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Old   September 27, 2007, 16:55
Default reversed flow
  #1
maria teresa
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hi,

im 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
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Old   September 27, 2007, 18:14
Default Re: reversed flow
  #2
Sham
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Usually reverse flow will go away after some iterations. If not try changing your BC.
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Old   September 28, 2007, 15:52
Default Re: reversed flow
  #3
bart weisser
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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
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Old   September 29, 2007, 15:28
Default Re: reversed flow
  #4
maria teresa
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how do i change the heigth-to-width aspect ratio???

where???

thank you!
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Old   September 29, 2007, 17:47
Default Re: reversed flow
  #5
maria teresa
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hi,

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

in your example wouldnt 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????

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Old   September 29, 2007, 21:29
Default Re: reversed flow
  #6
K.Baker
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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

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Old   September 30, 2007, 12:55
Default Re: reversed flow
  #7
Bart Weisser
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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.
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Old   October 2, 2007, 03:09
Default Re: reversed flow
  #8
maria teresa
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hi,

well im modeling an axial ompressor rotor blade, i dont know if i have to expect reverse flow.

what do i do in that case?
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Old   October 2, 2007, 17:41
Default Re: reversed flow
  #9
bart weisser
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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.
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Old   October 3, 2007, 20:42
Default Re: reversed flow
  #10
maria teresa
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i dont know what you mean with a reasonably sized section, can you explained to me a little bit more please.
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Old   October 4, 2007, 14:42
Default Re: reversed flow
  #11
bart weisser
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Woudl you please show us what your current solution domain is like? BW.
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Old   October 4, 2007, 16:35
Default Re: reversed flow
  #12
maria teresa
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Im sorry, i dont know how to uplode something to this page, can you tell me how??? im really sorry for the trouble
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