UDM derivatives for - for EHD flow
 

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

Re: UDM derivatives for - for EHD flow
 

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

Re: UDM derivatives for - for EHD flow
 

Have you tried manually calculating the derivatives in the the source code?

Re: UDM derivatives for - for EHD flow
 

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?

Re: UDM derivatives for - for EHD flow
 

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

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

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)
}
}

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