Flow Entrainment Research Articles

Volumetric wake investigation of a free-flying quadcopter using Shake-The-Box Lagrangian particle tracking

The Shake-The-Box technique was applied to experimentally quantify the time-resolved volumetric flow field around a free-flying quadcopter UAV with an overall span of about 0.5 m. State-of-the-art LED illumination and high-speed camera equipment was combined with modern Lagrangian tracer particle tracking and data assimilation techniques, facilitating a measurement volume larger than 1.5m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${1.5}\\,{\\hbox {m}^3}$$\\end{document}. The setup allowed for both hover and limited maneuvering of the quadcopter, while resolving even small details of the complex interactional aerodynamics. In hover out of ground effect, the four individual rotor wakes merged into a single jet within a few rotor radii below the rotor planes. Evaluating the mass and momentum fluxes over suitable control volumes yields accurate estimates for the quadcopter’s total thrust, the asymmetric thrust distribution between front and back rotors, and the entrainment of external flow through turbulent mixing. Hover in ground effect decreases the power requirement and induces recirculating flow in the center of the four rotors. The outwash pattern is non-uniform with jets developing between the rotors and pointing in radially outward directions. Forward flight cases result in a skewed, rapidly merging wake flanked by the roll-up of two “supervortices” similar to the wingtip vortices of fixed-wing vehicles.

Growth of organized flow coherent motions within a single-stream shear layer: 4D-PTV measurements

This study investigates the evolution of a single-stream shear layer (SSSL) originating from a wall boundary layer past a backward-facing step. Utilizing a time-resolved 3D-Particle Tracking Velocimetry (4D-PTV) technique, we track the trajectories of fluorescent particles to gain insight into the flow characteristics of the SSSL. A compact water tunnel facility (Reτ=1240\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{Re}_\ au =1\\,240$$\\end{document}) is fabricated to obtain an SSSL with a perpendicular slow entrainment stream past the separation edge. A hybrid interpolation approach that combines ensemble binning and Gaussian weighting is implemented to derive minimally filtered mean and instantaneous lower- and higher-order flow field parameters. Spanwise-dominant coherent motion accompanied by finer flow scales is observed to grow due to flow entrainment through “nibbling” actions of small-scale vortices, “engulfing” by large-scale vortices, and vortex pairing events. Furthermore, the non-zero-speed stream edge grows relatively faster than the zero-speed stream edge, showing a strong asymmetry in mixing composition across a mixing layer. The SSSL reaches self-similarity at a streamwise distance of ≈55θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx 55\\,\ heta _{0}$$\\end{document}, where θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta _0$$\\end{document} is the initial momentum thickness from the separation edge, i.e., considerably shorter than reported in previous studies. A literature comparison of growth rate parameters raises intriguing questions regarding a potential inclusive growth scaling unifying the free shear layers. A turbulent kinetic energy (TKE) budget analysis reveals a negative production region immediately downstream of the separation edge attributed to a large positive streamwise gradient of streamwise velocity. In the self-similar region, the phase-averaged flow mapping demonstrates a larger concentration of turbulence production rate around the outer edges of spanwise vortices, specifically at the intersection of braids and vortices. Furthermore, a spatial separation exists in the regions of peak production and dissipation rates within the vortex core region favoring dissipation. The braids exhibit a larger concentration of turbulence diffusion rates, indicating their function as a conduit for exchanging turbulence between neighboring coherent motions.

A Computational Analysis of Turbocharger Compressor Flow Field with a Focus on Impeller Stall

