How Fluent distinguish between incompresible liquid and compresible gas? Both are set up as fluids but surely behave differently while flowing. Is it just a matter of density (e.g. fluid (<~5kg/m^3) assumpted as compresible gas and over taht assumpted as incompresible liquid)?

Tab example: What solver settings or operating conditions should i use to distinguish between two fluids which one of them is compresible ~air and the other one not ~watter? (external aerodynamocs of hydroplane flight + landing)

Thanks

very good question.

No CFD software do partiality with any fluid (gas or liquid). If you are talking about free surface modelling, it is the density (heavyness) which keep one fluid below (or above) the other. You need to have gravity switched on with its proper direction in case of free surface modelling.

Let us know how this question generated in you mind. This will help to answer you correctly.

In short laws of physics (NS equations) are same for liquids as well as gases as far as CFD is concerned.

Can anybody through more light on this.

Ok, i'll give more thorough explanation what i mean. How about the gas injection into the liquid (bubble generation, growth and shape change). Let's supose two similar looking problems.

1) TRACING PATHLINES ON REAL DEVICES BY MEANS OF INK injected secondary phase is liquid so this should treat the secondary fluid phase incompresible ~ constant entire volume of 2nd phase.

GAS INJECTION IN BUBBLE REACTORS injected secondary phase is gas so this should treat the secondary fluid phase compresible ~ variable volume of 2nd phase dependent on the buyoance force involving pressure (bubbles).

Let's say that i want to model these two problems with imaginary liquid and gas with the same density. How do i influence the way the Fluent will go?