[nandiganavishal]

Dear Foamers,

I would like to know, if we can get the time derivative of a variable as the output. For instance, if I am solving the transient Navier Stokes, is it possible to get the acceleration (du/dt) too at every time step as an output.

Kindly let me know.

Regards

Vishal Nandigana

[alberto]

Use fvc::ddt(yourField) to store it in a volField of the same type of your field. Setting the IOobject to AUTO_WRITE will have the time derivative stored.

[nandiganavishal]

Thanks Alberto. It worked well.

[Andrea_85]

Hi,
how can i get the same in a postprocessing? If i use this code:

read U
read rho

volVectorField ddt
(
IOobject
(
"ddt",
runTime.timeName(),
mesh,
IOobject::NO_READ,
IOobject::AUTO_WRITE
),

fvc::ddt(rho,U)
);

ddt.write()

it returns always zero.

best
andrea

[alberto]

Originally Posted by Andrea_85

Hi,
how can i get the same in a postprocessing? If i use this code:

read U
read rho

volVectorField ddt
(
IOobject
(
"ddt",
runTime.timeName(),
mesh,
IOobject::NO_READ,
IOobject::AUTO_WRITE
),

fvc::ddt(rho,U)
);

ddt.write()

it returns always zero.

best
andrea

If you only declared the field in createFields.H, it is normal. You have to

a) declare the field as you did in createFields.H

b) add the line

ddt == fvc::ddt(rho,U);

at the end of pEqn.H, so you have the corrected velocity. You do not need the .write() call if you define the field with AUTO_WRITE in the global scope.

Best,