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

Time needed to make the flow steady again

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 17, 2020, 23:53
Default Time needed to make the flow steady again
  #1
New Member
 
Muhammad Alfatih
Join Date: Feb 2020
Posts: 5
Rep Power: 3
Fatih14 is on a distinguished road
Hi all,
I want to simulate a transient water vapor pipe flow that the mass flow inlet is changed at a certain time. I have done the simulation with the boundary condition: mass-flow-inlet using UDF to change the mass flow, and pressure outlet. I want to know the time that needed to make the mass flow at the outlet is equal again with the inlet. I have plotted the mass flow at the outlet at each time, but the time that needed is smaller than it should. The average velocity is around 20 m/s and the length of the pipe is 100 m, so the time needed should be around 5 s, but based on my plot, it is only 0.3 s. What's wrong with my simulation?
Thank you
Fatih14 is offline   Reply With Quote

Old   February 18, 2020, 14:52
Default Compressible or incompressible
  #2
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 32
vinerm will become famous soon enough
Are you modeling the vapor as compressible phase? If not, then the results are as expected. For an incompressible fluid, speed of sound is ideally infinite, i.e., any disturbance is transferred almost immediately. Now, a tool does not know that but it also does not know the speed at which it is to be transferred. Hence, if the objective is to predict a behavior, such as, gas filling in a container, transient flow boundary, etc., fluid has to be considered compressible.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   February 18, 2020, 19:17
Default
  #3
New Member
 
Muhammad Alfatih
Join Date: Feb 2020
Posts: 5
Rep Power: 3
Fatih14 is on a distinguished road
Hi Vinerm
The vapor in my simulation was compressible. I used real gas Redlich Kwong for the density of the fluid
Fatih14 is offline   Reply With Quote

Old   February 19, 2020, 05:33
Default Check SoS
  #4
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 32
vinerm will become famous soon enough
Check the Speed of Sound for your system then. Information does not travel at the speed of flow rather at the speed of sound. Take ratio of length and SoS and that should give you an estimate of time required.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   February 19, 2020, 05:44
Default
  #5
New Member
 
Muhammad Alfatih
Join Date: Feb 2020
Posts: 5
Rep Power: 3
Fatih14 is on a distinguished road
How to check the speed of sound of my system?
Fatih14 is offline   Reply With Quote

Old   February 19, 2020, 05:53
Default Fluent calculates it
  #6
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 32
vinerm will become famous soon enough
This is calculated by Fluent and you can check it in Contour plots or use volume reporting. This is available under Properties. You can also check Mach number under Velocity and then take ratio of velocity and Mach number.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   February 19, 2020, 06:11
Default
  #7
New Member
 
Muhammad Alfatih
Join Date: Feb 2020
Posts: 5
Rep Power: 3
Fatih14 is on a distinguished road
Ok I know that the mach number is around 0.05 and I can calculate the speed of sound. But how can I know that the fluid travel at the speed of sound?
Fatih14 is offline   Reply With Quote

Old   February 19, 2020, 06:40
Default Fluid Velocity
  #8
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 32
vinerm will become famous soon enough
Fluid travels at the speed you want it to. That's as per the conditions specified at the boundary. If Mach is 0.05, then fluid is traveling at 5% of the speed of sound. But the information in the fluid travels at the speed of sound and not at the speed of flow. If you change pressure at any boundary, it would be felt at another point depending upon the speed of sound and not based upon the speed of fluid. Otherwise, the information will never travel through a static fluid. Do note, this is true only for pressure and not for temperature or species.
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Old   February 19, 2020, 07:23
Default
  #9
New Member
 
Muhammad Alfatih
Join Date: Feb 2020
Posts: 5
Rep Power: 3
Fatih14 is on a distinguished road
Sorry, I can't understand what you mean, can you explain it more?
Fatih14 is offline   Reply With Quote

Old   February 19, 2020, 07:28
Default Article
  #10
Senior Member
 
vinerm's Avatar
 
Vinerm
Join Date: Jun 2009
Location: Nederland
Posts: 2,946
Blog Entries: 1
Rep Power: 32
vinerm will become famous soon enough
Though you can go through a lot of literature on compressible flow but I just searched and found this one to be rather simple one for explanation of what is happening in your case

http://thermopedia.com/content/646/
__________________
Regards,
Vinerm

PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.
vinerm is offline   Reply With Quote

Reply

Tags
pipe, time, transient 3d

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
Issues on the simulation of high-speed compressible flow within turbomachinery dowlee OpenFOAM Running, Solving & CFD 9 February 5, 2021 10:15
Moving mesh Niklas Wikstrom (Wikstrom) OpenFOAM Running, Solving & CFD 122 June 15, 2014 06:20
mixerVesselAMI2D's mass is not balancing sharonyue OpenFOAM Running, Solving & CFD 6 June 10, 2013 09:34
compiling firefoam Farshad_Noravesh OpenFOAM 27 December 24, 2012 04:21
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 04:03


All times are GMT -4. The time now is 15:17.