[tmy]

Hi all,

I was trying to output the time change rate of one species. As I searched online, there exists two methods, either by using fvm:: or fvc::. However, I found big differences between these two methods. Which one is correct? Or more accurate? Please find the codes below:

DDT_[i].field() = (fvm::ddt(rho, Y[i])()).source();
DDTfvc_[i].field() = fvc::ddt(rho, Y[i]);

I also attached the values of DDT and DDTfvc of C7H16 at t=1e-6s. As you can see they have the difference with order of 2.

DDT_C7H16.txt

DDTfvc_C7H16.txt

[simrego]

Hi!


fvm refers to finiteVolumeMethod or Matrix? not sure which one. But it will return an equation system for you since it is an implicit operator.
fvm is finiteVolumeCalculus. This is an explcit operator, which will do what you want.
So fvm will give you A*b=c. Actually it will give you the A matrix, and the c source vector, since b is the unknown, this is what we are looking for. when you use fvm(...).source, it should return the c vector from the equation system.
While fvc is the explicit operator, it will return a vector with the quantity you are interested in.


So shortly:
fvm: implicit, it will return the discretised equation system.

fvc: explicit operator, it will return the quantity.