Learning Forum

[Sebastian Sarabia]

Hi!, I have a problem defining the pressure at an operational depth. The exercise is simple, it is a pipe that discharges at different depths to see the dispersion. I define the operaion pressure in my BC, but when I want to see the pressure contour it doesn't change by the height.

[Rob]

I suspect it's covered here, /courses/index.php/courses/fluid-statics/  Putting hydrostatic into the box marked "Search" may give you additional materials. 

[Sebastian Sarabia]

Hi Rob, Thanks for taking your time to respond. I have reviewed the link you shared with me. I have been able to get pressure profiles consistent with the problem, but have found that no matter what operating pressure I use, the results are all similar and the velocity profiles do not vary. I'm testing every 10 meters deep. (Pressure operating 1 bar, 2 bar, 3 bar, 4 bar)

[NickFL]

What is the boundary condition you have at the inlet? Is it a velocity or a pressure?

The inlet pressure boundary condition is a total gauge pressure, meaning this is measured against the operating pressure that you are using. Therefore if you define the pressure as 1 [bar], this is actually 2 [bar] absolute (assuming default opeating pressure). When you change the opeating pressure to 2 [bar] the inlet pressure would also adjust to have the value of 3 [bar]. Maybe this is why the contours look identical.

If you defined the inlet as a velocity, the absolute pressure at the inlet should increase.

[Sebastian Sarabia]

Hi Nick, In the BC I have pipe velocity input and tidal current velocity input on the left, outlet pressure on the right and outlet pressure at the top, floor on the bottom.

I tried using operating pressure 2, 3, 4 bar leaving all gauge pressure at 0, then I tried leaving operating pressure at 0 and only using the gauge pressure for the pressure outputs for 2, 3, 4 bar, velocity contours all gave similar.

Should I use gauge pressure for velocity inputs?

I am looking for velocity contours for flow dispersion at different depths but there is almost no difference in the results I got.

 

[NickFL]

Let us talk a bit of theory here first. If we have a fixed velocity, we will compute a flow field that has this velocity inlet condition. The computed solution will give us the pressure at the inlet. Now look back at the equations we are solving. Where does the pressure come into play? Hint, it really doesn't (for simple cases). It is the pressure GRADIENT that drives the flow.

With that in our minds, let us return to your question. If you change the operating pressure, what are you doing? You are simply scaling the pressure up to whatever depth you are at. The pressure and velocity fields will be the same. What will be different is the absolute pressure field, but this is going to be simply whatever the difference is between the given operating pressures.