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This paper describes an experimental study on shear-based spray formation. A laminar liquid jet was ejected inside co-annular non-swirling and swirling air streams. The aerodynamic Weber numbers (WeA) and swirl numbers (S) of the flow cases ranged from 4 to 1426 and from 0 to 3.9, respectively. High-speed shadowgraphy was utilized to obtain data on the first droplet locations, breakup lengths of the liquid jets, and two-dimensional wave spatiotemporal spectra for the jets. In order to detect the large-scale instabilities of the central liquid jet, proper orthogonal decomposition (POD) was performed on the high-speed shadowgraphic images. Stereo particle image velocimetry was utilized to investigate the annular air flow fields with S in the range of 0–2.5. It was found that air swirl promotes the morphological development of the jets with S in the range of 1.2–2.5. Both the breakup length and axial distance between the first droplet separation and the nozzle exit reduce as WeA and S increase. Scaling of the first droplet locations and breakup lengths is also evaluated in this paper. In terms of the air flow fields, radial expansion of the annular swirling air jets was observed, and the annular swirling jets expand radially further as S goes up. Central reversal air flows appear near the nozzle exit when S ≥ 1.2 , and some small droplets are blown upward to the nozzle exit by these central reversal air flows. In terms of large-scale instabilities, flapping is the dominant instability across most of the flow cases (as revealed by the first POD mode). Wavy and explosive breakup appear as the secondary breakup modes when WeA is low (≤ 110). In the absence of the central reversal air flows, the temporal frequencies of the instabilities of the air–water interfaces increase as S goes up. It was found that the central reversal air flows tend to stabilize the air–water interfaces. The spatial frequencies of the instabilities of the air–water interfaces remain low (≤ 0.06 mm−1) across all the flow cases, which produce long-wave structures. [ABSTRACT FROM AUTHOR]

The paper presents the visualization results of the sudden contraction flow field with an annular gap downstream of the contraction cross section. The contraction ratio directly influences the flow phenomena. No stationary vortices were observed upstream of the contraction cross section, and flow separation divided the flow field within the annular gap into a recirculation zone, a shear layer, and a mainstream zone. Vena-contracta point and reattachment point occurred within the flow field of the annular gap. The production of Reynolds stress varied with the contraction ratio, and the momentum exchange mainly occurred in the recirculation zone and the shear layer, while the flow state in the mainstream zone tended to become isotropic. Rapid changes in turbulent kinetic energy primarily occurred in the shear layer, and the production term of turbulent kinetic energy contributed the most. Applying computational fluid dynamics and experimental results, this study has formulated an expression for the contraction coefficient of the proposed model. The axial positions of the vena-contracta point and reattachment point have a strong correlation with the contraction ratio, which can be represented by an exponential function. [ABSTRACT FROM AUTHOR]

The unsteady flow field in the aerostatic bearing always induces micro-vibrations, which are severely detrimental to the stability and precision of the bearing. Extensive research has been conducted on the mechanism of micro-vibration, but a consensus has not yet been reached. To this end, the large eddy simulation (LES) and proper orthogonal decomposition methods were employed to analyze the flow field of an annular aerostatic bearing in this paper. A mechanism for inducing micro-vibration and the identification of a novel flow behavior were ultimately revealed. First, the accuracy of our LES method has been validated through quantitative comparison with experimental data. Then, the mode decomposition has been conducted to analyze the flow field under various gas supply pressures. The results demonstrate that when the supply pressure Ps = 0.4 MPa, the micro-vibration is dominated by a pair of adjacent large-scale vortices with low frequencies in the recess. However, when Ps = 0.5 and 0.6 MPa, the convection and shearing processes near the orifice outlet and the rectangular recess inlet become intense, resulting in the displacement of large-scale vortices. Eventually, the small-scale high-frequency pressure fluctuation structures have been also observed, which are closely related to the convection process within recess. With the increase in gas supply pressure, the high-frequency pressure fluctuations at the circular recess outlet gradually diminish, while those at the orifice outlet emerge and gradually enlarge. Meanwhile, the mode dominant frequency is transferred from around 200 kHz to around 1000 kHz. The energy fraction of the high-frequency pressure fluctuations is also greatly increased. [ABSTRACT FROM AUTHOR]

In this paper, the transient ignition process of an annular combustor with 16 centrally staged swirling burners is experimentally investigated to study the mechanism of burner–burner flame propagation. The flame propagation patterns are studied by high-speed imaging. Three typical patterns of the burner–burner flame propagation are identified: the kindled-swirling pattern, entrained-swirling pattern, and sweeping pattern. The patterns are featured with different flame paths of motion. For fixed flow rates, the paths of motion are mainly determined by the overall equivalence ratio Φ. Furthermore, during the burner–burner flame propagation, the effect of the flow field on the local flame fronts is analyzed by Mie scattering and particle image velocimetry (PIV) methods. The PIV results show that the flame paths of motion are greatly influenced by the flow structure of the annular combustor. The optical diagnosis of the flame–flow interaction provides new insights into the ignition dynamics of the centrally staged annular combustor. [ABSTRACT FROM AUTHOR]

Modern low-emissions gas turbine combustors commonly employ a twin annular swirling flow configuration that consists of a central annular inner jet and a surrounding annular outer jet. This paper investigates the instability dynamics of helical vortices of such a flow configuration in non-reacting laminar setting with a varying outer jet swirling ratio S. The corresponding base flow features a centerbody wake (CBW), an outer recirculation zone, and a lip recirculation zone at low swirl ratios, whereas at high swirl ratios, the CBW is replaced by a central recirculation zone (CRZ). The azimuthal mode with wavenumber m = 1 is found to be absolutely unstable in the CBW region at low swirl ratios (S < 0.8), though not large enough to trigger global oscillations. With further increased swirl ratio (S ≥ 0.8), the CBW is suppressed and the CRZ supports a large region of absolute instability for both m = 1 and m = 2 modes. A three-dimensional nonlinear time stepping performed at S = 0.8 confirms that the absolute instability of m = 1 mode near the nozzle exit leads to the formation of a single-helix vortex in the near-field. Downstream of the CRZ, the m = 1 mode transits to convective instability, whereas the m = 2 mode is absolutely unstable. The single-helix vortex is consistently found to disappear in the far-field, where the flow dynamics is dominated by a double-helix vortex counter-winding around the tail of the CRZ. [ABSTRACT FROM AUTHOR]

The vortex ring state (VRS) of a rotor is associated with the development of the trailed vortex system in powered descending flight, where the topology of the vortex wake changes from its usual helical form into a toroidal form. In the VRS, the toroidal vortex ring envelops the entire rotor, and it sheds and reforms in an unsteady manner. In previous attempts to understand the basic phenomenology of the VRS, the focus was on the role of the trailed vortices in the transition to the VRS: computational and experimental work utilized rotor models to generate the trailing vortex wake, and mechanisms for the emergence of the VRS were postulated based on the interaction of the trailed vortices. In this paper, a different approach is taken: a set of experiments on a core annular jet flow are described, where the jet flow in counterflow is used to simulate a rotor in powered descent. It is shown that this leads to the formation of a flow field that shares many of the features of the VRS of a rotor system. This brings into question the role of the rotor blade trailing vortices in the development of the rotor wake VRS, and it is proposed instead that the interaction between the mean flow and counterflow drives the VRS phenomenon. [ABSTRACT FROM AUTHOR]

