Forced Turbulence Research Articles

Hydrodynamic Study of an Ebullated-bed Reactor in the H-oil Process

The effectiveness and performance of industrial hydro-processing ebullated bed reactors (EBRs) are highly dependent on the bed hydrodynamics and operating conditions. Hydrodynamics of ebullated bed reactors was studied in a cold model experimental setup. The results of a dynamic similarity test showed that the experimental data could be applied in the study of a large scale unit (Refinery of Lukoil, Burgas, Bulgaria) within a reasonable accuracy. Air and magnesium sulfate 20 wt% (MgSO4 + H2O) solution and solid catalyst particles were used as the gas, liquid and solid phases, respectively. For the design of experiments in the lab-scale cold-flow column, factorial method was introduced to study the influence of operating variables on the individual holdups and bubble characteristics. Pressure gradient method was used to estimate the individual holdups and bed porosity along the column, while photographic method was utilized to obtain images of the moving gas bubble. The images were analyzed using Ai Adobe illustrator CC (64 Bit) software to determine the bubbles geometric characteristics. Large gas bubbles were broken to smaller ones due to the increased turbulent intensity and shear forces at higher liquid velocities, reducing mass transfer resistances. Empirical correlations were developed for prediction of phase holdups and bed porosity with high accuracy. The results showed that liquid internal reflux ratio, which characterized the ebullated bed reactors has a predominant effect on the individual holdups and bubble size. A good agreement was observed between the results and available data in the literature.

Spatial Location of Regional Resources within the Strategic Drift of Globalization: Problems, Tendencies, Procedures

From the beginning of the XXI century between the processes of regional systems globalization and location development resources bigger contradictions can be seen. Balanced spatial development of territories is in ideal concepts and models for economics and society formation. But their implementation is under the influence of risks and threads. The uncial properties of regional systems are underestimated and their resource potential is used not reasonably. Thereby the strategic drift of globalization itself becomes not the fillip, but thread of perspective regional systems development. This investigation is dedicated to grounding ways to overcome its negative influence. Within this investigation we formulate a problem of global order tendencies analyses; development of new management tasks, including the formats of institutional norms, transforming the process of reproduction; forming of adaptive space reorganization procedure considering the lack of localized resources factor. Scientific novelty of the investigation is on optimization of the actual methods and tools for strategic management the territories and grounding the perspectives regional systems polistructural modelling. This allows to reduce strategic drift of globalization risks, modify regional economics evolution driving force and decrease human resources investigation process turbulence, assets, plots of land, preserve the integrality of the formed space and minimize its disbalance.

Inter-scale transfer of the large-scale background anisotropy under periodic conditions

In numerical simulations of three-dimensional isotropy turbulence, low-wave-number truncation leads to large-scale anisotropy, but little anisotropy is generated at small scales. As the generally accepted energy cascade theory suggests an energy transfer from large to small scale, we phenomenally guess that an inter-scale anisotropy evolution might occur in the energy transfer process. With analyses in Kolmogorov’s equation and DNS validations, we demonstrate that the large-scale anisotropy is effectively transferred by convective effect, but this anisotropy is then diminished at small scales by viscous dissipation. The inter-scale anisotropy transfer process is synchronised with the inter-scale energy transfer. Time evolution of anisotropy distribution is then investigated in free-decaying, forced and non-equilibrium isotropic turbulence. It is shown that the anisotropy distribution remains stationary if the shape of energy spectrum does not change. The objective of this work is to provide a theoretical and numerical support for the further study on anisotropy correction in post-processing results.

Numerical simulation of heat transfer improvement in the divertor of fusion reactors by using Al2O3 nanofluid

The objective of this work is to study the effect of using nanofluids as coolant on divertor system of fusion reactors which is known to be subjected to high heat loads coming from the plasma. Turbulent force convective heat transfer of water-based Al2O3 nanofluid flowing through the CuCrZr cooling tube of a small scale of mock up made of five tungsten monoblocks has been numerically investigated using single phase model. Computational fluid dynamic approach has been applied by using CFD software FLUENT 6.3.26. The computed results have been validated by traditional corrections expression reported by previous works. The dependence of temperature contours and profiles on volume fraction of nanofluids for different walls of this monoblock has been studied and compared with pure water. The maximum allowed temperature of the candidate material under unusual situation of a fusion reactor has been considered and compared with the maximum temperatures resulted from the CFD results. The effects of various nanofluid concentrations and Reynolds numbers on average Nusselt number have been also investigated. The results show a significant improvement in heat removal from the divertor under the cooling of alumina/water nanofluid with respect to pure water.

