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I am currently learning about cavitation in my fluid mechanics study. I am confused about the definition of Vapor Pressure, does it exist only in the saturation line? I am also confused about how can a cavitation bubble form just because the fluid pressure dropped below vapor pressure.

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  • $\begingroup$ What is your understanding of the definition of vapor pressure? $\endgroup$ Dec 10 '19 at 12:59
  • $\begingroup$ @ChetMiller Vapor pressure is the pressure exerted by a pure substance in vapor state in 2 phase (vapor and liquid) equilibrium. $\endgroup$
    – Nazvix
    Dec 10 '19 at 13:12
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Vapor pressure is a physical property of a given substance that depends only on the substance's temperature. One way to calculate it involves the Antoine equation (see https://en.wikipedia.org/wiki/Antoine_equation). For a given substance under equilibrium conditions in a closed container, and under ambient pressures higher than its vapor pressure (i.e., there is a mixture in the container), the substance still exerts its vapor pressure. For cases where the associated vapor-liquid equilibrium is ideal, that vapor pressure is the partial pressure of the material inside the container, per Raoult's Law (see https://en.wikipedia.org/wiki/Raoult%27s_law). This means that vapor pressure equals saturation pressure only in the case of a pure component.

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  • $\begingroup$ Thank you for the answer. $\endgroup$
    – Nazvix
    Dec 10 '19 at 16:30
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We need two parameters to specify the thermodynamic state of a pure material. They are temperature and pressure.

At any temperature, all materials have a defined vapor pressure. The vapor pressure of a material is the pressure at which the material will be in exact equilibrium with its pure vapor at that total pressure.

As we decrease the temperature on a material, the vapor pressure of the material also decreases. Liquid water boils at 1 atm at 100 °C. Liquid water boils at 23.8 Torr pressure at 25 °C.

When we have liquid water at 25 °C; in a room at 1 atm pressure, the water still has a vapor pressure of 23.8 Torr. When we would decrease the pressure on the liquid water (e.g. through cavitation) to a pressure below 23.8 Torr, the liquid water will boil. In cavitation, this is the formation of bubbles.

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  • $\begingroup$ Okay, I get it now regarding the formation of the cavitation bubble. About vapor pressure, does that mean vapor pressure = saturation pressure? $\endgroup$
    – Nazvix
    Dec 10 '19 at 15:16

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