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In article <921215012955@avistic.demon.co.uk> lorcan@avistic.demon.co.uk writes:
>In article <1992Dec9.234916.10094@retix.com> bmartin@retix.com writes:
>> Has anyone knoledge of the Cd value of an open box shape, with the
>> open side toward the airflow? Is it better/worse than a flat plate,
>>for example? 
>Worse (ie higher drag). If you've ever seen pictures of a Pelton wheel
>(a type of water turbine) the "paddles" are specially-shaped buckets
>[...]

I think the original question applied to a box without a "bottom".  i.e, a
length of rectangular duct aligned with the stream.  In a reasonably simple
analysis, the only difference in drag between the duct/box and an equivalent
area of flat plate will be due to boundary layer interference in the
interior corners.  If the box is large compared to the boundary layer
thickness, this effect could possibly be neglected.

It should be possible to make a reasonable approximation using standard data
for a rectangular duct for the interior flow, and use flat plate drag on the
outside of the box.  Try Fox & MacDonald or Schlichting for the data.

				Marc Leonard
				leonam@rpi.edu

>>> Has anyone knoledge of the Cd value of an open box shape, with the
>>> open side toward the airflow? Is it better/worse than a flat plate,
>>>for example? 
>>Worse (ie higher drag). If you've ever seen pictures of a Pelton wheel
>>(a type of water turbine) the "paddles" are specially-shaped buckets
>>[...]
>I think the original question applied to a box without a "bottom".  i.e, a
>length of rectangular duct aligned with the stream.  In a reasonably simple
>analysis, the only difference in drag between the duct/box and an equivalent
>area of flat plate will be due to boundary layer interference in the
>interior corners.  If the box is large compared to the boundary layer
>thickness, this effect could possibly be neglected.
>It should be possible to make a reasonable approximation using standard data
>for a rectangular duct for the interior flow, and use flat plate drag on the
>outside of the box.  Try Fox & MacDonald or Schlichting for the data.


True... but wouldn't the thickness of the sides add to the drag force,
should they be thick enough? I'm sure that the box/duct used would
have plate thickness that is <<< than the length and/or width... but
it's something to consider when figuring the Cd for that particular
piece, as there could be a non-negligible drag due to a small frontal
area... might only shift the result anywhere from .1 to 5% (maybe),
but it's something I would want to take into consideration if I was
performing a windtunnel experiment...


pf


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