Ras 2D Steady Flow & Courant

[Rocko Brown]

Howdy fellow river modelers

I had two basic questions I could use some help with:

1) I have been using Ras2D for steady flow simulations by simply prescribing a steady flow series over a number of days until the WSE and velocity profiles stabilize. Is there a better way to do this? I’m used to using SRH2D where you can simply put in a fixed flow and look for convergence and stabilization through monitoring points and lines.

2) I was also curious if courant values were important for quasi-steady runs as mentioned above. For the courant ranges that are recommended, do they apply to each cell or an average over the domain? For example, I have some models where a few cells have high courant numbers (~5-13) but the majority of the cells are within the recommended ranges.

Thanks very much for your time

[Scott Miller]

Rocko. RAS 2D is unsteady, so you’re doing quasi-steady right by using a timeseries. The courant number tells you how well flow is calculated in each cell. Particular cells with high courant numbers control the time step, if you’re using courant condition to control the timestep. Whether it is accuracy for a steady time step, or time step control, you’ll want to modify the size of cells with high courant numbers. Courant numbers can be mapped for a timestep, just like other flow properties, in Mapper.

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