Calculating turbulent transport tensors by averaging single-plume dynamics and application to dynamos

ABSTRACT Transport coefficients in turbulence are comprised of correlation functions between turbulent fluctuations and efficient methods to calculate them are desirable. For example, in mean-field dynamo theories used to model the growth of large-scale magnetic fields of stars and galaxies, the turbulent electromotive force is commonly approximated by a series of tensor products of turbulent transport coefficients with successively higher order spatial derivatives of the mean magnetic field. One ingredient of standard models is the kinematic coefficient of the zeroth-order term, namely the averaged kinetic pseudo-tensor $\boldsymbol \alpha$, that converts toroidal to poloidal fields. Here we demonstrate an efficient way to calculate this quantity for rotating stratified turbulence, whereby the pre-averaged quantity is calculated for the motion of a single plume, and the average is then taken over an ensemble of plumes of different orientations. We calculate the plume dynamics in the most convenient frame, before transforming back to the lab frame and averaging. Our concise configuration space calculation gives essentially identical results to previous lengthier approaches. The present application exemplifies what is a broadly applicable method.

On the influence of forced homogeneous-isotropic turbulence on the settling and clustering of finite-size particles

We investigate the motion of heavy particles with a diameter of several multiples of the Kolmogorov length scale in the presence of forced turbulence and gravity, resorting to interface-resolved direct numerical simulation based on an immersed boundary method. The values of the particles’ relative density (1.5) and of the Galileo number (180) are such that strong wake-induced particle clustering would occur in the absence of turbulence (Uhlmann and Doychev in J Fluid Mech 752:310–348, 2014. https://doi.org/10.1017/jfm.2014.330 ). The forced turbulence in the two present cases (with Taylor-scale Reynolds number 95 and 140) would lead to mild levels of clustering in the absence of gravity (Uhlmann and Chouippe in J Fluid Mech 812:991–1023, 2017. https://doi.org/10.1017/jfm.2016.826 ). Here we detect a tendency to cluster with an intensity (quantified via the standard deviation of the distribution of Voronoi cell volumes) which is intermediate between these two limiting cases, meaning that forced background turbulence decreases the level of clustering otherwise observed under ambient settling. However, the clustering strength does not monotonically decay with the relative turbulence intensity. Various mechanisms by which coherent structures can affect particle motion are discussed. It is argued that the reduced interaction time due to particle settling through the surrounding eddy (crossing trajectories) has the effect of shifting upward the range of eddies with a time-scale matching the characteristic time-scale of the particle. In the present cases this shift might bring the particles into resonance with the energetic eddies of the turbulent spectrum. Concerning the average particle settling velocity, we find very small deviations (of the order of one percent) from the value obtained for an isolated particle in ambient fluid when defining the relative velocity as an apparent slip velocity (i.e., as the difference between the averages computed separately for the velocities of each phase). This is consistent with simple estimates of the nonlinear drag effect. However, the relative velocity based upon the fluid velocity seen by each particle (computed via local averaging over a particle-attached sphere) has on average a smaller magnitude (by 5–7%) than the ambient single-particle value.

Numerical analysis for interphase forces of gas-liquid flow in a multiphase pump

PurposeThe modeling of interphase forces plays a significant role in the numerical simulation of gas–liquid flow in a rotodynamic multiphase pump, which deserves detailed study.Design/methodology/approachNumerical analysis is conducted to estimate the influence of interphase forces, including drag force, lift force, virtual mass force, wall lubrication force and turbulent dispersion force.FindingsThe results show that the magnitude of the interphase forces can be sorted by: drag force > virtual mass force > lift force > turbulent dispersion force > wall lubrication force. The relations between interphase forces and velocity difference of gas–liquid flow and also the interphase forces and gas volume fraction are revealed. The distribution characteristics of interphase forces in the passages from impeller inlet to diffuser outlet are illustrated and analyzed. According to the results, apart from the drag force, the virtual mass force, lift force and turbulent dispersion force are required, whereas wall lubrication force can be neglected for numerical simulation of gas–liquid flow in a rotodynamic multiphase pump. Compared with the conventional numerical method which considers drag force only, the relative errors of predicted pressure rise and efficiency based on the proposed numerical method in account of four major forces can be reduced by 4.95 per cent and 3.00 per cent, respectively.Originality valueThe numerical analysis reveals the magnitude and distribution of interphase forces inside multiphase pump, which is meaningful for the simulation and design of multiphase pump.

