# Time derivative

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 February 18, 2009, 09:24 Time derivative #1 Dmitry Guest   Posts: n/a Hi. I want to calculate dp/dt (static pressure time-derivative) in my transient calculation and store it. Is it possible to access to pressure value at previous time step in UDF?

 February 18, 2009, 10:27 Re: Time derivative #2 Grzegorz Guest   Posts: n/a Yes, it is possible. You can use C_P_M1(c,t) or C_P_M2(c,t) for previous and second previous time step, respectively (see User's Guide - 3.2.3 Cell Macros). You can also use memory storage C_UDMI to save current pressure field and in next time step acces to it and refill it with new values. Hope this help.

 February 19, 2009, 14:48 Re: Time derivative #3 Dmitry Guest   Posts: n/a Thank you. But the problem stil remains. I use C_UDMI to save previous time step pressure: C_UDMI(c,t,0)=C_P_M1(c,t) in my UDF in DEFINE_ADJUST macros. I get error message: FLUENT received fatal signal (ACCESS_VIOLATION) after first iteration.

 February 19, 2009, 14:50 Re: Time derivative #4 Grzegorz Guest   Posts: n/a Did your solution is calculated or did you start it from very begining?

 February 19, 2009, 15:09 Re: Time derivative #5 Dmitry Guest   Posts: n/a Yes, my solution calculated some time steps. Firstly I did 5 time steps without using of C_P_M1, then I put it in my UDF, reinterpreted it and tried to run calculation. But I have error.

 February 19, 2009, 15:17 Re: Time derivative #6 Grzegorz Guest   Posts: n/a Send me your udf - kondora@imc.pcz.czest.pl

 February 20, 2009, 08:54 Re: Time derivative #7 mange Guest   Posts: n/a did you remember to allow memory access for your C_UDMI ? /m

 February 20, 2009, 10:27 Re: Time derivative #8 Dmitry Guest   Posts: n/a What does it mean /m? I use other C_UDMI(c,t,1), C_UDMI(c,t,2) and so on to store my variables and I have no problems to access them.

 February 20, 2009, 10:33 Re: Time derivative #9 mange Guest   Posts: n/a I mean in the panel of user memory you have to specify how many fields you want to allocate memory for. by default it is 0.

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