disease state to a healthy one. In particular, by adopting QED, it is argued that in the preparation of homeopathic medicines, the progressive dilution/succussion processes create the conditions for the emergence of coherence domains (CDs) in the aqueous solution. Those domains code the original substance information (in terms of phase oscillations) and therefore they can transfer said information (by phase resonance) to the multi-level coherent structures of the living organism. We encourage that QED principles and explanations become embodied in the fundamental teachings of the homeopathic method, thus providing the homeopath with a firm grounding in the practice of rational medicine. Systematic efforts in this direction should include multiple disciplines, such as quantum physics, quantum biology, conventional and homeopathic medicine and psychology.

A modified proper orthogonal decomposition method for flow dynamic analysis

The numerical analysis method of Proper Orthogonal Decomposition (POD) is usually used to decompose the correlated nonlinear flow field and extract the structures with maximum energy contents (also called POD modes). However, it tends to mix the temporal frequencies in most modes, which confuses the physical interpretation and makes the flow dynamic characteristics unclear. For the transient flow field with discrete frequencies, to obtain the coherent structures with single-frequency dynamic characteristics, a new single-frequency POD method (F-POD) is proposed by utilizing Discrete Fourier Transform (DFT) and the corresponding inverse operation. In this way, the POD modes can be re-decomposed from the perspective of frequency. Particularly, for the spectra with a band of adjacent frequencies around some isolated peaks, the selection criterion of the frequency range in the inverse operation process is discussed to obtain the growth rate information of the modes. To illustrate the feasibility of F-POD, several multi-frequencies analytical cases with various growth patterns and a transitional cylinder wake case are investigated. In addition, the results are quantitatively compared with dynamic mode decomposition (DMD). It shows that the single-frequency POD modes with temporal evolutions can be well distinguished. Therefore, for the transitional flow scenarios with discrete frequencies, the F-POD could be an alternative method to extract the dynamic coherent structure with single-frequency and growth rate. We also found that the F-POD has challenges to deal with broadband turbulence, since it is hard to find main frequency.

Near‐Earth Solar Wind: Plasma Characteristics From ARTEMIS Measurements

AbstractThe interaction between the solar wind and Earth's magnetosphere is the main driver of magnetosphere dynamics because many important processes in Earth's inner and outer magnetosphere can be considered as responses to solar wind parameter variations. Therefore, accurate measurements of these parameters (density, velocity, ion, and electron temperature) in the near‐Earth solar wind is critical for understanding and forecasting magnetosphere dynamics. Moreover, the solar wind is a natural laboratory for investigation of plasma turbulence, including the configuration and dynamics of coherent plasma structures (such as large‐amplitude waves, discontinuities, and interplanetary shocks). A major solar wind parameter database, the OMNI database, consists of solar wind proton characteristics and magnetic field parameters but does not provide electron characteristics. We consider a supplementary solar wind data set compiled by observations of the two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes. We compare characteristics available in both the ARTEMIS and OMNI data sets. We also describe characteristics of solar wind electron components measured by ARTEMIS (density, temperature, and anisotropy). Finally, we discuss opportunities for investigation of the near‐Earth solar wind provided by the ARTEMIS data set.

Coexistence of solitons and extreme events in deep water surface waves

We study experimentally, in a large-scale basin, the propagation of unidirectional deep water gravity waves stochastically modulated in phase. We observe the emergence of nonlinear localized structures that evolve on a stochastic wave background. Such a coexistence is expected by the integrable turbulence theory for the nonlinear Schr{\"o}dinger equation (NLSE), and we report the first experimental observation in the context of hydrodynamic waves. We characterize the formation, the properties and the dynamics of these nonlinear coherent structures (solitons and extreme events) within the incoherent wave background. The extreme events result from the strong steepening of wave train fronts, and their emergence occurs after roughly one nonlinear length scale of propagation (estimated from NLSE). Solitons arise when nonlinearity and dispersion are weak, and of the same order of magnitude as expected from NLSE. We characterize the statistical properties of this state. The number of solitons and extreme events is found to increase all along the propagation, the wave-field distribution has a heavy tail, and the surface elevation spectrum is found to scale as a frequency power-law with an exponent --4.5 $\pm$ 0.5. Most of these observations are compatible with the integrable turbulence theory for NLSE although some deviations (e.g. power-law spectrum, asymmetrical extreme events) result from effects proper to hydrodynamic waves.

