# how to obtain time derivative

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 August 23, 2004, 01:15 how to obtain time derivative #1 soubhik Guest   Posts: n/a hi friends, I am solving some transient diffusion problem where I have to define a time derivative of an additional variable (concentration) in the source time. Has anyone of u tried obtaining time derivative such as dc/dt . I have used 2 additional variables C1 and C2 which calls the values of concentration at the beginning and end of time step (junction box) C (current timestep) - C(earlier timestep)/timestep this doesnot work because the expression yields values of zero for each timestep. presumably it reads the same value for C1 and C2 at each timestep . please suggest me any other ways. soubhik

 August 25, 2004, 23:43 Re: how to obtain time derivative #2 Jeff Guest   Posts: n/a Try calling your junction box just at the end of the time step with the following algorithm: Loop over all nodes: dCdt = (NewValue - C1)/dt C1 = NewValue C2 = dCdt End loop Jeff

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