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1
Li, Ning, e Qi Hong Zeng. "Direct Numerical Simulation on Transition of an Incompressible Boundary Layer on a Flat Plate". Applied Mechanics and Materials 268-270 (dicembre 2012): 1143–47. http://dx.doi.org/10.4028/www.scientific.net/amm.268-270.1143.
Testo completoAbstract (sommario):
Direct Numerical Simulation(DNS) was carried out for laminar-turbulent transition of an incompressible boundary layer on a flat plate based on disturbance Navier-Stokes(N-S) equation in spatial mode with Massage Passing Interface(MPI) technology. Study on breakdown mechanism of laminar-turbulent transition was carried on. The effect of mean flow distortion on the process of breakdown in laminar-turbulent transition was investigated. Results indicate that change of instability characteristic of mean flow profile plays a key role during process of breakdown.
2
Kadyirov, A. I., e B. R. Abaydullin. "Vortex Breakdown under Laminar Flow of Pseudoplastic Fluid". Journal of Physics: Conference Series 899 (settembre 2017): 022009. http://dx.doi.org/10.1088/1742-6596/899/2/022009.
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3
Zhou, Teng, Zaijie Liu, Yuhan Lu, Ying Wang e Chao Yan. "Direct numerical simulation of complete transition to turbulence via first- and second-mode oblique breakdown at a high-speed boundary layer". Physics of Fluids 34, n. 7 (luglio 2022): 074101. http://dx.doi.org/10.1063/5.0094069.
Testo completoAbstract (sommario):
Complete transition to turbulence via first- and second-mode oblique breakdown in a high-speed boundary layer at Mach 4.5 is studied by direct numerical simulations (DNS) and linear stability theory (LST). The initial frequency and spanwise wavenumbers for both types of oblique breakdown are determined from LST. Then, DNS is employed to study the main features of the two oblique breakdown types in detail, which has rarely been discussed in previous studies. This includes the main flow structures and evolution of various modes during the linear, nonlinear, and breakdown stages, and both different and similar features for the two oblique breakdown types are summarized. Compared with only one type of low-speed streak existing for first-mode oblique breakdown, two types occur in the second-mode oblique breakdown, and the generation mechanism, evolution process, and role of the low-speed streaks are studied. Subsequently, the generation mechanism of both the heat transfer and skin-friction overshoot during both oblique breakdowns is illustrated with emphasis on the heat transfer overshoot for the second mode, which occurs at the laminar stage. Finally, both types of oblique breakdown are the likely path to a fully developed turbulent flow, although the unstable region for the second-mode oblique waves is short and for the first-mode oblique waves is amplified slowly.
4
Seifi, Zeinab, Mehrdad Raisee e Michel J. Cervantes. "Optimal flow control of vortex breakdown in a laminar swirling flow". Journal of Physics: Conference Series 2707, n. 1 (1 febbraio 2024): 012129. http://dx.doi.org/10.1088/1742-6596/2707/1/012129.
Testo completoAbstract (sommario):
Abstract In highly swirling flows, such as hydraulic turbines operating under part-load (PL) conditions, vortex breakdown occurs and performance is impaired. Consequently, it is imperative that mitigation measures are taken. In the present study, a laminar swirling flow with a vortex breakdown at a Reynolds number of 180 is investigated. At the inlet, a swirling velocity profile with a swirl number of 1.095 is set. A stability analysis is conducted to identify unstable modes based on the assumption that vortex breakdown is a global instability. The results indicate that spiral modes with wave number 1 are unstable. An optimal flow control method based on the Adjoint method is then utilized to mitigate vortex breakdown. In the present study, the control method targets vorticity using a minimization algorithm. Control variables include radial and axial body forces. According to the results, the method was effective in mitigating vortex breakdown. A stability analysis conducted during the control process revealed that as the vorticity decreased, the growth-rate of the eigenvalue decreased, indicating that the flow is stabilized.
5
Kachanov, Yu S. "On the resonant nature of the breakdown of a laminar boundary layer". Journal of Fluid Mechanics 184 (novembre 1987): 43–74. http://dx.doi.org/10.1017/s0022112087002805.
Testo completoAbstract (sommario):
The first part of this paper (§2) briefly reviews the history of the idea of the resonant nature of laminar-boundary-layer breakdown. In the second part a new wave-resonance concept of the breakdown mechanism is proposed. The existing experimental data on the laminar boundary layer (and plane channel flow) breakdown are analysed from the viewpoint of this concept and are compared with the well-known local high-frequency secondary-instability concept. The results testify to the correctness of the proposed wave-resonant concept for the initial stages of breakdown, in particular for the K-regime of transition up to the spike formation and its doubling.Within the framework of the wave-resonance concept, before constructing the corresponding theory, many important features of the disturbance development can be qualitatively explained and understood. Concerning the understanding of the spike appearance, the wave-resonance concept complements the local high-frequency secondary-instability one and represents by itself a new fruitful viewpoint on this phenomenon. The development of the wave-resonance concept and its application to the analysis of numerical and physical experiments, together with the construction on this basis of the corresponding theory, can give an essential impetus towards the better understanding of the breakdown nature.
6
Bottaro, Alessandro, Inge L. Ryhming, Marc B. Wehrli, Franz S. Rys e Paul Rys. "Laminar swirling flow and vortex breakdown in a pipe". Computer Methods in Applied Mechanics and Engineering 89, n. 1-3 (agosto 1991): 41–57. http://dx.doi.org/10.1016/0045-7825(91)90036-6.
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7
Ozdemir, Celalettin E., Tian-Jian Hsu e S. Balachandar. "Direct numerical simulations of instability and boundary layer turbulence under a solitary wave". Journal of Fluid Mechanics 731 (28 agosto 2013): 545–78. http://dx.doi.org/10.1017/jfm.2013.361.
Testo completoAbstract (sommario):
AbstractA significant amount of research effort has been made to understand the boundary layer instability and the generation and evolution of turbulence subject to periodic/oscillatory flows. However, little is known about bottom boundary layers driven by highly transient and intermittent free-stream flow forcing, such as solitary wave motion. To better understand the nature of the instability mechanisms and turbulent flow characteristics subject to solitary wave motion, a large number of direct numerical simulations are conducted. Different amplitudes of random initial fluctuating velocity field are imposed. Two different instability mechanisms are observed within the range of Reynolds number studied. The first is a short-lived, nonlinear, long-wave instability which is observed during the acceleration phase, and the second is a broadband instability that occurs during the deceleration phase. Transition from a laminar to turbulent state is observed to follow two different breakdown pathways: the first follows the sequence of $K$-type secondary instability of a near-wall boundary layer at comparatively lower Reynolds number and the second one follows a breakdown path similar to that of free shear layers. Overall characteristics of the flow are categorized into four regimes as: (i) laminar; (ii) disturbed laminar; (iii) transitional; and (iv) turbulent. Our categorization into four regimes is consistent with earlier works. However, this study is able to provide more specific definitions through the instability characteristics and the turbulence breakdown process.
