热流科学与工程西步文通大学G教育部重点实验室If theneglectofthediffusion at anoutflowboundaryappears,forsomcreason, to be serious, then we should conclude that the analyst has placedAparticular badthe outflow boundary at an inappropriate location,A repositioning of theboundary would normally make the outflow treatment acceptableA partic-choice of anularly bad choice of an outflow-boundary location is the one in which thcre is an"inflow"overapartof it.Anexampleof this is shown inFig.5.12.Forsu-bad choice of the boundary,no meaningful solution can be obtained.outflow-boundaryThis may be a convenient place to review the boundary-conditionpractices for convection-diffusion problems.Whenever there is no fluid flowlocation is the oneacross the boundary of the calculation domain,theboundary flux is purelyadiffusion flux,and the practices described in Chapter 4apply.For thosepartsin which there is anof the boundary where the fluid flows into the domain, usually the values ofareknown.(Theproblemisnotproperlyspecifiedif wedonotknowthevalue of that a fluid stream brings with it.)The parts of the boundary"inflow" over awhere the fluid leaves the calculation domain form the outflow boundary,which we have already discussed.part of it. ...Forsuch a bad choiceof the boundary, nomeaningfulsolution can beobtained.(1980)BadGoodFigure 5.12 Good and bad choices of the location of the outflow boundary.CFD-NHT-EHG6/34CENTER
6/34 A particular bad choice of an outflow-boundary location is the one in which there is an “inflow” over a part of it. . For such a bad choice of the boundary, no meaningful solution can be obtained .(1980)
热流科学与工程西步文通大学C教育部重点实验室出口边券Cooling of plateTV screenCFD-NHT-EHTG7/34CENTER
7/34 Cooling of plate TV screen
热流科学与工程亚步文源大堂E教育部重点实验室2. Suggestion(1) Outlet normal velocity---treated according to localmassconservation(2) Outlet parallel velocity---treated by homogeneousNeumanncondition（齐次诺曼条件）Total mass conservationU,T.Local massVi.M1conservation个出口边界MIM212福i+1.M2.M25Li,M2CFD-NHT-EHTG8/34CENTER
8/34 (1) Outlet normal velocity-treated according to local mass conservation 2. Suggestion (2) Outlet parallel velocity-treated by homogeneous Neumann condition (齐次诺曼条件) Total mass conservation Local mass conservation
热流科学与工程西步文源大堂E教育部重点实验室VUi+l,M2 -Ui,M2 = 0 -Vi,M1 -Vi,M2i,M1Ui.MAxAyAyAylu2Vi,M1i+1,M2i.M2+1.M2△xThe resulted Vi,M1 has to bei,M2Axcorrected by total mass conservationcondition.auU.M=U.M2=0Tangential velocity6.7.4 Methods for outlet normal velocity to satisfytotalmassconservation1. Two situations1) Outlet without recirculation(1) Relative changes of outlet normal velocity =constant中HFO-NHTCEH9/34CENTER
9/34 , 1 , 2 1, 2 , 2 0 i M i M i M i M v v u u y x * * * , 1 , 2 1, 2 , 2 ( ) i M i M i M i M y v v u u x The resulted has to be corrected by total mass conservation condition. i M, 1 v 6.7.4 Methods for outlet normal velocity to satisfy total mass conservation 1. Two situations 1) Outlet without recirculation (1) Relative changes of outlet normal velocity =constant Vi M, 1 , 1 ) 0 i M U y Tangential velocity * U U i M i M , 1 , 2 Ui M, 1 x y
热流科学与工程亚步文源大堂E教育部重点实验室Di,MiVi,M1-Vi,m2 = k = constu+1M144.5Vi,M2Vi,M1 = Vi,M2(1+k)= f vi,M2 *f is determined according to total mass conservation :L2L2ZpimilimAx, =Zpvi,M2Ax, = FLOWINi=2i=2FLOWINTViMi = fVL2Vi.M2ZPi,miV,m2Ax,It is taken as the boundary condition for next iterationΦCFD-NHT-EHT10/34CENTER
10/34 , 1 , 2 , 2 i M i m i M v v k const v , 1 , 2 2 , (1 ) i M i M i M v v k f v f is determined according to total mass conservation : 2 2 , 1 , 1 , 1 , 2 2 2 L L i M i M i i M i M i i i v x v x FL f OWIN 2 , 1 , 2 2 L i M i M i FLOWIN f v x , 1 * i M i M, 2 v f v It is taken as the boundary condition for next iteration