[Prateek garg]

Hi there,

I am Prateek, I am trying to simulate reacting flow in a shock tube. I have two shock tubes with a diaphragm in between, first is closed at one end and second one is open to air. Now the problem is I want to define first tube with H2-O2 reacting mixture at 50 atm and another tube that is other side of diaphragm with air at STP. I want that when the simulation starts at t=0 the reaction starts at closed end and diaphragm get ruptured and flows starts in another tube due to pressure difference. I have taken boundary between two shock tubes as domain interface due to which I cant specify different fluid in two domains connected via interface. So, how can I specify different fluid in differnt domain and getting fluid flowing from one tube to another. CFX shows error saying H2 O2 must be present in this domain too. So, please help me what shud I do to proceed from here. It wud be a great help.

[Opaque]

A "fluid" in ANSYS CFX is a modeling abstraction for mixture, liquid, gases, etc. You should benefit from reading the documentation and understand how ANSYS CFX defines and uses the concepts of fluids, solids and materials.

For your specific setup, you can do it with a single fluid by creating a non-reacting mixture made of H2-O2-AirSTP. Then, on side 1 you define non-zero mass fractions for H2-O2, and zero for AirSTP. On side 2 you do the opposite and you should be set.

Notice that side 1, and side 2 could be within the same domain. For initializing mesh regions within the same domain, you should read about the "inside()@REGION:mesh name" functionality as well.

Hope the above helps,

[Prateek garg]

Thanks a lot Opaque. I will try and get back to u soon.

[Prateek garg]

There is another problem came up...if someone can help me finding the possible solution for this

****** Notice ****** |
| Newtons method failed to converge in 150 iterations. This |
| occurred while computing the following variable: |
| |
| Variable Name : Total Pressure |
| Location Name : coldzone |
| Mesh location : VERTICES |
| Mesh entity : |
| Last 3 Changes : 1.59880E+01 1.59880E+01 1.59880E+01 |
| Tolerance : 1.0000E+00 |
| |
| The Newton iteration was either slowly converging or has stalled. |
| The solver will continue with the variable set as it was on the |
| final iteration. If this situation continues you might try |
| increasing the number of iterations allowed for Newtons method. |
| This can be changed by setting one of the parameters: |
| |
| Temperature : "Constitutive Relation Iteration Limit" |
| Pressure : "Newton Pressure Iteration Limit" |
| |
| for your mixture using the definition file editor. |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| While evaluating |
| Total Pressure |
| on domain "reactionzone", |
| the variable |
| Absolute Pressure |
| went outside of its upper limit. Its maximum value was |
| 5.1679E+06. The bounds error was handled by clipping. |
| If this situation persists, consider increasing the table range

[Prateek garg]

Can u help me regarding this error?

****** Notice ****** |
| Newtons method failed to converge in 150 iterations. This |
| occurred while computing the following variable: |
| |
| Variable Name : Total Pressure |
| Location Name : coldzone |
| Mesh location : VERTICES |
| Mesh entity : |
| Last 3 Changes : 1.59880E+01 1.59880E+01 1.59880E+01 |
| Tolerance : 1.0000E+00 |
| |
| The Newton iteration was either slowly converging or has stalled. |
| The solver will continue with the variable set as it was on the |
| final iteration. If this situation continues you might try |
| increasing the number of iterations allowed for Newtons method. |
| This can be changed by setting one of the parameters: |
| |
| Temperature : "Constitutive Relation Iteration Limit" |
| Pressure : "Newton Pressure Iteration Limit" |
| |
| for your mixture using the definition file editor. |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| ****** Notice ****** |
| While evaluating |
| Total Pressure |
| on domain "reactionzone", |
| the variable |
| Absolute Pressure |
| went outside of its upper limit. Its maximum value was |
| 5.1679E+06. The bounds error was handled by clipping. |
| If this situation persists, consider increasing the table range

[Opaque]

This is a warning. As long as the solution converges, you should be fine.

What kind of boundary conditions do you have ? It seems there is a total pressure to static pressure conversion based on the message.

Did you follow the suggestions in the warning ?

[Prateek garg]

I initialised the first domain with 50 atm and another domain i.e the other side of the diaphragm is at 1 atm and the reference pressure in both the cases is 0 atm.

[ghorrocks]

In general the Newton's method solver problem indicates poor numerical stability. The approaches described in this FAQ are therefore relevant: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F