[Hossein1]

I am using 'dynamic gradient adaption' for mesh refinement. However, I'd like to be able to apply refinement not only based on phase gradient but also wherever there is a velocity gradient as well.
Please let me know whether it is possible and how.

Thanks

[torjo]

Hi,

It is possible, but its a bit of work.

Specify a UDS and use a DEFINE_AT_END udf to fill it with VOF.
This gives you access to the VOF gradient via the UDSI_G macro.

Use this to make another UDF that takes UDSI_G and whatever other scalar you wish to adapt with, and flag a user defined memory (UDM) such that it either adaps or coarsens.

e.g.

If UDSI_G>0.1 || dv/dx>0.1
UDM=1
else
UDM=0

then just set the adaption to be depending on the UDM (adapt if UDM>0.5 coarsen if UDM<0.5).


I hope this makes somewhat sence, else i will be happy to elaborate.

BR.
Torjo

[rupak504]

Originally Posted by torjo

Hi,

It is possible, but its a bit of work.

Specify a UDS and use a DEFINE_AT_END udf to fill it with VOF.
This gives you access to the VOF gradient via the UDSI_G macro.

Use this to make another UDF that takes UDSI_G

I hope this makes somewhat sence, else i will be happy to elaborate.

BR.
Torjo

Hello torjo

This is a similar problem you described above. I wish to find gradients of alpha variables in the VOF model. I already checked the expert activation and tried C_VOF_G, it still gives the error. One of the papers, I read they used UDS to define the alpha and then UDSI_G to find the gradient of alpha.


Above you have written "Specify a UDS and use a DEFINE_AT_END udf to fill it with VOF" what do you mean by this statement?

What I did was, I defined a UDS in the fluent, choosing alpha as the new variable, then I used UDSI_G to find the gradient of the alpha variable but it still gives the error.

Any help would be highly appreciated.

regards

[vinerm]

If using C_VOF_G gives segmentation violation, then the memory has not been assigned. However, if the value is 0, then the arguments passed to C_VOF_G could be wrong.

[rupak504]

Originally Posted by vinerm

If using C_VOF_G gives segmentation violation, then the memory has not been assigned. However, if the value is 0, then the arguments passed to C_VOF_G could be wrong.

#include "udf.h"
#include "sg.h"
#include "sg_mphase.h"
#include "flow.h"
#include "metric.h"
#include "mem.h"
#define CON 1

DEFINE_ADJUST(store_gradient, domain)
{
	Thread *t;
	Thread **pt;
	cell_t c;
	int phase_domain_index = 0.;
	Domain *pDomain = DOMAIN_SUB_DOMAIN(domain, phase_domain_index);
	{
		Alloc_Storage_Vars(pDomain, SV_VOF_RG, SV_VOF_G, SV_NULL);
		Scalar_Reconstruction(pDomain, SV_VOF, -1, SV_VOF_RG, NULL);
		Scalar_Derivatives(pDomain, SV_VOF, -1, SV_VOF_G, SV_VOF_RG,Vof_Deriv_Accumulate);
	}

	mp_thread_loop_c(t, domain, pt)
		if (FLUID_THREAD_P(t))
		{
			Thread *ppt = pt[phase_domain_index];

                        begin_c_loop(c, t)
			{
				C_UDMI(c, t, 0) = C_VOF_G(c, ppt)[0];
                                Message("UDMI0: %g\n", C_UDMI(c, t, 0));
			}
			end_c_loop(c, t)
		}
	Free_Storage_Vars(pDomain, SV_VOF_RG, SV_VOF_G, SV_NULL);
}

With this, I am not getting any segmentation errors.

UDMI0: 0
UDMI0: 0
UDMI0: 0
UDMI0: 0
UDMI0: 0
UDMI0: 0
UDMI0: -1.95115e-41
UDMI0: 0
UDMI0: 0
UDMI0: 0
UDMI0: 0

I am getting messages like this. Here, I have not switched on the expert option.

Now, I tried to use the expert option and set keep memory "yes".

#include "udf.h"
#include "sg.h"
#include "sg_mphase.h"
#include "flow.h"
#include "metric.h"
#include "mem.h"
#define CON 1

/************************************************** *******************/
/* Finding dot product of gradT and gradalpha(liq) */
/************************************************** *******************/

DEFINE_ADJUST(store_gradient, domain)
{
	Thread *t;
	Thread **pt;
	cell_t c;
	int phase_domain_index = 0.;
	Domain *pDomain = DOMAIN_SUB_DOMAIN(domain, phase_domain_index);

	mp_thread_loop_c(t, domain, pt)
		if (FLUID_THREAD_P(t))
		{
			Thread *ppt = pt[phase_domain_index];

			begin_c_loop(c, t)
			{

                C_UDMI(c, t, 0) = NV_DOT(C_VOF_G(c, ppt), C_T_G(c,t));
				Message("UDMI0: %g\n", C_UDMI(c, t, 0));

			}
			end_c_loop(c, t)
		}
}

But, here I got segmentation error.

[vinerm]

You appear to be getting correct values based on your first code with allocation of storage. Do note that most of these values would be 0 since most of the cells either have phase-1 or phase-2. Only at the interface the values would be positive or negative. So, run this UDF, go to Contour Plots and then plot the value for UDM. You should observe non-zero values at the interface.

[rupak504]

Originally Posted by vinerm

You appear to be getting correct values based on your first code with allocation of storage. Do note that most of these values would be 0 since most of the cells either have phase-1 or phase-2. Only at the interface the values would be positive or negative. So, run this UDF, go to Contour Plots and then plot the value for UDM. You should observe non-zero values at the interface.

I have got a doubt regarding the source term, that we add to the continuity equation. For example, let's say boiling problem, many a place people have given if (T(l)>Tsat) condition before mass transfer rate, many a place they did not. Which one is correct?


regards

[vinerm]

Mass transfer by evaporation is always due to vapor pressure being different from saturation pressure. Evaporation temperature is a function of pressure above the water. E.g., water boils at 373 K at sea level but same water boils at 345 K at the top of Mount Everest. The reason being low pressure. For the same reason, water in a pond is colder than atmosphere, due to same phenomenon fruit or vegetable or wet clothes dry even if not kept in the sun and without any wind; the vapor pressure in the atmosphere decides at what temperature the water should evaporate. There is a corresponding saturation temperature for each vapor pressure. If temperature of liquid is greater than the saturation temperature for a given pressure, then water evaporates. So, if we are talking about evaporation, mass transfer from liquid to vapor is possible only if liquid temperature is greater than the saturation temperature.