Extremes, intermittency, and time directionality of atmospheric turbulence at the crossover from production to inertial scales

The effects of mechanical generation of turbulent kinetic energy and buoyancy forces on the statistics of air temperature and velocity increments are experimentally investigated at the cross over from production to inertial range scales. The ratio of an approximated mechanical to buoyant production (or destruction) of turbulent kinetic energy can be used to form a dimensionless stability parameter $\zeta$ that classifies the state of the atmosphere as common in many atmospheric surface layer studies. Here, we assess how $\zeta$ affects the scale-wise evolution of the probability of extreme air temperature excursions, their asymmetry and time reversibility. The analysis makes use of high frequency velocity and air temperature time series measurements collected at $z$=5 m above a grass surface at very large Reynolds numbers $Re=u_* z/\nu > 1\times 10^5$ ($u_*$ is the friction velocity and $\nu$ is the kinematic viscosity of air). Using conventional higher-order structure functions, temperature exhibits larger intermittency and wider multifractality when compared to the longitudinal velocity component, consistent with laboratory studies and simulations conducted at lower $Re$. Moreover, deviations from the classical Kolmogorov scaling for the longitudinal velocity are shown to be reasonably described by the She-Leveque vortex filament model that has no 'tunable' parameters and is independent of $\zeta$. The work demonstrates that external boundary conditions, and in particular the magnitude and sign of the sensible heat flux, have a significant impact on temperature advection-diffusion dynamics within the inertial range. In particular, atmospheric stability affects both the buildup of intermittency and the persistent asymmetry and time irreversibility observed in the first two decades of inertial sub-range scales.

Electromotive force in strongly compressible magnetohydrodynamic turbulence

Fully compressible magnetohydrodynamic (MHD) turbulence is investigated in the framework of the multiple-scale direct-interaction approximation. With the aid of the propagators (correlation and Green’s functions), fluctuating fields are solved, and turbulent correlations are estimated in highly compressible turbulence. We focus on the expression of the turbulent electromotive force (EMF). Obliqueness between the mean magnetic field and the mean-density gradient, the mean internal density gradient and the non-equilibrium mean velocity contributes to the EMF in the presence of the density variance, which is ubiquitous in turbulence in strongly variable density flows such as the shock-front region. This density-variance effect is expected to locally enhance the turbulence intensity across the shock front, leading to a fast reconnection.

Modelling the vertical migration of different-sized Microcystis colonies: coupling turbulent mixing and buoyancy regulation.

Exceptional vertical migration ability provides the cyanobacterium Microcystis with competitive advantages in bloom formation and dominance establishment. Studies have been conducted on the vertical migration patterns of Microcystis since the 1970s; however, bloom simulations remain limited. Here, we used a simple model based on the viscous drag force of turbulence and analysed the motion characteristics of Microcystis colonies. The vertical distribution of turbulent kinetic energy (KZ), cell concentration and colony size profiles in Lake Taihu (Meiliang Bay and Gonghu Bay) and the critical vertical turbulent kinetic energy of colonies (TKZ, i.e. the anti-turbulence ability of colonies) were tested. The results showed that, under steady KZ profiles, colonies had relative rest positions (RRPs) where KZ ≈ TKZ and colonies of the same size gathered together. The vertical migration patterns were affected more by turbulence than by density (colony mass density) if the average KZ of the water column (MKZ) was not equal to TKZ. If MKZ ≈ TKZ, the colonies could exhibit a diurnal pattern of sinking at noon and floating upwards before dawn without steady RRPs. Our results suggest that studies on RRPs may offer optimizations for bloom forecast and control in the future due to easier simulation of KZ profiles than that of flow fields.

