Intensity Of Turbulent Fluctuations Research Articles

Weak magnetohydrodynamic turbulence theory revisited

Two recent papers, P. H. Yoon and G. Choe, Phys. Plasmas 28, 082306 (2021) and Yoon et al., Phys. Plasmas 29, 112303 (2022), utilized in the derivation of the kinetic equation for the intensity of turbulent fluctuations the assumption that the wave spectra are isotropic, that is, the ensemble-averaged magnetic field tensorial fluctuation intensity is given by the isotropic diagonal form, ⟨δBiδBj⟩k=⟨δB2⟩kδij. However, it is more appropriate to describe the incompressible magnetohydrodynamic turbulence involving shear Alfvénic waves by modeling the turbulence spectrum as being anisotropic. That is, the tensorial fluctuation intensity should be different in diagonal elements across and along the direction of the wave vector, ⟨δBiδBj⟩k=12 ⟨δB⊥2⟩k(δij−kikj/k2)+⟨δB∥2⟩k(kikj/k2). In the present paper, we thus reformulate the weak magnetohydrodynamic turbulence theory under the assumption of anisotropy and work out the form of nonlinear wave kinetic equation.

An experimental study of coherent swirling vortex rings in a turbulent pulsed jet

This experimental study examines the coherent swirling vortex rings in a turbulent pulsed jet at a Reynolds number Re=1.0×104 using planar particle image velocimetry. The coherent vortex rings are produced by a stepper-motor-driven piston successively injecting fluid into a background jet flow. The swirl is generated by axial swirlers which are made by three-dimensional printing technology and mounted behind the nozzle. The swirl number S generated in the experiments ranges from 0 to 1.10, which covers the critical swirl number of a swirling jet. The stroke-to-nozzle diameter ratio L/d=1.56 produces compact swirling vortex rings in a single pulsed jet without a background jet flow. Two pulsing frequencies (St=0.16 and 0.32) that generate coherent vortex rings are investigated, with the free jet also measured at the same Re for comparison. The combination of vortex rings and swirling flows is found to enhance the turbulent fluctuation intensity; this effect is positively related to the compactness of the vortex rings (St) and central recirculation zone (S). At the early stage, the generated swirling vortex ring develops until it reaches its limit, which decreases as S increases but increases with St. The axial distance at which this limit is achieved decreases with both increasing S and St. Then, the ring decays and is eventually dissipated at a decay rate that grows larger in proportion to S and St. Moreover, the swirling vortex rings evolve at their corresponding constant axial and radial velocities in the near field. The axial velocity decreases as S increases and increases with St, while the radial velocity increases with S and is independent of St. In addition, the axial distance between two adjacent swirling vortex rings decreases with increased S and St. Furthermore, the temporal variation of the axial distance also decreases at higher S and St, indicating that the axial propagation velocity decreases downstream. Finally, detailed structures of the swirling vortex ring cores and the corresponding structures induced by the rolling-up process are presented.

Study of the Air‐Sea Momentum Flux of the Coastal Marine Boundary Layer During Typhoons

AbstractWe analyzed the variations in the mean wind field, turbulence, and turbulent flux of three typhoons using observational data obtained from an offshore monitoring platform. The turbulent fluctuation intensity and friction velocity are found to increase with wind speed both before and after landfall with two different relationships as two distinct branches on the turbulence‐wind diagram. Turbulent fluctuation intensity is found to decrease with increasing wind speed during landfall because of the transition of land‐based turbulence to sea‐based turbulence. The relationships between the friction velocity and drag coefficient and the wind speed were similarly affected by whether the typhoon makes landfall, and the relative position of the landfall site of the typhoon to the observation site.

Floating particles mixing characteristics in an eccentric stirred tank coupled with dislocated fractal impellers

Abstract The mixing characteristics of floating particle dispersion process in an eccentric stirred tank with dislocated fractal impellers were investigated using computational fluid dynamics (CFD) and experimental analyses. Solid concentration distribution, axial solid concentration profile, cloud height, solid integrated velocity, power consumption and just drawdown speed were investigated. Results showed that dislocated fractal impeller can enhance solid integrated velocity and fluid turbulent fluctuation intensity compared with dislocated pitched blade impeller, and eccentric agitation coupled with dislocated fractal impeller could destroy the typical circulation loops and symmetric flow field and improve the axial circulation efficiency of floating particles on the basis of dislocated fractal impeller. Eccentric agitation coupled with dislocated fractal impeller could further enhance the floating particle dispersion homogeneity (MI) and decrease the just drawdown speed (N jd) on the basis of dislocated fractal impeller and dislocated pitched blade impeller at the specific power consumption. Meanwhile, eccentric ratio of 0.3 or 0.4 was optimal for the floating particle mixing process in this work.

