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In this video, we explain what a boundary layer is.

Let’s look at two extremes first:
Noslip condition: no matter how smooth the surface is, the flow will always stick to it, having a flow velocity of zero on the surface of the object.
Free stream velocity: the velocity of the undisturbed air, far away from the object

To understand what happens in between these two extremes, let’s look at air flowing across a flat plate. As the undisturbed air meets the leading edge of the plate, it will locally stick to it because of the noslip condition. As the air moves or slides across the plate, this layer or air sticking to it will grow thicker. This region, where the air moves slower than the free stream velocity, is called the boundary layer and it is mainly determined by the viscous forces. Outside of the boundary layer, the viscous forces are much less important and sometimes even ignored in fluid modeling

You can have a laminar boundary layer, with layers of air moving parallel to the surface, or a turbulent boundary layer, filled with vortices featuring velocity variations in all directions.
If the incoming airflow is laminar itself and if there are no disturbances that can trip the flow, the boundary layer will start off as laminar. As the air continues to move along the plate, so does the distance traveled from the leading edge. If we use this distance to calculate the Reynolds number, it’s clear that the Reynolds number will continue to rise as we move further down the plate.

When the Reynolds number crosses a critical value, which can be different for each application or geometry, the laminar boundary layer will start to transition into a turbulent one. The velocity profile of both types is very different and can have a large impact on the total friction & pressure force on the object.

In applications like airplanes, where shapes are smooth and streamlined, the goal is to maintain a laminar boundary layer as long as possible: a turbulent boundary layer increases the friction drag and typically is thicker than a laminar one, which increases the effective thickness and thus the pressure drag of the wing.

In applications with less streamlined shapes, like sports athletes or golf balls, it can be beneficial to trip the boundary layer and make it turbulent: turbulent boundary layers carry more momentum and will push the separation point further downstream – that’s the reason golf balls have dimples and athletes wear special suits or even shin tape, reducing the wake they leave behind.