Экспериментальное исследование пульсационных характеристик закрученного потока в модели четырехвихревой топки

Актуальность исследования обусловлена необходимостью вовлечения в топливно-сырьевую базу низкосортных высокозольных углей в связи с ограниченностью запасов высококачественного топлива. Для их эффективного и экологически безопасного использования требуется разработка новых и модернизация имеющихся котельных агрегатов с предварительным физическим и численным моделированием. Целью работы является экспериментальное исследование пульсационных характеристик турбулентного закрученного потока в модели перспективного топочного устройства, использующего четырёхвихревую схему сжигания пылеугольного топлива. Объектом исследования является изотермическая лабораторная модель четырёхвихревой топки, выполненная из оптически прозрачного оргстекла для проведения измерений в рамках современных бесконтактных методов диагностики потоков. Методы: физическое лабораторное моделирование аэродинамики и процессов переноса в четырехвихревой топке; бесконтактное исследование осредненных и пульсационных характеристик в объеме модели топки при различных режимных параметрах; исследование пульсаций давления с использованием конденсаторного микрофона. Результаты. С использованием метода цифровой трассерной визуализации получены распределения осредненных и пульсационных характеристик скорости потока в ряде сечений модели в широком диапазоне режимных параметров. Определено положение зон с высоким значением турбулентных пульсаций и застойных зон, установлены особенности их расположения в зависимости от режимов. С использованием конденсаторного микрофона получены данные о пульсациях давления в локальных областях объема модели. Анализ спектральных характеристик с применением к полученным данным преобразования Фурье показал, что в изучаемой модели отсутствуют нестационарные периодические колебания, связанные с прецессией вихревого ядра потока. Измеренные пульсации давления вызваны акустикой, пульсации скорости носят случайный характер и не связаны с динамикой когерентных структур.

Dynamics of Coherent Structures and Random Turbulence in Pressure Side Film Cooling on a First Stage Turbine Vane

This paper collects the final results of a combined experimental and numerical investigation on pressure side (PS) film cooling in a high-pressure turbine vane, including two staggered rows of cylindrical holes and a trailing edge cutback, fed by one plenum. Having learned that the scale resolving simulation technique is essential to get reasonable predictions of adiabatic film cooling effectiveness, the stress-blended eddy simulation (SBES) model has been selected as the best among the available hybrid RANS–LES options. Mainstream conditions were limited to low speed and low turbulence intensity due to the need of high temporal and spatial resolution. The choice of one only coolant-to-mainstream mass flow ratio equal to MFR = 1.5% was dictated by the hole discharge: on the one side, mainstream injection into the cooling holes and, on the other side, jet liftoff were avoided to get an effective thermal coverage downstream of the holes. SBES potential was evaluated on the basis of qualitative and quantitative characteristics of the flow along the interface between coolant and mainstream because of their ultimate effect on vane surface temperature. The focus was set on shape and dynamics of coherent structures: SBES provided evidence of shear layer Kelvin–Helmholtz instability and hairpin vortices, downstream of cooling holes, with a Strouhal number (St) of 1.3 and 0.3–0.4, respectively. Simulated vortex shedding in the cutback region was characterized by St of 0.32 to be compared against the measured St value of 0.40.

