Turbulent Entrainment Research Articles

Universal modulations of large-scale motions on entrainment of turbulent boundary layers

The modulations of high/low-speed large-scale motions (H/L-SMs) on the turbulent/non-turbulent interface (TNTI) and turbulent entrainment are investigated in turbulent boundary layers via both experimental and numerical studies. The spanwise locations of large-scale motions can be locked by the spanwise heterogeneity, so the boundary layers over such a configuration are investigated first as an instructive case. In the engulfment process, it is found that irrotational ‘bubbles’ near the TNTI are more likely to originate from engulfment, while bubbles far from the TNTI could be produced by the local turbulence itself. Additionally, H-SMs are found to enhance the engulfment by the sweep flow. In the nibbling process, a competition relationship is observed: L-SMs induce stronger instantaneous ‘nibbling’ events by transporting more fluids towards the TNTI, while the H-SMs induced a more distorted TNTI. Consequently, the integral nibbling flux is greater above H-SMs. Furthermore, the explored mechanisms are verified to be insensitive to the wall shapes such as smooth and homogeneous roughness walls, which demonstrates that these modulations are universal for turbulent boundary layers. Finally, a conceptual modulation model is proposed to illustrate the entrainment process above the large-scale motions.

Random Forest Predictions of Fine Ash Concentration and Charging Processes From Experimentally Generated Volcanic Discharges

AbstractExplosive volcanic plume rise is governed by the rate at which ambient air is ingested and heated by turbulent entrainment and mixing processes. A daunting observational challenge is to constrain the character of the underlying physical processes and their dependence on complex particle‐particle and particle‐gas interactions. Important clues may lie in the particle‐particle momentum exchange that gives rise to related electrical discharges near the vent during supersonic eruptions. Recent laboratory studies of positive and negative shock‐tube‐generated volcanic discharges show a correlation between fine ash concentration and the magnitude and number of positive discharges. Charge generation via collisions (triboelectrification) is hypothesized to be more efficient with high ash concentrations and at high decompression rates because collisions between particles become more frequent under these conditions. To understand the experimental data in greater detail, we train and implement a regression‐based random forest algorithm to quantitatively predict concentrations of fine ash using discharge count, magnitude and polarity as predictors. Using a metric for variable importance, we find in all, high pressure (HP, ≥10 MPa), and high ash mass experiments (HM, >22g) that positive discharge properties are most important when predicting fine ash concentration, consistent with triboelectrification as the predominant charging process under these conditions. This mechanism is not constrained for low‐pressure (LP, <10 MPa) conditions, suggesting a potential threshold decompression rate condition for this class of charging. This result provides insight into why near‐vent lightning is not a ubiquitous feature of explosive eruptions.

Understanding Turbulent Flows With High-Speed Imaging

How the simultaneous application of flow measurement techniques and high-speed imaging sheds light on turbulent entrainment

3D hydrodynamics simulations of massive stars with the PROMPI code

Evolutionary models of massive stars are affected by uncertainties related to three-dimensional processes like convection. However, it is not computationally possible to simulate at the same time both the entire lifetime of a star and its fluid motions. 3D hydrodynamics simulations can be employed to study the fluid behaviour in very high detail for a short time-scale, which can be used to improve 1D models. The code PROMPI is a multi-dimensional hydrodynamics software developed specifically for simulations of the late phases in massive star interiors. In these proceedings, we present results from a new PROMPI simulation of the neon-burning shell in a 15 M© star, focusing on the interplay between convective turbulence and nuclear reactions. In particular, we show how it is possible to use the chemical composition to study the time evolution of the convective zone, which is subjected to turbulent entrainment.

