Trapped air in fluid

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Adam
Adam on 27 Sep 2013
Answered: Yifeng Tang on 19 Aug 2022
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

Answers (1)

Yifeng Tang
Yifeng Tang on 19 Aug 2022
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.

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