Is friction drag more or less in laminar boundary layer vs turbulent boundary layer?
It is written in this page of wikipedia:
The laminar boundary is a very smooth flow, while the turbulent boundary layer contains swirls or "eddies." The laminar flow creates less skin friction drag than the turbulent flow, but is less stable.
But in another page, it is said as:
The flow over a body may begin as laminar. As a fluid flows over a surface shear stresses within the fluid slow additional fluid particles causing the boundary layer to grow in thickness. At some point along the flow direction, the flow becomes unstable and becomes turbulent. Turbulent flow has a fluctuating and irregular pattern of flow which are made obvious by the formation of vortices. While the turbulent layer grows, the laminar layer thickness decreases. This results in a thinner laminar boundary layer which, relative to laminar flow, depreciates the magnitude of friction force as fluid flows over the object.
So what's the correct statement?
Best Answer
You are focusing on skin friction. This is but one component of parasitic drag, which in turn is but one component of the total drag on an object. You need to look at all aspects of drag, and these vary dramatically based on object shape, surface texture, and speed.
A golf ball has dimples because skin friction is a small component of drag for blunt objects. Form drag (pressure drag) dominates over skin friction in the case of a golf ball. Consider an undimpled golf ball. The flow remains laminar over a good portion of a smooth ball, up to the point where the flow separates from the ball. This separation occurs early in a smooth ball, resulting in a rather large form drag. Now consider a dimpled golf ball. The dimples make the flow turbulent over almost all of the surface. This in turn moves the separation point moves well toward the rear of the ball. Skin friction increases somewhat, but pressure drag is significantly smaller for a dimpled vs undimpled golf ball. The total drag on a dimpled ball is much less than on an undimpled ball.
Now consider a wing. A wing is anything but a blunt object. Form drag is, by design, a minor contributor to the total drag force. Skin friction is much more important for a wing, so a design that maintains a laminar flow wins over a design that results in a turbulent flow.