Air entrainment is a concern of paramount importance in the process industries, including nuclear power plant, hydraulic machines, water treatment plants, hydrocyclones, and power generation turbines. The ingression of the air into the equipment results in the reduction of the hydraulic efficiency. In the current paper, a combined experimental and numerical investigation has been performed using the commensurate high-speed photography and volume of fluid-based numerical simulations. The mitigation strategies for the reduction of the air ingression include the reduction of the critical height by increasing the number of intakes and changing the intake configuration. The viability of these solutions and the hydrodynamics behind the interfacial evolution leading to the air entrainment have been studied in detail. The air ingression progresses with the establishment of the flow patterns ranging from bubbly flow, slug flow, to the annular flow. The reduction of the critical height can be accomplished by increasing the number of intakes for the same cross-sectional area leading to the reduction in the Froude number F r = V down / g H local < 1 . Further increasing the number of intakes beyond four leads to the minimal reduction in the critical height in the tank. [ABSTRACT FROM AUTHOR]

This paper highlights a direct numerical simulation study on the flow field of a coaxial synthetic jet (CSJ) generated from two independently controlled synthetic jet actuators, which are combined coaxially with 0 ° orientation angle. The jet is issued into a quiescent environment from inner and annular openings (orifices) with equal hydraulic diameters, employing an oscillating boundary. Seven different mass flux ratios ( M r ) such as 0.25 , 0.5 , 0.75 , 1.0 , 1.5 , 2.0 , and 3.0 are considered for the study. The average velocity ( U avg ) of inner jet, measured at orifice exit, is kept at 0.7 m/s (Reynolds number, R e = 135), and the same is varied for the annular jet to achieve the desired M r s. The influence of M r s on the vortex rings, evolved from inner and annular orifices, along with their dynamics, is predicted by furnishing the instantaneous flow field. Also, we examine the effect of M r s on the mean flow parameters of the CSJ. Moreover, the CSJ flow field is compared with the inner cavity synthetic jet (SJ), and annular cavity SJ under identical conditions, to demonstrate the superior performance of the CSJ over the single cavity SJs. For CSJ, the azimuthal instability of the evolved vortex rings can be triggered by decreasing the M r , which results in a wide jet. For M r ≥ 1.0 , the CSJ retains its axisymmetric nature, and the interaction of vortex rings emanating from the inner and annular cavities influences the strength and spreading of the CSJ. The modal decomposition of the instantaneous flow field is also performed using proper orthogonal decomposition method to gain insight of the coherent vortical structures present in the modes. The study will be useful for deploying such novel coaxial synthetic jets in various applications. [ABSTRACT FROM AUTHOR]

Stability control of the convection flow field has always been a focal issue. The annular flow discussed in this work is a typical research model of microgravity fluid physics, which is extracted from the industrial crystal growth by the Czochralski method. It is believed that the instability of thermal convection is the key factor affecting the quality of crystal growth. Combining the reinforcement learning algorithm with the neural network, this paper proposes a control policy that makes forced convection compete with thermocapillary convection by changing the dynamic boundary conditions of the system. This control policy is successfully applied to the control of the quasi-equilibrium state of annular flow, and the global stability of the flow field is well maintained. It first experimentally makes the annular flow field under low and medium M a numbers achieve a quasi-equilibrium state, which is different from that before the onset of flow oscillations. Then, a simulation environment is created to imitate the experimental conditions. After training in the simulation environment, with the self-optimized algorithm, the machine learning approach can successfully maintain the simulation environment in a quasi-equilibrium state for a long period of time. Finally, the learning method is validated in the experimental environment, and a quasi-equilibrium state control policy is completely optimized by using the same optimization policy and similar neural network structure. This work demonstrates that the model can understand the physical environment and the author's control objectives through reinforcement learning. It is an important application of reinforcement learning in the real world and a clear demonstration of the research value of microgravity fluid physics. [ABSTRACT FROM AUTHOR]

This paper describes the breakage behaviors of oil droplets under different flow conditions when flowing turbulently in a modified concentric cylinder rotating device. The annular flow field in the modified device is locally isotropic turbulence, and the oil droplet diameter is only influenced by the turbulent kinetic energy (TKE) dissipation rate. The TKE dissipation rate distribution under experimental conditions is obtained by the Reynold stress turbulence model. The droplet-size distribution of each sampling tube is studied by experiments, and the influence rules of oil concentration, inlet droplet diameter, and TKE dissipation rate on the droplet Sauter diameter are obtained. Based on the Hinze model, the model of the maximum stable diameter of droplets under medium turbulence intensity is established, and the accuracy of the model is verified by experiments. The new model provides a scientific basis for predicting the oil droplet breakage and has a wide range of applications. [ABSTRACT FROM AUTHOR]

In this paper, an improved probability model is introduced to provide a more comprehensive prediction of annular flow. Unlike previous work, which did not consider atomization and deposition, as well as breakup and coalescence, simultaneously, the improved model integrates all four processes into its framework. The mechanisms of these processes are described in detail by the present model. When annular flow is fully developed, the four processes reach dynamic equilibrium. A numerical program is compiled based on the influence of the four processes on the droplets. Five important parameters, including the droplet-diameter probability density distribution, characteristic droplet diameters, entrainment ratio, liquid film thickness, and interfacial shear-stress coefficient, are calculated when the annular flow is in dynamic equilibrium. The validity and accuracy of the improved model are assessed by comparison with 377 cases from 12 experiments. For most cases used to validate the predicted droplet-diameter probability density distribution, the prediction curves closely match the experimental data. The mean absolute percentage errors (MAPEs) for the two experiments used to validate the predicted characteristic droplet diameters are 20.74% and 24.04%, respectively. Additionally, the mean MAPEs of the entrainment ratio, liquid film thickness, and interfacial shear-stress coefficient are found to be 34.11%, 20.60%, and 27.28%, respectively. This demonstrates the effectiveness and reliability of the improved probability model in predicting annular flow. [ABSTRACT FROM AUTHOR]

Characterization of viscous, non-Newtonian atomization by means of internal waves is presented for a twin-fluid injector. Atomization of such fluids is challenging, especially at low gas–liquid mass ratios. This paper details mechanisms that enhance their disintegration in a "wave-augmented atomization" process. The working fluid, banana puree, is shear-thinning and described by the Herschel–Bulkley model. Unlike a conventional airblast injector, an annular flow of banana puree is injected into a core steam flow, encouraging regular puree waves to form inside the nozzle. A pulsing flow develops with three distinct stages: stretch, bulge, and burst, leading to an annular puree sheet stretching down from the nozzle exit. Rayleigh–Taylor instabilities and viscosity gradients destabilize the surface. During wave collapse, the puree sheet bulges radially outward and ruptures violently in a radial burst. Near-nozzle dynamics propagate axially as periodic Sauter mean diameter fluctuations in a wave pattern. Numerical simulations reveal three atomization mechanisms that are a direct result of wave formation: (1) wave impact momentum, (2) pressure buildup, and (3) droplet breakaway. The first two are the forces that exploit puree sheet irregularities to drive rupture. The third occurs as rising waves penetrate the central steam flow; steam shear strips droplets off, and more droplets break away as the wave collapses and partially disintegrates. Waves collapse into the puree sheet with a radial momentum flux of 1.7 × 105 kg/m s2, and wave-induced pressure buildup creates a large pressure gradient across the puree sheet prior to bursting. [ABSTRACT FROM AUTHOR]

