What is isotropic turbulence and why is it important?
This article begins with an overview, then gives the precise definition of isotropic turbulence, and follows that with the basic conservation equations, in both real space and wavenumber space. These provide the foundations of all theoretical approaches, both fundamental and phenomenological. After that, my intention is to try to highlight the main unresolved issues and give some indication of what progress there has been over decades (in all cases), and what still needs to be done. I should emphasise that I am not trying to provide either a conventional review or even a pedagogical treatment. Instead I am giving concise summaries, supplemented (where I can) by my own observations, which make substantial points that I believe are original, and which have not been made in the literature. To take just one example, it is known by some people that Kolmogorov's 1962 theory is not correctly described as a 'refinement' of his 1941 theory. This was pointed out by Kraichnan in 1974. However, what does not appear to have been recognized is that the 1962 theory is physically invalid, and also that a plausible implementation of it destroys the Kolmogorov (1941) scaling of energy spectra which has been widely observed over many years. This is discussed in Section 4 below. Lastly, I have tried to give an informal treatment in order to make everything easily accessible, to reach the widest possible audience. In particular, the section on renormalization methods is written without giving the equations of the various theories, merely stating in words what has been done, what are the different methods and also what still needs to be done.