Anisotropic Turbulent Fields Research Articles

Inflow turbulence generator for large eddy simulation based on a novel block‐vorticity vortex method: Application on a tall building wind effect

AbstractAccurately simulating turbulent wind fields is a significant challenge in wind engineering. This study proposes a novel block‐vorticity method aimed at overcoming the limitations of traditional turbulence generation methods. By superimposing a blocked vortex field at the inlet boundary of large eddy simulation (LES), the proposed method enables the generation of highly precise and anisotropic turbulent wind fields. To validate the effectiveness of the proposed method, the study investigates wind pressures on a high‐rise building structure and performs a comparative analysis with LES narrowband synthesis random flow generator (NSRFG), traditional LES, and SST k‐ω turbulence inlet models. The results demonstrate that the proposed method can effectively simulate turbulence characteristics of atmospheric boundary layer flow, including vortex structure and stochastic fluctuating wind field. Compared to traditional methods, the wind field characteristics of turbulence intensity, instantaneous vorticity, and turbulence self‐equilibrium had obvious advantages over traditional methods. Moreover, the LES vortex method is more accurate in simulating the mean wind pressure and fluctuating wind pressure of the high‐rise building compared to traditional models and is closer to the wind tunnel test results. The proposed method provides an effective approach for generating turbulent wind fields with anisotropic characteristics and can be used to predict the wind‐induced response of civil engineering structures.

Theory of the momentum source method for synthetic turbulence

The interaction between turbulence and blade leading edges is known to have a significant impact on the aerodynamic and aeroacoustic performance of propellers. In addition to directly simulating turbulence, synthetic turbulence, such as the momentum source method, has been developed as a popular method for studying this interaction process in computational fluid dynamics and computational aeroacoustics. However, it is found that for non-periodic disturbances, although the induced velocity field is divergence-free, spurious noise may be generated in the source region and contaminate simulation results. To address this issue, the present work proposes adding a correction term so that the divergence-free condition is satisfied globally and the unwanted acoustic waves are suppressed, as an extension to our previous work for time-periodic gusts [H. Jiang, Phys. Fluids 35, 096115 (2023)]. The strength of the proposed approach lies in its simplicity, flexibility, and generality. First, it derives explicit source terms, which are straightforward for numerical implementations, to generate unsteady flow fluctuations. Second, the sources can be added inside the computational domain, saving computational costs for turbulence convection and being compatible with most existing boundary conditions. Third, the proposed method can obtain analytical expressions for the needed momentum source of the Navier–Stokes equation subject to any desired isotropic or anisotropic divergence-free turbulence fields. The method has been verified by examples of synthesizing harmonic gusts, Gaussian eddies, and random turbulence. The synthetic velocity results characterized by different spectral components are directly compared to target velocity fields, verifying the proposed approach and showing its capability. Parameters that influence the distribution of added sources are systematically investigated to identify an optimal combination for different scenarios. Finally, the model is employed to evaluate the aerodynamic interaction between an incoming turbulence and a thin airfoil. The obtained results exhibit good correspondence with analytical solutions.

A coherence-improved and mass-balanced inflow turbulence generation method for large eddy simulation

Realistic inflow turbulence is crucial in large eddy simulations for obtaining accurate results. In this paper, an improved inflow turbulence generation method is proposed, called the coherence-improved and mass-balanced random flow generation (CMRFG) method. First, a study of the effect of the inlet mass-balanced condition is presented. Subsequently, the proposed method is explicitly deduced to satisfy arbitrary spatial coherence functions and the mass-balanced condition, so the generated turbulent fields effectively preserve the original turbulence characteristics, eliminating the need for additional inlet mass flux corrections. Meanwhile, the novel method retains the advantages of the previous method, satisfying arbitrary turbulence intensities, temporal spectra, temporal correlations, and spatial correlation coefficients. Additionally, the proposed method also ensures compliance with the divergence-free condition and Taylor's frozen hypothesis, which are essential for accurate flow field development. Finally, large eddy simulations of both homogeneous isotropic and anisotropic turbulent fields demonstrate that the turbulent fields generated by CMRFG closely align with experimental results and exhibit no artificial pressure fluctuations within the computational domain.

