Learning Forum

[prabu]

Hello, I am working on a 2d fluid flow VOF problem.
when i plot velocity magnitude at a section in console it appears as arrows both positive and negative direction(reverse flow). But when i extract the solution data and print it as ascii file, it shows all positive values. How would i find the reverse flow direction values. Kindly help. 
My email id: rabu.peterz@gmail.com" target="_blank" data-mt-detrack-inspected="true">prabu.peterz@gmail.com
Thanks in advance

[seeta gunti]

Hello Prabu,

How do you display the vectors/contours? I guess you might have turned on global range. Deselect the global range and display the vectors/contours.

Regards,

Seeta

[Keyur Kanade]

Hi,

Can you please give more details. 

When you plot it, you can see reverse flow. It is shown by reverse direction arrow. 

You can see min and max magnitude in panel as well as in graphics window. 

Do you want magnitude for each cell? May I know why do you want to see magnitude of reverse flow?

Please see if you have sufficient mesh count. 

Generally extend your outlet so that you will have reverse flow. Please define reverse flow properties correctly so that you dont have any reverse flow. 

Also in some cases, depending on inputs, the reverse flow occurs for few iterations and then goes off. These are very generic guidelines. 

Hope this helps. 

Regards,

Keyur

[Karthik R]

Hello,

Just to add one more point to all the good stuff Keyur mentioned earlier, you might have reversed flow because of the physics as well. Sometime, in case of incompressible multiphase flow simulations, the shape of the interface decides the direction of flow at the outlet. Just wanted to mention this.

Please plot the vector plots similar to what Keyur is showing and let us know.

Thank you.

Best Regards,

Karthik

[prabu]

 

Thanks Seeta, kkanade and Kremella for your reply. I will explain my problem further.

1. figure 2 (a) shows the the console plot of x velocity showing positive and negative direction and Figure 2b shows the solution data extracted from (File->Export->Solution Data) where it show both positive and negative values

however

2. In figure 1(a) the console plot of velocity magnitude showing positive and negative direction and Figure 1(b)  shows the solution data extracted from (File->Export->Solution Data) where it show only positive values.

I know Velocity SQRT(Ux^2 + Uy^2  which always gives a positive value. But I want to know its direction also when i extract them. Can you help.

[Raef.Kobeissi]

Hi,

Did you try to extract the velocity components rather than the magnitude?

Regards

[prabu]

hello Raef,

Yes i have tried extracting velocity components and succeeded as shown in the last two figure.

Thanks

[Keyur Kanade]

Hello, 

As you showed in second figure, its the correct way. You also explained why it would be positive. I dont know any other method how to get direction of velocity in tabular form. May be others can pitch in. 

Can you please explain, why do you want to have it direction. 

Regards,

Keyur

[Karthik R]

Hello,

I agree with both Keyur and Raef. You will have to export the x-magnitude and it will give you an indication of the velocity vector direction. 

Velocity vector by itself has a direction, only it is being colored by the velocity magnitude. Because you are plotting a vector plot, it has both negative and positive directions (indicated by the arrows). The arrows are merely being colored by the velocity magnitude. When you are exporting the velocity magnitude data however, you are truly exporting just the magnitude and hence no direction. In the second case, you are plotting the velocity vector and are asking Fluent to color them by the x-velocity component. Since the x-velocity component resembles closely with the velocity magnitude (y and z components are really small), the vector plot in this case is very similar to the previous vector plot. Also, x-component of the velocity has a magnitude and direction and so you are seeing positive and negative values.

To summarize, vector plots always indicate the direction (because you can only use these to plot vector quantities). The legend is what is being colored by the velocity magnitude.

I hope this helps.

Best Regards,

Karthik

[prabu]

Hello,

Thanks Keyur and Karthik for the explanation. I understood the difference between Velocity Magnitude and X-component velocity. Thanks.

Regards 

Prabu