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Flow simulation on Boat

Question asked by Joseph Kim on Oct 2, 2015
Latest reply on Oct 5, 2015 by Andrei Popov

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Hello,

I am a Mech Eng student and I am only learning to use solidworks as I work through a number of projects in Uni.
Anyways, I have been looking all over the forum but it does not seem to have the question I need answered.
I am trying to calculate a drag force using the flow simulation on the boat. (a floating body)
I know it can easily calculate the force on bodies entirely submerged in water. But is it possible to calculate the force if the body is partially submerged in water?
Two phase fluid external flow can not be done with Solidworks(as I read somewhere), but would it be possible to calculate the force due to each fluid(air/water) separately and add them up?? (given I can figure out where the boat is submerged). I understand the results would not be accurate. However as long as I can get somewhat reasonable value, that will do for the current project I am doing.

Thanks in advance

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Amit Katz Oct 2, 2015 9:21 AM

I think what you're asking for is theoretically feasible, however it comes with some big caveats that depend on the length scale of your boat and the relative speeds. You can have gas and liquid subdomains in the same simulation, but they will not mix, nor will their surfaces interact. That means you will not be able to model bow waves and a true wake. The amount of drag that these phenomena contribute depends on how big your boat is, and how fast it's going. Really the only legitimate simulation I can imagine for this thing is extremely low Reynolds number flow (and that value will of course vary widely between the air and water as well.) Otherwise you're going to be ignoring too many mechanics to get a real accurate model.

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Joseph Kim @ Amit Katz on Oct 2, 2015 9:46 AM

Yes. I understand. The thing is, this approach would be also ignoring the effects of planning as the speed of the boat increases. (which I assume will result in less drag) I am only trying it this way as I can not think of any other viable way to predict the drag force. (All the journal articles I looked into seem to use dimensional analysis with experimental values from model) I would use any method even if it does not involve CFD as long as I can get some approximate values.

To be more specific, I am modelling an RC boat travelling at the maximum speed. so the overall length would be something like 1m with the speed of 85km/h.

Anyways, if I was to just simulate it with two phases, do I need to first make a plane surface separating the water and air and use that to split the computational domain into two subdomains?

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Andrei Popov @ Joseph Kim on Oct 5, 2015 1:28 PM

Solidworks can't do two phase simulation.

OpenFoam can: CFD, OpenFoam, floating ship - YouTube

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Brian Stranyak Oct 2, 2015 9:34 AM

I showed this to a colleague who's son is in naval architecture school. His response was this "There are lots of resistance prediction models depending on hull type. That's probably the fella's best bet. SNAME has a publication called Principles of Naval Architecture. Volume 2 is all about resistance." I don't know what any of this is for my line of work

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Joseph Kim @ Brian Stranyak on Oct 2, 2015 10:06 AM

I just skimmed through the publication and it seems to suggest three methods, numerical, experimental and empirical. I won't be able to do a numerical or experimental as I have got no idea how. (formulas presented seem to require computational calculation). Maybe the empirical method could be done thru hand calculation? Anyways Thanks heaps I will look into it more

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Amit Katz Oct 2, 2015 10:06 AM

To be honest, I don't believe that you're going to get any sort of benefit from modeling the boat like this. A more powerful CFD package may help, but I think you might be stuck with practical experiments in this case.

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6 Replies

Amit Katz Oct 2, 2015 9:21 AM
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I think what you're asking for is theoretically feasible, however it comes with some big caveats that depend on the length scale of your boat and the relative speeds. You can have gas and liquid subdomains in the same simulation, but they will not mix, nor will their surfaces interact. That means you will not be able to model bow waves and a true wake. The amount of drag that these phenomena contribute depends on how big your boat is, and how fast it's going. Really the only legitimate simulation I can imagine for this thing is extremely low Reynolds number flow (and that value will of course vary widely between the air and water as well.) Otherwise you're going to be ignoring too many mechanics to get a real accurate model.
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- Joseph Kim @ Amit Katz on Oct 2, 2015 9:46 AM
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  Yes. I understand. The thing is, this approach would be also ignoring the effects of planning as the speed of the boat increases. (which I assume will result in less drag) I am only trying it this way as I can not think of any other viable way to predict the drag force. (All the journal articles I looked into seem to use dimensional analysis with experimental values from model) I would use any method even if it does not involve CFD as long as I can get some approximate values.
  
  To be more specific, I am modelling an RC boat travelling at the maximum speed. so the overall length would be something like 1m with the speed of 85km/h.
  
  Anyways, if I was to just simulate it with two phases, do I need to first make a plane surface separating the water and air and use that to split the computational domain into two subdomains?
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  - Andrei Popov @ Joseph Kim on Oct 5, 2015 1:28 PM
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    Solidworks can't do two phase simulation.
    OpenFoam can: CFD, OpenFoam, floating ship - YouTube
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Brian Stranyak Oct 2, 2015 9:34 AM
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I showed this to a colleague who's son is in naval architecture school. His response was this "There are lots of resistance prediction models depending on hull type. That's probably the fella's best bet. SNAME has a publication called Principles of Naval Architecture. Volume 2 is all about resistance." I don't know what any of this is for my line of work
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- Joseph Kim @ Brian Stranyak on Oct 2, 2015 10:06 AM
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  I just skimmed through the publication and it seems to suggest three methods, numerical, experimental and empirical. I won't be able to do a numerical or experimental as I have got no idea how. (formulas presented seem to require computational calculation). Maybe the empirical method could be done thru hand calculation? Anyways Thanks heaps I will look into it more
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Amit Katz Oct 2, 2015 10:06 AM
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To be honest, I don't believe that you're going to get any sort of benefit from modeling the boat like this. A more powerful CFD package may help, but I think you might be stuck with practical experiments in this case.
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Flow simulation on Boat

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