How to calculate velocity

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Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

How to calculate velocity

Post by Hirof » Fri Feb 26, 2021 6:11 am

Hello, everyone.

I'm working on simulation regarding subsurface water movement in tephra-mantled hillslope using HYDRUS 2D
Now, I would like to know "time series of velocity".
I understand that we can obtain the results of velocity at a specific time, but cannot obtain time series of velocity like pressure head and water content when we set observation node.
I think we can calculate velocity using the pressure head around a specific node.
Could anyone please tell me how to calculate velocity at a node and establish time series of change in velocity?

Thank you,
Hirof

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to calculate velocity

Post by Jirka » Fri Feb 26, 2021 6:49 am

I do not think that there is anyway to get velocities between print times. For observation nodes, we print only water contents, pressure heads, temperatures, and concentrations. You can perhaps use Meshlines, to get continuous fluxes through a specified cross-section. J.

Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

Re: How to calculate velocity

Post by Hirof » Fri Feb 26, 2021 7:36 am

Hello, Jirka.

Thank you very much for your quick reply.
Now, I want to try "Meshline" command to obtain time series of fluxes through a specified cross section.
Then, I have found the Mesh-Lines in the Auxiliary Objects, but I don't understand how to use use or open Mesh-line command.
Despite I clicked "Mesh-lines" and have tried to open it, I cannot continue to next procedure.
Could you please teach me how to use Mesh-Lines and get continuous fluxes?

Thank you,
Hirof

Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

Re: How to calculate velocity

Post by Hirof » Fri Feb 26, 2021 8:23 am

Dear jirka.

I have a question about the unit of water flux.
I found how to insert cross section for measuring fluxes.
Then, I displayed the results of water flux across the mesh line.
There is no unit in the vertical axis on the graph window.
Is the unit L/T or V/T?
Thank you very much for your help in advance.

Hirof

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to calculate velocity

Post by Jirka » Fri Feb 26, 2021 5:52 pm

It depends whether your project is 2D or axisymmetrical 3D. For the 2D project, it is L^2/T, i.e., flux [L/T] multiplied (integrated) over the length [L] of the segment. J.

Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

Re: How to calculate velocity

Post by Hirof » Sat Feb 27, 2021 9:34 am

Dear Prof. Jirka

Thank you for your reply.
I have one more question.
Can we calculate velocity using Meshline command, approximately?
I understand that we can obtain water flux along mesh line.
But, as you know, water flux is different from velocity, because it contains mass of water.
So I think we can calculate velocity by dividing water flux by water content around the segment.
Is it correct?
Do you have any suggestion to get velocity value along mesh line or at a node?
Thank you very much for your help in advance.

Hirof

Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

Re: How to calculate velocity

Post by Hirof » Sat Feb 27, 2021 9:55 am

Hi, everyone.

I mean I would like to know time series of velocity at a node or across mesh line.
I understand we can display "velocity" and "velocity vectors" in the results window.
I want to know the exact value of velocity at a node or across mesh line.

Thank you,
Hirof

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to calculate velocity

Post by Jirka » Sat Feb 27, 2021 8:50 pm

If you divide the meshline flux [L^2/T] by the length [L] of the meshline, you get Darcy’s flux [L/T]. If you divide it by the volumetric water content, you get the average pore velicity [L/T]. J.

Hirof
Posts: 22
Joined: Thu Mar 21, 2019 4:24 am
Location: Japan

Re: How to calculate velocity

Post by Hirof » Sun Feb 28, 2021 5:37 am

Thank you so much, Prof. Jirka.
That helps a lot.

Hirof

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