8
ZAKI, TAMER A., JAN G. WISSINK, WOLFGANG RODI e PAUL A. DURBIN. "Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence". Journal of Fluid Mechanics 665 (27 ottobre 2010): 57–98. http://dx.doi.org/10.1017/s0022112010003873.
Testo completoAbstract (sommario):
The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes laminar separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the laminar separation point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it laminar or a moderate free-stream turbulence of intensityTu~ 3%. Upstream of the separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the laminar boundary layer to separate. The separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased,Tu~ 6.5%, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer separation, is observed in the wake of the turbulent events.
9
Jost, Dominic, e Kai Nagel. "Probabilistic Traffic Flow Breakdown in Stochastic Car-Following Models". Transportation Research Record: Journal of the Transportation Research Board 1852, n. 1 (gennaio 2003): 152–58. http://dx.doi.org/10.3141/1852-19.
Testo completoAbstract (sommario):
Whether traffic displays multiple phases (e.g., laminar, jammed, synchronized) has been much discussed. Computational evidence is presented that a stochastic car-following model can be moved from two phases (laminar and jammed) to one phase by changing one of its parameters. Models with two phases show three states. Two of them are homogeneous and correspond to the two phases. The third state consists of a mix of the two phases (phase coexistence). Although the gas–liquid analogy to traffic models has been widely discussed, no traffic-related model ever displayed a completely understood stochastic version of that transition. A stochastic model is important to the understanding of the potentially probabilistic nature of the transition. If indeed two-phase models describe certain aspects correctly, predictions for breakdown probabilities can be made. Alternatively, if one-phase models describe these aspects better, there is no breakdown. Interestingly, such one-phase models can still allow for jam formation on a small scale, which may give the impression of two-phase dynamics.
10
Zang, Thomas A., e M. Yousuff Hussaini. "Multiple paths to subharmonic laminar breakdown in a boundary layer". Physical Review Letters 64, n. 6 (5 febbraio 1990): 641–44. http://dx.doi.org/10.1103/physrevlett.64.641.
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Sansica, Andrea, Neil D. Sandham e Zhiwei Hu. "Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble". Journal of Fluid Mechanics 798 (31 maggio 2016): 5–26. http://dx.doi.org/10.1017/jfm.2016.297.
Testo completoAbstract (sommario):
Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.
12
Incropera, F. P., A. L. Knox e J. R. Maughan. "Mixed-Convection Flow and Heat Transfer in the Entry Region of a Horizontal Rectangular Duct". Journal of Heat Transfer 109, n. 2 (1 maggio 1987): 434–39. http://dx.doi.org/10.1115/1.3248100.
Testo completoAbstract (sommario):
Entry-region hydrodynamic and thermal conditions have been experimentally determined for laminar mixed-convection water flow through a horizontal rectangular duct with uniform bottom heating. Direct heating of 0.05 mm stainless steel foil was used to minimize wall conduction, and the foil was instrumented to yield spanwise and longitudinal distributions of the Nusselt number. Flow visualization revealed the existence of four regimes corresponding to laminar forced convection, laminar mixed convection, transitional mixed convection, and turbulent free convection. The laminar mixed-convection regime was dominated by ascending thermals which developed into mushroom-shaped longitudinal vortices. Hydrodynamic instability resulted in breakdown of the vortices and subsequent transition to turbulent flow. The longitudinal distribution of the Nusselt number was characterized by a minimum, which followed the onset of mixed convection, and subsequent oscillations due to development of the buoyancy-driven secondary flow.
13
SHAIKH, F. N. "Investigation of transition to turbulence using white-noise excitation and local analysis techniques". Journal of Fluid Mechanics 348 (10 ottobre 1997): 29–83. http://dx.doi.org/10.1017/s0022112097006629.
Testo completoAbstract (sommario):
Weak free-stream turbulence excites modulated Tollmien–Schlichting (T–S) waves in a laminar boundary layer that grow in magnitude with downstream distance and ultimately lead to the formation of turbulent spots and then fully turbulent flow. Hot-wire experiments have indicated that the development of localized large-amplitude ‘events’ in the velocity records are the essential precursor to the eventual formation of turbulent spots in the flow field. Traditional global Fourier techniques are unable to resolve the localized nature of these events and hence provide little useful information concerning the physical processes responsible for this breakdown process.This investigation used sequences of computer-generated deterministic white noise to excite a laminar boundary layer via a loudspeaker embedded in a flat-plate model. This form of excitation generated the modulated disturbance waves of interest a short distance downstream from the source in a repeatable and deterministic manner. Further downstream the pattern of flow breakdown and subsequent generation of turbulent spots was similar to that observed in naturally excited situations. By repeatedly exciting the boundary layer with a single white-noise sequence it was possible to examine the highly nonlinear stages of ‘event’ development and breakdown with a single hot-wire probe.Two local analysis techniques, the wavelet transform (WT) and singular spectrum analysis (SSA), were used in conjunction with the white-noise excitation technique to examine the highly nonlinear flow mechanisms responsible for the localized formation of events that lead to the eventual breakdown to turbulence.
14
Franko, Kenneth J., e Sanjiva K. Lele. "Breakdown mechanisms and heat transfer overshoot in hypersonic zero pressure gradient boundary layers". Journal of Fluid Mechanics 730 (1 agosto 2013): 491–532. http://dx.doi.org/10.1017/jfm.2013.350.
Testo completoAbstract (sommario):
AbstractA laminar Mach 6 flat plate boundary layer is perturbed using three different types of disturbances introduced through blowing and suction. The linear and nonlinear development and eventual breakdown to turbulence are investigated using direct numerical simulation. The three different transition mechanisms compared are first mode oblique breakdown, second mode oblique breakdown and second mode fundamental resonance. The focus of the present work is to compare the nonlinear development and breakdown to turbulence for the different transition mechanisms and explain the heat transfer overshoot observed in experiments. First mode oblique breakdown leads to the shortest transition length and a clear peak in wall heat transfer in the transitional region. For all three transition mechanisms, the development of streamwise streaks precedes the breakdown to fully turbulent flow. The modal linear and nonlinear development are analysed including the breakdown of the streaks. The effect of wall cooling is investigated for second mode fundamental resonance and no qualitative differences in the nonlinear processes are observed. Finally, the development towards fully turbulent flow including mean flow, turbulent spectra, and turbulent fluctuations is shown and the first mode oblique breakdown simulation shows the furthest development towards a fully turbulent flow.
