Buoyancy - or getting rid of it..... (Geeks please)

[arryb]

Mods, I didn't know where to put this so it's here....

I'm designing something which is supposed to flap up to parallel with the flow of fluid and then back down to perpendicular to the flow again. I don't want buoyancy to be an issue here so that it floats and remains parallel with the flow - if the buoyancy force acting on an object is equal to the weight of the mass of water displaced by the object, does this mean that in order to get the behaviour I want, the object in the water has to have a density equal to that of the fluid in which it's submerged in?

Thanks in advance for any help

[mybrainhurts]

Dirty boy...

[Simpo Two]

Equal density sounds like neutral buoyancy to me, yes. So what drives the flap up and down?

[shirt]

ok, its the end of a long day so i may be wrong

i take it you mean the mass of the object is equal to the acting bouyancy force? i would take this to mean the forces are effectively cancelled and a small additional applied force will cause the object/flap to move.

in that case, if the flap is closed and liquid flows, the flap will open. when liquid stops flowing it will remain open until a force is applied.

so, either you can close the flap with a small control force, or calculate the force acting upon the flap by the fluid flow and make the flap sufficiently light as to be opened by this force yet heavy enough to oppose buoyancy and close by itself.

i may have misunderstood. where's the MS paint diagram man!?

Edited by shirt on Wednesday 6th May 17:45

[arryb]

shirt said:

ok, its the end of a long day so i may be wrong

i take it you mean the mass of the object is equal to the acting bouyancy force? i would take this to mean the forces are effectively cancelled and a small additional applied force will cause the object/flap to move.

in that case, if the flap is closed and liquid flows, the flap will open. when liquid stops flowing it will remain open until a force is applied.

so, either you can close the flap with a small control force, or calculate the force acting upon the flap by the fluid flow and make the flap sufficiently light as to be opened by this force yet heavy enough to oppose buoyancy and close by itself.

i may have misunderstood. where's the MS paint diagram man!?

Edited by shirt on Wednesday 6th May 17:45

Sorry, just read the OP again, I did indeed mean mass Your understanding is good enough I think from your response so no need for the MS paint diagram, haha.

I was either thinking of using the flow of water to bring the flap closed (i.e. putting a slight angle on it so that it never completely reaches parallel with the flow), or the other option is as you say in bold above.

But surely if the force from the flow is constant then you'll never get it back down again purely by weighting it as such and therefore and external force is needed in one direction (either to open it, or to close it)?

[arryb]

I'm being a muppet, sorry, just realised what you've said. Ok, so if the force of the flow was 5kN and I made the flap at greater than the buoyancy force but less than this, then when the flow force stops, it will close I think my best option is to go for neutral buoyancy. Thanks for your help

[V8mate]

Lead. Lots of it. Or gold.

Not wood. Or plastic.

Probably.