Boundary Numerical InstabilityBoundary Numerical Instabilityhttps://oss.deltares.nl/c/message_boards/find_thread?p_l_id=1806746&threadId=3245772020-06-01T19:58:01Z2020-06-01T19:58:01ZRE: Boundary Numerical InstabilitySteven Ayreshttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=4187452014-04-28T16:21:40Z2014-04-28T16:21:40ZThe problem was definitely related to boundary reflections. When I set alpha to 10000, the boundary velocity jets disappear.Steven Ayres2014-04-28T16:21:40ZRE: Boundary Numerical InstabilityFernando Barretohttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=4174732014-04-25T18:24:48Z2014-04-25T18:24:48ZHi Steven, <br /><br />When I use WL boundaries I usually put alpha as 1000. I didn't know how to calculate alpha, it's really to know. In Riemman we didn't use alpha, Riemann is a weaky reflective boundary condition, so I don't know a lot about alpha. <br />There are many possibilities to cause your error. Could you attach your model ?, I think it's easier to help.<br /><br />Regards, <br /><br />Fernando BarretoFernando Barreto2014-04-25T18:24:48ZRE: Boundary Numerical InstabilitySteven Ayreshttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=4170762014-04-24T19:03:17Z2014-04-24T19:03:17ZI have water level boundaries for the tide boundaries and an inflowing river total discharge boundary. I think the problem is short wave reflections in the water level boundaries. I raised the alpha coefficient to 1000 and this helped extend the simulation time to a few days, but I still seem to get re-circulation around the open boundary. I again raised the alpha coefficient to 10,000 and am testing now.<br /><br />A formula is given in the manual to determine the alpha coefficient - <br /><br />Equation 9.76, Water level boundary: alpha = Td * sqrt(H/g), where Td is the time it takes for a free surface wave to travel from the left boundary to the right boundary of the model area.<br /><br />Since the model is around 30 miles (48 km) across the shortest open tide boundary, and assuming the tidal wave travels around 30 miles/hour or 13.4 m/s, Td would equal 3600 seconds. Using a typical depth of 200 meters at the open boundary, alpha would be on the order of 16,000. Does that sound right?Steven Ayres2014-04-24T19:03:17ZRE: Boundary Numerical InstabilityFernando Barretohttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=4161472014-04-22T21:46:32Z2014-04-22T21:46:32ZDear Ayres,<br />my problem was the input of different boundary conditions (velocity and wl) in adjacent boundaries. This was resolved when I used riemann boundary conditions. What's your model configuration? boundary type, dimension?<br /><br />regards, <br />FernandoFernando Barreto2014-04-22T21:46:32ZRE: Boundary Numerical InstabilitySteven Ayreshttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=4156392014-04-21T18:10:34Z2014-04-21T18:10:34ZFernando, <br /><br />Were you able to resolve the boundary problems in your model? I have something similar going on in my 18-level Z-model with salinity. The model is creating velocity jets in the middle of an open stage boundary. I assume this has something to do with the salinity constituent but I'm not sure how to go about fixing the problem.<br /><br />Thanks,<br />SteveSteven Ayres2014-04-21T18:10:34ZBoundary Numerical InstabilityFernando Barretohttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=3245762013-09-09T22:45:48Z2013-09-09T22:45:48ZI'm doing a simulation of a meso scale region to understand the baroclinic circulation of this place. My model contains 32 Z-layers and 3 bounds (north, east, south). To the boundary configuration I chose to impose current in the top and east boundary, and elevation in the south boundary. At this configuration I got bizarre instabilities in the boundary. It was generated a velocity jet in the interface between boundary types (elevation and current)<br />In another configuration I imposed current in north bound., and in the half of the east boundary; the remaning sections was elevation type (south and the rest of east). In this configuration was also generated a jet in the interface between bound types (fig). The jet can be seen in the east boundary.<br />Someone knows how to solve this. Can this be related with the z-model? if not, any advice?Fernando Barreto2013-09-09T22:45:48Z