Understanding the flow instabilities encountered by the turbocharger compressor is an important step toward improving its overall design for performance and efficiency. While an experimental study using Particle Image Velocimetry was previously conducted to examine the flow field at the inlet of the turbocharger compressor, the present work complements that effort by analyzing the flow structures leading to stall instability within the same impeller. Experimentally validated three-dimensional computational fluid dynamics predictions are carried out at three discrete mass flow rates, including 77 g/s (stable, maximum flow condition), 57 g/s (near peak efficiency), and 30 g/s (with strong reverse flow from the impeller) at a fixed rotational speed of 80,000 rpm. Large stationary stall cells were observed deep within the impeller at 30 g/s, occupying a significant portion of the blade passage near the shroud between the suction surface of the main blades and the pressure surface of the splitter blades. These stall cells are mainly created when a substantial portion of the inlet core flow is unable to follow the impeller’s axial to radial bend against the adverse pressure gradient and becomes entrained by the reverse flow and the tip leakage flow, giving rise to a region of low-momentum fluid in its wake. This phenomenon was observed to a lesser extent at 57 g/s and was completely absent at 77 g/s. On the other hand, the inducer rotating stall was found to be most dominant at 57 g/s. The entrainment of the tip leakage flow by the core flow moving into the impeller, leading to the generation of an unstable, wavy shear layer at the inducer plane, was instrumental in the generation of rotating stall. The present analyses provide a detailed characterization of both stationary and rotating stall cells and demonstrate the physics behind their formation, as well as their effect on compressor efficiency. The study also characterizes the entropy generation within the impeller under different operating conditions. While at 77 g/s, the entropy generation is mostly concentrated near the shroud of the impeller with the core flow being almost isentropic, at 30 g/s, there is a significant increase in the area within the blade passage that shows elevated entropy production. The tip leakage flow, its interaction with the blades and the core forward flow, and the reverse flow within the impeller are found to be the major sources of irreversibilities.

Study on dynamic mechanism of granular flow erosion and entrainment based on DEM theory

BackgroundGranular flows are common on the Qinghai–Tibet Plateau and the Hengduan Mountains in China, and their dynamic process processes have obvious erosional and entrainment effects. On the one hand, the volume of the granular flow increases by a factor of several or ten, which significantly increases its ability to cause a catastrophe; on the other hand, the eroded loose material affects the granular flow dynamics process and changes its state of motion.MethodsIn this paper, the dynamic mechanism of granular flow erosion and entrainment is investigated by DEM simulation.Purpose The effects of different substrate materials and substrate boundary conditions on granular flow erosion and entrainment are analyzed, and the effects of material mixing caused by erosion and entrainment on the state of motion of granular flow are discussed. It was verified that the kinetic mechanisms of granular flow erosion and entrainment includes impact erosion, ploughing, and shear abrasion.ResultsAnd discovered that small matrix particle size, small matrix boundary friction, and small matrix thickness lead to stronger ploughing and shear abrasion; Large matrix fractal dimensions result in stronger ploughing and weaker shear abrasion, and the granular flow does not entrain large amounts of material to the accumulation zone. Meanwhile, the dynamics of erosion and entrainment of granular flow were investigated, and the results showed that: 1. The greater the erosion rate, the greater the velocity and kinetic energy of the granular flow, the greater the distance traveled, and the smaller the apparent friction angle (i.e., the greater the mobility). 2. The amount of small granules in a granular flow changes its fluidity, the more small granules there are, the more fluid it is. 3. The fit reveals that the substrate fractal dimension has the strongest effect on the velocity and kinetic energy of granular flow, followed by substrate thickness and substrate boundary friction.

Flow structure and heat transfer characteristics of sweeping and steady impingement jets with/without chevrons on a semi-circular concave surface

Striving for improved gas turbine performance requires operating at higher gas flow temperatures, posing challenges in preserving the structural integrity of the gas turbine. To respond to these challenges, gas turbine manufacturers have turned to internal cooling and jet impingement to provide an effective solution for cooling the leading edge of the gas turbine blades. Fluidic oscillator is known for its sweeping behavior and expansive coverage of targeted surface and, thus, it can efficiently remove heat. In this study, the author numerically simulated the cooling performance of the leading edge of the gas turbine blades under constant heat flux while using four different configurations of jet impingement: a sweeping jet, a sweeping jet with chevrons, a steady jet, and a steady jet with chevrons. The results showed that the sweeping jet configuration with chevrons outperformed the steady jet configurations owing to oscillating jet impingement and a higher intensity of turbulence that increased the entrainment of jet flow. Under the configuration of a sweeping jet with chevrons, the targeted surface recorded an average Nusselt number that is 19.2% higher than the one with a steady jet without chevrons, along with a more uniform distribution of the surface temperature. The outstanding behavior of the sweeping jet with chevrons is due to the its internal flow behavior, i.e., oscillating flow nature of the sweeping jet with augmented turbulence at the exit of the chevron's nozzle.

