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Dzhaugashtin, K. E. "The critical jet flow regime". Fluid Dynamics 25, nr 3 (1990): 335–39. http://dx.doi.org/10.1007/bf01049812.
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Gryparis, Evgenios, i Georgios C. Georgiou. "Annular Poiseuille flow of Bingham fluids with wall slip". Physics of Fluids 34, nr 3 (marzec 2022): 033103. http://dx.doi.org/10.1063/5.0086511.
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We consider the annular Poiseuille flow of a Bingham fluid with wall slip. First, the analytical solution is derived for the case in which Navier-slip conditions are applied at the two cylinders. A sliding (pure plug) regime is observed below a critical pressure gradient, and a yielding regime is eventually encountered above another critical pressure gradient in which the material yields near the two walls and moves as a plug in a core region. An intermediate semi-sliding regime is observed when different slip laws apply at the two walls in which the material yields only near the wall corresponding to weaker slip and the unyielded plug slides along the other. Next, we consider the case where wall slip occurs above a critical wall shear stress, the slip yield stress, which is taken to be less than the yield stress, in agreement with experimental observations. In this case, a no-flow regime is observed below a critical pressure gradient, followed by the sliding and yielding regimes. The critical values of the pressure gradient defining the various flow regimes are determined, and the closed-form solutions are provided for all cases. These are compared with available theoretical and experimental results in the literature.
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Viebahn, Jan, i Henk A. Dijkstra. "Critical Transition Analysis of the Deterministic Wind-Driven Ocean Circulation — A Flux-Based Network Approach". International Journal of Bifurcation and Chaos 24, nr 02 (luty 2014): 1430007. http://dx.doi.org/10.1142/s0218127414300079.
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A new method for constructing complex networks from fluid flow fields is proposed. The approach focuses on spatial properties of the flow field, namely, on the topology of the streamline field. The network approach is applied to a model of the wind-driven ocean circulation, which exhibits the prototype of a critical transition, that is, a back-to-back saddle-node bifurcation related to two separate dynamical regimes. The network analysis enables a structural characterization of, on the one hand, the viscous regime as a weakly-connected and highly-assortative regime, and, on the other hand, of the inertial regime as a highly-connected and weakly-assortative regime. Moreover, the network analysis enables a robust early-warning signal of the critical transition emerging from the viscous regime: The upcoming global regime change induced by the critical transition may be anticipated by a drastic decrease in the overall closeness of the network, which reflects a preceding local regime change in the flow field. Hence, the results support the application of network-based topology measures complementary to time-series based statistical properties as leading indicators of critical transitions.
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Lamia, Hachemi Rachedi, Lakehal Moussa i Achour Bachir. "Modern vision for critical flow in an egg-shaped section". Water Science and Technology 84, nr 4 (12.07.2021): 840–50. http://dx.doi.org/10.2166/wst.2021.274.
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Abstract The critical regime plays a primordial role in the study of gradually varying flows by classifying flow regimes and slopes. Through this work, a new approach is proposed to analyze critical flow regime in an egg-shaped channel. Based on both the definition of Froude number and Achour and Bedjaoui general discharge relationship, a relation between critical and normal depths is derived and then graphically represented for the particular case of a smooth channel characterized by a generating diameter equal to 1 m. The results show the influence of the slope on the frequency of occurrence of the critical regime. At the same time and independently of the flow rate, a very advantageous approach for the calculation of the Froude number has been proposed. The study shows that there are six zones to differentiate the various flow states, namely: on the one hand for steep slopes two subcritical zones interspersed by a supercritical zone and on the other hand for mild slopes a zone corresponding to uniform flow, an area where the flow is probably gradually varied and finally an area where the flow is abruptly varied. Based on the specific energy equation, a validation process concluded that the proposed relationships were reliable.
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Yu, Liyuan, Richeng Liu i Yujing Jiang. "A Review of Critical Conditions for the Onset of Nonlinear Fluid Flow in Rock Fractures". Geofluids 2017 (6.07.2017): 1–17. http://dx.doi.org/10.1155/2017/2176932.
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Selecting appropriate governing equations for fluid flow in fractured rock masses is of special importance for estimating the permeability of rock fracture networks. When the flow velocity is small, the flow is in the linear regime and obeys the cubic law, whereas when the flow velocity is large, the flow is in the nonlinear regime and should be simulated by solving the complex Navier-Stokes equations. The critical conditions such as critical Reynolds number and critical hydraulic gradient are commonly defined in the previous works to quantify the onset of nonlinear fluid flow. This study reviews the simplifications of governing equations from the Navier-Stokes equations, Stokes equation, and Reynold equation to the cubic law and reviews the evolutions of critical Reynolds number and critical hydraulic gradient for fluid flow in rock fractures and fracture networks, considering the influences of shear displacement, normal stress and/or confining pressure, fracture surface roughness, aperture, and number of intersections. This review provides a reference for the engineers and hydrogeologists especially the beginners to thoroughly understand the nonlinear flow regimes/mechanisms within complex fractured rock masses.
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Kunii, Kohei, Takahiro Ishida, Yohann Duguet i Takahiro Tsukahara. "Laminar–turbulent coexistence in annular Couette flow". Journal of Fluid Mechanics 879 (1.10.2019): 579–603. http://dx.doi.org/10.1017/jfm.2019.666.
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Annular Couette flow is the flow between two coaxial cylinders driven by the axial translation of the inner cylinder. It is investigated using direct numerical simulation in long domains, with an emphasis on the laminar–turbulent coexistence regime found for marginally low values of the Reynolds number. Three distinct flow regimes are demonstrated as the radius ratio $\unicode[STIX]{x1D702}$ is decreased from 0.8 to 0.5 and finally to 0.1. The high-$\unicode[STIX]{x1D702}$ regime features helically shaped turbulent patches coexisting with laminar flow, as in planar shear flows. The moderate-$\unicode[STIX]{x1D702}$ regime does not feature any marked laminar–turbulent coexistence. In an effort to discard confinement effects, proper patterning is, however, recovered by artificially extending the azimuthal span beyond $2\unicode[STIX]{x03C0}$. Eventually, the low-$\unicode[STIX]{x1D702}$ regime features localised turbulent structures different from the puffs commonly encountered in transitional pipe flow. In this new coexistence regime, turbulent fluctuations are surprisingly short-ranged. Implications are discussed in terms of phase transition and critical scaling.
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Sarraf Shirazi, Alireza, i Ian Frigaard. "SlurryNet: Predicting Critical Velocities and Frictional Pressure Drops in Oilfield Suspension Flows". Energies 14, nr 5 (25.02.2021): 1263. http://dx.doi.org/10.3390/en14051263.
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Improving the accuracy of the slurry flow predictions in different operating flow regimes remains a major focus for multiphase flow research, and it is especially targeted at industrial applications such as oil and gas. In this paper we develop a robust integrated method consisting of an artificial neural network (ANN) and support vector regression (SVR) to estimate the critical velocity, the slurry flow regime change, and ultimately, the frictional pressure drop for a solid–liquid slurry flow in a horizontal pipe, covering wide ranges of flow and geometrical parameters. Three distinct datasets were used to develop machine learning models with totals of 100, 325, and 125 data points for critical velocity, and frictional pressure drops for heterogeneous and bed-load regimes respectively. For each dataset, 80% of the data were used for training and the rest 20% for evaluating the out of sample performance. The K-fold technique was used for cross-validation. The prediction results of the developed integrated method showed that it significantly outperforms the widely used existing correlations and models in the literature. Additionally, the proposed integrated method with the average absolute relative error (AARE) of 0.084 outperformed the model developed without regime classification with the AARE of 0.155. The proposed integrated model not only offers reliable predictions over a wide range of operating conditions and different flow regimes for the first time, but also introduces a general framework of how to utilize prior physical knowledge to achieve more reliable performances from machine learning methods.
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CHURILOV, S. M. "Nonlinear stability of a stratified shear flow in the regime with an unsteady critical layer. Part 2. Arbitrary stratification of asymmetric flow". Journal of Fluid Mechanics 392 (10.08.1999): 233–75. http://dx.doi.org/10.1017/s0022112099005443.
