Fundamentals of Boundary Layers

In the Fluid Dynamics video, Fundamentals of Boundary Layers, we learn how fluids behave upon encountering an obstacle. In general, a boundary layer is formed when a fluid with viscosity passes a solid plate. The video consists of black and white montage of fluid experiments along boundary layers with accompanying narration.

Throughout the video we observe the formation of formation of boundary layers. While this phenomena was mentioned in class with reference to the Blasius Problem, I found it difficult to visualize the actual physical representation. To see demonstrated by real fluids that the boundary layer grows along the length of the boundary was a valuable addition to my intuitive knowledge of transport phenomena. The proportion given below, and the physical phenomena it represents, is shown in the video

δ/l ∝ v/(U_o l)

where δ is the boundary layer thickness, l is the distance along the length of the boundary, v is dynamic viscosity and U_o is the free stream velocity.

One class concept that I learned to think about in a different way was the relationship between vorticity and the boundary layer. The video describes the local boundary layer thickness as a measure of the vorticity that has diffused away from the plate. Prior to this insight I had thought of vorticity as strictly a measure of fluid spin and the boundary layer as a function of only velocities and the distance along the boundary.

One section of the video that I was able to relate to more than others was the demonstration of fluid motion around an elongated cylinder. Near the end of the video, an airplane wing is shown to give the example more relevance. I happen to thoroughly enjoy airplanes and airplane rides; oftentimes I sit near the wings so I can watch as they glide through the air. It was interesting to look at this commonplace example of fluid passing around an object as vortex generation and the disturbance of an otherwise laminar flow.