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February 22, 2012, 07:05 
PKweir simulation

#1 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Dear All,
I am beginner in flow3d , i want to simulate PKWeir but i have some difficulties in mesh and boundaries. Help me please. Thanks, Bassem 

February 22, 2012, 10:46 

#2 
Member
MC
Join Date: Mar 2009
Posts: 56
Rep Power: 9 
What problems are you having?


February 22, 2012, 17:19 

#3 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Hello melissa,
1 I simulate PKweir (phisical model). The result 3d, the flow comes at once but i want constant flow and fill the basin from the bottom to the up gradually until spill. I think there is a mistake in boundaries.[ Xmin: pressure .fluid elevation (0.3) , Xmax: pressure (0), Y min/max:symmetry, Zmin:wall, Zmax Pressure (0) ].Could you tell me what is the property that allows us to control the flow rate? 2 I import the stl file to flow3d and create the mesh .When i click to render in FAVOR, the form is not like the real form . But when i increase the number of cells, the form becomes similar. I need to know the way to determine the number of cells and size of cells exactly ? Thank you for taking the time to answer my questions. I look forward to your response. Bassem 

February 22, 2012, 17:27 

#4 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Hello melissa,
1 I simulate PKweir (phisical model). The result 3d, the flow comes at once but i want constant flow and fill the basin from the bottom to the up gradually until spill. I think there is a mistake in boundaries.[ Xmin: pressure .fluid elevation (0.3) , Xmax: pressure (0), Y min/max:symmetry, Zmin:wall, Zmax Pressure (0) ].Could you tell me what is the property that allows us to control the flow rate? 2 I import the stl file to flow3d and create the mesh .When i click to the render in FAVOR, the form is not like the real form . But when i increase the number of cells, the form becomes similar. I need to know the way to determine the number of cells and size of cells exactly ? Thank you for taking the time to answer my questions. I look forward to your response. Bassem 

February 22, 2012, 19:19 

#5 
Member
MC
Join Date: Mar 2009
Posts: 56
Rep Power: 9 
Hi Bassem,
1. If you know the flowrate then you can define it at the boundary with a varying height or you can define a volumetric flow rate. If you are trying to do a spatial velocity then you will need to do a customization. What information do you have? 2. You should have at least 3 cells in the smallest geometry thickness to achieve mesh resolution. Typically meshing and Favorizing is an iterative method and depends not only on mesh resolution but on capturing important flow structures. Melissa 

February 23, 2012, 07:09 

#6 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Hi Melissa,
1 I have a volumetric flow rate (0.08 m 3 / s) and height corresponding to this flow rate (in the upstream: 0.24 m). Where i can enter these values? 2 When i run simulation i have errors messages " restaring cycle with smaler time step " ,could you tell me where is the errors? Thanks, Bassem 

February 23, 2012, 10:41 

#7 
Member
MC
Join Date: Mar 2009
Posts: 56
Rep Power: 9 
If you have v10 you enter them at a boundary. If you have earlier version you will need to add velocity with height.
The solver has automatic controls so it will adjust the time step if it is not converging. If this is happening at the beginning of your simulation then you can add an initial time step to help the numerics. If it is happening in the middle of the simulation then I would not worry unless the simulation terminates 

March 5, 2012, 05:35 

#8 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Hi Melissa,
Sorry for the slow reply. .Thanks a lot for your help. I think i have some difficulties in mesh and boundaries. If you have any document on the mesh please send me. Thank you, Bassem Athmani 

March 6, 2012, 16:28 

#9 
Member
MC
Join Date: Mar 2009
Posts: 56
Rep Power: 9 
Meshing is an iterative process. Mesh>FAVORize to view whether the geometry is well defined (meaning that all parts are resolved or can be seen)
The number of cells depends on the type of problem you are running (physical models that will be used) . Typically 3 cells across the smallest width will resolve most flow structures that the user wishes to capture. If you know the size of eddies then you should have that region resolved well enought to capture it. Geometry resolution can be 1 cell. but the gridlines must line along the surface (x, y or z plane) So a rule of thumb is to begin meshing with a cell size that is 3 cells across the smallest width. A good simulation requires that mesh dependency study be done on the simulation. This means you get the same results using smaller or greater cell size. As far as know there is no real method of how to chose the levels of dependency but if you change your cell size from 0.1 to 0.11 and the results look different then you do not have mesh dependency meaning results may not be accurate. Flow3D uses law of the wall which defines the boundary layer at which turbulence begins. There is a tech note on the dependency of the mesh which can be found on flow3d website. Boundary conditions are found in Meshing and Boundary tab in v10 under meshing. To define a volumetric flow rate you would select the dialog next to the boundary where you want flow to enter>select volumetric flow rate>define vector direction +1 in y direction will have flow moving toward the positive y. Where it shows fluid height you would define a height if you want it to be constant at that boundary. Fraction of fluid is 1 for fluid 1. 

March 8, 2012, 07:55 

#10 
New Member
Athmani Bassem
Join Date: Dec 2011
Posts: 14
Rep Power: 6 
Thanks a lot Melissa for your help.


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