## Fluids

#### mass flow rate given vs mass flow rate output

• tatimaes
Subscriber

Hi everyone, I am still around trying to figure out the last steps of my model, I am very close to my thesis submission and still have to go through the results phase, I'll appreciate your help on the following:

For my model, I am using the open channel submodel and pressure inlet/outlet as boundary conditions where I specified velocity and water level from my geometry.

I am struggling a lot on understanding why the discharge that implicitly I'm giving as an input with these boundary conditions, is not the same after the simulation? The discharge - Q50 I have from the river is 1600m3/s, but in the report fluxes mass flow rate the discharge is ~1800m3/s (I reached mass balance). What does it mean?

1. I tried several simulations, and the discharge does not get closer to 1600m3/s
2. What about roughness? my domain is very large, my mesh is not fine everywhere, only in desired points (I'm using a student license). I did simulations with smooth walls and also with values of ks for wall banks and wall bed (they are different)
3. ks - wall river bed: I got it from D50 of sand grain size for the river
4. ks - wall river banks: typical values of ks for this type of river
5. I computed water heigh (the free surface level at the inlet) from the manning formula knowing discharge-Q50 and slope. Does it affect my solution? the fact that I am using 'n' of manning from literature for this type of river? and fluent takes roughness from the law wall equation?
6. I know very little about it, but can that wall lifting be happening?

What I am missing? or what should I look at carefully?

Thank you very much

• Rob
Ansys Employee
You're adding 1600m3/s through the inlet. As VOF tends to be transient, how's the volume of water in the domain doing? Ie if you're losing more than you're adding is the domain water volume also decreasing?
• tatimaes
Subscriber
Hello dear Rob,
Let me show you the volume water mass graph and flux report:

• Rob
Ansys Employee
That's still stabilising. You'll see a mismatch until the volume report stabilises. The question you now need to consider is whether that alters the flow around the bit you're interested in.
• tatimaes
Subscriber
Thanks, Rob
So, if I get your point
I should check the mass flow rate with a higher simulation time to reach stationary?
I am guessing the difference in discharge will affect water level and velocities around the structures I placed. But I am not sure, I guess for that I need the results on both cases to compare...
Just to be sure: those comments I made above about roughness do they interfere with it? Or not for this case.