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

reversed flow

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 27, 2007, 17:55
Default reversed flow
  #1
maria teresa
Guest
 
Posts: n/a
hi,

i´m having trouble modeling an axial compressor rotor.

i begin the simulations without speed, and i have tried to increase it little by little, but everytime reversed appears after some iterations,

how do i fixed it?????

thank you so much, maria teresa
  Reply With Quote

Old   September 27, 2007, 19:14
Default Re: reversed flow
  #2
Sham
Guest
 
Posts: n/a
Usually reverse flow will go away after some iterations. If not try changing your BC.
  Reply With Quote

Old   September 28, 2007, 16:52
Default Re: reversed flow
  #3
bart weisser
Guest
 
Posts: n/a
If your solution domain is initially quiescent, and you are experiencing problems with outlet reversed flow after a large number of iterations, I believe your solution domain is not long enough to allow the recirculation to be modelled inside the domain. This usually happens quite a bit in vertical open channel natural convection simulations.

The height-to-width aspect ratio will, of course, depend on the Rayleigh number, but I usually use (from previous experience) an aspect ratio between 10 to 20, and I have no reverse flow problems.

BW
  Reply With Quote

Old   September 29, 2007, 16:28
Default Re: reversed flow
  #4
maria teresa
Guest
 
Posts: n/a
how do i change the heigth-to-width aspect ratio???

where???

thank you!
  Reply With Quote

Old   September 29, 2007, 18:47
Default Re: reversed flow
  #5
maria teresa
Guest
 
Posts: n/a
hi,

i think that is the problem, but i have a doub...

in your example wouldn´t the aspect ratio be 1?, because 1000/20= 50, and 500/10=50, so 50/50=1

my domain is 87x288 mm

axial direction has 8 nodes in the 87 mm, with succesive ratio1: 1.352 and ratio2:1

radial direction has 10 nodes in the 288 mm, with succesive ratio of 1

can you help me??? how do i do it with a succecsive ratio diferent than 1????

  Reply With Quote

Old   September 29, 2007, 22:29
Default Re: reversed flow
  #6
K.Baker
Guest
 
Posts: n/a
Hi The aspect ratio is you have to calculate manually when u genrate the mesh in gambit. suppose if u have a domain of (1x0.5)m) . lenght is 1 m domain and have 20 nodes in the axial direction with successive ratio as 1, each cell will be 50 mm in length in axial direction. now in the radial direction if u have 10 nodes defined with successive ratio of 1,then each cell widht will be 5. the aspect ratio will become 50/5 = 10. this aspect ratio should not be more than 20.

Regards K.Baker

  Reply With Quote

Old   September 30, 2007, 13:55
Default Re: reversed flow
  #7
Bart Weisser
Guest
 
Posts: n/a
Hello:

The aspect ratio I am referring to is the geometric aspect ratio. Is your geometry confined (as in, flow confined between two non-infinite parallel plates), or is it a simple free channel flow. From your previous message I gathered that the plate sizes are fixed. Is there another way to reformulate the problem so that this aspect ratio cant be more flexible (e.g., include sections outside of your plates into your solution domain).

Another thing, is that from this geometry, are you expecting reversed flow (just think about the problem for a second). If not, I think you may want to review your boundary conditions.

As for mesh aspect ratio, while it will affect the general trend your solution little (unless your grid density is very low), I don't think you need to worry about it too much.

BW.
  Reply With Quote

Old   October 2, 2007, 04:09
Default Re: reversed flow
  #8
maria teresa
Guest
 
Posts: n/a
hi,

well i´m modeling an axial ompressor rotor blade, i don´t know if i have to expect reverse flow.

what do i do in that case?
  Reply With Quote

Old   October 2, 2007, 18:41
Default Re: reversed flow
  #9
bart weisser
Guest
 
Posts: n/a
Hello again:

Based on what you have told us about the nature and geometry of the problem, I believe you would expect a recirculation zone on the back side of the blade (i.e., the side facing away from the incident flow). So yes, if the solution domain is limited, you are likely to experience reverse flow along the boundary.

To circumvent this, you may have to incorporate a reasonably sized sections upstream and downstream of the rotor blade, where recirculation is less likely to take place at the extended boundaries. It does not really affect your experimental setup (if you have one), but this is really a numerical construct. Let me know if this is an acceptable solution.

BW.
  Reply With Quote

Old   October 3, 2007, 21:42
Default Re: reversed flow
  #10
maria teresa
Guest
 
Posts: n/a
i don´t know what you mean with a reasonably sized section, can you explained to me a little bit more please.
  Reply With Quote

Old   October 4, 2007, 15:42
Default Re: reversed flow
  #11
bart weisser
Guest
 
Posts: n/a
Woudl you please show us what your current solution domain is like? BW.
  Reply With Quote

Old   October 4, 2007, 17:35
Default Re: reversed flow
  #12
maria teresa
Guest
 
Posts: n/a
I´m sorry, i don´t know how to uplode something to this page, can you tell me how??? i´m really sorry for the trouble
  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
reversed flow free FLUENT 4 February 27, 2014 02:56
reversed flow during RNG in turbulence-k epsilon Suman Kandula FLUENT 4 April 2, 2012 08:12
Reversed flow...>>>Please HELP...! sushii FLUENT 14 October 29, 2007 05:27
reversed flow at velocity inlet / mass flow inlet ib FLUENT 1 March 26, 2007 14:11
reversed flow Sastry FLUENT 5 March 22, 2007 09:05


All times are GMT -4. The time now is 08:02.