Liquid droplet entrainment in an annular flow boiling regime—A Bayesian regularization algorithm based study

Accurate prediction of the entrained liquid droplet fraction in an annular two-phase flow regime plays a crucial role for estimating the dryout type critical heat flux to identify the optimized flow characteristics in the thermal systems across different industries. Existing studies have provided different correlations based on the limited experimental data. However, these correlations are applicable to certain operating conditions. Therefore, the present study aims at applying a deep learning method, specifically an artificial neural network (ANN), to enhance the prediction of the entrained liquid droplet fraction. Experimental data from various works on annular flow, covering a wide spectrum of pressure and flow conditions, are utilized for training the ANN model. Eight input variables, viz, superficial gas velocity (JSG), superficial liquid velocity (JSL), gas viscosity (μG), liquid viscosity (μL), gas density (ρG), liquid density (ρL), pipe diameter (d) and liquid surface tension (σLV) are considered as input features. The entrained liquid droplet fraction is the single output feature. The present model employs the Bayesian regularization backpropagation algorithm for training. The present ANN model is compared against the performance of linear regression, decision tree and support vector machine algorithms, and found that the performance of the present Bayesian regularization neural network (BRNN) model is superior within ∼7.5% deviation. Further, the BRNN model is coupled with the film mass flow rate model to obtain the axial variation of the liquid film mass flow rate and good agreement is noticed when compared against the experimental data.

Turbulent flow around submerged foundation arrays for ocean energy

Submerged ocean energy foundations experience dynamic loading due to the non-uniform approach velocity and wake propagation behind them. This paper investigates the unsteady turbulent flow dynamics and propagation of large-scale vortical structures around submerged foundations using a large eddy simulation. The computational domain constituted an array of eight foundations arranged in an inline configuration of two columns. Two streamwise spacing, x = 6D and 8D, are investigated at an approach Reynolds number, based on the channel hydraulic diameter, of 8.5 × 104. The parametric study reveals the expected changes in wake recovery and their impact on the downstream foundations. The narrow spacing promotes intense wake interactions, while larger inline spacing causes the entrainment of ambient flow, thereby limiting the wake interaction with the downstream foundations. A two-point correlation analysis reveals larger integral turbulent timescales for the 6D spacing compared to the 8D spacing. The separated flow around the foundations is characterized by eddies shed at varying frequencies. Multiple spectra peaks dominate at low frequencies, signifying dominant energy-containing large-scale vortices coupled with periodic excitation of the vortices as energy is transferred from the large-to small-scale turbulent structures.

Effect of Entrainment on the Liquid Film Behavior in Pipe Elbows

Multiphase flow entrainment in natural gas engineering significantly influences the safety and efficiency of oil companies since it affects both the flow and the heat transfer process, but its mechanisms are not fully understood. Additionally, current computational fluid dynamics (CFD) methodologies seldom consider entrainment behavioral changes in pipe elbows. In this article, a verified CFD method is used to study the entrainment behavior, mechanism, and changes in an elbow. The results show that droplet diameter in a developed annular flow follows a negative skewness distribution; as the radial distance (from the wall) increases, the fluctuation in the droplets becomes stronger, and the velocity difference between the gas and the droplets increases linearly. Turbulence bursts and vortices sucking near the wall jointly contribute to droplet entrainment. As the annular flow enters the elbow, the secondary flow promotes the film expansion to the upper and lower parts of the pipe. Droplets re-occur near the elbow exit intrados, and their size is much smaller than those in the upstream pipe. Vortices sucking under low gas velocity play an important role in this process. These findings provide guidelines for safety and flow assurance issues in natural gas production and transportation and bridge the gap between multiphase flow theory and natural gas engineering.

