Artykuły w czasopismach: „Instability and transition”

1

Morkovin, Mark V. "Instability and Transition". International Journal of Heat and Fluid Flow 12, nr 4 (grudzień 1991): 384. http://dx.doi.org/10.1016/0142-727x(91)90029-u.

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Men, Hongyuan, Xinliang Li i Hongwei Liu. "Direct numerical simulations of hypersonic boundary layer transition over a hypersonic transition research vehicle model lifting body at different angles of attack". Physics of Fluids 35, nr 4 (kwiecień 2023): 044111. http://dx.doi.org/10.1063/5.0146651.

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This paper performs direct numerical simulations of hypersonic boundary layer transition over a Hypersonic Transition Research Vehicle (HyTRV) model lifting body designed by the China Aerodynamic Research and Development Center. Transitions are simulated at four angles of attack: 0°, 3°, 5°, and 7°. The free-stream Mach number is 6, and the unit Reynolds number is 107 m−1. Four distinct transitional regions are identified: the shoulder cross-flow and vortex region and the shoulder vortex region on the leeward side, the windward vortex region and the windward cross-flow region on the windward side. As the angle of attack increases, the transition locations on the leeward side generally move forward and the transition ranges expand, while the transition locations generally move backward and the transition ranges decrease on the windward side. Moreover, the shoulder vortex region moves toward the centerline of the leeward side. At large angles of attack (5° and 7°), the streamwise vortex on the shoulder cross-flow and vortex region will enable the transition region to be divided into the cross-flow instability region on both sides and the streamwise vortex instability region in the middle. In addition, the streamwise vortex also leads to a significant increase in cross-flow instability in their upper region, which can generate a new streamwise vortex instability region between the two transition regions on the leeward side. Furthermore, since the decrease in the intensity and the range for the cross-flow on the windward side, the windward cross-flow region tends to become narrow and ultimately disappears.

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Marshall, Victor W., Philippa J. Clarke i Peri J. Ballantyne. "Instability in the Retirement Transition". Research on Aging 23, nr 4 (lipiec 2001): 379–409. http://dx.doi.org/10.1177/0164027501234001.

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Dagaut, J., M. E. Negretti, G. Balarac i C. Brun. "Linear to turbulent Görtler instability transition". Physics of Fluids 33, nr 1 (1.01.2021): 014102. http://dx.doi.org/10.1063/5.0033944.

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Lee, S. Y., i J. M. Wang. "Microwave Instability across the Transition Energy". IEEE Transactions on Nuclear Science 32, nr 5 (październik 1985): 2323–25. http://dx.doi.org/10.1109/tns.1985.4333900.

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COOK, ANDREW W., WILLIAM CABOT i PAUL L. MILLER. "The mixing transition in RayleighTaylor instability". Journal of Fluid Mechanics 511 (25.07.2004): 333–62. http://dx.doi.org/10.1017/s0022112004009681.

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Bayly, B. J., S. A. Orszag i T. Herbert. "Instability Mechanisms in Shear-Flow Transition". Annual Review of Fluid Mechanics 20, nr 1 (styczeń 1988): 359–91. http://dx.doi.org/10.1146/annurev.fl.20.010188.002043.

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CHAURASIA, HEMANT K., i MARK C. THOMPSON. "Three-dimensional instabilities in the boundary-layer flow over a long rectangular plate". Journal of Fluid Mechanics 681 (16.06.2011): 411–33. http://dx.doi.org/10.1017/jfm.2011.205.

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A detailed numerical study of the separating and reattaching flow over a square leading-edge plate is presented, examining the instability modes governing transition from two- to three-dimensional flow. Under the influence of background noise, experiments show that the transition scenario typically is incompletely described by either global stability analysis or the transient growth of dominant optimal perturbation modes. Instead two-dimensional transition effectively can be triggered by the convective Kelvin–Helmholtz (KH) shear-layer instability; although it may be possible that this could be described alternatively in terms of higher-order optimal perturbation modes. At least in some experiments, observed transition occurs by either: (i) KH vortices shedding downstream directly and then almost immediately undergoing three-dimensional transition or (ii) at higher Reynolds numbers, larger vortical structures are shed that are also three-dimensionally unstable. These two paths lead to distinctly different three-dimensional arrangements of vortical flow structures. This paper focuses on the mechanisms underlying these three-dimensional transitions. Floquet analysis of weakly periodically forced flow, mimicking the observed two-dimensional quasi-periodic base flow, indicates that the two-dimensional vortex rollers shed from the recirculation region become globally three-dimensionally unstable at a Reynolds number of approximately 380. This transition Reynolds number and the predicted wavelength and flow symmetries match well with those of the experiments. The instability appears to be elliptical in nature with the perturbation field mainly restricted to the cores of the shed rollers and showing the spatial vorticity distribution expected for that instability type. Indeed an estimate of the theoretical predicted wavelength is also a good match to the prediction from Floquet analysis and theoretical estimates indicate the growth rate is positive. Fully three-dimensional simulations are also undertaken to explore the nonlinear development of the three-dimensional instability. These show the development of the characteristic upright hairpins observed in the experimental dye visualisations. The three-dimensional instability that manifests at lower Reynolds numbers is shown to be consistent with an elliptic instability of the KH shear-layer vortices in both symmetry and spanwise wavelength.

