Download or read book Linear and Nonlinear Instabilities of Blasius Boundary Layer Perturbed by Streamwise Vortices Part II Intermittent Instability Induced by Long wavelength Klebanoff Modes written by and published by . This book was released on 2001 with total page 44 pages. Available in PDF, EPUB and Kindle. Book excerpt: This report presents theoretical results on the stability properties of a Blasius boundary layer perturbed by Klebanoff distortions with a relatively long spanwise scale. Even relatively weak Klebanoff modes can alter the near-wall curvature of the underlying flow by O(1) and, hence, introduce linear instabilities with larger characteristic growth rates and frequencies than those of the Tollmien-Schlichting waves in an unperturbed Blasius flow. A localised distortion supports both sinuous and varicose modes of instability, with the growth rates of the sinuous modes being likely to be larger, in general. Overall, the instability is intermittent in time and localised in space, being confined to a finite part of the Klebanoff mode cycle and to a specific window(s) along the streamwise direction. A spanwise periodic distortion supports spatially quasi-periodic modes (via the parametric resonance mechanism), which may be viewed as modified T-S waves with excess growth rates when the Klebanoff modes are weak. In spite of the simplifications involved in this theory, its predictions appear qualitatively consistent with some of the unusual characteristics of the high-frequency wavepackets observed during previous experiments. The nonlinear development of a localised sinuous mode is followed across a sequence of asymptotic regimes using the non-equilibrium critical-layer theory.
Download or read book Linear and Nonlinear Instabilities of Blasius Boundary Layer Perturbed by Streamwise Vortices written by Xue-Song Wu and published by . This book was released on 2001 with total page 48 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Seventh IUTAM Symposium on Laminar Turbulent Transition written by Philipp Schlatter and published by Springer Science & Business Media. This book was released on 2010-03-11 with total page 628 pages. Available in PDF, EPUB and Kindle. Book excerpt: The origins of turbulent ?ow and the transition from laminar to turbulent ?ow are the most important unsolved problems of ?uid mechanics and aerodynamics. - sides being a fundamental question of ?uid mechanics, there are numerous app- cations relying on information regarding transition location and the details of the subsequent turbulent ?ow. For example, the control of transition to turbulence is - pecially important in (1) skin-friction reduction of energy ef?cient aircraft, (2) the performance of heat exchangers and diffusers, (3) propulsion requirements for - personic aircraft, and (4) separation control. While considerable progress has been made in the science of laminar to turbulent transition over the last 30 years, the c- tinuing increase in computer power as well as new theoretical developments are now revolutionizing the area. It is now starting to be possible to move from simple 1D eigenvalue problems in canonical ?ows to global modes in complex ?ows, all - companied by accurate large-scale direct numerical simulations (DNS). Here, novel experimental techniques such as modern particle image velocimetry (PIV) also have an important role. Theoretically the in?uence of non-normality on the stability and transition is gaining importance, in particular for complex ?ows. At the same time the enigma of transition in the oldest ?ow investigated, Reynolds pipe ?ow tran- tion experiment, is regaining attention. Ideas from dynamical systems together with DNS and experiments are here giving us new insights.
Download or read book ICASE Semiannual Report written by and published by . This book was released on 2002 with total page 64 pages. Available in PDF, EPUB and Kindle. Book excerpt: This report summarizes research conducted at ICASE in applied mathematics, computer science, fluid mechanics, and structures and material sciences during the period October 1, 2000 through March 31, 2001.