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A weakly nonlinear analysis of the downstream evolution of weakly unstable disturbances in a stably stratified mixing layer with a large Reynolds number is carried out. No other requirements are imposed upon velocity and density profiles, thus making it possible to overcome the restrictions placed in earlier studies (Brown & Stewartson 1978; Brown et al. 1981; Churilov & Shukhman 1987, 1988) by a particular choice of weakly supercritical flow models assuming symmetry. For each of the two critical layer regimes possible here, viscous and unsteady, evolution equations are obtained, their solutions and competition between nonlinearities in the course of instability development are analysed, and evolution scenarios for unstable disturbances are constructed for different levels of their supercriticality. It is established that the regime with a nonlinear critical layer does not arise in an evolutionary manner, except for the previously studied case of a weak stratification (Shukhman & Churilov 1997). It is shown that while in the viscous critical layer regime the relaxation of assumptions of the symmetry and weak supercriticality of the flow produces no fundamental changes in the theory, in the unsteady critical layer regime a new (non-dissipative cubic) nonlinearity appears which governs the instability development on equal terms with two already known nonlinearities. Results are illustrated by calculations for two families of flow models with a controlled degree of asymmetry.
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Ahmed, A., R. Manzoor, S. U. Islam i H. Rahman. "Numerical investigation for flow over a square rod through a passive control method at various Reynolds numbers". Canadian Journal of Physics 98, nr 5 (maj 2020): 425–32. http://dx.doi.org/10.1139/cjp-2019-0155.
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This work presents a numerical simulation performed to study the effect of Reynolds number (Re = 80–200) on fluid flow over a square rod attached to two small controlling rods using the Lattice Boltzmann method. For this reason, the spacing ratio between the control rods and main rod varies systematically from g = 0.5–5. Flow has been subdivided into three flow regimes based on spacing ratios. The first flow regime is considered at a small gap (g = 0.5, 1, and 1.5), the second flow regime is obtained at a moderate gap (g = 2, 2.5, and 3), and the third flow regime is considered a at large gap (g = 4–5). Five different types of flow modes were noticed in the given flow regimes. The values Re = 200 and g = 5 were found to be critical due to a sudden change in flow characteristics. The maximum value of Cdmean is 0.869 and the largest percent reduction (65.15%) in the mean drag coefficient was found at Re = 200 and g = 2.
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Merritt, Angela, Belize Lane i Charles Hawkins. "Classification and Prediction of Natural Streamflow Regimes in Arid Regions of the USA". Water 13, nr 3 (1.02.2021): 380. http://dx.doi.org/10.3390/w13030380.
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Understanding how natural variation in flow regimes influences stream ecosystem structure and function is critical to the development of effective stream management policies. Spatial variation in flow regimes among streams is reasonably well understood for streams in mesic regions, but a more robust characterization of flow regimes in arid regions is needed, especially to support biological monitoring and assessment programs. In this paper, we used long-term (41 years) records of mean daily streamflow from 287 stream reaches in the arid and semi-arid western USA to develop and compare several alternative flow-regime classifications. We also evaluated how accurately we could predict the flow-regime classes of ungauged reaches. Over the 41-year record examined (water years 1972–2013), the gauged reaches varied continuously from always having flow > zero to seldom having flow. We predicted ephemeral and perennial reaches with less error than reaches with an intermediate number of zero-flow days or years. We illustrate application of our approach by predicting the flow-regime classes at ungauged reaches in Arizona, USA. Maps based on these predictions were generally consistent with qualitative expectations of how flow regimes vary spatially across Arizona. These results represent a promising step toward more effective assessment and management of streams in arid regions.
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Yao, Jianfeng, Wenjuan Lou, Guohui Shen, Yong Guo i Yuelong Xing. "Influence of Inflow Turbulence on the Flow Characteristics around a Circular Cylinder". Applied Sciences 9, nr 17 (2.09.2019): 3595. http://dx.doi.org/10.3390/app9173595.
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To study the influence of turbulence on the wind pressure and aerodynamic behavior of smooth circular cylinders, wind tunnel tests of a circular cylinder based on wind pressure testing were conducted for different wind speeds and turbulent flows. The tests obtained the characteristic parameters of mean wind pressure coefficient distribution, drag coefficient, lift coefficient and correlation of wind pressure for different turbulence intensities and of Reynolds numbers. These results were also compared with those obtained by previous researchers. The results show that the minimum drag coefficient in the turbulent flow is basically constant at approximate 0.4 and is not affected by the turbulence intensity. When the Reynolds number is in the critical regime, the lift coefficient increased sharply to 0.76 in the smooth flow, indicating that flow separation has an asymmetry; however, the asymmetry does not appear in the turbulent flow. Drag coefficient decreases sharply at a lower critical Reynolds number in the turbulent flow than in the smooth flow. In the smooth flow, the separation point is about 80° in the subcritical regime; it suddenly moves backwards in the critical regime and remains almost unchanged at about 140° in the supercritical regime. However, the angular position of the separation point will always be about 140° for turbulent flow for the Reynolds number in these three regimes. Turbulence intensity and Reynolds number have a significant effect on the correlation of wind pressures around the circular cylinder. Turbulence will weaken the positive correlation of the same side and also reduce the negative correlation between the two sides of the circular cylinder.
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Cheng, K. C., i Y. W. Kim. "Flow Visualization Studies on Vortex Instability of Natural Convection Flow Over Horizontal and Slightly Inclined Constant-Temperature Plates". Journal of Heat Transfer 110, nr 3 (1.08.1988): 608–15. http://dx.doi.org/10.1115/1.3250536.
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Flow visualization experiments were performed in a low-speed wind tunnel to study vortex instability of laminar natural convection flow along inclined isothermally heated plates having inclination angles from the horizontal of θ = 0, 5, 10, 15 and 20 deg. The temperature difference between plate surface and ambient air ranged from ΔT = 15.5 to 37.5°C and the local Grashof number range was Grx = 1.02×106 to 2.13×108. Three characteristic flow regimes were identified as follows: a two-dimensional laminar flow, a transition regime for developing longitudinal vortices, and a turbulent regime after the breakdown of the longitudinal vortices. Photographs are presented of side and top views of the flow and of cross-sectional views of the developing views of the developing secondary flow in the postcritical regime. Instability data of critical Grashof number and wavelength are presented and are compared with the theoretical predictions from the literature.
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Kosmatskiy, Ya Ig, M. Yu Egorov i V. D. Lychakov. "Flow analysis in seamless hot-extruded pipes with helical inner ribbing surface". Journal of Physics: Conference Series 2088, nr 1 (1.11.2021): 012011. http://dx.doi.org/10.1088/1742-6596/2088/1/012011.
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Abstract The article is devoted to numerical simulation of heat transfer processes occurring during the flow of a coolant in seamless hot-extrusion pipes with a spiral inner fin surface (TMK-IRS). A description of the numerical modeling technique is given along with the interface of the program used to create different types of internal fins. Thermohydraulic analysis of finned pipes for transient, turbulent and laminar flow regimes has been carried out. An estimate of the critical Reynolds number characterizing the transition to a turbulent regime, the nature of the transient flow regime in comparison with other classical cases is given.
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Takeda, Kazuki, Yohann Duguet i Takahiro Tsukahara. "Intermittency and Critical Scaling in Annular Couette Flow". Entropy 22, nr 9 (4.09.2020): 988. http://dx.doi.org/10.3390/e22090988.
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The onset of turbulence in subcritical shear flows is one of the most puzzling manifestations of critical phenomena in fluid dynamics. The present study focuses on the Couette flow inside an infinitely long annular geometry where the inner rod moves with constant velocity and entrains fluid, by means of direct numerical simulation. Although for a radius ratio close to unity the system is similar to plane Couette flow, a qualitatively novel regime is identified for small radius ratio, featuring no oblique bands. An analysis of finite-size effects is carried out based on an artificial increase of the perimeter. Statistics of the turbulent fraction and of the laminar gap distributions are shown both with and without such confinement effects. For the wider domains, they display a cross-over from exponential to algebraic scaling. The data suggest that the onset of the original regime is consistent with the dynamics of one-dimensional directed percolation at onset, yet with additional frustration due to azimuthal confinement effects.