Wake flow characteristics and unsteady performance of a pump-jet propulsor under hull condition

The propulsor is one of the main noise sources of an underwater vehicle. The pump-jet operating under hull condition is in a typical non-uniform strong anisotropic turbulent flow field. In this paper, the wake flow characteristics and unsteady performance of a pump-jet propulsor under hull condition are numerically investigated. Reynolds-averaged Navier–Stokes (RANS) method and large eddy simulation (LES) method are used to evaluate and compare the ability of solving vortex structure, pressure distribution and unsteady force. LES method can capture the interaction between different vortex systems, and the vortex shedding process of the stator wake and appendage wake. These lead to that the frequency spectra of fluctuation pressure have peaks in the low-frequency range for LES method. For unsteady force spectrum, the frequency of maximum amplitude of a single blade is mainly affected by relative intensities of the stator wake and appendage wake. “Humps” appear near the blade passing frequency and its multiples for unsteady force spectrum of the rotor for LES method. The blade passing frequency in the rotor force spectrum is generated by the phase cancelation between different blades. The completeness of rotor blade phase cancelation is mainly affected by the intensity of the non-stationarity in the flow field. Overall, it is sufficient to clarify the hydrodynamic performance and the characteristics of fluctuation pressure distribution of the pump-jet with RANS method. The LES method is necessary to obtain reasonable transient characteristics of the unsteady force and vortex system interaction for the pump-jet under hull condition.

The study on the broadband thrust spectrum of propeller in anisotropic and inhomogeneous turbulence

Considering the effects of stretching of turbulent eddies and inhomogeneous distribution of turbulence parameters, such as turbulence length scale, a new analytical approach to calculating the unsteady broadband thrust of propeller in anisotropic and inhomogeneous turbulence was introduced based on the correlation methods conducted in isotropic turbulence. Stephens's experiment was used to verify the theoretical approach to modeling in anisotropic and inhomogeneous turbulence. The effects of turbulence parameters on propeller broadband thrust in anisotropic and inhomogeneous turbulent field were discussed. The results showed that the haystack amplitudes at blade passage frequencies increase with the nonuniform level of the turbulent intensity and stretching parameters of turbulence eddy, while the effects of nonuniform distribution of inflow velocity and turbulence length scale can be neglected. Additionally, the haystack frequencies of propeller thrust spectrum are only dependent on blade passage frequency.

Effects of isotropic and anisotropic turbulent structures over spray atomization in the near field

Sprays and atomization processes are extremely diffused both in nature and in industrial applications. In this paper we analyze the influence of the nozzle turbulence on primary atomization, focusing on the resulting turbulent field and atomization patterns in the Near Field (NF). In order to do so, a Synthetic Boundary Condition (SBC) and a Mapped Boundary Condition (MBC), producing respectively isotropic and anisotropic turbulent fields, have been generated as inflow conditions for the spray Direct Numerical Simulations (DNS). We present a specific methodology to ensure consistency on turbulence intensity and integral lengthscale between the two inflows. The analysis performed on the turbulent field (using one-point statistics and spectrum analysis) reveals a significantly stronger turbulent field generated by the inflow boundary conditions with anisotropic structures. While the increased turbulence field generated in the MBC case results in a higher number of droplets generated, the probability functions of both cases are extremely similar, leading to the non-obvious conclusion that the atomization patterns are only slightly affected by the inflow condition. These considerations are supported by the analysis of droplet size distributions, radial distribution functions, axial and radial distributions, highlighting extremely similar behaviors between the MBC and the SBC cases. Finally, these analyses and their computations are presented in detail, underlining how this type of point-process characterization shows interesting potential in future studies on sprays.

Impact of difference between explicit and implicit second-order time integration schemes on isotropic/anisotropic steady incompressible turbulence field

This study presents the impact of the difference between the implicit and explicit time integration methods on a steady turbulent flow field. In contrast to the explicit time integration method, the implicit time integration method may produce significant kinetic energy conservation error because the widely used spatial difference method for discretizing the governing equations is explicit with respect to time. In this study, the second-order Crank-Nicolson method is used as the implicit time integration method, and the fourth-order Runge-Kutta, second-order Runge-Kutta and second-order Adams-Bashforth methods are used as explicit time integration methods. In the present study, both isotropic and anisotropic steady turbulent fields are analyzed with two values of the Reynolds number. The turbulent kinetic energy in the steady turbulent field is hardly affected by the kinetic energy conservation error. The rms values of static pressure fluctuation are significantly sensitive to the kinetic energy conservation error. These results are examined by varying the time increment value. These results are also discussed by visualizing the large scale turbulent vortex structure.

