{"action":"created","issue":{"url":"https://api.github.com/repos/firemodels/fds/issues/6738","repository_url":"https://api.github.com/repos/firemodels/fds","labels_url":"https://api.github.com/repos/firemodels/fds/issues/6738/labels{/name}","comments_url":"https://api.github.com/repos/firemodels/fds/issues/6738/comments","events_url":"https://api.github.com/repos/firemodels/fds/issues/6738/events","html_url":"https://github.com/firemodels/fds/issues/6738","id":347049789,"node_id":"MDU6SXNzdWUzNDcwNDk3ODk=","number":6738,"title":"Correlation between the wall roughness and friction factor","user":{"login":"florinmtl","id":41639792,"node_id":"MDQ6VXNlcjQxNjM5Nzky","avatar_url":"https://avatars0.githubusercontent.com/u/41639792?v=4","gravatar_id":"","url":"https://api.github.com/users/florinmtl","html_url":"https://github.com/florinmtl","followers_url":"https://api.github.com/users/florinmtl/followers","following_url":"https://api.github.com/users/florinmtl/following{/other_user}","gists_url":"https://api.github.com/users/florinmtl/gists{/gist_id}","starred_url":"https://api.github.com/users/florinmtl/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/florinmtl/subscriptions","organizations_url":"https://api.github.com/users/florinmtl/orgs","repos_url":"https://api.github.com/users/florinmtl/repos","events_url":"https://api.github.com/users/florinmtl/events{/privacy}","received_events_url":"https://api.github.com/users/florinmtl/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":{"login":"rmcdermo","id":4418497,"node_id":"MDQ6VXNlcjQ0MTg0OTc=","avatar_url":"https://avatars2.githubusercontent.com/u/4418497?v=4","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},"assignees":[{"login":"rmcdermo","id":4418497,"node_id":"MDQ6VXNlcjQ0MTg0OTc=","avatar_url":"https://avatars2.githubusercontent.com/u/4418497?v=4","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},{"login":"mcgratta","id":11333911,"node_id":"MDQ6VXNlcjExMzMzOTEx","avatar_url":"https://avatars0.githubusercontent.com/u/11333911?v=4","gravatar_id":"","url":"https://api.github.com/users/mcgratta","html_url":"https://github.com/mcgratta","followers_url":"https://api.github.com/users/mcgratta/followers","following_url":"https://api.github.com/users/mcgratta/following{/other_user}","gists_url":"https://api.github.com/users/mcgratta/gists{/gist_id}","starred_url":"https://api.github.com/users/mcgratta/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/mcgratta/subscriptions","organizations_url":"https://api.github.com/users/mcgratta/orgs","repos_url":"https://api.github.com/users/mcgratta/repos","events_url":"https://api.github.com/users/mcgratta/events{/privacy}","received_events_url":"https://api.github.com/users/mcgratta/received_events","type":"User","site_admin":false}],"milestone":null,"comments":24,"created_at":"2018-08-02T14:57:42Z","updated_at":"2018-08-07T22:21:07Z","closed_at":null,"author_association":"NONE","body":"Hello all,\r\n\r\nI am presently in the situation of validating the design of the ventilation system for a road tunnel. The use of FDS has been challenged by one of our international consultant concerning the computing of the linear pressure losses, i.e. the correlation between the wall roughness and Darcy’s friction factor “f”? \r\nSo, in order to clarify the correlation between the roughness of the walls and Darcy’s friction factor value, I performed several simulations without fire, in the empty tunnel, and could not reach any conclusion (see 180720PressLossNoSlip5_devcLambda.fds).  \r\n\r\nModel’s input and simulation parameters: \r\n\r\nTunnels geometry and characteristics:\r\n\r\n1.\t1390 m (L) x12.8 m (W) x5 (H) m tunnel;\r\n2.\thydraulic diameter Dh=7191 mm \r\n3.\tforced ventilation:  2 fans x 250 m3/s exhaust, situated at 950 m from the entry portal;\r\n4.\tno vehicles;\r\n5.\tno fire; \r\n6.\t24 meshes;\r\n7.\tcell dimensions: 1 m * 0.512 m * 0.5 m;\r\n8.\tWalls’ material: Concrete (characteristics from FDS library);\r\n9.\tWall roughness: 5 mm.\r\nHere it is the description of the surface:\r\n\r\n&SURF ID='WallConc',\r\n      BACKING='INSULATED',\r\n      MATL_ID(1,1)='CONCRETE',\r\n      MATL_MASS_FRACTION(1,1)=1.0,\r\nTHICKNESS(1)=0.25,\r\nNO_SLIP=.TRUE.,\r\nROUGHNESS=5.0E-3/\r\n\r\n(The use of NO_SLIP=.TRUE. seems to be an error; if so, my understanding of the user guide was erroneous)\r\n\r\n10.\tStandard ambient conditions: \r\na.\ttemperature 20 deg C;\r\nb.\tdensity rho=1.206 kg/m3; \r\nc.\tatmospheric pressure  p= 1325E5 Pa, \r\nd.\tkinematic viscosity of the air nu = 1.506*10^-5 m2/s\r\n\r\n11.