Numerical study on effect of operating pressures on CRKEC-based two-stage ejector

The two-stage ejector mixing-diffuser section in this study was computed using the Redlich-Kwong equation of state. The ejector was designed based on the constant rate of kinetic energy change (CRKEC) approach. The water vapor mixing diffuser profile and flow properties were calculated using a one-dimensional gas dynamic model. For the numerical investigation, the estimated geometrical profile based on the input design and operating conditions was utilized. ANSYS-Fluent 14.0 was em-ployed for the numerical study. The analysis was conducted under both on-design and off-design scenarios using the standard k-ε turbulence model. The impact of operating factors on flow behavior and entrainment ratios was investigated at off-design conditions. The findings demonstrated that the operational total pressures of the primary, secondary, and exit flows are a function of the two-stage ejector (TSE) entrainment ratio. With a higher exit pressure and more secondary/entrained flows, the entrain-ment ratio increases. However, altering the primary flow pressure in ways other than for the design conditions reduces the entrainment ratio.

Numerical modeling of self-aerated flows: Turbulence modeling and the onset of air entrainment

Numerical modeling of self-aerated flows is essential for understanding water systems and designing hydraulic structures. This work discusses and extends a theoretical/numerical model to represent air entrainment in self-aerated flows, which includes a criterion to define the occurrence of air entrainment, based on a balance between disturbing and stabilizing energies. The impact of the turbulence closure on the modeling of the onset of air entrainment and the distribution of bubble concentration is studied with particular emphasis. This work shows that uniform-density formulations of Reynolds-averaged Navier–Stokes closures lead to severe overprediction of air entrainment due to unphysical transport of turbulent kinetic energy generated in the air phase to the water phase. In contrast, combining variable-density turbulence closures with the energy balance criterion enables accurate prediction of the regions where air is entrained for stepped spillways and plunging jets. The choice of turbulence closure significantly influences the proposed criterion, emphasizing the importance of selecting an appropriate closure for an accurate description of the air entrainment process. Based on the conducted tests, the standard k−ε and buoyancy modified k−ε models better predict the onset of air entrainment and bubble distribution in stepped spillways, while the k−ω SST model proves to be more effective in capturing air entrainment at the impingement point in plunging jets. This study expands the capabilities of numerical models in predicting air entrainment and provides valuable insights into the effects and interrelations of the turbulence modeling, the air entrainment occurrence criteria, and the bubble transport equations.

An experimental study on air-oil flow patterns in horizontal pipes using two synthetic oils

Investigations on the gas-oil mixture flow of highly viscous oils have become increasingly significant in various industries for the past couple of years. This study examines the characteristics of air-oil flow patterns to understand the effect of the oil phase's thermophysical properties and flow rates on flow patterns. An experimental study was conducted to visualize the air-oil mixture flow in horizontal pipes. The diameters of the horizontal pipes were selected as 20 and 40 mm, respectively. Two different oils having substantially different viscosities were employed as the liquid phase. Based on the visualization results, air-oil mixture flow is categorized into seven flow patterns: stratified smooth, stratified wavy, annular, plug, slug, bubbly, and dispersed bubble flows. Along with representative images, this study demonstrates the essential characteristics of individual flow patterns. Furthermore, it discusses the influence of each phase's flow rate, oil viscosity, and surface tension on flow patterns. The processes of liquid entrainment in stratified wavy flow, bubble tip breakup in plug flow, and liquid slug aeration in slug flow are also illustrated, respectively, with the corresponding sequential representative images and schematic diagrams.

