124 research outputs found
Direct versus noise-induced optimal transitions for a model shear flow
No full textThe transition from laminar to turbulent flow in parallel shear flows like pipe flow or plane Couette flow is not due to a linear instability of the laminar profile but requires finite amplitude perturbations. The perturbations have to be strong enough that they cross the boundary to the basin of attraction of the turbulent regime. We use a 2-dimensional model of the transition to turbulence to explore the underlying phase space structure for optimal perturbations with respect to three different criteria: the energy of the initial condition, the energy dissipation of the initial condition and the amplitude of noise in a noise-induced transition. We find that the optimal transition states are different, but that the scaling with Reynolds number is the same in all three cases. Implications for full simulations will be discussed
Intergenerationale Betreuung von älteren Menschen mit Demenz
No full textCan caring communities address the crisis in health and care? Annalena Eckhardt analyses the practices and models of caring communities. These aim to improve the quality of life of people with dementia through intergenerational support. In a comprehensive literature review, the author analyses the social, emotional and practical aspects of intergenerational care models, including intercultural aspects. She concludes that people with dementia can benefit greatly from social participation and autonomy within communities, but that obstacles such as idealised assumptions and barriers to access still need to be overcome
Stability and exact coherent structures of the asymptotic suction boundary layer with temperature gradient
No full textThe asymptotic suction boundary layer with a temperature gradient is a good point of entry to study the dynamics of thermal boundary layers by means of dynamical systems theory. The laminar flow without heating is parallel and its properties have been studied before. We add a temperature difference between the bottom plate and the free stream flow, and study the stability in dependence on Reynolds, Rayleigh and Prandtl number. In marked contrast to the usual Rayleigh-B\'enard problem, the onset of convection is subcritical. Tracking secondary bifurcations we identify time-periodic, spanwise, and doubly-localized exact coherent states for this flow
Mean field model for turbulence transition in plane Poiseuille flow
No full textIn the pipe flow model of Dwight Barkley the main idea is to model pipe flow as an excitable, bistable medium. Using a one-dimensional FitzHugh-Nagumo-type reaction-advection-diffusion system with two variables the model captures qualitatively a surprising number of features of the turbulence transition in pipe flow. Motivated by this success, we here describe a derivation of a set of two 1+1-dimensional coupled differential equations for the closely related system of plane Poiseuille flow from the Navier-Stokes equation. The model contains terms for the production of turbulent kinetic energy, its transfer between the modes and its dissipation by viscous terms. The model shows a bifurcation to a non-trivial state and reflects some of the complex dynamics observed in direct numerical simulations
Bypass transition in boundary layers as an activated process
No full textWe consider the spatio-temporal aspects of the transition to turbulence in a boundary layer above a flat plate exposed to free-stream turbulence. Combining results from the receptivity to free-stream turbulence with the observation of a double threshold from transition studies in e.g. pipe flow we arrive at a physically motivated prediction for the spatial distribution of nucleation events in boundary layers. We use a cellular automaton to implement a complete model for the spatial and temporal evolution of turbulent patches and show that the model reproduces the statistical features of the boundary layer remarkably well. The success of the modeling shows that bypass transition occurs as a spatiotemporally activated process, where transition is triggered by critical fluctuations imported from the free-stream turbulence
Fully localised edge states in boundary layers
No full textInvestigation of the laminar-turbulent boundary is performed in a boundary-layer flow. Constant homogeneous suction is applied at the wall in order to prevent the spatial growth of the layer, leading to the parallel Asymptotic Suction Boundary Layer (ASBL). Edge tracking is performed in a large computational domain allowing for full spatial localisation of the structures on the laminar-turbulent separatrix. The obtained dynamics of the state goes through calm and bursting phases. During the latter the structure grows in size, shedding vortices downstream of its core which viscously decay during the calm phases. Comparison with the computation in spatially growing boundary layer is made. The influence of the Reynolds number and the path leading from the edge state to turbulent flow are considered
Imágenes de autor e imágenes de la Patagonia: Trelew de Marcelo Eckhardt
No full textTrelew (1997) of the Argentine writer Marcelo Eckhardt is composed byessays, stories and a novel that they develop in an inserted way. Characterized by the generic mixture, and by a insistent description of the geographic and cultural space of the Patagonia, Trelew develops the mitografìa of the city homonym simultaneously that constructs that of the own author who expands and diversifies his speech in other voices, turning the text in a plural text by the forms and by the voices that form it.Trelew (1997) del escritor argentino Marcelo Eckhardt está compuesto porensayos, relatos y una novela que se desarrollan de modo intercalado. Caracterizado por la mixtura genérica, así como por una insistente descripción del espacio geográfico y cultural de la Patagonia, Trelew despliega la mitografía de la ciudad homónima a la vez que construye la del propio autor que expande y diversifica su enunciación en otras voces, convirtiendo el texto en un texto plural no sólo en cuanto a las formas sino, también, en cuanto a las voces que lo configuran
Turbulence Transition in Shear Flows: Chaos in High-Dimensional Spaces
Full text linkThe study of the transition to turbulence in parallel shear flows without linear instability of the laminar profile has profited immensely from the application of dynamical systems ideas. Studies of the transition in plane Couette flow and pipe flow, in particular, have shown that the transition is connected with the appearance of 3-d coherent structures that form a chaotic saddle which shows up in a transient turbulent dynamics. It is remarkable that these concepts, initially developed for low-dimensional systems, also work in such a high-dimensional setting. The present note contains a brief summary of key features and a short list of references for further reading.Process and EnergyMechanical, Maritime and Materials Engineerin
VIBRATIONAL ASSIGNMENTS FOR THE LOW FREQUENCY RAMAN ACTIVE PHONONS IN TCNQ SINGLE CRYSTALS
No full text1A, Girlando and C. Pecile, Spectrochim. Acta, 29A, 1859 (1973). Address of White, Brose, and Eckhardt: Department of Chemistry, University of Nebraska, Lincoln, Nebraska, 68588-0304.Author Institution:A satisfactory assignment for the low frequency, Raman active phonons of 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) in the solid state has eluded spectroscopists ever since the first polarized Raman spectra of the single crystal were Several attempts have been made to assign the observed transitions as either intramolecular vibrations or lattice phonons, but it has been thought that significant coupling prevents a well defined demarcation in frequency between the internal and external modes in the TCNQ crystal. In order to resolve this problem, we have measured the single crystal Raman spectra of TCNQ in the range for six different polarization configurations. The data exhibit some significant differences from results that have already been reported, particularly for the bands observed at 40, 63, and . A lattice dynamical calculation using a minimized potential for the TCNQ lattice has also been performed. A comparison of the theoretical and experimental results shows excellent agreement and offers a complete determination of the lattice phonon frequencies, symmetries, and eigenvectors of TCNQ
Poetisches gedenkbuch; festgeschenck zur feier des hundertjährigen jubiläums der vereinigung des herzogthums Bukowina mit dem österr. kaiserstaate und der Inauguration der Universität in Czernowitz;
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