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Old   October 8, 2012, 04:23
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Ham Ta
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Hi everybody,
See the attached file, I have a sloped solid body, when I favorize the model with inefficient mesh size. I see the stepped slope and itís not good, I know about FAVOR action, but how I can improve my model (except increasing mesh).
Thanks
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Old   October 8, 2012, 07:06
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mderosca
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Hello,

As I know is just visualization. Favor algorithm creates a plane in each cell to determinate the surfaces, but when makes the render don't shown the planes and apears littles steps.

So run the model and you will see no problems.
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Old   October 8, 2012, 10:16
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Ham Ta
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dear mderosca
I think your answer is not correct, after I run the simulation still have problems, please check it and tell me if i wrong.
so many thanks
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Old   October 9, 2012, 03:29
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Hi,

For such a simple case you can make the geometry horizontal and change the gravity instead. Just a workaround for simple geometries, otherwise gridrefinement and a simulation restart.
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Old   October 9, 2012, 05:04
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Ham Ta
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dear robert_cfd,
thank you for answer
But my model is so complicated (Ogee spillway with chute)
even use 2 million volume still I have step in my model that is bad influence on my real model.
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Old   October 9, 2012, 07:12
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Ok, I see. Is the spillway straight? Just the spillway, not the whole powerplant? When I look at your attached picture, you might also use symmetry or even just one cell in width (quasi 2D). Then you can use more cells without loosing time.
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Old   October 9, 2012, 17:37
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Ham Ta
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I have 3D model and you can see the whole 3d solid in attached file.
with 2 million mesh still I have problem,
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Old   October 10, 2012, 08:16
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For me it looks like you could test
-how it works with one cell in y direction
-or rotate the geometry around y axis till the flat main part is horizontal, using the angle for calculation of the x and z component of the new gravity field. Then you only have to take care of the in- and outflow area with a finer grid.
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Old   October 17, 2012, 04:49
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Daniel Valero
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Dear Hammed,
in my experience, you would be impressed with results. Maybe you don't expect such good ones.. I would launch some 2D simulations only to prove it.
But maybe you have tried yet. If you are asking for something like unstructures body fitted meshes you won't find it. Just give it a chance

Quote:
Originally Posted by Hamed&honey View Post
I have 3D model and you can see the whole 3d solid in attached file.
with 2 million mesh still I have problem,
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Old   October 17, 2012, 05:39
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Ham Ta
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dear Davahue
I have investigating cavitation on spillway chute, and we know that the rough surface can be increase cavitation index, I have straight slope in real, but stepped slope in model. is this ok? I think its not going to accurate result. am I wrong?
Thanks
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Old   October 17, 2012, 10:47
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Daniel Valero
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You are wright,
but I suppose you are using a RANS model, so the way it faces the boundary layer has nothing to do with what you are thinking (from a classical hydraulical point of view). I would recommend you to read about wall functions in David Wilcox's (or others) or the FlowScience Technical Note about Roughness. The aproximation all RANS turbulence models do about the surface roughness is very important, if you take it into account, you won't be so demanding on surface resolution. I mean, for a numerical model of N-S equations to reflect exact gradients nearby the surface would need thousands of cells in the boundary layer. If you have not that resolution, that artificial roughness created by FAVOR, in my opinion, is not afecting definitively the main flow because the model doesn't catch it.

I'm not an expert, but i'm sure Michael Barkhudarov or many other experts in this forum could tell you if i'm wright or wrong or how to face your problem better. Anyway, try it and compare the results with experimental data.

Hope it helps,
D.

QUOTE=Hamed&honey;387049]dear Davahue
I have investigating cavitation on spillway chute, and we know that the rough surface can be increase cavitation index, I have straight slope in real, but stepped slope in model. is this ok? I think its not going to accurate result. am I wrong?
Thanks[/QUOTE]
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Old   October 17, 2012, 11:48
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Ham Ta
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dear Davahue
I think you are an expert
in fact, basic problem is about velocity,
Those steps are very effective on velocity
cause to reduction.
I compare with experimental model.

velocity at the jump location
Exp=25 m/s
CFD=20 m/s
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