# How could I get the gradient of UDM?

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 February 7, 2011, 10:27 How could I get the gradient of UDM? #1 New Member   Join Date: Mar 2009 Posts: 19 Rep Power: 9 Hi, everyone, I am trying to get the second derivative of the velocity using UDF. I give the value of the gradient of velocity to a UDM, then I need to know how to get the gradient of the UDM? Could anyone help? Thanks very much! Fireman

 February 7, 2011, 20:57 #2 Senior Member   Real Name :) Join Date: Jan 2010 Location: United States Posts: 193 Rep Power: 8 You could write the derivative of velocity (C_DUDX, etc) to a UDS, then use C_UDSI_G(c,t,0) to get the derivative of the scalar. That's kind of a hack way to do it, but it's there. You'll need to enable the User-defined scalar, and turn off solution of the transport equation for your scalar in order to keep the values you write fixed. You could also calculate the numerical derivative yourself by looking at the cell's neighboring values for Du/Dx (etc) and coming up with some approximation. Hope this helps. ComputerGuy

 February 8, 2011, 06:36 #3 New Member   Join Date: Mar 2009 Posts: 19 Rep Power: 9 ComputerGuy, thank you very much!

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