Download or read book NASA Langley Scientific and Technical Information Output 2001 written by and published by . This book was released on 2002 with total page 166 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Mathematical Reviews written by and published by . This book was released on 2004 with total page 906 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Program and Abstracts for the U S National Congress of Theoretical and Applied Mechanics written by and published by . This book was released on 2002 with total page 658 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Interaction of Oblique Instability Waves with Weak Streamwise Vortices written by Marvin E. Goldstein and published by . This book was released on 1994 with total page 56 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Numerical Study of the Instability of a Blasius Boundary Layer Distorted by Streamwise Vortices written by Yan Cui and published by . This book was released on 1999 with total page 131 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book On the Instability of Goertler Vortices to Nonlinear Travelling Waves written by Institute for Computer Applications in Science and Engineering and published by . This book was released on 1990 with total page 52 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Fully Nonlinear Development of the Most Unstable Goertler Vortex in a Three Dimensional Boundary Layer written by Institute for Computer Applications in Science and Engineering and published by . This book was released on 1992 with total page 32 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book On the Stability of Nonlinear Viscous Vortices in Three dimensional Boundary Layers written by Institute for Computer Applications in Science and Engineering and published by . This book was released on 1992 with total page 56 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Streamwise Vortices in Heated Boundary Layers written by National Aeronautics and Space Administration (NASA) and published by Createspace Independent Publishing Platform. This book was released on 2018-07-11 with total page 44 pages. Available in PDF, EPUB and Kindle. Book excerpt: The nonlinear instability of the boundary layer on a heated flat plate placed in an oncoming flow is investigated. Such flows are unstable to stationary vortex instabilities and inviscid traveling wave disturbances governed by the Taylor-Goldstein equation. For small temperature differences the Taylor-Goldstein equation reduces to Rayleigh's equation. When the temperature difference between the wall and free stream is small the preferred mode of instability is a streamwise vortex. It is shown in this case that the vortex, assumed to be of small wavelength, restructures the underlying mean flow to produce a profile which can be massively unstable to inviscid traveling waves. The mean state is shown to be destabilized or stabilized to inviscid waves depending on whether the Prandtl number is less or greater than unity. Hall, Philip Unspecified Center NAS1-18605; RTOP 505-90-52-01...
Download or read book Taylor Goertler Instabilities of Tollmien Schlichting Waves and Other Flows Governed by the Interactive Boundary Layer Equations written by Institute for Computer Applications in Science and Engineering and published by . This book was released on 1985 with total page 38 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Mechanisms for Transition to Turbulence in the Bottom Boundary Layer Under a Surface Solitary Wave written by Mahmoud Mostafa kamel Saad Sadek and published by . This book was released on 2015 with total page 220 pages. Available in PDF, EPUB and Kindle. Book excerpt: Transition mechanisms to turbulence in the unsteady bottom boundary layer (BBL) driven by a soliton-like pressure gradient in an oscillating water tunnel (an approximation for the BBL under surface solitary waves) are investigated in a hydrodynamic instability context. This investigation aims to establish connections across theoretical and numerical analysis and the experimental findings of Sumer et al., 2010 (J. Fluid Mech. vol. 646, 2010,p. 207). Transition to turbulence in the surface solitary wave-driven BBL can take place according to two different scenarios. The primary scenario is associated with the classical transition resulting from the breakdown of the exponentially growing 2-D Tollmien-Schlichting waves. The alternative scenario consists of a characteristically different path to transition resulting from the formation of localized turbulent spots. The formation of these turbulent spots, which are the manifestation of an algebraic growth of infinite streamwise disturbance, i.e., zero streamwise wave number, leads to a bypass transition to turbulence. In regards to the first scenario, a detailed map for the temporal instability is established. Both linear stability analysis and fully nonlinear two- and threedimensional simulations using high-order numerical methods have been carried out. The process of delineation of the stability regions as a function of boundary layer thickness-based Reynolds number of the temporally evolving base flow, Re[delta] consists of two parts. The first part aims to assess the lower limit of the Re[delta] range within which the standard, quasi- steady, linear stability analysis is applicable as it considers individual profiles sampled during the base flow transient evolution. Below this limit, transient linear stability analysis serves as a more accurate predictor of the stability properties of the base flow. In the