# Is anyone familiar with the concept of "active streamlining"

Discussion in 'Hydrodynamics and Aerodynamics' started by lumberjack_jeff, May 21, 2011.

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### lumberjack_jeffSawdust sweeper

From Shultz Engineering.
This is fascinating. Has it ever been tried on boat hulls? Suck water through holes in the hull near the point at which laminar flow becomes a turbulent boundary layer?

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### StumbleSenior Member

I could maybe see this working on a submarine, or an airplane, but as boats move through the water and is lifted and rolled by waves/wind the seperation point movesaround the hull. I don't know but I would guess that while it may work great in stable environments in dynamic ones it probably adds to drag. Plus weight and complexity.

I have no specific information on a system like this though.

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### gonzoSenior Member

That is the problem with using systems developed for a homogenous system on a boat. Boats operate on the boundary between two mediums: air and water. The boundary itself is turbulent.

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### markdrelaSenior Member

Most of these concepts are misguided. Ignoring induced drag , the net profile drag is
D_net = D_friction + D_pressure
What usually sink suction or blowing concepts is the neglect of pressure drag, which absolutely cannot be ignored.

A better way to look at the problem is via dissipation due to viscous stresses in the flowfield, including all the suction passages. The relation is
D_net * V_freestream = Dissipation
or equivalently
D_net = Dissipation / V_freestream

The only way suction can reduce dissipation is by
1) Preventing separation (separated flow regions are very dissipative)
2) Delaying transition (turbulent flow is more dissipative than laminar flow)

Sucking away a boundary layer which does not accomplish 1) or 2) is almost certainly counterproductive, since there will be additional dissipation within the suction passages, so D_net has to increase. Specifically, the total onboard power needed for propulsion and the suction pumps will increase over the simple non-suction case.

So one way to determine whether a suction system makes sense is to answer:
a) Will it reduce total viscous dissipation?
b) Will it reduce it more than a simple reshaping of the geometry?
If the answer is "no" to either a) or b), then using suction doesn't make sense.

All of the above applies also to blowing, except for 2) above.

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