Question

What happens to the integral ∫dS when the area of exhaust is less than the area of inlet?

(a) Positive

(b) Negative

(c) Zero

(d) Infinity

I have been asked this question during an internship interview.

Query is from Jet Propulsion Engine Thrust topic in portion Integral Form of Conservation Equation for Inviscid Flow of Aerodynamics

Answer 1

The correct choice is (b) Negative

For explanation: The sign of the integral ∫dS in the momentum equation is determined by the area of exhaust and inlet. When the exit area is less than the inlet area, the sum of negative components of pressure is more than the sum of positive components of the pressure. Thus, the sign becomes negative.