Varying Velocity at Boundary Condition of 2D Model is Creating an Eddy

[lrief]

I have a 2D model in a wide river. My upstream boundary is a flow hydrograph, and downstream is a stage hydrograph. The river has a deep navigation channel near the right bank, and is shallow on the left side of the river. At the upstream boundary condition, the flow is entering the model with a varying velocity. The velocity is highest in the deep navigation channel, and slow in the shallow areas. This is causing a “jet” of water to enter the model at the navigation channel, and generating large eddys as the water swirls around this jet. One large eddy causes a complete reversal of the river flow direction on the left side. I am using the SWE-ELM equation set. If i switch to the Diffusion Wave equation set, the eddys do go away. However, one purpose of this model is to evaluate the local effects of dredging sediment from the river bottom, and I believe the SWE-ELM equation set will be best for this. Is there something I should be doing at the boundary condition to help the flow enter the model more smoothly and evenly?

[lrief]

I’ve posted this same question on Reddit, where I included a screenshot from the model, showing the issue: https://www.reddit.com/r/HECRAS/comments/1isolxa/varying_velocity_at_boundary_condition_of_2d/

[Eric R]

It may be associated with the “EG slope” in the unsteady file. If the “EG slope” value is high, I suggest activating the “TW Check” option to avoid instabilities.

Other things I would test:
– I would improve the size of the mesh, orienting it, as best as possible, in the direction of the flow;
– I would activate the “Conservative” turbulence model;
– I would expand the modeled area upstream, to check with greater certainty whether the vortex in your image is real or just a problem with the model. As a rule of thumb, I always try to move points of interest (of some project) away from the flow inlets and outlets, because, in my experience, they do not always have reasonable flow patterns.

[Author]

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