CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Problems with Simulation of a Valve with Cavitation

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 3, 2018, 06:54
Default Problems with Simulation of a Valve with Cavitation
  #1
New Member
 
Wilbert S
Join Date: Mar 2018
Posts: 3
Rep Power: 8
WilbertS is on a distinguished road
Hi everyone,

I am working on a simulation of a valve connected to an discharge pipe, where I try to find the mass flow and velocity profile at the exit of this discharge pipe.
I keep the valve at a certain fixed opening (30%). At the inlet I set up a total pressure of 40 bar and at the outlet I set an opening pressure of 1 bar (absolute).

I initially did a run (Steady State - no Cavitation) with a smaller valve and everything seemed reasonable (reasonable expected massflow and the velocity profile was similar to what I expected and to literature).

With a larger valve, however, I noticed an unrealistic exit velocity profile and large zones with negative (absolute) pressure. As a result I turned on cavitation (water + water vapor) with steady state. With cavitation I get however:

- At the outlet of the discharge pipe (opening type, that allows only inflow of water) an inflow/backflow of water. There is no mass flow going out of the discharge pipe, hence large mass imbalances;
- Very large zones of water vapor (cavitation), especially near the outlet. The amount of water vapor does seem excessive.

I tried turning on total energy, but that did not change anything.

I searched online and in this forum and often I read that the backflow with cavitation can be solved by:
- Setting a lower opening pressure, or;
- Performing a transient simulation;
- I read that putting the outlet further downstream often does not work / resolve the problem.

The problem that I see with a transient simulation is that my model is quite large (discharge tube has a diameter of 2 meters and 8 meters long. The valve is relatively small. Element size 8-12 mm. In total I have 100 million elements. The flow velocity is locally very high, especially at the valve opening speeds can go above 50 m/s) and I read that for cavitation you need small elements and very small timesteps.

So to summarize:
- Does my backflow problem stem indeed from the fact that for accurate simulation of the cavitation you need a transient simulation?
- Using transient flow, I presume I should use adaptive time step. What is a suggested minimum time step or at least starting point for my time step for cavitation (I read someone posting 1E-6 sec, that seems very low)? The total flow-through time of the valve + discharge tube is around 3 seconds.
- Does it make sense to increase the element size (thus reducing the total amount of elements) to be able to run a simulation within a reasonable time? As increase the element size, reduces the accuracy of capturing caviation. Also the valve opening is relatively small, thus I need at least local refinement to capture the flow accurately.

I ran the 100 million steady state model with 100 iterations in 15 hours, so my computer is quite capable of handling a bit of calculations.

I hope I gave enough information. In case something is missing or needs more explanation, please let me know!
WilbertS is offline   Reply With Quote

Old   April 3, 2018, 18:53
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,690
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Why do you say increasing the length to the outlet does not work? That is the normal way to respond to these issues and it usually does work (or at least help).

Only run a transient simulation if you are sure it is transient as they are very expensive for simulations like this.

Your mesh size should be determined by a sensitivity study where you look at the accuracy you require. You should not determine mesh size based on your opinion of what the run time should be.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   April 4, 2018, 04:23
Default
  #3
New Member
 
Wilbert S
Join Date: Mar 2018
Posts: 3
Rep Power: 8
WilbertS is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Why do you say increasing the length to the outlet does not work? That is the normal way to respond to these issues and it usually does work (or at least help).

Only run a transient simulation if you are sure it is transient as they are very expensive for simulations like this.

Your mesh size should be determined by a sensitivity study where you look at the accuracy you require. You should not determine mesh size based on your opinion of what the run time should be.
Thank you for your response.

- I read many posts / threats and almost all stated that simply increasing the length of the outlet did not help for cases with cavitation. That is why I dismissed it. For normal cases indeed, the first step for stabilizing the outlet condition is increasing the outlet length. Furthermore, due to the large outlet diameter, it is very computational expensive to make the outlet longer (although larger elements can be used towards the end of course).

- Exactly, I fear that in the limited time I have a transient run is not possible.

- I maybe posted it a bit crude, but initially I started with a coarser mesh, 1-10 million elements, to find the right opening grade of the valve. Then I used a finer mesh to see the sensitivity of the study and see how much the outlet mass flow changed (that is my most important variable). However, also with the cavitation model I keep having large mass imbalances (as a result of the backflow) Moreover due to very high local speeds (small first inflation layer), small openings and large size of the model I ended up with a lot of elements.

I also feared that using too large elements would not be able to capture the cavitation. I will try at least start a simulation with less elements and with a longer outlet to see what happens and then start to refine.
WilbertS is offline   Reply With Quote

Old   April 4, 2018, 06:12
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,690
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Well I can say if you have large sections of cavitated flow on your outlet boundary you are going to have a hard time converging. Before dismissing it out of hand you really should try it.

You will find that obtaining mesh convergence for a cavitation model will be challenging. If you have obtained an adequate mesh for a single phase model then that is a very good start for a cavitation model.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   April 4, 2018, 09:22
Default
  #5
New Member
 
Wilbert S
Join Date: Mar 2018
Posts: 3
Rep Power: 8
WilbertS is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Well I can say if you have large sections of cavitated flow on your outlet boundary you are going to have a hard time converging. Before dismissing it out of hand you really should try it.

You will find that obtaining mesh convergence for a cavitation model will be challenging. If you have obtained an adequate mesh for a single phase model then that is a very good start for a cavitation model.
Thank you again for your suggestion. Tomorrow I'll do some trials, it can't hurt!
WilbertS is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Transient simulation of flow through a valve aadit.shroff Main CFD Forum 12 April 12, 2018 04:20
Convergence cavitation simulation anon_p CONVERGE 2 May 16, 2017 09:32
Species transport simulation problems AWalmsley11 FLUENT 0 September 8, 2013 11:44
Centrifugal Pump Cavitation problem or not. ismael.s CFX 13 February 27, 2012 08:00
Problems about multi swirl tubes simulation using Jason FLUENT 4 May 9, 2008 00:33


All times are GMT -4. The time now is 21:45.