second step, above the Re[delta] limit where the BBL is determined to be linearly unstable, the base flow is further classified as unconditionally stable, conditionally unstable or unconditionally unstable in terms of its sensitivity to the amplitude and the insertion time of perturbations. Two distinct modes of instability exist in this case: post- and pre- flow-reversal modes. At a moderate value of Re[delta], both modes are first observed in the wave deceleration phase. The post flow reversal mode dominates for relatively low Re and it is the one observed in Sumer et al. (J. Fluid Mech. vol. 646, 2010,p. 207). For Re above a threshold value of the base flow in the unconditionally unstable regime, the pre-flow reversal mode, which has a larger wavelength than its post-reversal counterpart, becomes dominant. In the same regime, the threshold Re[delta] value above which instability is observed in the acceleration phase of the wave is also identified. In this case, the base flow velocity profiles lack any inflection point, suggesting that the origin of such an instability is viscous. Finally, the lower Re[delta] limit above which quasi-steady linear stability analysis is valid may be independently obtained by adapting to the surface solitary wave BBL framework an instability criterion which links the average growth rate and wave event timescale, as previously proposed in studies of the instability of the interior of progressive and solitary internal waves. This part of the study is a recap to what have been published by Sadek et al. (2015). For the second transition scenario, the linear stability analysis is reformulated in the non- modal framework where the three-dimensional nature of the base flow is considered. By doing such a reformulation, an optimum initial disturbance is identified. Such a disturbance can lead to very large short-term energy growth that can not be captured by the classical modal stability analysis. The optimum initial disturbance is first calculated for different velocity profiles following a quasi-steady approach where the individual profiles are assumed steady within the wave event. Additionally, the resulting short-term maximum energy growth is determined as a function of the streamwise ([alpha]) and spanwise ([beta]) wave number. It is shown that the maximum growth is associated with the disturbance with no streamwise dependence ([alpha] = 0). The formation of streaks is simulated when the three-dimensional DNS is perturbed with the pre-determined optimum initial disturbance. In order to trigger the transition to turbulence, a secondary instability is required. The characteristics of this secondary perturbation is chosen in accordance with the flat plate boundary layer flow. This approach can successfully simulate the bypass transition. Sample result from a representative Re[delta] case are presented in the thesis hereafter.
Download or read book Instabilities Chaos and Turbulence written by Paul Manneville and published by World Scientific. This book was released on 2010 with total page 456 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book (2nd edition) is a self-contained introduction to a wide body of knowledge on nonlinear dynamics and chaos. Manneville emphasises the understanding of basic concepts and the nontrivial character of nonlinear response, contrasting it with the intuitively simple linear response. He explains the theoretical framework using pedagogical examples from fluid dynamics, though prior knowledge of this field is not required. Heuristic arguments and worked examples replace most esoteric technicalities. Only basic understanding of mathematics and physics is required, at the level of what is currently known after one or two years of undergraduate training: elementary calculus, basic notions of linear algebra and ordinary differential calculus, and a few fundamental physical equations (specific complements are provided when necessary). Methods presented are of fully general use, which opens up ample windows on topics of contemporary interest. These include complex dynamical processes such as patterning, chaos control, mixing, and even the Earth's climate. Numerical simulations are proposed as a means to obtain deeper understanding of the intricacies induced by nonlinearities in our everyday environment, with hints on adapted modelling strategies and their implementation.
Download or read book New Approaches and Concepts in Turbulence written by T. Dracos and published by Springer Science & Business Media. This book was released on 1993-09-01 with total page 450 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book contains the proceedings of a colloquium held in Monte Verità from September 9-13, 1991. Special care has been taken to devote adequate space to the scientific discussions, which claimed about half of the time available. Scientists from all over the world presented their views on the importance of kinematic properties, topology and fractal geometry, and on the dynamic behaviour of turbulent flows. They debated the importance of coherent structures and the possibility to incorporate these in the statistical theory of turbulence, as well as their significance for the reduction of the degrees of freedom and the prospective of dynamical systems and chaos approaches to the problem of turbulence. Also under discussion was the relevance of these new approaches to the study of the instability and the origin of turbulence, and the importance of numerical and physical experiments in improving the understanding of turbulence.