15
Jovanovic, Jovan, e Mira Pashtrapanska. "On the evolution of laminar to turbulent transition and breakdown to turbulence". Thermal Science 7, n. 2 (2003): 59–76. http://dx.doi.org/10.2298/tsci0302059j.
Testo completoAbstract (sommario):
Starting from the basic conservation laws of fluid flow, we investigated transition and breakdown to turbulence of a laminar flat plate boundary layer exposed to small, statistically stationary, two-component, three-dimensional disturbances. The derived equations for the statistical properties of the disturbances are closed using the two-point correlation technique and invariant theory. By considering the equilibrium solutions of the modeled equations, the transition criterion is formulated in terms of a Reynolds number based on the intensity and the length scale of the disturbances. The deduced transition criterion determines conditions that guarantee maintenance of the local equilibrium between the production and the viscous dissipation of the disturbances and therefore the laminar flow regime in the flat plate boundary layer. The experimental and numerical databases for fully developed turbulent channel and pipe flows at different Reynolds numbers were utilized to demonstrate the validity of the derived transition criterion for the estimation of the onset of turbulence in wall-bounded flows.
16
Nering, Konrad, e Kazimierz Rup. "An improved algebraic model for by-pass transition for calculation of transitional flow in pipe and parallel-plate channels". Thermal Science 23, Suppl. 4 (2019): 1123–31. http://dx.doi.org/10.2298/tsci19s4123n.
Testo completoAbstract (sommario):
Modified algebraic intermittency model developed by E. Dick and S. Kubacki was used to describe laminar-turbulent transition. In this work a modification of this model was made for simulating internal flows in pipes and parallel-plate channel. In particular, constants present in this model were modified. These modified constants are the same for different flows in pipes and parallel-plate channels. In this work, a dependence of friction factor on Reynolds number and turbulence intensity were determined as well as the localization of laminar breakdown and fully developed flow. Obtained results were compared with theoretical and experimental data presented in the literature.
17
Lopez, J. M. "Axisymmetric vortex breakdown Part 1. Confined swirling flow". Journal of Fluid Mechanics 221 (dicembre 1990): 533–52. http://dx.doi.org/10.1017/s0022112090003664.
Testo completoAbstract (sommario):
A comparison between the experimental visualization and numerical simulations of the occurrence of vortex breakdown in laminar swirling flows produced by a rotating endwall is presented. The experimental visualizations of Escudier (1984) were the first to detect the presence of multiple recirculation zones and the numerical model presented here, consisting of a numerical solution of the unsteady axisymmetric Navier-Stokes equations, faithfully reproduces these phenomena and all other observed characteristics of the flow. Further, the numerical calculations elucidate the onset of oscillatory flow, an aspect of the flow that was not clearly resolved by the flow visualization experiments. Part 2 of the paper examines the underlying physics of these vortex flows.
18
Wu, Xiaohua, Parviz Moin, Ronald J. Adrian e Jon R. Baltzer. "Osborne Reynolds pipe flow: Direct simulation from laminar through gradual transition to fully developed turbulence". Proceedings of the National Academy of Sciences 112, n. 26 (15 giugno 2015): 7920–24. http://dx.doi.org/10.1073/pnas.1509451112.
Testo completoAbstract (sommario):
The precise dynamics of breakdown in pipe transition is a century-old unresolved problem in fluid mechanics. We demonstrate that the abruptness and mysteriousness attributed to the Osborne Reynolds pipe transition can be partially resolved with a spatially developing direct simulation that carries weakly but finitely perturbed laminar inflow through gradual rather than abrupt transition arriving at the fully developed turbulent state. Our results with this approach show during transition the energy norms of such inlet perturbations grow exponentially rather than algebraically with axial distance. When inlet disturbance is located in the core region, helical vortex filaments evolve into large-scale reverse hairpin vortices. The interaction of these reverse hairpins among themselves or with the near-wall flow when they descend to the surface from the core produces small-scale hairpin packets, which leads to breakdown. When inlet disturbance is near the wall, certain quasi-spanwise structure is stretched into a Lambda vortex, and develops into a large-scale hairpin vortex. Small-scale hairpin packets emerge near the tip region of the large-scale hairpin vortex, and subsequently grow into a turbulent spot, which is itself a local concentration of small-scale hairpin vortices. This vortex dynamics is broadly analogous to that in the boundary layer bypass transition and in the secondary instability and breakdown stage of natural transition, suggesting the possibility of a partial unification. Under parabolic base flow the friction factor overshoots Moody’s correlation. Plug base flow requires stronger inlet disturbance for transition. Accuracy of the results is demonstrated by comparing with analytical solutions before breakdown, and with fully developed turbulence measurements after the completion of transition.
19
Jovanovic, Jovan, e Mina Nishi. "The origin of turbulence in wall-bounded flows". Thermal Science 21, suppl. 3 (2017): 565–72. http://dx.doi.org/10.2298/tsci160413184j.
Testo completoAbstract (sommario):
The motion of liquids and gases can be either laminar, flowing slowly in orderly parallel and continuous layers of fluid that cannot mix, or turbulent in which motion exhibits disorder in time and space with the ability to promote mixing. Breakdown of ordered to disordered motion can follow different scenarios so that no universal mechanism can be identified even in similar flow configurations [1]. Only under very special circumstances can the mechanism associated with the appearance of turbulence be studied within the deterministic theory of hydrodynamic stability [2] or employing direct numerical simulations [3] which themselves cannot provide the necessary understanding [4]. Here we show that the representative mechanism responsible for the origin of turbulence in wallbounded flows is associated with large variations of anisotropy in the disturbances [5]. During the breakdown process, anisotropy decays from a maximum towards its minimum value, inducing the explosive production of the dissipation which logically leads to the appearance of small-scale three-dimensional motions. By projecting the sequence of events leading to turbulence in the space which emphasizes the anisotropic nature in the disturbances [6], we explain why, demonstrate how and present what can be achieved if the process is treated analytically using statistical techniques [7]. It is shown that the statistical approach provides not only predictions of the breakdown phenomena which are in fair agreement with available data but also requirements which ensure persistence of the laminar regime up to very high Reynolds numbers.
20
LUO, Jisheng. "Inherent mechanism of breakdown in laminar-turbulent transition of plane channel flows". Science in China Series G 48, n. 2 (2005): 228. http://dx.doi.org/10.1360/04yw0168.
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Tian, Zhaohua, Meirong Dong, Shishi Li e Jidong Lu. "Spatially resolved laser-induced breakdown spectroscopy in laminar premixed methane–air flames". Spectrochimica Acta Part B: Atomic Spectroscopy 136 (ottobre 2017): 8–15. http://dx.doi.org/10.1016/j.sab.2017.08.001.
