Effect of free stream velocity on thickness of boundary layer - Fluid Mechanics

Q.  What is the effect of free stream velocity on thickness of boundary layer?
- Published on 04 Aug 15

a. Increase in free stream velocity increases the boundary layer thickness
b. Increase in free stream velocity decreases the boundary layer thickness
c. The boundary layer thickness does not get affected by any change in free stream velocity
d. unpredictable

ANSWER: Increase in free stream velocity decreases the boundary layer thickness

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    Discussion

  • Pankaj   -Posted on 05 Oct 15
    The boundary layer starts with the leading edge of the solid boundary and the boundary layer thickness increases with the distance x along the surface.

    The thickness of velocity boundary layer can be defined as the distance from the solid body at which the viscous flow velocity is 99% of the free stream velocity.

    Boundary layer varies inversely with the free stream velocity. As we know that the velocity gradients are large over the boundary layer. When free stream velocity is small, the upper layer does not pull the adjacent lower layer with it. But when the free stream velocity is sufficiently more enough to pull the lower layer of the fluid by its adjacent upper layer, then the velocity gradient reduces resulting decrease in boundary layer thickness.

    Therefore, if free stream velocity increases, the thickness of boundary layer decreases and vice-versa.

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