
[Sponsors] 
December 4, 2009, 06:47 
NEW surface tension model for Fluent

#1 
New Member
Join Date: Mar 2009
Posts: 15
Rep Power: 8 
Hello there!
I have made a new surface tension model. It gives no spurious currents within the volume of secondary phase, but there is always spurious velocity in one or two cells on interface. This velocity is increasing with time. What is it caused by? I think I should make a change to pressure correction equation, but I am afraid that it is not possible in Fluent. right? Please, any advice! best regards bohis 

December 4, 2009, 10:31 
surf model

#2  
Senior Member

What is your model based on? Has it been published (just the model etc?) It would be interesting to see your surf. Ten. Force model.
Quote:


December 4, 2009, 12:31 
just brief description

#3 
New Member
Join Date: Mar 2009
Posts: 15
Rep Power: 8 
Hi, it is based on Height Functions and I reconstruct the length of interface in 2D cell. Ten. force is applied only where interface is present.
and what is your view on my mass imbalance in very few cells  spurious velocity? cia! Honza 

April 8, 2013, 07:14 

#4 
Member
Indrajit
Join Date: Nov 2012
Posts: 33
Rep Power: 4 
Hi bohis.
Its been a long time now since you posted this thread, but can you share your surface tension model for fluent??? Actually I have been doing some simulations and parasitic currents at the interface have troubled me for some time. Your help would be of great value to me. Regards, Indrajit 

April 8, 2013, 07:43 
surface tension

#5 
New Member
Join Date: Mar 2009
Posts: 15
Rep Power: 8 
Hello!
U are right, it has been really a long time since I last did something with surface tension. Fluent calculates normals and curvatures from gradient of volume fraction and we can say that this is very inaccurate! I used socalled Height Functions to tackle this. The second thing is to formulate surface tension force as a volumetric force. I suggest U use Brackbills approach. Even with this strategy U should get better results. good luck! 

April 8, 2013, 08:46 

#6  
Member
Indrajit
Join Date: Nov 2012
Posts: 33
Rep Power: 4 
Quote:
Even if I calculate the normals by height functions, how can I modify the surface tension expression in fluent?? Fluent uses Brackbill's model but I have found out that with this model parasitic currents increase for small drop radius and low speeds!!! These currents totally alter the shape of my drop giving unrealistic results!! Please suggest me something I could possibly do. Is there a way to modify fluent's surface tension expression??? Regards, Indrajit 

April 9, 2013, 02:39 

#7  
New Member
Join Date: Mar 2009
Posts: 15
Rep Power: 8 
Quote:
Switch of Fluent surface tension model and write it youself (DEFINE_SOURCE(c,t)) I will do the same what FLUENT does, but U will use normals and curvatures computed using HEIGHT FUNCTIONS. Note that U can apply HF only on structured meshes...good luck, I cannot help you more.. 

April 9, 2013, 03:05 

#8  
Member
Indrajit
Join Date: Nov 2012
Posts: 33
Rep Power: 4 
Quote:
I will see how that works out!! As also could you please tell me how do I change the smoothing value of VOF in fluent??? I read that after ('rpsetvar patch/vof? #t) I must set smoothing value to 2 to avoid spuriou currents.However I have not been able to find how it is done? Regards, Indrajit 

