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ditto May 2, 2003 02:41

aspect ratio
 
halo friends,

I have a model of a typical valve meshed in CFX- 5.5.1 in which the aspect ratio shoots high as upto 830. I tried to reduce this as much as possible.But i want to know the effect of this high aspect ratio elements when we are solving it.since my flow path being complex i am unable to reduce the ratio further .Any help in sorting it out , please?

- DITTO

Robin May 3, 2003 22:13

Re: aspect ratio
 
Hi ditto,

If the high aspect ratio elements are aligned with the flow, such at along a wall, the effect may not be significant. If the flow is not aligned, you may run into local convergence difficulties and the accuracy in this region will be somewhat reduced. Just how much will be problem dependent.

Robin

ditto May 3, 2003 23:55

Re: aspect ratio
 
halo robin,

The problem i have is about the flow of steam through a valve where it looses its pressure and temperature by mixing with water.so now how much the high aspect ratio elements will affect this?.Moreover in cfx upto what aspect ratio we can proceed?. Thank you for your reply.

- ditto

Robin May 5, 2003 10:16

Re: aspect ratio
 
Without seeing the model, anything would be a guess. I suggest contacting your support rep for help.

Robin

ramkumar May 6, 2003 06:27

Re: aspect ratio
 
hi ditto,

Even though there is a thumb rule that the aspect ratio of unstructured mesh must be less than 5 and for structured mesh it can be upto 200, in solving typical high aspect ratio problems such as tunnels, etc which are very long compared to diameters cfd experts have comfortably used aspect ratio as high as 400 so i think ,it wont affect much but for some convergence problems which you can overcome by proper selection of timestep.

Robin May 6, 2003 12:09

Re: aspect ratio
 
Hi Ramkumar,

...in solving typical high aspect ratio problems such as tunnels, etc which are very long compared to diameters cfd experts have comfortably used aspect ratio as high as 400...

This is only because the flow is aligned with the grid, which may not be the case here.

...there is a thumb rule that the aspect ratio of unstructured mesh must be less than 5 and for structured mesh it can be upto 200...

This is a good point, but should not be generalized to structured or unstructured. Rather, it has to do with element type (and really more to do with skew than aspect ratio). You can have a high aspect ratio hex or prism element which still has reasonable skew, however if you stretch a tet element, the skew will always be poor.

Ditto: what element type are your high aspect ration elements? How is the skew angle?

Regards, Robin

ditto May 8, 2003 23:51

Re: aspect ratio
 
halo robin,

I am using unstructured tet element for the model.As you asked the maximum face angle is 172 and minimum face angle is 0.6.so how to go about?

- ditto

Robin May 11, 2003 11:19

Re: aspect ratio
 
Ditto,

A minimum face angle of 0.6 degrees will cause problems. Your best bet is to go back to the mesher and add mesh controls in this region, or modify your geometry to improve the mesh quality.

Robin


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