Results 1 - 10 of 31407
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[en] A general theory for the fluctuation spectrum of the onset of turbulence is developed, applying to systems that approach turbulence through a cascade of subharmonic bifurcations. Applied to Rayleigh-Benard flow, we find excellent quantitative agreement with the recent experimental data of Libchaber and Maurer. (orig.)
[en] We use a thin flux tube model in a rotating spherical shell of turbulent convective flows to study how active region scale flux tubes rise buoyantly from the bottom of the convection zone to near the solar surface. We investigate toroidal flux tubes at the base of the convection zone with field strengths ranging from 15 kG to 100 kG at initial latitudes ranging from 10 to 400 with a total flux of 1022 Mx. We find that the dynamic evolution of the flux tube changes from convection dominated to magnetic buoyancy dominated as the initial field strength increases from 15 kG to 100 kG. At 100 kG, the development of Ω-shaped rising loops is mainly controlled by the growth of the magnetic buoyancy instability. However, at low field strengths of 15 kG, the development of rising Ω-shaped loops is largely controlled by convective flows, and properties of the emerging loops are significantly changed compared to previous results in the absence of convection. With convection, rise times are drastically reduced (from years to a few months), loops are able to emerge at low latitudes, and tilt angles of emerging loops are consistent with Joy's law for initial field strengths of ∼>40 kG. We also examine other asymmetries that develop between the leading and following legs of the emerging loops. Taking all the results together, we find that mid-range field strengths of ∼40-50 kG produce emerging loops that best match the observed properties of solar active regions.
[en] The important question of the relation between theory and experiment in different physical problems is discussed. A number of examples, both widely and little known, are used to show that some physical theories, considered by many as correct because they corresponded to many experimental facts and to the existing level of science, have been found to be false. One of the important reasons for this is that it is very difficult to distinguish between the causes and consequences of phenomena observed in experiments. The best example is the study of turbulent flows, where the causes and consequences are often erroneously interchanged in relation to the properties and development mechanisms of turbulence. At the same time, some counterexamples are described where phenomena that are impossible from the standpoint of universally accepted theoretical concepts turn out to be reality in special cases. (methodological notes)
[en] A new enhanced radiation process from plasma turbulence (plasma-maser) is interpreted as 'dissipative structure' in plasma turbulence. The plasma-maser process is effective in an open plasma system where some of the input energy is dissipated as anomalous radiation. The validity of the linear response theory which neglects the ensemble averaged second order electric field is given under the random phase approximation. The ordinary mode growth rate in the presence of the enhanced stationary Langmuir turbulence is obtained and the results have potential importance to clarify the mechanism of the anomalous radiation in plasma astrophysics. (author)
[en] We describe the development of an accurate yet computationally tractable statistical dynamical closure theory for general inhomogeneous turbulent flows, coined the quasi-diagonal direct interaction approximation closure (QDIA), and its application to problems in data assimilation. The QDIA provides prognostic equations for evolving mean fields, covariances and higher-order non-Gaussian terms, all of which are also required in the formulation of data assimilation schemes for nonlinear geophysical flows. The QDIA is a generalization of the class of direct interaction approximation theories, initially developed by Kraichnan (1959 J. Fluid Mech. 5 497) for isotropic turbulence, to fully inhomogeneous flows and has been further generalized to allow for both inhomogeneous and non-Gaussian initial conditions and long integrations. A regularization procedure or empirical vertex renormalization that ensures correct inertial range spectra is also described. The aim of this paper is to provide a coherent mathematical description of the QDIA turbulence closure and closure-based data assimilation scheme we have labeled the statistical dynamical Kalman filter. The mathematical formalism presented has been synthesized from recent works of the authors with some additional material and is presented in sufficient detail that the paper is of a pedagogical nature.
[en] Fractality, as we introduce it, is an attribute relating to any object or system where the existence of self-similar replication of the whole is present in any order and scale. This phenomenon can be realized in any turbulent flow due to the self-similar flow structures in its energy cascade. This intrinsic natural fractality is dominant in any turbulent flow. This paper reports the effect of a forced fractality externally superimposed on a turbulent flow in a circular wind tunnel on this natural fractality. The forced fractality was created by a set of fractal orifice plates. The time correlation and energy spectra showed that the forced fractality significantly excites the natural fractality and increases flow mixing. Simultaneously, we found that the fractal orifice plate is much more efficient than the classical orifice plate with equal flow area in terms of the flow mixing