- Oct 23 2019
- c.niroshinie

I Analyzed the velocity fields and water level fluctuations of a river with Nays2dFlood. I used very fine geographic data of the river and results seems reasonable. However, I want to confirm is it appropriate to use Nays2Dflood for low flow conditions as well.

I can’t use Nays2DH, as I need to add the inflows from tributaries. That’s why I used Nays2Dflood.

## コメント

Hello,

If you are not satisfied with results for low flow conditions.

I suggest that calculation grid size reduce smaller.

Thank you very much for your suggestion.

Hello,

I used Nays2Dflood and I use small grid size (W16000 dj12.5 nj1280 and ni4500 di 14) then the calculation was fail, so I would like to know how much smallest gird size we can determine or limitation of grid size. thank you very much

khanvixay

How is the time step for calculation? You should change the time step for the calculation when you change the grid size.

I always use output time interval 600 second, calculate time step 0.2 second, i just follow the guideline but i do not know how to determine these time also, could you please clarify. thank you

dt (calculation time step) should be given to satisfy CFL condition.

It should be the minimum of dt < dx / { [u] + (gh)^1/2 } and dt < dy / { [v] + (gh)^1/2 }

you can reduce your time step to 0.1 or 0.05 and see. How about the inflow and the water depths?

If you send your model I will check the mistake.

thank you very much sir, finally i can calculated since i reduce grid size and increase grid cell base on dj < di, output time interval 600 second, calculate time step 0.2 second. but i still would like to know, why since i decrease grid cell (dj 12.5, di 14) base on (dj < di, output time interval 600 second, calculate time step 0.2 second) the calculation was fail.

i also can not understand you suggestion like dt < dx / { [u] + (gh)^1/2 } and dt < dy / { [v] + (gh)^1/2 }

i do not know how to send my model to you to check also, if i have your email address is will be fine.

thank you very much for your kindness

khanvixay

dt (calculation time step) should be given to satisfy CFL condition. https://en.wikipedia.org/wiki/Courant–Friedrichs–Lewy_condition

It should be the minimum of dt < dx / { [u] + (gh)^1/2 } and dt < dy / { [v] + (gh)^1/2 }

here dt is time step

dx is grid size in x direction

[u] is absolute value of velocity in x direction

g is gravitational acceleration

h is water depth

dy is grid size in y direction

[v] is absolute value of velocity in y direction

thank you very much sir, it take a lot of time to learn but finally i understood.