Average of time on streamline in Transient analysis

sonu rao · September 6, 2014, 14:02

I did a transient analysis of flow in blood vessel. There is an option in function calculator "average". I used that for "streamline" as location and then "time on streamline" as variable. It gave me some time values. Is it residence time?

ghorrocks · September 7, 2014, 06:19

No. You are calculating streamlines on a transient flow. Streamlines only use the instantaneous flow field, so time varying flow fields will not be accurate.

To get residence time in a transient flow you should use massless particles or a additional variable (the additional variable approach is described here: http://www.cfd-online.com/Forums/cfx...ution-cfx.html)

sonu rao · September 7, 2014, 07:16

Ok thanks...I'll try out that...
I want to calculate the absolute mass flow rate on a plane...I have done CFD in aneurysm but i am not able to get mass flow rate on the plane at neck of aneurysm...since whatever is going inside through this plane is coming out also so mass flow rate is coming near to zero...so i need to calculate absolute mass flow rate...can you please tell me how to do that?

ghorrocks · September 7, 2014, 07:43

This question appears unrelated to your initial question on streamlines.

In a transient flow what goes into the domain does not necessarily equal what goes out. There can be accumulation (or loss) of stuff within the domain as well.

So if you are getting no flow at one point then are you sure the flow is not accumulating somewhere?

sonu rao · September 7, 2014, 07:54

I just want to know flow rate in aneurysm...both mass flow rate and residence time can give me that...hence i asked the second question though both appears to you unrelated...
Mass flow rate at inlet and outlet are equal and opposite hence there should be no accumulation or any losses...problem is at a plane (on the neck of aneurysm) I want inflow rate...how to get that?

ghorrocks · September 7, 2014, 08:20

Can you post an image of what you are trying to do? Your question does not make sense to me - you say there is no accumulation in the domain, so the inflow equals the outflow. But that means that any cross section through the flow will also have the same flow - so why are you trying to calculate it?

sonu rao · September 7, 2014, 08:29

may be I am not saying correctly..here is the picture...in this picture one plane is there at the neck of aneurysm...I want to calculate how much flow is going in the aneurysm only...so created a plane at the neck and want to calculate flow on that...

ghorrocks · September 7, 2014, 09:03

Oh, I think I understand now. You could use something like massFlowAveAbs(plane_normal_velocity)@plane where plane_normal_velocity is the velocity field at the plane which is normal to the plane.

But have a think about what you are trying to do. Is the absolute value of the mass flow rate meaningful? Would something like RMS (root mean square) be better?

sonu rao · September 7, 2014, 09:11

ok thanks I'll try out with that expression...In case I want to calculate RMS also could you tell me how to do that?

ghorrocks · September 7, 2014, 18:28

areaAve(Velocity^2)@plane, or you could do a mass average if that is more physical for you.

sonu rao · September 8, 2014, 01:04

What is the direction of Velocity in the expression "areaAve(Velocity^2)@plane" ? Is it normal to the plane?I think just taking the velocity like this will give magnitude of velocity not direction... Don't you think I should take velocity normal to the plane ...I may be wrong just need your suggestion...

ghorrocks · September 8, 2014, 01:17

Velocity returns the magnitude of the velocity. The magnitude normal to the plane may be more appropriate in your case.

I cannot comment on the appropriateness of any of these approaches as I do not know what you are trying to do with it. All these approaches sound somewhat artificial to me.

sonu rao · September 8, 2014, 01:25

How to get velocity normal to the plane as I have showed in image? In this expression "massFlowAveAbs(plane_normal_velocity)@plane where plane_normal_velocity is the velocity field at the plane which is normal to the plane" is velocity normal to the plane I have created or is in a plane which is normal to plane I have created in image?... I want some measure of flow only for aneurysm... Which function to use for that and in which way is where I am having problem...

ghorrocks · September 8, 2014, 06:09

You can define a local coordinate frame on the plane and use the _x, _y, _z modifiers of velocity to get the normal. Have a look in the CFX reference manual for the available functions, variables and modifiers.

sonu rao · September 8, 2014, 06:28

okk thanks...

sonu rao · September 9, 2014, 07:21

I made a local coordinate frame on the plane and tried to use the expression "areaAve_z_Coordinate Frame 1(Velocity)@Plane"...I saw that the values are same for X, Y and Z axis....Could you tell me by seeing images where I am doing wrong?

ghorrocks · September 9, 2014, 07:29

I do not know what is wrong there.