Dynamic Migration Modes of Collective Cells

Collective cell migration occurs in a diversity of physiological processes such as wound healing, cancer metastasis, and embryonic morphogenesis. In the collective context, cohesive cells may move as a translational solid, swirl as a fluid, or even rotate like a disk, with scales ranging from several to dozens of cells. In this work, an active vertex model is presented to explore the regulatory roles of social interactions of neighboring cells and environmental confinements in collective cell migration in a confluent monolayer. It is found that the competition between two kinds of intercellular social interactions—local alignment and contact inhibition of locomotion—drives the cells to self-organize into various dynamic coherent structures with a spatial correlation scale. The interplay between this intrinsic length scale and the external confinement dictates the migration modes of collective cells confined in a finite space. We also show that the local alignment–contact inhibition of locomotion coordination can induce giant density fluctuations in a confluent cell monolayer without gaps, which triggers the spontaneous breaking of orientational symmetry and leads to phase separation.

Grid sensitivity of flow field and noise of high-Reynolds-number jets computed by large-eddy simulation

Three isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 105 are computed by large-eddy simulation using four different meshes in order to investigate the grid sensitivity of the jet flow field and noise. The jets correspond to two initially fully laminar jets and one initially strongly disturbed jet considered in previous numerical studies. At the exit of a pipe nozzle of radius r0, they exhibit laminar boundary-layer mean-velocity profiles of thickness [Formula: see text] and [Formula: see text], respectively. For the third jet, a peak turbulence intensity close to 9% is also imposed by forcing the boundary layer in the nozzle. The grids contain up to one billion points, and, compared to the grids used in previous simulations, they are finer in the axial direction downstream of the nozzle and in the radial direction on the jet axis and in the outer region of the mixing layers. The main flow field and noise characteristics given by the simulations, including the mixing-layer thickness, the centerline mean velocity, the turbulence intensities on the nozzle lip line and the jet axis, spectra of velocity and far-field pressure obtained from the jet near field by solving the isentropic linearized Euler equations, are presented. With respect to those from previous studies, the results are very similar for the initially laminar jet with thick boundary layers, but they differ significantly for the initially laminar jet with thin boundary layers and for the initially disturbed jet. For the latter two jets, using a finer grid leads to a faster flow development, to higher turbulence intensities in the shear layers and at the end of the potential core, to stronger large-scale structures, and to the generation of more low-frequency noise. Moreover, very small mesh spacings appear to be necessary all along the jet mixing layers, and in particular during their early stages of growth, to properly capture the formation and dynamics of the flow coherent structures and thus obtain results in good agreement with measurements available for high-Reynolds-number jets.

Large-scale coherent flow structures in rough-bed open-channel flow observed in fluctuations of three-dimensional velocity, skin friction and bed pressure

ABSTRACTThe dynamics of large-scale coherent structures (LSCS) are investigated in a rough-bed flow over the whole water column and longitudinal and transverse turbulent fluxes of momentum are compared. The relation between LSCS and fluctuations of skin friction and near-bed surface pressure is examined in order to understand the effects of LSCS on the bed. The study was carried out in an open channel with a non-moving permeable bed of screeded loose-mixed gravel (D50 = 1.5 cm) at Reynolds numbers ranging from 3.19 × 104 to 1.08 × 105. Quasi-instantaneous full-depth three-dimensional (3D) velocity profiles with high spatial and temporal resolution measured with an acoustic Doppler velocity profiler (ADVP) are combined with the signals of a hot-film sensor mounted on the top of the gravel and a piezoresistive pressure transducer placed in the top layer of the bed particles at the centre of the channel. Vertical turbulent momentum flux is clearly dominated by ejections and sweeps over most of the water column. Near the bed, contributions of sweeps are larger than those of ejections. This is reversed at the mid-depth (z/h = 0.5). No preferential quadrant orientation is seen for the transverse momentum flux. A correlation between LSCS dynamics in the water column and data from the sensors on the bed is made evident. Sweeps correlate with positive fluctuations of bottom pressure and skin friction while ejections correlate with negative ones.