Learning Forum

[Nathan Blackmore-Wells]

Hi,

A tiny bit of context: I'm running an open channel 2D Fluent simulation to try to find an outlet velocity profile to use as an inlet in a different case.

My inlet for the first case is a mass flow inlet of water, with a surface height 1m in a 2m channel with a small slope (0.0005). I want it to run such that there are no surface waves (Fr<1). I have this figured out and choose to run a mass flow inlet of 2100kg/s and get a flat, wave-free channel with a mass flow at outlet of almost, 2100kg/s. Happy days. So i export the velocity profile.

I then want to reuse this velocity profile in the same geometry with a pressure inlet specified by free surface height and velocity magnitude. When i put this v-prof into the v-mag box the mass flow i got is only around 1700kg/s and seems to change as the solution converges.

i need this mass flow rate to remain constant to control the simulation and avoid surface waves.

a) Why is the mass flow not constant if i have a profile for velocity and a set height that doesnt change between the two cases? Have i not set both the U and the A (=h) of the inlet (Q=UA)?

b) Why does this change during convergence?

 

I'd hugely appreciate any help or suggestions for alternate ways i can do this

 

-Nate

[Rob]

If you're using the open channel the downstream flow will alter the inlet mass flow. How does the free surface look? Is there a hydraulic jump?  I know why profiles are used, but it's sometimes better to just accept the compute overhead and model the whole lot in one go.