# Boundary layer thickness - Fluid Mechanics

Q.  Boundary layer thickness is the distance from the boundary to the point where velocity of the fluid is
- Published on 05 Aug 15

a. equal to 10% of free stream velocity
b. equal to 50% of free stream velocity
c. equal to 90% of free stream velocity
d. equal to 99% of free stream velocity

ANSWER: equal to 99% of free stream velocity

#### Discussion

• Pankaj   -Posted on 05 Oct 15
The thickness of boundary layer can be defined as the distance of wall or the solid surface to the point where velocity of the fluid if 99% of the free stream velocity or the velocity in the middle of the pipe when fluid flowing through pipe.

The boundary layer thickness increases with distance from the point where the fluid fluid starts to pass over the bounded surface. It increases to maximum in fully developed flow.

As fluid passes from the bounded surface over more length, more fluid is slowed by friction between the fluid layer close to the bounded surface. Hence the thickness of the slower layer increases.

The velocity gradient becomes smaller, as the boundary layer thickness becomes greater. In this case, shear stress decreases until it is no longer enough to drag the slow fluid near the surface along.

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