Question

The mean velocity gradient is zero at the centreline for ___________

(a) jets

(b) mixing flows

(c) mixing flows and wakes

(d) jets and wakes

I had been asked this question in an international level competition.

I'd like to ask this question from Free and Wall Turbulence topic in chapter Turbulence Modelling of Computational Fluid Dynamics

Answer 1

Correct answer is (d) jets and wakes

The explanation is: For turbulent flows at jets and wakes, the sign must change at the symmetry line. The symmetry line is the centreline here. For the sign change to be possible, the velocity gradients should reach zero at this line.