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Basics of Fluid Mechanics, 2014a

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388 CHAPTER 11. COMPRESSIBLE FLOW ONE DIMENSIONAL<br />

Differentiation <strong>of</strong> the equation state (perfect gas), P = ρRT , and dividing the results<br />

by the equation <strong>of</strong> state (ρRT) yields<br />

dP<br />

P<br />

= dρ<br />

ρ + dT T<br />

(11.35)<br />

Obtaining an expression for dU/U from the mass balance equation (11.32) and using<br />

it in equation (11.34) reads<br />

dP<br />

ρ − U 2<br />

dU<br />

U<br />

[{ }} {<br />

dA<br />

A + dρ ]<br />

=0 (11.36)<br />

ρ<br />

Rearranging equation (11.36) so that the density, ρ, can be replaced by the static<br />

pressure, dP/ρ yields<br />

dP<br />

ρ<br />

= U 2 ( dA<br />

A + dρ<br />

ρ<br />

)<br />

dP<br />

dP<br />

⎛<br />

1<br />

c {}}{<br />

2<br />

= U 2 dA<br />

⎜<br />

⎝ A + dρ<br />

dP<br />

dP<br />

ρ<br />

⎞<br />

⎟<br />

⎠<br />

(11.37)<br />

Recalling that dP/dρ = c 2 and substitute the speed <strong>of</strong> sound into equation (11.37) to<br />

obtain<br />

[ ( ) ] 2<br />

dP U<br />

1 − = U 2 dA (11.38)<br />

ρ c A<br />

Or in a dimensionless form<br />

dP<br />

ρ<br />

(<br />

1 − M<br />

2 ) = U 2 dA A<br />

(11.39)<br />

Equation (11.39) is a differential equation for the pressure as a function <strong>of</strong> the cross section<br />

area. It is convenient to rearrange equation (11.39) to obtain a variables separation<br />

form <strong>of</strong><br />

dP = ρU2<br />

A<br />

11.4.3.1 The pressure Mach number relationship<br />

dA<br />

1 − M 2 (11.40)<br />

Before going further in the mathematical derivations it is worth looking at the physical<br />

meaning <strong>of</strong> equation (11.40). The term ρU 2 /A is always positive (because all the<br />

three terms can be only positive). Now, it can be observed that dP can be positive or<br />

negative depending on the dA and Mach number. The meaning <strong>of</strong> the sign change for<br />

the pressure differential is that the pressure can increase or decrease. It can be observed

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