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maria teresa September 27, 2007 17:55

reversed flow
 
hi,

iīm 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

Sham September 27, 2007 19:14

Re: reversed flow
 
Usually reverse flow will go away after some iterations. If not try changing your BC.

bart weisser September 28, 2007 16:52

Re: reversed flow
 
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

maria teresa September 29, 2007 16:28

Re: reversed flow
 
how do i change the heigth-to-width aspect ratio???

where???

thank you!

maria teresa September 29, 2007 18:47

Re: reversed flow
 
hi,

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

in your example wouldnīt 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????


K.Baker September 29, 2007 22:29

Re: reversed flow
 
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


Bart Weisser September 30, 2007 13:55

Re: reversed flow
 
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.

maria teresa October 2, 2007 04:09

Re: reversed flow
 
hi,

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

what do i do in that case?

bart weisser October 2, 2007 18:41

Re: reversed flow
 
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.

maria teresa October 3, 2007 21:42

Re: reversed flow
 
i donīt know what you mean with a reasonably sized section, can you explained to me a little bit more please.

bart weisser October 4, 2007 15:42

Re: reversed flow
 
Woudl you please show us what your current solution domain is like? BW.

maria teresa October 4, 2007 17:35

Re: reversed flow
 
Iīm sorry, i donīt know how to uplode something to this page, can you tell me how??? iīm really sorry for the trouble


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