Immersed Solid

hamed.majeed · September 17, 2012, 11:29

Hi,
I have questions regarding the immersed solid in ANSYS.
What is the immersed boundary method used in it. Nothing specifically is mentioned in ANSYS Help. However, the modifications of the cells and finding the near wall nodes indicates that it is discrete direct forcing.
I am greatly confused about how does it identify the boundary.

Please help.

ghorrocks · September 17, 2012, 19:49

This is well described in the manual.

For non-wall function flow the boundary does not need to be found. The momentum source to move the body goes to zero as you move away from the body naturally.

For the modified forcing option it does need to find the boundary - is this what you are asking about?

hamed.majeed · September 18, 2012, 03:45

Hi, thnx for replying.
But when "ANSYS help" explains about the Boundary Model being used, it discusses this.
Boundary model > Modified Forcing have two options
1. Extrusion Distance: The Boundary is extruded along normal based on Extrusion Length specified, and an imaginary volume is created. The near-wall nodes that lie in this imaginary volume are then projected onto the plane of the Immersed boundary.
2. Search Through Elements: The elements of the fluid domain near the immersed boundary are searched and near-wall nodes are found. These nodes are then projected onto the plane of immersed boundary.

This thing shows that the boundary is being located by ANSYS CFX. Does not it??
Also kindly tell me the method ANSYS uses to deal with immersed solids.

Thank you
Regards
Hamed

ghorrocks · September 18, 2012, 21:21

So you are talking about the modified forcing options.

I do not know what method it uses to detect the boundary. This is a pretty standard numerical application so I guess they did not think it worth documenting. There are lots of methods of how to do this described on the web so do a google search to find some examples.

Why is it important for you? Why do you need to know details of it?

hamed.majeed · September 21, 2012, 15:31

Hi,
Well I am simulating a rigid body drop in water under gravity. I have experimented using different shapes and calculating the drop time. There are two important factors which affect my simulation results other than the standard cfd parameters (time step, turbulence model, etc.). These are:
1. momentum source scaling factor
2. boundary model.
Boundary model also has two options: those discussed in above post.
By changing 1 and 2 makes hell of difference in simulating the drop time. The main issue of concern is:
Q. Does 1 and 2 have optimum value based on the analysis/flow details. Like for eg. turbulence model depends on flow conditions etc. So, is there any limits on 1 and 2 ??

ghorrocks · September 24, 2012, 21:14

Is this a two phase model - so dropping the body on a air/water interface?

But your question appears to simply say you need to do a sensitivity analysis on these parameters to work out what is important.

September 17, 2012, 11:29	Immersed Solid	#1
hamed.majeed Senior Member Hamed Abdul Majeed Join Date: May 2012 Location: New Orleans, LA, US Posts: 147 Rep Power: 14	Hi, I have questions regarding the immersed solid in ANSYS. What is the immersed boundary method used in it. Nothing specifically is mentioned in ANSYS Help. However, the modifications of the cells and finding the near wall nodes indicates that it is discrete direct forcing. I am greatly confused about how does it identify the boundary. Please help.

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September 17, 2012, 19:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	This is well described in the manual. For non-wall function flow the boundary does not need to be found. The momentum source to move the body goes to zero as you move away from the body naturally. For the modified forcing option it does need to find the boundary - is this what you are asking about?

September 18, 2012, 03:45		#3
hamed.majeed Senior Member Hamed Abdul Majeed Join Date: May 2012 Location: New Orleans, LA, US Posts: 147 Rep Power: 14	Hi, thnx for replying. But when "ANSYS help" explains about the Boundary Model being used, it discusses this. Boundary model > Modified Forcing have two options 1. Extrusion Distance: The Boundary is extruded along normal based on Extrusion Length specified, and an imaginary volume is created. The near-wall nodes that lie in this imaginary volume are then projected onto the plane of the Immersed boundary. 2. Search Through Elements: The elements of the fluid domain near the immersed boundary are searched and near-wall nodes are found. These nodes are then projected onto the plane of immersed boundary. This thing shows that the boundary is being located by ANSYS CFX. Does not it?? Also kindly tell me the method ANSYS uses to deal with immersed solids. Thank you Regards Hamed

September 18, 2012, 21:21		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	So you are talking about the modified forcing options. I do not know what method it uses to detect the boundary. This is a pretty standard numerical application so I guess they did not think it worth documenting. There are lots of methods of how to do this described on the web so do a google search to find some examples. Why is it important for you? Why do you need to know details of it?

September 21, 2012, 15:31		#5
hamed.majeed Senior Member Hamed Abdul Majeed Join Date: May 2012 Location: New Orleans, LA, US Posts: 147 Rep Power: 14	Hi, Well I am simulating a rigid body drop in water under gravity. I have experimented using different shapes and calculating the drop time. There are two important factors which affect my simulation results other than the standard cfd parameters (time step, turbulence model, etc.). These are: 1. momentum source scaling factor 2. boundary model. Boundary model also has two options: those discussed in above post. By changing 1 and 2 makes hell of difference in simulating the drop time. The main issue of concern is: Q. Does 1 and 2 have optimum value based on the analysis/flow details. Like for eg. turbulence model depends on flow conditions etc. So, is there any limits on 1 and 2 ??

September 24, 2012, 21:14		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Is this a two phase model - so dropping the body on a air/water interface? But your question appears to simply say you need to do a sensitivity analysis on these parameters to work out what is important.