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Salas, M. D., e G. Kuruvila. "Vortex breakdown simulation: A circumspect study of the steady, laminar, axisymmetric model". Computers & Fluids 17, n. 1 (gennaio 1989): 247–62. http://dx.doi.org/10.1016/0045-7930(89)90020-0.
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Pruett, C. D., e T. A. Zang. "Direct numerical simulation of laminar breakdown in high-speed, axisymmetric boundary layers". Theoretical and Computational Fluid Dynamics 3, n. 6 (settembre 1992): 345–67. http://dx.doi.org/10.1007/bf00417933.
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Sivasubramanian, Jayahar, e Hermann F. Fasel. "Direct numerical simulation of transition in a sharp cone boundary layer at Mach 6: fundamental breakdown". Journal of Fluid Mechanics 768 (10 marzo 2015): 175–218. http://dx.doi.org/10.1017/jfm.2014.678.
Testo completoAbstract (sommario):
Direct numerical simulations (DNS) were performed to investigate the laminar–turbulent transition in a boundary layer on a sharp cone with an isothermal wall at Mach 6 and at zero angle of attack. The motivation for this research is to make a contribution towards understanding the nonlinear stages of transition and the final breakdown to turbulence in hypersonic boundary layers. In particular, the role of second-mode fundamental resonance, or (K-type) breakdown, is investigated using high-resolution ‘controlled’ transition simulations. The simulations were carried out for the laboratory conditions of the hypersonic transition experiments conducted at Purdue University. First, several low-resolution simulations were carried out to explore the parameter space for fundamental resonance in order to identify the cases that result in strong nonlinear interactions. Subsequently, based on the results from this study, a set of highly resolved simulations that proceed deep into the turbulent breakdown region have been performed. The nonlinear interactions observed during the breakdown process are discussed in detail in this paper. A detailed description of the flow structures that arise due to these nonlinear interactions is provided and an analysis of the skin friction and heat transfer development during the breakdown is presented. The controlled transition simulations clearly demonstrate that fundamental breakdown may indeed be a viable path to complete breakdown to turbulence in hypersonic cone boundary layers at Mach 6.
25
Jovanovic´, J., e M. Pashtrapanska. "On the Criterion for the Determination Transition Onset and Breakdown to Turbulence in Wall-Bounded Flows1". Journal of Fluids Engineering 126, n. 4 (1 luglio 2004): 626–33. http://dx.doi.org/10.1115/1.1779663.
Testo completoAbstract (sommario):
Starting from the basic conservation laws of fluid flow, we investigated transition and breakdown to turbulence of a laminar flat plate boundary layer exposed to small, statistically stationary, two-component, three-dimensional disturbances. The derived equations for the statistical properties of the disturbances are closed using the two-point correlation technique and invariant theory. By considering the equilibrium solutions of the modeled equations, the transition criterion is formulated in terms of a Reynolds number based on the intensity and the length scale of the disturbances. The deduced transition criterion determines conditions that guarantee maintenance of the local equilibrium between the production and the viscous dissipation of the disturbances and therefore the laminar flow regime in the flat plate boundary layer. The experimental and numerical databases for fully developed turbulent channel and pipe flows at different Reynolds numbers were utilized to demonstrate the validity of the derived transition criterion for the estimation of the onset of turbulence in wall-bounded flows.
26
Gumowski, K., e S. Kubacki. "Experimental study of laminar-to-turbulent transition in an adverse pressure gradient flow". Journal of Physics: Conference Series 2367, n. 1 (1 novembre 2022): 012018. http://dx.doi.org/10.1088/1742-6596/2367/1/012018.
Testo completoAbstract (sommario):
Abstract Experimental results on laminar-to-turbulent transition in a separated laminar boundary layer formed over a flat plate in an adverse pressure gradient are provided. Experiments have been performed for a range of Reynolds numbers (1.4 − 2.4 · 105) and for two freestream turbulence levels (Tu =3.6 and 5.3%). The measurements of mean and fluctuating velocity profiles have been carried out at inlet to the test section using Constant Temperature Anemometry (CTA). The data might be useful for validation and development of transition models. In order to characterize the dynamics leading to the transition process in the separated boundary layer on the plate surface, the Particle Image Velocimetry (PIV) measurements have been performed. The results show that the transition process in the separated boundary layer is influenced by breakdown of the Klebanoff streaks.
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Zuikov, Andrey L., e Elena V. Bazhina. "Viscous stress tensor and stability of laminar contravortical flows". Vestnik MGSU, n. 7 (luglio 2019): 870–84. http://dx.doi.org/10.22227/1997-0935.2019.7.870-884.
Testo completoAbstract (sommario):
Introduction: coaxial layers in contravortical flows rotate in the opposite directions. This determines their complicated spatial structure. The relevance of the subject is in the uniquely effective mixing of the moving medium. This property has a great potential of application from microbiology and missile building for obtaining highly dispersed mixtures to heat engineering for increasing the intensity of heat transfer. However, contravortical flows have a high degree of hydrodynamic instability. This hinders effective development of these technologies. Contravortical flows are observed behind Francis hydroturbines, whose derated operation causes modes with a significant increase of hydraulic unit vibrations up to destruction of the units. The purpose of the study is to identify physical laws of the contravortical flow hydrodynamics, common for both laminar and turbulent fluid flow modes.
Materials and methods: theoretical analysis of the viscous stress tensor and local stability zones of contravortical laminar flows.
Results: the article provides a mathematical description of the tensor of viscous tangential (τij) and normal (σii) stresses as well as local stability zones of the flow according to Rayleigh (Ra) and Richardson (Ri) criteria. The graphs of the radial-axial distributions of the viscous stress components are given, local stability zones are shown and the point of “vortex breakdown” is indicated. The solutions are obtained in the form of Fourier – Bessel series. The hydrodynamic structure of the flow is analysed.
Conclusions: it is established that the most significant viscous stresses are observed at the beginning of the interaction zone of contrarotating layers. It is established that the areas with the most unstable flow are localized in the flow vortex core. Three zones can be distinguished in the vortex core: a zone of weak instability with local Richardson numbers to Ri = –1, passing into a zone of flow destabilization with high negative values of Richardson numbers Ri = –10 to –100, in turn, transforming into a zone with rapidly increasing instability up to Ri = –1000. This is a zone of loss of flow stability, culminating in the “ortex breakdown”.
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Thomson, K. D. "Some comments on the later stages of transition from laminar to turbulent flow in the flat plate boundary layer". Aeronautical Journal 92, n. 918 (ottobre 1988): 309–14. http://dx.doi.org/10.1017/s0001924000016341.
Testo completoAbstract (sommario):
Pseudo-streamwise vortices develop during the transition of a boundary layer from the laminar to the turbulent state. It is suggested that the induced velocities associated with these vortices cause a gross change in the flow, and that the initial uniform flow is no longer tenable. The character of the flow in the boundary layer must change suddenly, and it is believed that this change provides the trigger for flow breakdown to turbulence.