The composition of massive white dwarfs and their dependence on C-burning modeling

Context. Recent computations of the interior composition of ultra-massive white dwarfs (WDs) have suggested that some WDs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive WDs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne WDs might form when convective boundary mixing is accounted for during the propagation of the C-flame in the C-burning stage. Both the Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. Aims. In this work, we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final WD compositions that have been found by different authors. Additionally, we explore the impact of such differences on the cooling times, crystallization, and pulsational properties of pulsating WDs. Methods. We performed numerical simulations of the evolution of intermediate massive stars from the zero age main sequence to the WD stage varying the adopted physics in the modeling. In particular, we explored the impact of the intensity of convective boundary mixing during the C-flash, extreme mass-loss rates, and the size of the adopted nuclear networks on the final composition, age, as well crystallization and pulsational properties of WDs. Results. In agreement with previous authors, we find that the inclusion of convective boundary mixing quenches the carbon flame leading to the formation of hybrid CO-Ne cores. Based on the insight coming from 3D hydro-dynamical simulations, we expect that the very slow propagation of the carbon flame will be altered by turbulent entrainment affecting the inward propagation of the flame. Also, we find that Ne-dominated chemical profiles of massive WDs recently reported appear in their modeling due to a key nuclear reaction being overlooked. We find that the inaccuracies in the chemical composition of the ultra-massive WDs recently reported lead to differences of 10% in the cooling times and degree of crystallization and about 8% in the period spacing of the models once they reach the ZZ Ceti instability strip.

Turbulent entrainment in viscoelastic fluids

Direct numerical simulations (DNS) of turbulent fronts spreading into an irrotational flow region are used to analyse the turbulent entrainment mechanism for viscoelastic fluids. The simulations use the FENE-P fluid model and are initiated from DNS of isotropic turbulence with Weissenberg and turbulence Reynolds numbers varying in the ranges $1.30 \le Wi \le 3.46$ and $206 \le Re_{\lambda }^{0} \le 404$, respectively. The enstrophy dynamics near the turbulent/non-turbulent interface (TNTI) layer, that separates regions of turbulent and irrotational flow, includes a new mechanism – the viscoelastic production – caused by the interaction between the vorticity field and the polymer stresses. This term can be a sink or a source of enstrophy in the turbulent core region of the flow, depending on the Weissenberg number, and contributes to the initial growth of the enstrophy in the viscous superlayer, together with the viscous diffusion, which is the only mechanism present for Newtonian fluids. For low and moderate Weissenberg numbers the scaling of the TNTI layer is similar to the scaling of TNTI layers for Newtonian fluids, but this is no longer the case at high Weissenberg numbers where the enstrophy tends to be concentrated into thin vortex sheets instead of vortex tubes. Finally, it is shown that the substantial decrease of the entrainment rates observed in turbulent flows of viscoelastic fluids, compared with Newtonian fluids, is caused by a reduction of the surface area and fractal dimension of the irrotational boundary, originated by the depletion of ‘active’ scales of motion in the fluid solvent caused by the viscoelasticity.

Exergy evaluation of equivalence ratio, compression ratio and residual gas effects in variable compression ratio spark-ignition engine using quasi-dimensional combustion modeling

This study is founded on previously validated, in-house, comprehensive, two-zone, quasi-dimensional combustion model, predicting performance and emissions in spark-ignition engines. The model is extended to include exergy terms complementing the first-law analysis results, which provide further useful information by implementation on test results acquired from an experimental, Ricardo E6, variable compression ratio (VCR) gasoline engine at the authors’ laboratory, operated under various CR and equivalence ratio (EQR) values at wide-open throttle. The model consists of unburned and burned zones simulating the combustion process, following strictly the flame-front movement in combustion chamber. The eventual combustion of the unburned mixture turbulent entrainment into burned zone through the flame-front area is followed closely. The exergy terms of each simulated zone are identified and computed, considering also chemical exergy components (diffusion plus reactive). The accurate account of temperature and species histories in burned zone after entrainment from the unburned one, should lead to more precise evaluation. This is important if irreversibility is computed from balance of all other exergy terms. The investigation examines and compares exergy-wise various EQR, CR and residual gas fractions. Presented diagrams of exergy terms for each zone and cylinder charge provide detailed information for chemical exergy, irreversibility and exergy losses.