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Nijjer, Japinder S., Duncan R. Hewitt i Jerome A. Neufeld. "The dynamics of miscible viscous fingering from onset to shutdown". Journal of Fluid Mechanics 837 (5.01.2018): 520–45. http://dx.doi.org/10.1017/jfm.2017.829.
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We examine the full ‘life cycle’ of miscible viscous fingering from onset to shutdown with the aid of high-resolution numerical simulations. We study the injection of one fluid into a planar two-dimensional porous medium containing another, more viscous fluid. We find that the dynamics are distinguished by three regimes: an early-time linearly unstable regime, an intermediate-time nonlinear regime and a late-time single-finger exchange-flow regime. In the first regime, the flow can be linearly unstable to perturbations that grow exponentially. We identify, using linear stability theory and numerical simulations, a critical Péclet number below which the flow remains stable for all times. In the second regime, the flow is dominated by the nonlinear coalescence of fingers which form a mixing zone in which we observe that the convective mixing rate, characterized by a convective Nusselt number, exhibits power-law growth. In this second regime we derive a model for the transversely averaged concentration which shows good agreement with our numerical experiments and extends previous empirical models. Finally, we identify a new final exchange-flow regime in which a pair of counter-propagating diffusive fingers slow exponentially. We derive an analytic solution for this single-finger state which agrees well with numerical simulations. We demonstrate that the flow always evolves to this regime, irrespective of the viscosity ratio and Péclet number, in contrast to previous suggestions.
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Liu, Haihu, i Yonghao Zhang. "Lattice Boltzmann Simulation of Droplet Generation in a Microfluidic Cross-Junction". Communications in Computational Physics 9, nr 5 (maj 2011): 1235–56. http://dx.doi.org/10.4208/cicp.231009.101110s.
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AbstractUsing the lattice Boltzmann multiphase model, numerical simulations have been performed to understand the dynamics of droplet formation in a microfluidic cross-junction. The influence of capillary number, flow rate ratio, viscosity ratio, and viscosity of the continuous phase on droplet formation has been systematically studied over a wide range of capillary numbers. Two different regimes, namely the squeezinglike regime and the dripping regime, are clearly identified with the transition occurring at a critical capillary number Cacr. Generally, large flow rate ratio is expected to produce big droplets, while increasing capillary number will reduce droplet size. In the squeezing-like regime (Ca ≤ Cacr), droplet breakup process is dominated by the squeezing pressure and the viscous force; while in the dripping regime (Ca ≤ Cacr), the viscous force is dominant and the droplet size becomes independent of the flow rate ratio as the capillary number increases. In addition, the droplet size weakly depends on the viscosity ratio in both regimes and decreases when the viscosity of the continuous phase increases. Finally, a scaling law is established to predict the droplet size.
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Nikushchenko, Dmitry, Valery Pavlovsky i Elena Nikushchenko. "Fluid Flow Development in a Pipe as a Demonstration of a Sequential Change in Its Rheological Properties". Applied Sciences 12, nr 6 (17.03.2022): 3058. http://dx.doi.org/10.3390/app12063058.
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The sequence of the process of changing the velocity profiles and the laws of resistance during the flow of a fluid in a pipe is considered. With the increasing of the Reynolds number, we obtain the transition of the flow regime from laminar to turbulent. In the presence of small additives of polymers, when the Toms effect is observed in the fluid flow, the turbulent regime changes with a further increase in the Reynolds number to another regime, the rheology of which leads to laminar velocity profiles and corresponding resistance laws. Then, with an increase in the Reynolds number for polymer solutions, the limiting Virk flow regime with its own rheology is reached. All the mentioned flow regimes and all types of rheology can be described using one rheological relation, which is a power-law generalization of Newton’s formula, by changing the values of the power value in this ratio upon reaching the corresponding critical Reynolds numbers. This generalization can be extended to the spatial case of flow and the rheological relation can be represented in tensor form with a further system of differential equations for a fluid flow with an arbitrary rheology. After that, boundary value problems in fluid mechanics can be solved for any fluid flow regime.
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Schippa, Leonardo, Ferruccio Doghieri, Anna Pellegrino i Elisa Pavesi. "Thixotropic Behavior of Reconstituted Debris-Flow Mixture". Water 13, nr 2 (11.01.2021): 153. http://dx.doi.org/10.3390/w13020153.
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Time-dependent rheological properties and thixotropy of reconstituted debris-flows samples taken from channel bank deposits are examined using a commercial rheometer equipped with a vane rotor geometric system. Sweep tests and creep tests were carried out involving mixtures having different grain concentrations ranging between 50% and 58%. Different initial conditions of the mixtures were considered in order to analyze the effects of aging and rejuvenation (thixotropy) over a short period of time and long period of time. Tested slurries show viscosity bifurcation, yield stress and time-dependent behavior. According to the experimental results, three different regimes were identified: a lower shear rate regime, corresponding to a shear rate lower than the critical value; an intermediate banding shear rate regime characterized by static and dynamic yield stress level; and a higher shear rate regime where the flowing debris behaves as a non-Newtonian fluid characterized by a constant steady state ultimate apparent viscosity. In any case, the initial state of the mixture and the sediment concentration affects the ultimate steady state rheology and the time-dependent (thixotropy) slurries’ behavior.
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Schippa, Leonardo, Ferruccio Doghieri, Anna Maria Pellegrino i Elisa Pavesi. "Thixotropic Behavior of Reconstituted Debris-Flow Mixture". Water 13, nr 2 (11.01.2021): 153. http://dx.doi.org/10.3390/w13020153.
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Time-dependent rheological properties and thixotropy of reconstituted debris-flows samples taken from channel bank deposits are examined using a commercial rheometer equipped with a vane rotor geometric system. Sweep tests and creep tests were carried out involving mixtures having different grain concentrations ranging between 50% and 58%. Different initial conditions of the mixtures were considered in order to analyze the effects of aging and rejuvenation (thixotropy) over a short period of time and long period of time. Tested slurries show viscosity bifurcation, yield stress and time-dependent behavior. According to the experimental results, three different regimes were identified: a lower shear rate regime, corresponding to a shear rate lower than the critical value; an intermediate banding shear rate regime characterized by static and dynamic yield stress level; and a higher shear rate regime where the flowing debris behaves as a non-Newtonian fluid characterized by a constant steady state ultimate apparent viscosity. In any case, the initial state of the mixture and the sediment concentration affects the ultimate steady state rheology and the time-dependent (thixotropy) slurries’ behavior.
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Ren, Jie, Song Fu i Rene Pecnik. "Linear instability of Poiseuille flows with highly non-ideal fluids". Journal of Fluid Mechanics 859 (16.11.2018): 89–125. http://dx.doi.org/10.1017/jfm.2018.815.
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The objective of this work is to investigate linear modal and algebraic instability in Poiseuille flows with fluids close to their vapour–liquid critical point. Close to this critical point, the ideal gas assumption does not hold and large non-ideal fluid behaviours occur. As a representative non-ideal fluid, we consider supercritical carbon dioxide ($\text{CO}_{2}$) at a pressure of 80 bar, which is above its critical pressure of 73.9 bar. The Poiseuille flow is characterized by the Reynolds number ($Re=\unicode[STIX]{x1D70C}_{w}^{\ast }u_{r}^{\ast }h^{\ast }/\unicode[STIX]{x1D707}_{w}^{\ast }$), the product of the Prandtl ($Pr=\unicode[STIX]{x1D707}_{w}^{\ast }C_{pw}^{\ast }/\unicode[STIX]{x1D705}_{w}^{\ast }$) and Eckert numbers ($Ec=u_{r}^{\ast 2}/C_{pw}^{\ast }T_{w}^{\ast }$) and the wall temperature that in addition to pressure determine the thermodynamic reference condition. For low Eckert numbers, the flow is essentially isothermal and no difference with the well-known stability behaviour of incompressible flows is observed. However, if the Eckert number increases, the viscous heating causes gradients of thermodynamic and transport properties, and non-ideal gas effects become significant. Three regimes of the laminar base flow can be considered: the subcritical (temperature in the channel is entirely below its pseudo-critical value), transcritical and supercritical temperature regimes. If compared to the linear stability of an ideal gas Poiseuille flow, we show that the base flow is modally more unstable in the subcritical regime, inviscid unstable in the transcritical regime and significantly more stable in the supercritical regime. Following the principle of corresponding states, we expect that qualitatively similar results will be obtained for other fluids at equivalent thermodynamic states.
