# Supersonic flow of non-compressible fluids

Will the non-compressible fluids, travelling at Mach 1, at the throat of a CD nozzle, accelerate after they leave the throat?

• Relevance to space exploration? – Organic Marble May 20 '20 at 17:36
• If you assume incompressibility, the speed of sound is infinite, so the concept of Mach 1 is meaningless. Supersonic flow is and must be compressible flow. – Tristan May 20 '20 at 19:06
• @Tristan "the speed of sound is infinite" - well say a factor of four or five higher than in air. I'm interested though, did you mean something else that had the same end result? – Puffin May 20 '20 at 21:44
• @Puffin The speed of a low amplitude pressure wave is $\sqrt{\frac{dP}{d\rho}}$. In a perfectly incompressible medium, $\rho$ is constant, meaning you divide by zero. "Incompressible" media like water, steel, etc., are still just a little bit compressible, hence the finite speed of sound. – Tristan May 21 '20 at 16:16
• @Niranjan "Why" type explanations lend themselves to oversimplifications, mischaracterizations, and confusion between cause and effect (see, for example, the classic Bernoulli misconceptions about how airfoils generate lift) – Tristan May 21 '20 at 16:25