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Granatosky, Michael C., Caleb M. Bryce, Jandy Hanna, Aidan Fitzsimons, Myra F. Laird, Kelsey Stilson, Christine E. Wall i Callum F. Ross. "Inter-stride variability triggers gait transitions in mammals and birds". Proceedings of the Royal Society B: Biological Sciences 285, nr 1893 (12.12.2018): 20181766. http://dx.doi.org/10.1098/rspb.2018.1766.

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Speed-related gait transitions occur in many animals, but it remains unclear what factors trigger gait changes. While the most widely accepted function of gait transitions is that they reduce locomotor costs, there is no obvious metabolic trigger signalling animals when to switch gaits. An alternative approach suggests that gait transitions serve to reduce locomotor instability. While there is evidence supporting this in humans, similar research has not been conducted in other species. This study explores energetics and stride variability during the walk–run transition in mammals and birds. Across nine species, energy savings do not predict the occurrence of a gait transition. Instead, our findings suggest that animals trigger gait transitions to maintain high locomotor rhythmicity and reduce unstable states. Metabolic efficiency is an important benefit of gait transitions, but the reduction in dynamic instability may be the proximate trigger determining when those transitions occur.

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Kobayashi, Ryoji. "Review: Laminar-to-Turbulent Transition of Three-Dimensional Boundary Layers on Rotating Bodies". Journal of Fluids Engineering 116, nr 2 (1.06.1994): 200–211. http://dx.doi.org/10.1115/1.2910255.

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The laminar-turbulent transition of three-dimensional boundary layers is critically reviewed for some typical axisymmetric bodies rotating in still fluid or in axial flow. The flow structures of the transition regions are visualized. The transition phenomena are driven by the compound of the Tollmien-Schlichting instability, the crossflow instability, and the centrifugal instability. Experimental evidence is provided relating the critical and transition Reynolds numbers, defined in terms of the local velocity and the boundary layer momentum thickness, to the local rotational speed ratio, defined as the ratio of the circumferential speed to the free-stream velocity at the outer edge of the boundary layer, for the rotating disk, the rotating cone, the rotating sphere and other rotating axisymmetric bodies. It is shown that the cross-sectional structure of spiral vortices appearing in the transition regions and the flow pattern of the following secondary instability in the case of the crossflow instability are clearly different than those in the case of the centrifugal instability.

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Loiseau, Jean-Christophe, Jean-Christophe Robinet, Stefania Cherubini i Emmanuel Leriche. "Investigation of the roughness-induced transition: global stability analyses and direct numerical simulations". Journal of Fluid Mechanics 760 (4.11.2014): 175–211. http://dx.doi.org/10.1017/jfm.2014.589.

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AbstractThe linear global instability and resulting transition to turbulence induced by an isolated cylindrical roughness element of height $h$ and diameter $d$ immersed within an incompressible boundary layer flow along a flat plate is investigated using the joint application of direct numerical simulations and fully three-dimensional global stability analyses. For the range of parameters investigated, base flow computations show that the roughness element induces a wake composed of a central low-speed region surrounded by a three-dimensional shear layer and a pair of low- and high-speed streaks on each of its sides. Results from the global stability analyses highlight the unstable nature of the central low-speed region and its crucial importance in the laminar–turbulent transition process. It is able to sustain two different global instabilities: a sinuous and a varicose one. Each of these globally unstable modes is related to a different physical mechanism. While the varicose mode has its root in the instability of the whole three-dimensional shear layer surrounding the central low-speed region, the sinuous instability turns out to be similar to the von Kármán instability in the two-dimensional cylinder wake and has its root in the lateral shear layers of the separated zone. The aspect ratio of the roughness element plays a key role on the selection of the dominant instability: whereas the flow over thin cylindrical roughness elements transitions due to a sinuous instability of the near-wake region, for larger roughness elements the varicose instability of the central low-speed region turns out to be the dominant one. Direct numerical simulations of the flow past an aspect ratio ${\it\eta}=1$ (with ${\it\eta}=d/h$) roughness element sustaining only the sinuous instability have revealed that the bifurcation occurring in this particular case is supercritical. Finally, comparison of the transition thresholds predicted by global linear stability analyses with the von Doenhoff–Braslow transition diagram provides qualitatively good agreement.

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Kleif, Helle Bendix. "A Typology of Transition Patterns Involving Long-Term NEET Episodes: Accumulation of Risk and Adversity". Youth 3, nr 1 (9.02.2023): 170–83. http://dx.doi.org/10.3390/youth3010012.

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This paper uses Danish population-based administrative registers to study contemporary school-to-work transitions among young adults who experience long-term NEET episodes between age 16 and 20. By applying sequence analysis and clustering, this paper identifies five distinct transition patterns. Using this typology as the outcome variable in multinomial regression the paper offers insight into how experiences and circumstances, developing until age 16, can affect the subsequently unfolding transition process. Finally, the paper looks ahead and describes whether transitional difficulty accumulates into early adulthood. While one transition pattern stands out as more stable and less worrying, three of the remaining four demonstrate how transitional difficulty between age 16 and 20 develops as precarious patterns of attachment to well-established systems within the Danish welfare state. It is further established that various childhood risk factors significantly increase the odds of experiencing precarious transition patterns. Finally, the analyses demonstrate how instability and risk during childhood and school-to-work transition extend into early adulthood for a large part of the study population.

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Hernandez, Rafael, i Konstantin I. Matveev. "Transition to Instability in Segmented Rijke Tube". Open Thermodynamics Journal 4, nr 1 (9.08.2009): 141–43. http://dx.doi.org/10.2174/1874396x010040100141.