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Luo, S. C., i T. L. Gan. "Flow past two tandem circular cylinders of unequal diameter". Aeronautical Journal 96, nr 953 (marzec 1992): 105–14. http://dx.doi.org/10.1017/s0001924000024647.
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SummaryThe present work investigates the structure associated with flow past two tandem circular cylinders with a diameter ratio of 0·33. The smaller cylinder is always upstream. The two parameters varied in the present work are the spacing between the cylinders denoted by l/d2where l is the centre to centre spacing and d2 is the diameter of the downstream cylinder, and the Reynolds number Re based on d2 Just like the case of flow past two equal size cylinders, a critical spacing was found to exist. For spacing less than the critical value, shear layers that separate from the upstream cylinder reattach onto the downstream cylinder (reattached flow regime) whereas for spacing larger than the critical value both cylinders shed vortices (co-shedding flow regime). For diameter ratio dl/d2 of 0·33 and at Re ≈ 6·27 × 104, the critical spacing is at l/d2 ≈ 1·8 to 2·2. In 1·8 ≤ l/d2 ≤ 2·2, bi-stable flow situation where the flow structure changes intermittently between the reattached and the co-shedding types was observed and the probability that the flow is co-shedding within the bi-stable regime is found to follow a normal distribution.It was also found that the reattached flow regime can be sub-divided into two sub-regimes. In 0·66 ≤ l/d2 ≤ 1·2 (sub-regime 1), the vortex formation length, lf, of vortices shed by the downstream cylinder appears to be shorter, resulting in larger drag force on the downstream cylinder and larger r.m.s. pressure around it. In 1·4 ≤ l/d2 ≤ 1·18 (subregime 2), lf appears to be larger which results in a smaller drag force and smaller r.m.s. pressure distribution when compared with sub-regime 1. Only the downstream cylinder has a Strouhal number S2 associated with it in the reattached flow regime. The general trend is that of a slight reduction in S2 with increasing l/d2 which is consistent with the relation of larger lf at larger l/d2 in the reattached flow regime. In the coshedding regime, the Strouhal numbers of both cylinders are similar.The Reynolds number was varied in the range 3·15 × 104 ≤ Re ≤ 8·81 × 104. It was observed that both the mean drag and the r.m.s. lift and drag forces reduce in magnitude when Re was increased. From the spectra of the lift force, reduction in C'L was found to be due to a reduction in the regularity of the vortex shedding process. From the pressure distribution of the downstream cylinder at constant l/d2, it was found that the reduction in mean drag with increasing Re is caused by an increase in base pressure which in turn is likely to be the consequence of an increase in vortex formation length. The upper and lower limits of the range in the lldi at which bistable flow was detected were found to decrease with increasing Re. At l/d2 = 2·0, the flow was of the reattached type at Re = 3·15 × 104 and 4·75 × 104 but became bi-stable at Re ≥ 6·27 × 104.
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Maxwell, James L., Nicholas Webb, Douglas Bradshaw, Marcie R. Black, Karlene Maskaly, Craig A. Chavez, Miguel Espinoza i in. "On “how to start a fire”, or transverse forced-convection, hyperbaric laser chemical vapor deposition of fibers and textiles". Textile Research Journal 84, nr 18 (17.04.2014): 1976–86. http://dx.doi.org/10.1177/0040517514527373.
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This work explores the transverse forced flow of precursor gases during hyperbaric pressure laser chemical vapor deposition (HP-LCVD). Axial and mass growth rates of carbon fibers are measured experimentally, and a numerical model is developed that provides fiber growth rates in both the mass-transport-limited (MTL) and kinetically limited (KL) regimes. It is found that the fiber’s transport-limited rate increases as the square root of the flow velocity, while simultaneously, the temperature drops with the inverse square root of the flow velocity. Growth is enhanced by forced flow so long as the reaction zone remains within the MTL regime; upon reaching a critical temperature and flow rate, however, fibers enter the KL regime, and the growth rate declines with rising flow rate. Molecular properties of the precursors employed and gas concentrations ultimately determine the range of the MTL and the locations of the critical temperature and flow rate. The growth rates of fibers can indeed be enhanced by transverse forced convection—to at least three times the zero-flow steady-state rate, provided an MTL regime exists. Complex three-dimensional structures may be grown from these fibers in a freeform manner, and the more rapidly such microstructures can be fabricated, the more practical HP-LCVD becomes for industrial use, including the fabrication of novel textiles.
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Kološ, Ivan, Vladimíra Michalcová i Lenka Lausová. "Numerical Analysis of Flow Around a Cylinder in Critical and Subcritical Regime". Sustainability 13, nr 4 (14.02.2021): 2048. http://dx.doi.org/10.3390/su13042048.
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Modeling the wind flow around cylindrical buildings is one of the problems within urban physics. Despite the simple geometry of the cylinder, it is an interesting physical phenomenon. Partial knowledge of flow field properties can be found in the literature, but in terms of their use for practical tasks, the data are still incomplete. The authors performed a numerical analysis of the flow around the smooth cylinder in the subcritical and critical regime for Reynolds numbers in the range of Re = 2.3 × 103 to 4 × 105. Turbulent flow was solved using LES model and the numerical solution was compared with available data from experiments or standard. Analysis of the mean stream velocity showed the elongation of the core of the wake with decreasing Re. The pressure coefficient evaluation showed a big difference between its distribution in the subcritical and critical regime. In the subcritical regime, a significant increase in the minimum value and a shift of the extreme close to the axis of the cylinder is proven. The results of the drag coefficient confirm a significant decrease in the transition from subcritical to critical regime, which is indicated in the cited experiments.
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Woo, Nam-Sub, Young-Ju Kim i Young-Kyu Hwang. "Experimental Study on the Helical Flow in a Concentric Annulus With Rotating Inner Cylinder". Journal of Fluids Engineering 128, nr 1 (22.09.2005): 113–17. http://dx.doi.org/10.1115/1.2136923.
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This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed laminar flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose, respectively, when the inner cylinder rotates at the speed of 0-600rpm. The results of the present study show the effect of the bulk flow Reynolds number Re and Rossby number Ro on the skin friction coefficients. They also point to the existence of a flow instability mechanism. The effect of rotation on the skin friction coefficient depends significantly on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change in skin friction coefficient, which corresponds to a variation of the rotational speed, is large for the laminar flow regime, whereas it becomes smaller as Re increases for transitional flow regime and, then, it gradually approaches to zero for turbulent flow regime. Consequently, the critical bulk flow Reynolds number Rec decreases as the rotational speed increases. The rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.
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CHANG, KEH-CHIN, i WEN-CHUNG WU. "A STUDY ON FLOW REGIME NEAR CRITICAL RAYLEIGH NUMBER FOR BUOYANCY-DRIVEN CAVITY FLOW". Modern Physics Letters B 19, nr 28n29 (20.12.2005): 1635–38. http://dx.doi.org/10.1142/s0217984905010098.
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A benchmark problem with air medium in a square cavity heated by a vertical side is studied in a Ra range near its critical value without using the usual model approximations, such as Boussinesq or low-Mach-number approximation. Two analysis methods of power spectrum and phase trajectory are adopted to distinguish the flow pattern in the cavity. It demonstrates how the flow evolves from transition to turbulent regimes with increasing Ra values.