September 6, 2014, 14:02	Average of time on streamline in Transient analysis	#1
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	I did a transient analysis of flow in blood vessel. There is an option in function calculator "average". I used that for "streamline" as location and then "time on streamline" as variable. It gave me some time values. Is it residence time?

September 7, 2014, 06:19		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	No. You are calculating streamlines on a transient flow. Streamlines only use the instantaneous flow field, so time varying flow fields will not be accurate. To get residence time in a transient flow you should use massless particles or a additional variable (the additional variable approach is described here: http://www.cfd-online.com/Forums/cfx...ution-cfx.html) rajann_786 likes this.

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September 7, 2014, 07:16		#3
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	Ok thanks...I'll try out that... I want to calculate the absolute mass flow rate on a plane...I have done CFD in aneurysm but i am not able to get mass flow rate on the plane at neck of aneurysm...since whatever is going inside through this plane is coming out also so mass flow rate is coming near to zero...so i need to calculate absolute mass flow rate...can you please tell me how to do that?

September 7, 2014, 07:43		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	This question appears unrelated to your initial question on streamlines. In a transient flow what goes into the domain does not necessarily equal what goes out. There can be accumulation (or loss) of stuff within the domain as well. So if you are getting no flow at one point then are you sure the flow is not accumulating somewhere?

September 7, 2014, 07:54		#5
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	I just want to know flow rate in aneurysm...both mass flow rate and residence time can give me that...hence i asked the second question though both appears to you unrelated... Mass flow rate at inlet and outlet are equal and opposite hence there should be no accumulation or any losses...problem is at a plane (on the neck of aneurysm) I want inflow rate...how to get that?

September 7, 2014, 08:20		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	Can you post an image of what you are trying to do? Your question does not make sense to me - you say there is no accumulation in the domain, so the inflow equals the outflow. But that means that any cross section through the flow will also have the same flow - so why are you trying to calculate it?

September 7, 2014, 09:03		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	Oh, I think I understand now. You could use something like massFlowAveAbs(plane_normal_velocity)@plane where plane_normal_velocity is the velocity field at the plane which is normal to the plane. But have a think about what you are trying to do. Is the absolute value of the mass flow rate meaningful? Would something like RMS (root mean square) be better?

September 7, 2014, 09:11		#9
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	ok thanks I'll try out with that expression...In case I want to calculate RMS also could you tell me how to do that?

September 7, 2014, 18:28		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	areaAve(Velocity^2)@plane, or you could do a mass average if that is more physical for you.

September 8, 2014, 01:04		#11
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	What is the direction of Velocity in the expression "areaAve(Velocity^2)@plane" ? Is it normal to the plane?I think just taking the velocity like this will give magnitude of velocity not direction... Don't you think I should take velocity normal to the plane ...I may be wrong just need your suggestion...

September 8, 2014, 01:17		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	Velocity returns the magnitude of the velocity. The magnitude normal to the plane may be more appropriate in your case. I cannot comment on the appropriateness of any of these approaches as I do not know what you are trying to do with it. All these approaches sound somewhat artificial to me.

September 8, 2014, 01:25		#13
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	How to get velocity normal to the plane as I have showed in image? In this expression "massFlowAveAbs(plane_normal_velocity)@plane where plane_normal_velocity is the velocity field at the plane which is normal to the plane" is velocity normal to the plane I have created or is in a plane which is normal to plane I have created in image?... I want some measure of flow only for aneurysm... Which function to use for that and in which way is where I am having problem...

September 8, 2014, 06:09		#14
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	You can define a local coordinate frame on the plane and use the _x, _y, _z modifiers of velocity to get the normal. Have a look in the CFX reference manual for the available functions, variables and modifiers.

September 8, 2014, 06:28		#15
sonu rao New Member Sonu rao Join Date: Sep 2014 Posts: 9 Rep Power: 11	okk thanks...

September 9, 2014, 07:29		#17
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,696 Rep Power: 143	I do not know what is wrong there.