Entrainment Process Research Articles

Experimental Investigation of Mechanisms of Droplet Entrainment in Annular Gas-Liquid Flows: A Review

Entrainment of liquid from the film surface by high-velocity gas stream strongly affects mass, momentum and heat transfer in annular flow. The construction of basic assumptions for simplified physical models of the flow, as well as validation of numerical models, requires detailed experimental investigation of droplet entrainment process and the preceding stages of film surface evolution. The present paper analyzes the achievements and perspectives of application of various experimental approaches to qualitative and quantitative characterization of droplet entrainment. Optical visualization in at least two planes simultaneously may provide enough information on transitional liquid structures and detaching droplets, given that the side-view image is not obscured by the wall film. A planar LIF technique is not suitable for this purpose, since real objects are hidden by curved agitated interface and replaced by optical artifacts. To characterize the waves evolving into the transitional liquid structures, film thickness measurements in the plane of the wall are necessary. Such measurements can be achieved by intensity-based optical techniques, such as Brightness-Based LIF, near-infrared or X-ray attenuation techniques, combined with the side-view observations.

Throttle and expansion characteristics of supercritical carbon dioxide during its venting

Supercritical CO2 has important applications in the fields of power cycle generation systems and carbon capture, utilization and storage (CCUS). The venting equipment represents a fundamental pressure-relief method for the purpose of depressurizing sections of those systems for inspection, maintenance or repair. The extremely low temperatures, dry ice blockage and overpressurization will appear in the system due to the throttling and expansion effect during venting. Meanwhile, the released CO2 may cause the exposure for the high density gaseous CO2, solid CO2 particles and cryogenics to the people in the venting area. The study of flow characteristics during CO2 venting is absolutely essential for assessing the hazard probability of dry ice formation and the blockage of relief valves during emergency or routine operational venting. In this paper, the theoretical analysis and experimental tests were conducted to study the throttle and expansion characteristics through the elbow, valve and orifice during straight vertical venting of supercritical CO2 with a 258 m long, 233 mm inner diameter pipeline system. The pressures and temperatures inside the main pipe and vent pipe, and the temperatures near field region were recorded during venting. After the valve was opened, the pressure in the vent pipe rose rapidly, and the rising range decreased with the decrease of the distance from the orifice. The temperature drops of CO2 were obvious after supercritical CO2 passed through the bend, the valve and the orifice. The supercritical CO2 in the main pipe changed into gas-liquid two-phase, and the liquid CO2 were produced in the vent pipe due to the throttling expansion effect. The CO2 temperature near the orifice were in quasi steady state after reaching the lowest value due to the continuous vaporization of liquid-phase CO2 and sublimation of dry ice particles, accompanied by the air entrainment process.

System COP of Ejector-Based Ground-Source Heat Pumps

Compared to mechanical compressors, ejector-based refrigeration systems can make direct use of many forms of thermal energy, including waste heat, solar thermal, or biogases. It is known that SE systems have a lower thermal efficiency compared to mechanical compressors because of their lower performance at high compression ratios. In the present work, binary fluid ejector heat pumps with high efficiency are presented based on a proper selection of the binary fluids and the ejector geometry for specific operating conditions of a ground-source heat pump cooling system (GSHP). The existing literature on ejector-based refrigeration systems considers the thermal COP and does not account for many energy losses across the system. In the present paper, the system COP of an ejector-based GSHP that accounts for all energy exchange processes is determined. A method for the calculation of the work done by the boiler feed pump, the refrigeration expansion valve, and the ground loop circulation pump is presented. The influence of the condenser temperature on the entrainment process and the system COP is also discussed. The estimated overall system COP for the three top-ranked binary fluid candidates under various operating conditions was found to range from 1.55 to 3.06.

