Flow of gas and water in concentric pipes

AS_Aero · October 1, 2018, 08:58

Hi All

I am trying to simulate a flow inside a circular channel with a needle which is having a wall thickness of 1mm and inside the needle gas is flowing and outside, water is flowing inside the channel. This water flow velocity will help to create the bubble from the needle and detach the bubble from the needle. But due to this thickness of the wall, My timestep is very less since I have this small wall thickness which controls the dt. So now my doubt is, whether its possible to have a small thin wall so that I can avoid the mesh size constraint.
I need an internal wall for gas flow and external wall for water flow.

I have attached an image.

urosgrivc · October 2, 2018, 04:41

Your timestep would be very small despite that thin wall,
because of the physics, you are resolving.

AS_Aero · October 2, 2018, 04:50

Yeah true, but this small thickness is 1mm is just a sample, I have one needle of thickness 0.002mm so this will be controlled by the thickness of the mesh in that region, so I was thinking if there is any possiblity to have a thin wall of zero thickness so that I can give mesh on both sides.

Gert-Jan · October 2, 2018, 07:51

This is a geometry/mesh problem, so better ask this in the meshing & geometry forum.
Nevertheless, sure this is possible. In ICEM, you can have an internal wall. When set as split wall, you will have a different mesh on both sides, allowing you to create different BC's in Pre.
Not sure if it will help you in reducing time step........

urosgrivc · October 2, 2018, 08:30

aha.. but if you can neglect the thickness of 0.002mm at the end you could probably bring your needle wall of small thickness to a point 0 at the end so it would be at a slight angle and the tip would not have thickness maybe

AS_Aero · October 2, 2018, 08:44

Oh yeah that is also possible in case of the needle geometry. but if its a micro pipe then its going to be hard.

ghorrocks · October 2, 2018, 18:13

I do CFD of MEMS devices and dimensions of a micron or two is common. Some of my simulations require time steps of 1e-10s due to the very small length dimensions and fast rate of the effects. So do not be afraid of using small time steps.

October 2, 2018, 18:13		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I do CFD of MEMS devices and dimensions of a micron or two is common. Some of my simulations require time steps of 1e-10s due to the very small length dimensions and fast rate of the effects. So do not be afraid of using small time steps. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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October 2, 2018, 04:41		#2
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	Your timestep would be very small despite that thin wall, because of the physics, you are resolving.

October 2, 2018, 04:50		#3
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Yeah true, but this small thickness is 1mm is just a sample, I have one needle of thickness 0.002mm so this will be controlled by the thickness of the mesh in that region, so I was thinking if there is any possiblity to have a thin wall of zero thickness so that I can give mesh on both sides.

October 2, 2018, 07:51		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	This is a geometry/mesh problem, so better ask this in the meshing & geometry forum. Nevertheless, sure this is possible. In ICEM, you can have an internal wall. When set as split wall, you will have a different mesh on both sides, allowing you to create different BC's in Pre. Not sure if it will help you in reducing time step........

October 2, 2018, 08:30		#5
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	aha.. but if you can neglect the thickness of 0.002mm at the end you could probably bring your needle wall of small thickness to a point 0 at the end so it would be at a slight angle and the tip would not have thickness maybe

October 2, 2018, 08:44		#6
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Oh yeah that is also possible in case of the needle geometry. but if its a micro pipe then its going to be hard.