Important Question And Answer: Civil - Mechanics Of Fluids - Boundary Layer

BOUNDARY LAYER

1. Mention the range of Reynold's number for laminar and turbulent flow in a pipe.

If the Reynolds number is less than 2000, the flow is laminar. But if the Reynold's number is greater than 4000, the flow is turbulent flow.

2. What does Haigen-Poiseulle equation refer to?

The equation refers to the value of loss of head in a pipe of length 'L' due to viscosity in a laminar flow.

3. What are the factors to be determined when viscous fluid flows through the circular pipe? The factors to be determined are:

i. Velocity distribution across the section.

ii. Ratio of maximum velocity to the average velocity. iii. Shear stress distribution.

iv. Drop of pressure for a given length

4. Define kinetic energy correction factor?

 Kinetic energy factor is defined as the ratio of the kinetic energy of the flow per sec based on  actual velocity across a section to the kinetic energy of the flow per sec based on average  velocity across the same section. It is denoted by (?).

 K.  E  factor  (?)  =  K.E  per  sec  based  on  actual  velocity  /  K.E  per  sec  based  on  Average velocity

 5. Define momentum correction factor (?):

 It is defined as the ratio of momentum of the flow per sec based on actual velocity to the  momentum of the flow per sec based on average velocity across the section.

 ?= Momentum  per  sec  based  on  actual  velocity/Momentum  Per  sec  based  on  average  velocity

 6. Define Boundary layer.

 When  a real  fluid  flow passed a solid  boundary,  fluid  layer is  adhered  to  the solid  boundary. Due to adhesion fluid undergoes retardation thereby developing a small region in the  immediate vicinity of the boundary. This region is known as boundary layer.

 7. What is mean by boundary layer growth?

 At  subsequent  points  downstream  of  the  leading  edge,  the  boundary  layer  region  increases because the retarded fluid is further retarded. This is referred as growth of boundary  layer.

 8. Classification of boundary layer.

 (i) Laminar boundary layer,

 (ii) Transition zone,

 (iii)Turbulent boundary layer.

 9. Define Laminar boundary layer.

 Near the leading edge of the surface of the plate the thickness of boundary layer is small  and flow is laminar. This layer of fluid is said to be laminar boundary layer.

 The length of the plate from the leading edge, upto which laminar boundary layer exists  is called as laminar zone. In this zone the velocity profile is parabolic.

 10. Define transition zone.

 After laminar zone, the laminar boundary layer becomes unstable and the fluid motion  transformed to turbulent boundary layer. This short length over which the changes taking place

is called as transition zone. 11. Define Turbulent boundary.

Further downstream of transition zone, the boundary layer is turbulent and continuous to grow in thickness. This layer of boundary is called turbulent boundary layer.

12. Define Laminar sub Layer

In the turbulent boundary layer zone, adjacent to the solid surface of the plate the velocity variation is influenced by viscous effects. Due to very small thickness, the velocity distribution is almost linear. This region is known as laminar sub layer.

13. Define Boundary layer Thickness.

It is defined as the distance from the solid boundary measured in y-direction to the point, where the velocity of fluid is approximately equal to 0.99 times the free stream velocity

(U) of the fluid. It is denoted by ?.

14.List the various types of boundary layer thickness.

Displacement thickness(?*), Momentum thickness(?), Energy thickness(?**)

15. Define displacement thickness.

The displacement thickness (?) is defined as the distance by which the boundary should be displaced to compensate for the reduction in flow rate on account of boundary

layer formation.

16. Define momentum thickness.

The momentum thickness (?) is defined as should be displaced to compensate for the reduction account of boundary layer formation. the distance by which the boundary in momentum of the flowing fluid on

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17. Define energy thickness

The energy thickness (?**) is defined as the distance by which the boundary should be displaced to compensate for the reduction in kinetic energy of the flowing fluid on account of boundary layer formation.