Effect Of Turbulent Fluctuations Research Articles

Experimental and numerical investigation on the bubble breakup and coalescence characteristics in the highly efficient static mixers

CFD-PBM and experiment methods were applied to study the bubble dispersion characteristics in the static mixers with three twisted blades (TKSM). The gas-liquid velocity ratio was selected as drag correction factors. The simulation data was compared with experimental values at different superficial gas velocity and the results revealed that the modified model could effectively enhance the calculation accuracy of gas holdup. Furthermore, bubble size distribution (BSD) was well predicted by Luo-Prince model and the mean relative error (MRE) of axial d32 between the prediction of Luo-Prince model and experimental value was 2.86 %. Meanwhile, the influences of bubble collision mechanisms and coalescence correctors on the numerical prediction were quantitatively analyzed. The effects of turbulent fluctuation, viscous shear, wake entrainment and buoyancy driven were considered for the prediction of bubble collision frequency in TKSM. The prediction precision of cumulative probability distribution (CPD) was promoted by 17.26 % when the correctors obtained from Energy-Minimization Multi-Scale (EMMS) method were used. In addition, empirical parameter CDas was evaluated and the results showed that the Das coalescence model could accurately describe the BSD when CDas was 0.01–0.1. The backmixing performance of the gas phase in the TKSM exceeds that of the liquid phase, which is distinct from that in vortex unit.

Effect of Turbulent Fluctuation on Settling Behavior of Dredged Slurry in Sheared Condition

For the land shortage problem upon the city development in coastal areas, it is of great importance to improve the overall understanding of the settlement behavior of dredged slurry and determine the required site water storage capacity as well as the mechanical properties of the newly formed dredged slurry. This paper presents an experimental investigation on the effect of turbulent fluctuation on the settling behavior of dredged slurry in sheared condition via a self-developed testing apparatus namely double cylinder settling column. Test results showed that the settling velocity of the soil-water interface initially increased then decreased with the increasing rotation speed of the inner column. The interface would disappear and a stable suspended layer was observed when the rotation speed was greater than 20 rad/min, where water contents and grain composition were not changed over the depth. The particle content was closer to that of the original sample as the rotation speed kept increasing in the suspended layer. It was found that the stabilized flocs were gradually broken down by the turbulence effect existing in the sheared slurry. Moreover, the stabilized flocs were losing the stability gradually as the flowing velocity increased till 10 rad/min and the floc structure was broken down completely once the flowing velocity was greater than 20 rad/min.

An evaluation of gas-phase micro-mixing models with differential mixing timescales in transported PDF simulations of sooting flame DNS

The use of transported probability density function (TPDF) models to predict soot has the strong advantage that the effects of turbulent fluctuations on soot source terms can be rigorously accounted for. However, soot processes are closely coupled to gas-phase composition. Among the open issues for gas-phase micro-mixing is the species-dependence of mixing timescales. The objective is to carry out an evaluation on the effect of incorporating differential mixing timescales among gas-phase species in a TPDF simulation for soot prediction. A DNS having the configuration of a temporally evolving, non-premixed ethylene flame with a four-step, three-moment soot model is considered as the target for evaluation. The DNS dataset is applied to provide key inputs for TPDF simulations to limit the sources of error to micro-mixing. TPDF simulations with the interaction by exchange with the mean (IEM) and modified Curl (MC) models, which impose the same mixing timescale to all species, underpredict soot mass fraction and overpredict extinction levels regardless of the prescribed mixing frequency. By incorporating differential mixing timescales among gas-phase species, IEM-DD and MC-DD models yield notable improvement in predictions of the overall extinction and soot levels, highlighting the benefit of accounting for differential mixing timescales. A TPDF simulation with the Euclidean minimum spanning tree (EMST) model yields even better predictions, illustrating that the localness in composition space remains a critical issue. The indicated species mixing frequencies by the EMST model are shown to follow the DNS results qualitatively, illustrating that the micro-mixing process based on the Euclidean distance in composition space reproduces to a certain extent the differential mixing timescales due to reaction. Finally, it is shown that incorporating differential mixing timescales of soot moments is expected to have limited value as the mixing timescales of soot moments are sufficiently large to safely neglect soot mixing.

