# UDM derivatives for - for EHD flow

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 January 15, 2008, 10:42 UDM derivatives for - for EHD flow #1 Sandilya Garimella Guest   Posts: n/a Hello All I am writing a UDF to solve for two scalars with source terms The details of user defined scalars are UDS0 is potential UDS1 is charge density I am solving for an EHD flow now using a formula I am writing a UDM such that UDM0 = f(UDS0) UDM1 = f(UDS1) where f() represents "function of" Now the source terms of my equations for UDS0 and UDS1 depend upon derivatives of UDM0 and UDM1 hence I need to calculate these derivatives Now, Derivatives for UDS in any cell can be got by UDSI_G(c,t,f)[] Similarly is there a macro to get derivatives of UDM in each cell Would be glad if someone could point me to correct source or solution to my requirement Thank you Regards Sandilya

 January 15, 2008, 10:44 Re: UDM derivatives for - for EHD flow #2 Sandilya Garimella Guest   Posts: n/a Hello All I am writing a UDF to solve for two scalars with source terms. The details of user defined scalars are UDS0 is potential UDS1 is charge density I am solving for an EHD flow Now using a formula I am writing a UDM such that UDM0 = f(UDS0) UDM1 = f(UDS1) where f() represents "function of" Now the source terms of my equations for UDS0 and UDS1 depend upon derivatives of UDM0 and UDM1 hence I need to calculate these derivatives. Now, Derivatives for UDS in any cell can be got by UDSI_G(c,t,f)[] Similarly is there a macro to get derivatives of UDM in each cell Would be glad if someone could point me to correct source or solution to my requirement Thank you Regards Sandilya

 January 15, 2008, 13:06 Re: UDM derivatives for - for EHD flow #3 RE13 Guest   Posts: n/a Have you tried manually calculating the derivatives in the the source code?

 January 15, 2008, 13:09 Re: UDM derivatives for - for EHD flow #4 Sandilya Garimella Guest   Posts: n/a I did and it works but the expressions are cumbersome and i want to make code easily understandable!!! Can we extract derivatives of UDM's?

 January 16, 2008, 03:23 Re: UDM derivatives for - for EHD flow #5 Reza Guest   Posts: n/a Hi,Sandilya. As I know for modeling of EHD with fluent you need to calculate E that is derivative of potential. For this you must define Ex and Ey in the Adjust part of your UDF like this:"C_UDMI(c,t,0)=C_UDSI(c,t,0);C_UDMI(c,t,1)=C_ UDSI_G(c,t,0)[0];C_UDMI(c,t,2)=C_UDSI_G(c,t,0)[1];" where C_UDSI(c,t,0) is potential. After this, you can use C_UDMI(c,t,1),C_UDMI(c,t,2) in other parts of your UDF for Ex and Ey. I hope this is useful for you. Regards

 July 9, 2009, 07:49 #6 Member   Akour Join Date: May 2009 Posts: 79 Rep Power: 9 Sandiya, This is very late...but I am trying to do a ehd simulation as well and am having problems with one of the source terms in the space charge transport equation, whenever I have a source term as dV/dX (gradient of potential), my solution diverges...I have tried storing the gradients in a define adjust and then using the C_UDMI values...but it doesnt make a difference...so when I add the source term Q*E_x on the x-direction momentum equation as: C_UDSI(c,t,1)*C_UDSI_G(c,t,0)[0] the solution diverges...and the same goes for the source term in the space charge transport equation which depends on electric field...please help if you can AK __________________ akour

 August 24, 2011, 11:50 #7 New Member   sarah Join Date: Oct 2009 Posts: 3 Rep Power: 8 Hi Reza - regarding your code above, why do you need the line C_UDMI(c,t,0) = C_UDSI(c,t,0) if you are not using it in calculating the components of electric fields. I am trying to do this also and I'm getting access errors even when I include this line. The following is my adjust function - if you spot any major errors please let me know. #include "udf.h" #include "sg.h" DEFINE_ADJUST(udf_adjust, domain) { Thread *t; cell_t c; thread_loop_c(t,domain) { begin_c_loop(c,t) { C_UDMI(c,t,0) = C_UDSI(c,t,0) ; /*-------------------------------------------------------------*/ /* udm 0 -> Electric field axial component */ /*-------------------------------------------------------------*/ C_UDMI(c,t,1) = - C_UDSI_G(c,t,0)[0] ; /*-------------------------------------------------------------*/ /* udm 1 -> Electric field radial component */ /*-------------------------------------------------------------*/ C_UDMI(c,t,2) = - C_UDSI_G(c,t,0)[1] ; /*-------------------------------------------------------------*/ /* udm 2 -> Electric field magnitude */ /*-------------------------------------------------------------*/ C_UDMI(c,t,3) = NV_MAG(C_UDSI_G(c,t,0)) ; } end_c_loop(c,t) } }

 September 9, 2013, 03:17 #8 New Member   ehsan Join Date: Jul 2013 Posts: 5 Rep Power: 5 Hi,can any body help me about ehd on fluent,i have some experimental data on ehd.i want to model it in fluent but i have not any code of ehd in fluent.please help me abut how to model ehd in fluent,thank you. e.shabahang@gmail.com

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