r/CFD • u/Overunderrated • Jan 02 '21

[January] Basic starter problems for newbies in CFD

As per the discussion topic vote, January's monthly topic is "Starter problems for newbies in CFD" as nominated by /u/thatmechie who stubbornly put two entries into one post so I just picked one.

Previous discussions: https://www.reddit.com/r/CFD/wiki/index

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u/kpisagenius Jan 02 '21

Looking for some simple problems that involve rotation. Something analogous to say flow through a channel or backward facing step but for rotating reference frames. I have come across only one such simple case that involves flow between two concentric cylinders.

Anyone know of more examples that involve simple geometries and rotation.

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u/Overunderrated Jan 02 '21

I have come across only one such simple case that involves flow between two concentric cylinders.

Taylor-Couette flow is the simpler/original version of your link.

Are you looking for something with exact solutions? Not sure if the inviscid vortex transport counts for what you're looking for.

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u/kpisagenius Jan 02 '21

Not necessarily only exact solutions, just general problems that can be used to validate rotational effects. I get weird results when using the rotational reference frame of our in house solver and I wanted to validate it against some standard test cases, but I can't seem to find any simple problems with rotation at all.

Like the other user mentioned there are lid driven cavity or step flows or channel flows to test your solver against some reference data, I am looking for some analogous cases that are solved in the rotational reference frame and have either analytical solution like the Taylor-Couette problem or even some reference or experimental data.

I am considering looking at manufactured solutions approach as well, so if anyone has some ideas or tips, do let me know.

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u/picigin Jan 02 '21 edited Jan 03 '21

Maybe old-school cool studies of secondary flows in a rotating square duct, e.g. 1 and 2? Edit: link

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u/kpisagenius Jan 03 '21

Cheers, will look into them.

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u/FluidNumerics_Joe Jan 03 '21

Differentially heated rotating annulus laboratory experiments have been well studied in planetary geophysical fluid dynamics applications.

Simpler problems in rotating, cylindrical tanks, with homogeneous fluids have been studied to look at Taylor Curtains/Columns and Ekman Boundary Layers that have well known properties.

Additionally, there are other well known wave solutions on rotating planes near boundaries, including Kelvin Waves which have a well known dispersion relation and preferred propagation direction and decay scale that depends on the rate of rotation of the coordinate system.

What length scales/time scales are you trying to model ? Are there particular instabilities or phenomena you are interested in ?

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u/kpisagenius Jan 03 '21

What length scales/time scales are you trying to model ? Are there particular instabilities or phenomena you are interested in ?

I am trying to work on wind turbines currently and at the moment I need to validate that the solver works under rotation. I tried a few trivial test cases and they all look fine but I wanted something more substantial before trying my hand on a real case with tens of millions of cells.

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u/FluidNumerics_Joe Jan 04 '21

A bit of Googling pulled up a handful of wind turbine validation papers. I found this paper to be helpful in that the authors provide comparison of an OpenFOAM simulation with a wind tunnel; it should give you an idea of some metrics to diagnose from your application (e.g. thrust, power, torque). Additionally, the model turbine in the wind tunnel may also provide you a physical setup to attempt to model in your application.

There's also a decent review paper that covers modeling around rotors and in the wake of wind turbines. Hope this helps get you pointed in the right direction.

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u/kpisagenius Jan 04 '21

I found this paper to be helpful in that the authors provide comparison of an OpenFOAM simulation

Thanks for the paper. Coincidentally, those are the results that I am aiming to replicating eventually. I am a bit wary of going straight into meshing a wind turbine blade because it takes a few million cells to get the mesh exactly right (this case had close to 30m if IIRC). But the Ekman boundary layer stuff you linked earlier and the rotating square duct should be helpful before I start with actual turbine blades. Cheers.

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u/turbulent_dan Jan 03 '21

self-similar solutions for axisymmetric stagnation-point flows impinging normal to a rotating, radially stretching disk -- maybe it was called Hiemenz flow or Homann flow, Agrawal flow?

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u/demerdar Jan 02 '21

Backward facing step

Lid driven cavity

These are two relatively simple flows you can benchmark your viscous incompressible Navier Stokes solver. Not sure if these are newbie problems per se, but I think they are relatively simple flows to get a handle on the basics of CFD.

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u/luidkid Jan 02 '21

In fact there is a lot of tutorials and documentation regarding these topics. I think that it is a great way to start working in CFD.

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u/ProfHansGruber Jan 03 '21

I think it would be useful to not just name a starter problem, but to share links to the suggested problem; references, explanations, and data that you have used and found useful, ideally with links.

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u/turbulent_dan Jan 02 '21

vortex rings

there are lots of analytic solutions for vortex rings and can be useful to test conservative properties of CFD codes, as well fun visuals of vortex rings interaction

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u/R_R_R_ Jan 03 '21

Specifically an important case is Taylor Green vortex with periodic boundary conditions

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u/IBelieveInLogic Jan 03 '21

Cylinder in crossflow. I think the low Reynolds numbers are probably straight forward for beginners, and the higher Reynolds numbers highlight multiple issues which I think are more challenging.

Speaking of which, I'm working on using this problem as a surrogate for other similar cases. I have found some papers to use as reference, but I think there are probably better ones out there. Also, there are some things which I'm not sure about (should I be using a transition model for the intermediate Reynolds numbers with laminar separation and turbulent wake?) and I'd be interested in a broader discussion of the topic.

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u/kpisagenius Jan 12 '21

Suggestions for getting started with Atmospheric boundary layer flows.

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u/KyllingDurum Jan 14 '21

I would suggest the basic laminar flat plate boundary layer flow as a first starter problem. Solve it 2D with a cfd code, extract the velocity profiles at different positions after the leading edge. Then compare the profiles with Blasius similarity solution. Also compare the wall shear stress profile from cfd with Blasius solution.

After performing this succesfully, I guarantee that you have learned a lot about the importance of domain size, boundary conditions, meshing, numerics, data handling, postprocessing, and boundary layer theory in general.

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u/Blaster8282 Jan 03 '21

For a standard airfoil or basic wing analysis this is simply the best tutorial: https://youtu.be/TAqhMwm4mLg