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!
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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
I’m not in this field. What do you think is the most exciting area of this. Bonus points if you can ELI5.
To me personally, the field is fun because it gives me a perspective on all the processes which drives the weather and climate. I like watching the skies and the seas.
Scientifically the most exciting areas right would be:
- mixing efficiency: The ocean has a layered structure and it takes 1000s of years for the deep ocean to rise up. Many are trying to understand at what limits the turbulence in ocean currents drives the cold saline deep ocean to starts mixing with warmer fresher waters on the top more effectively. An important question in our changing climate.
- convection: a classic problem where the heated walls drive convective motion. People are still trying to understand different parameters and how it affects the kind of convection we get. Convection is everywhere: ocean currents transporting heat between the latitudes and, atmosphere which determine where the deserts and rainforests are etc.
How do you see climate change impacting brackish waters?
Brackish waters are basins where inland freshwater meet the seas. With rising sea levels I would expect them to move further upstream or cease to exist - depending on the basin.
Can you give further insight to the possibility of the Gulf Stream collapse?
Not just the Gulf stream, but the whole Atlantic Meridional Overturning Circulation (AMOC). AMOC, as you may know transports heat keeping the tropics cool, north warm and melting the Arctic. From what I understand the observations are only a few decades old, but it suggests the AMOC has been slowing down.
As an extreme case (and I repeat extreme) scenario it begs the question would it shut down or flips direction. Theoretically there are “thermohaline” circulation modes where that may happen. Paleoclimate evidence suggest that there were times that there was no AMOC and we had a PMOC (a Pacific one) instead. So many open questions…
Thank for doing this.
I have a Master’s in geotechnical engineering and I want to learn how to program fluid-solid interaction. How can I learn it? Know any good codebases I can learn from?
Also, I have been trying to work in scientific computing. You know of any companies that works in this space?
Take a look at FLUSI in Github. I am not familiar with that space. I bet wind turbine companies and aerospace sector might have some applications.
What exactly is geotechnical engineering? Is it like dams and bridges?
Mostly it deals with soils and it’s suitability for constructions. So yeah, I also covers dams and bridges too…
I used to work with a guy with a doctorate in Hydrogeology. Is that the same thing? “It’s like geology, just add water…” was his description of his field.
Umm…no.
It is like physics but of liquids and gases.
To be precise study of motion and related phenomena in fluids. It used to be in the realm of classical physics. Then most physicists went over to relativity and quantum physics. So we engineers took over the subject.
If I want to build a DIY large (multi thousand liters) stratified hot water tank to store heat for my house for up to a couple of days, what would be your tips for such a system?
And: what is your favorite aspect of fluid mechanics?
That’s quite an engineering question. I am not an expert, but usually solar water heaters do a great job in keeping them hot.
Favourite aspect? Instabilities and waves in fluids.
Oh damn, I completely missed the “geophysical” part 🤦♂️. Sorry!
Do you sit in a chair all day or a standing desk?
As a graduate student I sat all day - bad life choices. After I graduated the new job had an adjustable desk.
I work in science computing and HPC on the admin side, so my questions might be a bit different but,
- what languages do you typically work in?
- what do you use to encapsulate the software environment/package management for your batch jobs?
- what sort of compute resources and time do your jobs typically require?
- do you use any sort of workflow managers or just rawdog it with bash scripts?
- Fortran, Python (with extensions) mostly. Some C and C++ for libraries.
- Conda / Pip + virtual environments. Uploading packages to PyPI if needed. Our HPC people try to promote Singularity, but I would like to rapidly prototype and version control while doing research.
- I have mostly done 2D simulations. Usually 32 cores but upto 128 cores in 4 nodes. Time can range from 2 days upto several weeks. It gets a lot more demanding with 3D simulations.
- Mostly Python scripting. Sometime Makefiles, and awk and bash scripts come into play.
Very nice. Conda is a good one. I recommend checking out Snakemake since you already use Conda and Makefiles. It’s like Make but supercharged with Conda and Python. It makes sharing workflows easy and you don’t need to fool around with containers like with Singularity (which is great too, but has a steeper learning curve)
Cheers.
Edit: also be sure to check out Mamba. It’s a community fork of Conda and performs way better.
Not sure if this is in your realm or not, but I’m working on a diversion tunnel for a dam and we’re installing orifice pieces of varying internal diameter to slow the flow rate of the water. The orifices are being installed in order from smallest diameter to largest which seems counterintuitive to me. I would think you would go in descending diameter size to slowly restrict the flow in steps. Could you shed any light on why this would be engineered this way?
Every orifice adds some drag into the flow, decreasing the flow rate. However if you would go for a descending size of orifices you would introduce a Venturi effect, increasing the velocity for a given flow rate. Think what happens when you squeeze the end of a gardening water hose; or what happens when you blow air out of your mouth.
Ahhhhh that would make sense, thanks for the answer!