Determining whether the swirling jet is applicable in bottom-hole cleaning: A CFD study

Borehole cleaning is a major concern in well drilling which significantly relevant to the drilling cost and safety. In this work, the bottom-hole cleaning performance of swirling jet induced by a drill bit nozzle was discussed. Computational fluid dynamics (CFD) simulations of cuttings transport have been conducted in a full three-dimensional, unsteady multiple-Euler framework by means of the commercial software ANSYS-FLUENT. A detailed comparison, including velocity and pressure fields, Reynolds numbers, cuttings trajectory and volume fraction variation has been made based on the simulation results. The results indicate that swirling jet showed bad bottom-hole cleaning performance compared with straight jet with conventional drilling control parameters. That is, the peculiar properties of swirling jet such as higher turbulent level and entrainment did not cause more efficient removal of drilled cuttings. Relevant conclusions are helpful to promote thorough understanding of the use of swirling jets.

Evidence of Microplastic Size Impact on Mobility and Transport in the Marine Environment: A Review and Synthesis of Recent Research

Marine Microplastics (MPs) exhibit a wide range of properties due to their variable origins and the weathering processes to which they are exposed. MP’s versatile properties are connected to their dispersal, accumulation, and deposition in the marine environment. MP transport and dispersion are often explained by analogy with sediments. For natural sediments, one of the key features linked to transport and marine morphology is particle size. There is, however, no size classification defined for MP particles and MPs constitute all plastic particles sized smaller than the threshold of 5 mm. In this study, based on existing knowledge in hydrodynamics and natural sediment transport, the impact of MP size on turbulent entrainment, particle settling, and resuspension is described. Moreover, by analyzing several quantitative studies that have provided size distribution, size-selective accumulation of MPs in various regions of the marine environment is reported on. The preferential presence of MPs based on their size in different marine compartments is discussed based on the governing hydrodynamic parameters. Furthermore, the linkage between polymer properties and MP shape and size is explored. Despite the evident connection between hydrodynamic transport and MP size presented, classification of MP size presents challenges. MP size, shape, and density appear simultaneously in the definition of many hydrodynamic parameters described in this study. Unlike mineral sediments that possess a narrow range of density and shape, plastics are manufactured in a wide variety of densities and marine MPs are versatile in shape. Classification for MP size should incorporate particle variability in terms of polymer density and shape.

Analysis of upward- and downward flame spread over vertical installed polyethylene-insulated electrical wires

Upward- (UFS) and downward (DFS) flame spread along a vertical installed electrical wire has been examined experimentally to gain better understandings of the heat transfer mechanism and burning behavior over electrical wires. UFS is considered as a much more hazardous fire scenario than DFS, however, UFS over electrical wires was very limited addressed. Fundamentally, the controlling heat transfer boundary over the preheating zone, hence the flame spread behaviors are quite different between DFS and UFS. This problem is more complicated for an electrical wire, for which the metal conduction is involved additionally, playing an important role in the heat transfer process. Thirteen types of polyethylene-insulated electrical wires with Cu- and NiCr-core were used to investigate the effect of metal core thermal conduction and wire configuration for both UFS and DFS. During the flame spread process, the pyrolysis length of UFS was enlarged, while only portion of virgin material of DFS were burnt as the result of distinct dripping. By further concerning the curvature effect, the dependence of the flame length on heat release rate per unit width (i.e. perimeter for cylinder) can be well explained by non-dimensional analysis. It turns out that the growth of flame length will be restrained as radius decreases significantly, and can be categorized in to two regions, one is controlled by laminar molecular diffusion followed the other controlled by turbulent entrainment. The variation of flame spread rate (FSR) with wire configuration and metal core thermal conduction are well interpreted by a theoretical model, which describes the heat recirculation through flame-insulation-core comprehensively based on thermodynamics theories, and attempts to consider the effects of temperature gap between core and insulation at pyrolysis front, burning fraction and melting heat. The validity of this new model is well confirmed by measured FSR. An important finding is that a highly conductive core will trigger an acceleration of FSR over the wire, primarily because heat flux in gas phase is more sensitive to the change of core thermal conductivity than heat conduction through core. Even though the heat diffusion length increases at the same time, however, the heat flux in gas phase grows faster than the corresponding heat loss through core, thereby compensate the corresponding heat loss. This work could facilitate understanding of heat transfer mechanism and burning behavior not only on the electrical wires, but also on the composite material with high thermal-conductive media.