\tPressure sensors all over the tunnel, at 0.1 m from the wall and 2.5 m high spaced at 100 m, with the following generic description:\r\n\r\n&DEVC ID='PresNi', QUANTITY='PRESSURE', XYZ=50.0,0.1,2.5, ORIENTATION=0.0,1.0,0.0\r\n\r\nFDS results: \r\n1.\tupstream the exhaust fans positive velocity of 2.6 m/s;\r\n2.\tdownstream negative velocity negative 4.92 m/s (air is drawn from the exterior).\r\n\r\nThe mean values of the pressure calculated in a stabilized regime, have been used to compute the “f” factor:\r\n\r\nf=2*deltaPressure_i,i+1*Dh /(2*rho*deltaLength_i, i+1).\r\n\r\nThe values are (see 180720PressLossNoSlip5_devcLambda.xlsx):\r\n\r\nUpstream (velocity 2.6 m/s): \r\n“f” calculated with   FDS results :  0.09400 ; 0.10904 ; 0.11788 ; 0.118980 ; 0.118578 ; 0.118098 ; 0.118413 ; 0.117669 for Re = 1251025\r\n\r\nDownstream (velocity of 4.92 m/s): \r\n“f” calculated with    FDS results : 0.13456 ; 0.11691for Re : 2349254\r\n\r\nThese values have been compared with those calculated with the Colebrook equation:\r\n\r\n f = {-2*log10[e/(D*3.7)+2.51/(Re*f^1/2)]}^-2\r\n\r\nComputed values:\r\n\r\n1.\tupstream: Re=1 251 025; f=0.01833564 \r\n\r\n2.\tdownstream: Re=2 349 254; f=0.01818287 \r\n\r\nThe differences are huge and the values of « f » computed from FDS results are way too big.\r\n\r\nOn the other hand, if there is no fire, the values of the static pressure measured by sensors must always be in my opinion negative because I assume that the devices are measuring gauge pressure, so inferior to the atmospheric pressure (In the simulation whose results are presented above the values were negative). This happens in some simulations but not in all. \r\n\r\nI studied the examples given for the Moody validation, and I ran some of them. \r\nFor example in  “z0=p0001_dpdx=-1_N8-press.fds” I modified the length of the mesh to 20m and I added some pressure devices. \r\nIn this case, I also find the pressure increases along the tunnel (so “f” results negative).\r\n\r\nThus being said, will you please help me to get in the good direction? \r\nThe examples given in the validation guide are all using WIND parameters, but pressure devices do not give the same pressure gradient (Pa/m). \r\nShould I use “Statistic” cross-sections instead of devices? \r\nOn the other hand, if the fire is present, the friction factor varies downstream because of the Re and viscosity, so the constant pressure gradient hypothesis does not function anymore. \r\nAlso, the dimensions of a real tunnel are not comparable with those used in the simulations, the air/gases are flowing in both directions, etc. \r\n\r\nSo, for a road tunnel with mechanical ventilation (exhaust fans, jet fans, Saccardos, etc) I would like you to indicate the best practice to correlate the roughness and the linear pressure losses.\r\nAlso, if I have to compare the values of “f” calculated from Colebrooke and Darcy, which is the best way to measure the mean values of the pressure in different cross sections?\r\n\r\nI thank in advance for your precious help.\r\nBest regards,\r\n\r\nFlorin\r\n\r\n[180720PressLossNoSlip5.zip](https://github.com/firemodels/fds/files/2253923/180720PressLossNoSlip5.zip)\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n"},"comment":{"url":"https://api.github.com/repos/firemodels/fds/issues/comments/411221912","html_url":"https://github.com/firemodels/fds/issues/6738#issuecomment-411221912","issue_url":"https://api.github.com/repos/firemodels/fds/issues/6738","id":411221912,"node_id":"MDEyOklzc3VlQ29tbWVudDQxMTIyMTkxMg==","user":{"login":"rmcdermo","id":4418497,"node_id":"MDQ6VXNlcjQ0MTg0OTc=","avatar_url":"https://avatars2.githubusercontent.com/u/4418497?v=4","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},"created_at":"2018-08-07T22:21:07Z","updated_at":"2018-08-07T22:21:07Z","author_association":"CONTRIBUTOR","body":"OK, I have to revise my conclusions somewhat.  The good news is that I think we are fine to *push* the flow through the tunnel with DEVC for pressure.  The problem I was seeing before was mainly due to erratic behavior caused by the CFL velocity norm resulting in a time step that was not truly stable.  So, the keys are:\r\n\r\n1. Use GLMAT solver for multi-mesh.\r\n2. Use CFL_VELOCITY_NORM=2 (or better, e.g., 1 would also work)\r\n\r\nWith these settings I get f_10 = 0.0088 and f_20=0.0085 and f_Colebrook = 0.0093.\r\n\r\n![screen shot 2018-08-07 at 6 20 48 pm](https://user-images.githubusercontent.com/4418497/43805665-a5552e70-9a6e-11e8-80ea-ccef64aaf4ac.png)\r\n"}}
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