Fluid Mechanics - Mechanical Engineering (MCQ) questions and answers for Q. 3954

Q.  The region in the turbulent boundary layer zone, adjacent to the solid surface of the plate is called as
- Published on 04 Aug 15

a. laminar sub layer
b. turbulent sub layer
c. solid sub layer
d. solid layer

ANSWER: laminar sub layer
 

    Discussion

  • Pankaj   -Posted on 05 Oct 15
    The region in the turbulent boundary layer zone, adjacent to the solid surface of the plate is called as laminar sub layer. As this laminar sub layer is very thin, the velocity gradient is assumed to be constant and the velocity variation is also assumed ti be linear.

    In the laminar sub layer, the velocity gradient becomes large at points very close to the boundary layer and again becomes large with the viscous shear forces to maintain the fluid in laminar flow. This occurs within the turbulent zone and very thin near to the wall.

    Even if the laminar sub layer is very thin, it plays an important role in the friction characteristics of the surface. If the height of roughness of a pipe in turbulent flow is greater than the thickness of the laminar sub layer, then this increases the amount of turbulence and energy losses in the flow.

    The pipe is said to be smooth when the height of roughness of the pipe is less than the thickness of the laminar sub layer.

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