Numerical and experimental investigation on droplet entrainment mechanism and closed equations of two-fluid model

As a clean and low-carbon energy source, natural gas is an important force in achieving carbon peaking and carbon neutrality goals. Affected by the strong force of gas and liquid during mining and transportation, it exhibits the phenomenon of droplet entrainment. Entrained droplets change the properties of gas and liquid phases, which is crucial for accurate measurement of liquid film thickness, gas holdup, pressure drop and flow rates in natural gas pipelines. The measurement of entrained droplets has always been a focus and difficulty in the two-phase flow research field. Therefore, numerical simulation combined with experimental measurement are presented to study the droplet entrainment mechanism and closed equations of two-fluid model. The specific parameter settings for simulating horizontal gas–liquid annular flow based on numerical method are discussed, and effective simulation of droplet entrainment and deposition in annular flow is achieved. By comparing liquid film thickness and wave velocity from the simulation results with the experimental results, the correctness of the simulation method is verified. Then, the mechanism and dynamic evolution process of entrained droplets in annular flow are revealed, and the generation and deposition characteristics of entrained droplets are studied. As a result, the closed equations of two-fluid model such as droplet velocity, gas–liquid interface velocity, and gas–liquid interface friction factor are established. The proposed two-fluid model is validated using dual mode ultrasound and differential pressure sensors, demonstrating its good applicability and extrapolation.

Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle

The velocity fields and impinging pressure characteristics of confined jet impinges on a flat surface subject to the influence of various axial stages of a flat plate were experimentally examined. The instantaneous flow patterns, time-averaged flow patterns and velocity characteristics, velocity histories, and impingement pressure distributions were obtained using flow visualisation, particle image velocimeter (PIV), hot-wire anemometer measurement, and pressure detection techniques, respectively. From the analysis of the flow patterns, three characteristics of flow regimes are identified – laminar vortical wake, transition, and turbulent vortical wake – in the domain of Reynolds number and jet-to-wall distance. In laminar vortical wake mode, a ‘fat' pair of counter-rotating vortices appears between the confining surface of the nozzle and the flat plate. This results in a large region of turbulent intensities due to the entrainment of the jet flow. In transition mode, shear-layer vortices appear on the shear layer of the bifurcated jet. In turbulent vortical wake mode, the large momentum of jet fluids impinges the flat plate and deflects radially. The counter-rotating vortices and shear-layer vortices no longer exist in the wake of the bifurcated jet. The large eddies quickly break up into small eddies as the momentum of the jet-dominated flow. No obvious peak value of oscillating frequency was detected in this flow mode along the shear layers of the bifurcated impinging jet. The momentum of the fluid structures is converted into stagnation pressure on the flat plate when a strong jet impinges on it; therefore, the impingement pressure is large, consequently. The impingement pressure in the mode of the turbulent vortical wake was significantly greater than that in the modes of laminar vortical wake and transition.

Heat transfer across an air curtain for heat confinement in road tunnels: Influence of the heat source

Experiments were carried out to study the influence of the heat source using double-flow double-jet curtains (DS-TJ) to confine heat in fire safety problems. For this purpose, a small-scale road tunnel facility in 1:34 was built. The heat source used is an electric resistance. Simultaneous two-dimensional temperature and velocity measurements were made across the DS-TJ air curtain and in its immediate vicinity using a fine thermocouple and a 2D laser Doppler anemometer (LDA), respectively. Four cases were studied, by modifying the air curtain velocities at the nozzle exits and the heat source power. It is noted that hot-cold jets and heat source play a role in the curtain heat confinement by modifying the development of the two jets. Both the entrainment of the upward flow rising from the heat source as well as the Kelvin-Helmholtz instabilities contribute to this. On the other hand, the nozzle exit velocities determine the behavior of the Reynolds stress profiles as well as the production of the velocities fluctuations across the curtain, showing a non-correlating effect on the hot side, so that the profiles take values around zero, suggesting a heat source influence. In contrast in the cold side negative values indicate energy-draining behavior. The intensity of thermal turbulence reveals that the hot jet has higher values than the cold jet, constituting a thermal barrier to turbulent heat flow, and all activity is relegated to the zone of interaction between the two jets. In fact, the production derived from temperature fluctuations occurs mainly in the interaction of hot-cold jets, and negative values are due to large turbulent eddies, such as Kelvin-Helmholtz instabilities and Görtler-type structures. The Richardson number shows that buoyancy forces prevail on both sides of the curtain (Ri > 0.1), especially on the hot side, due to the heat source. Lower Ri values within the curtain indicate the presence of a thermal barrier, attenuating heat transfer through the curtain and favoring thermal sealing.

