I didn't see the beginning of this thread, but eddys as air flows over a
rapid change in cross section of a pipe are a common and relatively well
understood phenomena - both going from large to small and small to large,
in both cases eddy's form at the end of the larger piece of pipe. This
vastly increases the overall back pressure in the pipe. The solution is to
make sure the cross section doesn't change abruptly by graduating the
change over a distance.

regards

alex rice

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