new variable defination

Reza.sbi · June 4, 2018, 06:47

I want to define a new variable in cfx , that's Eq is:
HI(%)=(ct^a)(shear stress )^b
this is hemolysis index
c,a,b is constant
t is time
The solution method is steady state
I want to get an hemolysis countor in cfx post .

Can someone help me?

ghorrocks · June 4, 2018, 07:32

This is easy to do, but the only trick is you will need to make the expressions (ct) and (shear stress) unitless before you raise them to a power. You can do this by just dividing by one, with a unit of the inverse of their units.

Gert-Jan · June 4, 2018, 08:02

Quote:

Originally Posted by Reza.sbi

I want to define a new variable in cfx , that's Eq is:
HI(%)=(ct^a)(shear stress )^b
this is hemolysis index
c,a,b is constant
t is time
The solution method is steady state
I want to get an hemolysis countor in cfx post .

Can someone help me?

If I read it correctly, your equation is: c*t^a*ShearStress^b.
So you have time to the power of a.
But you mention it is steady state, i.e. time is not a variable. So, that doesn't match. Please clarify.........

Reza.sbi · June 5, 2018, 05:17

Hi
In Navier Stokes
In steady state
iteration step are used instead of time steps.
I would use
*length of time each particle is placed under stress
as time in this equation:
HI(%)=(ct^a)(shear stress )^b
my project is hemolysis estimtion in ventricular assist device( centrifugal pump).
I have obtained the pressure and shear stress counter from cfx
I want to get a hemolysis counter
I think
This equation must be solved simultaneously with the contunity equation

but I do not know what to do?

Gert-Jan · June 5, 2018, 05:43

If you open your case in CFD-Post, you can create streamlines. Then you obtain a new variable "Time on Streamline". Since you only want to postprocess this equation, I guess this variable is sufficient for what you need, not?

Reza.sbi · June 5, 2018, 12:28

Yes
Thank you
I have done this
This method is Lagrangian
But I want to run Eulerian
And in this way it is necessary to define the variable before it is run.
in Lagrangian method, We can not get hemolysis counter. we can get Discrete data.
I want to compare two Lagrangian and Eulerian methods.

Gert-Jan · June 5, 2018, 19:06

I can't follow your comment completely. Please read it over and rearrange your sentences.

You mentioned: "This equation must be solved simultaneously with the contunity equation". Why? Is the equation used somewhere else?

Nevertheless, streamlines are some kind of Lagrangian, as it ignores Turbulent dispersion. But it is done in the postprocessing, so, you can do everything after the run is done. What's wrong with it? Again: Why must the equation be solved simultaneously with the continuity equation?

To run Eulerian, you have to define a scalar that describes the residence time. This is doable.:
1) define a Additional Variable (volumetric)
2) Create a subdomain that covers your whole domain and set a source for your additional variable equal to 1.
3) Set 0 at the inlet.
4) run the calculation
If you are sure this variable is solved completely (Check the overall balances!!!), you obtain a variable representing time that you can use for your equation. A problem/challenge can be that this scalar is subject to mixing.

A better way could be the use of Lagrangian particles with turbulent dispersion in the solver. Then you get discrete data, where mixing doesn't interfere, and have the possibility to create a residence time distribution. But evaluating your equation can only be done in the postprocessing. Also, there is no guarantee that in every cell a particle will be present, so yous time will be undefined at these locations.

In other words, every method has its pros and cons........

June 4, 2018, 06:47	new variable defination	#1
Reza.sbi New Member Reza Join Date: Jun 2018 Posts: 5 Rep Power: 8	I want to define a new variable in cfx , that's Eq is: HI(%)=(ct^a)(shear stress )^b this is hemolysis index c,a,b is constant t is time The solution method is steady state I want to get an hemolysis countor in cfx post . Can someone help me?

June 4, 2018, 07:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	This is easy to do, but the only trick is you will need to make the expressions (ct) and (shear stress) unitless before you raise them to a power. You can do this by just dividing by one, with a unit of the inverse of their units. VIKAS.PASSION.SINGH and Reza.sbi like this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

June 5, 2018, 05:17		#4
Reza.sbi New Member Reza Join Date: Jun 2018 Posts: 5 Rep Power: 8	Hi In Navier Stokes In steady state iteration step are used instead of time steps. I would use *length of time each particle is placed under stress as time in this equation: HI(%)=(ct^a)(shear stress )^b my project is hemolysis estimtion in ventricular assist device( centrifugal pump). I have obtained the pressure and shear stress counter from cfx I want to get a hemolysis counter I think This equation must be solved simultaneously with the contunity equationbut I do not know what to do?

June 5, 2018, 05:43		#5
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,913 Rep Power: 28	If you open your case in CFD-Post, you can create streamlines. Then you obtain a new variable "Time on Streamline". Since you only want to postprocess this equation, I guess this variable is sufficient for what you need, not? Reza.sbi likes this.

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June 5, 2018, 12:28		#6
Reza.sbi New Member Reza Join Date: Jun 2018 Posts: 5 Rep Power: 8	Yes Thank you I have done this This method is Lagrangian But I want to run Eulerian And in this way it is necessary to define the variable before it is run. in Lagrangian method, We can not get hemolysis counter. we can get Discrete data. I want to compare two Lagrangian and Eulerian methods.

June 5, 2018, 19:06		#7
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,913 Rep Power: 28	I can't follow your comment completely. Please read it over and rearrange your sentences. You mentioned: "This equation must be solved simultaneously with the contunity equation". Why? Is the equation used somewhere else? Nevertheless, streamlines are some kind of Lagrangian, as it ignores Turbulent dispersion. But it is done in the postprocessing, so, you can do everything after the run is done. What's wrong with it? Again: Why must the equation be solved simultaneously with the continuity equation? To run Eulerian, you have to define a scalar that describes the residence time. This is doable.: 1) define a Additional Variable (volumetric) 2) Create a subdomain that covers your whole domain and set a source for your additional variable equal to 1. 3) Set 0 at the inlet. 4) run the calculation If you are sure this variable is solved completely (Check the overall balances!!!), you obtain a variable representing time that you can use for your equation. A problem/challenge can be that this scalar is subject to mixing. A better way could be the use of Lagrangian particles with turbulent dispersion in the solver. Then you get discrete data, where mixing doesn't interfere, and have the possibility to create a residence time distribution. But evaluating your equation can only be done in the postprocessing. Also, there is no guarantee that in every cell a particle will be present, so yous time will be undefined at these locations. In other words, every method has its pros and cons........