Mean drag reduction of a square cylinder using self-adaptive flaps

We report an experimental study about the control of the turbulent wake of a square cylinder. The control device consists in a couple of compliant flaps implemented on the sides of the square cylinder. Both flaps are hinged so that they can freely rotate around their leading edge. Particular emphasis is given to the analysis of the wake through the investigation of both mean and turbulent velocity fields. The contribution to drag reduction of each of these physical parameters is evaluated from experimental data. The main conclusions are twofold: i. the control essentially modifies the turbulent velocity field leading to a reduction of the lateral flow entrainment, ii. this change is due to the synchronization of the vortex shedding on each side of the square cylinder.

Investigating discrepancies between jet and plume in mean and turbulent flow of 45o inclined dense jets

Rapid expansion in the desalination industry in the last few years has led to the release of substantial quantities of concentrate and highly saline wastewater (brine) into marine environments. To mitigate the environmental consequences, the flow is discharged in depth from single or multi-port diffusers with inclined jets. This technique is used to enhance flow mixing and entrainment in the receiving water. To study flow turbulent characteristics along the jet-to-plume transition zone, a Laser Induced Fluorescence technique has been used. The dynamics of concentration fluctuations in the 45o inclined dense jets were investigated, and the extent of entrainment into the jet core or the length of the non-zero intermittency zone was identified. Intensity and intermittency of the fluctuations across the flow path together with energy spectrums at different points were plotted. The results show that turbulence kinetic energy spectrums resemble the −5/3 power law of Kolmogorov theory for the inertial subrange.

Modelling erosion, entrainment and deposition in cohesive granular flows: Application to dense snow avalanches

Gravitational mass movements may erode and/or entrain a significant amount of bed material that can strongly affect the flow dynamics until the moving mass eventually deposits and comes to rest. Snow avalanches generally release on slopes covered by a metastable and thus potentially erodible snow cover that can have a wide range of strength – or cohesion – depending on the type of snow and its physical properties. As the avalanche flows, the snow cover is fully or partially entrained at the front and at the base of the flow, increasing the mass of the avalanche. Conversely, at the tail, snow may be deposited along the track, reducing the overall flowing mass. The balance between entrainment and deposition therefore determines the growing or decaying of the avalanche in terms of mass. To date, it remains unclear how cohesion influences these processes and what consequences it has for avalanche dynamics and run-out. Here, we perform simulations based on the Discrete Element Method (DEM) to analyze the influence of cohesion and slope angle on the erosion, entrainment, mixing and deposition processes. This method makes it possible to follow the dynamics of the particles within the flow very precisely, something that cannot be done in real experiments. In the model, the cohesion is represented as the combined effects of a fragmentation potential associated with the strength of the bonds, and an aggregation potential associated with the stickiness of the particles. For various combinations of input parameters and material properties, we release a heap of particles over an erodible bed and simulate the entrainment and deposition mechanisms. Our results show on the one hand that a low strength (< 3 kPa) promotes a ploughing entrainment mechanism and a large entrainment velocity, up to 3 m/s. On the other hand a high strength (> 3 kPa) favors basal abrasion as the flow front is not able to destabilize the erodible bed at once. In this case, the entrainment velocity decreases typically below 1 m/s. This has important consequences on mixing: the front in granular flows with low strength and adhesion is typically made of freshly entrained material coming from the whole depth of the bed, while remains of the released material can be found at the front of highly cohesive avalanches. Finally, the deposition process is analyzed by evaluating the relationship between the deposit thickness hstop and slope angle θ which extends the framework of the model of hstopθ from cohesionless to cohesive granular flows. We find that a higher bond strength of the flowing material increases the deposition height. Our work improves our understanding of the mechanics of cohesive granular flows and may contribute to improving parameterizations in depth-averaged models used to simulate geophysical mass flows such as rock, ice, snow avalanches, debris flows and landslides.

