Momentum Equation for compressible 1D flow

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SUMMARY

The discussion focuses on deriving the differential momentum equation for compressible 1D flow, specifically the equation dp = -ρu du. The key equation referenced is the integral form of momentum conservation, expressed as ∮_{CS} ρU(U·n)dA = -∮_{CS} pndA, applicable for steady-state flow. A participant seeks clarification on the origin of the term p + (1/2)dp, which represents the force exerted by the solid boundary on the fluid within the control volume.

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Homework Statement


Derive the differential momentum equation: dp=-\rho udu.

Homework Equations


\oint_{ CS}^{ } \rho \vec{U}(\vec{U}\cdot \vec{n})dA=-\oint_{ CS}^{ } p\vec{n}dA for steady state flow...

The Attempt at a Solution


I tried to solve it like in the attached picture, but can someone explain where the p+\frac{1}{2}dp (marked with red line in the picture) comes from? Is it a linear interpolation or something?
Untitled1.png


Thanks!
 

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That is a term for representing the force that the solid boundary exerts (in the x direction) on the fluid passing through the control volume.
 

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