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Mollaabbasi, R., i S. M. Taghavi. "Buoyant displacement flows in slightly non-uniform channels". Journal of Fluid Mechanics 795 (22.04.2016): 876–913. http://dx.doi.org/10.1017/jfm.2016.232.
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We consider displacement flows in slightly diverging or converging plane channels. The two fluids are miscible and buoyancy is significant. We assume that the channel is oriented close to horizontal. Employing a classical lubrication approximation, we simplify the governing equations to furnish a semi-analytical solution for the flux functions. Then, we demonstrate how the non-uniformity of the displacement flow geometry can affect the propagation of the interface between the heavy and light fluids in time, for various parameters studied, e.g. the viscosity ratio, a buoyancy number and rheological features. By setting the molecular diffusion effects to zero, certain solution behaviours at longer times can be practically predicted through the associated hyperbolic problem, using which it becomes possible to directly compute the interfacial features of interest, e.g. leading and trailing front heights and speeds. For a Newtonian displacement flow in a converging or uniform channel, as the buoyancy number increases from zero, we are able to classify three flow regimes based on the behaviour of the trailing front near the top of the channel: a no-back-flow regime, a stationary interface flow regime, and a sustained back-flow regime. For the case of a diverging channel flow, the sustained back-flow regime is replaced by an eventually stationary interface flow regime. In addition, as the displacement flow progresses, the leading front speed typically increases (decreases) in a converging (diverging) channel, while the opposite is usually true for the front height. For the no-back-flow regime (i.e. with small buoyancy), the solution of the displacement flow at long times in all the geometries considered converges to a similarity form, while no similarity form is found for the other flow regimes. As the displacement flow develops, frontal diffusive effects are reduced (enhanced) in a converging (diverging) channel and multiple fronts are progressively less (more) present in a converging (diverging) channel. Regarding non-Newtonian effects, a shear-thinning fluid displacing a Newtonian fluid exhibits an increasingly fast front that has a short height in a converging channel. When a yield stress is present in the displaced fluid, it is possible to find residual wall layers of displaced fluid that are completely static. These layers disappear at a certain critical downstream distance in a converging channel while they appear at a critical distance in a diverging channel. Finally, the combination of strong buoyant and yield-stress effects can modify the destiny of a second front that follows the leading front.
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Ota, T., H. Nishiyama i Y. Taoka. "Flow Around an Elliptic Cylinder in the Critical Reynolds Number Regime". Journal of Fluids Engineering 109, nr 2 (1.06.1987): 149–55. http://dx.doi.org/10.1115/1.3242635.
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Flow around an elliptic cylinder of axis ratio 1:3 has been investigated experimentally in the critical Reynolds number regime on the basis of mean static pressure measurements along the cylinder surface and of hot-wire velocity measurements in the near wake. The critical Reynold number has been found to vary with the angle of attack α and attains a minimum around α = 5 to 10 deg. At the critical Reynolds number, the drag, lift, and moment coefficients change discontinuously, and the Strouhal number based on the upstream uniform flow velocity and the major axis length of the cylinder reaches a maximum of about 1.0 to 1.5 depending on α. It is found, however, that the universal Strouhal number based on the velocity along the separated shear layer and the wake width is nearly equal to 0.19, on average, even in the critical Reynolds number regime. The pressure distribution along with the surface oil flow pattern revealed the existence of a small separation bubble near the leading edge accompanying a turbulent boundary layer.
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Mocaër, Philippe, Michel Laguës, Uri Dai i Guy Deutscher. "YBaCuO critical magnetic field Hc2 determination from the flow regime dissipation". Journal of the Less Common Metals 164-165 (październik 1990): 1055–60. http://dx.doi.org/10.1016/0022-5088(90)90517-n.
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Jacimovski, Darko, Radmila Garic-Grulovic, Zeljko Grbavcic, Mihal Djuris i Nevenka Boskovic-Vragolovic. "Momentum, heat, and mass transfer analogy for vertical hydraulic transport of inert particles". Chemical Industry 68, nr 1 (2014): 15–25. http://dx.doi.org/10.2298/hemind130207025j.
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Wall-to-bed momentum, heat and mass transfer in vertical liquid-solids flow, as well as in single phase flow, were studied. The aim of this investigation was to establish the analogy among those phenomena. Also, effect of particles concentration on momentum, heat and mass transfer was studied. The experiments in hydraulic transport were performed in a 25.4 mm I.D. cooper tube equipped with a steam jacket, using spherical glass particles of 1.94 mm in diameter and water as a transport fluid. The segment of the transport tube used for mass transfer measurements was inside coated with benzoic acid. In the hydraulic transport two characteristic flow regimes were observed: turbulent and parallel particle flow regime. The transition between two characteristic regimes (?*=0), occurs at a critical voidage ??0.85. The vertical two-phase flow was considered as the pseudofluid, and modified mixture-wall friction coefficient (fw) and modified mixture Reynolds number (Rem) were introduced for explanation of this system. Experimental data show that the wall-to-bed momentum, heat and mass transfer coefficients, in vertical flow of pseudofluid, for the turbulent regime are significantly higher than in parallel regime. Wall-to-bed, mass and heat transfer coefficients in hydraulic transport of particles were much higher then in single-phase flow for lower Reynolds numbers (Re<15000), while for high Reynolds numbers (Re>15000), there was not significant difference. The experimental data for wall-to-bed momentum, heat and mass transfer in vertical flow of pseudofluid in parallel particle flow regime, show existing analogy among these three phenomena.
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XUE, MING, i DICK K. P. YUE. "Nonlinear free-surface flow due to an impulsively started submerged point sink". Journal of Fluid Mechanics 364 (10.06.1998): 325–47. http://dx.doi.org/10.1017/s0022112098001335.
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The unsteady fully nonlinear free-surface flow due to an impulsively started submerged point sink is studied in the context of incompressible potential flow. For a fixed (initial) submergence h of the point sink in otherwise unbounded fluid, the problem is governed by a single non-dimensional physical parameter, the Froude number, [Fscr ]≡Q/4π(gh5)1/2, where Q is the (constant) volume flux rate and g the gravitational acceleration. We assume axisymmetry and perform a numerical study using a mixed-Eulerian–Lagrangian boundary-integral-equation scheme. We conduct systematic simulations varying the parameter [Fscr ] to obtain a complete quantification of the solution of the problem. Depending on [Fscr ], there are three distinct flow regimes: (i) [Fscr ]<[Fscr ]1≈0.1924 – a ‘sub-critical’ regime marked by a damped wave-like behaviour of the free surface which reaches an asymptotic steady state; (ii) [Fscr ]1<[Fscr ]<[Fscr ]2≈0.1930 – the ‘trans-critical’ regime characterized by a reversal of the downward motion of the free surface above the sink, eventually developing into a sharp upward jet; (iii) [Fscr ]>[Fscr ]2 – a ‘super-critical’ regime marked by the cusp-like collapse of the free surface towards the sink. Mechanisms behind such flow behaviour are discussed and hydrodynamic quantities such as pressure, power and force are obtained in each case. This investigation resolves the question of validity of a steady-state assumption for this problem and also shows that a small-time expansion may be inadequate for predicting the eventual behaviour of the flow.
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SÁNCHEZ-SANZ, M., i A. VELAZQUEZ. "Vortex-induced vibration of a prism in internal flow". Journal of Fluid Mechanics 641 (13.11.2009): 431–40. http://dx.doi.org/10.1017/s0022112009991893.
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In this article, we study the influence of solid-to-fluid density ratio m on the type of vortex-induced oscillation of a square section prism placed inside a two-dimensional channel. We assume that the solid body has neither structural damping nor spring restoring force. Accordingly, the prism equation of motion contains only inertia and aerodynamics forces. The problem is considered in the range of Reynolds numbers Re ∈ [50 200] (based on the prism cross-section height h) and channel widths H = H′/h ∈ [2.5 10]. We found that, for each Re and H, there is a critical mass ratio mc that separates two different oscillation regimes. For m > mc, the prism oscillation is periodical and contains a single harmonic. For m < mc, the prism oscillation changes completely and assumes an irregular pattern that is characterized by multiple harmonics that appear to belong to a uniform spectrum. The change from one regime to the other is abrupt and we were not able to observe a transitional regime in which the number of response harmonics grew by finite steps. The value of the critical mass ratio grows along with the Reynolds number and the channel width.
