3 ms·

Dope! Can anyone help me with the following, as this has been 'floating' around in my head for nearly a decade ; Wales (the animal) oft have barnacles on the l

Dope! Can anyone help me with the following, as this has been 'floating' around in my head for nearly a decade ; Wales (the animal) oft have barnacles on the leading edges of their flippers, which results in an eddy effect which increases efficiency/thrust. Da Vinci was the earliest known documentor of eddy-based pumps and predicted the eddies in the ventricular systems of the hearts pumping of blood... What I would like to model is a toroidal propeller with leading edge bumps ('barnacles') while also having the dimpling pattern of a golf ball to reduce drag... and I want to measure if this idea holds water. I just dont know how to model this using this tool... help?

b-fg 3y ago
As long as you can define a signed distance function, and the function governing the motion of the propeller, WaterLily can simulate it!
- elil17 3y ago
  I don’t think that’s true. What about cavitation?
  - b-fg 3y ago
    Cavitation has nothing special other than multi-phase flow. And implementing a VOF method is definitely in our roadmap. You can currently simulate it without cavitation to get a feeling of the unsteady flow solution, or wait for us to implement VOF.
  - samstave 3y ago
    Cavitation is precisely what I would like to test... Also, will 'waterlilly' work with a viscosity to Air? Meaning - toroidal props are a newer entering thing in drones... and I'd like to find out if the above applies equally to fluid/air?
    - b-fg 3y ago
      Then you should is a different solver that fills your needs. WaterLily is an incompressible flow solver that works in non-dimensional units (assuming constant unit density). So you can change the viscosity of the fluid by modifying the Reynolds number (with set a set characteristic velocity and length scale).
- JonChesterfield 3y ago
  What turbulence models do you have implemented? It's been ~ a decade since I played with CFD, wondering if DNS is computationally tractable these days.
  - elil17 3y ago
    DNS is not used much (at all?) for real world problems, but LES has become much more common.
  - b-fg 3y ago
    There is no explicit LES model, aka implicit LES. All the additional dissipation required when running coarse meshes relies on the numerical dissipation of the implemented schemes.
elil17 3y ago
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.
- pge 3y ago
  +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.
  - Nanana909 3y ago
    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..
    - pge 3y ago
      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.
- DNF2 3y ago
  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.
all2 3y ago
Leading edge bumps has been done! - https://en.wikipedia.org/wiki/Tubercle_effect https://en.wikipedia.org/wiki/Tubercle_effect - https://arc.aiaa.org/doi/abs/10.2514/1.J050631 https://arc.aiaa.org/doi/abs/10.2514/1.J050631 - https://physics.paperswithcode.com/paper/effect-of-leading-edge-tubercles-on https://physics.paperswithcode.com/paper/effect-of-leading-e... Enjoy!
- samstave 3y ago
  curious ; are you invalidating, or validating my comment?
  - all2 3y ago
    I was providing information in case you didn't know this stuff was out there. If you're going to do some CFM, it might be useful to see what prior art there is.
    - samstave 3y ago
      Thank you so much, super surprised that info is out there along the same thought lines!! I really think there is something here
- samstave 3y ago
  I have a really weird hypotho So we look at the actions of the faster currents as the flipper cuts through, and where does the current flow, such that you can direct micro currents to the other bumps and managing overall flow... such that certain bumps 'feed' others... and if they are maleable, and you can manage each...
  - all2 3y ago
    Sooo, in terms of malleable wing sections there's some neat research done by MIT that might be interesting to you [0]. There's also some new work that pops up when I was searching for [0]; apparently the same researchers have been working on this idea for some time [1]. The idea is simple in concept, produce small building blocks and assemble large shapes from those building blocks. If you design your building blocks right and assemble them correctly you can achieve some pretty impressive macro properties, including compliant mechanisms [2]. And then there's this 3D printer I stumbled across yesterday that prints on successive sheets of carbon fiber [3]. This could be relevant for the manufacturing process (they currently use injection molding in [1]?) as it would offer composite material strengths at a high speed (they claim 17 seconds for a relatively complex part). [0] https://news.mit.edu/2016/morphing-airplane-wing-design-1103 https://news.mit.edu/2016/morphing-airplane-wing-design-1103 [1] https://news.mit.edu/2019/engineers-demonstrate-lighter-flexible-airplane-wing-0401 https://news.mit.edu/2019/engineers-demonstrate-lighter-flex... [2] https://www.science.org/doi/10.1126/science.1240889 https://www.science.org/doi/10.1126/science.1240889 [3] https://www.youtube.com/watch?v=0BKzikfssTM https://www.youtube.com/watch?v=0BKzikfssTM

b-fg 3y ago

As long as you can define a signed distance function, and the function governing the motion of the propeller, WaterLily can simulate it!

