Question

For subsonic flows, the characteristics through a flow field point are imaginary.

(a) True

(b) False

This question was addressed to me at a job interview.

I need to ask this question from Two Dimensional Irrotational Flow topic in portion Numerical Techniques for Steady Supersonic Flow of Aerodynamics

Answer 1

The correct choice is (a) True

The best explanation: When the Mach number is less than 1 i.e. the flow is subsonic, the numerator in the characteristic curve is imaginary as the terms inside the square root sign is negative. Thus, methods of characteristics is not used for subsonic flows.

\(\frac {dy}{dx_{char}} = \frac {- \frac {uv}{a^{2}} ± \sqrt {(-ve \, term)}}{1 – \frac {u^{2}}{a^{2}}}\)