29
Watmuff, Jonathan H. "Effects of Weak Free Stream Nonuniformity on Boundary Layer Transition". Journal of Fluids Engineering 128, n. 2 (4 aprile 2005): 247–57. http://dx.doi.org/10.1115/1.2169813.
Testo completoAbstract (sommario):
Experiments are described in which well-defined weak Free Stream Nonuniformity (FSN) is introduced by placing fine wires upstream of the leading edge of a flat plate. Large amplitude spanwise thickness variations form in the boundary layer as a result of the interaction between the steady laminar wakes from the wires and the leading edge. The centerline of a region of elevated layer thickness is aligned with the centerline of the wake in the freestream and the response is shown to be remarkably sensitive to the spanwise length-scale of the wakes. The region of elevated thickness is equivalent to a long narrow low speed streak in the layer. Elevated Free Stream Turbulence (FST) levels are known to produce randomly forming arrays of long narrow low speed streaks in laminar boundary layers. Therefore the characteristics of the streaks resulting from the FSN are studied in detail in an effort to gain some insight into bypass transition that occurs at elevated FST levels. The shape factors of the profiles in the vicinity of the streak appear to be unaltered from the Blasius value, even though the magnitude of the local thickness variations are as large as 60% of that of the undisturbed layer. Regions of elevated background unsteadiness appear on either side of the streak and it is shown that they are most likely the result of small amplitude spanwise modulation of the layer thickness. The background unsteadiness shares many of the characteristics of Klebanoff modes observed at elevated FST levels. However, the layer remains laminar to the end of the test section (Rx≈1.4×106) and there is no evidence of bursting or other phenomena associated with breakdown to turbulence. A vibrating ribbon apparatus is used to examine interactions between the streak and Tollmien-Schlichting (TS) waves. The deformation of the mean flow introduced by the streak is responsible for substantial phase and amplitude distortion of the waves and the breakdown of the distorted waves is more complex and it occurs at a lower Reynolds number than the breakdown of the K-type secondary instability that is observed when the FSN is not present.
30
Chew, J. W. "Computation of Forced Laminar Convection in Rotating Cavities". Journal of Heat Transfer 107, n. 2 (1 maggio 1985): 277–82. http://dx.doi.org/10.1115/1.3247411.
Testo completoAbstract (sommario):
Finite difference solutions are presented for forced laminar convection in a rotating cylindrical cavity with radial outflow. This forms a simple model of the cooling flow between two compressor disks in a gas turbine engine. If the fluid enters the cavity from a uniform radial source, it is shown that the local Nusselt number changes from that of a “free disk” near the center of the cavity to that for Ekman layer flow at larger radii. With an axial inlet, the flow, and consequently, the heat transfer, is more complex. If vortex breakdown occurs, then the results are very similar to those for the radial inlet case, but otherwise a wall jet forms on the downstream disk, and the heat transfer from this disk may be several times that for the upstream disk. Variation of mean Nusselt number with rotational speed is qualitatively similar to previously published experimental measurements in turbulent flow. The effect of Prandtl number on heat transfer has also been demonstrated.
31
Wang, Meng, Sanjiva K. Lele e Parviz Moin. "Sound radiation during local laminar breakdown in a low-Mach-number boundary layer". Journal of Fluid Mechanics 319, n. -1 (luglio 1996): 197. http://dx.doi.org/10.1017/s0022112096007318.
Testo completo
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Kro¨ner, M., J. Fritz e T. Sattelmayer. "Flashback Limits for Combustion Induced Vortex Breakdown in a Swirl Burner". Journal of Engineering for Gas Turbines and Power 125, n. 3 (1 luglio 2003): 693–700. http://dx.doi.org/10.1115/1.1582498.
Testo completoAbstract (sommario):
Flame flashback from the combustion chamber into the mixing zone limits the reliability of swirl stabilized lean premixed combustion in gas turbines. In a former study, the combustion induced vortex breakdown (CIVB) has been identified as a prevailing flashback mechanism of swirl burners. The present study has been performed to determine the flashback limits of a swirl burner with cylindrical premixing tube without centerbody at atmospheric conditions. The flashback limits, herein defined as the upstream flame propagation through the entire mixing tube, have been detected by a special optical flame sensor with a high temporal resolution. In order to study the effect of the relevant parameters on the flashback limits, the burning velocity of the fuel has been varied using four different natural gas-hydrogen-mixtures with a volume fraction of up to 60% hydrogen. A simple approach for the calculation of the laminar flame speeds of these mixtures is proposed which is used in the next step to correlate the experimental results. In the study, the preheat temperature of the fuel mixture was varied from 100°C to 450°C in order to investigate influence of the burning velocity as well as the density ratio over the flame front. Moreover, the mass flow rate has been modified in a wide range as an additional parameter of technical importance. It was found that the quenching of the chemical reaction is the governing factor for the flashback limit. A Peclet number model was successfully applied to correlate the flashback limits as a function of the mixing tube diameter, the flow rate and the laminar burning velocity. Using this model, a quench factor can be determined for the burner, which is a criterion for the flashback resistance of the swirler and which allows to calculate the flashback limit for all operating conditions on the basis of a limited number of flashback tests.
33
Skripkin, S. G. "Parametric study of cone angle influence on bubble vortex breakdown onset in laminar conical flow at various swirl numbers". Journal of Physics: Conference Series 2119, n. 1 (1 dicembre 2021): 012019. http://dx.doi.org/10.1088/1742-6596/2119/1/012019.
Testo completoAbstract (sommario):
Abstract The current work studies a swirling laminar viscous pipe flow with a controllable swirl number and varying pipe divergence cone angle. Such flows are widely used in various engineering applications. When a certain level of flow swirl is reached, a phenomenon called vortex breakdown occurs, the characteristics of which depend on the intensity of swirling of the flow and the Reynolds number. However, in addition to these two parameters, an important influence is exerted by the pipe opening angle, which often does not allow generalizing the results obtained in the pipe flow with even slightly different angles. Since experimentally it is quite difficult and expensive to change the pipe angle, especially considering the water as working fluid, this issue could be solved using CFD techniques. Using the design study, 63 different combinations of S and α are considered. The effect of the pipe divergence angle on the position of the bubble vortex breakdown and its properties is demonstrated. It is shown that there is a nonlinear relationship between the position of the bubble breakdown onset and the minimum value of the axial velocity at the axis depending on the opening angle of the cone.
34
Xu, Guoliang, e Song Fu. "A Four-Equation Eddy-Viscosity Approach for Modeling Bypass Transition". Advances in Applied Mathematics and Mechanics 6, n. 4 (agosto 2014): 523–38. http://dx.doi.org/10.4208/aamm.2013.m266.
