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Stationary Water-jet wall impingement

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Old   July 8, 2014, 03:04
Default Stationary Water-jet wall impingement
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John
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Hello all,

I am a quite new to CFX and I would like to know if it is possible to evaluate shearing forces on wall generated during water-jet impingement.

Currently I am able to identify and determine water droplets velocities, pressure on my volume of control and pressure on the wall but there is nothing which concerns shearing forces on the wall.

The configuration is very simple : a nozzle and a wall or a flat panel.

My scope is to study the correlation between the inlet pressure and shearing forces generated which represent the base of my surface cleaning study by spray technology.

In the variable section of CFD-Post I see "water shear stain rate" and "water wall shear" but I cannot understand how to implement them into my simulation.

I had a deep look in the Setup section, especially at the wall boundary condition where I see I have a multiple choice in the Mass and Momentum option between No slip wall, free slip wall, specified shear and fluid dependent.

So far, I have carried out my simulation with the "No slip" option that assumes velocities nearby the wall are zero by default. So in my perspective it should not consider any shearing generation in this way. Am I right ?

On the other side considering the " specified shear" option I have to insert a value of pressure along the x,y and z axis but I am not sure about the sense of specifying shears stress in this way.

Could you please suggest me an easy way to evaluate shearing stress on the wall ?

I am running Ansys CFX v15 .

Thanks in advance.

Last edited by Triccio; July 8, 2014 at 04:19.
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Old   July 8, 2014, 06:19
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Glenn Horrocks
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Quote:
if it is possible to evaluate shearing forces on wall generated during water-jet impingement
Yes, it is.

Just include all variables in the results file and have a look at the results in CFD-Post.


Quote:
So far, I have carried out my simulation with the "No slip" option that assumes velocities nearby the wall are zero by default. So in my perspective it should not consider any shearing generation in this way. Am I right ?
Umm - no, you are (very) wrong. Wall shear is the derivative of velocity WRT the normal direction and that can have a value when the velocity is zero.

Quote:
On the other side considering the " specified shear" option I have to insert a value of pressure along the x,y and z axis but I am not sure about the sense of specifying shears stress in this way.
If you do not know the shear then you cannot use a wall shear stress boundary condition.
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Old   July 9, 2014, 19:43
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Originally Posted by ghorrocks View Post
Yes, it is.

Just include all variables in the results file and have a look at the results in CFD-Post.
Hello,
thanks for you answer.

Please, can you explain me how to include all variables in the results file ?
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