Triple Hill’s Vortex Synthetic Eddy Method

The generation of initial or inflow synthetic turbulent velocity or scalar fields reproducing statistical characteristics of realistic turbulence is still a challenge. The synthetic eddy method, previously introduced in the context of inflow conditions for large eddy simulations, is based on the assumption that turbulence can be regarded as a superposition of coherent structures. In this paper, a new type of synthetic eddy method is proposed, where the fundamental eddy is constructed by superposing three Hill's vortices, with their axes orthogonal to each other. A distribution of Hill's vortices is used to synthesize an anisotropic turbulent velocity field that satisfies the incompressibility condition and match a given Reynolds stress tensor. The amplitudes of the three vortices that form the fundamental eddy are calculated from known Reynolds stress profiles through a transformation from the physical reference frame to the principal-axis reference frame. In this way, divergence-free anisotropic turbulent velocity fields are obtained that can reproduce a given Reynolds stress tensor. The model was tested on both isotropic and anisotropic turbulent velocity fields, in the framework of grid turbulence decay and turbulent channel flow, respectively. The transition from artificial to realistic turbulence in the proximity to the inflow boundary was found to be small in all test cases that were considered.

Time-series observation of the effects of kinetic energy conservation error on isotropic and anisotropic steady incompressible turbulence

The purpose of this study was to examine the effects of kinetic energy conservation error on isotropic and anisotropic steady incompressible turbulence fields. Results were obtained from time series of turbulent kinetic energy and static pressure fluctuation. A standard large-eddy simulation (LES) was used in the numerical analysis. The conservation error was generated using the Crank-Nicolson method. The result obtained using the fourth-order Runge-Kutta method was used to set results of reference. The effects of the conservation error on the mean value of the global turbulent kinetic energy were found to be small. On the other hand, it was found that the time series and mean values of static pressure fluctuation root mean square (rms) were sensitive to the conservation error. This discrepancy in the static pressure fluctuation rms was able to be reduced by adjusting the value of the model constant used in the LES. However, this adjusted model constant caused an error in the global turbulent kinetic energy. These results were found in both isotropic and anisotropic turbulence fields.

Multi-wave band Synchrotron Polarization of Gamma-Ray Burst Afterglows

Abstract Multi-wave band synchrotron linear polarization of gamma-ray burst (GRB) afterglows is studied under the assumption of an anisotropic turbulent magnetic field with a coherence length of the plasma skin-depth scale in the downstream of forward shocks. We find that for typical GRBs, in comparison to optical polarization, the degree of radio polarization shows a similar temporal evolution but a significantly smaller peak value. This results from differences in observed intensity image shapes between the radio and optical bands. We also show that the degree of the polarization spectrum undergoes a gradual variation from the low- to the high-polarization regime above the intensity of the spectral peak frequency, and that the difference in polarization angles in the two regimes is zero or 90°. Thus, simultaneous multi-wave band polarimetric observations of GRB afterglows would be a new determinative test of the plasma-scale magnetic field model. We also discuss theoretical implications from the recent detection of radio linear polarization in GRB 171205A with the Atacama Large Millimeter/submillimeter Array and other models of magnetic field configuration.

Numerical Study of Hydrogen Auto-Ignition Process in an Isotropic and Anisotropic Turbulent Field

The physical mechanisms underlying the dynamics of the flame kernel in stationary isotropic and anisotropic turbulent field are studied using large eddy simulations (LES) combined with a pdf approach method for the combustion model closure. Special attention is given to the ignition scenario, ignition delay, size and shape of the flame kernel among different turbulent regimes. Different stages of ignition are analysed for various levels of the initial velocity fluctuations and turbulence length scales. Impact of these parameters is found small for the ignition delay time but turns out to be significant during the flame kernel propagation phase and persists up to the stabilisation stage. In general, it is found that in the isotropic conditions, the flame growth and the rise of the maximum temperature in the domain are more dependent on the initial fluctuations level and the length scales. In the anisotropic regimes, these parameters have a substantial influence on the flame only during the initial phase of its development.

Generation of a random anisotropic incompressible turbulent velocity field in accordance with the calculated flow statistics

On the base of Randomized Spectral Method (RSM), a new stochastic algorithm for the generation of homogeneous anisotropic turbulent velocity fields has been developed. This technique provides incompressibility of the obtained velocity field realizations simultaneously with the two main required statistical properties—the Reynolds-stress tensor and the turbulent energy spectrum. The method is computationally efficient and can easily be extended to the case of inhomogeneous turbulence.