This paper proposes a data-driven method named stacked long short-term memory (S-LSTM) for predicting the future growth of acoustic pressure signals to detect precursors of combustion instability. The application of S-LSTM is investigated using the acoustic pressure data obtained from an annular combustor. The S-LSTM method is compared with the support vector machine (SVM) in terms of the predictive performance and also provides detailed insights into the influence of input choice by interpreting the results of S-LSTM. It is demonstrated that S-LSTM can effectively predict future pressure signals with a better error control performance compared to the SVM method. Furthermore, the feasibility of the S-LSTM in the thermoacoustic instability problem is verified using acoustic pressure data obtained from industrial combustion tests with a low-emission aero-engine. It is expected that the implementation of S-LSTM provides an early prediction solution to avoid thermoacoustic instability. [ABSTRACT FROM AUTHOR]

One method and two results have contributed to the complete understanding of magnetohydrodynamic laminar flow in an annular channel with a transverse magnetic field in this paper. In terms of the method, a computationally cheap semi-analytic algorithm is developed based on the spectral method and perturbation expansion. By virtue of the fast computation, dense cases with almost continuous varying Hartmann number M, Reynolds number Re, and cross section ratio η are calculated to explore the flow patterns that are missed in previous research. In terms of the results of the inertialess regime, we establish the average velocity map and electric-flow coupling delimitation in η-M space. Seven phenomenological flow patterns and their analytical approaches are identified. In terms of the results of the inertial regime, we examine the law of decreasing order-of-magnitude of inertial perturbation on primary flow with increasing Hartmann number. The proposed semi-analytic solution coincides with the R e 2 / M 4 suppression theory of J. A. Baylis and J. C. R. Hunt ["MHD flow in an annular channel; theory and experiment," J. Fluid Mech. 43, 423–428 (1971)] in the case of M < 40. When M > 40, the pair of trapezoid vortices of secondary flow begins to crack, and there is, therefore, a faster drop in inertial perturbation as R e 2 / M 5 , which is a new suppression theory. When M > 80, the anomalous reverse vortices are fully developed near Shercliff layers resulting in the weaker suppression mode of R e 2 / M 2.5 , which confirms the theoretical prediction of P. Tabeling and J. P. Chabrerie ["Magnetohydrodynamic secondary flows at high Hartmann numbers," J. Fluid Mech. 103(1), 225–239 (1981)]. [ABSTRACT FROM AUTHOR]

The paper discusses the formation and characteristics of stratified air–water downflow in the mesodomain. The stratified pattern, observed at low phase velocities, is characterized by air and water flowing side by side and both wetting the conduit wall, similar to stratified distributions in horizontal conduits. Such flow segregation without the influence of gravity is counterintuitive and no detailed investigation on the formation and flow physics of this distribution is reported till date. We have performed extensive experiments in glass conduits of 2.5–12.5 mm diameter where the two phases are introduced through T and Y junctions with different included angles between the entry arms. Our experiments reveal that stratified flows are formed up to a critical angle ϕ c subtended by the edges of the circumferential liquid film and beyond the critical wetting angle, the flow pattern is annular with liquid completely wetting the conduit wall. We further note ϕ c to depend on liquid properties, included angle of Y-entry, conduit diameter, and phase flow rates. Based on experimental observations, we propose a simplistic analysis to relate the liquid properties, conduit dimension, and Y-entry included angle and the liquid and gas flow rates to describe the formation of stratified and annular flow at the junction. The analysis also estimates the in situ void fraction during stratified gas–liquid downflow. [ABSTRACT FROM AUTHOR]

In this paper, the flow dynamics in the wake of a turbulent annular jet is studied using Time-Resolved Stereoscopic Particle Image Velocimetry and Proper Orthogonal Decomposition (POD). In this wake, a central recirculation zone is present which, under certain conditions, shows a low-frequency precessing motion. POD analysis of the measured velocity data shows that at zero swirl, an asymmetry is present in the wake, which motion is random in time. This asymmetry originates from a bifurcation of the flow once a threshold Reynolds number is exceeded. For low-swirl numbers, ranging from 0 < S < 0.12, the asymmetry is still present and its motion becomes structured into a well defined precession. For S > 0.12, the precession is gone and the motion of the asymmetric wake is again random in time, similar like the non-swirling jet. In this paper, a model is developed to describe the influence of swirl on the wake dynamics. The model assumes that perturbations in the inner shear layer near the bluff body wall are convected towards the stagnation point. These perturbations cause a shift in the stagnation points position. This shift is convected back to the inner shear layer through convection in the recirculating flow. The dynamics of this feedback mechanism can be modeled by the nonlinear delayed saturation model. In this paper, the model is adapted for swirling flow and simulations show that good agreement is found with the experiments. [ABSTRACT FROM AUTHOR]

In this paper, the stability of two co-axial immiscible fluids flowing in an annular duct is investigated. The inner layer consists of a shear-thinning fluid, which is surrounded by a Newtonian liquid annulus in the outer layer. A constant pressure gradient is applied to drive the flow in the annular channel. Linear stability analysis is employed to investigate the shear-thinning effect on the Rayleigh-Plateau instability and the interface wave instability. Results show that the Rayleigh-Plateau mode can be enhanced and the topological structures of the marginal stability curve of the Rayleigh-Plateau mode can be significantly changed by the shear-thinning effect. When the shear-thinning effect is strong, a case study shows that the Rayleigh-Plateau instability can be slightly suppressed by the viscosity stratification in the inner layer. The shear-thinning effect has a dual influence on the interface wave instability. It can either enhance or suppress the interface wave instability, depending on the thickness ratio and viscosity ratio between the outer layer and the inner layer. [ABSTRACT FROM AUTHOR]

In this paper, we are concerned with the transition to dynamical chaos and related anomalous transport of a passive scalar in the annular Kolmogorov flow, which is considered as a model of the barotropic zonal flows in the Earth's atmosphere and ocean or their laboratory analogs. The investigation of the anomalous transport is conducted within a dynamically consistent flow model describing the saturation of barotropic instability. The analysis is based on the numerical solution of equations of a quasi-two-dimensional flow in an annular channel with rigid walls taking into account the beta-effect and external (bottom) friction. It is supposed that the sinusoidal velocity profile of the Kolmogorov flow has three periods inside a channel and the sticking condition on the channel walls is satisfied. Four basic regimes arising with increasing flow supercriticality, the last of which corresponds to dynamical chaos, are distinguished. It is found that five modulated chains of wave-vortex structures with closed streamlines are formed in the channel and their temporal behavior is studied by making videos. The frequency–wavenumber spectra of the longitudinal velocity at certain values of radial coordinates are drawn and the largest Lyapunov exponent is determined in the regime of dynamical chaos. The relationship between the streamlines behavior and the discrete peaks of frequency–wavenumber spectra is elucidated. The occurrence of anomalous transport of a passive scalar is confirmed by drawing trajectories of tracer particles, as well as by determining exponents of the time dependence of mean particle displacement and its variance. [ABSTRACT FROM AUTHOR]

