User Forum of Software BASEMENT

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Steady flow becomes unstable

Hi

I have a channelized reach about 11 km long (nothing really special, some bends). After successfully creating a steady initial condition (Qin = Qout = 85 m3/s) I am trying to get a steady high flow through the reach. At first, I managed to get up to 2000 m3/s without issues, after which it became unstable (Qout becomes negative in 1 timestep) and an error message was shown "The velocity 'v= 200.009 m/s' in the element # 736 exceeded the maximum velocity 'vmax=200 m/s' and is set to this maximum. No more messages are shown."  I changed this particular mesh element, and also improved the mesh in some other areas when this error re-occurred.
After changing the mesh, I retried and now the Qout starts going up and down a little bit already at 600 m3/s and then after 800 m3/s it just goes down (to 400 m3/s). This time no abrupt instability with negative Q, but steadily decrease... I get no error messages.

I'm not sure what I'm doing wrong, is it the mesh? I tried changing minimum water depth and CFL, so far it seems to show the same behaviour.

thanks for any advice

Alyssa

Stephan Kammerer

Developer

Registered: 2015-03-31

Posts: 67

Re: Steady flow becomes unstable

It sounds like this problem is related to your mesh. Does the mesh fulfill certain quality criteria such as minimum triangel angle and minimum cell size? The fact that 85 m3/s run without issues indicates that problematic cells get wetted at higher discharge. Check your mesh for very small or very steep (inclined) cell especially close to your inlet and outlet boundaries.

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Re: Steady flow becomes unstable

Hi
Thanks for the reply. I used the default of 28° triangle angle when creating the mesh. I don't find any very small cells in the quality mesh, it looks quite even. As for slope, I don't have steep areas in general, I tried to look for local issues. I changed the number of points in the inflow and outflow boundary, putting only the minimum, or putting a lot, and it seems to impact the outcome, but I am not getting over the instability issue. I also tried adding a simple cross section extra at both boundaries (simple trapezoid). The slope of my reach is low, 0.0005m/m. Are there any recommendations for the boundaries? I used 50m2 max area for the quality mesh in the river bed (width is 150m) and 100m2 for the embankments, but I also tried with smaller. I'm not sure what I am doing wrong.

thanks

Alyssa

User

From: Aix-en-Provence, France

Registered: 2019-10-28

Posts: 41

Re: Steady flow becomes unstable

Hi
I fixed it, with changing the boundaries (adding cross section) and adding some slope there. Maybe because my slope is low in my reach it didn't work before.
Thanks for the help!

cheers
Alyssa