Low Re Flow in periodic (sinusoidal) roughness piping

[tiwanaku]

Looking for information/insights as to why piping that contains sinusoidal roughness patterns (6" diameter pipe, 1/8" wave amplitude, 1/2" wavelength) may have a lower resistance than smooth piping at very low Reynolds Numbers.
tiwanaku

 

[briand2]

Are you dead certain that the flow is laminar throughout the pipe section?

What is happening at the peaks and troughs of the ripples in the pipe wall? Could local separation be causing flow to be turbulent locally, thus reducing the friction factor?

Brian

 

[prex]

Can you be more specific on how much is the change?
Is the 1/8" ripple (total or half?) depth outside or inside of the inner pipe base diameter?

prex

http://www.xcalcs.com

Online tools for structural design

 

[tiwanaku]

Ripple amplitude from a zero reference plane is 1/8"; wavelength (peak-to-peak) is about 1/2", pipe diameter is about 6". Flow is most likely open channel for low water velocities less than 15 ft/sec in the inclined pipe (about 3 degrees). The problem reduces to understanding if an open channel flow in a pipe with a periodically rippled, wetted zone bottom somehow reduces wall friction over a smooth wall, wetted zone bottom.
tiwanaku