free falling cylinder

rasool motamedi · July 21, 2015, 06:27

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

ghorrocks · July 21, 2015, 20:36

FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

rasool motamedi · July 22, 2015, 03:49

Quote:

Originally Posted by ghorrocks

FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

thanks regards
i knew this.
my problem is different between this two picture.
maybe i did not consider impact load. how can do it?

ghorrocks · July 22, 2015, 06:18

Quote:

i knew this.

A vague question gets a vague answer.

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?

rasool motamedi · July 22, 2015, 06:51

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.

rasool motamedi · July 23, 2015, 03:10

there is no idea

ghorrocks · July 23, 2015, 03:17

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.

rasool motamedi · July 23, 2015, 03:30

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.

ghorrocks · July 23, 2015, 06:12

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:

i applied a lot of mesh type.

I do not know what you mean.

rasool motamedi · July 27, 2015, 11:15

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???

ghorrocks · July 27, 2015, 18:19

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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July 21, 2015, 20:36		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

July 23, 2015, 03:10		#6
rasool motamedi Member rasool Join Date: Apr 2013 Posts: 41 Rep Power: 13	there is no idea

July 23, 2015, 03:17		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.

July 23, 2015, 03:30		#8
rasool motamedi 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.

July 27, 2015, 18:19		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.