Combined direct numerical simulation and long-wave simulation of a liquid film sheared by a turbulent gas flow in a channel

In this work, the dynamics of a thin liquid film sheared by a turbulent gas flow are investigated numerically. It is known that even a constant interfacial shear stress affects film stability and dynamics. We are interested in the effect of turbulent fluctuations on the film development. A combination of a direct numerical simulation (DNS) of the turbulent gas flow and a long-wave theory for the liquid film evolution is used to study the effect of the turbulent shear stress fluctuations on the liquid film. The simulation is carried out in two steps. First, a DNS of a single-phase turbulent channel flow is conducted. The time-dependent turbulent shear stress at the lower wall is stored. In the second step, the time- and location-dependent turbulent shear stress serves as a boundary condition in a one-sided long-wave simulation of the liquid film to identify the effect of the turbulent gas flow on the film stability and dynamics. The resulting film deformation is simulated for different Reynolds numbers, and an analysis of the film deformation and stability as a function of the turbulent shear stress fluctuations is given. The numerical simulations are accompanied by a simplified linear analysis. The results show that the dynamics of the liquid film sheared by a turbulent gas flow depend not only on the average shear stress at the liquid-gas interface but also on the amplitude as well as the temporal and spatial scales of the shear stress fluctuations.

Effect of turbulent fluctuations on neutral particles transport with the TOKAM3X-EIRENE turbulence code

The effect of turbulent fluctuations on the transport of neutral particles (atoms, molecules) in tokamak plasmas is investigated with the 3D global turbulence code TOKAM3X-EIRENE in limiter geometry. The statistical properties of turbulent fields relevant to this work are discussed, including the recycling flux. The neutral particle transport is recalculated on the mean field plasma, and compared to the mean neutral particle density/flows obtained from the turbulent simulation, so as to assess the effects of the fluctuations, in particular on the ionization balance. The latter effects are remarkably modest in the simulation presented here, but are expected to become more and more pronounced as the high recycling regime is approached, in particular because the plasma temperature becomes low enough so that ionization is strongly non-linear. However, the turbulent fluctuations in the SOL do have a substantial effect on the neutral densities on the low field side of the limiter, including in the confined plasma. These effects are traced back to non-linearities in the plasma flux at the wall, and the fluctuations in the latter are identified as an important contributor both to neutral particle density fluctuations and to deviations from mean neutral particle density/flows in the turbulent simulation with respect to the same quantities recalculated on the mean plasma fields.

A method for reducing mean flow in oscillating-grid turbulence

Oscillating-grid turbulence (OGT) is an experimental tool that has been widely used to study the role of turbulent fluctuations under conditions of small mean flow. We report experiments to investigate the structure of the turbulent flow produced by an oscillating grid, using velocity measurements obtained through the application of two-dimensional particle image velocimetry in the vertical plane through the centre of the grid. Ensemble averages of the fluid velocity measurements at specific stages of the grid’s oscillation indicate that mean flow is induced in OGT by the merging of grid-induced jets close to the tank sidewalls. The installation of an open-ended ‘inner box’ (with its top edge positioned just below the bottom of the grid’s oscillation) is shown to inhibit the merging of the jets, thereby resulting in a reduction in the magnitude of the mean flow within the interior of the inner box region. Measurements of the time-averaged root-mean-square turbulent velocity components and the time-averaged turbulent kinetic energy flux indicate that the installation of the inner box results in turbulence that is in good agreement with the well-established models of OGT across the central 50% of the inner box’s width, but that distinct anisotropic regions exist adjacent to the vertical sidewalls. We anticipate that this simple amendment to reduce the mean flow present in OGT can be readily used in future work that utilises OGT to isolate the effects of turbulent fluctuations from those of the mean flow.Graphical abstract

Two-fluid sub-grid-scale viscosity in nonlinear simulation of ballooning modes in a heliotron device

A large eddy simulation (LES) approach is introduced to enable the study of the nonlinear growth of ballooning modes in a heliotron-type device, by solving fully 3D two-fluid magnetohydrodynamic (MHD) equations numerically over a wide range of parameter space, keeping computational costs as low as possible. A model to substitute the influence of scales smaller than the grid size, at sub-grid scale (SGS), and at the scales larger than it—grid scale (GS)—has been developed for LES. The LESs of two-fluid MHD equations with SGS models have successfully reproduced the growth of the ballooning modes in the GS and nonlinear saturation. The numerical results show the importance of SGS effects on the GS components, or the effects of turbulent fluctuation at small scales in low-wavenumber unstable modes, over the course of the nonlinear saturation process. The results also show the usefulness of the LES approach in studying instability in a heliotron device. It is shown through a parameter survey over many SGS model coefficients that turbulent small-scale components in experiments can contribute to keeping the plasma core pressure from totally collapsing.