Turbulent Characteristics and Air Entrainment Patterns in Breaking Surge Waves

Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity at the front. This paper examined the turbulent structures in breaking surge waves with Froude numbers of 1.71 and 2.13 by investigating the air entrainment and perturbation patterns across the surge front. A combination of the Volume Of Fluid (VOF) method and Large Eddy Simulation (LES) was utilized to capture air entrainment and turbulent structures simultaneously. The 3D nature of the vortical structures was simulated by implementing a spanwise periodic boundary. The water surface perturbation and air concentration profiles were extracted, and the averaged air concentration profiles obtained from the numerical simulations were consistent with laboratory observations reported in the literature. The linkage between turbulent kinetic energy distribution and air entrainment was also explored in this paper. Finally, using quadrant analysis and the Q-criterion, this paper examined the role of the spanwise perturbations in the development of turbulent structures in the surge front.

Numerical experiments on turbulent entrainment and mixing of scalars – ERRATUM

Numerical experiments on turbulent entrainment and mixing of scalars – ERRATUM

Turbulent thermohaline hydraulic jumps

Turbulent entrainment properties of thermohaline internal hydraulic jumps in quiescent ambient water are investigated. Underflow and overflow jumps are considered. The study is mainly concerned with thermohaline buoyancy effects on flow development. The thermohaline buoyancy is determined from a seawater equation of state, which is nonlinear in temperature and linear in salinity, and is accurate throughout the range of temperature and salinity of interest (T = 0 to 40 °C and S = 0 to 40 ppt). The results indicate that thermohaline buoyancy produces smaller less diluted underflow jumps and larger more diluted overflow jumps. These nonlinear buoyancy effects are particularly significant when buoyancy arising from temperature and salinity act in opposite directions. Hydraulic controls by vertical and/or horizontal constrictions downstream reveal, using a matching technique, unique stationary miscible jump solutions.

Numerical experiments on turbulent entrainment and mixing of scalars

Numerical experiments on the turbulent entrainment and mixing of scalars in a incompressible flow have been performed. These simulations are based on a scale decomposition of the velocity field, thus allowing the establishment from a dynamic point of view of the evolution of scalar fields under the separate action of large-scale coherent motions and small-scale fluctuations. The turbulent spectrum can be split into active and inactive flow structures. The large-scale engulfment phenomena actively prescribe the mixing velocity by amplifying inertial fluxes and by setting the area and the fluctuating geometry of the scalar interface. On the contrary, small-scale isotropic nibbling phenomena are essentially inactive in the mixing process. It is found that the inertial mechanisms initiate the process of entrainment at large scales to be finally processed by scalar diffusion at the molecular level. This last stage does not prescribe the amount of mixing but adapts itself to the conditions imposed by the coherent anisotropic motion at large scales. The present results may have strong repercussions for the theoretical approach to scalar mixing, as anticipated here by simple heuristic arguments which are shown able to reveal the rich dynamics of the process. Interesting repercussions are also envisaged for turbulence closures, in particular for large-eddy simulation approaches where only the large scales of the velocity field are resolved.

Numerical modelling of study the effect of the entrainment velocity, the number of Nusselt and the thickness of the non-convective zone on the stability of the pond solar

In this paper, the effect of the entrainment velocity, the Nusselt number, and the thickness of the salinity gradient zone $(NCZ)$ on the stability of the solar pond are studied. The modelling equations of thermal energy and mass transfer in a salt gradient solar pond are discretized by finite difference methods in the transient regime. A new border condition applicable near the equilibrium of interface between the $(NCZ)$ and the $(LCZ)$ region is proposed. We take account of the effects of both turbulent entrainment and diffusion on the growth or erosion of the gradient zone $(NCZ)$. The obtained numerical results show an additional condition of solar pond's stability which links between the salinity gradient $\left( \Delta C\right) \ $ and the temperature gradient $\left( \Delta T\right)$ in the $(NCZ)$ region.

