Thomas
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 16h revised Dependance of diffusion coefficient on size? added 1167 characters in body 1d answered Dependance of diffusion coefficient on size? 1d answered Navier Stokes : what about angular momentum? Feb 4 answered 1-D Fick's first law - partial derivative? Feb 3 comment 1-D Fick's first law - partial derivative? Fick's law also applies to non-steady states. The only requirement is that the $t$ and $x$ dependence is slowly varying compared to collisional time and length scales. Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics I should explain why I am so insistent on this point. I have written a number of papers (such as this one arxiv.org/abs/1404.6843) in which I compute transport coefficients using a variety of microscopic theories. The results violate "frame invariance" (although I had not heard of this concept when I wrote them). That's not a surprise, because "frame invariance" is not a property of Newton's equation, or the Boltzmann equation, or quantum field theory. Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics To me, wikipedia pages like this one en.wikipedia.org/wiki/Objectivity_%28frame_invariance%29 illustrate the problem. The page states what "frame invariance of material response" is, but not why we expect it to be true. Muller (for all his faults) actually gives an explicit argument why we don't expect it to be true. Jan 27 revised Viscous forces with asymmetric gradient velocity in fluid mechanics added 385 characters in body Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics If you want to ponder this further; Take a look at the book by Muller that I found using google (mentioned above). In Insert 8.1 he provides am example for the frame dependence of heat flux which (I think) is completely correct. Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics You keep saying this, but that's not a physical argument. If the microscopic laws of physics look different to an inertial and a rotating observer, why would the laws of fluid dynamics look the same (which is what this principle asserts)? Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics Yes, but Newton's equations are not invariant under rotations of the observer. A rotating observer sees a Coriolis force, a non-rotating observer does not. Jan 27 comment Does nature really follow the heat equation? The heat equation is not a primitive ad-hoc equation. It is an exact consequence of energy conservation in the limit of small temperature gradients. Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics To me, the point is this: Why would we expect the equation of continuum (fluid) mechanics to form-invariant (covariant) under rotations, given that the laws of mechanics are not? Jan 27 comment Viscous forces with asymmetric gradient velocity in fluid mechanics That's interesting -- I was unaware of this. Some googling shows that there was (and maybe still is) an argument between Truedell, Noll, and others and various members of the physics community about whether such a principle exist. For some particularly nasty comments about Truesdell see Ingo Muller "A History of Thermodynamics." Jan 26 comment Viscous forces with asymmetric gradient velocity in fluid mechanics Is that a known (valid?) principle of physics? If it is, it should follow from a symmetry of the laws of physics. Jan 26 comment Derivation of the diffusion coefficient? Not sure what you are asking. The book you refer to is called "Mathematical Theory" and it does, indeed, provide detailed, rigorous, derivations of transport coefficients, The page you refer is from an introductory chapter -- just keep reading. (Although the Chapman and Cowling is, admittedly, somewhat heavy going. I provide the a brief summary of the calculation here physics.stackexchange.com/questions/230380/… ) Jan 26 comment Viscous forces with asymmetric gradient velocity in fluid mechanics This sounds sort of right, but I am not quite sure if the argument is entirely correct. A rotating observer is non-inertial (and may see funny forces). I think the correct version of this argument is that there should not be a stress in a rigidly rotating fluid, because this in an equilibrium state. Jan 26 answered Viscous forces with asymmetric gradient velocity in fluid mechanics Jan 25 revised Is the speed of sound almost as high as the speed of light in neutron stars? added 129 characters in body Jan 25 comment Yang-Mills theories, confinement and chiral symmetry breaking Any text book on particle physics will have a short discussion on quarkonia (charmonium, most notably the J/psi, bottomonium, in particular the upsilon). A more technical discussion is in chapter 6 of Yndurain, or reviews like this arxiv.org/abs/1111.0165 .