Effect of particle concentration on turbulent modulation inside hydrocyclone using coupled MPPIC-VOF method

We discuss the implementation of a coupled Multi-phase particle in cell method (MPPIC) and Volume of Fluid (VOF) flow solver to simulate particles in a fluid flow field having a free surface. Using the solver developed using OpenFOAM 4.1 library, we study the flow of particles inside a highly anisotropic turbulent flow field of a hydrocyclone. The free surface from the air-core inside the hydrocyclone is resolved using the VOF method coupled with the LES turbulence model. We test multiple sub-grid scale model and find the dynamic k-equation model to have the best-predicting capability of the mean and turbulent flow field as well as the air-core shape. Using the established flow field, we study the particle flow properties and its effect on the fluid flow field using a four-way coupled description between MPPIC and VOF. Important flow properties such as the particle flow field velocity and turbulence modulation with varying particle concentration were studied. It was found that even with low solid concentration (0.3–1.5% by volume) there is a quite substantial increase in the fluctuating flow field (around 12%) and with a further increase (1.8%) we observe a decrease in the turbulent levels by approximately 6%.

CHANG-ES

Context. The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field can be well explained by dynamo action, the three-dimensional structure of the halo field and its physical nature are still unclear. Aims. As first steps towards understanding the halo fields, we want to clarify whether or not the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies. We also aim to investigate whether these halo fields are simply turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure. Methods. Analysis of the Faraday rotation in the halo is used as a tool to distinguish anisotropic turbulent fields from large-scale magnetic fields. However, this has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations in C-band and L-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform rotation measure synthesis (RM synthesis) in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies in C-band. Results. Though the stacked edge-on galaxies were of different Hubble type, and had differing star formation activity and interaction activity, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized emission in their halos. We detected large-scale field reversals in all of their halos. In six galaxies, these are along lines that are approximately perpendicular to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 did we observe vertical giant magnetic ropes (GMR) similar to those detected recently in NGC 4631. Conclusions. The observed X-shaped structure of the halo field seems to be an underlying feature of spiral galaxies. It can be regarded as the two-dimensional projection of the regular magnetic field which we found to have scales of typically 1 kpc or larger observed over several kiloparsecs. The ordered magnetic field extends far out in the halo and beyond. We detected large-scale magnetic field reversals in the halo that may indicate that GMR are more or less tightly wound. With these discoveries, we hope to stimulate model simulations for the halo magnetic field that should also explain the determined asymmetry of the polarized intensity (PI).

Стохастический алгоритм генерации бездивергентного анизотропного однородного поля турбулентных пульсаций скоростей

Стохастический алгоритм генерации бездивергентного анизотропного однородного поля турбулентных пульсаций скоростей

Magnetic fields and cosmic rays in M 31

Context. Magnetic fields play an important role in the dynamics and evolution of galaxies; however, the amplification and ordering of the initial seed fields are not fully understood. The nearby spiral galaxy M 31 is an ideal laboratory for extensive studies of magnetic fields. Aims. Our aim was to measure the intrinsic structure of the magnetic fields in M 31 and compare them with dynamo models of field amplification. Methods. The intensity of polarized synchrotron emission and its orientation are used to measure the orientations of the magnetic field components in the plane of the sky. The Faraday rotation measure gives information about the field components along the line of sight. With the Effelsberg 100-m telescope three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 2.645, 4.85, and 8.35 GHz (wavelengths of 11.3, 6.2, and 3.6 cm). The λ3.6 cm survey is the first radio survey of M 31 at such small wavelengths. Maps of the Faraday rotation measures (RMs) are calculated from the distributions of the polarization angle. Results. At all wavelengths the total and polarized emission is concentrated in a ring-like structure of about 7–13 kpc in radius from the centre. Propagation of cosmic rays away from the star-forming regions is evident. The ring of synchrotron emission is wider than the ring of the thermal radio emission, and the radial scale length of synchrotron emission is larger than that of thermal emission. The polarized intensity from the ring in the plane of the sky varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is oriented almost along the ring, with a pitch angle of −14 ° ±2° at λ6.2 cm. The RM varies systematically along the ring. The analysis shows a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (ASS) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BSS) regular field with about six times smaller amplitude. The RM amplitude of (118 ± 3) rad m−2 between λ6.2 cm and λ3.6 cm is about 50% larger than between λ11.3 cm and λ6.2 cm, indicating that Faraday depolarization at λ11.3 cm is stronger (i.e. with a larger Faraday thickness) than at λ6.2 cm and λ3.6 cm. The phase of the sinusoidal RM variation of −7 ° ±1° is interpreted as the average spiral pitch angle of the regular field. The average pitch angle of the ordered field, as derived from the intrinsic orientation of the polarized emission (corrected for Faraday rotation), is significantly smaller: −26 ° ±3°. Conclusions. The dominating ASS plus the weaker BSS field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The difference in pitch angle of the regular and the ordered fields indicates that the ordered field contains a significant fraction of an anisotropic turbulent field that has a different pattern than the regular (ASS + BSS) magnetic field.

