# Derivative

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 August 25, 2014, 04:50 Derivative #1 Member   Mansoor Saleem Join Date: Jun 2014 Posts: 31 Rep Power: 5 Hello Good Morning I want to define derivative in my expressions, I have seen in the post of 2011, Singer1812 has suggested someone to use TRANS_LOOP as an alternative of derivative. I want to introduce Density in this way. Is anyone here who has any idea how to use this how we introduce the Old Variable?? Thank you so much.

 August 25, 2014, 06:49 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,097 Rep Power: 109 If you just need the time derivative then use the time derivative modifier. This is described in the reference manual. There are several thread son the forum which describe how to use the TRANS_LOOP approach, do a search to find them.

 August 25, 2014, 06:52 #3 Member   Mansoor Saleem Join Date: Jun 2014 Posts: 31 Rep Power: 5 Thank you so much ghorrocks I will try

 August 25, 2014, 08:17 #4 Member   Mansoor Saleem Join Date: Jun 2014 Posts: 31 Rep Power: 5 Hello Ghorrocks If I want to introduce an expression like this CEL = Density at time 0 - Density of current time step is it possible to introduce in this way. I think for 2nd value we can use the simple variable Density but for first variable how can we introduce time reference? Thank you so much

 August 25, 2014, 08:37 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,097 Rep Power: 109 If you want to reference it against the density at time zero I see two options: * If your time zero density can be calculated then you can simply calculate it when you need it. Then you do not need to store it. * The TRANS_LOOP trick only gives you the variables value at the last time step. If you want to store the first time step value then you will have to write a fortran routine to do this.

 August 25, 2014, 09:23 #6 Member   Mansoor Saleem Join Date: Jun 2014 Posts: 31 Rep Power: 5 Thank you ghorrocks I am using the 1st option but I dont have any idea of User Fortran, Can you please recommend some tutorial or example because from user Guide its really difficult to understand.

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