cross-posted from: https://discuss.tchncs.de/post/479621
Hi all! I defended my Ph.D. thesis back in 2019 and I also served as the creator and moderator for the subreddit r/FluidMechanics for a long time. I think with that I have gathered enough experience and courage to answer some of your queries. Some broad topics that I can answer questions on are:
- computation fluid mechanics
- scientific programming and HPC
- nonlinear shallow water equations
- statistical description of turbulence: spectra, energy budget etc.
- experimental methods: PIV
- stratified turbulence
- academia
- navigating your career pre- and post-Ph.D.
Ask away!
What is the difference between a solid and a fluid? Is it a discreet thing, or a spectrum?
I’m thinking of the pitch drop experience, and non-newtonian fluids.
It is a discreet thing if you consider pure materials. Phase transitions are sharp for all materials.
Non-newtonian fluids are usually a mixture of fine solid particles and liquid creating funky physics. Stress and shear rate are not linear as Newton’s law dictates. They can be shear thickening like corn starch in water or shear thining like blood. Shear thinning fluids are considered pseudo plastics, which is also a property in some solids.
That’s super interesting. Where does vibrating sand to make it look like a liquid fall in the spectrum? Is sand considered a mixture of materials because sand is so large and mixing with air?
https://www.youtube.com/watch?v=My4RA5I0FKs
What does shear rate mean for a liquid? How attractive each particle is to each other? What would you consider a good resource for people to learn the basics or at least the basic vocabulary of fluid dynamics?
I have also wondered the same and I don’t have an answer to why sand behaves as such.
In solids, for one talks of displacement a.k.a shear. Since fluids are often in a state of motion shear rate means the gradient of velocity (for example for a flow over a solid flat plate how horizontal velocity changes as you move away from a flat plate, du/dy).
Liquids have both fairly strong cohesive (liquid-liquid) and adhesive (liquid-solid) forces. This is why surface tension exists allowing drops to form. But these forces are weak compared to the shear forces a typical Newonian fluid experiences.
Kundu and Cohen is my go to book. Feynman Lectures also has some Fluid Mechanics sections.
What you should aim to understand is how the Navier-Stokes equation work and what different terms represent.
Does this mean a consistent fluid can’t have a shear rate against itself? And one of your other comments you mentioned the different layers of the ocean mixing do the different layers have shear rates with respect to each other?
Mentally I can see how different salinity levels would act as different surfaces in terms of velocity interaction. I suppose flow turbulence would be a fluid internal shear force. It’s really interesting way of looking at things thank you for defining it