[kenzosenzo]

Hey guys,
i'm newbie and i hope i post in the correct section. i'm right now try to study the mass flow rate of air at the outlet of a diffuser. We know that to get mass flow rate, we need the average of normal velocity multiply to its cross sectional area.

my problem is,
how to get air velocity magnitude and direction with respect to surface area (at a certain point)?

i know fluent can produce a vector contour,
but can i get to the database of the velocity magnitude and direction (for each point)?

thx

-kenzo-

[Zaktatir]

Yes you can access them by UDF but I dunno why you need that!

[flotus1]

Would it not be easier to take the mass flow rate calculated by CFD Post?
This way, you wont even get into trouble when the incompressibility assumption in the formula you described breaks down.

[kenzosenzo]

Originally Posted by Zaktatir

Yes you can access them by UDF but I dunno why you need that!

Can you please explain more about UDF? My study is actually involve in the accuracy by comparing cfd method with anemometer instrument. So i need cfd data to find velocity magnitude and direction to compare with anemometer reading.

At the end i'll produce correction factor

[kenzosenzo]

Originally Posted by flotus1

Would it not be easier to take the mass flow rate calculated by CFD Post?
This way, you wont even get into trouble when the incompressibility assumption in the formula you described breaks down.

That's one of the thing,
But my study need to know the velocity magnitude and direction at a certain point. Because i need to compare cfd method with anemometer gauge(where anemometer detect velocity at certain place).

[Zaktatir]

then you do not need UDF (own subroutines)

You can plot a line/surface and then post-process the velocity profile along this line for example

[kenzosenzo]

Originally Posted by Zaktatir

then you do not need UDF (own subroutines)

You can plot a line/surface and then post-process the velocity profile along this line for example

i manage to plot line and surface in the post processing
but i still cant figure out how to get the angle and direction of the velocities.
i hope these pictures will help.



the last picture is the angle that i need in my study,
does the post processing can produce this?
or any idea how can i get it?

[flotus1]

So the angle you are searching for is the one between the positive x-axis and the flow in z-direction?

In this special case, you should be able to create an expression like

atan2(Velocity w,Velocity u)

Then you can create a variable from this expression that can be post-processed directly in CFD-Post.
If this is not exactly the angle you are searching for, you can surely find the correct adjustments to the formula.

[kenzosenzo]

Originally Posted by flotus1

So the angle you are searching for is the one between the positive x-axis and the flow in z-direction?

In this special case, you should be able to create an expression like

atan2(Velocity w,Velocity u)

Then you can create a variable from this expression that can be post-processed directly in CFD-Post.
If this is not exactly the angle you are searching for, you can surely find the correct adjustments to the formula.

thank you so much for the reply,
i'll try to use that code.

But the exact case is the angle at the surface of my diffuser in each blade (red arrow). I'll provide you this pic to be clear. So basically for a special case, i need to create an expression or code?

Thank you again.

[flotus1]

If you add a coordinate system to your last picture you will be much closer to a solution.

If the directions do not coincide with the main cartesian directions, you can still add a custom coordinate system.

[kenzosenzo]

Originally Posted by flotus1

If you add a coordinate system to your last picture you will be much closer to a solution.

If the directions do not coincide with the main cartesian directions, you can still add a custom coordinate system.

sorry for late reply,
i'm a bit confuse to use your given code.
can you explain to me more clearly?

i mean the basic things to perform that code. Is it udf?
i'm really new to this.

[flotus1]

First you need to create an expression:
expression.png

To make the result of the expression available for post-processing, you need to convert it to a variable:
variable.jpg

Then you can use it just like any other variable to do the post-processing:
contour.jpg

You should at least do some of the basic tutorials.

[kenzosenzo]

Originally Posted by flotus1

First you need to create an expression:
Attachment 28606

To make the result of the expression available for post-processing, you need to convert it to a variable:
Attachment 28607

Then you can use it just like any other variable to do the post-processing:
Attachment 28608

You should at least do some of the basic tutorials.

okay i think now i get it.
so we make some custom variable with that expression to project
the velocity at certain plane/line right?

i'm trying this with simple geometry first and i manage to get
the clearer view of the velocities.

but how do we extract the information after doing that?
i mean the velocity magnitude and direction at that plane/line ?

[flotus1]

Have you tried to click on "file" -> "export"?

[kenzosenzo]

Originally Posted by flotus1

Have you tried to click on "file" -> "export"?

ohh, so now i get it.
the data from the specific area/line can be obtain
in excel format after we "export" it.

thank you so much,
if i have more question i'll notify here and i'll show my results for others as a guide.

[kenzosenzo]

Originally Posted by flotus1

First you need to create an expression:
Attachment 28606

To make the result of the expression available for post-processing, you need to convert it to a variable:
Attachment 28607

Then you can use it just like any other variable to do the post-processing:
Attachment 28608

You should at least do some of the basic tutorials.

may i know where to find some basic tutorial for this post processing? i mean i need to understand more on the expression "atan2(Velocity w,Velocity u)" to make sure it's what i need actually.

thank you so much!

[flotus1]

I hardly think that there are tutorials for any specific expression.
It just math, so the possible combinations of expressions and variables are infinite.
You will have to figure out for yourself if your equations do what you had in mind.

[kenzosenzo]

Originally Posted by flotus1

I hardly think that there are tutorials for any specific expression.
It just math, so the possible combinations of expressions and variables are infinite.
You will have to figure out for yourself if your equations do what you had in mind.

Okay,
to check whether the expression is valid or not,
i try another diffuser with 30 degree of blade angle. This is the result.



So then, i plot a line in the middle of the diffuser
to export for the post processing. I have also draw the angle of velocity
that i need to make you understand.



noted that with the blade angle of 30 degree, probably the file that in the excel form will get nearly at 30 degree angle of velocity. (Angle normal to the surface of the diffuser outlet).

but later i get this "single" (should it be in x,y and z direction?) velocity angle and only between 0-9.45 degree.

Oh and this result i tried by using expression "atan2(Velocity w, Velocity u)"
i really need to understand how to make correct expressions.



Hope you can help me.