Question

Is strength of a vortex tube vary with time?

(a) False

(b) True

I have been asked this question during an interview for a job.

My doubt is from The Vortex Filament, Biot-Savart Law, Helmholtz Theorem topic in chapter Flow over Airfoils over Finite Wing of Aerodynamics

Answer 1

Right answer is (a) False

To explain: The strength of a vortex tube does not vary with time, fluid elements lying on a vortex line at some instant continue to lie on that vortex line. More simply, vortex lines move with the fluid, also vortex lines and tubes must appear as a closed loop.