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Temme, Franziska, Jenny V. Turton, Thomas Mölg i Tobias Sauter. "Flow Regimes and Föhn Types Characterize the Local Climate of Southern Patagonia". Atmosphere 11, nr 9 (25.08.2020): 899. http://dx.doi.org/10.3390/atmos11090899.
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The local climate in Southern Patagonia is strongly influenced by the interaction between the topography and persistent westerlies, which can generate föhn events, dry and warm downslope winds. The upstream flow regime influences different föhn types which dictate the lee-side atmospheric response regarding the strength, spatial extent and phenomenology. We use a combination of observations from four automatic weather stations (AWSs) and high-resolution numerical modeling with the Weather Research and Forecasting (WRF) model for a region in Southern Patagonia (48° S–52° S, 72° W–76.5° W) including the Southern Patagonian Icefield (SPI). The application of a föhn identification algorithm to a 10-month study period (June 2018–March 2019) reveals 81 föhn events in total. A simulation of three events of differing flow regimes (supercritical, subcritical, transition) suggests that a supercritical flow regime leads to a linear föhn event with a large spatial extent but moderate intensity. In contrast, a spatially limited but locally strong föhn response is induced by a subcritical regime with upstream blocking and by a transition regime with a hydraulic jump present. Our results imply that the hydraulic jump-type föhn event (transition case) is the most critical for glacier mass balances since it shows the strongest warming, drying, wind velocities and solar radiation over the SPI. The consideration of flow regimes over the last 40 years shows that subcritical flow occurs most frequently (78%), however transitional flow occurs 14% of the time, implying the potential impact on Patagonian glaciers.
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Zheng, Chuntai, Peng Zhou, Siyang Zhong i Xin Zhang. "On the cylinder noise and drag reductions in different Reynolds number ranges using surface pattern fabrics". Physics of Fluids 35, nr 3 (marzec 2023): 035111. http://dx.doi.org/10.1063/5.0138074.
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This study experimentally investigates the potential of using surface pattern fabrics for the cylinder noise and drag control in different Reynolds number ranges. The aerodynamic and aeroacoustic effects were evaluated through the noise and force measurements in an anechoic wind tunnel. It was observed that the noise and drag reductions take place simultaneously but in different Reynolds number ranges, corresponding to the cylinder flow in different flow regimes, e.g., sub-critical, critical, and supercritical flow regimes. Microphone arc array measurements reveal that the suppression of the Aeolian tone in the critical regime is the major cause of noise reductions, and the noise directivity gradually loses dipole features in the critical and supercritical flow regimes, which is probably related to the reduced lift fluctuation coefficient and the spanwise segment of the sound sources. Further hotwire wake survey revealed significant changes in flow dynamics, which explain the variations of noise and drag in different flow regimes. We have shown for the first time that fabric with different surface patterns can effectively reduce cylinder drag and noise in different Reynolds number ranges. Since the Reynolds number is a key factor that determines the flow state in practical engineering applications, e.g., cycling aerodynamics, this study suggests that optimal drag and noise reductions can be realized by employing the combinations of different surface pattern fabrics to account for the Reynolds number effects.
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Sumner, Chloe, i Youra Taroyan. "Amplification of magnetic field twisting by a stagnation point flow". Astronomy & Astrophysics 642 (październik 2020): A181. http://dx.doi.org/10.1051/0004-6361/202038761.
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Context. Flows are a common feature of many processes occurring in the solar atmosphere, such as the formation of prominences where evaporated plasma from the chromosphere condensates along thin prominence threads that are seen to twist and oscillate. Aim. We aim to investigate the twisting of these threads by plasma condensation during their formation. Methods. We introduce a simple model with fixed critical points where the flow speed matches the Alfvén speed. This allows us to study the problem separately in the sub-Alfvénic and super-Alfvénic regimes. The temporal and spatial evolution of small amplitude initial twists along a thread is investigated analytically and numerically. Results. Analytical solutions are constructed in terms of the generalised hypergeometric functions. The solutions grow in time, despite the absence of any influxes of energy or magnetic fields. These results are confirmed numerically: We find oscillations with an amplifying amplitude and increasing period in the sub-Alfvénic regime. In the super-Alfvénic regime, we find twist amplification without any accompanying oscillations. An interesting result is the convergence of the twists at the critical points that leads to the formation of steep gradients and small scales. Energy is transferred from the flow to the amplifying twists. Conclusions. Magnetic field lines may be twisted by a stagnation point flow without the influx of any azimuthal field or energy. This twisting could assist in the formation of topology that is able to support the growth of prominences. The formation of steep gradients and small scales at the critical point is a new phenomenon which requires further investigation in the non-linear regime with the inclusion of magnetic diffusion.
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Manneville, Paul, i Masaki Shimizu. "Transitional Channel Flow: A Minimal Stochastic Model". Entropy 22, nr 12 (29.11.2020): 1348. http://dx.doi.org/10.3390/e22121348.
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In line with Pomeau’s conjecture about the relevance of directed percolation (DP) to turbulence onset/decay in wall-bounded flows, we propose a minimal stochastic model dedicated to the interpretation of the spatially intermittent regimes observed in channel flow before its return to laminar flow. Numerical simulations show that a regime with bands obliquely drifting in two stream-wise symmetrical directions bifurcates into an asymmetrical regime, before ultimately decaying to laminar flow. The model is expressed in terms of a probabilistic cellular automaton of evolving von Neumann neighborhoods with probabilities educed from a close examination of simulation results. It implements band propagation and the two main local processes: longitudinal splitting involving bands with the same orientation, and transversal splitting giving birth to a daughter band with an orientation opposite to that of its mother. The ultimate decay stage observed to display one-dimensional DP properties in a two-dimensional geometry is interpreted as resulting from the irrelevance of lateral spreading in the single-orientation regime. The model also reproduces the bifurcation restoring the symmetry upon variation of the probability attached to transversal splitting, which opens the way to a study of the critical properties of that bifurcation, in analogy with thermodynamic phase transitions.
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ANSONG, JOSEPH K., PATRICK J. KYBA i BRUCE R. SUTHERLAND. "Fountains impinging on a density interface". Journal of Fluid Mechanics 595 (8.01.2008): 115–39. http://dx.doi.org/10.1017/s0022112007009093.
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We present an experimental study of an axisymmetric turbulent fountain in a two-layer stratified environment. Interacting with the interface, the fountain is observed to exhibit three regimes of flow. It may penetrate the interface, but nonetheless return to the source where it spreads as a radially propagating gravity current; the return flow may be trapped at the interface where it spreads as a radially propagating intrusion or it may do both. These regimes have been classified using empirically determined regime parameters which govern the relative initial momentum of the fountain and the relative density difference of the fountain and the ambient fluid. The maximum vertical distance travelled by the fountain in a two-layer fluid has been theoretically determined by extending the theory developed for fountains in a homogeneous environment. The theory compares favourably with experimental measurements. We have also developed a theory to analyse the initial speeds of the resulting radial currents. The spreading currents exhibited two different flow regimes: a constant-velocity regime and an inertia-buoyancy regime in which the front position, R, scales with time, t, as R ∼ t3/4. These regimes were classified using a critical Froude number which characterized the competing effects of momentum and buoyancy in the currents.
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CLARKE, S. R., i E. R. JOHNSON. "Topographically forced long waves on a sheared coastal current. Part 1. The weakly nonlinear response". Journal of Fluid Mechanics 343 (25.07.1997): 131–51. http://dx.doi.org/10.1017/s0022112097005685.