April 14, 2013, 06:00 

#9  
Member
Indrajit
Join Date: Nov 2012
Posts: 33
Rep Power: 4 
Quote:
I wrote the UDF for height functions but it gives me segmentation violation error. I put the calculation of height functions and curvature in DEFINE_EXECUTE_AT_END so that they are not calculated every iteration ( DEFINE_SOURCE might do that!!) Please take a look at the following UDF and let me know how can I deal with segmentation violation error. I guess nesting of loops is causing the trouble.: DEFINE_EXECUTE_AT_END(heightfunc) { cell_t cell1,c; Thread *t,*pt,*pt1,*thread1; Domain *d; real xc[ND_ND],xc1[ND_ND],xc2[ND_ND]; d = Get_Domain(1); real VOF,VOF1,sum=0,e=0,w=0,q=0,h1,h2,k; /****************************************This part calulates the height functions***************************************** *************************/ thread_loop_c(thread1,d) /*loops over all cell threads in domain*/ { printf("first change loop"); begin_c_loop_all (cell1,thread1) { pt= THREAD_SUB_THREAD(thread1,1); if( C_VOF(cell1,pt)!=0 && C_VOF(cell1,pt)!=1) { sum=0; C_CENTROID(xc,cell1,thread1); VOF=C_UDMI(cell1,pt,0); VOF1=C_UDMI(cell1,pt,1); if(abs(VOF1)>= abs(VOF)) { thread_loop_c(thread1,d) /*loops over all cell threads in domain*/ { begin_c_loop_all (cell1,thread1) { pt1= THREAD_SUB_THREAD(thread1,1); if( C_VOF(cell1,pt1)!=0 && C_VOF(cell1,pt1)!=1) { C_CENTROID(xc1,cell1,thread1); if( xc1[0]==xc[0] && xc1[1]>=(xc[1](3*dely)) && xc1[1]<=(xc[1]+(3*dely))) { sum=sum+ (C_VOF(c,t)*dely); } } } end_c_loop_all (cell1,thread1) } } C_UDMI(cell1,thread1,3)=sum; } } end_c_loop_all (cell1,thread1) } /**********************************************This part caluclates the curvature***************************************** ******************************/ thread_loop_c(thread1,d) /*loops over all cell threads in domain*/ { printf("second loop"); begin_c_loop_all (cell1,thread1) { pt1= THREAD_SUB_THREAD(thread1,1); if( C_VOF(cell1,pt1)!=0 && C_VOF(cell1,pt1)!=1) { C_CENTROID(xc2,cell1,thread1); thread_loop_c(t,d) /*loops over all cell threads in domain*/ { begin_c_loop_all (c,t) { pt= THREAD_SUB_THREAD(t,1); if( C_VOF(c,pt)!=0 && C_VOF(c,pt)!=1) { /*C_UDSI(c,t,0)=C_UDMI(c,t,0); C_UDSI(c,t,1)=C_UDMI(c,t,1); C_UDMI(c,t,6)= NV_MAG(C_UDSI_G(c,t,0))*C_UDMI(c,t,0)*0.0702*C_R(c ,t); C_UDMI(c,t,7)=NV_MAG(C_UDSI_G(c,t,1))*C_UDMI(c,t,1 )*0.0702*C_R(c,t);*/ C_CENTROID(xc1,c,t); if((xc2[0]+delx)==xc1[0]) { e= C_UDMI(c,t,3); } if((xc2[0]delx)==xc1[0]) { w= C_UDMI(c,t,3); } if(xc2[0]==xc[1]) { q= C_UDMI(c,t,3); } } } end_c_loop_all (c,t) } h1=(ew)/delx; h2=(e(2*q)+w)/(delx*delx); k=h2/pow((1+(h1*h1)),1.5); C_UDMI(cell1,thread1,4) = 0.0702*k*(2e6)*(h1)/volume; C_UDMI(cell1,thread1,5)= 0.0702*k*(2e6)*(1)/volume; } } end_c_loop_all (cell1,thread1) } } 

November 8, 2013, 16:05 

#10 
Member
Shashank
Join Date: Apr 2011
Posts: 70
Rep Power: 6 
Were you able to solve the segmentation error in your UDF?


November 8, 2013, 19:04 

#11 
Member
Shashank
Join Date: Apr 2011
Posts: 70
Rep Power: 6 
You haven't specified VOF, VOF1, delx and dely anywhere in your code. Can you explain what they are?


January 9, 2014, 20:49 

#12 
Member
Shashank
Join Date: Apr 2011
Posts: 70
Rep Power: 6 
Could you please tell me what C_UDMI(c,t,0) and C_UDMI(c,t,1) are in your UDF? They seem to have appeared from nowhere. I see no calculation done for these two terms.


July 17, 2014, 17:40 

#13 
New Member
Rajesh Singh
Join Date: Jun 2010
Posts: 9
Rep Power: 7 
Hi Indrajit,
Can you share the working correct form of this code. I will really appriciate for this. Regards 

July 23, 2014, 01:47 

#14 
Member
Join Date: Dec 2012
Posts: 79
Rep Power: 4 
Hi
A short hint for those who want do use the height function. The calculation for that is of order O(n^2). For large meshes this can take heaps of time. There is a function built in called LevelSet which does pretty much the same. 

August 20, 2014, 21:24 

#15 
New Member
Bill Wang
Join Date: Aug 2014
Posts: 21
Rep Power: 3 
Hi IndrajitW,
Have you got the correct result with the UDF you have mentioned? I have been working on the wickvapor interface for a long time but not with a good answer. Will you give me some hint? Bill 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem with Gmsh  nishant_hull  Open Source Meshers: Gmsh, Netgen, CGNS, ...  23  August 5, 2015 02:09 
Continuum Surface Force Model  Miguel  Main CFD Forum  4  August 4, 2012 14:56 
Surface tension problem.  Kes  FLUENT  0  May 19, 2008 22:49 
Surface tension  Continuum Surface Force model  Robert  Main CFD Forum  0  May 2, 2002 07:34 
surface tension model in EX26  Junhua Wu  FLUENT  0  June 7, 2000 10:49 