#### Characteristics of a Boundary Layer over a Flat Surface: ConcepTest and Example Problem

Try to answer this ConcepTest and solve the example problem before using this module. Studies show that trying to answer the questions before studying material improves learning and retention. We suggest that you write down the reasons for your answers. By the end of this module, you should be able to answer these on your own. Answers will be given at the end of this module.

##### Example Problem 1

Water flows over the flat surface. The surface is infinitely wide in the z-direction. Far above the surface (at large values of y), the water flows with a speed of U_{∞} = 0.3 m/s. Experiments have shown that the flow is laminar for Reynolds numbers smaller than 500,000 and turbulent for Reynolds numbers larger than 500,000.

1. Describe in words where the flow is laminar.

2. Describe in words where the flow is turbulent.

3. At what value of x does the flow transition from laminar to turbulent? Report your answer in meters. Properties for water are ρ = 1000 kg/m^{3} and μ = 0.001 Pa-s.

4. Instead of water, air flows over the plate at the same speed. Does the flow transition from laminar to turbulent closer to the leading edge of the plate (at a smaller value of x), or further from the leading edge of the plate (at a larger value of x)? Why? Properties for air are ρ = 1.2 kg/m^{3} and μ = 1.8 × 10^{-5} Pa-s.