Dynamics of coherent vortex rings in a successively generated turbulent pulsed jet

This work focuses on the dynamics of coherent vortex rings in a successively generated turbulent pulsed jet at a Reynolds number (Re) of 1.0×104 using planar particle image velocimetry (PIV). In the experiments, the vortex rings are issued into the free jet mainstream by the reciprocation of a stepper-motor driven piston. The stroke-to-nozzle diameter ratio L/d is 1.56, which produces compact vortex rings without a trailing jet in single pulsed jet flow without a free-jet mainstream. The flow fields generated by four pulsing frequencies are investigated, and the free jet at the same Re is also measured for comparison; the maximum dimensionless pulsing frequency (St) is 0.32. The effect of enhancement of turbulent fluctuation intensity (turbulence enhancement) is found to be closely related to the generation and structure of the vortex rings in a flow field. The compactness of the vortex rings is positively related to St, as more compact vortex rings exhibit stronger entrainment and turbulence enhancement. The generated vortex rings convect at a speed of 5St∙U0 in the near field (x/d<7), and then convect at a slightly decreased speed downstream. In addition, the distance between two adjacent vortex rings decreases with increased St. Finally, the spatial dynamic mode decomposition (DMD) results show that the decay rate and the propagation time for an entire flow field subjected to vortex ring generation decrease almost linearly with increased St. This indicates that vortex ring generation increases the propagation time and decay rate of these types of flow fields.

Effects of turbulence fluctuation intensity in bioreactor of sewage treatment on physical and chemical properties of biofilms.

The flow velocity plays an important role in the growth and characteristics of biofilm in the bioreactor as well as its treatment efficiency, which has been a hot research topic. In a sewage treatment bioreactor, the type of flow is usually turbulence. According to the methods of Reynolds decomposition, the flow velocity of turbulence consists of the time-mean velocity and fluctuation velocity, which usually exist simultaneously in real flow. However, the current research on the influence of turbulence flow velocity mainly focuses on the time-mean velocity, while the fluctuation velocity has not been reported because of very difficult to control. To this end, in this paper, a laboratory oscillating-grid turbulence (OGT) bioreactor with zero time-mean velocity and only fluctuation velocity was designed. In this bioreactor, the fluctuation velocity could be easily manipulated by varying the operational parameters of the grid. Based on the numerical simulation of Gas-liquid two-phase flow, the distributions of fluctuation velocity and corresponding turbulence fluctuation intensity, gas holdup, and Reynolds stress were obtained. After that, the effects of the turbulent fluctuation intensity on the biofilm thickness, density, and composition of extracellular polymeric substances (EPS) were studied experimentally. The results showed that turbulent fluctuation had a significant effect on the physical and chemical properties of biofilms, and the fluctuation velocity promoted the increase in the biofilm density and the content of protein and carbohydrates in EPS. This study was intended to provide theoretical support for the design and operation optimization of bioreactors.

Studies of combustion effects on near-wall turbulence in supersonic flows by large eddy simulation

In scramjet combustors, combustion occurring inside supersonic boundary layers can affect the near-wall turbulence, which can further reduce wall skin friction substantially. To reveal the mechanisms of the skin-friction reduction by boundary-layer combustion, high-accuracy large eddy simulations (LES) of the Burrows-Kurkov supersonic reacting wall-jet experiment are conducted in the present research. The LES results are then employed to investigate combustion effects on characteristics of near-wall turbulence, including flow structures, intensity of turbulent fluctuations, wall skin friction and heat flux, et al. The studies show that ignition generates turbulent kinetic energy and Reynolds shear stress dramatically but they will be depressed quickly by heat release at the downstream stable combustion region. The combustion chemical reactions affect the characteristics of the near-wall turbulence to further influence wall skin friction and heat flux. Due to volumetric expansion and turbulence-generation effects of ignition, distributions of wall skin friction at the ignition region exhibit the feature of a sudden decrease following by a drastic increase. Furthermore, the changes of wall skin friction and heat flux at the ignition region and downstream stable combustion region are mainly due to the changes of wall normal gradients of mean velocity and temperature. It is also found that ignition is a dominant factor for combustion effects on near-wall turbulence, which was generally ignored in the previous studies. Therefore, ignition reaction should be taken into consideration seriously in future application of the skin-friction reduction techniques by boundary-layer combustion.

