The critical velocity for non-viscous fluids:
A. 0
B. Infinite
C. Small but not zero
D. Arbitrary

The transition between laminar and turbulent flow is somewhere between Reynolds number 1000 and 2000 (the higher the more turbulent). The transition is not very sharp and at Reynolds numbers close to 1000 you will have laminar flow with just occasional turbulence. You can however set an arbitrary valueRec of Reynolds number where you consider the flow to be turbulent. The speed at which the flow begins to be turbulent is called critical speed.

We can calculate the critical speed vc by inverting the formula for the Reynolds number.

vc=Reη/ϱl
The critical velocity of non-viscous fluid is 0 .
Hope this helps