# Re: Cost in Aquatic Birds (the big one)

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Mokele mbembe wrote:

>  If the aerodynamics are such that the boundary layer would
> separate from the body in question, use of wires, hairs, etc to cause the
> smooth boundary layer to become turbulent will reduce the drag by preventing
> rather nasty vorticies from forming.

It can also work where the boundary layer is already turbulent, no longer
laminar but still attached, in order to prevent turbulent seperated flow.  As an
aside, in the range of Reynolds numbers where a shark's skin texture can help --
placing the rows in a regular array can increase the drag up to about 10% even
in a turbulent attached layer; using the same basic pattern, placing the
adjacent rows with a slightly chaotic spacing can  reduce drag up to about 10%.
A fascinating effect.

> Highly aerodynamic things don't need this, as their boundary layer just slides
> off.

Doesn't that sort of depend upon Reynolds number, aoa, and a few other things of
that ilk?  It is quite easy to get turbulent seperated flow on aerodynamic
objects (just ask my airplane (even though it isn't all that aerodynamic)).

> , without hairs or barbs, the boundary layer separates and forms a
> drag-inducing vortex (aka "parasite drag").

Isn't this just one sort of parasite drag?  As an aside, the drag increases when
the boundary layer transitions from laminar flow to turbulent attached flow, and
increases again (for a different reason) when it transitions from turbulent
attached flow to turbulent seperated flow.  The purpose of vg's (of whatever
nature) is usually to insert additional energy into the boundary layer to
prevent the formation of a seperation bubble.

>  With the hairs, this additional drag force doesn't come into effect, since
> what was once a flat sheet of
> almost isobaric air

I may be misinterpreting you here.  My impression is that you are saying the
entire boundary layer whether laminar or turbulent attached is at very nearly
constant pressure. Is this the case?  If so, then are you talking about total
pressure, static pressure, or dynamic pressure (since the boundary layer
velocity at the surface is zero, and by definition, the outer edge of the
laminar boundary layer is taken to be the streamline at which the velocity is
99% of the freestream velocity -- consequently, the static and dynamic pressure
vary across the depth of the laminar boundary layer)?

> becomes turbulent and unable to form single, large vorticies.

Isn't it usually already turbulent attached?  Or by turbulent, were you
referring only to turbulent seperated flow as opposed to turbulent attached
flow?

> Hope this helps. or at least doesn't waste too much bandwidth. :)

I think it helped.  In my mind, this subject never wastes bandwidth.  Don't know
how many others agree with me.

Jim

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