As fluid flows over a submerged object a boundary layer is formed. The boundary layer is due to the shear stress caused by the viscous effects of the fluid as it moves over the object. Depending on the length of the object and the speed of the fluid, the boundary layer could exhibit both laminar and turbulent flow. Refer to the figure below.

To determine if the fluid has a laminar or turbulent flow as it moves over the object Reynolds number would be used. Depending upon the surface roughness, the transition from laminar flow to turbulent flow occurs at a Reynolds number of 2X10^{5} to 3X10^{6}. To calculate the Reynolds number for fluid flow over a flat plate the following equation would be used.

In addition to determining if the flow is laminar or turbulent Reynolds number also determines the importance of the viscous effects. As Reynolds number becomes larger the viscous effects aren't as important at the front of the boundary layer, but become much more important as you near the end of the boundary layer. Also, the larger the Reynolds number the thinner the boundary layer becomes.

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