elil17 3y ago
I don’t think that’s true. What about cavitation?
- b-fg 3y ago
  Cavitation has nothing special other than multi-phase flow. And implementing a VOF method is definitely in our roadmap. You can currently simulate it without cavitation to get a feeling of the unsteady flow solution, or wait for us to implement VOF.
- samstave 3y ago
  Cavitation is precisely what I would like to test... Also, will 'waterlilly' work with a viscosity to Air? Meaning - toroidal props are a newer entering thing in drones... and I'd like to find out if the above applies equally to fluid/air?
  - b-fg 3y ago
    Then you should is a different solver that fills your needs. WaterLily is an incompressible flow solver that works in non-dimensional units (assuming constant unit density). So you can change the viscosity of the fluid by modifying the Reynolds number (with set a set characteristic velocity and length scale).
JonChesterfield 3y ago
What turbulence models do you have implemented? It's been ~ a decade since I played with CFD, wondering if DNS is computationally tractable these days.
- elil17 3y ago
  DNS is not used much (at all?) for real world problems, but LES has become much more common.
- b-fg 3y ago
  There is no explicit LES model, aka implicit LES. All the additional dissipation required when running coarse meshes relies on the numerical dissipation of the implemented schemes.

elil17 3y ago

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.

pge 3y ago
+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.
- Nanana909 3y ago
  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..
  - pge 3y ago
    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.
DNF2 3y ago
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.

all2 3y ago

Leading edge bumps has been done! - https://en.wikipedia.org/wiki/Tubercle_effect https://en.wikipedia.org/wiki/Tubercle_effect - https://arc.aiaa.org/doi/abs/10.2514/1.J050631 https://arc.aiaa.org/doi/abs/10.2514/1.J050631 - https://physics.paperswithcode.com/paper/effect-of-leading-edge-tubercles-on https://physics.paperswithcode.com/paper/effect-of-leading-e... Enjoy!

samstave 3y ago
curious ; are you invalidating, or validating my comment?
- all2 3y ago
  I was providing information in case you didn't know this stuff was out there. If you're going to do some CFM, it might be useful to see what prior art there is.
  - samstave 3y ago
    Thank you so much, super surprised that info is out there along the same thought lines!! I really think there is something here
samstave 3y ago
I have a really weird hypotho So we look at the actions of the faster currents as the flipper cuts through, and where does the current flow, such that you can direct micro currents to the other bumps and managing overall flow... such that certain bumps 'feed' others... and if they are maleable, and you can manage each...
- all2 3y ago
  Sooo, in terms of malleable wing sections there's some neat research done by MIT that might be interesting to you [0]. There's also some new work that pops up when I was searching for [0]; apparently the same researchers have been working on this idea for some time [1]. The idea is simple in concept, produce small building blocks and assemble large shapes from those building blocks. If you design your building blocks right and assemble them correctly you can achieve some pretty impressive macro properties, including compliant mechanisms [2]. And then there's this 3D printer I stumbled across yesterday that prints on successive sheets of carbon fiber [3]. This could be relevant for the manufacturing process (they currently use injection molding in [1]?) as it would offer composite material strengths at a high speed (they claim 17 seconds for a relatively complex part). [0] https://news.mit.edu/2016/morphing-airplane-wing-design-1103 https://news.mit.edu/2016/morphing-airplane-wing-design-1103 [1] https://news.mit.edu/2019/engineers-demonstrate-lighter-flexible-airplane-wing-0401 https://news.mit.edu/2019/engineers-demonstrate-lighter-flex... [2] https://www.science.org/doi/10.1126/science.1240889 https://www.science.org/doi/10.1126/science.1240889 [3] https://www.youtube.com/watch?v=0BKzikfssTM https://www.youtube.com/watch?v=0BKzikfssTM