There is laminar flow and turbulent flow. The rougher the surface, the more turbulent flow with greater lift.
At least for aircraft.
But, I may be wrong. That class was a long time ago and involved flow in fuel pipelines.
Cool link Monkeyhead.
Really nice discussion and summary carldelo.
@string said:
There is laminar flow and turbulent flow. The rougher the surface, the more turbulent flow with greater lift.
At least for aircraft.
But, I may be wrong. That class was a long time ago and involved flow in fuel pipelines.
Well, in certain situations roughness can decrease drag - but it’s a narrow range of conditions. Generally speaking, turbulence does not help - it usually increases drag, except over certain geometries and flow conditions. Laminar flow (smooth flow) is generally preferable and will generate less drag, but unfortunately most real-world flows are turbulent,
For example, the dimples (roughness elements) on a golf ball help by initiating an early transition from laminar to turbulent boundary layer (TBL) flow over the ball. The TBL is better able to remain attached to the ball at that speed, and so the TBL separates from the ball farther along the surface of the ball. This reduces the size of the low pressure wake region caused by flow separation from the ball surface - a smaller low-pressure zone means less drag. This is only functional over a narrow range of conditions, which a golf ball happens to operate in when driven. This was a big conundrum until it was understood, termed the ‘drag crisis’:
https://thatsmaths.com/2015/12/03/the-flight-of-a-golf-ball/
https://www.grc.nasa.gov/www/K-12/airplane/dragsphere.html
On wings, there are often small tabs sticking up from the wing, aligned with the flow - they are vortex generators, termed ‘turbulators’ (a silly word):
http://farm3.static.flickr.com/2625/3724840086_3fde70e475.jpg
The purpose is to induce streamwise vortex flow that enables the boundary layer to remain attached to the upper surface of the wing when it would normally separate. Delaying flow separation enables the wing to continue to produce lift, rather than stalling (the normal result of flow separation). There are now many types of turbulators available commercially and they seem popular in the model airplane world.
Tubercles on the leading edge of humpback whale fins are also presumed to improve flow by vortex generation, although they operate in a highly 3D, unsteady flow regime which is much harder to analyze. Vortex generators can operate in laminar or turbulent flow - their purpose is not to initiate turbulence, but to modify the flow field over the wing (or fin) in a beneficial way.
There are also interesting flow effects due to the seams on baseballs and the big ridge around the middle of a cricket ball - it’s all pretty interesting to look into.
Thanks. I know a lot of people know more about it than I do.