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July 21, 2015, 06:27 |
free falling cylinder
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#1 |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
hi
i am simulating 2D free falling cylinder in water. initial velocity =3 m/s i do it with a lot of structured and unstructured good mesh with sweep method but i have not good answer only i have near answer with this bad mesh and and when i decrease the element size, answers are bad but mesh motion is good and have not negative volume mesh of walls are stationary what is the problem? how can apply the impact loads? thanks regard realy i am hopeless of cfx because of depending on mesh i am sure my time step is ok it is 0.0001 Last edited by rasool motamedi; July 21, 2015 at 19:08. |
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July 21, 2015, 20:36 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,819
Rep Power: 144 |
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July 22, 2015, 03:49 |
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#3 | |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
Quote:
i knew this. my problem is different between this two picture. maybe i did not consider impact load. how can do it? |
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July 22, 2015, 06:18 |
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#4 | |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,819
Rep Power: 144 |
Quote:
Before we talk about how to implement this - what are you trying to do? Does the cylinder hit the water with constant velocity? Or does it stop when it hits the water and float? How far underwater does it go? What size is the sphere and what speed is it going at? |
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July 22, 2015, 06:51 |
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#5 |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
what are you trying to do?We model the particular problems of cylinder and wedge slam into still water. slamming and impact is important
Does the cylinder hit the water with constant velocity? no Or does it stop when it hits the water and float? Cylinder of half buoyant mass dropping into initially still water How far underwater does it go? like picture What size is the sphere and what speed is it going at? initial speed is 3m/s and diameter oo cylinder is 11 cm thanks regards. |
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July 23, 2015, 03:10 |
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#6 |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
there is no idea
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July 23, 2015, 03:17 |
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#7 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,819
Rep Power: 144 |
You are probably going to have to do some tuning of the CFX free surface model to get this working nicely. The defaults often need a bit of a spruce up for different applications. Also - do you need surface tension for this model? You might need it to form the splashed droplets. Your model is also likely to be highly mesh sensitive. You will need to do a careful validation of your mesh size.
The FAQ I linked to covers the negative volume element error so I will not discuss it further. |
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July 23, 2015, 03:30 |
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#8 |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
do you need surface tension for this model? i applied it but answer is not true.
You might need it to form the splashed droplets. how can apply it? and i applied a lot of mesh type. |
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July 23, 2015, 06:12 |
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#9 | |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Surface tension is a difficult model to work with so only turn it on if you need to. But as I said, if you are forming droplets I suspect you need to.
Quote:
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July 27, 2015, 11:15 |
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#10 |
Member
rasool
Join Date: Apr 2013
Posts: 41
Rep Power: 13 |
ihi
i do not know why the water have no diffusion. true behavior show in right picture. i applied surface tension coefficient. how can define right value of surface tension coefficient? in wiki it is 0.071 N/m i must use additional variable??? |
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July 27, 2015, 18:19 |
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#11 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,819
Rep Power: 144 |
You are probably going to have to adjust the default surface modelling properties, as I mentioned in post #7.
Surface tension models are highly sensitive to mesh quality. There is no need for an additional variable. If you have set surface tension that is all you need to do. |
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