Test: Boundary Layer Theory, Laminar Flow - 1 - Mechanical Engineering MCQ

Ans. (c) Ratio =
for fully developed laminar viscous flow
through a circular tube has value of 2.0

Ans. (b) It can be said that in a fully developed flow, the pressure gradient
balances the wall shear stress only and has a constant value at any section.

Ans. (d)

Ans. (a) The Reynolds number at which the turbulent flow changes to laminar flow
is known as lower critical Reynolds number. The lower critical Reynolds number
for a pipe flow is different for different fluids.

 It is in the x-direction only. Particles of fluid adhere to the plate surface as they approach it and the fluid is slowed down considerably.

Explanation:

A boundary layer is a thin layer of viscous fluid close to the solid surface of a wall in contact with a moving stream.

In scientific terms, A boundary layer is a thin region in the fluid adjacent to a surface where velocity and temperature gradients normal to the surface are significant.

The difference in velocity between adjacent layers of the fluid is known as a velocity gradient.

The difference in temperature between adjacent layers of the fluid is known as a temperature gradient.

The boundary layer thickness (δ) is the distance across a boundary layer from the wall to a point where the flow velocity has essentially reached the free stream velocity.

Thus velocity gradient will be large and at the surface, due to less velocity difference, it will be a thin shear layer.

Ans. (a)

Ans. (c)

Ans. (d) That so why we are using Reynold’s number for analysis.

Ans. (b) Thickness of boundary layer at 4 mm from leading edge = 2 × (4/2)^1/2 = 2 × 2^1/2

Ans. (b)

Ans. (b)

Ans. (d) In the region of 2 × 10⁵ (Reynolds number), the boundary layer on the cylinders and sphere begins to become unstable and thus boundary layer is said
to reattach and the separation point moves back along the cylinder. Due to flow reattachment, a pressure recovery takes place over the back side and thus the
drag force decreases.

Ans. (a) At the stagnation point pressure is maximum
∴ For flow past ideal fluid over a cylinder from front stagnation point pressure first decreases then increases.

Ans. (b)

Ans. (b)

Concept:

Critical Velocity

Critical Velocity is the maximum velocity of a fluid, above which, the streamline flow get the change to turbulent flow.

• Lower critical Velocity
  • It is the velocity at which the laminar flow stops.
  • In this, the flow enters from laminar to transition period which exists between the laminar and turbulent flow.
• Upper Critical velocity
  • It is the velocity at which the turbulent flow starts.
  • In this, the flow enters from the transition period to turbulent flow.