Testo completoAbstract (sommario):
AbstractIt is very important to predict the bypass transition in the simulation of flows through turbomachinery. This paper presents a four-equation eddy-viscosity turbulence transition model for prediction of bypass transition. It is based on the SST turbulence model and the laminar kinetic energy concept. A transport equation for the non-turbulent viscosity is proposed to predict the development of the laminar kinetic energy in the pre-transitional boundary layer flow which has been observed in experiments. The turbulence breakdown process is then captured with an intermittency transport equation in the transitional region. The performance of this new transition model is validated through the experimental cases of T3AM, T3A and T3B. Results in this paper show that the new transition model can reach good agreement in predicting bypass transition, and is compatible with modern CFD software by using local variables.
35
YU, PENG, T. S. LEE, Y. ZENG e H. T. LOW. "EFFECT OF VORTEX BREAKDOWN ON MASS TRANSFER IN A CELL CULTURE BIOREACTOR". Modern Physics Letters B 19, n. 28n29 (20 dicembre 2005): 1543–46. http://dx.doi.org/10.1142/s0217984905009869.
Testo completoAbstract (sommario):
The present work shows that vortex breakdown may also occur in a bioreactor for animal cell or tissue culture. The aim is to study the effect of vortex breakdown on the fluid environment for cell growth, particularly hydrodynamic stress and mass transfer. A numerical simulation of the fluid flow and oxygen transfer in a cylindrical bioreactor is presented, in which a rotating base wall is used to generate the medium flow and mixing. The steady and laminar, axisymmetric Navier-Stokes and species equations are solved by the numerical model based on finite volume method. The hydrodynamic stress and oxygen transfer conditions are systematically studied by varying the two key parameters which determine the flow behavior: bioreactor aspect ratio H/R and a rotation Reynolds number Re=ΩR2/ν. It is found that the oxygen concentration at the attached breakdown vortex center is significantly higher than that at the main recirculation center but the hydrodynamic stress level is almost similar in the two regions. The results would provide guidance on choosing the proper operating parameters for cell or tissue culture.
36
WILLIAMSON, N., N. SRINARAYANA, S. W. ARMFIELD, G. D. McBAIN e W. LIN. "Low-Reynolds-number fountain behaviour". Journal of Fluid Mechanics 608 (11 luglio 2008): 297–317. http://dx.doi.org/10.1017/s0022112008002310.
Testo completoAbstract (sommario):
Experimental evidence for previously unreported fountain behaviour is presented. It has been found that the first unstable mode of a three-dimensional round fountain is a laminar flapping motion that can grow to a circling or multimodal flapping motion. With increasing Froude and Reynolds numbers, fountain behaviour becomes more disorderly, exhibiting a laminar bobbing motion. The transition between steady behaviour, the initial flapping modes and the laminar bobbing flow can be approximately described by a function FrRe2/3=C. The transition to turbulence occurs at Re > 120, independent of Froude number, and the flow appears to be fully turbulent at Re≈2000. For Fr > 10 and Re≲120, sinuous shear-driven instabilities have been observed in the rising fluid column. For Re≳120 these instabilities cause the fountain to intermittently breakdown into turbulent jet-like flow. For Fr≲10 buoyancy forces begin to dominate the flow and pulsing behaviour is observed. A regime map of the fountain behaviour for 0.7≲Fr≲100 and 15≲Re≲1900 is presented and the underlying mechanisms for the observed behaviour are proposed. Movies are available with the online version of the paper.
37
HAIN, R., C. J. KÄHLER e R. RADESPIEL. "Dynamics of laminar separation bubbles at low-Reynolds-number aerofoils". Journal of Fluid Mechanics 630 (10 luglio 2009): 129–53. http://dx.doi.org/10.1017/s0022112009006661.
Testo completoAbstract (sommario):
The laminar separation bubble on an SD7003 aerofoil at a Reynolds numberRe= 66000 was investigated to determine the dominant frequencies of the transition process and the flapping of the bubble. The measurements were performed with a high-resolution time-resolved particle image velocimetry (TR-PIV) system. Contrary to typical measurements performed through conventional PIV, the different modes can be identified by applying TR-PIV. The interaction between the shed vortices is analysed, and their significance for the production of turbulence is presented. In the shear layer above the bubble the generation and amplification of vortices due to Kelvin–Helmholtz instabilities is observed. It is found that these instabilities have a weak coherence in the spanwise direction. In a later stage of transition these vortices lead to a three-dimensional breakdown to turbulence.
38
ASAI, MASAHITO, MASAYUKI MINAGAWA e MICHIO NISHIOKA. "The instability and breakdown of a near-wall low-speed streak". Journal of Fluid Mechanics 455 (25 marzo 2002): 289–314. http://dx.doi.org/10.1017/s0022112001007431.
Testo completoAbstract (sommario):
The instability of the three-dimensional high-shear layer associated with a near-wall low-speed streak is investigated experimentally. A single low-speed streak, not unlike the near-wall low-speed streaks in transitional and turbulent flows, is produced in a laminar boundary layer by using a small piece of screen set normal to the wall. In order to excite symmetric and anti-symmetric modes separately, well-controlled external disturbances are introduced into the laminar low-speed streak through small holes drilled behind the screen. The growth of the excited symmetric varicose mode is essentially governed by the Kelvin–Helmholtz instability of the in ectional velocity profiles across the streak in the normal-to-wall direction and it can occur when the streak width is larger than the shear-layer thickness. The spatial growth rate of the symmetric mode is very sensitive to the streak width and is rapidly reduced as the velocity defect decreases flowing to the momentum transfer by viscous stresses. By contrast, the anti-symmetric sinuous mode that causes the streak meandering is dominated by the wake-type instability of spanwise velocity distributions across the streak. As far as the linear instability is concerned, the growth rate of the anti-symmetric mode is not so strongly affected by the decrease in the streak width, and its exponential growth may continue further downstream than that of the symmetric mode. As for the mode competition, it is important to note that when the streak width is narrow and comparable with the shear-layer thickness, the low-speed streak becomes more unstable to the anti-symmetric modes than to the symmetric modes. It is clearly demonstrated that the growth of the symmetric mode leads to the formation of hairpin vortices with a pair of counter-rotating streamwise vortices, while the anti-symmetric mode evolves into a train of quasi-streamwise vortices with vorticity of alternate sign.
39
Kumar, Vivaswat, Federico Pizzi, André Giesecke, Ján Šimkanin, Thomas Gundrum, Matthias Ratajczak e Frank Stefani. "The effect of nutation angle on the flow inside a precessing cylinder and its dynamo action". Physics of Fluids 35, n. 1 (gennaio 2023): 014114. http://dx.doi.org/10.1063/5.0134562.
