{"action":"created","issue":{"url":"https://api.github.com/repos/firemodels/fds-smv/issues/2440","labels_url":"https://api.github.com/repos/firemodels/fds-smv/issues/2440/labels{/name}","comments_url":"https://api.github.com/repos/firemodels/fds-smv/issues/2440/comments","events_url":"https://api.github.com/repos/firemodels/fds-smv/issues/2440/events","html_url":"https://github.com/firemodels/fds-smv/issues/2440","id":91898002,"number":2440,"title":"Initial values affect solution","user":{"login":"SimonDrake","id":13106725,"avatar_url":"https://avatars.githubusercontent.com/u/13106725?v=3","gravatar_id":"","url":"https://api.github.com/users/SimonDrake","html_url":"https://github.com/SimonDrake","followers_url":"https://api.github.com/users/SimonDrake/followers","following_url":"https://api.github.com/users/SimonDrake/following{/other_user}","gists_url":"https://api.github.com/users/SimonDrake/gists{/gist_id}","starred_url":"https://api.github.com/users/SimonDrake/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/SimonDrake/subscriptions","organizations_url":"https://api.github.com/users/SimonDrake/orgs","repos_url":"https://api.github.com/users/SimonDrake/repos","events_url":"https://api.github.com/users/SimonDrake/events{/privacy}","received_events_url":"https://api.github.com/users/SimonDrake/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":{"login":"rmcdermo","id":4418497,"avatar_url":"https://avatars.githubusercontent.com/u/4418497?v=3","gravatar_id":"","url":"https://api.github.com/users/rmcdermo","html_url":"https://github.com/rmcdermo","followers_url":"https://api.github.com/users/rmcdermo/followers","following_url":"https://api.github.com/users/rmcdermo/following{/other_user}","gists_url":"https://api.github.com/users/rmcdermo/gists{/gist_id}","starred_url":"https://api.github.com/users/rmcdermo/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/rmcdermo/subscriptions","organizations_url":"https://api.github.com/users/rmcdermo/orgs","repos_url":"https://api.github.com/users/rmcdermo/repos","events_url":"https://api.github.com/users/rmcdermo/events{/privacy}","received_events_url":"https://api.github.com/users/rmcdermo/received_events","type":"User","site_admin":false},"milestone":null,"comments":4,"created_at":"2015-06-29T20:13:00Z","updated_at":"2015-07-01T11:45:23Z","closed_at":null,"body":"Fire Dynamics Simulator, Compilation Date: Fri, 26 Sep 2014, Version: FDS 6.1.2, SVN Revision No.: 20564, MPI Enabled; Number of MPI Processes: 8, OpenMP Disabled\r\n\r\nI have produced a scaled-down tunnel simplification of a new build, in order to demonstrate an apparent problem in which the prescribed initial value of longitudinal velocity, is influencing the resulting air flow through the tunnel. The tunnel is 100m (100 cells) long, 12m (12 sells) wide, and 2m (4 cells) high. An open boundary with a dynamic pressure is specified at each portal, 0Pa at the left hand portal where x=0m, and 20Pa at the right hand portal where x=100m. The case is prescribed thus, in which the initial longitudinal velocity value U0=XX is the only parameter that is varied:\r\n\r\n&HEAD CHID='simple_dp_test001a'  /\r\n&MISC P_INF=84300.0, TMPA=38.0, U0=XX, V0=0.0, W0=0.0, ISOTHERMAL=.TRUE., BAROCLINIC=.TRUE.  /\r\n&DUMP DT_RESTART=15.0 /\r\n&TIME T_END=1200.0 /\r\n&MESH ID='Mesh000', FYI='1mx1mx0.5m'   , IJK= 100,12,4, XB=0.0,100.0,-6.0,6.0,0.0,2.0 / count =4800\r\n** Left hand portal:\r\n&VENT SURF_ID='OPEN', DYNAMIC_PRESSURE=0.0, XB=  0.0,  0.0,-6.0,6.0,0.0,2.0  / [XMIN]\r\n** Right hand portal:\r\n&VENT SURF_ID='OPEN', DYNAMIC_PRESSURE=20.0, XB=100.0,100.0,-6.0,6.0,0.0,2.0 /  [XMAX] \r\n** On vertical centreline:\r\n&SLCF PBY=0.0, QUANTITY='VELOCITY', VECTOR=.TRUE. /\r\n&SLCF PBY=0.0, QUANTITY='U-VELOCITY' /\r\n&SLCF PBY=0.0, QUANTITY='PRESSURE' /\r\n** Left hand portal:\r\n&SLCF PBX=0.0, QUANTITY='VELOCITY', VECTOR=.TRUE. /\r\n&SLCF PBX=0.0, QUANTITY='U-VELOCITY' /\r\n&SLCF PBX=0.0, QUANTITY='PRESSURE' /\r\n** Right hand portal:\r\n&SLCF PBX=100.0, QUANTITY='VELOCITY', VECTOR=.TRUE. /\r\n&SLCF PBX=100.0, QUANTITY='U-VELOCITY' /\r\n&SLCF PBX=100.0, QUANTITY='PRESSURE' /\r\n** 1m above road:\r\n&SLCF PBZ=1.0, QUANTITY='VELOCITY', VECTOR=.TRUE. /\r\n&SLCF PBZ=1.0, QUANTITY='U-VELOCITY' /\r\n&SLCF PBZ=1.0, QUANTITY='PRESSURE' /\r\n** Flow into domain:\r\n&DEVC ID='FLOW-LH-IN' , QUANTITY='VOLUME FLOW +', XB=  0.0,   0.0,-6.0 , 6.0, 0.0, 2.0  /\r\n&DEVC ID='FLOW-RH-IN'   , QUANTITY='VOLUME FLOW -', XB=100.0, 100.0,-6.0, 6.0, 0.0, 2.0 /\r\n** Flow out of domain:\r\n&DEVC ID='FLOW-LH-OUT', QUANTITY='VOLUME FLOW -', XB=  0.0,   0.0,-6.0 , 6.0, 0.0, 2.0  /\r\n&DEVC ID='FLOW-LH-OUT'  , QUANTITY='VOLUME FLOW +', XB=100.0, 100.0,-6.0, 6.0, 0.0, 2.0 /\r\n&TAIL /\r\n\r\nThe