[wolverinmi]

I am running CFX for investigate the hydrodynamic characteristic for my concrete block. I found that the buoyancy model results more accurate than non-buoyancy mode, and its picture is attached in this mail as well.

I realized that in buoyancy mode, my result is wrong due to automatically setting for default material for the block. Do any body know how to change the density for the block?


images upload





Thanks so much if you can help me to solve out this problems.

[ghorrocks]

You can set the density in the material properties. But why are you modelling the density of the concrete block? It is only relevant if you are modelling CHT or rigid body dynamics - but your simulation looks like just flow over the block and density of the block is not significant for that.

[wolverinmi]

Originally Posted by ghorrocks

You can set the density in the material properties. But why are you modelling the density of the concrete block? It is only relevant if you are modelling CHT or rigid body dynamics - but your simulation looks like just flow over the block and density of the block is not significant for that.

Because I wanted to investigate the Lift acting on the concrete block. However, the buoyancy model is applied in this test, so we need to put exact weight of the block in order to point out the good results. Do u know what model that we can modify the material properties?

I had a look on geometry and CFX, but they seem like have only solid material. If i add data engineering and set concrete as main material for all solid. We need to define some unexpected factors in mesh model like stress analysis, so i find other easy way to solve the problem.

[ghorrocks]

You predict the lift on a body by integration the wall force acting on the body by the fluid. This gives you the fluid force vector. You don't model the weight of the body itself. You seem to have misunderstood this type of simulation. Have you looked at the CFX tutorial examples of fluid flows over bodies?

[wolverinmi]

Originally Posted by ghorrocks

You predict the lift on a body by integration the wall force acting on the body by the fluid. This gives you the fluid force vector. You don't model the weight of the body itself. You seem to have misunderstood this type of simulation. Have you looked at the CFX tutorial examples of fluid flows over bodies?

Actually I am in the final year student in AMC for Naval Artichecture, I did CFD last year about the fluids flow over bodies. In contract, I found that applied buoyancy mode we got the better results because of applying the hydrostatic pressure in the Navier-Stock equation. And the problem is the total forces acting on the vertical body is depending on the weight of the body, buoyancy force and the lift. So If I can not model the mass, i think the graph result from CFX is not right.

Have u ever meet this sistuation before? I am not quite clear about this point, so If you can explain a bit for me. I am so happy and appreciate.

Regard
Viet Hung

[ghorrocks]

The weight of the body is easily added by hand to the buoyancy force calculated by CFD. So you don't model the weight but add it afterwards.

Unless you are a rigid body motion simulation then you do need the body mass - but you have not mentioned that so I assume you are not doing a rigid body simulation.

[wolverinmi]

Originally Posted by ghorrocks

The weight of the body is easily added by hand to the buoyancy force calculated by CFD. So you don't model the weight but add it afterwards.

Unless you are a rigid body motion simulation then you do need the body mass - but you have not mentioned that so I assume you are not doing a rigid body simulation.

I think I understand what you mean? So if I active my model to Buoyancy model, the force acting on vertical direction on block is including the lift force and buoyancy force without weight. And I can calculate the total force later by hand applying the mass of block. Is it what you mean?

For the buoyancy mode, the result of lift is about 18N at velocity of 0.6 m/s.
But the buoyancy mode got the total force is -28N.
The hand calculation for buoyancy is about 586N (Volume = 0.6 cubic metter), the mass is 145,6 kg, so I think I apply the wrong boundary layer.

My project is very simple, just investigating the lift and drag due to the fluid acting on body. So i defined the boundary such as:

Inlet (inlet ) = 0.6 m/s
Out let (Opening) Pa =0 atm. I just worry Pa = 0 (touch with environment or Pa= rho.g.h (hydrostatic pressure) )
Other boundary is defined wall function.

Can you check it again for me please?

Regard

[wolverinmi]

Originally Posted by ghorrocks

The weight of the body is easily added by hand to the buoyancy force calculated by CFD. So you don't model the weight but add it afterwards.

Unless you are a rigid body motion simulation then you do need the body mass - but you have not mentioned that so I assume you are not doing a rigid body simulation.

I also tried to add the rigid motion, but they said that even i add the rigid body. But i did not define any mesh motion, so the mode is not running.

If i want to add rigid motion, do i need to seperate two solid without using distract Boolean methodology?

[ghorrocks]

Do not consider doing a rigid body simulation until you can do accurate fixed mesh simulations. So stay with fixed mesh simulations for now.

You will probably have to account for the hydrostatic pressure in your outlet boundary. have a look at how the flow over a bump tutorial does it.

[wolverinmi]

Originally Posted by ghorrocks

Do not consider doing a rigid body simulation until you can do accurate fixed mesh simulations. So stay with fixed mesh simulations for now.

You will probably have to account for the hydrostatic pressure in your outlet boundary. have a look at how the flow over a bump tutorial does it.

Can u please give me a link for this tutorial. I tried to apply the hydrostatic pressure at the outlet, but the result is not accurate. I used to make model for multiphases of air and water for ship s resistance, but this case is quite complicated

[ghorrocks]

The example is in the CFX tutorials, provided with the standard ANSYS installation.

[wolverinmi]

Originally Posted by ghorrocks

The example is in the CFX tutorials, provided with the standard ANSYS installation.

Thanks you very much for your support.
Finally I have done my work with buoyant force, and CFX did a good job. I just misunderstood about the define of hydrostatic pressure, but Ansys automatically calculate this kind of pressure for me. So in the outlet i just defined relative pressure at 0 Pa and it is working.

Thanks and best regard
Viet Hung