Understanding Displacement Thickness and Flow Types — Lesson 6

This lesson covers the concept of displacement thickness in boundary layer flows and the characteristics of laminar and turbulent flows. It delves into the velocity profile within the boundary layer, the effect of velocity deficit, and the concept of displacement thickness. The lesson also discusses the differences between laminar and turbulent flows, their characteristics, and the conditions under which they occur. For instance, it explains how turbulent flows can withstand higher adverse pressure gradients, delaying flow separation and reducing pressure drag.

Video Highlights

02:03 - Explanation of the implications of velocity deficit in the boundary layer.
09:02 - Discussion on the formation of boundary layer and the transition from laminar to turbulent flow.
13:16 - Explanation of the concept of Reynolds decomposition and its application in turbulent flow.
17:15 - Discussion on the impact of turbulent flow on the boundary layer and the concept of Reynolds stress.
24:10 - Explanation of the impact of turbulent flow on drag and flow separation.
31:52 - Discussion on factors that can augment turbulence.

Key Takeaways

- Displacement thickness is the distance by which the external potential flow is displaced outwards due to the decrease in velocity within the boundary layer.
- Laminar flow is smooth and orderly, typically occurring at lower Reynolds numbers, while turbulent flow is random, chaotic, and unsteady, usually happening at higher Reynolds numbers.
- Turbulent flows can withstand higher adverse pressure gradients, delaying flow separation and reducing pressure drag.
- Turbulent boundary layers have higher velocities closer to the wall, leading to higher skin friction.
- Factors such as surface roughness, ambient disturbances, wall heating, and adverse pressure gradient can augment turbulence.