-140 kPa pressure, how is it possible?
 

Hi everyone,

I am modeling an open channel flow using Fluent. When I plot floor pressure on a chart, one location shows -140 kPa pressure. How can pressure be less than absolute 0 pressure? The channel bed is 20 m wide, and I use 0.5m tet mesh on the bed.

Thanks!

Hi
According to my experience in turbo machinery modeling when the water tends to cavitate at the suction side of blade, if u dont activate cavitation and multiphase simulation then the pressure in that part is unreal.
I think fluent and other codes are not limited by absolute pressure 0, so the codes dont know reality.
Anyway in your simulation I think it is because of convergence problem. Check velocity contour in that part and see if the velocity is unnaturally high in that place.

What boundary conditions and models u are using.

Sorry for the late reply. I use VOF model, velocity inlet for inlet BC and pressure outlet for outlet BC.

CFD -Fellow: the velocity on that edge is the highest within the channel, and I expected the pressure is about -80 to -100 kPa only.

What is ur operating pressure and operating density boundary condition

I use atmospheric pressure 101.3 KPa and air density 1.225 kg/m^3

There is a default limiter on Minimum Absolute Pressure of 1 pascal. Unless you see this error message, then the absolute pressure isn't less than 0.

Why don't you just plot the absolute pressure?

Pay attention to the reference location. You are specifying the operating pressure and operating density at that location. Is gravity enabled? I think it is because otherwise there is no operating density.

The pressure plot display in Fluent does not include the hydrostatic component (because Fluent redefines pressure based on the reference pressure and density). As you go down in elevation, the hydrostatic component adds (but it is missing in the display). If you wanted absolute pressure, you should have plotted it.

Hi Lucky Tran,

Thanks for your comment. I specified reference pressure (101.3 kPa) at the default location (x,y,z = 0), and this point is actually on the channel bed, not in the atmosphere, this may be the reason why I got the wrong value of pressure.

You need to define the operating density of the working fluid (like for water 998 at desired temperature)

Why do we need to use water density for operating density? I think we always use the lighter fluid density and in this case it's air density.