This study analyses the flow of Taylor bubbles through vertical and inclined annular pipes using high-fidelity numerical modeling. A recently developed phase-field lattice Boltzmann method is employed for the investigation. This approach resolves the two-phase flow behavior by coupling the conservative Allen–Cahn equation to the Navier–Stokes hydrodynamics. This paper makes contributions in three fundamental areas relating to the flow of Taylor bubbles. First, the model is used to determine the relationship between the dimensionless parameters (Eötvös and Morton numbers) and the bubble rise velocity (Froude number). There currently exists no surrogate model for the rise of a Taylor bubble in an annular pipe that accounts for fluid properties. Instead, relations generally include the diameter of the outer and inner pipes only. This study covered Eötvös numbers between 10 and 700 and Morton numbers between 10−6 and 100. As such, the proposed correlation is applicable to concentric annular pipes within this range of parameters. An assessment of the correlation to parameters outside of this range was made; however, this was not the primary scope for the investigation. Following this, the effect of pipe inclination was introduced with the impact on rise velocity measured. A correlation between the inclination angle and the rise velocity was proposed and its performance quantified against the limited experimental data available. Finally, the high-fidelity numerical results were analyzed to provide key insights into the physical mechanisms associated with annular Taylor bubbles and the shape they form. To extend this work, future studies on the effect of pipe eccentricity, diameter ratios, and pipe fittings (e.g., elbows and risers) on the flow of Taylor bubbles will be conducted. [ABSTRACT FROM AUTHOR]

This paper is devoted to the instability mechanisms of the thermocapillary flow in an annular pool. The stability limit of the axisymmetric basic state was studied over a wide range of Prandtl numbers (0.001 ≤ Pr ≤ 6.7) using linear stability analysis based on the spectral element method. The results demonstrate five types of instabilities, and the corresponding instability mechanisms were revealed by disturbance energy analysis. In particular, in the narrow range 1.4 ≤ Pr ≤ 1.53, with the increase in the Marangoni number, three transitions between the axisymmetric steady flow and the three-dimensional oscillatory flow were found, owing to the coupling and interaction of the hydrodynamic and hydrothermal instability mechanisms. [ABSTRACT FROM AUTHOR]

In this paper different flow patterns of an annular jet with a stepped-conical nozzle as well as the transition between these patterns are numerically investigated as a function of the swirl number S which is the ratio of tangential momentum flux to axial momentum flux. The Reynolds number of the jet based on the axial velocity and the nozzle hydraulic diameter is 180. The 3D Navier Stokes equations are solved using the direct numerical simulation. Four different flow patterns are identified and their associated flow structures are discussed. Starting from an annular jet at zero swirl, spinning vortices around the central axis originate with increasing swirl. As the swirl is further increased, the onset of vortex breakdown occurs, followed by jet attachment to the nozzle. Decreasing the swirl number back from this flow pattern, the Coanda effect near the nozzle outlet creates a wall jet. This wall jet remains till the decreasing swirl number equals to zero, showing hysteresis in flow patterns between an increase and a subsequent decrease in swirl. The determined flow states are experimentally validated. Potential applications related to these flow patterns and their hysteretic behavior are also briefly discussed. [ABSTRACT FROM AUTHOR]

This paper reports annular shear cell measurements granular flows with an eye towards experimentally confirming the flow regimes laid out in the elastic theory of granular flow. Tests were carried out on four different kinds of plastic spherical particles under both constant volume flows and constant applied stress flows. In particular, observations were made of the new regime in that model, the elastic-inertial regime, and the predicted transitions between the elastic-inertial and both the elastic-quasistatic and pure inertial regimes. [ABSTRACT FROM AUTHOR]

There is a widely popular perception in the cementing community that the turbulent regime should be used where possible in primary cementing of oil and gas wells. We conduct a detailed numerical study and analyze turbulent annular displacement flows. In particular, we show what roles viscous and buoyancy stresses play in the displacement efficiency. More importantly, we argue that the long-standing industry practice of preferring a turbulent displacement flow over laminar is over-simplistic and questionable. Indeed, we demonstrate that if the flow rate is increased very much when turbulent, steady displacements will be lost. [ABSTRACT FROM AUTHOR]

Annular flow is one of the most frequently observed flow patterns with gas–liquid two-phase flows in tubes or channels. In the annular flow pattern, a thin liquid film flows along the channel wall, while the gas flows in the center of the channel carrying liquid droplets in suspension. The fraction of the liquid flow rate that is transported as suspended droplets is quantified using the entrained liquid fraction (ELF), which is a key flow parameter in the analysis and modeling of annular flows. This review provides a critical assessment of ELF experimental data available in the open literature and of ELF prediction methods proposed to date. The experimental data assessment is carried out by means of a large ELF data bank collected from the literature (4175 data points from 53 literature studies; 10 fluids combinations; operating pressures from atmospheric to 20 MPa; experiments carried out with adiabatic, evaporating, and condensing flows through circular tubes, and non-circular channels with diameters from 3.02 to 155.7 mm), which is critically analyzed devoting special attention to important aspects not adequately addressed in previous studies, such as a cross-comparison between different ELF measuring techniques, and the analysis of flow development and gravity effects. The assessment of the ELF prediction methods focuses on 15 widely quoted methods, which are critically analyzed and whose prediction performance is evaluated against the measured data. The curated ELF experimental data bank is provided in full and usable form. Research gaps for further investigations are identified and discussed. [ABSTRACT FROM AUTHOR]

Modeling of annular flow with the computational fluid dynamics (CFD) is challenging as one has to consider several, rather different, phenomena simultaneously: the continuous liquid film, continuous gas core, and dispersed droplets. A morphology-adaptive multifield two-fluid model (MultiMorph) developed by Meller et al. ["Basic verification of a numerical framework applied to a morphology adaptive multifield two-fluid model considering bubble motions," Int. J. Numer. Methods Fluids 93(3), 748–773 (2021)], with three numerical phase fields, is well suited to simulate such multiple flow structures. Droplet formation plays an important role in annular flow, and a new droplet entrainment model is proposed, expressed as a phase morphology transfer term from the continuous liquid film to dispersed droplets phase field. The new model is developed based on the shear-off entrainment mechanism on the interfacial wave, implying that the droplet formation is dominated by the balance between the shear forces and the surface tension forces at the gas–liquid interface. In contrast to the existing entrainment models, the new model considers the flow parameters locally at the interface, and it is suitable for phase-resolving CFD frameworks without input of global parameters such as a pipe diameter. The proposed model is implemented in the MultiMorph framework based on the OpenFOAM Foundation release open-source CFD library. The performance of the new model is evaluated by conducting own annular flow experiments with void fraction measurements using electrical resistance tomography, as well as with comparison to published models from the literature. Qualitatively, the model can adequately resolve the formation of interfacial waves on the liquid film downstream from the inlet. The simulated droplets are primarily generated at the tip of such waves, which is consistent with the physical understanding and experimental observations of droplet entrainment. Quantitatively, the modeled entrained droplet fraction matches well the experimental observation in the developing entrainment region. The liquid film fraction obtained with the new model is analyzed and compared with the experimental data. Good agreement between measured and simulated statistics of the liquid film fraction, i.e., the mean, standard deviation, probability density function, and power spectral density, is demonstrated. [ABSTRACT FROM AUTHOR]

Draining from a closed-top tube occurs by downward displacement of liquid by air. The air volume grows inside the tube as an axisymmetric bullet-shaped finger similar to the Taylor bubble observed in gas–liquid slug flows, and the liquid drains as an annular film between the finger and the tube wall. The present study investigates the draining of shear-thinning vis-à-vis Newtonian liquids from closed-top circular millichannels. Numerical simulations using the phase-field method suggest that both the power law and Carreau models give close predictions of draining behavior in the investigated domain, i.e., for shear rate >0.1 s−1. The results are validated against experimental measurements based on high-speed photography and particle image velocimetry during the draining of aqueous solutions of carboxymethyl cellulose, xanthan gum, and glycerol. Further validations are performed for shear-thinning (using the power law, Carreau, and Carreau–Yasuda models) and Newtonian liquids with literature data on Taylor bubble rise in stationary liquid columns. The simulations using the power law model are used to explore additional insights into the flow physics. An increase in the apparent viscosity of shear-thinning liquids (by increasing the flow behavior index and/or flow consistency index) slows down the rate of Taylor finger growth. Increased liquid viscosity also results in a slender Taylor finger and leaves a higher amount of undrained liquid at the end of film-wise draining. The draining rate is a more significant function of flow behavior index n of the power law model for highly shear-thinning liquids (n < 0.6). [ABSTRACT FROM AUTHOR]