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Tohline, J. E. "Star formation - Phase transition, not Jeans instability". Astrophysical Journal 292 (maj 1985): 181. http://dx.doi.org/10.1086/163144.

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Williamson, C. H. K. "Mode A secondary instability in wake transition". Physics of Fluids 8, nr 6 (czerwiec 1996): 1680–82. http://dx.doi.org/10.1063/1.868949.

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Sharma, Prateek, Gregory W. Hammett i Eliot Quataert. "Transition from Collisionless to Collisional Magnetorotational Instability". Astrophysical Journal 596, nr 2 (20.10.2003): 1121–30. http://dx.doi.org/10.1086/378234.

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Fomby, Paula, i Stacey J. Bosick. "Family Instability and the Transition to Adulthood". Journal of Marriage and Family 75, nr 5 (3.09.2013): 1266–87. http://dx.doi.org/10.1111/jomf.12063.

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Finlay, W. H., J. B. Keller i J. H. Ferziger. "Instability and transition in curved channel flow". Journal of Fluid Mechanics 194, nr -1 (wrzesień 1988): 417. http://dx.doi.org/10.1017/s0022112088003052.

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Freeman, Walter J., i Mark D. Holmes. "Metastability, instability, and state transition in neocortex". Neural Networks 18, nr 5-6 (lipiec 2005): 497–504. http://dx.doi.org/10.1016/j.neunet.2005.06.014.

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YURI, MICHIKO. "Phase transition, non-Gibbsianness and subexponential instability". Ergodic Theory and Dynamical Systems 25, nr 4 (8.06.2005): 1325–42. http://dx.doi.org/10.1017/s0143385704000926.

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Mohamadou, Alidou, A. Kenfack-Jiotsa i T. C. Kofané. "Modulational instability and spatiotemporal transition to chaos". Chaos, Solitons & Fractals 27, nr 4 (luty 2006): 914–25. http://dx.doi.org/10.1016/j.chaos.2005.04.039.

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Avery, Montie, Cedric Dedina, Aislinn Smith i Arnd Scheel. "Instability in large bounded domains—branched versus unbranched resonances". Nonlinearity 34, nr 11 (13.10.2021): 7916–37. http://dx.doi.org/10.1088/1361-6544/ac2a15.

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Abstract We study transitions from convective to absolute instability near a trivial state in large bounded domains for prototypical model problems in the presence of transport and negative nonlinear feedback. We identify two generic scenarios, depending on the nature of the linear mechanism for instability, which both lead to different, universal bifurcation diagrams. In the first, classical case of a linear branched resonance the transition is hard, that is, small changes in a control parameter lead to a finite-size state. In the second, novel case of an unbranched resonance, the transition is gradual. In both cases, the bifurcation diagram is determined by interaction of the leading edge of an invasion front with upstream boundary conditions. Technically, we analyze this interaction in a heteroclinic gluing bifurcation analysis that uses geometric desingularization of the trivial state.

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Li, Xiao-guang, Changhong Li, Yuan Li i Pu-jin Zhang. "Research of Transitional Failure Mode as Damage Evolution in Rock Wall". Advances in Civil Engineering 2020 (7.09.2020): 1–12. http://dx.doi.org/10.1155/2020/8864074.

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The stress condition of tunnel surrounding rock mass is complex. The stress concentration of in situ brittle rock mass caused by excavation results in localized damage evolution parallel to the free face, which is called surface instability. The rock wall shows the transition characteristics of the failure mode with the distance from the surface to the depth. Low strength surface instability and transition failure modes of the tunnel’s rock wall are common in deep condition but cylindrical specimens cannot simulate stress state of rock wall surface well in conventional rock mechanics tests. This paper conducted the indoor experimental study of the biaxial stress state and studied the surface instability of samples. An indoor test device for the simulation of transitional surface failure of the rock wall was developed. Through a biaxial stress loading test on the rectangular rock sample, the damage process and crack development of rock samples were analyzed, and the law of stress and strain related to the failure mode transition was characterized as well. Based on test results and strength analysis, an explanation of the failure theory and its corresponding model are proposed based on the maximum strain strength theory. Furthermore, this paper concludes that the failure mode of surface instability presents transition feature from brittle to ductile with the increase of distance from the surface to depth.

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Motsay, R. W., K. E. Anderson i R. P. Behringer. "The onset of convection and turbulence in rectangular layers of normal liquid 4He". Journal of Fluid Mechanics 189 (kwiecień 1988): 263–86. http://dx.doi.org/10.1017/s0022112088001004.

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We have carried out high-precision measurements of the heat transport in intermediate-size rectangular layers of convecting normal liquid 4He with Prandtl numbers of 0.52 and 0.70. The containers used for these experiments had horizontal dimensions, in units of the height d, of 13.4 × 5.95 (cell I) and 18.2 × 8.12 (cell II). The slopes N1 of the Nusselt curves were 0.56 and 0.70 respectively for cell I and cell II. These values are significantly lower than predictions for N1 for horizontally unbound layers, but comparable with results obtained in cylindrical containers of liquid helium with roughly the same number of convection rolls. For the two containers, the onset of the first instability after the onset of convection occurred at Rayleigh numbers R1 that were in reasonable quantitative agreement with the predictions of Busse and Clever for the skewed-varicose instability. For both containers, the transition at R1 was characterized by long transients ranging from ∼ 102 to ∼ 103 vertical-thermal-diffusion times. A decrease in the Nusselt number was also observed. As the Rayleigh number was increased above R1, a new steady state evolved and then additional transitions were observed. These transitions occurred at Rayleigh numbers labelled R2, R3,…, with a total of five transitions seen in cell I and a total of three transitions seen for cell II. The transition for each cell at R2 can be related quantitatively to the skewed-varicose instability, and the transition at R3 is associated with an oscillatory instability. For cell II, the time-dependence beginning at R3 persisted to the highest Rayleigh number studied, R = 11.7Rc. However, for container I, two more regimes of time-independent flow were observed; the last of these was at an unexpectedly high Rayleigh number of 6.7Rc. This work extends to lower Prandtl number recent studies made on moderate-size rectangular layers of convecting water and alcohol.

