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June 16, 2017, 12:14 
Assigning mass flow rate at the outlet

#1 
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cyln
Join Date: Jul 2016
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Hello,
I am doing a steady state incompressible flow simulation with a laminar flow model. My computational domain is a pipe. There are two concentric outlets at the outflow boundary. At the outlets, I have to assign velocity as a massflowrate. I couldnt really find the exact name of this code. So, just to do a test run, i used flowRateInletVelocity with a negative mass flow rate. 1. First of all, what would you recommend for the type? How can I define the mass flow rate at the outlet? OUTLET1 { type flowRateInletVelocity; massFlowRate 10.0; rho rho; rhoInlet 1.225; } 2. When I run the simulation, I get the following error. As I read the threads in the forum, it is caused by the mesh. #0 Foam::error:rintStack(Foam::Ostream&) at ??:? #1 Foam::sigFpe::sigHandler(int) at ??:? #2 ? in "/lib/x86_64linuxgnu/libc.so.6" #3 Foam::symGaussSeidelSmoother::smooth(Foam::word const&, Foam::Field<double>&, Foam::lduMatrix const&, Foam::Field<double> const&, Foam::FieldField<Foam::Field, double> const&, Foam::UPtrList<Foam::lduInterfaceField const> const&, unsigned char, int) at ??:? #4 Foam::symGaussSeidelSmoother::smooth(Foam::Field<d ouble>&, Foam::Field<double> const&, unsigned char, int) const at ??:? #5 Foam::smoothSolver::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const at ??:? #6 ? in "/opt/openfoam4/platforms/linux64GccDPInt32Opt/bin/simpleFoam" #7 ? in "/opt/openfoam4/platforms/linux64GccDPInt32Opt/bin/simpleFoam" #8 ? in "/opt/openfoam4/platforms/linux64GccDPInt32Opt/bin/simpleFoam" #9 ? in "/opt/openfoam4/platforms/linux64GccDPInt32Opt/bin/simpleFoam" #10 __libc_start_main in "/lib/x86_64linuxgnu/libc.so.6" #11 ? in "/opt/openfoam4/platforms/linux64GccDPInt32Opt/bin/simpleFoam" Floating point exception (core dumped) 3. When I check my mesh using "checkMesh", I get the following error. My mesh is converted from fluent.msh (ICEM) using fluentMeshToFoam. It says Failed 1 Mesh Checks but I cannot really see where the error is. I would be grateful if you could tell me how to interpret this. Time = 0 Mesh stats points: 1902 faces: 18200 internal faces: 16704 cells: 8726 faces per cell: 4 boundary patches: 5 point zones: 0 face zones: 0 cell zones: 0 Overall number of cells of each type: hexahedra: 0 prisms: 0 wedges: 0 pyramids: 0 tet wedges: 0 tetrahedra: 8726 polyhedra: 0 Checking topology... Boundary definition OK. Cell to face addressing OK. ***Unused points found in the mesh, number unused by faces: 40 number unused by cells: 40 <<Writing 40 unused points to set unusedPoints Upper triangular ordering OK. Face vertices OK. Number of regions: 1 (OK). Checking patch topology for multiply connected surfaces... Patch Faces Points Surface topology INLET1 43 32 ok (nonclosed singly connected) INLET2 131 90 ok (nonclosed singly connected) CYLWALL 1143 604 ok (nonclosed singly connected) OUTLET1 42 31 ok (nonclosed singly connected) OUTLET2 137 95 ok (nonclosed singly connected) Checking geometry... Overall domain bounding box (0 0.199986 0.2) (0.75 0.2 0.199917) Mesh has 3 geometric (nonempty/wedge) directions (1 1 1) Mesh has 3 solution (nonempty) directions (1 1 1) Boundary openness (1.6148e16 9.11285e18 2.03217e17) OK. Max cell openness = 2.10748e16 OK. Max aspect ratio = 4.11226 OK. Minimum face area = 0.000221586. Maximum face area = 0.00274932. Face area magnitudes OK. Min volume = 1.83909e06. Max volume = 4.18896e05. Total volume = 0.0935029. Cell volumes OK. Mesh nonorthogonality Max: 51.574 average: 17.1314 Nonorthogonality check OK. Face pyramids OK. Max skewness = 0.624006 OK. Coupled point location match (average 0) OK. Failed 1 mesh checks. End Cheers! 