КАВИТАЦИЯ НА РИФЛЕНОМ ДВУМЕРНОМ ГИДРОКРЫЛЕ ПРИ МАЛОМ УГЛЕ АТАКИ

Кавитация представляет собой один из основных источников неустойчивостей течения, возникающих при эксплуатации гидравлического оборудования, а также является причиной эрозионного износа его рабочих элементов. В этой связи разработка и развитие различных методов управления кавитационными течениями является актуальной задачей для задержки развития кавитации и снижения ее негативного влияния. Одним из таких методов является модификация поверхности гидрокрыла. Цель работы: экспериментальное исследование кавитационного обтекания рифленого гидрокрыла с продольными бороздками полукруглого профиля на поверхности (НЛ2), представляющего собой уменьшенную модель направляющей лопатки высоконапорной гидротурбины, в сравнении с моделью оригинальной направляющей лопатки (НЛ1). Методы. Для анализа пространственной структуры и временной эволюции паровых каверн и оценки их интегральных характеристик была применена высокоскоростная визуализация. Скорость течения над гидрокрыльями и в их следе измерялась с помощью метода PIV, на основе измеренных ансамблей полей мгновенной скорости были получены распределения средних и турбулентных характеристик течения. Результаты. На лопатке с модифицированной поверхностью (НЛ2) кавитация зарождается в виде одиночных изолированных пузырей в канавках, которые при уменьшении числа кавитации переходят в кавитирующие стрики. Пока стрики локализованы в канавках и не взаимодействуют друг с другом, режим обтекания остается стационарным. Однако когда их размер становится больше диаметра желобков, они выходят за пределы этих углублений, взаимодействуют и образуют единую каверну, которая теряет устойчивость и начинает пульсировать. В целом бороздки на поверхности гидрокрыла позволяют в некоторой степени задержать развитие кавитации и воспрепятствовать переходу к нестационарным режимам обтекания. На нестационарном режиме динамика каверн на оригинальном гидрокрыле (НЛ1) и НЛ2 сильно отличается. Так, на НЛ1 в отличие от НЛ2 каверна никогда не исчезает полностью, каверна на НЛ1 в среднем оказывается длиннее и пульсирует с большей частотой (St=0,09) по сравнению с НЛ2 (St=0,06). Кроме того, поведение каверны в течение одного периода пульсаций оказалось довольно необычным для обеих моделей: сначала она увеличивается до максимального размера, затем несколько уменьшается и снова вырастает до максимума, после чего возвращается в исходное состояние. Причина такой динамики пока остается невыясненной. На переходном режиме обтекания, когда внутри бороздок формируются кавитирующие стрики, интенсивность турбулентных флуктуаций скорости над поверхностью НЛ2 снижается по сравнению с режимом пузырьковой кавитации. Это происходит потому, что изолированные каверны в желобках как бы восстанавливают форму модифицированного гидрокрыла, делая геометрию его поверхности более приближенной к оригинальной (НЛ1). Таким образом, профиль НЛ2 становится более заполненным благодаря локальной кавитации в бороздках. Вместе с тем наличие канавок на поверхности НЛ2 приводит к дополнительной турбулизации течения вблизи поверхности лопатки для всех рассматриваемых режимов течения, что, вероятно, и является причиной задержки развития кавитации на гидрокрыле с бороздками.

Precessing spherical shells: flows, dissipation, dynamo and the lunar core

SUMMARYPrecession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet’s spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies. We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence and magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations. We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations. We extend previous precession dynamo studies towards lower viscosities, at the limits of today’s computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precession-driven dynamo in a spherical core. But this question remains open as planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity. Moreover, the fully turbulent dissipation regime has not yet been reached in simulations. Our results suggest that the melted lunar core has been in a turbulent state throughout its history. Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.

Do magnetic fields enhance turbulence at low magnetic Reynolds number?

