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So I am learning for thermodynamics now and there is one thing that is bugging me. In the book it shows a laval nozzle (very simplified) and a diffuser.

laval nozzle diffuser

with the formulas: formulas

(a is the speed of sound)

According to those images and its explanation that would mean, if I make a paper cone with a hole in it and blow into it, at the hole speed of sound would be reached. That is obviously not the case. So what am I missing?

At the beginning it says "Isentropic flows" but how does one actually know that the flow is isentropic? Or is it because of turbulence?

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  • $\begingroup$ The assumption of isentropic is made to simplify the analysis. You could analyse it not assuming isentropic and see the difference. $\endgroup$ – Solar Mike Jun 24 at 12:16
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According to those images and its explanation that would mean, if I make a paper cone with a hole in it and blow into it, at the hole speed of sound would be reached. That is obviously not the case. So what am I missing?

You are not blowing hard enough. There needs to be enough pressure drop from the big part of the cone to atmospheric to accelerate the air to the speed of sound. Do the math to figure out how hard to blow, and blow that hard. (You may want to figure out the bursting strength of your cone. And your lungs).

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  • $\begingroup$ yep, I get it now. However one cannot blow with room temperature through a cone that exhausts into roomtemperature again. That would no longer be isentropic and thus make calculating it a lot harder. I think $\endgroup$ – RIJIK Jul 1 at 12:54

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