At the microfluidic scale, the utilization of helically coiled channels (HCCs), also known as a spiral channel, for two-phase flow offers numerous advantages in various applications. Existing articles mainly focus on the macro-scale transport, examining secondary flows induced in curved channels. The increasing demand, however, for innovative miniature equipment for thermal energy management emphasizes the importance of comprehending gas–liquid micro-scale flow in curved channels. Unfortunately, despite a vast body of literature on this paradigm, there is still a lack of systematic investigations into the underlying facets of two-phase micro-scale transport in HCCs. To address this gap, our study conducted experiments on adiabatic two-phase air–water flow inside an up-flow helical micro-scale tube. The tube had a hydraulic diameter of 0.87 mm, a coil diameter of 50 mm, and a helical pitch of 20 mm. The primary aim was to explore the impact of centrifugal force on flow pattern, void fraction, and frictional pressure drop characteristics. Additionally, we carefully examined the phase separation phenomenon influenced by the secondary flows induced by the curved channel. In particular, we compared the gas-core flow pattern (either throat-annular flow or annular flow), void fraction, and frictional pressure drop obtained from our experiments on the helical tube with corresponding results based on straight micro-scale channel configurations for an Eötvös number of approximately 0.01. In summary, this study delves deep into the crucial aspects of two-phase micro-scale transport in HCCs, contributing to a better understanding of these systems for future advancements in micro-channel applications. [ABSTRACT FROM AUTHOR]

Aerodynamic forces created on the lifting body, like an aircraft wing, can be optimized by modifying the flow field around it. These aerodynamic forces directly influence fuel consumption, thus the economy of flight. Various techniques have been developed and tested on wings for improving aerodynamic efficiency. The present work deals with the integration schemes of fanjets with aircraft wings for the same reason. Fanjets, like bladeless fans, can induce and entrain surrounding air for generation of lift and thrust; however, they are not yet integrated with the wing for modification of aerodynamic forces. In this novel research, the flow physics of the fanjet is explored by varying various geometric parameters. With an increase in the fanjet radius, the mass flow rate decreases, whereas the drag increases. A similar trend was observed for an increase in the jet width. For variation in the angle of attack, the maximum mass flow rate and minimum drag were observed at an angle of 0°. A similar analysis was carried out for semi-annular fanjets. Based on the results, the preferred selection for geometric parameters of annular and semi-annular fanjets was documented for integration with the aircraft wing. [ABSTRACT FROM AUTHOR]

High heat flux electronic devices put forward new requirements for heat dissipation, and boiling heat transfer technology is widely used because of its higher heat dissipation capacity. In this study, the volume of fluid method was employed, along with the incorporation of the Lee phase-change mass transfer model, to investigate two-phase flow and heat transfer in vertical upward rectangular microchannels. The heat flux was varied within the range of 10–40 kW/m2, while the mass flux was varied within the range of 200–600 kg/m2 s. With the increase in heat flux, bubble flow, slug flow, churn flow, and annular flow were found successively. A phase diagram was established to predict the flow pattern transition during the boiling process. When the flow pattern changes to the churn and the annular flow, the active nucleation site density increases obviously with the Boiling number (Bo). A new correlation was proposed for two-phase flow boiling heat transfer, suitable for vertical upward channels in microscale fluids. The friction factor obtained using the Darcy friction factor equation agrees well with the simulation results at a high-pressure drop. The instability in microchannels increases with the increase in heat flux, particularly in annular flow, resulting in more severe wall temperature fluctuations. [ABSTRACT FROM AUTHOR]

The flow characteristics of an annular expansion–deflection (ED) nozzle are investigated numerically during an ascending–descending trajectory over a large nozzle pressure ratio span. The shock pattern evolution, nozzle operation mode transition, nozzle flow hysteresis, and thrust variation during this trajectory are examined, and the interactions between them are discussed. A new criterion for distinguishing the open and closed wake modes of the ED nozzle is proposed based on a perturbation front in combination with sonic lines. Using this criterion, an exact boundary between the open and closed wake modes can be readily drawn. The present study shows that the interaction between the shock pattern transition and nozzle operation mode transition is indirect. During the ascent, the open-to-closed wake mode transition lays the foundation of the downstream shock pattern variation. During the descent, however, the stretching of the dominating Mach stem delays the nozzle closed-to-open wake mode transition. The different flow mechanisms during the ascent and descent result in an overall hysteresis of nozzle operation mode transition. The nozzle thrust undergoes dropping-rising developments (the thrust troughs) in the ascent and descent. These thrust troughs are also found to be the results of two very different flow mechanisms, namely, the forward–backward movement of shock separation point on nozzle shroud and the pressure rise on pintle base after closed-to-open mode transition. [ABSTRACT FROM AUTHOR]

Two-phase flow patterns are critical in falling film heat devices. Hydrodynamic characteristics of turbulent falling films across a horizontal triangular tube bundle were studied under a range of film Reynolds numbers and contact angles. A flow pattern map based on the k-means clustering approach was proposed after the intercolumn two-phase flow patterns were grouped. The results demonstrated that that the intercolumn liquid-vapor flow patterns in a horizontal tube bundle could be effectively and reasonably grouped using the k-means clustering approach according to the results of the void fraction and interface area. There are four intercolumn liquid-vapor flow patterns that were identified: bubbles flow, annular-slug-column flow, half-annular flow, and multi-bridges flow. The annular-slug-column flow pattern and the multi-bridges flow pattern are next to one another in the flow pattern map and are capable of transitioning one into the other. It is only through the two transitional flow patterns that the half-annular-column flow pattern and bubbles flow pattern can turn to one another. In the bubbles' flow pattern and half-annular-column flow pattern, respectively, the highest and least averaged heat transfer coefficients are attained. Simply raising the film Reynolds number or lowering the contact angle will not produce the best heat transfer results. [ABSTRACT FROM AUTHOR]

Turbulent structures in a concentric annular pipe within a uniform transverse magnetic field are examined for a liquid metal flow. Large-eddy simulations are performed to study the effect of magnetic field on turbulence suppression and heat transfer within this geometry. At the characteristic Prandtl number of liquid metals, the smallest scales based on temperature fluctuations are much larger than those of the velocity, which allows to resolve all the temperature scales with sufficient accuracy. The calculations are run at Reynolds number 8900 for three different Hartmann numbers, H a = 40 , 60 , 120. The comparison with available direct numerical simulation data shows encouraging agreement. The main findings of this work show a circumferential dependency of the flow characteristics on the local orientation of the magnetic field, with increased anisotropy observed at all Hartmann numbers studied. Anisotropic effects of the magnetic field are predominant for Ha = 60 and Ha = 120 causing turbulence to deviate from its conventional state. At these Hartmann numbers, a partial redistribution of the turbulent kinetic energy from the axial and radial components to the azimuthal component is observed. This effect, observed here for the first time, appears to be related to the appearance of coexisting quasi two-dimensional (2D) and three-dimensional (3D) turbulence states. Moreover, large skin friction increments are also observed at Ha = 60 and Ha = 120, while coherent structures stretching and streak suppression are found for all three Hartmann numbers. [ABSTRACT FROM AUTHOR]