Assessment of improved Schnerr-Sauer model in cavitation simulation around a hydrofoil

To enhance the capability of Schnerr-Sauer model in simulating cavitating flows, in this study, we developed an improved Schnerr-Sauer model based on the Rayleigh-Plesset equation and homogeneous flow assumption. The model considers the effects of turbulent fluctuation and noncondensable gas. The unsteady cavitating flow over a two-dimensional Clark-Y hydrofoil was numerically investigated combined with the SST k-ω turbulence model, in which the turbulence eddy viscosity is corrected. We obtain the time-averaged lift and drag coefficients, the cavity shape evolution at cloud cavitation, and its x-velocity profiles from these calculations. Compared with available experimental data in the literature, the time-averaged lift and drag coefficients predicted from the improved Schnerr-Sauer model agree better with the experimental results than with those of the original. In addition, the modified model can accurately predict the characteristics of the time-averaged lift and drag coefficients during different cavitation conditions. Moreover, the improved model can better simulate the inception, growth, shedding, and collapse of cloud cavitation than the original model. The overall results prove the reliability and accuracy of the improved Schnerr-Sauer model in cavitating flow simulations over a hydrofoil.

English

The effect of turbulent fluctuations on the threshold criteria for incipient grain motion over a wide range of sediment size is investigated. In this work, attention is paid to the comparison of the critical Shields parameter θc profile obtained when the near-bed fluid forces induced sediment motion are oscillating-grid turbulence and a single idealised eddy of vortex ring. For experimental work, near-spherical monodisperse sediments were used throughout with relative densities of 1.2 and 2.5 and mean diameters d ranging between 80 and 1087 μm. The measured values of θc on a horizontal bed α = 0 (hence denoted as θc0), were compared to the θc0 profiles obtained by grid turbulence and vortex ring experiments. Although different in magnitude, the θc0 profiles were comparable, that is the θc0 were seen to increase monotonically for hydraulically smooth bedforms and to be approximately constant for hydraulically rough bedforms. However the limit of hydraulically smooth region was found to vary between the oscillating-grid turbulence experiments, where wider smooth region was found when the turbulent fluctuations used to calculate θc0 is not the near-bed velocity.

Effect of Turbulent Fluctuations on Neutral Particle Penetration and Charge Exchange Sputtering

AbstractIn this contribution we present results on the effect of turbulent plasma fluctuations on the transport of atoms and molecules as well as on impurities sputtered due to charge exchange atomic bombardment. These phenomena are investigated using the kinetic Monte‐Carlo particle transport code EIRENE and a statistical model of plasma fluctuations. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical analysis of stochastic dispersion of micro-particles in turbulent flows in a realistic model of human nasal/upper airway

Turbulent flow field and particle deposition in a realistic model of human upper airway system including: nasal cavity, nasopharynx, oropharynx, larynx and trachea were analyzed numerically. The Lagrangian approach was used to find the trajectories of micron-size particles for the breathing rates of 30, 45 and 60l/min. The continuous phase flow was evaluated using a RANS (Reynolds Average Navier-Stokes) turbulence model and the effect of turbulent fluctuations on particle trajectories was modeled using a continuous random walk (CRW) stochastic model, which is based on the normalized Langevin equation. The total and regional deposition fractions of micro-particles were evaluated and the effect of turbulent fluctuations on the particle deposition rate was investigated.