Roles of raindrop impact in detachment and transport processes of interrill soil erosion

To date the roles of raindrop impact in sediment entrainment and transport processes of interrill soil erosion are not yet fully quantified. The objectives are to 1) evaluate the effects of raindrop impact on sediment entrainment, 2) systematically quantify the relative importance of raindrop-driven and flow-driven transport, and 3) characterize sediment size distributions in different sediment transport regimes. A loam soil with 48.4% sand and 23% clay was packed into flumes (L × W × H: 1.8 × 0.5 × 0.1 m) and subjected to intensities of 60, 90, 120 mm h−1 at gradients of 5, 10, and 16°. Air filter and tarp covers were used to vary impact energy and flow length. Results show that sediment is largely entrained by raindrop impact and transported by raindrop-impacted flow. Interrill erosion consists of two composite processes: raindrop-induced detachment/entrainment and raindrop-impacted flow transport. The former includes direct detachment by raindrop impact and ‘flow detachment’. The latter includes raindrop-driven and flow-driven transports. The proportions between the two transports vary with slope steepness, slope length, flow depth (or flow discharge), rainfall intensity, and sediment characteristics. Raindrop-driven transport is competent but inefficient, while the opposite is true for flow-driven transport. Because raindrop impact plays dual roles in detaching soil (and/or entraining sediment) and enhancing sediment transport, a drop energy factor must be included in interrill erosion models to adequately simulate the dual roles. More studies are required to derive a drop energy function that takes into consideration the rainfall energy dissipation by canopies of various plants under natural conditions.

Circadian clocks signal future states of affairs

On receiver-based teleosemantic theories of representation, the chemical states of the circadian clocks in animal, plant and cyanobacterial cells constitute signals of future states of affairs, often the rising and setting of the sun. This signalling is much more rigid than sophisticated representational systems like human language, but it is not simple on all dimensions. In most organisms the clock regulates many different circadian rhythms. The process of entrainment ensures that the mapping between chemical states of the clock and the daily light-dark cycle is adjusted to deal with seasonal changes. In regulating anticipatory behaviour, the states of the clock look forward both to the time the behaviour is supposed to happen and the later time when the anticipated circumstances are supposed to arise. The case of the circadian clock shows that purely indicative signals can arise in very basic biological systems and brings into sharp relief the trade-offs involved in characterizing representational systems. On receiver-based teleosemantic theories, future-directed signals are not restricted to complex multicellular organisms but are ubiquitous in the biological world.

Music of infant-directed singing entrains infants’ social visual behavior

Infant-directed singing is a culturally universal musical phenomenon known to promote the bonding of infants and caregivers. Entrainment is a widely observed physical phenomenon by which diverse physical systems adjust rhythmic activity through interaction. Here we show that the simple act of infant-directed singing entrains infant social visual behavior on subsecond timescales, increasing infants' looking to the eyes of a singing caregiver: as early as 2 months of age, and doubling in strength by 6 months, infants synchronize their eye-looking to the rhythm of infant-directed singing. Rhythmic entrainment also structures caregivers' own cueing, enhancing their visual display of social-communicative content: caregivers increase wide-eyed positive affect, reduce neutral facial affect, reduce eye motion, and reduce blinking, all in time with the rhythm of their singing and aligned in time with moments when infants increase their eye-looking. In addition, if the rhythm of infant-directed singing is experimentally disrupted-reducing its predictability-then infants' time-locked eye-looking is also disrupted. These results reveal generic processes of entrainment as a fundamental coupling mechanism by which the rhythm of infant-directed singing attunes infants to precisely timed social-communicative content and supports social learning and development.

Air entrainment mechanism of chute bottom aerators in high-speed chute flows

In high-head dam projects, chute aerators are commonly applied as artificial aeration devices to prevent cavitation erosion damage in high-speed spillway and tunnel flows. These specific chute air–water flows are generated by jet lower-surface aeration above the bottom cavity and jet impact aeration on the bottom floor. These air entrainment processes determine the total air flux transport downstream of the chute flow. However, the differential air fluxes caused by the two different aeration mechanisms remain unclear. Based on physical model investigations, the two air entrained processes are observed by a high-speed camera, and detailed air flux data are measured in the aerator cavity and impact areas. The effects of the jet lower-surface disintegrating and then instantaneously reattaching to the chute floor are discussed. The measured air flux data indicate that the inertial jet impact entrainment dominates with increasing the Froude number, while the air flux proportion of the jet lower-surface aeration increases with the aerator height. A general prediction model for aerator cavity entrainment is proposed, considering the effects of the Froude number and jet lower-surface aeration. The scale effects of the coupling mechanism on chute aerator air entrainment highlight that aerator cavity air entrainment primarily depends on the inertial jet impact, and the main effects of aerator design on entrainment performance are manifested in the air cavity properties.

