Why should an ideal fluid be incompressible?

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Discussion Overview

The discussion revolves around the characteristics of an ideal fluid, specifically focusing on the reasons for assuming incompressibility in fluid dynamics. Participants explore the implications of this assumption in both theoretical and practical contexts.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • Some participants suggest that the assumption of incompressibility simplifies the mathematical treatment of fluid dynamics.
  • One participant notes that the bulk modulus of most materials is high, leading to negligible compression under typical pressures, which may justify the incompressibility assumption.
  • Another participant emphasizes that the term "ideal" refers to simplifications that facilitate analysis rather than a reflection of real fluid behavior.
  • A humorous analogy is made comparing an ideal fluid to an 18-year-old single malt Scotch whisky, suggesting a subjective interpretation of the term "ideal."

Areas of Agreement / Disagreement

Participants generally agree on the utility of the incompressibility assumption for simplifying analysis, but there is no consensus on the implications or the nature of "ideal" fluids.

Contextual Notes

Some limitations include the dependence on specific definitions of ideal fluids and the potential complexity introduced by compressibility in real-world scenarios.

KingDaniel
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Hi,

I read that an ideal fluid needs to be frictionless and incompressible.
Please explain why, especially the incompressible part?

Thanks
 
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It makes their mathematical treatment substantially easier.
 
KingDaniel said:
Hi,

I read that an ideal fluid needs to be frictionless and incompressible.
Please explain why, especially the incompressible part?Thanks
The bulk modulus of most materials is very high compared to usual pressures, resulting in insignificant compression of most materials. Solids usually are even more incompressible than liquids. Wiki the bulk modulus to get a feel for the forces involved.
 
As boneh3ad said, "ideal", in the context of a lot of math and science isn't saying that we wish real fluids behaved like that, it's about making assumptions/simplifications that make analysis easier (or in some cases, that make analysis possible at all). Compressibility dramatically increases the difficulty of fluid dynamics, so if you can ignore it, it makes a lot of sense to do so.
 
As my PhD advisor often said, an ideal fluid is an 18-year-old, single malt Scotch whisky.
 
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I think that's a definition I can get behind.
 
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