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Liam September 1, 2005 08:27

Tip Clearance
 
I am having a few problems generating a grid with tip clearance in CFX Turbogrid. Whenever I export a set of Turbogrid input files from BladeGen and import them into Turbogrid it seems as though Turbogrid has trouble detailing the blade tip at the shroud. I have tried various blade geometries and in some cases it seems to add an extra piece of material to the leading edge at the shroud and in other cases it seems to remove a piece of material. I have attached a word file containing a picture of an example were it seems to have removed some material. I am unable to generate a grid with tip clearance without it containing a very low skew angle.

Any help would be greatly appreciated.

Kind Regards


Darcy September 5, 2005 12:37

Re: Tip Clearance
 
You have to offset your shroud curve outward in order to use tip clearance. The distance to offset equals the clearance you want to have.

new@cfx September 7, 2005 04:54

Re: Tip Clearance
 
I have experienced similar problems. I think the Turbogrid data import routine is not at all robust. You can try adding more points in bladegen to the place where you find the problem is occuring, e.g., leading edge or trailing edge whichever the case may be. This sometimes helps. also look at what interpolation you are using for the blade curves. make sure that it is set to linear. that ensures accuracy in case your blade has sharp trailing edges. try setting all interpolation types to linear. its the safest type of interpolation and will let you know immediately if the problem is with the way Turbogrid reads those points or whether the problems is with interpolation.


Liam September 7, 2005 05:30

Re: Tip Clearance
 
Thank you very much for your reply. After one very long week of working on this I finally got a decent grid created in TurboGrid; minimum skew angle was 31 degrees. I added an extra control curve at 98% of the blade height, close to where the problem was. I also added a control curve on the shroud of the casing. This gave me a bit more control over the grid within the tip clearance region as there was now a control curve on the rotor shroud and the casing shroud. If you are going to add a shroud curve to lie on the casing then make sure that you select the 'Profile at shroud provided' from the Clearance options in the Template menu. I also found that slighlty lowering the number of cells within the tip clearance region increased the minimum skew angle. However, the number of cells within the tip clearance region must not be too low otherwise the secondary flows created form tip leakage will not be capture in enough detail.



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