There are a few reports about supercritical water circulating fluidized beds (SCWCFBs), and simulations were conducted via a two-fluid model to investigate flow dynamics in a riser of SCWCFB across different flow velocities, solid circulation rates, pressures, and temperatures. The investigated characteristics include the void fraction distributions, fluid velocities, particle velocities, and drag forces. The results show that the flow characteristics in the SCWCFB riser are similar to those in the traditional gas–solid riser. In the supercritical water fluidized bed riser, the void fractions are mainly in the range of 0.85–0.95. They are low at the bottom but high at the top and low near the wall but high at the center. Particle velocities are mainly distributed between 0 and 1 m s−1. They are upward near the axis of the riser and downward near the walls, showing the annular-core flow structures. Fluid paths are tortuous at low fluid velocities but straight at high fluid velocities. Particle drag forces are mainly around 0.7–0.8 times particle weights. The effects of pressures on flow dynamics in the SCWCFB riser can be ascribed to the changes in density and viscosity of supercritical water, affecting the ability of fluid to carry particles. [ABSTRACT FROM AUTHOR]

Experimental studies of air–water two-phase flows in a mini tube with an inner diameter of 3.12 mm have been conducted in terms of the flow pattern, void fraction, and pressure gradients. The fluid velocities range from 0.065 to 21.78 m/s and from 0.109 to 1.835 m/s, respectively. A right-angle prism is applied to capture images simultaneously from two perpendicular directions. Three-dimensional gas–liquid interfaces are reconstructed with the obtained images from two directions. The gas void fraction is investigated using this method, which is experimentally validated with the quick closing valves method. Three flow patterns are obtained in the present work, which is bubbly, slug, and annular flows. The Probability Density Functions of the cross-sectional void fraction show that the intermittent flow has two peaks and the annular flow has only one peak. Volumetric void fraction is affected by the flow pattern and the flow rate. However, at very high gas velocities, volumetric void fraction is independent of the liquid rate. A new frictional pressure drop correlation is proposed based on the experimental data. [ABSTRACT FROM AUTHOR]

Flooding may take place for in-tube condensation, causing unstable flow and deteriorated heat transfer. Here, the phase separation principle is proposed to eliminate flooding. Comparative experiments of condensation were performed in both bare tube (BT) and modulated heat transfer tube (MHTT) with a mesh membrane tube (MMT) insert. The working fluid is water-steam under a sub-atmospheric pressure. It was observed that slug flow exists at small mass fluxes and vapor mass qualities in BT. Due to the periodic formation of liquid column over the tube cross section, flooding indeed takes place, causing unstable flow, deteriorated heat transfer, and large pressure drop. The MHTT completely eliminated flooding, converting the unstable flow into a stable flow. Heat transfer coefficients are 7.47 times of those in BT, maximally, accompanying reduced pressure drops. High-speed visualization and theoretical analysis indicated that smaller pore size provides larger capability to prevent the vapor phase penetrating the mesh screen, resulting in larger driving force for liquid suction toward the MMT inside. The MMT provided a tunnel for liquid transportation in the upward direction, which is the mechanism to eliminate flooding. The modulation of the annular flow pattern was also performed and analyzed by using the phase separation principle, and the results indicated the effectiveness of MMT in the annular flow regime. This work is benefit for applications, such as air-cooled condenser, whose performance is important to influence the whole system performance for power generation. [ABSTRACT FROM AUTHOR]

The characteristics of twin-fluid atomization operating in the annular flow regime were studied experimentally under various gas-to-liquid ratios (GLRs) and injection pressures. The macroscopic morphology of the spray was obtained by shadowgraph, while the droplet size and velocity were measured using a phase-Doppler particle analyzer technique. It was found that the spray cone angle increases almost linearly with the GLR, and the axial distance required for droplet coalescence to outweigh the breakup decreases with increasing GLR. The Sauter mean diameter (SMD) first decreases and then increases along the axial direction due to the competition between turbulent breakup and droplet coalescence. The droplet size follows a lognormal distribution; the droplet velocity distribution is closer to a lognormal distribution under large GLRs, while it follows normal distribution with GLR = 3%. Regarding the radial distribution, low GLRs (3% and 5%) lead to a bimodal spatial velocity distribution, while for large GLRs, the droplet velocity decreases monotonically toward the far field. The spray tends to become more stable with increasing GLR and injection pressure P inj , whereas the SMD increases with increasing P inj . The underlying atomization mechanism in a twin-fluid injector in the annular flow state can be regarded as the disintegration of the initial liquid sheet by longitudinal Kelvin–Helmholtz instability followed by transverse Rayleigh–Taylor instability, which yields a direct proportionality of the droplet size to the initial liquid sheet thickness Δ L . Subsequently, for high P inj , the gas core shrinks and Δ L increases, which results in an increased SMD but enhanced atomization efficiency Δ L / SMD . [ABSTRACT FROM AUTHOR]

Longitudinal low-frequency thermoacoustic unstable modes in combustion chambers have been intensively studied experimentally, numerically, and theoretically, leading to significant progress in both understanding and controlling these acoustic modes. However, modern annular gas turbines may also exhibit azimuthal modes, which are much less studied and feature specific mode structures and dynamic behaviors, leading to more complex situations. Moreover, dealing with 10-20 burners mounted in the same chamber limits the use of high fidelity simulations or annular experiments to investigate these modes because of their complexity and costs. Consequently, for such circumferential acoustic modes, theoretical tools have been developed to uncover underlying phenomena controlling their stability, nature, and dynamics. This review presents recent progress in this field. First, Galerkin and network models are described with their pros and cons in both the temporal and frequency framework. Then, key features of such acoustic modes are unveiled, focusing on their specificities such as symmetry breaking, non-linear modal coupling, forcing by turbulence. Finally, recent works on uncertainty quantifications, guided by theoretical studies and applied to annular combustors, are presented. The objective is to provide a global view of theoretical research on azimuthal modes to highlight their complexities and potential. [ABSTRACT FROM AUTHOR]

Flow characteristics in combustion chambers of several non-circular cross-sectional configurations were numerically investigated. Different cross-sectional configurations, including annular and multi-sector annulus configurations with different fillets, were proposed, examined, and compared to the conventional circular arrangement. A numerical model was established and validated against other findings. The model was then used to study the effects of different design parameters, including convex and fillet radii, as well as some operational parameters, such as the angular velocity on the total pressure drop and the secondary flow characteristics like the swirl number, secondary vortex intensity, and helicity. The results revealed the optimum number of annulus sectors to be six, at which the highest hydraulic diameter was achieved for the same cross-sectional area. Optimum values of the inner and outer corner fillets were also obtained. Several valve arrangements were also investigated to achieve the highest possible valve area ratio for the proposed configurations. One of the proposed configurations, which incorporates six annular chambers with filleted corners with a hydraulic diameter ratio of 0.991 compared to the conventional circular design, denoted AFC4, achieved a more compact design while maintaining very comparable flow characteristics to those of the circular design. Although it needs more thorough investigations, it could present a viable alternative design option for the combustion chambers. [ABSTRACT FROM AUTHOR]