Dynamics of magnetic flux tubes and IR-variability of young stellar objects

We simulate the dynamics of slender magnetic flux tubes (MFTs) in the accretion disks of T Tauri stars. The dynamical equations of our model take into account aerodynamic and turbulent drag forces, and the radiative heat exchange between the MFT and ambient gas. The structure of the disk is calculated with the help of our MHD model of the accretion disks. We consider the MFTs formed at distances of 0.027−0.8 au from the star with various initial radii and plasma betas β0. The simulations show that MFTs with a weak magnetic field (β0 = 10) rise slowly with speeds less than the speed of sound. MFTs with β0 = 1 form an outflowing magnetized corona above the disk. Strongly magnetized MFTs (β0 = 0.1) can cause outflows with velocities 20 – 50 km s−1. The tubes rise periodically over times from several days to several months according to our simulations. We propose that periodically rising MFTs can absorb stellar radiation and contribute to the IR-variability of young stellar objects.

Separation of gas and dust in the winds of AGB stars

AbstractWe present first results from a project aiming at a better understanding of how gas and dust interact in dust-driven winds from Asymptotic Giant Branch (AGB) stars. We are at the final stage of developing a new parallelised radiation-hydrodynamics (RHD) code for AGB-wind modelling including a new generalised implementation of drift. We also discuss first results from high-resolution box simulations of forced turbulence intended to give quantitative “3D corrections” to dust-driven winds from AGB stars. It is argued that modelling of dust-driven winds of AGB stars is a problem that may need to be treated in a less holistic way, where some parts of the problem are treated separately in detailed simulations and are parameterised back into a less detailed (1D spherically symmetric) model describing the entire picture.

Computational Fluid Dynamics Simulations of the Air–Water Two-Phase Vertically Upward Bubbly Flow in Pipes

In this study, computational fluid dynamics (CFD) simulation of air–water two-phase flow in vertical pipe has been carried out. Wide ranges of superficial liquid and gas velocities, 0.376–3.53 and 0.0036–1.275 m/s, and dispersed phase holdup range from 0.3 to 38.74% is used in the simulation. In this study, for drag, lift, wall lubrication, and turbulent dispersion force the models of Grace, Tomiyama, Hosokawa, and Burns are used, respectively. The Eulerian–Eulerian multiphase approach with the k–ω shear stress transport (SST) turbulence model was used. Bubble diameter is an essential parameter to perform the Eulerian simulation. In the present study, we developed new correlation for bubble diameter and with this predicted bubble diameter simulations were performed. The developed CFD model is well capable of predicting gas void fraction, interfacial area concentration, pressure drop, gas/liquid velocity, and turbulent kinetic energy.

Aerodynamic robustness in owl-inspired leading-edge serrations: a computational wind-gust model

Owls are a master to achieve silent flight in gliding and flapping flights under natural turbulent environments owing to their unique wing morphologies. While the leading-edge serrations are recently revealed, as a passive flow control micro-device, to play a crucial role in aerodynamic force production and sound suppression (Rao et al 2017 Bioinspiration Biomim. 12 1–13), the characteristics of wind-gust rejection associated with leading-edge serrations remain unclear. Here we address a large-eddy simulation-based study of aerodynamic robustness in owl-inspired leading-edge serrations, which is conducted with clean and serrated wing models through mimicking wind-gusts under a longitudinal fluctuation in free-stream inflow and a lateral fluctuation in pitch angle over a broad range of angles of attack (AoAs) over 0° ⩽ Φ ⩽ 20°. Our results show that the leading-edge serration-based passive flow control mechanisms associated with laminar-turbulent transition work effectively under fluctuated inflow and wing pitch, indicating that the leading-edge serrations are of potential gust fluctuation rejection or robustness in aerodynamic performance. Moreover, it is revealed that the tradeoff between turbulent flow control (i.e. aero-acoustic suppression) and force production in the serrated model holds independently to the wind-gust environments: poor at lower AoAs but capable of achieving equivalent aerodynamic performance at higher AoAs >15° compared to the clean model. Our results reveal that the owl-inspired leading-edge serrations can be a robust micro-device for aero-acoustic control coping with unsteady and complex wind environments in biomimetic rotor designs for various fluid machineries.

Characterisation of flexible fibre deformations in turbulence

The transport of deformable objects by a turbulent flow is common in environmental sciences which are interested, for instance, in the dynamics of plankton in the ocean, and in industry, such as the papermaking or textile industries. In this study, the deformations of flexible fibres in homogeneous isotropic turbulence are experimentally and numerically investigated, focusing on the local curvature κ. By comparing our results to the predictions for worm-like chain polymers in an ideal solvent, we are able to identify the role of the spatial and temporal correlations of the turbulent forcing. In particular, we show that these correlations are responsible for a straightening of long fibres which become statistically less distorted by turbulence as their length increases.