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The flow of a constant-vorticity current past coastal topography is investigated in the long-wave weakly nonlinear limit. In contrast to other near-critical weakly nonlinear systems this problem does not exhibit hydraulically controlled solutions. It is shown that near criticality the evolution of the vorticity interface is governed by a forced BDA (Benjamin–Davis–Acrivos) equation. The solutions of this equation are discussed and two distinct near-critical flow regimes are identified. Owing to the non-local nature of the forcing, the first of these regimes is characterized by quasi-steady solutions controlled at the topography with some blocking of the upstream rotational fluid, while in the second regime steady nonlinear wavetrains form downstream of the obstacle with no upstream influence. In the hydraulic limit the velocity band for both of these critical regimes approaches zero.
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Wang, Qi Wen, Chao Zou i Han Feng Wang. "Aerodynamic Force of a Cantilevered Cylinder in Uniform Flow and Turbulent Boundary Layer". Applied Mechanics and Materials 470 (grudzień 2013): 499–503. http://dx.doi.org/10.4028/www.scientific.net/amm.470.499.
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The aerodynamic force and pressure distribution on a cantilevered cylinder with aspect ratio of 5 mounted in smooth uniform oncoming flow and turbulent boundary layer have been experimentally investigated. The tested Reynolds number ranges from 0.64×105 to 5.81×105, covering subcritical, critical and supercritical regimes. The results indicate that the aerodynamic drag of a cantilevered cylinder is smaller that that of 2D cylinder. The drag differs significantly along cylinder span because of the three dimensionality of the flow. The presence of turbulent boundary layer makes cylinder back pressure more uniform along cylinder span relative to that in uniform flow. Transition from subcritical to critical regime occurs earlier near cylinder free end in uniform flow, while this behavior disappears with the presence of turbulent boundary layer.
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Kordi, E., S. A. Ayyoubzadeh, M. Z. Ahmadi i A. Zahiri. "Prediction of the lateral flow regime and critical depth in compound open channels". Canadian Journal of Civil Engineering 36, nr 1 (styczeń 2009): 1–13. http://dx.doi.org/10.1139/l08-095.
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In this study, the common critical depth calculation in compound channels has been modified considering the effect of momentum transfer between the interface of a main channel and its floodplains. In noncorrected specific energy curves of a given slope, the flow is not necessarily entirely sub- or supercritical as it is in a single cross section and there is a possibility of both flow regimes at a certain stage, called the lateral mixed flow regimes, which makes the application of specific energy equation to determine the critical depth and transitional zone calculations questionable. In the present research, the flow distribution in a main channel and floodplains has been corrected by combining the corrected hydraulic flow in compound cross sections using the coherence method. The specific energy has been subsequently modified in the subsections. The results seem satisfactory when compared with the results based on the available laboratory data.
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Zheng, Chuntai, Peng Zhou, Siyang Zhong i Xin Zhang. "Experimental investigation on cylinder noise and its reductions by identifying aerodynamic sound sources in flow fields". Physics of Fluids 35, nr 3 (marzec 2023): 035103. http://dx.doi.org/10.1063/5.0138080.
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Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameter-based Reynolds numbers ranging from [Formula: see text] to [Formula: see text], covering the sub-critical, critical, and supercritical regimes. The force and noise measurement results show that dimple fabric simultaneously reduces noise and drag in the critical regime. The changes in flow structures were characterized by the Time-resolved Particle Image Velocimetry (TR-PIV) measurements. Based on the vortex sound theory, the flow analysis shows that the dominant sound sources are found to be concentrated near the cylinder surface, which is caused by the unsteady vortex motions near the separation locations during the process of vortex shedding. The cross-correlation between the synchronized TR-PIV and microphone measurements further supports the conclusions. Moreover, the cylinder noise reductions controlled by the dimples are directly associated with the reduced sound sources in the critical and supercritical regimes, corresponding to the reduced strength of the vortex shedding.
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OTA, Terukazu, Takayuki MORI i Toshihiko TAKAHASHI. "Unsteady Flow Around an Elliptic Cylinder in the Critical Reynolds Number Regime." Transactions of the Japan Society of Mechanical Engineers Series B 62, nr 598 (1996): 2101–5. http://dx.doi.org/10.1299/kikaib.62.2101.
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TADIĆ, BOSILJKA, G. J. RODGERS i STEFAN THURNER. "TRANSPORT ON COMPLEX NETWORKS: FLOW, JAMMING AND OPTIMIZATION". International Journal of Bifurcation and Chaos 17, nr 07 (lipiec 2007): 2363–85. http://dx.doi.org/10.1142/s0218127407018452.
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Many transport processes on networks depend crucially on the underlying network geometry, although the exact relationship between the structure of the network and the properties of transport processes remain elusive. In this paper, we address this question by using numerical models in which both structure and dynamics are controlled systematically. We consider the traffic of information packets that include driving, searching and queuing. We present the results of extensive simulations on two classes of networks; a correlated cyclic scale-free network and an uncorrelated homogeneous weakly clustered network. By measuring different dynamical variables in the free flow regime we show how the global statistical properties of the transport are related to the temporal fluctuations at individual nodes (the traffic noise) and the links (the traffic flow). We then demonstrate that these two network classes appear as representative topologies for optimal traffic flow in the regimes of low density and high density traffic, respectively. We also determine statistical indicators of the pre-jamming regime on different network geometries and discuss the role of queuing and dynamical betweenness for the traffic congestion. The transition to the jammed traffic regime at a critical posting rate on different network topologies is studied as a phase transition with an appropriate order parameter. We also address several open theoretical problems related to the network dynamics.
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Fang, Ranran, Hongbo Zhu, Zekai Li, Xiaohui Zhu, Xianhang Zhang, Zhiyu Huang, Ke Li i in. "Temperature Effect on Capillary Flow Dynamics in 1D Array of Open Nanotextured Microchannels Produced by Femtosecond Laser on Silicon". Nanomaterials 10, nr 4 (21.04.2020): 796. http://dx.doi.org/10.3390/nano10040796.
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Capillary flow of water in an array of open nanotextured microgrooves fabricated by femtosecond laser processing of silicon is studied as a function of temperature using high-speed video recording. In a temperature range of 23–80 °C, the produced wicking material provides extremely fast liquid flow with a maximum velocity of 37 cm/s in the initial spreading stage prior to visco-inertial regime. The capillary performance of the material enhances with increasing temperature in the inertial, visco-inertial, and partially in Washburn flow regimes. The classic universal Washburn’s regime is observed at all studied temperatures, giving the evidence of its universality at high temperatures as well. The obtained results are of great significance for creating capillary materials for applications in cooling of electronics, energy harvesting, enhancing the critical heat flux of industrial boilers, and Maisotsenko cycle technologies.
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Dolgopolov, S. I. "Mathematical simulation of choking under self-oscillations in hydraulic systems with cavitating pumps of liquid-propellant rocket engines". Technical mechanics 2020, nr 4 (10.12.2020): 35–42. http://dx.doi.org/10.15407/itm2020.04.035.
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As known from the study of cavity flows in fixed channels (Venturi tube), with decreasing channel outlet pressure there comes a point where the flow rate ceases to increase. To increase the flow rate, the inlet pressure must be increased. This phenomenon of flow rate limitation at a fixed inlet pressure is due to a critical regime of cavity flow at the narrowest cross-section and is termed choking. Impeller pumps also exhibit choking regimes described by the so-called chocking characteristic, which relates the critical pump flow rate to the inlet pressure. This work is aimed at extending a hydrodynamic model of cavitating pumps of liquid-propellant rocket engines (LPREs) by including a mathematical simulation of chocking regimes. A mechanism of realization of the chocking process in pumps is proposed. The mechanism is as follows. When the parameter oscillation amplitudes are high enough, the inlet flow rate and pressure computed at integration step i may be in the inadmissible range, i.e., below the chocking regime characteristic. In this case, the flow rate and the pressure must be refined. It is found that the computed decrease in the cavitation self-oscillation frequency in comparison with the eigenfrequency of a hydraulic system with a cavitating pump is close to its experimental value in the case where the inlet flow rate and pressure are assumed to be coordinates of the point of intersection of the choking characteristic and the line that connects the values of the pump inlet flow rate and pressure computed at integration steps i-1 and i. It is shown that the LPRE pump choking characteristic is a specific nonlinearity associated with the critical cavity flow in the pump and may manifest itself at high parameter oscillation amplitudes. It is found that the choking characteristic of an LPRE pump affects the cavitation oscillation parameters to a greater extent than the cavity volume vs. pump inlet pressure and flow rate relationship does and is the governing nonlinearity in the pump system in choking.