In the transonic flow over a hemisphere-on-cylinder turret, strong aero-optical effects can be caused by local shock/boundary-layer interactions and separation shear layers at the turret's zenith. The effects of an annular rough wall on the passive control of fluid and aero-optics are investigated by experimental measurements and numerical simulations. The local shock/boundary-layer interaction and separated shear layer at the zenith of the turret are recorded using shadowing and Mach–Zehnder interferometer measurements. The aero-optics are measured using a Shack–Hartmann wavefront sensor. The experimental results show that the annular rough wall on the turret weakens the local shock wave, moves the flow separation point forward, and reduces the wavefront distortion at the zenith. The rough wall functions for the shear stress transport (SST) k-ω turbulence model proposed by B. Aupoix ["Roughness corrections for the k–ω shear stress transport model: Status and proposals," J. Fluids Eng. 137, 021202 (2014)] and C.-H. Lee ["Rough boundary treatment method for the shear-stress transport k–ω model," Eng. Appl. Comput. Fluid 12, 261–269 (2018)] are used to further study the control effect of different roughnesses. Numerical simulations based on both rough wall functions show good agreement with the experimental measurements. For various transonic flows, the steady wavefront distortions at the zenith with the rough wall at roughness k s = 1 mm are 21%–50% smaller than those with smooth walls. The smaller the supersonic region, the more effective the rough wall is in reducing wavefront distortion. [ABSTRACT FROM AUTHOR]

The annular flow field between the car and the hoistway of an elevator is an important part of the piston effect. It is, thus, important to study the law of airflow in the annular space to reduce the piston effect. The study establishes a theoretical model of annular flow between the car and the hoistway of an ultra-high-speed elevator based on the semi-empirical theory of turbulence and the Bernoulli principle to obtain the law of distribution and the characteristic position of annular flow velocity. The correctness of the theoretical model was verified by comparing its calculations with the results of experimental measurements and numerical simulations. The effects of the speed of the car and its length as well as the blocking ratio and length of the hoistway on the distribution of annular flow velocity are analyzed, and the general law of influence of parametric changes on annular flow is summarized. The results show that the distribution of flow velocity in the annulus was positive, then became negative, and finally tended to zero with increasing distance from the wall. The characteristic velocity in the annular flow field was reliant on many factors, and its distribution was most significantly affected by the velocity of the car, followed by the blocking ratio and height of the hoistway. [ABSTRACT FROM AUTHOR]

Freely falling or rising objects in quiescent Newtonian fluid have been frequently encountered in nature or industry, such as the spreading of seeds from a tree or the movement of ores in deep sea mining. The dynamic behaviors of freely moving objects can provide a significant understanding of the evolution of the body wake and the resulting path instability. In this study, we present numerical simulations of freely falling annular disks released from quiescent water for relatively low Reynolds numbers from 10 to 500 while keeping the non-dimensional moment of inertia I * and inner to outer diameter ratio η constant. The falling stage experiences a variation from quasi-one-dimensional mode, steady oblique motion (SO motion), to the fully three-dimensional mode, helical motion. The stage diagram is plotted to show the variation tendency with the increment of Reynolds numbers. The detailed characteristics of the trajectories and orientation of the annular disks for different motions are analyzed. The corresponding vortical structures are presented, and an analog of the wingtip vortex is found at the outer rim of the disk for transitional and helical motion. A steady recirculation region of SO motion is observed, which is similar to that of a stationary disk but with complex multilayer structures formed by the combined effects of both the inner and outer rims. The limit streamline and pressure coefficient are investigated, demonstrating that the asymmetrical pressure distribution that exerts fluid forces and torques on the disk plays a crucial role in the dynamic response of the disk. Furthermore, combining the flow fields and fluid forces, the physical mechanism responsible for the diverse falling patterns is explored in detail. [ABSTRACT FROM AUTHOR]

The accurate simulation of upward multiphase flow of hydrate slurry in the annulus is one of the key scientific unsolved issues in natural gas hydrate solid fluidization exploitation. In this work, the upward multiphase flow of hydrate slurry in a vertical concentric annulus is simulated. The hydrate slurry hydrodynamic models suitable for pseudo-single-phase flow, bubbly flow, slug flow, and annular flow are proposed, respectively. Finally, the hydrate decomposition kinetic model is combined with the established annulus hydrate slurry multiphase flow model to simulate the multiphase flow of hydrate slurry in the annulus. The factors affecting flow behaviors are analyzed. During the upward flow in the annulus, the hydrate slurry temperature first decreases and then increases. As the inlet temperature increases, the fluid temperature, hydrate decomposition rate, and gas superficial velocity increase. During the upward flow in the annulus, hydrate may be formed again, which indicates that the error may be magnified due to ignoring hydrate formation. The larger the flow rate, the smaller the length of the slug flow. The larger the hydrate volume fraction, the higher the starting point of hydrate decomposition. These findings are of practical value to give a further understanding of hydrate slurry multiphase flow, which can promote further engineering application of natural gas hydrate solid fluidization exploitation. [ABSTRACT FROM AUTHOR]

The present study is devoted to a two-phase, gas–solid flow in an annular pipe (hollow cylinder) at an elevated pressure of 15 bars and moderate Reynolds number of circa 6000. The influence of the particle loading, the interaction between the phases, and turbulence dispersion on the flow dynamics is systematically studied by means of computational fluid dynamics simulations, employing the Ansys FLUENT commercial package. The cases with a particle volumetric fraction of 1.2% are referred to as "high particle loading," and those with 0.13% are denoted as "low particle loading." The following cases are investigated: (1) pure gas flow; (2) low particle loading two-phase flow with one-way coupling and with turbulence dispersion; (3) low particle loading two-phase flow with two-way coupling but without turbulence dispersion; (4) low particle loading two-phase flow with two-way coupling and with turbulence dispersion; (5) high particle loading two-phase flow with one-way coupling and with turbulence dispersion; (6) high particle loading two-phase flow with two-way coupling but without turbulence dispersion; and (7) high particle loading two-phase flow with two-way coupling and with turbulence dispersion. The boundary layer is found to grow without fluctuations of the turbulent kinetic energy (TKE) for cases 1, 2, and 5. For case 4, the TKE fluctuations have been identified, although they appear to be less substantial than those in cases 6 and 7. The authors attribute the semi-chaotic nature of the TKE fluctuations to the particle loading and two-way coupling. In addition, the onset and development of the flow instability have been observed at a random axial distance in cases 4, 6, and 7. Such instability is also attributed to the two-way coupling with turbulence dispersion in the flow. It is concluded that the particle loading, one-way, or two-way coupling between the phases, and the turbulence dispersion models significantly influence the development of the flow dynamics with the same inlet and boundary conditions. Consequently, it is not a trivial question, which result a user should trust. The present computational results inspire to perform verification as well as experimental validation of the simulations, so the simulation results can subsequently be used with confidence for design analysis. [ABSTRACT FROM AUTHOR]

The effect of radius ratio on turbulent convective heat transfer within a concentric annular pipe has been studied using direct numerical simulation. Four radius ratios ( R i / R o = 0.1–0.7) have been compared at a fixed Reynolds number, where Ri and Ro denote the radii of the inner and outer pipes, respectively. The statistical moments of the temperature field, budget balances of the temperature variance and turbulent heat fluxes, and turbulence structures that dominate the heat transfer process have been thoroughly studied in both physical and spectral spaces. It is observed that the radius ratio has a significant impact on the Nusselt numbers and skin friction coefficients of the inner and outer cylinder walls, and on the interaction of thermal boundary layers developed over these two curved walls. Owing to the curvature difference between the two cylinder surfaces, the thermal boundary layer developed over the outer cylinder wall is thicker than that over the inner cylinder wall. Also, turbulent heat transfer is more intense on the outer cylinder side than on the inner cylinder side. As the radius ratio decreases, the difference in turbulence statistics between the inner and outer cylinder sides becomes increasingly pronounced. It is also observed that both axial and azimuthal characteristic length scales of the most energetic turbulent thermal structures are larger on the inner cylinder side than on the outer cylinder side. [ABSTRACT FROM AUTHOR]