Imposing a magnetic field on a turbulent flow of electrically conducting fluid incurs the Joule effect. A current paradigm is that the corresponding dissipation increases with the intensity of the magnetic field, and as a result turbulent fluctuations are all the more damped as the magnetic field is strong. While this idea finds apparent support in the phenomenology of decaying turbulence, measurements of turbulence in duct flows and other, more complex configurations have produced seemingly contradicting results. The root of the controversy is that magnetic fields promote sufficient scale-dependent anisotropy to profoundly reorganise the structure of turbulence, so their net effect cannot be understood in terms of the additional dissipation only. Here we show that when turbulence is forced in a magnetic field that acts on turbulence itself rather than on the mechanisms that generate it, the field promotes large, nearly 2D structures capturing sufficient energy to offset the loss due to Joule dissipation, with the net effect of increasing the intensity of turbulent fluctuations. This change of paradigm potentially carries important consequences for systems as diverse as the liquid cores of planets, accretion disks and a wide range of metallurgical and nuclear engineering applications.

A DNS study of aerosol and small-scale cloud turbulence interaction

Abstract. The purpose of this study is to investigate the interaction between small-scale turbulence and aerosol and cloud microphysical properties using direct numerical simulations (DNS). We consider the domain located at the height of about 2000 m from the sea level, experiencing transient high supersaturation due to atmospheric fluctuations of temperature and humidity. To study the effect of total number of particles (Ntot) on air temperature, activation and supersaturation, we vary Ntot. To investigate the effect of aerosol dynamics on small-scale turbulence and vertical air motion, we vary the intensity of turbulent fluctuations and the buoyant force. We find that even a small number of aerosol particles (55.5 cm−3), and therefore a small droplet number concentration, strongly affects the air temperature due to release of latent heat. The system comes to an equilibrium faster and the relative number of activated particles appears to be smaller for larger Ntot. We conclude that aerosol particles strongly affect the air motion. In a case of updraught coursed by buoyant force, the presence of aerosol particles results in acceleration of air motion in vertical direction and increase of turbulent fluctuations.

Comparison of Direct and Spectral Methods for Evaluation of the Temperature Structure Parameter in Numerically Simulated Convective Boundary Layer Flows

Abstract In many engineering and meteorological applications, atmospheric turbulence within the planetary boundary layer is described in terms of its representative parameters. One such parameter is the structure-function (or structure) parameter that is used to characterize the intensity of turbulent fluctuations of atmospheric flow variables. Structure parameters are derivatives of structure functions, but are used more frequently than the latter ones for practical needs as they do not explicitly include dependence on the separation distance. The structure parameter of potential temperature, which is the subject of this study, describes the spatial variability of the temperature fluctuations. It is broadly represented in theories and models of electromagnetic and acoustic wave propagation in the atmosphere, and forms the basis for the scintillometer measurement concept. The authors consider three methods to compute the potential temperature structure function and structure parameter: the direct method, the true spectral method, and the conventional spectral method. Each method is tested on high-resolution potential temperature datasets generated from large-eddy simulations of a variety of convective boundary layer flow cases reproduced by two representative numerical codes. Results indicate that the popular conventional spectral method routinely exaggerates the potential temperature structure-function parameter, likely due to the unrealistic assumptions underlying the method. The direct method and true spectral method are recommended as the more suitable approaches.

Effect of surface roughness element on near wall turbulence with zero-pressure gradient

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

Wind tunnel investigation of sound attenuation in turbulent flow

Wind tunnel investigation of the sound wave attenuation by grid-generated turbulence is performed. The most influential parameters, such as the propagation distance, intensity of turbulent fluctuations and integral scale of the fluctuations are studied using an ultrasonic technique. The results are compared to the theoretical predictions available on the wave statistics. Theoretical predictions are well confirmed and partly extended. It is demonstrated that the ultrasonic technique provides the possibility of reproducing the main effects of atmospheric turbulence on sound propagation while benefiting from isolating the role of various parameters therefore sets of experimental data can be generated under laboratory conditions to benchmark further extensions of theoretical models and numerical simulations.

