Question

Choose the correct option regarding a streamline.

(a) Speed, not velocity, at all points of a streamline is same

(b) Two streamlines can intersect in case of laminar flow

(c) There is no friction between streamlines in case of steady flow

(d) In a given streamline, velocity of a point can vary with time

I had been asked this question in an interview for job.

Question is from Fluids Mechanical Properties in division Mechanical Properties of Fluids of Physics – Class 11

Answer 1

The correct choice is (d) In a given streamline, velocity of a point can vary with time

Easiest explanation: If we consider a streamline in a pipe of varying diameter we can say that speed of different points in that streamline are different. The tangent to a streamline gives the direction of velocity at that point, so two streamlines cannot intersect as at the point of intersection we will not be able to draw a single tangent. In case of steady flow across a pipe, there could be streamlines next to each other having different velocities and hence there will be friction between the streamlines which we call viscosity. If flow is unsteady, but not turbulent, the velocity of a point in a streamline can change. For eg: If we slowly increase the speed of a tap flow will be laminar and speed of a point (on a streamline) will increase with time.