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How to simulate a hull in Fluent using VOF

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Old   November 12, 2012, 07:51
Default How to simulate a hull in Fluent using VOF
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Hello everybody, I'm a university student and I'm trying to simulate a ship hull on a water channel in Fluent.
I have realized the mesh in Gambit and it seems to have a good quality.
My problem is how to set the parameters and the simulation in Fluent 14.
I read I have to use a VOF model for a simulation concerning both the water and the air.
I did the follow commands but you can see the results in figure 2: the water and the air seems to be mixed..

My commands in Fluent14:
  • import the gambit mesh
  • set a steady solver
  • set the VOF model - two phases - implicit scheme - open channel flow
  • set the model viscous k-epsilon (2eq)
  • define materials - water liquid
  • define phases - phase 1=air and phase 2 = water liquid.
  • define operating conditions - gravity=-9.81 m/s2 and specified operating density.

regarding the boundary conditions they are reported in figure1. I used symmetry to simulate half fluid volume, wall on the hull, ocean floor and on the right surfaces of the fluid volume, pressure outlet for air in the air surfaces and velocity_inlet for the entrance surface of water. For the water pressure outlet, I set an udf. It describes the idrostatic pressure depending by the z-coordinate :
  • define-user defined-functions-interpreted and chose the following Udf:
/************************************************** *********************
UDF for specifying steady-state parabolic pressure profile boundary
profile for a turbine vane
************************************************** **********************/
#include "udf.h"
DEFINE_PROFILE(pressure_profile,thread,index)
{
real x[ND_ND];
real z;
face_t f;
begin_f_loop(f,thread)
{
F_CENTROID(x,f,thread);
z=x[2];
F_PROFILE(f,thread,index) = 998.2*9.81*(-z);
}
end_f_loop(f,thread)
}
  • set the boundary condition for the water outlet: boundary conditions - pressure_oultet_water. phase: mixture. edit - I chose the udf_pressure_profile
  • phase water - multiphase - backflow volume fraction=1
  • boundary conditions - pressure_oultet_air: phase:mixture - gauge pressure=0
  • boundary conditions - velocity_inlet_air: phase:mixture magnitude normal to bound velocity magnitude:2m/s
  • phase water - multiphase-backflow volume fraction=1

now I set the position of the water when the simulation start
  • adapt-region and I insert the values for the volume of water. then I press mark.
  • solve -controls-solution: I chose the PISO model and all the first order equations
  • solve monitors-residual all the residual set to 10^-5
  • solve initialize - init
  • solve initialize - patch - water

where I'm wrong? Thanks to everyone will help me!

BC-hull.jpg

results.png
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Old   November 15, 2012, 08:53
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Anyone can help me?
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Old   April 29, 2013, 04:19
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I have a similar problem, can anyone help please?
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Old   April 29, 2013, 06:53
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http://www.youtube.com/watch?v=y4Sd__Viw-0
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Old   November 17, 2013, 15:19
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hello,
i also did what it is shown in that video,but with diff domain n ship,so i get net mass flow rate -2.478946e+9.but it should be zero known,pls any 1 will help in suggesting the boundary condition n solution method,thanking u
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Old   November 18, 2013, 04:58
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Hi dmaz,
Your problem has dynamic condition and it's incorrect to consider steady solver. It's possible to achieve a steady state solution by VOF method but it is difficult. You should choose small under relaxation values. I recommend you to use unsteady solver. It's may be time consuming but your result will be correct.
Be successful.
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Old   November 18, 2013, 05:49
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Hi dmaz
please read this tutorial in Fluent (it is very similar to your problem):

Heave and Pitch Simulation of Ship hull moving through head sea waves
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Old   September 3, 2014, 05:14
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hi dmaz
in boundary condition after you set all boundaries
change phase from mixture to water
then change volume fraction for all water boundaries from 0 to 1;

you should set the volume fraction of water in initialize to 0; and in initialize, u should compute from all zones

Please, do not hesitate to ask further questions.

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Old   November 14, 2014, 14:21
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I would like to simulate a multiphase flow ( air/water) model with fluent, however i don't know that it is possible to add contaminants to water in Fluent. for example instead of having pure water, i would like to add nitrogen to water and simulate it with fluent .
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Old   November 15, 2014, 13:30
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Quote:
Originally Posted by azna View Post
I would like to simulate a multiphase flow ( air/water) model with fluent, however i don't know that it is possible to add contaminants to water in Fluent. for example instead of having pure water, i would like to add nitrogen to water and simulate it with fluent .
Hi azna
Yes, it's possible. Please study Fluent user guide -> Modeling Species Transport and Finite-Rate Chemistry.

Good luck.
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Old   December 1, 2014, 03:46
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You have enabled open channel flow.

Why do not you use open channel supported boundary conditions.
For inlet , pressure inlet or Mass flow inlet
For outlet, Pressure outlet

Pressure inlet required specification of Total pressure, which is prescribed by the user inputs of free surface level and velocity magnitude for calculating dynamci head.
Mass flow inlet asks for the inout for free surface level and mass flow rates of different phases.

You need not to specify UDF too, as open channel boundary conditions automatically take care of hydrostatic pressure profile based on the input of free surface level.
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Old   August 4, 2015, 14:54
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Thanks for sharing
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