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1D/2D instability

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Basically, I have a model with a gradually increasing mass error and eventually going unstable. I have tried everything I can think of, looked at the message files, check files, changed the arrangement, time steps and others with no success.

I suspect it is to do with the 1D-2D connection at the upstream end of the channel, it is a messy arrangement. There is a roadway acting like a weir with flow along and across the downstream face, and, a transition from pipe to channel in the same location. Further, it is difficult to get a HX line at this point perpendicular to flow for all flows. I have done the best I can but suspect it could be improved. I have included an image below with ME grid in the background for a very extreme event (PMP) but the problem also occurs for lesser events.

post-2708-1277278225_thumb.jpg

CD = green cross hatching XS = brown QT bc = magenta HX = orange (broken) (also obsured by CD perimeter) CN = light blue (also obscured by XS) pipe = blue (solid) channel = blue (broken),

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Hi Ade,

Is there a reason you haven't continued the1D pipe directly to the 1D channel? It might improve things if you extended the pipe to snap to the upstream end of the 1D channel, and then you could get rid of the 4 cn lines extending from the end of the 1D pipe (the 4 visible ones on the image). I assume there are also 2 cn lines obscured behind the most upstream cross section lines - these would be the only ones you'd need.

The extended hx lines at the upstream end of the open channel may be causing problems also. It's probably better to only extend them as far as the edge of your 1D code boundary. You could wrap the hx lines around the upper end of the 1D channel though (so that they meet), to allow overland flow to fall straight back into channel (in line with flow) rather than being forced to approach from the sides.

Having said all that it looks like it is quite a steep fall between the road and the 1D channel, which may be causing the problems...?

Cheers

Richard

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Hi Richard

Most of those arrangements I have tried but was not sure which to settle on. Now i do. I still get instability but it takes some unknowns (for me) out.

Thanks Richard. With any luck your suggestions, along with pm's, will hopefully be enough to sort it out.

Ade

Hi Ade,

Is there a reason you haven't continued the1D pipe directly to the 1D channel? It might improve things if you extended the pipe to snap to the upstream end of the 1D channel, and then you could get rid of the 4 cn lines extending from the end of the 1D pipe (the 4 visible ones on the image). I assume there are also 2 cn lines obscured behind the most upstream cross section lines - these would be the only ones you'd need.

The extended hx lines at the upstream end of the open channel may be causing problems also. It's probably better to only extend them as far as the edge of your 1D code boundary. You could wrap the hx lines around the upper end of the 1D channel though (so that they meet), to allow overland flow to fall straight back into channel (in line with flow) rather than being forced to approach from the sides.

Having said all that it looks like it is quite a steep fall between the road and the 1D channel, which may be causing the problems...?

Cheers

Richard

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Hi Ade

I agree with Richard's suggestions. The pipe must definitely connect directly to the open channel. I'd also be considering moving the connection into the open channel downstream a bit (the northern HX line is likely to be particularly problematic with its oblique angle to the steep terrain).

If you're still having issues, probably easiest to contact support@tuflow.com and we'll get the model off you to have a closer look at the setup.

Cheers

Bill

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