Well, even for gasses, your results are accurate enough as long as the Mach number is below 0.3: <a href="http:&#x2F;&#x2F;encyclopedia2.thefreedictionary.com&#x2F;incompressible+flow" rel="nofollow">http:&#x2F;&#x2F;encyclopedia2.thefreedictionary.com&#x2F;incompressible+fl...</a>