Effects of pressure and inlet temperature on coaxial gaseous methane/liquid oxygen turbulent jet flame under transcritical conditions

This study has investigated numerically turbulent flames of cryogenic oxygen and methane under supercritical pressures relevant to liquid propellant rocket engines. A real-fluid version of the flamelet equations is employed to accommodate simultaneously non-equilibrium chemistry of hydrocarbon fuel and non-ideal thermodynamics in local flame structures while the effect of turbulent fluctuations is accounted for via a presumed probability density functions. The present model reproduced qualitatively well the experimentally observed unique feature of a transcritical flame of coaxial gaseous methane/liquid oxygen injector, which is characterized by sudden flame expansion, abruptly terminated flame tip, and expansion induced flow recirculation. Numerical results reveal that pseudo-boiling phenomena occurred in the transcritical mixing layer between the cryogenic oxygen core and the surrounding hot gas play a crucial role in mixing and combustion processes. It is also found that the transcritical flame structure is drastically affected by elevating the chamber pressure or increasing the oxygen inlet temperature in terms of flame length, sudden expansion angle, and reverse flow strength. Detailed discussions are made for effects of the real-fluid behaviors on the turbulent flame field as well as on the local flame structure in mixture fraction space.

Effect of turbulent fluctuations on the drag and lift forces on a towed sphere and its boundary layer

AbstractThe impact of turbulent fluctuations on the forces exerted by a fluid on a towed spherical particle is investigated by means of high-resolution direct numerical simulations. The measurements are carried out using a novel scheme to integrate the two-way coupling between the particle and the incompressible surrounding fluid flow maintained in a high-Reynolds-number turbulent regime. The main idea consists of combining a Fourier pseudo-spectral method for the fluid with an immersed-boundary technique to impose the no-slip boundary condition on the surface of the particle. This scheme is shown to converge as the power $3/ 2$ of the spatial resolution. This behaviour is explained by the ${L}_{2} $ convergence of the Fourier representation of a velocity field displaying discontinuities of its derivative. Benchmarking of the code is performed by measuring the drag and lift coefficients and the torque-free rotation rate of a spherical particle in various configurations of an upstream-laminar carrier flow. Such studies show a good agreement with experimental and numerical measurements from other groups. A study of the turbulent wake downstream of the sphere is also reported. The mean velocity deficit is shown to behave as the inverse of the distance from the particle, as predicted from classical similarity analysis. This law is reinterpreted in terms of the principle of ‘permanence of large eddies’ that relates infrared asymptotic self-similarity to the law of decay of energy in homogeneous turbulence. The developed method is then used to attack the problem of an upstream flow that is in a developed turbulent regime. It is shown that the average drag force increases as a function of the turbulent intensity and the particle Reynolds number. This increase is significantly larger than predicted by standard drag correlations based on laminar upstream flows. It is found that the relevant parameter is the ratio of the viscous boundary layer thickness to the dissipation scale of the ambient turbulent flow. The drag enhancement can be motivated by the modification of the mean velocity and pressure profile around the sphere by small-scale turbulent fluctuations. It is demonstrated that the variance of the drag force fluctuations can be modelled by means of standard drag correlations. Temporal correlations of the drag and lift forces are also presented.

Effects of Turbulent Fluctuations on Nanoparticle Coagulation in Shear Flows

The effects of fluid turbulence on the coagulation of aerosols are studied quantitatively and qualitatively. Direct numerical simulation data is used to isolate the effect of the small or subgrid-scale (SGS) particle–particle interactions on nanoparticle coagulation in three-dimensional flows. The rate of particle growth is decomposed into the contribution of the large-scales and small-scales interactions. The contribution of the small-scale interactions is presented as a function of time, space, flow dynamics, and coagulation Damkohler number. Results show that small-scale interactions act to both increase and decrease particle growth. The probability density functions (PDFs) of the SGS growth rate exhibit a negative bias, which increases with time and coagulation Damkohler number. Additionally, PDFs conditioned on the Q-criterion suggest that the contribution of the small-scale interactions primarily act to reduce particle growth in regions characterized by fluid rotation.

Effects of turbulent fluctuations with prescribed statistics on passive neutral particle transport in plasmas

This work addresses the transport of neutral particles (atoms, molecules) in magnetized fusion plasmas, in the presence of density fluctuations with given statistics. The latter are described by a multivariate gamma distribution. The geometry is a 2D slab and turbulence is assumed to be statistically homogeneous. The average neutral density and ionization source, which are the quantities relevant for integrated simulations and diagnostic applications, are calculated analytically in the scattering free case. The boundary conditions and the ratio of the turbulence correlation length to the neutral mean free path are identified as the main control parameters in the problem. The non-trivial relationship between the average neutral density and the ionization source is investigated. Monte Carlo calculations including scattering are then presented, and the main trends obtained in the scattering free case are shown to be conserved.