Entrainment and bulk flow characteristics of heated and unheated transient plumes using direct numerical simulations

An off-source volumetrically heated turbulent transient plume, known as transient diabatic plume (TDP), is compared with an unheated transient plume (TP). Both the TDP and TP are simulated using direct numerical simulations at a source Reynolds number (Re) of 2000. The flow is sufficiently resolved till the Kolmogorov (η) scale. The radius of the TP shows a self-similar behavior of linear increase with height after about five diameters from the source hot-patch. However, the velocity does not show a self-similar behavior. The addition of off-source buoyancy in the TDP triples the vertical velocity and also causes an order of magnitude increase in the vorticity magnitude (ω) due to the effect of the baroclinic torque. Compared to the TP, the entrainment coefficient and radius of the TDP increase and this shows enhanced entrainment due to heating. The transient flows fields are analyzed using local scales based on integral quantities of volume, momentum, and buoyancy. Normalizing the vorticity with these integral scales results in a more uniform scaled ωl for both flows. Hence, for both TP and TDP, we use the same values of ωl to quantify the inner and outer boundaries of the turbulent non-turbulent layer (TNTL). The width of the TNTL (δTNTL) decreases by ∼25% due to heating. The conditionally averaged velocity profiles suggest that very close to the outer irrotational boundary (IB) of the TNTL, the TDP exhibits stronger updrafts and downdrafts in comparison to the TP. Entrainment is studied using the mean relative velocity (⟨vn⟩) defined at the IB. The analysis of the components of ⟨vn⟩ reveal that the viscous diffusion component, which is of the order of the Kolmogorov velocity (uη), is balanced by the baroclinic and viscous dissipation components, while the inviscid term is negligible in comparison.

Effect of spindle position on the performance and two-phase characteristics of controllable ejector for transcritical CO[formula omitted] refrigeration system

Spindle-controlled variable-geometry ejectors can adjust the system capacity to meet varying load requirements in transcritical-CO2 refrigeration systems. The present study numerically explores the effect of spindle position on the performance and flow characteristics of a controllable two-phase CO2 ejector. Spindle-controlled ejectors achieved entrainment ratios higher than that of fixed geometry ejector. Motive and suction flows exhibited high sensitivity to the spindle position near the critical operating range because of the transition between operating modes. The two-phase and shock characteristics associated with ejector flow and the variations observed for different spindle positions are discussed. Critical discharge pressures and entrainment ratios are estimated for various spindle positions. The critical compression and entrainment ratios correlate with the respective normalized spindle position in the ejector. The impact of the effective nozzle throat area on the flow entrainment range and achievable pressure lift is investigated. The results showed that the critical entrainment ratio bears a common linear relationship with the critical compression ratio for all positions of the spindle. The study relates the most significant performance and geometric parameters of the spindle-controlled two-phase CO2 ejector.

Entrainment effect of dry and saturated path material on granular flow movement and the force of granular flow impacting on the retaining wall: A numerical study

In this study, the effects of granular flow on dry and saturated entraining materials along the flow path and the force of granular flow impacting on the retaining wall are numerically investigated. Numerical simulations of the existing granular flow entrainment experiment are conducted to evaluate the accuracy and effectiveness of the adopted discrete element method (DEM). Subsequently, the movement and entrainment process of the granular flow aimed at investigating the entrainment effect and the normal force acting on the retaining wall, is studied. Velocity of the granular flow decreases a lot due to entrainment. As a result, many particles are accumulated along the flume bottom and the peak/residual normal force acting on the rigid wall decrease significantly. Finally, DEM and computational fluid dynamics (CFD) are coupled to simulate the granular flow entrainment with a saturated path material. The result shows that after adding fluid to the entrainment area, the entrainment area has a more significant deceleration effect on the quickly passing source particles due to fluid resistance. Furthermore, the interaction between the fluid and the original particles in the entrainment area makes the particles in the entrainment area being less susceptible to be entrained by the granular flow.