LearnChemE

#### Characteristics of a Boundary Layer over a Flat Surface: Example Problems

Try to solve these problems before watching the solutions in the screencasts.

##### 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/m3 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/m3 and μ = 1.8 × 10-5 Pa-s.

##### Example Problem 2

U = 0.05 m/s in all four cases.
Why do the velocity profiles look different?