On the elimination of the sweeping interactions from theories of hydrodynamic turbulence

In this paper, we revisit the claim that the Eulerian and quasi-Lagrangian same time correlation tensors are equal. This statement allows us to transform the results of an MSR quasi-Lagrangian statistical theory of hydrodynamic turbulence back to the Eulerian representation. We define a hierarchy of homogeneity symmetries between incremental homogeneity and global homogeneity. It is shown that both the elimination of the sweeping interactions and the derivation of the 4/5-law require a homogeneity assumption stronger than incremental homogeneity but weaker than global homogeneity. The quasi-Lagrangian transformation, on the other hand, requires an even stronger homogeneity assumption which is many-time rather than one-time but still weaker than many-time global homogeneity. We argue that it is possible to relax this stronger assumption and still preserve the conclusions derived from theoretical work based on the quasi-Lagrangian transformation.Comment: v1: submitted to Physica D. v2: major revisions; resubmitted to Physica D. v3: minor revisions requested by referee

Turbulence for (and by) amateurs

Series of lectures on statistical turbulence written for amateurs but not experts. Elementary aspects and problems of turbulence in two and three dimensional Navier-Stokes equation are introduced. A few properties of scalar turbulence and transport phenomena in turbulent flows are described. Kraichnan's model of passive advection is discussed a bit more precisely. {Part 1: Approaching turbulent flows.} Navier-Stokes equation. Cascades and Kolmogorov theory. Modeling statistical turbulence. Correlation functions and scaling. {Part 2: Deeper in turbulent flows.} Turbulence in two dimensions. Dissipation and dissipative anomalies. Fokker-Planck equations. Multifractal models. {Part 3: Scalar turbulence.} Transport and Lagrangian trajectories. Kraichnan's passive scalar model. Anomalous scalings and universality. {Part 4: Lagrangian trajectories.} Richardson's law. Lagrangian flows in Kraichnan's model. Slow modes. Breakdown of Lagrangian flows. Batchelor limit. Generalized Lagrangian flows and trajectory bundles.Comment: 37 pages, 6 figures, lecture note

Exact Solutions of a Remarkable Fin Equation

A model "remarkable" fin equation is singled out from a class of nonlinear (1+1)-dimensional fin equations. For this equation a number of exact solutions are constructed by means of using both classical Lie algorithm and different modern techniques (functional separation of variables, generalized conditional symmetries, hidden symmetries etc).Comment: 6 page