Characteristics of a Boundary Layer over a Flat Surface: Summary
The answers to the ConcepTests are given below and will open in a separate window.
Key points from this module:
- A boundary layer over a flat plate is a somewhat arbitrary way to indicate where shear forces (viscous effects) are significant. Shear forces are significant
inside the boundary layer and insignificant outside the boundary layer. - When shear forces are significant, a differential element of fluid tends to deform as it moves along the length of the plate. Only regions of fluid that are within the boundary layer can “feel” the presence of the plate.
- The thickness of the boundary layer increases as the fluid flows downstream from the leading edge.
- Boundary layers are laminar for Reynolds numbers less than about 500,000 and turbulent for Reynolds numbers larger than that.
- For a given location on a plate, higher Reynolds number flows typically have thinner boundary layers.
From studying this module, you should now be able to:
- Describe in words the velocity profile over a flat surface.
- Identify where the shear stress is highest and where it is negligibly small for laminar flow over a flat surface.
- Look at a velocity profile over a flat surface and have a fair understanding of where the boundary layer exists.
- Make a list of the fluid properties that affect the velocity profile over a flat surface and describe how changes in each property affect the velocity profile and the shape of the boundary layer.
Prepared by Jeffrey Knutsen, Department of Mechanical Engineering, University of Colorado Boulder