June 16, 2017, 14:47 

#2 
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Join Date: Feb 2014
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you boundary condition should work, i guess.
your main problem is the mesh. are you trying to convert a 3D mesh? Try fluent3DMeshToFoam for the conversion... 

June 16, 2017, 14:56 

#3 
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cyln
Join Date: Jul 2016
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yea, my mesh is 3D. I used both fluentMeshToFoam and fluent3DMeshToFoam. I get the same error at the end when I check the mesh using checkMesh.
So what is wrong in my mesh? 

June 16, 2017, 15:04 

#4 
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not sure with the infos you gave. as a first step i would try to reduce the non orthogonality.


June 16, 2017, 15:27 

#5 
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cyln
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I have reduced the nonorthogonality. Getting the same error unfortunately.


June 17, 2017, 11:29 

#6 
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cyln
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Does anyone else another suggestion?


June 17, 2017, 12:17 

#7 
Senior Member

Hi,
@cyln The error, you have posted, can be caused by MANY different things. Your pressure equation is diverging (since you posted just printStack output, it is just a guess). It can be caused by:  mesh  wrong boundary conditions  discretisation schemes Since your post provides almost no information (though checkMesh output is surely useful), we need to guess, what is wrong. 1. Did you try PCG instead of smoothSolver? 2. What is you OTHER boundary conditions? 3. What are your discretisation schemes? 4. Are you sure flow is laminar? 

June 18, 2017, 01:34 

#8 
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cyln
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Hi alexeym, thank you for your reply,
Yes exactly, my pressure eqn is diverging.  I tried changing my mesh but I got the same at the end.  My BCs are as follows: Velocity: INLET1 { type fixedValue; value uniform (25 0 0); } OUTLET1 { type flowRateInletVelocity; massFlowRate 3.0; rho rho; rhoInlet 1.225; } OUTLET2 { type flowRateInletVelocity; massFlowRate 6.0; rho rho; rhoInlet 1.225; } PIPEWALL { type slip; }  Pressure INLET1 { type zeroGradient; } OUTLET1 { type fixedValue; value uniform 0; } OUTLET2 { type fixedValue; value uniform 0; } PIPEWALL { type zeroGradient; } To me, boundary conditions looks normal. I think flowRateInletVelocity BC at the outlets should also be fine.  Here are my schemes: ddtSchemes { default steadyState; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phi,U) bounded Gauss upwind; div(phi,k) bounded Gauss upwind; div(phi,epsilon) bounded Gauss upwind; div(phi,R) bounded Gauss upwind; div(R) Gauss linear; div(phi,nuTilda) bounded Gauss upwind; div((nuEff*dev2(T(grad(U))))) Gauss linear; } laplacianSchemes { default Gauss linear corrected; } interpolationSchemes { default linear; }  1. Yes, I have tried PCG and some other solvers instead of smoothSolver. Number of iterations varied bu I got the same error at the end. 4. You are right about the flow. The nature of my flow is turbulent. However, I am running my simulation purposely as a laminar flow. I solved the same problem using CFX and I have to compare my results. OF should still be able to solve it. 

June 18, 2017, 03:21 

#9 
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Santiago Lopez Castano
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Your system is overdetermined. Put all pressure BC to zeroGradient
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June 18, 2017, 04:04 

#10 
Senior Member

@cyln
Small additions 1. Rephrasing Santiago in general you cannot arbitrarily fix value of pressure and velocity at a boundary. Since flowRateInletVelocity is a child of fixedValue, you should set pressure BC to something that does not fix value (ex. fixedFluxPressure). 2. Could you also post your SIMPLE dictionary from fvSolution? Do you have nonorthogonal corrector iteration there? 

June 18, 2017, 12:28 

#11 
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cyln
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yes, fixedValue pressure was my major mistake. I changed it to zeroGradient. Then it was asking me to define ref cell and pressure, so did I.
 Now it is giving continuity error cuz I can never exactly match the outflow mass flow rate if I do not define mass flow rate at the inflow boundary too. I need to get rid of continuity error while still using fixedValue of velocity at the inlet. I am trying to figure that out. And here is my SIMPLE dictionary: SIMPLE { pRefCell 0; pRefValue 0; nNonOrthogonalCorrectors 10; //0 consistent yes; residualControl { p 1e2; U 1e3; } } 

