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

cavitation

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 29, 2001, 04:12
Default cavitation
  #1
Perelli Maxime
Guest
 
Posts: n/a
Hi, Does anybody use the cavitation model from Star-CD? Because my work goes on a hydraulics valve with 2 pressure boudaries (one for the inlet with a pressure about 500 bar, the second for the outlet with a pressure of 1 bar), but as I do a steady calculation: first I don't have the convergence; second I have crazy values ( velocities about 2000 m/s!!! and negative absolute pressure!) So I tried to turn on the cavitation model and followed the manual's instructions but the calculations don't seem to start! Could anybody help me? Thanks

Maxime
  Reply With Quote

Old   May 29, 2001, 11:54
Default Re: cavitation
  #2
Chinor
Guest
 
Posts: n/a
The last time I used the Cavitation model you had to run Transiently (not Steady State). You can use the steady state solution as a starting point for the time dependent solution. However, you need a decent steady solution first.

The negative absolute pressure is a good indicator that you will see some cavitation.

The 2000 m/s is not a good thing. Check your cell quality (warpage, internal angles, etc..)
  Reply With Quote

Old   May 31, 2001, 03:34
Default Re: cavitation
  #3
MvD
Guest
 
Posts: n/a
Negative densities and high velocities are quite usual in high pressure flows in hydraulic systems. You can check you values by a simple estimation using the Bernoulli-quation. Unusual high velocities can also occure due to coarse or inadequate grid structures near sharp edges.

The cavitation model can only be used in transient mode. Be sure to use the latest version; earlier version had some bugs. Sometimes restarts from single phase solution are not converging because StarCD does not make a real restart, but only uses some initial values. It could be necessary to start with a lower pressure difference and then use a ramp to reach the high pressure inlet condition. The default value for the cavitation model to set in is 80 time steps which is usually too low. If the model does not set in, you have probably messed up your load step definitions. You also should use the cavprop user-function and check the values for vapour pressure and velocity of sound. Subcyling has to be turned on, a maximal value of 10 cycling steps shoud be enough. The BTF model is quite unstable so you have to live with the barotropic model. Surface tension values have to be set in rcon 187. The model sets its own time step ! The actual time step can be calculated by using the postdat user-function.

Good luck.

Michael
  Reply With Quote

Reply


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
cavitation konstantin Main CFD Forum 0 March 10, 2008 08:45
cavitation dc FLUENT 5 March 8, 2008 23:38
UDF and Cavitation amir FLUENT 3 December 14, 2004 03:04
Cavitation Maniram CFX 1 July 13, 2004 16:00
cavitation Hans Phoenics 1 March 31, 2003 08:41


All times are GMT -4. The time now is 09:23.