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Hello everyone,

I perform some simple steady flow calculations with fixed discharge of 10 m3/s but
in Listing it shows following for FE scheme:
FLUX BOUNDARY 1: -10.07835 M3/S ( >0 : ENTERING <0 : EXITING )
FLUX BOUNDARY 2: 10.07800 M3/S ( >0 : ENTERING <0 : EXITING )
and for FV scheme:
FLUX BOUNDARY 1: -10.01988 M3/S ( >0 : ENTERING <0 : EXITING )
FLUX BOUNDARY 2: 10.01203 M3/S ( >0 : ENTERING <0 : EXITING )

I would like to know the reasons why these value differ from 10 m3/s.

Thank you.

Kind regards
Sandip

Hello Sandip,

With what you have quoted in the listing, I would have said the values differ from max 0.07 m3/s do not they? Less than 1 %.

Anyway, hard to tell more without looking at at least the 2 steering files you use.

Maybe SOLVER ACCURACY is let to default value or a too coarse value. You can try to decrease it (1.E-6, 1.E-8, 1.E-10 or even 1.E-15 if you can).

Otherwise, are you sure steady state is reached for both simulations?

Chi-Tuan

Hi Chi-Tuan,

I have uploaded my steering file.
I wanted to know if the given discharge is increased by some safety factor during calculation to avoid loss during FV and FE calculation.
I believe they reached steady state because I calculated for one hour and the flux were not changing since 42-44 minutes.
As you mentioned Solver accuracy but what about FV scheme?


Kind regards
Sandip

Hello Sandip,

Are you sure the prescribed elevation of 51.5 m fits the prescribed flowrate of 10 m3/s?
If you decrease it, I think the written values in the output file will decrease.

The FE method needs the solving of a linear system, that is why solver accuracy is required, contrary to FV method.

Hope this helps,

Chi-Tuan

Hi Chi-Tuan,

51.5m is an estimated elevation. As I did not have Outflow Boundary Condition information, I measured the slope of the river and estimated water depth using Manning Formula to get the water elevation.
I have tried decreasing this value and increasing this value but all gives more or less similar Flux Boundary of 10.07 which is higher than prescribed discharge.

My assumption is that since I am calculating dry condition, some water will remain in the Flow area hence the Outflow FLUX BOUNDARY should be less than 10m3/s and it should not be higher than Inflow.

Kind regards
Sandip

Hello Sandip,

If you have dry areas or tidal flats, it may explain light differences between expected values and computed values.

You can reach a steady state and that is the main goal.

Chi-Tuan