June 18, 2017, 16:06 

#12 
Senior Member

@cyln
1. 10 is too much (though, if you want, you can leave as it is). 2 would be enough. Maybe you could use leastSquares instead of Gauss linear. 2. Since for you case flowRateInletVelocity will do the following: Code:
const scalar avgU = flowRate_>value(t)/gSum(rho*patch().magSf()); Code:
postProcess func 'patchAverage(name=OUTLET1,U)' 

June 18, 2017, 16:43 

#13 
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cyln
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1. I reduced it from 10 to 2.
2. I run the code "postProcess func 'patchAverage(name=OUTLET1,U)' " to learn exact value of the patch area and set the fixedValue velocity accordingly. Even though I am getting very close value, it is still not exactly the same. See the error below: Time = 1 smoothSolver: Solving for Ux, Initial residual = 1, Final residual = 1.63795e05, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1, Final residual = 0.0268419, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 1, Final residual = 0.0265898, No Iterations 1 > FOAM FATAL ERROR: Continuity error cannot be removed by adjusting the outflow. Please check the velocity boundary conditions and/or run potentialFoam to initialise the outflow. Total flux : 16.6829 Specified mass inflow : 5.84553 Specified mass outflow : 5.8449 Adjustable mass outflow : 1.00136e18 From function bool Foam::adjustPhi(Foam::surfaceScalarField&, const volVectorField&, Foam::volScalarField&) in file cfdTools/general/adjustPhi/adjustPhi.C at line 107. FOAM exiting Do you suggest me to add the first code in U file? 

June 18, 2017, 17:43 

#14 
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Santiago Lopez Castano
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Using the socalled 'non orthogonal correctors' does have an impact in your velocity field. Incrementing the intermediate steps on the predictor of PISO/SIMPLE without correcting the velocity generates artificial enstrophy (or "dissipation", as some call it) thus messing the prediction of TKE
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June 18, 2017, 17:48 

#15 
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cyln
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Hi Santiago,
I had the same error even without using that nonorthogonal correctors. What is your suggestion then? 

June 18, 2017, 18:10 

#16 
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Santiago Lopez Castano
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Your error is not related to my comment. You can perfectly get a divergencefree velocity field where the TKE is not correct. I just wanted to point out that using the non orthogonal correctors is not inconsequential. Regarding your problem it seems to me that setting the outlets instead of setting the inlet is a bit odd, both from a physical and mathematical perspective. Although numerically is desirable to get an uniform field as an outlet, that's never really the case. Imagine turbulent flow behind a cylinder. Now, on the mathematics part, the equations you are solving are three parabolic and one elliptic, momentum travels unidirectionally, whereas mass conservation is 'omnipresent' this disparity is not well taken by projection methods. Solution of Momentum conservation becomes stiff since information has to travel 'upstream'. Is there any reason you set your exits instead of the inlet?
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June 18, 2017, 18:26 

#17 
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cyln
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I see, thanks a lot for your explanatory reply.
I need to set the inlet velocity BC to 'fixedValue' and outlet velocity BC to 'mass flow rate'. At the outflow boundary, the velocity is not uniform. I searched for the name of the BC so that I could set the outlet mass flow rate, but I couldnt find it. So, I did a test run with "flowRateInletVelocity" with a negative mass flow rate. Anyways, I got stuck with the continuity problem 

June 19, 2017, 02:46 

#18 
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Santiago Lopez Castano
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You are a FLUENT user, right?
The outflow BC as used in ANSYS does not exist in FOAM, as far as I know. Besides, using such boundaries constraint the physics resolvable by the code. An advective (or even zeroGradient) BC on the fluxes at the outlet is preferred. In the end depends on how much physics are you expecting to see. Sent from my Lenovo A5000 using CFD Online Forum mobile app 

June 19, 2017, 03:23 

#19 
Senior Member

@cyln
Could you decrease relTol in velocity linear solver? Guess, it is 0.1 now, and this is not enough to satisfy BCs. Also maybe you can disclose what outflow Bfs did you utilise for CFX simulation? 

June 19, 2017, 11:56 

#20 
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cyln
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@alexeym
I varied relTol between 0.1 and 0.01. I am getting the same error. In CFX, I assigned mass flow rate at the outflow boundary. It is so robust, converge no matter what. I wanna assign the same BC in OF. @Santiago I am a CFX user, however, switching to OF. I need to capture the velocity gradient at the outlet as accurate as possible. 

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