Direct measurement of the inertial drag and lift forces on entrained coarse particles at various protrusion heights

AbstractLaboratory experiments were performed to study the impact of relative particle protrusion P/D (P is the protrusion height and D is the diameter of the spherical target particle) on the entrainment of sediment particles from a rough bed of spherical grains. The target particle to be entrained was instrumented with electronic sensors to measure the triaxial linear acceleration, and consequently the particle inertial forces, during the entrainment process. The velocity field around the particle was obtained using two‐dimensional particle tracking velocimetry (PTV) technique and the velocity data were synchronised with the force data relative to the moment of entrainment. Experimental results show that the inertial drag and lift forces systematically decreased as relative particle protrusion increased. The ratio of inertial lift force to drag force revealed that drag force slightly dominated the entrainment process at P/D > 0.7, while lift force slightly dominated at P/D < 0.62. Inertial drag coefficient was found to be independent of particle protrusion for low P/D, but rapidly declined for P/D > 0.62–0.7. Differently, inertial lift coefficient remained relatively constant within the tested protrusion ranges. Additionally, the critical Shields number was linearly related to the normalised inertial forces over the present particle protrusion range. In summary, the particle protrusion demonstrates a significant impact on the inertial forces and, further, the entrainment process.

Numerical simulation of impact and entrainment behaviors of debris flow by using SPH–DEM–FEM coupling method

Abstract Increasing rain levels can easily destabilize and destroy particulate matter in mountainous areas, which can cause natural disasters, such as debris flow and landslides. Constitutive equations and numerical simulation are the theoretical bases for understanding the behavior of these disasters. Thus, this study aimed to investigate the impact of the debris flow and its entrainment behavior on gully bed sediments. We adopted a coupled analysis method based on elastic–plastic constitutive equations by considering the elasto-plasticity of slurry and the elastic characteristics of debris materials. The coupled method consisted of smooth particle hydrodynamic (SPH), discrete element method (DEM), and finite element method (FEM) (SPH–DEM–FEM). SPH particles represented fluid, DEM particles denoted solid immersed in fluid, and FEM elements represented the terrain and structures. The coupling analysis model was used to simulate the coupling contact of solid, liquid, and structures and to describe the entrainment behavior between solid and liquid phases. The model feasibility was verified by comparing the basic simulation results with experimental values of the dam break model and the rotating cylindrical tank model. The coupled model was then combined with the data management and modeling of geographic information system to simulate the 2010 Yohutagawa debris flow event. Finally, we explored the influence of debris shape-related parameters on the debris flow erosion entrainment process.

Transport properties of fast ice within the nearshore

Fast ice affects many coastlines in high- and mid-latitude regions, shaping the coastal morphology by acting as both a protective barrier from winter storms and as an erosive agent through sediment entrainment and transport. It has been hypothesized that when sediment-rich ice is dislodged and transported beyond the depth of closure by storm waves, sediment is permanently removed from the nearshore, starving the littoral system and facilitating future erosion. However, despite this large potential impact on coastal sediment budgets, the rates and mechanisms of sediment-rich ice transport are poorly constrained. This study investigates the sediment entrainment process by fast ice, the mechanisms needed for ice mobilization, and the force needed to breakup and transport sediment-rich ice. We find that the Coulomb frictional strength of the sediment regulates the fast ice system and that the viscous rheology of the ice has no direct mechanical effect. Importantly, these results show that only the frictional coefficient of the beach sediment needs to be measured to estimate the resistive strength of the fast ice, significantly simplifying the requirements for evaluating the potential for fast ice mobilization and associated sediment transport. • Investigations on sediment entrainment and mobilization processes for fast ice. • Experiments reveal the force needed to breakup and transport sediment-rich ice. • The frictional strength of the sediment, not ice, regulates the fast ice system. • Blocks of ice with adhered sediment can be treated as a Coulomb material.

