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Is it possible to find position of separation point in CFX?

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Old   September 28, 2016, 13:19
Default Is it possible to find position of separation point in CFX?
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Hi,

The following figure is tangential velocity contour in a turbine. Now i want to find exactly position of separation point in that. Is it possible? How?



Thanks.

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Old   September 29, 2016, 01:04
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If you plot vectors of velocity you are going to find that vectors turn in the other direction (forward) near the wall, Than you can pinpoint where backward facing flow and forward facing flow meet eachother and some kind of stagnation point with almost no welocity is present this is the flow seperation point.
That is one way of doing it, some other way includes inverse pressure gradients but I am not familiar with those.
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Old   September 29, 2016, 01:23
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Or check the wall shear stress component in the flow direction on the wall.
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Old   September 29, 2016, 02:05
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Hi Lance, that is a good one I havent thought about that one, thanks.

It probably reaches 0 or some lowest walue where seperation ocures, corect?
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Old   September 29, 2016, 03:48
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Thanks.

I think that there are not wall shear stress components in CFX! But there are wall shear x,y,z in CFX. Where does wall shear stress components in CFX?

I used turbo charts for this case as follows:



Is it correct?

The turbine rotates in direction of -X ( according to right hand rule, it rotates in direction of +Y) as follows and inlet velocity is in direction of Y.



According above explanations, how can i find separation point in my case exactly?

Last edited by aja1345; October 3, 2016 at 16:05.
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Old   September 29, 2016, 05:20
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Separation occurs theoretically at the point where is zero. So, if i have a graph, i can find this point easily. How can i plot graph in CFX?

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Old   September 29, 2016, 06:12
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Hi again,

I plotted Velocity .Gradient as follows:



Is it correct for separation point?

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Old   September 29, 2016, 06:17
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i have to identify point corresponding to the condition d=0. This comes from shear stress value being zero at that point. Therefore, Is above post correct?

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Old   September 29, 2016, 06:22
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In the following figure, There is not zero value in Velocity .Gradient but there is lowest value near to zero.....in this position, is there separation point?

Last edited by aja1345; October 3, 2016 at 16:06.
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Old   September 29, 2016, 07:53
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I was wrong!

I think that i should apply velocity gradient . The plot is as follows:

This plot is clearer! and separation point is clear in plot. separation point is near to 0.m.

Is it correct?

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Old   September 30, 2016, 04:58
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No-one guide?
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Old   September 30, 2016, 07:18
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Where is the problem and why are you posting so meny graphs here on this thread? I had a feeling that you were talking to yourself or diferent topic in the last 7 posts.

Why dont you listen to Lance and me.

If you want to pinpoint the seperation point you can do what we told you to, days ago.

Lances (shear) aproach and my (vector) aproach both are shown in these three figures:

1 http://https://drive.google.com/open?id=0Bw9ueWZA9k_QOGpVS0tHMDZldW8
2 zoomed in - http://https://drive.google.com/open?id=0Bw9ueWZA9k_QcTd2eWNoQU94eDA
3 zoomed in further - http://https://drive.google.com/open?id=0Bw9ueWZA9k_QUXVTTE9TV2tZTzQ

Both aproaches show wery good agrement with eachother.
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Old   September 30, 2016, 07:47
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Hi urosgrivc,

Thanks for your answers, but i don't want to discomfiture you.

In fact, I want to know that approach of post 10 is OK or not?

I want to learn different approach in this case, urosgrivc. I would like all of the ideas discussed here.

however, I am grateful that allocating your time for me.

Thanks.
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Old   October 4, 2016, 04:41
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And a further complication is that separation points can be exactly defined in 2d flows but there is not a general definition of separation point (or even separation line) in 3d flows. If your flow is 3d you might be able to define something, but it will not be universal for all flows.
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