Aspects of helicity in turbulent flows.
Item
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Title
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Aspects of helicity in turbulent flows.
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Identifier
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AAI9029969
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identifier
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9029969
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Creator
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Polifke, Wolfgang Herbert.
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Contributor
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Adviser: Andreas Acrivos
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Date
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1990
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, General | Physics, Fluid and Plasma | Statistics
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Abstract
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Helicity, the scalar product of velocity and vorticity, is a quantity of great potential significance for our understanding of turbulent flows, as it possesses a number of intriguing properties related to the nonlinear dynamics and the structure of turbulent flows. In this study we investigate several aspects of helicity via direct numerical simulations of decaying and quasi-stationary turbulent flows: The influence of helicity on the energy transfer and the nonlinearity, the statistics and dynamical significance of helicity fluctuations, and the relation of helicity to the local 'entangledness' of vortex lines.;A new initialization and forcing procedure has been developed, which allows to control the mean helicity in numerical simulations of turbulence. It is observed in agreement with earlier work by Andre & Lesieur (J. Fluid Mech. 81, 1977), that strong mean helicity impedes the transfer of energy towards smaller scales, depresses the nonlinearity of a turbulent flow, and reduces dissipation in decaying turbulence.;A random-phase or quasi-Gaussian approximations (QGA) is employed to gauge the fluctuations of helicity observed in simulations of turbulent flows. It is shown that the QGA is compatible with Kolmogorov-type scaling arguments supplemented by elementary statistical considerations. Sweeping effects on helicity fluctuations are discussed. In decaying and stationary flows with small mean helicity it is found that the fluctuations of the helicity of the large scales are well described by the QGA, and as such not strong enough to directly influence the energy transfer. No evidence of an inverse cascade of mean-square helicity (Levich & Tsinober, Phys. Lett. A 93, 1983) was found. The helicity of the small scales is observed to fluctuate anomalously, i.e. stronger than Gaussian, which indicates the presence of small scale phase coherence. However, the nature of the observed fluctuations suggests that the adiabatic invariance of helicity fluctuations at large Reynolds numbers is not the cause of the phase coherence (Levich, Phys. Rep. 151, 1987, and Levich & Shtilman, Phys. Lett. A 126, 1988). The mechanism of the depression of nonlinearity in isotropic turbulence (Kraichnan & Panda, Phys. Fluids 31, 778, 1988) is investigated. It is found that the main source of the depression in non-helical flows is a tendency of the Lamb vector to develop a significant potential component. The possibility of probing the local structure or 'entangledness' of the vorticity field of turbulent flows with an appropriately defined 'relative helicity' (Levich, Phys. Rep. 151, 1987) is discussed. Preliminary numerical results show a correlation of entangledness with turbulent activity.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