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Lee, Sang Jin, i Tamer A. Zaki. "Simulations of natural transition in viscoelastic channel flow". Journal of Fluid Mechanics 820 (5.05.2017): 232–62. http://dx.doi.org/10.1017/jfm.2017.198.

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Orderly, or natural, transition to turbulence in dilute polymeric channel flow is studied using direct numerical simulations of a FENE-P fluid. Three Weissenberg numbers are simulated and contrasted to a reference Newtonian configuration. The computations start from infinitesimally small Tollmien–Schlichting (TS) waves and track the development of the instability from the early linear stages through nonlinear amplification, secondary instability and full breakdown to turbulence. At the lowest elasticity, the primary TS wave is more unstable than the Newtonian counterpart, and its secondary instability involves the generation of $\unicode[STIX]{x1D6EC}$-structures which are narrower in the span. These subsequently lead to the formation of hairpin packets and ultimately breakdown to turbulence. Despite the destabilizing influence of weak elasticity, and the resulting early transition to turbulence, the final state is a drag-reduced turbulent flow. At the intermediate elasticity, the growth rate of the primary TS wave matches the Newtonian value. However, unlike the Newtonian instability mode which reaches a saturated equilibrium condition, the instability in the polymeric flow reaches a periodic state where its energy undergoes cyclical amplification and decay. The spanwise size of the secondary instability in this case is commensurate with the Newtonian $\unicode[STIX]{x1D6EC}$-structures, and the extent of drag reduction in the final turbulent state is enhanced relative to the lower elasticity condition. At the highest elasticity, the exponential growth rate of the TS wave is weaker than the Newtonian flow and, as a result, the early linear stage is prolonged. In addition, the magnitude of the saturated TS wave is appreciably lower than the other conditions. The secondary instability is also much wider in the span, with weaker ejection and without hairpin packets. Instead, streamwise-elongated streaks are formed and break down to turbulence via secondary instability. The final state is a high-drag-reduction flow, which approaches the Virk asymptote.

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Benedek, András, i Péter Klekner. "Managing Economic Transition". Industry and Higher Education 11, nr 3 (czerwiec 1997): 182–88. http://dx.doi.org/10.1177/095042229701100312.

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In the early stages of economic transition in Hungary, stagnation was the dominant characteristic of the country, accompanied by crisis indicators such as marked internal and especially external economic instability, instability in foreign trade, and structural impediments to development. Massive unemployment, a decrease in real wages, evident even today, and the intensification of social inequalities contributed additional tensions to the initial transition phase. In this paper, the authors first outline the main characteristics of the Hungarian economy. Then, against this background, they discuss the status of human resources development during the transition period, the economic and legal reforms taking place in the context of education and vocational training, and finally the role of the social partnership in vocational training. The discussion and conclusions drawn are intended to indicate key issues in workforce development not only in Hungary but in countries throughout the region of Eastern and Central Europe

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Lavagno, A. "Nuclear phase transition and thermodynamic instabilities in dense nuclear matter". EPJ Web of Conferences 182 (2018): 03007. http://dx.doi.org/10.1051/epjconf/201818203007.

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We study the presence of thermodynamic instabilities in a nuclear medium at finite temperature and density where nuclear phase transitions can take place. Such a phase transition is characterized by pure hadronic matter with both mechanical instability (fluctuations on the baryon density) that by chemical-diffusive instability (fluctuations on the electric charge concentration). Similarly to the liquid-gas phase transition, the nucleonic and the Δ-matter phase have a different isospin density in the mixed phase. In the liquid-gas phase transition, the process of producing a larger neutron excess in the gas phase is referred to as isospin fractionation. A similar effects can occur in the nucleon-Δ matter phase transition due essentially to a Δ- excess in the Δ-matter phase in asymmetric nuclear matter. In this context we also discuss the relevance of Δ-isobar and hyperon degrees of freedom in the bulk properties of the protoneutron stars at fixed entropy per baryon, in the presence and in the absence of trapped neutrinos.

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Lenz, Brenda. "The Transition From Adolescence to Young Adulthood: A Theoretical Perspective". Journal of School Nursing 17, nr 6 (grudzień 2001): 300–306. http://dx.doi.org/10.1177/10598405010170060401.

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Life transitions are periods in time when individuals experience major changes. Transitions may occur during periods between two relatively stable states of human development. The associated changes with the transition bring instability as the person passes through the period. During this period, the individual is typically required to make major adjustments, to develop new skills, or to learn to cope with new experiences. One major life transition begins during the final year or years of high school. This transition, unlike childhood transitions, for many individuals will include a move from one’s childhood home and away from their family of origin and from an established network of friends. A successful transition to young adulthood will form a foundation for the individual in future stages of development and transitions. Three frameworks of transition, developmental psychology, counseling, and nursing, are described.