tunnel has an adverse pressure gradient in the positive x (longitudinal) direction, so that for positive initial velocities the flow should reverse, and likewise for negative initial velocities the flow direction should be unchanged. The following parameter runs have been made for a 1200s duration in order to reach as near steady conditions as possible:\r\n\r\ntest001a  U0=  +1.0 resulting volume flux from right to left 135.91m3/s\r\ntest002a  U0=  0.0 resulting volume flux from right to left 134.32m3/s\r\ntest003a  U0=  -1.0 resulting volume flux from right to left 135.94m3/s\r\ntest004a  U0=  +2.0 resulting volume flux from right to left 140.61m3/s\r\ntest005a  U0= +10.0 resulting volume flux from left to right 155.15m3/s **changed direction!\r\ntest006a  U0= -10.0 resulting volume flux from right to left 192.64m3/s\r\n\r\nLink goes to image of volume flow rate vs time (limited to 100s on horizontal scale) and described below.\r\n![test](https://cloud.githubusercontent.com/assets/13106725/8417194/b630d348-1ea2-11e5-8ae3-35a39e0f452a.png)\r\n\r\nAll the results were compared by way of volume flow rates at the portals, written to a device file. In the first instance, a small difference in the resulting flows of the first two tests (U0=+1.0 and U0=0.0) raised my curiosity. So I decided to reverse the initial flow (U0=-1.0) and replicated the final result of (U0=+1.0) which is what I expected, it didn't however explain why when (U0=0.0) the solution differed. So I increased the initial flow a bit more (U0=+2.0) creating yet another solution, and then a lot more (U0=+10.0) which resulted this time in a reversal of the final solution, which was clearly wrong as flow took place against the pressure gradient. A final test with (U0=-10,0) provided yet another large deviation in the final solution, albeit in the right direction.\r\n\r\nIn the actual tunnel model I have been starting with U0=+10.0 and I think this is the reason I have not been able to reach a sensible isothermal solution, prior to later adding a fire. Hence the need to investigate using the simplified model. In all probability I have made some simple error in setting up my model but it is far from obvious to me. Would you kindly investigate what is an apparent influence of initial values on the final solution. Many thanks, in advance.\r\n"},"comment":{"url":"https://api.github.com/repos/firemodels/fds-smv/issues/comments/117626345","html_url":"https://github.com/firemodels/fds-smv/issues/2440#issuecomment-117626345","issue_url":"https://api.github.com/repos/firemodels/fds-smv/issues/2440","id":117626345,"user":{"login":"SimonDrake","id":13106725,"avatar_url":"https://avatars.githubusercontent.com/u/13106725?v=3","gravatar_id":"","url":"https://api.github.com/users/SimonDrake","html_url":"https://github.com/SimonDrake","followers_url":"https://api.github.com/users/SimonDrake/followers","following_url":"https://api.github.com/users/SimonDrake/following{/other_user}","gists_url":"https://api.github.com/users/SimonDrake/gists{/gist_id}","starred_url":"https://api.github.com/users/SimonDrake/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/SimonDrake/subscriptions","organizations_url":"https://api.github.com/users/SimonDrake/orgs","repos_url":"https://api.github.com/users/SimonDrake/repos","events_url":"https://api.github.com/users/SimonDrake/events{/privacy}","received_events_url":"https://api.github.com/users/SimonDrake/received_events","type":"User","site_admin":false},"created_at":"2015-07-01T11:45:23Z","updated_at":"2015-07-01T11:45:23Z","body":"Since I will be allowing the smoke to back-layer first, and then activate the jet fans to reverse the smoke layer, I will need to maintain a pressure boundary at the entry portal as well as the exit portal, in order for the model to permit smoke to leave at the entry portal if the conditions dictate before the jet fans are activated.\r\n\r\nI would also like to point out that I think the User Guide is misleading in this instance, see section 6.4.1 describing U0: \r\n\r\n\"U0, V0, W0 Initial values of the gas velocity in each of the coordinate directions. Normally, these are all 0 m/s, but there are a few applications where it is convenient to start the flow immediately, like in an outdoor simulation involving wind.\"\r\n\r\nI had earlier used U0=10m/s in the isothermal simulation, in order to accelerate the computation that had been painfully slow, my logic being that it might have been quicker to solve if the tunnel air slowed from a high momentum condition, rather than the jet fans having to overcome the inertia of the initially still tunnel air. However, I am on the right path now, and thank you for pointing me to the U0 definition."}}