Testo completoAbstract (sommario):
The effect of the nutation angle on the flow inside a precessing cylinder is experimentally explored and compared with numerical simulations. The focus is laid on the typical breakdown of the directly forced m = 1 Kelvin mode for increasing precession ratio (Poincaré number) and the accompanying transition between laminar and turbulent flows. Compared to the reference case with a 90° nutation angle, prograde rotation leads to an earlier breakdown, while in the retrograde case, the forced mode continues to exist also for higher Poincaré numbers. Depending largely on the occurrence and intensity of an axisymmetric double-roll mode, a kinematic dynamo study reveals a sensitive dependence of the self-excitation condition on the nutation angle and the Poincaré number. Optimal dynamo conditions are found for 90° angle which, however, might shift to slightly retrograde precession for higher Reynolds numbers.
40
Moise, Pradeep, e Joseph Mathew. "Bubble and conical forms of vortex breakdown in swirling jets". Journal of Fluid Mechanics 873 (24 giugno 2019): 322–57. http://dx.doi.org/10.1017/jfm.2019.401.
Testo completoAbstract (sommario):
Experimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile’s swirl parameter $S$ (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ($S=1$) a helical mode of azimuthal wavenumber $m=-2$ (co-winding, counter-rotating mode) was observed. A ‘swelling’ appeared at $S=1.38$, and a steady bubble breakdown at $S=1.4$. On further increase to $S=1.5$, a helical, self-excited global mode ($m=+1$, counter-winding and co-rotating) was observed, originating in the bubble’s wake but with little effect on the bubble itself – a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ($S=1.55$), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown – transition to a conjugate state, and the dominance of negative azimuthal vorticity – was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable.
41
Cheng, K. C., e 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, n. 3 (1 agosto 1988): 608–15. http://dx.doi.org/10.1115/1.3250536.
Testo completoAbstract (sommario):
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.
42
Walker, G. J., e J. P. Gostelow. "Effects of Adverse Pressure Gradients on the Nature and Length of Boundary Layer Transition". Journal of Turbomachinery 112, n. 2 (1 aprile 1990): 196–205. http://dx.doi.org/10.1115/1.2927633.
Testo completoAbstract (sommario):
Existing transition models are surveyed and deficiencies in previous predictions, which seriously overestimate transition length under an adverse pressure gradient, are discussed. A new model for transition in an adverse pressure gradient situation is proposed and experimental results are provided that confirm its validity. A correlation for transition length is advanced that incorporates both Reynolds number and pressure gradient effects. Under low free-stream turbulence conditions the basic mechanism of transition is laminar instability. There are, however, physical differences between zero and adverse pressure gradients. In the former case, transition occurs randomly, due to the breakdown of laminar instability waves in sets. For an adverse pressure gradient, the Tollmien–Schlichting waves appear more regularly with a well-defined spectral peak. As the adverse pressure gradient is increased from zero to the separation value the flow evolves continuously from random to periodic behavior and the dimensionless transition length progressively decreases.
43
Di Giovanni, Antonio, e Christian Stemmer. "Cross-flow-type breakdown induced by distributed roughness in the boundary layer of a hypersonic capsule configuration". Journal of Fluid Mechanics 856 (5 ottobre 2018): 470–503. http://dx.doi.org/10.1017/jfm.2018.706.
Testo completoAbstract (sommario):
Direct numerical simulations are undertaken to investigate the nature of instability mechanisms induced by singular and distributed roughnesses on a blunt-capsule configuration. On the base of a capsule-like hemispherical forebody at wind-tunnel conditions ($M=5.9$), we analyse the development of unsteady disturbances behind a patch of two different roughness geometries. First, spanwise periodic roughness elements are considered and cross-validation with other methods of the stability analysis is achieved. Two main unstable modes are found in the roughness wake, corresponding to the symmetric and antisymmetric modes already known for single roughness elements. Second, the case of a patch of (pseudo-)randomly distributed roughness is presented. A new type of roughness-induced cross-flow-like instability is observed for the blunt-capsule configuration. The rapid growth of primary and secondary instabilities in the cross-flow-type vortex is analysed and quantified in both the linear and nonlinear stages up to the laminar–turbulent breakdown. Spatio-temporal Fourier analysis is performed to track the onset of secondary instabilities, whereas laminar–turbulent transition is identified by the steep increase of the wall heat flux.
44
Brinkerhoff, Joshua R., e Metin I. Yaras. "Numerical investigation of transition in a boundary layer subjected to favourable and adverse streamwise pressure gradients and elevated free stream turbulence". Journal of Fluid Mechanics 781 (16 settembre 2015): 52–86. http://dx.doi.org/10.1017/jfm.2015.457.
Testo completoAbstract (sommario):
Laminar-to-turbulent transition of a boundary layer subjected to streamwise pressure gradients and elevated free stream turbulence is computed through direct numerical simulation. The streamwise pressure distribution and elevated free stream turbulence levels mimic the conditions present on the suction side of highly-cambered airfoils. Longitudinal streamwise streaks form in the laminar boundary layer through the selective inclusion of low-frequency disturbances from the free stream turbulence. The spanwise spacing normalized by local inner variables indicates stabilization of the streaks occurs by the favourable pressure gradient and prevents the development of secondary streak instability modes until downstream of the suction peak. Two distinct processes are found to trigger transition to turbulence in the adverse pressure gradient region of the flow. One involves the development of varicose secondary instability of individual low-speed streaks that results in their breakdown and the formation and growth of discrete turbulent spots. The other involves a rapid amplification of free stream disturbances in the inflectional boundary layer in the adverse pressure gradient region that results in a largely homogeneous breakdown to turbulence across the span. The effect of high-frequency free stream disturbances on the streak secondary instability and on the nonlinear processes within the growing turbulent spot are analysed through the inviscid transport of instantaneous vorticity. The results suggest that free stream turbulence contributes to the growth of the turbulent spot by generating large strain rates that activate vortex-stretching and tilting processes within the spot.
45
Nering, Konrad, e Kazimierz Rup. "Modified algebraic model of laminar-turbulent transition for internal flows". International Journal of Numerical Methods for Heat & Fluid Flow 30, n. 4 (21 gennaio 2019): 1743–53. http://dx.doi.org/10.1108/hff-10-2018-0597.