In Situ Monitoring of Dispersion Dynamics of Carbon Nanotubes during Sonication Using Electrical Conductivity Measurements

The main challenge in the fabrication of carbon nanotube‐ (CNT‐) based composite materials is the optimization of the sonication time in order to obtain homogenous and uniform dispersion of CNTs. Past studies mostly relied on postprocessing characterization techniques to address this issue. In the present, however, in situ monitoring of dispersion dynamics of CNTs in distilled water is carried out using instantaneous conductivity measurements. Using a computer controlled data acquisition system, the time evolution of the solution conductivity was carefully recorded. The data were then used to evaluate the intensity of turbulent fluctuations, which clearly highlighted the existence of three distinct sonication phases. During the first phase, the conductivity fluctuations initially increased attaining ultimately a maximum, thus indicating the occurrence of large agglomerates of CNTs. During the second phase of sonication, the solution conductivity showed a rather steep increase while fluctuations steadily declined. This phenomenon can be attributed to the breakdown of large CNT agglomerates, resulting in greater dispersion homogeneity of CNTs. During the third phase, after almost 650 kJ/L of sonication energy, the conductivity increase was almost negligible. The fluctuation intensity also remained constant during this phase signifying that the further sonication was no longer required.

Passive Control of Near-wall Turbulence By Means of Roughness Elements

The present work experimentally investigates the effect of roughness elements onto zero-pressure-gradient turbulent boundary layer. We use an array of spanwisely-aligned cylindrical roughness elements attached on the wall surface to regulate the near-wall low-speed streaky structures. With both qualitative visualization and quantitative measurement, it is found that the regularization only occurs in the region below the height of the roughness element, where the local streamwise turbulent fluctuation intensity can be reduced with about 10%. Moreover, it is also found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence, there is no streak-regularization effect in the buffer region, so that the near-wall streamwise turbulent fluctuation intensity doesn’t reduce.

Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.

Reconstruction of time-averaged temperature of non-axisymmetric turbulent unconfined sooting flame by inverse radiation analysis

A multi-wavelength inversion method is extended to reconstruct the time-averaged temperature distribution in non-axisymmetric turbulent unconfined sooting flame by the multi-wavelength measured data of low time-resolution outgoing emission and transmission radiation intensities. Gaussian, β and uniform distribution probability density functions (PDF) are used to simulate the turbulent fluctuation of temperature, respectively. The reconstruction of time-averaged temperature consists of three steps. First, the time-averaged spectral absorption coefficient is retrieved from the time-averaged transmissivity data by an algebraic reconstruction technique. Then, the time-averaged blackbody spectral radiation intensity is estimated from the outgoing spectral emission radiation intensities. Finally, the time-averaged temperature is approximately reconstructed from the multi-wavelength time-averaged spectral emission radiation data by the least-squares method. Noisy input data have been used to test the performance of the proposed inversion method. The results show that the time-averaged temperature distribution can be estimated with good accuracy, even with noisy input data. The accuracy of the estimation decreases with the increase of turbulent fluctuation intensity of temperature and the effects of assumed PDF on the reconstruction of temperature are small.

Methodic Errors in Laser Measurements of Euler Statistical Moments of a Nonuniform Turbulent Velocity Field

Probability distributions of the velocity obtained by means of laser Doppler spectroscopy with different ratios between the dimensions of the probe volume and the scales of the nonuniformity are studied using a model of a locally normal, spatially nonuniform turbulent field. Sample calculations of the measured probability distributions of the velocity and the statistical moments of these distributions up to the fourth order are presented for model examples that take into account the nonuniformity of the mean flow velocity and the intensity of turbulent fluctuations of the velocity. It is shown that the substantial deviations in the velocity distributions from the normal law which are observed in a number of experiments may be related to noncompliance with the locality condition.

Turbulence intensity in dilute two-phase flows—1 Effect of particle-size distribution on the turbulence of the carrier fluid

We deal with the effect of particle size and concentration on the intensity of turbulent fluctuations in two-phase flows. An interaction between polydisperse particles and the turbulence of the carrier fluid is considered. The theoretical analysis of that process is based on Prandtl's mixing-length theory, modified to account for the peculiarities of the viscous interaction of small particles and carrier fluid as well as an effect of the admixture inertia. For a particle-laden flow with a bidisperse particle-size distribution, the intensity of turbulent fluctuations of the particles and fluid is determined as a function of the particles sizes and their mass contents. It is shown that at a fixed total mass content of the admixture, the turbulence intensity in polydisperse flow is essentially different from that in monodisperse flow. Some results on the turbulence properties of a turbulent particle-laden jet with a polydisperse admixture are obtained.