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Hendi, Seyed Hossein, Shahram Panahiyan i Behzad Eslam Panah. "Geometrical Method for Thermal Instability of Nonlinearly Charged BTZ Black Holes". Advances in High Energy Physics 2015 (2015): 1–12. http://dx.doi.org/10.1155/2015/743086.

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We consider three-dimensional BTZ black holes with three models of nonlinear electrodynamics as source. Calculating heat capacity, we study the stability and phase transitions of these black holes. We show that Maxwell, logarithmic, and exponential theories yield only type one phase transition which is related to the root(s) of heat capacity, whereas, for correction form of nonlinear electrodynamics, heat capacity contains two roots and one divergence point. Next, we use geometrical approach for studying classical thermodynamical behavior of the system. We show that Weinhold and Ruppeiner metrics fail to provide fruitful results and the consequences of the Quevedo approach are not completely matched to the heat capacity results. Then, we employ a new metric for solving this problem. We show that this approach is successful and all divergencies of its Ricci scalar and phase transition points coincide. We also show that there is no phase transition for uncharged BTZ black holes.

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Thurlow, M. S., B. J. Brooks, P. G. J. Lucas, M. R. Ardron, J. K. Bhattacharjee i A. L. Woodcraft. "Convective instability in rotating liquid 3He-4He mixtures". Journal of Fluid Mechanics 313 (25.04.1996): 381–407. http://dx.doi.org/10.1017/s002211209600225x.

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Thermal convection is investigated experimentally in a dilute liquid mixture of 3He in 4He at four temperatures between 20 and 100 mK above the superfluid transition temperature, chosen for their proximity to the codimension-two and hydrodynamic tricritical points. Two experimental cells of aspect ratio 2.76 and 1.00 were used. For the cell with the higher aspect ratio, two convective transitions at each of the four temperatures were observed above a critical angular velocity, and only one observed below. At temperatures lower than that of the hydrodynamic tricritical point the transition with the lower critical Rayleigh number is hysteretic for all angular velocities; above this temperature hysteresis is absent. The critical Rayleigh numbers are compared with theoretical predictions that take into account the existence of convection modes with azimuthal angular dependence. In the case of the cell with the smaller aspect ratio thermal relaxation oscillations were observed when heating from below. Convective thresholds were again observed but their critical Rayleigh numbers are almost independent of angular velocity. Some suggestions are advanced for this unexpected behaviour.

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Jia, Lingchun, Hongchun Yuan, Yingli Chang, Mu Gu i Jiajie Zhu. "Dynamic instability of lithiated phosphorene". RSC Advances 10, nr 53 (2020): 32259–64. http://dx.doi.org/10.1039/d0ra04885b.

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Jacob, Niya Mary, i Tiju Thomas. "Nanorod to quantum dot conversion in ZnO dispersions with co-surfactants". RSC Advances 5, nr 20 (2015): 15154–58. http://dx.doi.org/10.1039/c4ra05778c.

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A chemically-induced nanorod to QD transition is achieved using co-surfactants. This is different from the physical instability driven transitions reported so far in nanowires and polymers. We propose a suitable mechanism for the observed phenomenon.

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DOU, HUA-SHU, i BOO CHEONG KHOO. "ENERGY SPECTRUM OF DISTURBANCE AT TURBULENT TRANSITION VIA ENERGY GRADIENT METHOD". International Journal of Modern Physics: Conference Series 19 (styczeń 2012): 293–303. http://dx.doi.org/10.1142/s2010194512008884.

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The energy gradient theory for flow instability and turbulent transition was proposed in our previous work. The theoretical result obtained accords well with some experimental data for pipe and channel flows in the literature. In the present study, the energy gradient theory is extended to examine the effect of disturbance frequency on turbulent transition. Then, the energy spectrum of disturbance at the turbulent transition is obtained, which scales with the wave number by an exponent of –2. This scaling is near to the K41 law of –5/3 for the full developed isentropic homogenous turbulence. The difference for the two energy spectra may be due to the intermittency of turbulence at the transition state. The intermittence causes the distribution of the energy spectrum to take on a steeper gradient (tending to –2 from –5/3). Finally, the flow instability leading to turbulent transition can be classified as two-dimensional (2D) or three-dimensional (3D) in terms of the wave number and the Re. It is found that there is an optimum wave number which separates the 2D and 3D transitions and at which the disturbance energy at transition is minimum.

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MALIK, MUJEEB R., FEI LI, MEELAN M. CHOUDHARI i CHAU-LYAN CHANG. "Secondary instability of crossflow vortices and swept-wing boundary-layer transition". Journal of Fluid Mechanics 399 (25.11.1999): 85–115. http://dx.doi.org/10.1017/s0022112099006291.

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Crossflow instability of a three-dimensional boundary layer is a common cause of transition in swept-wing flows. The boundary-layer flow modified by the presence of finite-amplitude crossflow modes is susceptible to high-frequency secondary instabilities, which are believed to harbinger the onset of transition. The role of secondary instability in transition prediction is theoretically examined for the recent swept-wing experimental data by Reibert et al. (1996). Exploiting the experimental observation that the underlying three-dimensional boundary layer is convectively unstable, non-linear parabolized stability equations are used to compute a new basic state for the secondary instability analysis based on a two-dimensional eigenvalue approach. The predicted evolution of stationary crossflow vortices is in close agreement with the experimental data. The suppression of naturally dominant crossflow modes by artificial roughness distribution at a subcritical spacing is also confirmed. The analysis reveals a number of secondary instability modes belonging to two basic families which, in some sense, are akin to the ‘horseshoe’ and ‘sinuous’ modes of the Görtler vortex problem. The frequency range of the secondary instability is consistent with that measured in earlier experiments by Kohama et al. (1991), as is the overall growth of the secondary instability mode prior to the onset of transition (e.g. Kohama et al. 1996). Results indicate that the N-factor correlation based on secondary instability growth rates may yield a more robust criterion for transition onset prediction in comparison with an absolute amplitude criterion that is based on primary instability alone.

