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Problem with Scale in Geometry

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Old   October 13, 2015, 14:04
Default Problem with Scale in Geometry
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Hamidreza Bijanyar
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Hello.
I have a new problem that is so strange.
I have a 120 inch Butter-fly valve.
I should study valve is size of 20 inch to 120 inch and find a difference in flow properties.
I use Scale function in geometry (Create --> Body Transformation --> Scale) to change the size of the valve.
There was no problem until.
After running and getting result the result of 20 , 30 , 40 , ... valve is too similar to the 120 inch valve !!!!!!
WHAT IS THE PROBLEM ??
I have checked the cfx-post and in function calculator the inlet area is correct for all size and there is no problem.
I didn't change the setup cell and boundary condition and result expression.
How can it happen ??
I attached the result file.
Thanks to your really important helps.
Attached Files
File Type: pdf Sigma i.pdf (40.8 KB, 6 views)
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Old   October 13, 2015, 20:21
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Glenn Horrocks
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That suggests in the range of Reynolds numbers you tested the Cd does not change much. This is quite normal. You will need to go to a Reynolds number where the flow changes significantly to get a different Cd.
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Old   October 14, 2015, 04:50
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Hamidreza Bijanyar
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I used 105,000 to 300,000 pa for input pressure and 101325 pa for outlet pressure since after valve flow goes to the atmosphere.
For all 12 inch to 120 inch, I used these pressure boundary.
The Reynolds number change since the pipe diameter change.
For Example for 10 degree opened valve it change from 2,732,771 for 12 inch valve to 29,074,178 for 120 inch valve.
How can i find that Reynolds number you mentioned ?
And what pressure range should i use ?
I want to find the change in incipient cavitation related to valve size.
Thank you for your help.
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Old   October 14, 2015, 07:33
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Glenn Horrocks
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Reynolds number is just a number to tell you what flow regime the flow is in. Low Re = laminar, high = turbulent; and for some flows it tells you details about the flow (such as around a cylinder it tells you what the wake is likely to be).

But I have no idea what are the important Re numbers in your flow. Flat plates, pipe flow, cylinders, spheres and things like that are well known; but other flows you have to work it out for yourself.
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Old   October 14, 2015, 07:53
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Hamidreza Bijanyar
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I have no idea what to do for this.
I know from tullis formula that if we decrease the size of a valve it's starting cavitation number will decrease too.
I chose pressure range from 105000 to 300000 since in this pressure range cavitation start on the valve.
I only want to find SSE ( Size Scale Effect ) in my valve.
It means in a fixed shape valve by changing size, cavitation number will change. by holding all other parameter constant.
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