Testo completoAbstract (sommario):
Purpose For internal flows with small values of the Reynolds number, there is often at a considerable distance from the pipe inlet cross-section a change of the flow form from laminar to turbulent. To describe this phenomenon of laminar-turbulent transition in the pipe, also parallel-plate channel flow, a modified algebraic intermittency model was used. The original model for bypass transition developed by S. Kubacki and E. Dick was designed for simulating bypass transition in turbomachinery. Design/methodology/approach A modification of mentioned model was proposed. Modified model is suitable for simulating internal flows in pipes and parallel-plate channels. Implementation of the modified model was made using the OpenFOAM framework. Values of several constants of the original model were modified. Findings For selected Reynolds numbers and turbulence intensities (Tu), localization of laminar breakdown and fully turbulent flow was presented. Results obtained in this work were compared with corresponding experimental results available in the literature. It is particularly worth noting that asymptotic values of wall shear stress in flow channels and asymptotic values of axis velocity obtained during simulations are similar to related experimental and theoretical results. Originality/value The modified model allows precision numerical simulation in the area of transitional flow between laminar, intermittent and turbulent flows in pipes and parallel-plate channels. Proposed modified algebraic intermittency model presented in this work is described by a set of two additional partial differential equations corresponding with k-omega turbulence model presented by Wilcox (Wilcox, 2006).
46
Zuikov, Andrey, e Genrikh Orekhov. "Hydrodynamic structure of laminar flows with oppositely-swirled coaxial layers". MATEC Web of Conferences 265 (2019): 02022. http://dx.doi.org/10.1051/matecconf/201926502022.
Testo completoAbstract (sommario):
The article is devoted to the theoretical study of hydrodynamics of laminar flows with coaxial layers swirled in opposite directions and moving along the pipe. Such flows in a turbulent range have a wide practical application potential in technologies of dissipation of mechanical energy and mixing multiphase and heterogeneous media in microbiology, chemistry, ecology, heat engineering, power engineering, engine and rocket engineering. The article describes the tensor of viscous tangents (τii) and normal (σii) stresses. The questions of stability of flow according to the Rayleigh (Ra) and Richardson (Ri) criteria are considered. Calculation formulas and graphs of radial-axial distributions of viscous stress components, local stability zones are given, the point of “crisis and decay of the flow” or “vortex breakdown” is indicated. The solutions are obtained in the form of Fourier-Bessel series. The analysis of the hydrodynamic structure of the flow is made.
47
MATTNER, T. W., P. N. JOUBERT e M. S. CHONG. "Vortical flow. Part 1. Flow through a constant-diameter pipe". Journal of Fluid Mechanics 463 (25 luglio 2002): 259–91. http://dx.doi.org/10.1017/s0022112002008741.
Testo completoAbstract (sommario):
This paper describes an exploration of the behaviour and properties of swirling flow through a constant-diameter pipe. The experiments reveal a complicated transition process as the swirl intensity Ω is increased at fixed pipe Reynolds number Re ≈ 4900. For Ω [les ] 1.09, the vortex was steady, laminar, axisymmetric, and developed slowly with streamwise distance. The upstream velocity profiles were similar to those commonly appearing in the literature in similar apparatus. Spiral vortex breakdown appeared in the test section for 1.09 [les ] Ω [les ] 1.31 and was associated with a localized transition from jet-like to wake-like mean axial velocity profiles. Further increase in Ω caused the breakdown to move upstream of the test section. Downstream, the core of the post-breakdown flow was unsteady and recovered toward jet-like profiles with streamwise distance. At Ω = 2.68, a global transition occurred in which the mean axial velocity profiles suddenly developed an annular axial velocity deficit. At the same time, disturbances began to appear in the outer flow. Further increase in Ω eventually led to an annulus of reversed axial flow and a completely unsteady vortex.
48
Valencia, Alvaro. "Pulsating Flow in a Channel With a Backward-Facing Step". Applied Mechanics Reviews 50, n. 11S (1 novembre 1997): S232—S236. http://dx.doi.org/10.1115/1.3101841.
Testo completoAbstract (sommario):
The incompressible laminar flow in a channel with a backward-facing step is studied for steady cases and for pulsating inlet flow conditions. For steady flows, the influrnce of the inlet velocity profile, the height of the step, and the Reynolds number on the reattachment length is investigated. A parabolic entrance profile was used for pulsating flow. It was found with amplitude of oscillation of one by Re = 100 that the primary vortex breakdown through one pulsatile cycle and the wall shear stress in the separation zone varied markedly with pulsating inlet flow.
49
Kamiyo, Ola, e Abimbola Dada. "Laminar Natural Convection in Attics of Rooftops with Depressed Walls". FUOYE Journal of Engineering and Technology 9, n. 2 (2 agosto 2024): 258–64. http://dx.doi.org/10.4314/fuoyejet.v9i2.15.
Testo completoAbstract (sommario):
Laminar natural convection in the attics of rooftops with depressed upper walls and heated base wall has been investigated numerically using a finite volume CFD package. Selected roofs referred to as Combination, Clerestory and Butterfly roofs are compared with standard isosceles triangular roof having the same pitch and base length. The results obtained show that the depressed wall distorts the multicellular air movement pattern within the attics. It compressed the cells, thereby reducing their sizes and damping their rotation. The depression brings the upper walls closer to the base wall resulting in further breakdown of the convection cells, distortion of the cell shape, modification of the velocity and temperature distribution, higher air pressure and increased heat transfer rate within the area under the depressed walls. Overall, the mean heat transfer rate along the base wall increases with the depth and length of the depressed wall. It is therefore recommended that roof designers should apply caution with depressed walls in order to minimize heat exchange across the ceiling.
50
Mishra, Pratima, Rohit Kumar e Awadhesh Kumar Rai. "Development and optimization of experimental parameters for the detection of trace of heavy metal (Cr) in liquid samples using laser-induced breakdown spectroscopy technique". Journal of Laser Applications 35, n. 2 (maggio 2023): 022021. http://dx.doi.org/10.2351/7.0000959.
Testo completoAbstract (sommario):
Detection of traces of impurities/elements present in liquid samples using laser-induced breakdown spectroscopy (LIBS) is challenging because the signal intensity is weaker than in the case of solid samples. The present paper deals with the optimization of experimental parameters for different phases of a liquid sample and the improvement of the limit of detection (LOD) in these LIBS experimental setups. LIBS spectra of chromium in the liquid sample have been recorded in three different configurations [laminar flow, i.e., liquid flowing through a small jet, frozen liquid (ice), and liquid deposited on a filter paper]. Experimental conditions for different phases were optimized to get a better signal-to-noise (S/N) ratio and signal-to-background ratio (S/B) in the LIBS spectra. The best S/N and S/B ratio is observed when LIBS spectra is recorded for the liquid deposited on the filter paper configuration. The spectral intensity of Cr is enhanced several folds (36 times) in the LIBS spectra recorded in the filter paper configuration as compared to the laminar flow. The calibration curve method is used to measure the LOD for three different configurations. The better LOD (9.7 ppm for 357.8 nm Cr line) is observed in the case of deposited liquid on the filter paper than the other liquid phase (laminar, 85.5 ppm and ice, 63.7 ppm) configurations.