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Hatman, A., i T. Wang. "A Prediction Model for Separated-Flow Transition". Journal of Turbomachinery 121, nr 3 (1.07.1999): 594–602. http://dx.doi.org/10.1115/1.2841357.

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The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers that are laminar at separation and those that are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation/short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation/long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

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Bharuthram, R., i M. A. Hellberg. "Low-frequency drift-induced instabilities in a magnetized two-ion plasma". Journal of Plasma Physics 35, nr 3 (czerwiec 1986): 393–412. http://dx.doi.org/10.1017/s0022377800011429.

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Numerical solutions of a dispersion relation for low-frequency electrostatic waves in a current-carrying, cold, weakly collisional, magnetized two-ion plasma are used to discuss the two-stream and resistive natures of the ion-ion hybrid instability. An instability with analogous behaviour is found to be associated with the light ion cyclotron frequency. Analytical results explain the behaviour. A numerically derived transition diagram summarizes the parameter values for which transitions between different modes take place.

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Ma, Wei. "Investigation of Material Instability Behaviors Caused by Combined Stress Loadings". Key Engineering Materials 535-536 (styczeń 2013): 168–72. http://dx.doi.org/10.4028/www.scientific.net/kem.535-536.168.

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This study involves the stability of plastic flow of thermoviscoplastic materials. The general instability criterion is proposed for determining the onset conditions of instability and the transition conditions among various plastic deformation behaviors. By using the phase diagram method, the transition of material instability modes under complex loading conditions is described and the analytical results are further validated with numerical examples.

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Riaz, Syed Shahed, Sandip Kar i Deb Shankar Ray. "Differential flow induced transition of Hopf instability to Turing instability and pattern formation". Physica D: Nonlinear Phenomena 203, nr 3-4 (kwiecień 2005): 224–32. http://dx.doi.org/10.1016/j.physd.2005.04.003.

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Williamson, C. H. K. "Three-dimensional wake transition". Journal of Fluid Mechanics 328 (10.12.1996): 345–407. http://dx.doi.org/10.1017/s0022112096008750.

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It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability (modes A and B), depending on the regime of Reynolds number (Re), although almost no understanding of the physical origins of these instabilities, or indeed their effects on near-wake formation, have hitherto been made clear. We address these questions in this paper. In particular, it is found that the two different modes A and B scale on different physical features of the flow. Mode A has a larger spanwise wavelength of around 3–4 diameters, and scales on the larger physical structure in the flow, namely the primary vortex core. The wavelength for mode A is shown to be the result of an ‘elliptic instability’ in the nearwake vortex cores. The subsequent nonlinear growth of vortex loops is due to a feedback from one vortex to the next, involving spanwise-periodic deformation of core vorticity, which is then subject to streamwise stretching in the braid regios. This mode gives an out-of-phase streamwise vortex pattern.In contrast, mode-B instability has a distinctly smaller wavelength (1 diameter) which scales on the smaller physical structure in the flow, the braid shear layer. It is a manifestation of an instability in a region of hyperbolic flow. It is quite distinct from other shear flows, in that it depends on the reverse flow of the bluff-body wake; the presence of a fully formed streamwise vortex system, brought upstream from a previous half-cycle, in proximity to the newly evolving braid shear layer, leads to an in-phase stream-wise vortex array, in strong analogy with the ‘Mode 1’ of Meiburg & Lasheras (1988) for a forced unseparated wake. In mode B, we also observe amalgamation of streamwise vortices from a previous braid with like-sign vortices in the subsequent braid.It is deduced that the large scatter in previous measurements concerning mode A is due to the presence of vortex dislocations. Dislocations are triggered at the sites of some vortex loops of mode A, and represent a natural breakdown of the periodicity of mode A instability. By minimizing or avoiding the dislocations which occur from end contamination or which occur during wake transition, we find an excellent agreement of both critical Re and spanwise wavelength of mode A with the recent secondary stability analysis of Barkley & Henderson (1996).Wake transition is further characterized by velocity and pressure measurements. It is consistent that, when mode-A instability and large-scale dislocations appear, one finds a reduction of base suction, a reduction of (two-dimensional) Reynolds stress level, a growth in size of the formation region, and a corresponding drop in Strouhal frequency. Finally, the present work leads us to a new clarification of the possible flow states through transition. Right through this regime of Re, there exist two distinct and continuous Strouhal frequency curves: the upper one corresponds with purley small- scale instabilities (e.g. denoted as mode A), while the lower curve corresponds with a combination of small-scale plus dislocation structures (e.g. mode A*). However, some of the flow states are transient or ‘unstable’, and the natural transitioning wake appears to follow the scenario: (2D→A*→B).

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Imayama, Shintaro, P. Henrik Alfredsson i R. J. Lingwood. "An experimental study of edge effects on rotating-disk transition". Journal of Fluid Mechanics 716 (28.01.2013): 638–57. http://dx.doi.org/10.1017/jfm.2012.564.

