Examples of nearly perfect fluids and gases When learning for physics (hydrodynamics and gases) I wanted to know what would be examples of nearly perfect fluids (no surface effects and no friction) and perfect gases (only elastic collisions and a simplified interaction  potential) that are used in experiments etc.
When searching for that in the Internet I only found things like ultra-hot quark-gluon plasma and optically trapped 6Li atoms for perfect fluids. I rather thought of more practicable examples like fluid helium, is it near to a perfect fluid?
For gases I think that would be hydrogen and light halogens (because the volume of the molecules is neglected), but at which temperatures?
 A: Effectively ideal gases are pretty easy to come by. Air at SATP would be a good example.
To be a little more precise, what you're looking for is that the mean free path $\lambda \gg \sigma$, the diameter of the molecules.  Basically this just means that molecules spend most of their time far away from  each other.
You can find $\lambda$ by:
$$\frac{m}{\sqrt{2} \pi \rho \sigma^2}$$
with the mass of a molecule ($m$) and density of the gas ($\rho$).  Look for stuff on kinetic theory of gases for more detai.  The above treats molecules hard spheres colliding with each other rather than dealing with attraction and repulsion that goes on.
The just have to watch out at elevated pressure and/or low temperature. 
I should add that perfect gases are not perfect fluids.  They definitely have viscosity and conductivity (in gases both just correspond to diffusion of momentum and heat).  But that diffusion is fairly slow.  In many cases viscous forces a insignificant relative to the momentum of the flow and the fluid will appear to be inviscid.
But if  Reynolds number ($L U / \nu$), you'll see the development of turbulence.  Turbulence is complex (understatement) but it will often produce effects (drag, average flow patterns etc.) very similar to those that would be present in a much more viscous fluid. In a way you've lost the 'perfect fluid' behavior again.
Both the inviscid assumption and turbulence are very much problem dependent rather than being purely determined by the properties of the fluid itself.
