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 antonio April 5, 2010 10:27

boundary conditions and turbulence intensity

Dear all. I am modeling a rectangular channel with water. At the present moment I am dealing with 2 difficulties.

1) I have created the following expressions for the initial settings:
DenWater= 997;
DownH = 0.14[m](height of water at downstream);
DownPres= DenWater*g*DownVFWater*(DownH-y) (hydrostatic distribution at outlet)
DownVFWater= if(y<DownH1,0)
UpH=0.14[m](height of water at upstream)
UpPres=DenWater*g*UpVFwater*(UpH-y)(hydrostatic distribution at inlet)
UpVFWater= if(y<UpH,1,0).

Basically I have set the DownH=UpH because I want to have an uniform regime so the water height should be constant along the domain. The problem is that the solver is creating artificial walls( I have already pushed the outlet boundary far away the recirculation zone). In this context I had analyzed the results in the CFX-Post and I have seen that the pressure in the outlet is larger than inlet!!!How is this possible (I have set reference pressure equal to 1 atm)?I really donīt know why the program is doing this.

2) It is possible to set different turbulence intensities in the 3 directions using the k-epsilon model?

Best Regards

 ghorrocks April 5, 2010 19:04

Try using a velocity boundary at the inlet rather than a pressure inlet. Do you know the flow rate? If so then this approach makes sense.

Quote:
 It is possible to set different turbulence intensities in the 3 directions using the k-epsilon model?
No. k-e is an isotropic turbulence model and therefore the turbulence is assumed equal in all directions. If you have anisotropic turbulence then you have to use a Reynolds Stress model or LES/DES.

 antonio April 6, 2010 05:24

Hi Horrocks.

I have done precisely that (by the way my channel has a width of 0.14 m a length of 1.7 m and a height of 0.14 m and an inclination of 9*10^-4). What you see above are the expressions that I have used in the "expressions field" in the CFX-Pre. However when I defined the boundary conditions for the inlet boundary I have used also the option cartesian velocity components. As the documentation say (and you also say that) defining a velocity at the inlet and the pressure at the outlet is usually the most robust way of defining the boundary conditions. Is the expression that I have used for the distribution of the pressure at inlet unnecessary (I think/thought that the solver need this information).

In what concerns the question related to turbulence intensities thanks for the answer..that what I was thinking but it always good to check.

Best regards

 brunoc April 6, 2010 11:57

Hi Antonio,

What you should do is set velocity at the inlet (as you are doing with the cartesian components) and then set free surface level using the volume fractions (the UpVFWater expression you have there).

At the outlet prescribe the static pressure (not the average static pressure) and use the expression you have for hidrostatic pressure.

That should do the job. But you should check the tutorial CFX has on free surface over a bump.

Cheers.

 antonio April 6, 2010 12:03

Dear All.

Analyzing the results that I have I can see that I have an adverse pressure gradient in the bottom of my channel.I think that this is what is causing the recirculation of the flow. How can I control this?Any suggestion?

I am thinking changing the boundaries conditions (at the present moment I have a velocity at inlet and Static Pressure at Outlet):specifying the total pressure at Inlet and the velocity of the flow at Outlet.

Best Regards

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