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AbstractThe onset of transition for the rotating-disk flow was identified by Lingwood (J. Fluid. Mech., vol. 299, 1995, pp. 17–33) as being highly reproducible, which motivated her to look for absolute instability of the boundary-layer flow; the flow was found to be locally absolutely unstable above a Reynolds number of 507. Global instability, if associated with laminar–turbulent transition, implies that the onset of transition should be highly repeatable across different experimental facilities. While it has previously been shown that local absolute instability does not necessarily lead to linear global instability: Healey (J. Fluid. Mech., vol. 663, 2010, pp. 148–159) has shown, using the linearized complex Ginzburg–Landau equation, that if the finite nature of the flow domain is accounted for, then local absolute instability can give rise to linear global instability and lead directly to a nonlinear global mode. Healey (J. Fluid. Mech., vol. 663, 2010, pp. 148–159) also showed that there is a weak stabilizing effect as the steep front to the nonlinear global mode approaches the edge of the disk, and suggested that this might explain some reports of slightly higher transition Reynolds numbers, when located close to the edge. Here we look closely at the effects the edge of the disk have on laminar–turbulent transition of the rotating-disk boundary-layer flow. We present data for three different edge configurations and various edge Reynolds numbers, which show no obvious variation in the transition Reynolds number due to proximity to the edge of the disk. These data, together with the application (as far as possible) of a consistent definition for the onset of transition to others’ results, reduce the already relatively small scatter in reported transition Reynolds numbers, suggesting even greater reproducibility than previously thought for ‘clean’ disk experiments. The present results suggest that the finite nature of the disk, present in all real experiments, may indeed, as Healey (J. Fluid. Mech., vol. 663, 2010, pp. 148–159) suggests, lead to linear global instability as a first step in the onset of transition but we have not been able to verify a correlation between the transition Reynolds number and edge Reynolds number.

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EL AKOURY, R., M. BRAZA, R. PERRIN, G. HARRAN i Y. HOARAU. "The three-dimensional transition in the flow around a rotating cylinder". Journal of Fluid Mechanics 607 (30.06.2008): 1–11. http://dx.doi.org/10.1017/s0022112008001390.

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The flow around a circular cylinder rotating with a constant angular velocity, placed in a uniform stream, is investigated by means of two- and three-dimensional direct numerical simulations. The successive changes in the flow pattern are studied as a function of the rotation rate. Suppression of vortex shedding occurs as the rotation rate increases (>2). A second kind of instabilty appears for higher rotation speed where a series of counter-clockwise vortices is shed in the upper shear layer. Three-dimensional computations are carried out to analyse the three-dimensional transition under the effect of rotation for low rotation rates. The rotation attenuates the secondary instability and increases the critical Reynolds number for the appearance of this instability. The linear and nonlinear parts of the three-dimensional transition have been quantified by means of the amplitude evolution versus time, using the Landau global oscillator model. Proper orthogonal decomposition of the three-dimensional fields allowed identification of the most energetic modes and three-dimensional flow reconstruction involving a reduced number of modes.

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BLACKBURN, H. M., i S. J. SHERWIN. "Instability modes and transition of pulsatile stenotic flow: pulse-period dependence". Journal of Fluid Mechanics 573 (luty 2007): 57–88. http://dx.doi.org/10.1017/s0022112006003740.

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The instability modes arising within simple non-reversing pulsatile flows in a circular tube with a smooth axisymmetric constriction are examined using global Floquet stability analysis and direct numerical simulation. The sectionally averaged pulsatile flow is represented with one harmonic component superimposed on a time-mean flow. We have previously identified a period-doubling global instability mechanism associated with alternating tilting of the vortex rings that are ejected out of the stenosis throat with each pulse. Here we show that while alternating tilting of vortex rings is the primary instability mode for comparatively larger reduced velocities associated with long pulse periods (or low Womersley numbers), for lower reduced velocities that are associated with shorter pulse periods the primary instability typically manifests as azimuthal waves (Widnall instability modes) of low wavenumber that grow on each vortex ring. Convective shear-layer instabilities are also supported by the types of flow considered. To provide an insight into the comparative role of these types of instability, which have still shorter temporal periods, we also introduce high-frequency low-amplitude perturbations to the base flows of the above global instabilities. For the range of parameters considered, we observe that the dominant features of the primary Floquet instability persist, but that the additional presence of the convective instability can have a destabilizing effect, especially for long pulse periods.

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Zhou, Ling, Chao Yan, Zi Hui Hao i Wei Xuan Kong. "A “Laminar + Transition Criteria” Model for Hypersonic Three-Dimensional Boundary Layer Transition Prediction". Applied Mechanics and Materials 798 (październik 2015): 627–31. http://dx.doi.org/10.4028/www.scientific.net/amm.798.627.

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A “laminar + transition criteria” model utilizingReθ/MeandReCFcriteria in conjunction with an intermittency functionΓis developed in this paper. With pretreated computational grid and total enthalpyh0=(h0,∞)maxcriteria the boundary layer edge and crossflow velocity can be obtained by using parallel methodology. Validation is accomplished via HIFiRE-5 and a blunt cone with small angle of attack. Results show that computedReθ/MeandReCFdistributions are similar to theN-factor for streamwise instability and crossflow instability from linear PSE methods. The shape and trend of transition regions predicted by the “laminar + transition criteria” model in HIFiRE-5 and blunt cone are in good agreement with the experiment and DNS. However, for the transition induced by inflection point on streamwise velocity profiles, using criteria related to boundary layer thickness is inappropriate and can predict transition onset prematurely.