Synthetic generation of the atmospheric boundary layer for wind loading assessment using spectral methods

The recently proposed PRFG3 method is extended in order to synthesise an atmospheric boundary layer flow, which can be used as inflow condition for Large Eddy Simulation (LES). Firstly, PRFG3 is used for generating homogeneous isotropic and homogeneous anisotropic turbulence fields. Based on the obtained results, the method is applied to generate an atmospheric boundary layer, which is used as inflow condition for a LES of a uniform rough terrain, specified in agreement with the Eurocode. Results are discussed based on the analysis of one and two points statistics of the flow field. Good agreement between the target values used in the inflow generation, the synthetic field and the LES, highlights the soundness of the proposed approach. A PRFG3 implementation is available for download at https://site.unibo.it/cwe-lamc/en.

The school of the turbulent swirling flow at the Faculty of Mechanical Engineering University of Belgrade

This review paper provides data about research activities at the School of the turbulent swirling flow at the Faculty of Mechanical Engineering, University of Belgrade, conducted in the period 1941 up to date. An overview is provided of the main directions in this research area. First papers dealt with the turbulent swirling flow in hydraulic turbines to be continued by the experimental and analytical approaches on the axial fans pressure side. The complexity of 3-D, non-homogeneous, anisotropic turbulent velocity fields required complex experimental and theoretical approach, associated with the complex numerical procedures. Analytical approaches, complex statistical analyses and experimental methods and afterwards CFD employed in the research are presented in this paper. The 150 scientific papers, numerous diploma works, several master of science (magister) theses, six Ph. D. theses and two in progress, 40 researchers, national and international projects are the facts about the School. Scientific references are chronologically presented. Numerous abstracts from scientific conferences, presentations, projects with industry and lectures are not given here.

AN EMPIRICAL RELATION BETWEEN THE LARGE-SCALE MAGNETIC FIELD AND THE DYNAMICAL MASS IN GALAXIES

The origin and evolution of cosmic magnetic fields as well as the influence of the magnetic fields on the evolution of galaxies are unknown. Though not without challenges, the dynamo theory can explain the large-scale coherent magnetic fields which govern galaxies, but observational evidence for the theory is so far very scarce. Putting together the available data of non-interacting, non-cluster galaxies with known large-scale magnetic fields, we find a tight correlation between the integrated polarized flux density, S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost linear correlation between the large-scale magnetic field B and v(rot), assuming that the number of cosmic ray electrons is proportional to the star formation rate, and a super-linear correlation assuming equipartition between magnetic fields and cosmic rays. This correlation cannot be attributed to an active linear alpha-Omega dynamo, as no correlation holds with global shear or angular speed. It indicates instead a coupling between the large-scale magnetic field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster rotating and/or more massive galaxies have stronger large-scale magnetic fields. The observed B-v(rot) correlation shows that the anisotropic turbulent magnetic field dominates B in fast rotating galaxies as the turbulent magnetic field, coupled with gas, is enhanced and ordered due to the strong gas compression and/or local shear in these systems. This study supports an stationary condition for the large-scale magnetic field as long as the dynamical mass of galaxies is constant.

Depolarization of synchrotron radiation in a multilayer magneto-ionic medium

Depolarization of diffuse radio synchrotron emission is classified in terms of wavelength-independent and wavelength-dependent depolarization in the context of regular magnetic fields and of both isotropic and anisotropic turbulent magnetic fields. Previous analytical formulas for depolarization due to differential Faraday rotation are extended to include internal Faraday dispersion concomitantly, for a multilayer synchrotron emitting and Faraday rotating magneto-ionic medium. In particular, depolarization equations for a two- and three-layer system (disk-halo, halo-disk-halo) are explicitly derived. To both serve as a `user's guide' to the theoretical machinery and as an approach for disentangling line-of-sight depolarization contributions in face-on galaxies, the analytical framework is applied to data from a small region in the face-on grand-design spiral galaxy M51. The effectiveness of the multiwavelength observations in constraining the pool of physical depolarization scenarios is illustrated for a two- and three-layer model along with a Faraday screen system for an observationally motivated magnetic field configuration.