Trapped air in fluid

Hello,

I am curious to the effects of the trapped air parameter.

I am using an ideal pressure source pumping flow into a dead ended system.

I have used values of 0.04 (I assume 4%),0.004, 0.0004, 0.00004 and 0.

The simulation for 0 gives very smooth results in analysing the harmonic of the system.

The issue I have is when I reduce the amount of trapped air (starting at 4%) the results actually get more erratic to the point where the pressure readings at the dead end read a constant negative pressure.

I was wonderings if I was doing anything wrong because I would have thought as the air was reduced the results would tend to the zero value.

Thank you.

Adam

Answer 1

[Yifeng Tang]

The entrained air affects how the bulk modulus (hence also density) changes as a function of the pressure. More air will make the fluid more "compliant" at low pressure. See the plot below, generated from theIsothermal Liquid Properties (IL) block.

For the hydraulic domain, the entrained air has a similar effect. The 3rd equation on this doc page shows how the bulk modulus changes with pressure and air fraction.