A two‐fluid single‐column model of turbulent shallow convection. Part II: Single‐column model formulation and numerics

AbstractThe two‐fluid single‐column model of Thuburn et al. (Quart. J. R. Meteorol. Soc., 2019, 145, 1535–1550) is extended to include moisture and horizontal wind shear. Turbulent kinetic energy is introduced as a prognostic variable, dependence on a diagnosed boundary‐layer height is removed, and subfilter fluxes are approximated using a two‐fluid version of a Mellor–Yamada scheme. Three mechanisms for entrainment and detrainment processes are introduced, which represent entrainment of unstable air at the surface, forced detrainment of air at the top of the boundary/cloud layers, and turbulent mixing that relaxes the convective fluid to a reference profile. A semi‐implicit Eulerian discretization replaces the semi‐implicit semi‐Lagrangian implementation of Thuburn et al. (Quart. J. R. Meteorol. Soc., 2019, 145, 1535–1550) to improve numerical stability and conservation. The equations for the implicit time step are solved using a quasi‐Newton method, which is shown to perform well in numerical tests for conservation and convergence. The two‐fluid single‐column model presented in this article will be applied to simulations of shallow cumulus convection in Part III.

Confirming an antiphasic bicyclic pattern of forward entrainment in signal detection: A reanalysis of Sun et al. (2021).

Forward entrainment refers to that part of the entrainment process that persists after termination of an entraining stimulus. Hickok et al. (2015) reported forward entrainment in signal detection that lasted for two post-stimulus cycles. In a recent paper, Sun et al. (2021) reported new data which suggested an absence of entrainment effects (Eur. J. Neurosci, 1-18, doi.org/10.1111/ejn.15367). Here we show that when Sun et al.'s data are analysed using unbiased detection-theoretic measures, a clear antiphasic bicyclic pattern of entrainment is observed. We further show that the measure of entrainment strength used by Sun et al., the normalized Fourier transform of performance curves, is not only erroneously calculated but is also unreliable in estimating entrainment strength due to signal-processing artifacts.

Investigation of the Cryogenic Nitrogen and Non-Cryogenic N-Dodecane and Ammonia Injections using a Real-Fluid Modelling Approach

<div class="section abstract"><div class="htmlview paragraph">In modern compression ignition engines, the dense liquid fuel is directly injected into high pressure and temperature atmosphere, so the spray transitions from subcritical to supercritical conditions. To gain better control of the spray-combustion heat release process, it is important to have a physically accurate description of the spray development process. This work explored the effect of real-fluid thermodynamics in the computational prediction of multiphase flow for two non-ideal situations: the cryogenic nitrogen and non-cryogenic n-dodecane and ammonia sprays. Three real-fluid equations of state (EoS) such as the Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), and Redlich-Kwong-Peng-Robinson (RKPR) coupled with the real-fluid Chung transport model were implemented in OpenFoam to predict the real-fluid thermodynamic properties. Validations against the CoolProp database were conducted. The RKPR EoS demonstrated an overall better predictive performance compared to the SRK and PR EoS. Due to miscalculations of the thermodynamic properties under supercritical conditions, the cases using the ideal-gas EoS predicted the significantly distinct spray features from the cases using real-fluid EoS. For the cryogenic nitrogen spray simulations, cases using various real-fluid EoS yielded similar spray features because of the low injection rate and thus the weak ambient entrainment process. The reduction of ambient pressure promoted the turbulent mixing process for the n-dodecane spray due to the smaller ambient density and resistance. Compared to ammonia, n-dodecane had higher density and viscosity under supercritical conditions, which led to its higher jet velocity and more concentrated spray feature.</div></div>

