Limitations of Henry law for fast acting systems

It is important to realize that this law refers to the equilibrium state.

If a hydraulic system is operating above the saturation pressure for a considerable period of time, all air/gas will be dissolved in the hydraulic fluid.

If the system pressure suddenly drops below the saturation pressure, air/gas bubbles start to form. This is not an instantaneous process and can involve a time constant of the order of minutes.

If a hydraulic system is operating below the saturation pressure and the pressure suddenly rises above the saturation pressure, the air/gas bubbles collapse and the air/gas dissolve in the liquid. This is a faster process but not instantaneous. It is therefore possible to have undissolved air/gas above the saturation pressure.

The process is further complicated by the fact that the fraction of air/gas and hence the saturation pressure may vary with time and with position within the system.

It would be interesting to take all these phenomena into account but for general simulation of hydraulic systems, this is out of the question. The dynamics of these processes are complex and the physics are not fully understood. The only solution is to make simplifying assumptions providing a compromise between physical realism and speed of calculations.

The primary objectives are to employ rigorous conservation of mass and to ensure that the bulk modulus of the liquid is reduced by the presence of air/gas and vapor.