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Girfoglio, M., F. De Rosa, G. Coppola i L. de Luca. "Unsteady critical liquid sheet flows". Journal of Fluid Mechanics 821 (18.05.2017): 219–47. http://dx.doi.org/10.1017/jfm.2017.241.
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The unsteady global dynamics of a gravitational liquid sheet interacting with a one-sided adjacent air enclosure (commonly referred to as nappe oscillation configuration) is addressed under the assumptions of potential flow and the presence of surface tension effects. From a theoretical viewpoint the problem is challenging, because from previous literature it is known that the equation governing the evolution of small disturbances exhibits a singularity at the vertical station where the local flow velocity equals the capillary wave velocity (local critical condition), although the solution to the problem has not yet been found. The equation governing the local dynamics resembles one featuring the forced vibrations of a string of finite length, formulated in the reference frame moving with the flow velocity, and exhibits both slow and fast characteristic curves. From the global system perspective the nappe behaves as a driven damped spring–mass oscillator, where the inertial effects are linked to the liquid sheet mass and the spring is represented by the equivalent stiffness of the air enclosure acting on the displacement of the compliant nappe centreline. A suited procedure is developed to remove the singularity of the integro-differential operator for Weber numbers less than unity. The investigation is carried out by means of a modal (i.e. time asymptotic) linear approach, which is corroborated by numerical simulations of the governing equation and supported by systematic comparisons with experimental data from the literature, available in the supercritical regime only. As regards the critical regime for the unit Weber number, the major theoretical result is a sharp increase in oscillation frequency as the flow Weber number is gradually reduced from supercritical to subcritical values due to the shift of the prevailing mode from the slow one to the fast one.
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Niino, Hiroshi, Atsushi Mori, Takehiko Satomura i Sayaka Akiba. "Flow Regimes of Nonlinear Heat Island Circulation". Journal of the Atmospheric Sciences 63, nr 5 (1.05.2006): 1538–47. http://dx.doi.org/10.1175/jas3700.1.
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Abstract Previous laboratory and numerical experiments show that the nonlinear heat island circulation has two different flow regimes: One has two maximums of updraft at both edges of the heat island (type E), while the other has a single maximum of updraft at the center of the island (type C). Our theoretical consideration shows that the heat island circulation is principally governed by two nondimensional parameters: a nonlinear parameter ɛN = Δθ/(Γδ) and a Prandtl number Pr = ν/κ, where Δθ is the surface temperature anomaly of the heat island, Γ the vertical gradient of the basic potential temperature, δ the thickness of the thermal boundary layer, ν the kinematic viscosity, and κ the temperature diffusivity. For a given fluid, the Prandtl number is fixed, so that the flow regime is principally determined by the nonlinear parameter ɛN. Numerical experiments are performed to confirm the theoretical prediction. The steady-state flows obtained from the numerical experiments are categorized into either of the two regimes. In accordance with the theoretical prediction, the transition between the two regimes occurs at a critical value of ɛN(∼3.4). When ɛN is larger (smaller) than this critical value, type C (type E) is realized. The physical mechanism for the transition is interpreted in the light of the authors' previous theoretical study.
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Nicks, Bradley Scott, Ernesto Barraza-Valdez, Sahel Hakimi, Kyle Chesnut, Genevieve DeGrandchamp, Kenneth Gage, David Housley i in. "High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes". Photonics 8, nr 6 (11.06.2021): 216. http://dx.doi.org/10.3390/photonics8060216.
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The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties for potential medical applications of electron acceleration. In the near-critical density regime, electrons are accelerated by the ponderomotive force followed by the electron sheath formation, resulting in a flow of bulk electrons. This behavior represents a qualitatively distinct regime from that of low-density LWFA. A quantitative entropy index for differentiating these regimes is proposed. The dependence of accelerated electron energy on laser amplitude is also examined. For the majority of this study, the laser propagates along the axis of the target of carbon nanotubes in a 1D geometry. After the fundamental high-density physics is established, an alternative, 2D scheme of laser acceleration of electrons using carbon nanotubes is considered.
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Salazar, Juan Fernando, Juan Camilo Villegas, Angela María Rendón, Estiven Rodríguez, Isabel Hoyos, Daniel Mercado-Bettín i Germán Poveda. "Scaling properties reveal regulation of river flows in the Amazon through a “forest reservoir”". Hydrology and Earth System Sciences 22, nr 3 (9.03.2018): 1735–48. http://dx.doi.org/10.5194/hess-22-1735-2018.
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Abstract. Many natural and social phenomena depend on river flow regimes that are being altered by global change. Understanding the mechanisms behind such alterations is crucial for predicting river flow regimes in a changing environment. Here we introduce a novel physical interpretation of the scaling properties of river flows and show that it leads to a parsimonious characterization of the flow regime of any river basin. This allows river basins to be classified as regulated or unregulated, and to identify a critical threshold between these states. We applied this framework to the Amazon river basin and found both states among its main tributaries. Then we introduce the “forest reservoir” hypothesis to describe the natural capacity of river basins to regulate river flows through land–atmosphere interactions (mainly precipitation recycling) that depend strongly on the presence of forests. A critical implication is that forest loss can force the Amazonian river basins from regulated to unregulated states. Our results provide theoretical and applied foundations for predicting hydrological impacts of global change, including the detection of early-warning signals for critical transitions in river basins.
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Bryanskaya, Yuliya, i Aleksandra Ostiakova. "The conditions for occurrence of critical flow regime of water in the open stream". E3S Web of Conferences 97 (2019): 05006. http://dx.doi.org/10.1051/e3sconf/20199705006.
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For the solution of engineering problems require increasingly accurate estimates of the hydraulic characteristics of the water streams. To date, it is impossible to consider sufficiently complete theoretical and experimental justification of the main provisions of the theory of turbulence, hydraulic resistance, channel processes. The composition of tasks related to flows in wide channels, turbulence problems are of scientific and practical interest. Various interpretations of the determination of the critical Froude number in wide open water flows based on observations and theoretical transformations are considered. The conditions for the emergence of a critical regime of water flow in an open wide channel are analyzed in order to estimate the critical Froude number and critical depth. Estimates of the critical Froude number for laboratory and field conditions are given. The estimations allow us to consider the proposed approach acceptable for determining the conditions of occurrence of the critical flow regime. The General, physical interpretation of conditions of occurrence of the critical regime of water flow on the basis of phenomenological approach is specified. The results take into account the values of the components of the total specific energy of the section. This shows the estimated calculation. The results obtained theoretically make it possible to compare the above interpretations and determine their applicability, and the results of the analysis can be useful for the estimated calculations of flows in channels and river flows in rigid, undeformable boundaries and with minor channel deformations.
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Robinet, J. C., i X. Gloerfelt. "Instabilities in non-ideal fluids". Journal of Fluid Mechanics 880 (4.10.2019): 1–4. http://dx.doi.org/10.1017/jfm.2019.719.
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The recent study of Ren et al. (J. Fluid Mech., vol. 871, 2019, pp. 831–864) investigated the hydrodynamic linear stability of a compressible boundary layer over an insulated flat plate for a non-ideal gas (supercritical $\text{CO}_{2}$). In particular, the authors showed that in the transcritical regime (across the pseudo-critical line) the flow is strongly convectively unstable due to the co-existence of two unstable modes: Mode I, related to Tollmien–Schlichting instabilities and a new inviscid two-dimensional mode (Mode II) with a spatial growth rate one order of magnitude larger than Mode I for high Eckert numbers. In contrast to the transcritical regime, in the sub- and supercritical regimes, Mode II does not exist. Only Mode I drives the instabilities: viscous and two-dimensional for the subcritical regime and inflectional and three-dimensional for the supercritical regime.