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Is it possible to use gradient adaption for two different parameters?

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Old   January 17, 2017, 20:20
Default Is it possible to use gradient adaption for two different parameters?
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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
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Old   April 19, 2018, 06:00
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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.
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Old   April 29, 2020, 10:35
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Quote:
Originally Posted by torjo View Post
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
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Old   April 29, 2020, 11:55
Default VF Gradient
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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.
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Old   April 30, 2020, 03:25
Default VF gradient
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Quote:
Originally Posted by vinerm View Post
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.
Code:
#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.

Code:
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".

Code:
#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.

Last edited by rupak504; April 30, 2020 at 04:05. Reason: update
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Old   April 30, 2020, 05:20
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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.
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Old   April 30, 2020, 18:33
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Quote:
Originally Posted by vinerm View Post
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?


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Old   May 1, 2020, 15:37
Default Mass Transfer Mechanism
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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.
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