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This mock test of Boundary Layer Theory, Laminar Flow - MCQ Test 1 for Mechanical Engineering helps you for every Mechanical Engineering entrance exam.
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QUESTION: 1

A fully developed laminar viscous flow through a circular tube has the ratio of maximum velocity to average velocity as

Solution:

Ans. (c) Ratio =

for fully developed laminar viscous flow

through a circular tube has value of 2.0

QUESTION: 2

Which one of the following is correct?

In a fully developed region of the pipe flow,

Solution:

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.

QUESTION: 3

The power consumed per unit length in laminar flow for the same discharge, varies directly as Dn where D is the diameter of the pipe.What is the value of ‘n’?

Solution:

Ans. (d)

QUESTION: 4

The lower critical Reynold number for a pipe flow is:

Solution:

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.

QUESTION: 5

The free stream undisturbed flow has a uniform velocity U _{INFINITY }in the

Solution:

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.

QUESTION: 6

The free stream undisturbed flow has a uniform velocity U _{INFINITY }in the

Solution:

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.

QUESTION: 7

For air near atmosphere conditions flowing over a flat plate, the laminar thermal boundary layer is thicker than the hydrodynamic boundary layer.

Solution:

QUESTION: 8

Boundary layer is defined as

Solution:

Ans. (b)

• The boundary layer of a flowing fluid is the thin layer close to the wall

• In a flow field, viscous stresses are very prominent within this layer.

• Although the layer is thin, it is very important to know the details of flow within it.

• The main-flow velocity within this layer tends to zero while approaching the wall (no-slip condition).

• Also the gradient of this velocity component in a direction normal to the surface is large as compared to the gradient in the stream wise direction.

QUESTION: 9

Assertion (A): In the boundary layer concept, the shear stress at theouter edge of the layer is considered to be zero.

Reason (R): Local velocity is almost equal to velocity in potential flow.

Solution:

Ans. (a)

QUESTION: 10

The critical value of Reynolds number for transition from laminar toturbulent boundary layer in external flows is taken as:

Solution:

Ans. (c)

QUESTION: 11

The predominant forces acting on an element of fluid in the boundarylayer over a flat plate placed in a uniform stream include

Solution:

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

QUESTION: 12

A laminar boundary layer occurs over a flat plate at zero incidence tothe flow. The thickness of boundary layer at a section 2 m from theleading edge is 2 mm. The thickness of boundary layer at a section 4 mfrom the leading edge will be:

Solution:

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

QUESTION: 13

Which of the following assumptions is/are correct to obtain an analytical solution for the problem on laminar boundary layer of fluid on flat plate?

Solution:

QUESTION: 14

The equation of the velocity distribution over a plate is given byu = 2y – y^{2} where u is the velocity in m/s at a point y meter from theplate measured perpendicularly. Assuming μ = 8.60 poise, the shearstress at a point 15 cm from the boundary is:

Solution:

Ans. (b)

QUESTION: 15

For turbulent boundary layer low, the thickness of laminar sublayer 'δ'is given by:

Solution:

Ans. (b)

QUESTION: 16

The boundary layer flow separates from the surface if

Solution:

Ans. (b)

QUESTION: 17

Drag on cylinders and spheres decreases when the Reynolds number isin the region of 2 × 10^{5} since

Solution:

Ans. (d) In the region of 2 × 10^{5} (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.

QUESTION: 18

Which one of the following is correct?

For flow of an ideal fluid over a cylinder, from the front stagnationpoint,

Solution:

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.

QUESTION: 19

Thermal boundary layer is a region where

Solution:

Ans. (b)

QUESTION: 20

For flow over a flat plate the hydrodynamic boundary layer thicknessis 0.5 mm. The dynamic viscosity is 25 × 10^{-6} Pa s, specific heat is 2.0kJ/(kgK) and thermal conductivity is 0.05 W/(m–K). The thermal boundary layer thickness would be:

Solution:

Ans. (b)

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