postProcess div(U)

jagan1mohan · July 5, 2020, 02:41

Hello Friends, I've simulated cavity case from tutorials and everything works fine. Last timeStep residuals are as given below.

Time = 0.5

Courant Number mean: 0.222158 max: 0.852134
smoothSolver: Solving for Ux, Initial residual = 2.3091e-07, Final residual = 2.3091e-07, No Iterations 0
smoothSolver: Solving for Uy, Initial residual = 5.0684e-07, Final residual = 5.0684e-07, No Iterations 0
DICPCG: Solving for p, Initial residual = 8.63844e-07, Final residual = 8.63844e-07, No Iterations 0
time step continuity errors : sum local = 8.8828e-09, global = 4.94571e-19, cumulative = 1.10417e-17
DICPCG: Solving for p, Initial residual = 9.59103e-07, Final residual = 9.59103e-07, No Iterations 0
time step continuity errors : sum local = 9.66354e-09, global = 1.13175e-18, cumulative = 1.21735e-17

It is important to note, div(U) as we know is continuity equation and its residual is in order of e-17 from above.

But when we issue postProcess -func 'div(U)', first observation is the following message for each time folder.

Time = 0.5

Reading fields: volVectorFields: U

Executing functionObjects
--> FOAM Warning : functionObjects::div div(U) cannot find required object U of type surfaceScalarField
functionObjects::div div(U) writing field: div(U)

However, this writes a field by the name div(U) to each time folder.

My second observation is when we see the values in these fields, values are orders of magnitude higher even at last time steps and we say case is converged.

1. What is this error / warning? How to address this?

2. What is the reason for discrepancy between div(U) and continuity residual?

Thank you,Jagan Mohan

Anup Singh · July 5, 2020, 06:37

fvm:div() or fvc:div() they both take first term as a surface scalar field as input whereas U is volume vector field which you are trying to take. Nevertheless, If you need to see output of div(U) you have to first write a surface scalar field of U which in this case is already written as surface scalar variable phi so there is no need to go to such length.

For further instruction about various operators and their uses you should refer to Openfoam user guide and Programmers Guide.

July 5, 2020, 02:41	postProcess div(U)	#1
jagan1mohan New Member Jagan Mohan Join Date: Dec 2019 Location: New York Posts: 27 Rep Power: 7	Hello Friends, I've simulated cavity case from tutorials and everything works fine. Last timeStep residuals are as given below. Time = 0.5 Courant Number mean: 0.222158 max: 0.852134 smoothSolver: Solving for Ux, Initial residual = 2.3091e-07, Final residual = 2.3091e-07, No Iterations 0 smoothSolver: Solving for Uy, Initial residual = 5.0684e-07, Final residual = 5.0684e-07, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.63844e-07, Final residual = 8.63844e-07, No Iterations 0 time step continuity errors : sum local = 8.8828e-09, global = 4.94571e-19, cumulative = 1.10417e-17 DICPCG: Solving for p, Initial residual = 9.59103e-07, Final residual = 9.59103e-07, No Iterations 0 time step continuity errors : sum local = 9.66354e-09, global = 1.13175e-18, cumulative = 1.21735e-17 It is important to note, div(U) as we know is continuity equation and its residual is in order of e-17 from above. But when we issue postProcess -func 'div(U)', first observation is the following message for each time folder. Time = 0.5 Reading fields: volVectorFields: U Executing functionObjects --> FOAM Warning : functionObjects::div div(U) cannot find required object U of type surfaceScalarField functionObjects::div div(U) writing field: div(U) However, this writes a field by the name div(U) to each time folder. My second observation is when we see the values in these fields, values are orders of magnitude higher even at last time steps and we say case is converged. 1. What is this error / warning? How to address this? 2. What is the reason for discrepancy between div(U) and continuity residual? Thank you,Jagan Mohan

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Incompressible div(U) validation errors	steampunc	OpenFOAM Post-Processing	5	April 21, 2020 05:00
postprocess: symbol lookup error	BenAsbo626	OpenFOAM Post-Processing	2	October 26, 2018 14:28
postProcess functionality in openFOAM 4	bullmut	OpenFOAM Post-Processing	23	July 21, 2017 10:11
[OpenFOAM] Take derivative of mean velocity in paraFoam	hiuluom	ParaView	13	April 26, 2016 07:44
Derivative of velocity and mean velocity	hiuluom	OpenFOAM Post-Processing	1	May 30, 2015 00:42

July 5, 2020, 06:37		#2
Anup Singh New Member Anup Singh Join Date: Mar 2020 Posts: 22 Rep Power: 6	fvm:div() or fvc:div() they both take first term as a surface scalar field as input whereas U is volume vector field which you are trying to take. Nevertheless, If you need to see output of div(U) you have to first write a surface scalar field of U which in this case is already written as surface scalar variable phi so there is no need to go to such length. For further instruction about various operators and their uses you should refer to Openfoam user guide and Programmers Guide.