Effect of turbulent fluctuations on the behaviour of fountains in stratified environments

The interaction between a turbulent fountain and its stratified environment was studied. A heavy fluid, cold water, was injected vertically upwards into a linearly stratified medium. The round heavy-fluid jet reaches a maximum height before it begins to fall due to the effect of gravity. Because of the effects of friction and mixing, the vertical momentum and density of the jet fluid decrease as it submerges to an intermediate height of zero buoyancy. At this point, the jet fluid spreads as a horizontal front, intruding into the stratified environment. The degree of fluctuation in the proximity of the injection point was studied under both unrestricted- and restricted-flow configurations at the injection, using two differently sized stainless-steel woven-wire screens at the injection port as flow-restricting means. Using visualization and velocimetry techniques, both maximum and spreading heights were found to decrease with increasing turbulence at the point of injection.

Simulation of Turbulent Reacting Flow in a Swirl Combustor

This article presents a numerical simulation of turbulent reacting flow in a swirl combustor. A new algebraic Reynolds stress model (ASM) is applied to the simulation. To explore the effects of turbulent fluctuation on the time-averaged reaction rate, the explicit algebraic expressions for the second-order-moments of concentration fluctuation are derived. They are utilized for the closure of the time-averaged reaction rate. The calculated results are compared with the measured test data and those predicted by the k − ϵ model. Agreement is achieved between the prediction and the measurement in terms of the gas velocity, temperature, species concentrations, and turbulent fluctuating velocity. The predicted results by the new ASM are improved over those obtained by the k − ϵ model.

Turbulent anti-resistivity and the zonal magnetic field dynamo in drift-ballooning turbulence

The effect of turbulent fluctuations on the zonal (flux surface averaged) magnetic field in drift-ballooning turbulence is shown to be equivalent to a turbulent anti-resistivity. The flux surface average of the convective nonlinearity in Ohm’s law is proportional to the flux surface average of the current. The coefficient of proportionality can be defined as a turbulent resistivity ηturb. The correlation of the flux surface average of the convective nonlinearity with the negative of the flux surface average of the current is nearly 100%. Because the convective nonlinearity is correlated with the negative of the current, and not the current, the turbulent resistivity is negative. The magnitude of ηturb is virtually identical to the magnitude of the collisional resistivity η, but opposite in sign, so that the total resistivity ηtotal=η+ηturb is nearly zero. The effect of the fluctuations is to balance the effect of collisional resistive diffusion. As a result, while the energy in the zonal flow increases to a large value as the fluctuations grow and saturate, the energy in the zonal magnetic field remains very small.

Expanded analogy between Boltzmann kinetic theory of fluids and turbulence

We demonstrate that the effects of turbulent fluctuations have a striking resemblance to those of microscale (thermal) fluctuations in laminar flows, even to higher order in the Knudsen number. This suggests that there may be a good basis for understanding turbulence in terms of Boltzmann kinetic theory. If so, turbulence may be better described in terms of ‘mixing times’ rather than the more classical ‘mixing lengths’. Comparisons are made to Reynolds-stress turbulence models.

Aerosol deposition in periodic shock waves

The paper considers deposition of small aerosol particles inside a closed tube filled with a gas at resonance, when the flow is accompanied by periodic shock waves traveling back and forth along the tube. It is found that resonance gas oscillations cause vigorous aerosol deposition leading to a rapid concentration decrease. We study the effect of the Saffman lift and thermophoretic forces on particle trajectory in the boundary layer near the tube wall. The thermophoretic force arises due to gas heating by periodic shock waves, while the lift force is caused by the interaction of the shock with the viscous boundary layer. Both forces move particles toward the wall causing their deposition. The effect of turbulence on aerosol deposition is also considered. It is found that for particles less than 1 μm in diameter, the effect of turbulent fluctuations on their deposition is dominant, while for larger particles, the Saffman lift force prevails. The effect of the thermophoretic force is found to be slight. It is also found that at resonance the Saffman lift force causes instability in motion of large particles resulting in their rapid deposition. Our experiments of aerosol deposition in a resonance tube compare favorably with the results of the numerical study.