Assessment of the CUPID Code for Bubbly Flows in Horizontal Pipes

Two-phase flow in a horizontal pipe has a pronounced feature; that is, two-phase-flow parameters are highly nonsymmetric because gravity is perpendicular to the mean flow direction. Thus, three-dimensional analysis is necessary for the accurate prediction of two-phase flow in a horizontal pipe, such as the hot leg and cold leg of a pressurized water reactor and the pressure tubes in a CANDU reactor. In this study, we simulated bubbly flows in horizontal pipes using the CUPID code, which adopts a two-fluid, three-field model for two-phase flow. In the preliminary calculations, it was found that the particle-averaged two-fluid momentum equation, rather than the standard two-fluid momentum equation, predicts a physically reasonable slip ratio and nondrag forces, except turbulent dispersion forces have negligible effects on the radial void distribution when the particle-averaged two-fluid momentum equation is used. Based on the results, we selected the physical models and computational mesh for subsequent code assessment using various bubbly flow experiments in horizontal pipes. The turbulent dispersion force model was improved to take into account the large void fraction change at the top. The results of the code assessment show good predictions for the axial pressure drop, liquid velocity, and turbulent kinetic energy profile and predict reasonably well the effects of jl and jg on two-phase-flow parameters. However, additional studies are needed for more accurate prediction of the nonsymmetric distribution of gas velocity and turbulent kinetic energy.

Direct numerical simulation of droplet breakup in homogeneous isotropic turbulence: The effect of the Weber number

We report the direct numerical simulation of droplet breakup in forced homogeneous isotropic turbulence with a mass-conserving level set method. In particular, the effect of the Weber number on the interface morphology of droplet breakup and the interaction between vortical structures and the interface are addressed. The densities and viscosities are the same for both phases in the present simulations. It is found that with increasing the Weber number, the initially large-scale spherical droplet tends to break down into dispersed small droplets. Compared with the flow local topology in the carrier-phase turbulence, the local topology of the bi-axial strain is suppressed at the statistical stationary state inside the droplet. During the droplet breakup process, the vorticity tends to be tangent to the large-scale interface at the early stage, and subsequently the alignment is mitigated for large Weber numbers, because the small droplets with relatively strong surface tension and small local Weber numbers can resist the deformation induced by local turbulent straining motion in the statistically stationary state.

Effects of finite spatial and temporal resolution in direct numerical simulations of incompressible isotropic turbulence

A study combining spectral filtering and numerical simulations at enhanced spatial and/or temporal resolution is used to clarify the proper scaling of dissipation and enstrophy in forced incompressible isotropic turbulence.

Effects of Discrete Energy and Helicity Conservation in Numerical Simulations of Helical Turbulence

Helicity is the scalar product between velocity and vorticity and, just like energy, its integral is an inviscid invariant of the three-dimensional incompressible Navier-Stokes equations. However, space- and time-discretization methods typically corrupt this property, leading to violation of the inviscid conservation principles. This work investigates the discrete helicity conservation properties of spectral and finite-differencing methods, in relation to the form employed for the convective term. Effects due to Runge-Kutta time-advancement schemes are also taken into consideration in the analysis. The theoretical results are proved against inviscid numerical simulations, while a scale-dependent analysis of energy, helicity and their non-linear transfers is performed to further characterize the discretization errors of the different forms in forced helical turbulence simulations.

A numerical tool for the study of the hydrodynamic recovery of the Lattice Boltzmann Method

We investigate the hydrodynamic recovery of Lattice Boltzmann Method (LBM) by analyzing exact balance relations for energy and enstrophy derived from averaging the equations of motion on sub-volumes of different sizes. In the context of 2D isotropic homogeneous turbulence, we first validate this approach on decaying turbulence by comparing the hydrodynamic recovery of an ensemble of LBM simulations against the one of an ensemble of Pseudo-Spectral (PS) simulations. We then conduct a benchmark of LBM simulations of forced turbulence with increasing Reynolds number by varying the input relaxation times of LBM. This approach can be extended to the study of implicit subgrid-scale (SGS) models, thus offering a promising route to quantify the implicit SGS models implied by existing stabilization techniques within the LBM framework.