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Test: Boundary Layer Theory, Laminar Flow - 1


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20 Questions MCQ Test GATE Mechanical (ME) 2023 Mock Test Series | Test: Boundary Layer Theory, Laminar Flow - 1

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Test: Boundary Layer Theory, Laminar Flow - 1 - Question 1

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

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 1

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

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 2

Which one of the following is correct? 
In a fully developed region of the pipe flow,

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 2

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.

Test: Boundary Layer Theory, Laminar Flow - 1 - 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’?  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 3

Ans. (d)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 4

The lower critical Reynold number for a pipe flow is:  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 4

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.

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 5

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

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 5

 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.

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 6

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

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 7

Boundary layer is defined as  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 7

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.

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 8

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.

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 8

Ans. (a)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 9

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

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 9

Ans. (c)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 10

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

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 10

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

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 11

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:  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 11

Ans. (b) Thickness of boundary layer at 4 mm from leading edge = 2 × (4/2)1/2 = 2 × 21/2

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 12

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?

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 13

The equation of the velocity distribution over a plate is given byu = 2y – y2 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: 

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 13

Ans. (b)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 14

The boundary layer flow separates from the surface if  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 14

Ans. (b)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 15

Drag on cylinders and spheres decreases when the Reynolds number isin the region of 2 × 105 since  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 15

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.

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 16

Which one of the following is correct?  
For flow of an ideal fluid over a cylinder, from the front stagnationpoint,

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 16

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.

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 17

Thermal boundary layer is a region where 

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 17

Ans. (b)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 18

Laminar developed flow at an average velocity of 5 m/s occurs in a pipe of 10 cm radius. The velocity at 5 cm radius is

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 19

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:  

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 19

Ans. (b)

Test: Boundary Layer Theory, Laminar Flow - 1 - Question 20

The velocity at which the laminar flow stops is known as

Detailed Solution for Test: Boundary Layer Theory, Laminar Flow - 1 - Question 20

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.
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