Pressure fluctuation produced in liquid rocket engines affects atomization of the annular liquid jet in the combustion chamber. A linear stability analysis of an annular liquid jet under acoustic oscillations was conducted in this work. Parametric instability was applied to study the problem, and the oscillations of the gas velocity were considered. The Floquet theory was used to solve the disturbance, from which the dispersion equation containing the coefficients of the infinite order matrix was derived; the order of the coefficient matrix affected the accuracy of the results. There were several unstable regions in the wavenumber range, obtained by solving the dispersion matrix. When the gas velocity oscillated, the amplitude of the surface wave displacement also oscillated. In various unstable regions, the growth of the surface wave of the annular liquid sheet was also different, but the crest never passed through the equilibrium position, which was more complicated than in the planar liquid sheet. Increasing oscillation frequency contributed to an increase in the wavenumber corresponding to the unstable area. Increasing the oscillation amplitude increased the maximum growth rate. The effect of physical parameters on the instability of the annular liquid jet was also discussed. [ABSTRACT FROM AUTHOR]

The atomization and the spray characteristics from an effervescent/aerated strut injector were studied experimentally using water as the liquid and nitrogen gas as the secondary fluid. The spray flow features were obtained through various experimental tools such as instantaneous Mie scattering technique, phase Doppler particle analyzer, and time-resolved shadowgraph images for different Weber numbers, momentum flux ratios, and gas to liquid ratios. A proper orthogonal decomposition method was used for an in-depth analysis of the spray structures. The instantaneous Mie scattering images showed the formation of the liquid clusters and the dominant two-phase flow interaction with increasing momentum flux and gas to liquid ratios in addition to the increased spray spread area. The statistical analysis of the Mie images revealed high fluctuating locations in the spray. It was observed that by increasing the momentum flux or gas to liquid ratios, the droplet diameter rapidly decreases due to the flow transition to the annular flow regime. Beyond a certain limit of momentum flux or gas to liquid ratios, no significant change in the diameter was observed. Furthermore, the spatio-temporal fluid dynamic features and the two-phase flow transition inside the spray region were brought out through the proper orthogonal decomposition. The power spectral density revealed the ligament breakup and the droplet formation below ≤500 Hz. Regardless of the momentum flux and gas to liquid ratios, a peak was observed between 2500 Hz and 3000 Hz due to the flow transition where the liquid fragment cluster formation and the flapping were found to be almost constant. [ABSTRACT FROM AUTHOR]

The slip of a small bubble (SB) from the annular film of the slug/Taylor bubble (TB) is often encountered in the chemical reactors and has intrigued many researchers. A combined experimental and numerical study has been performed to investigate the interaction of the SB and the slug bubble in a rectangular column with viscous fluids. The interaction behavior of the SB depends upon its diameter, deq, and thermo-physical properties of the fluid. The SB sprints away from the slug bubble at low Morton numbers, Mo = [(ρl − ρg)gμ 4 ]/[ρ1²Ïƒ ³ ] (sprint-away regime). On the other hand, SB interacts with TB due to its lower terminal velocity at higher Mo (bubble slip regime). The SB behaves independently ahead of the TB nose but accelerates linearly into its annular film. A regime map has been proposed to differentiate between the bubble slip and the sprint-away regime. The entrapped film between TB and SB is continuously fed from the annular film and avoids the coalescence. An ad hoc pressure jump model has been proposed to explain the repulsion of SB in the annular film. Furthermore, a modified lubrication theory based model predicted the stability of the entrapped film due to interfacial velocities and curvature. [ABSTRACT FROM AUTHOR]

Rotating cavity flows have been widely studied for years because of many implications that these have on industrial applications. These flows can indeed generate, under specific conditions, self-sustained oscillations that can be noisy or even dangerous for the integrity of a system. The coherent structures or flow modes composing this unsteady phenomenon usually called "pressure band phenomenon" are misunderstood and therefore difficult to control. In the present study, the dynamics of an annular rotor/stator cavity is investigated to shed some light on the flow organization and identify control strategies based on reliable theory and analysis to stabilize the observed undesired flow modes. No specific tool is known today to control a multi-frequency phenomenon. To address this first issue, the mode dominance and interactions appearing in this multi-frequency problem are investigated, thanks to dynamic mode tracking and control [M. Queguineur et al., "Dynamic mode tracking and control with a relaxation method," Phys. Fluids 31, 034101 (2019)]. The benefit of this method is to be able to follow in time several modes while controlling them one by one and observe mode dominance and interactions. This purely numerical controller shows that, here, the dominant mode of the annular cavity is at the source of another low frequency mode. Based on this information and to develop a physically relevant control strategy, the global linear stability framework previously used by Queguineur et al. ["Large eddy simulations and global stability analyses of an annular and cylindrical rotor/stator cavity limit cycles," Phys. Fluids 31, 104109 (2019)] is further developed to make use of the sensitivity to a base flow modification theory. This specific analysis indeed enables us to point out the exact location where the base flow should be modified to shift the dominant mode frequency and/or growth rate. In this context, passive controller positioning is identified for the studied annular cavity flow. Such strategies are then validated through new large eddy simulations of a controlled cavity using low amplitude injection/suction demonstrating the adequacy of the analysis and control strategy. [ABSTRACT FROM AUTHOR]

We present an experimental study of the flow of yield stress materials through annular abrupt expansions–contractions, to evaluate the flow invasion into the cavity formed in the larger cross section region. Steady inertialess flows of Carbopol® aqueous dispersions were investigated. The flow pattern reveals yielded and unyielded regions, which were visualized using tracer particles, laser sheets, and a digital camera. The yield surfaces were identified in the experiments by choosing large enough exposure times that allow sufficient particle displacement in the yielded region. To estimate the amount of fluid that remains stagnant in the cavity, we defined the invasion ratio, a quantity that was determined through image processing for different combinations of the governing parameters. The influence of the cavity diameter and axial length, eccentricity, and inlet velocity on the invasion ratio was investigated. Fore-aft asymmetric yield surfaces were observed for all tests, probably due to elastic effects. [ABSTRACT FROM AUTHOR]

In micro/nano-devices, the low-speed transport of mass, momentum, and energy through long-ducts is frequently encountered, thereby necessitating scientific investigations. Here, long-ducts of various annular cross sections conducting low-speed gas flows under the influence of a small pressure gradient are considered, in order to understand how the mass flow rate is affected by rarefaction, variations in the radius ratio, and eccentricity of annular geometries. The Boltzmann model equation is treated by a low-variance formulation and simulated by a stochastic kinetic particle-based approach, which addresses the deviation of the molecular distribution function from equilibrium to reduce computational cost significantly. An efficient parallel solver has also been developed and utilized in this research, which is validated against the reported results in the literature. The efficient kinetic particle treatment provides a powerful simulation tool to reveal multi-scale flow physics, which is essential to develop and optimize micro/nano-fluidic devices. [ABSTRACT FROM AUTHOR]