A Mixing Process Influenced by Wall Jet-Induced Shock Waves in Supersonic Flow

With the development of hypersonic air-breathing propulsion systems, such as the supersonic combustion ramjet (Scramjet) and rocket-based combined cycle (RBCC) engines, the mixing process of supersonic airstream with fuel in the engine combustor has been drawing more and more attention. Due to the compressibility effects, the mixing process in a supersonic condition is significantly inhibited. In the present paper, the novel strategy of wall-jet induced shock waves (WJISW) is put forward to realize mixing enhancement. The interaction process between WJISW and the supersonic mixing layer is researched and the enhanced-mixing mechanism is revealed, employing large eddy simulation (LES) methods. The fine vortex structures of the flow field are well captured and presented, utilizing the numerical schlieren technique. Detailed visualization results indicate that WJISW in a low frequency condition can result in the ‘region action mode’ (RAM) never reported before. The drastic dynamic behaviors including growth, deformation, and distortion in the interaction region can undoubtedly promote the mixing of upper and lower streams. The Reynolds stress distributions along the streamwise x-direction suggest that more intense fluctuations can be achieved with a low frequency WJISW. Moreover, a sharp increase in mixing layer thickness can be realized in the interaction region. The dynamic mode decomposition (DMD) analysis results show that the mixing layer evolution process is dominated by the mode induced by WJISW, which leads to the coexistence of both large- and small-scale structures in the flow field. The entrainment process corresponding to large-scale vortices and the nibbling process corresponding to small-scale vortices can obviously promote mixing enhancement. It is suggested that the present proposed strategy is a good candidate for enhanced-mixing with application to Scramjet and RBCC combustors.

Smart Sediment Particle: A novel approach to investigating fluvial bed entrainment using instrumented sensors

The Smart Sediment Particle (SSP) instrumented with multiple sensors to obtain tri-axial linear accelerations is used for studying the mechanism of coarse grain entrainment. Three bed arrangements are tested to examine their influences on entrainment processes and the threshold force and impulse conditions. The SSP shows satisfactory precision to capture the imperceptible movement tendencies immediately (e.g., 0.15 s) after the dislodgement. The experimental results show that bed packing can significantly affect the particle's entrainment threshold and the path of movement. Lateral movement is predominant for some cases because the entrainment path is based on the least work done by the flow, even if the direction is not aligned with the flow. The concept of effective protrusion height is proposed to quantify the effect of local bed arrangement on entrainment. The mean velocity field around the SSP is also measured together with the acceleration data. The mean threshold velocity and resultant impulse show consistent dependence on the bed arrangement. In general, the SSP shows a good prospect of being used to study small-scale entrainment mechanism.

Lagrangian coherent structures in flow past a backward-facing step

This paper investigates flow past a backward-facing step (BFS) in a duct at Reynolds number $Re = 5080$ based on step height, mean inflow velocity and fluid kinematic viscosity. The flow configuration matches a combustion experiment conducted by Pitz and Daily in 1983. High-resolution velocity fields are obtained in OpenFOAM by direct numerical simulation, and the flow field analysed by Lagrangian approaches. Trajectories of fluid particles in areas of interest are obtained by high-order numerical integration, and used to compute finite-time Lyapunov exponents (FTLEs) and polar rotation angles. Lagrangian coherent structures (LCSs) are extracted using geodesic theory, including hyperbolic LCSs and elliptic LCSs. We use complementary qualitative and quantitative LCS analyses to uncover the underlying flow structures. Notably, we find that a flow pathway in which fluid particles rarely diverge from adjacent particles is opened and closed by FTLE ridges determined by the periodic shedding of vortices from the BFS. Two dominant vortices with significant Lagrangian coherence, generated respectively by the separated boundary layer and shear layer, are self-sustaining and of comparable strength. Hyperbolic repelling LCSs act as transport barriers between the pathway and cores of the coherent vortices, thus playing a major part in the fluid entrainment process. Interactions between these different geometric regions partitioned by LCSs lead to intrinsic complexity in the BFS flow.

Aerosol-boundary layer interaction modulated entrainment process

Aerosol-boundary layer interactions play an important role in affecting atmospheric thermodynamics and air pollution. As a key factor in dictating the development of the boundary layer, the entrainment process in the context of aerosol-boundary layer interactions is still poorly understood. Using comprehensive field observations made at a superstation in Beijing, we gain insight into the response of the entrainment process to aerosols. We found that high aerosol loading can significantly suppress the entrainment rate, breaking the conventional linear relationship between sensible heat fluxes and entrainment fluxes. Related to aerosol vertical distributions, aerosol heating effects can alter vertical heat fluxes, leading to a strong interaction between aerosols and the entrainment process in the upper boundary layer. Such aerosol-entrainment coupling can inhibit boundary layer development and explains the great sensitivity of observed entrainment rates to aerosols than can traditional calculations. The notable impact of aerosols on the entrainment process raises holistic thinking about the dynamic framework of the boundary layer in a polluted atmosphere, which may have a significant bearing on the dispersion of air pollutants and the land-atmosphere coupling.

