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Mochel, J.M. Gillis, Keith Alan 2011-06-02T16:42:28Z 2011-06-02T16:42:28Z 10000-01-01 1987 A model for the onset of nonlinear dissipation in AC and DC flows is presented as an extension of the linear theory of Ambegaokar, Halperin, Nelson, and Siggia to include to flow velocity. The renormalization group calculation is carried out on a computer to determine the dissipation as a function of superfluid velocity. We find the onset of nonlinear dissipation is driven by two mechanisms whose relative contributions are temperature dependent. The free vortex contribution is most important at the higher temperatures whereas the bound pair contribution is dominant at low temperatures. The model is fitted to experimental measurements with two weakly coupled adjustable paramters: the vortex diffusivity and a free vortex creation time. Our model does not predict a critical velocity in the usual sense except at T=O, but we define a characteristic velocity in order to compare the theory with experiment. At T=O we find a critical velocity given by the Feynman criterion, where the frequency dependent vortex diffusion length coincides with the zero in the vortex energy. In addition, a novel experimental apparatus using a 3p. I.D. capillary to allow us to change the helium film thickness at low temperatures is described. Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-06-02T16:42:28Z No. of bitstreams: 1 1987_gillis.pdf: 7717209 bytes, checksum: 4e05e6ec7d07db8626b643b18d8357c3 (MD5) Made available in DSpace on 2011-06-02T16:42:28Z (GMT). No. of bitstreams: 1 1987_gillis.pdf: 7717209 bytes, checksum: 4e05e6ec7d07db8626b643b18d8357c3 (MD5) Previous issue date: 1987 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-06-02T16:42:28Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:14:03-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Thesis U of I Only 1269150 http://hdl.handle.net/2142/25223 en 1987 Keith Alan Gillis nonlinear flow dissipation 2D superfluid helium films flow velocity renormalization group calculation Model for the onset of nonlinear flow dissipation in 2D superfluid helium films Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.