Experimental study of entrainment vortex characteristic at steam-water interface of a water-submerged steam jet

Interfacial fluctuation and turbulent entrainment in the vapour-liquid mixing layer have great influence on heat and mass transfer in direct contact condensation. In this study, an ingenious experimental design was adopted to explore the characteristic microstructure at the steam-water interface of a water-submerged steam jet. Firstly, the periodic evolution process of large-scale two-phase coherent entrainment vortexes in the upper and lower steam-water turbulent mixing layers was captured. This process was divided into four stages: vortex formation, vortex growth, vortex pairing and vortex shedding. At the vortex growth stage, the central axial displacement of vortex was approximately linear with time, whereas the variation of vortex scale was nonlinear with time. Secondly, the periodic fluctuation of jet penetration length was caused by the pairing and shedding of periodic vortexes. Finally, the vortex formation process and distribution law of mixing layer thickness were analysed. The average scale of vortex blob at the pairing position was predicted and showed good agreement with the experimental data.

Universality of small-scale motions within the turbulent/non-turbulent interface layer

Abstract

Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High‐Resolution Digital Holography

AbstractMarine stratocumulus clouds contribute significantly to the Earth's radiation budget due to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties, such as aerosol concentration, droplet number, and precipitation rates, lead to considerable errors in global climate models. While these clouds usually have small vertical extent, turbulent entrainment‐mixing and precipitation can generate significant variations in droplet number, size, and relative dispersion with altitude. Here, we analyze turbulent entrainment‐mixing processes and the variability in cloud microphysical properties as a function of height within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. We use high‐resolution airborne holographic measurements and compare them with local turbulence measurements. We find that entrainment‐mixing is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further analysis of Damköhler number and transition scale number are able to explain the mixing mechanisms at different cloud heights using phase relaxation but not droplet evaporation as the microphysical time scale. A modified droplet evaporation time scale that considers local saturation deficit using a simple linear mixing model is developed, and it is able to reliably explain the observed mixing mechanisms. This study reinforces the importance of turbulent mixing and the use of appropriate microphysical time scales in determining cloud microphysical processes.

3D simulations of oxygen shell burning with and without magnetic fields.

We present a first 3D magnetohydrodynamic (MHD) simulation of convective oxygen and neon shell burning in a non-rotating [Formula: see text] star shortly before core collapse to study the generation of magnetic fields in supernova progenitors. We also run a purely hydrodynamic control simulation to gauge the impact of the magnetic fields on the convective flow and on convective boundary mixing. After about 17 convective turnover times, the magnetic field is approaching saturation levels in the oxygen shell with an average field strength of [Formula: see text], and does not reach kinetic equipartition. The field remains dominated by small-to-medium scales, and the dipole field strength at the base of the oxygen shell is only [Formula: see text]. The angle-averaged diagonal components of the Maxwell stress tensor mirror those of the Reynolds stress tensor, but are about one order of magnitude smaller. The shear flow at the oxygen-neon shell interface creates relatively strong fields parallel to the convective boundary, which noticeably inhibit the turbulent entrainment of neon into the oxygen shell. The reduced ingestion of neon lowers the nuclear energy generation rate in the oxygen shell and thereby slightly slows down the convective flow. Aside from this indirect effect, we find that magnetic fields do not appreciably alter the flow inside the oxygen shell. We discuss the implications of our results for the subsequent core-collapse supernova and stress the need for longer simulations, resolution studies, and an investigation of non-ideal effects for a better understanding of magnetic fields in supernova progenitors.

Convective core entrainment in 1D main-sequence stellar models

ABSTRACT 3D hydrodynamics models of deep stellar convection exhibit turbulent entrainment at the convective-radiative boundary which follows the entrainment law, varying with boundary penetrability. We implement the entrainment law in the 1D Geneva stellar evolution code. We then calculate models between 1.5 and 60 M⊙ at solar metallicity (Z = 0.014) and compare them to previous generations of models and observations on the main sequence. The boundary penetrability, quantified by the bulk Richardson number, RiB, varies with mass and to a smaller extent with time. The variation of RiB with mass is due to the mass dependence of typical convective velocities in the core and hence the luminosity of the star. The chemical gradient above the convective core dominates the variation of RiB with time. An entrainment law method can therefore explain the apparent mass dependence of convective boundary mixing through RiB. New models including entrainment can better reproduce the mass dependence of the main-sequence width using entrainment law parameters A ∼ 2 × 10−4 and n = 1. We compare these empirically constrained values to the results of 3D hydrodynamics simulations and discuss implications.