A critical analysis of Lin et al.'s (2021) failure to observe forward entrainment in pitch discrimination.

Forward entrainment refers to that part of the entrainment process that outlasts the entraining stimulus. Several studies have demonstrated psychophysical forward entrainment in a pitch-discrimination task. In a recent paper, Lin et al. (2021) challenged these findings by demonstrating that a sequence of 4 entraining pure tones does not affect the ability to determine whether a frequency modulated pulse, presented after termination of the entraining sequence, has swept up or down in frequency. They concluded that rhythmic sequences do not facilitate pitch discrimination. Here, we describe several methodological and stimulus design flaws in Lin et al.'s study that may explain their failure to observe forward entrainment in pitch discrimination.

Leader-Follower Emotional Entrainment and Daily Interaction Quality: The Role of Follower Attachment

Followers’ high-quality interactions with their leader are a cornerstone of leadership research. Despite a growing understanding of leader emotions as static predictors of high-quality leader-follower relationships, research has yet to pay attention to the micro-level, dynamic interplay of leader and follower emotions for the emergence of daily interaction quality. We develop an episodic emotional entrainment perspective stating that leaders’ expressions of positive (joy, calmness) and negative emotions (fear, sadness) serve as affective events influencing corresponding follower emotions. Over the course of a day, this manifests in affective entrainment patterns (i.e., the leaders’ and followers’ affective states fluctuating in a common rhythm), which in turn predict followers’ perceived interaction quality with their leaders. The strength of the entrainment process, however, is influenced by the follower’s attachment style. We conducted a daily diary study across ten workdays with four measurements each day (N=72 employees; N=497 daily responses). Results of within-day multilevel latent growth models support an emotional entrainment process such that followers’ felt emotions oscillated with the observed leader emotional expression over the course of the day. Within-day changes in followers’ felt high-activated emotions (i.e., joy, fear) predicted perceived end-of-day interaction quality, whereas changes in low-activated emotions (i.e., calmness, sadness) did not. The relationship between leaders’ expressions of joy (fear) and corresponding follower felt emotions at the end of the day via emotional entrainment was weaker (stronger) for followers with high attachment avoidance (attachment anxiety), which in turn was positively (negatively) associated with end-of-day interaction quality.

Dust-planetary boundary layer interactions amplified by entrainment and advections

Mineral dust contributes to more than half of the global aerosol loading. However, the radiative impacts of dust aerosols on planetary boundary layer (PBL) structure have not been explored sufficiently. During a typical dust storm event over Tarim Basin, dust aerosols exhibit a well-mixed distribution during the daytime in spite of a shallow layer of dust particles accumulated at higher altitudes. By contrast, nocturnal dust plumes are located near the surface due to stable stratification. We demonstrate that these differentiated vertical distributions determine the spatial heterogeneity of dust loading, radiative fluxes and PBL height variations. Dust aerosols cause daytime PBL suppression and nighttime PBL promotion through modulating surface and atmospheric radiative budgets. Specifically, dust-induced cooling effect within PBL directly inhibits the daytime PBL development. PBL suppression effect is then amplified by entrainment processes resulting in excessively low PBL height, especially for dust particles below but near the PBL top. Dust plumes weaken updrafts from PBL and downdrafts of the free atmosphere, which further reduce the entrainment mixing through attenuating horizontal and vertical advection, and eventually amplify PBL suppression. At night, near-surface dust aerosols stimulate a warm and unstable lower atmosphere, generate warm advection heating and promote the PBL development. Our study highlights the importance of specifying entrainment parameters and nighttime advection activities in quantifying the dust-PBL interactions.