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Comment by elil17 | Hacker News Reader

Why use this tool? Why not simply build and test the propeller? Your results would be much more reliable - CFD is famously finicky, especially for a situation like a propeller where you have rotating flows.

The point of golf ball dimples is to act as vortex generators and improve flow attachment, so having both barnacles and dimples is redundant.

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+1 to this - Though it has been many years, at one point I helped write CFD code for the US Navy. The disconnect between real-world measurements and simulations was vast for any structure remotely complicated (ie anything but an axially symmetric simple shape like a torpedo). While the CFD code has gotten way better (largely via Moore's law), I expect propellers are still quite hard to simulate, and your best path is to build a model and see how it performs.

But the CFD isn't to replace building the model...it's to give you a head start and reasonable starting location, and to find glaring errors. Otherwise you're going to be spending thousands and thousands running back to the wind tunnel every time you want to iterate..

Agreed, and that is exactly how we used it with the Navy - the CFD simulations narrowed the universe of options so we could be more efficient with the wind tunnels and water tanks. In this case, however, he has a specific shape that he wants to test. For that, going straight to a physical model makes sense.

Is building and testing an actual propeller going to be easier? I would have thought that setting up and running a simulation could be done in a moderate amount of hours, and could then be quickly iterated on. The only requirements are a laptop and an internet connection.

Building a model, on the other hand, could potentially involve a multi-year effort to re-educate yourself to learn how to build models, having to acquire hardware and materials, and setting up a lab for testing. And then building many different actual models.

Yep.

and i didnt give the following context:

Like JWST "variential dimples and bumps via hydrolic mechanisms, for optimised flow" now we have the hard part added in. (dimple flexing) and there are more levels than that.....

for example you want to be able to shape your dimples in order to precisely control your eddies and currents...

Trying to optimize across a bunch of different dimple/bump patterns is a great use case for CFD. However, you'd want to validate a CFD model like that against real world test data (either by running your own tests or by finding test data in the literature).

To be clear, I think adding vortex generators to boat propellers is a great idea. But to me the starting place would be a building physical propeller with vortex generators (even if they aren't the perfect vortex generators) and seeing if it has a noticeable impact on boat performance. That would justify the difficulty/time required to build a CFD model which you could use to optimize the design. (I'm saying this given that you don't have experience in CFD and it would be a large outlay of your own time to learn how to use it.)

No, that's backwards. Running a CFD simulation would involve a multi-year effort to re-educate yourself to learn how to build an accurate CFD model.

Building a physical model, on the other hand, merely requires 3D printing the desired shape, doing some sort of casting process to get that out of metal, putting it on a boat, and seeing if you get a noticeable performance impact.

It's one of these situations: https://xkcd.com/1425/

How do you figure that, when the CFD software is already built and available, and the premise in the question is that that the asker is already in a position to start using it? It's pretty clear that the background of the asker is aligned with a software approach.

> Building a physical model, on the other hand, merely requires 3D printing the desired shape, doing some sort of casting process to get that out of metal, putting it on a boat, and seeing if you get a noticeable performance impact.

I love how you sneak the word "merely" in there, when for people like me, and presumably the asker, this would be a Herculean task :D

>the premise in the question is that that the asker is already in a position to start using it

That's an incorrect premise. The asker is clearly not in a position to use an off the shelf CFD package because they don't have the solid basis in fluid mechanics required to interpret the results.

I think maybe you/the asker are thinking of CFD like a tool that simulates fluids. That's not what it is. It's a set of approximations which, sometimes, are applicable to specific circumstances. Even LES, the most general tractable model, requires you to make informed assumptions about the boundary conditions.

> I love how you sneak the word "merely" in there, when for people like me, and presumably the asker, this would be a Herculean task

I'm sure building a physical prop would be challenging if you had no experience, but it is the sort of thing a lot of people can learn from Youtube and do in their garage.

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