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Janssen, R. J. A., i R. A. W. M. Henkes. "Influence of Prandtl number on instability mechanisms and transition in a differentially heated square cavity". Journal of Fluid Mechanics 290 (10.05.1995): 319–44. http://dx.doi.org/10.1017/s0022112095002539.

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The transition from laminar to turbulent of the natural-convection flow inside a square, differentially heated cavity with adiabatic horizontal walls is calculated, using the finite-volume method. The purpose of this study is firstly to determine the dependence of the laminar-turbulent transition on the Prandtl number and secondly to investigate the physical mechanisms responsible for the bifurcations observed. It is found that in the square cavity, for Prandtl numbers between 0.25 and 2.0, the transition occurs through periodic and quasi-periodic flow regimes. One of the bifurcations is related to an instability occurring in a jet-like fluid layer exiting from those corners of the cavity where the vertical boundary layers are turned horizontal. This instability is mainly shear-driven and the visualization of the perturbations shows the occurrence of vorticity concentrations which are very similar to Kelvin–Helmholtz vortices in a plane jet, suggesting that the instability is a Kelvin–Helmholtz-type instability. The other bifurcation for Prandtl numbers between 0.25 and 2.0 occurs in the boundary layers along the vertical walls. It differs however from the related instability in the natural-convection boundary layer along an isolated vertical plate: the instability in the cavity is shear-driven whereas the instability along the vertical plate is mainly buoyancy-driven. For Prandtl numbers between 2.5 and 7.0, it is found that there occurs an immediate transition from the steady to the chaotic flow regime without intermediate regimes. This transition is also caused by instabilities originating and concentrated in the vertical boundary layers.

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Nimura, Tomohiro, i Takahiro Tsukahara. "Viscoelasticity-Induced Instability in Plane Couette Flow at Very Low Reynolds Number". Fluids 7, nr 7 (13.07.2022): 241. http://dx.doi.org/10.3390/fluids7070241.

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Elasto-inertial turbulence (EIT), a new turbulent state found in polymer solutions with viscoelastic properties, is associated with drag-reduced turbulence. However, the relationship between EIT and drag-reduced turbulence is not currently well-understood, and it is important to elucidate the mechanism of the transition to EIT. The instability of viscoelastic fluids has been studied in a canonical wall-bounded shear flow to investigate the transition process of EIT. In this study, we numerically deduced that an instability occurs in the linearly stable viscoelastic plane Couette flow for lower Reynolds numbers, at which a non-linear unstable solution exists. Under instability, the flow structure is elongated in the spanwise direction and regularly arranged in the streamwise direction, which is a characteristic structure of EIT. The regularity of the flow structure depends on the Weissenberg number, which represents the strength of elasticity; the structure becomes disordered under high Weissenberg numbers. In the energy spectrum of velocity fluctuations, a steep decay law of the structure’s scale towards a small scale is observed, and this can be recognized as a ubiquitous feature of EIT. The existence of instability in viscoelastic plane Couette flow supports the idea that the transitional path toward EIT may be mediated by subcritical instability.

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Lavagno, A., D. Pigato i G. Gervino. "Thermodynamic instabilities in high energy heavy-ion collisions". Modern Physics Letters B 29, nr 18 (10.07.2015): 1550092. http://dx.doi.org/10.1142/s021798491550092x.

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One of the very interesting aspects of high energy heavy-ion collisions experiments is a detailed study of the thermodynamical properties of strongly interacting nuclear matter away from the nuclear ground state. In this direction, many efforts were focused on searching for possible phase transitions in such collisions. We investigate thermodynamic instabilities in a hot and dense nuclear medium where a phase transition from nucleonic matter to resonance-dominated [Formula: see text]-matter can take place. Such a phase transition can be characterized by both mechanical instability (fluctuations on the baryon density) and by chemical-diffusive instability (fluctuations on the strangeness concentration) in asymmetric nuclear matter. In analogy with the liquid–gas nuclear phase transition, hadronic phases with different values of antibaryon–baryon ratios and strangeness content may coexist. Such a physical regime could be, in principle, investigated in the future high-energy compressed nuclear matter experiments which will make it possible to create compressed baryonic matter with a high net baryon density.

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Singh, N. K. "Instability and Transition in a Laminar Separation Bubble". Journal of Applied Fluid Mechanics 12, nr 5 (1.09.2019): 1511–25. http://dx.doi.org/10.29252/jafm.12.05.29607.

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Gorton, Gary, i Andrew Winton. "Banking in Transition Economies: Does Efficiency Require Instability?" Journal of Money, Credit and Banking 30, nr 3 (sierpień 1998): 621. http://dx.doi.org/10.2307/2601261.

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Link, Bennett. "Deflagration instability in the quark-hadron phase transition". Physical Review Letters 68, nr 16 (20.04.1992): 2425–28. http://dx.doi.org/10.1103/physrevlett.68.2425.

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Tanaka, Toyoichi, Shao-Tang Sun, Yoshitsugu Hirokawa, Seiji Katayama, John Kucera, Yoshiharu Hirose i Takayuki Amiya. "Mechanical instability of gels at the phase transition". Nature 325, nr 6107 (luty 1987): 796–98. http://dx.doi.org/10.1038/325796a0.

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