Expert: Paul Soderman - 8/20/2008

Question
Two spherical buoys are attached by two short lines of same length to a single anchor point. What orientation will the main axis of the doublet take in relation to a steady horizontal current?
What will be the drag force on the doublet compared to that on a single isolated buoy?


Answer
Marc
I hope I am not helping with a homework problem.  A quick analysis of the problem leads me to believe that the main axis of the doublet will be perpendicular to the freestream.  Because of first order interference effects, the flow about each sphere will be similar to that of a sphere attached to a wind tunnel wall (boundary layer neglected).  The stagnation point will move toward the wall and change the flow field.  According to  the following reference, a cylinder on a wall will have 2/3 the drag of a free cylinder.  Since cylinders and spheres are somewhat similar, I assume that the doublet will have 4/3 the drag of a single isolated buoy.  What do you think ?
Paul

http://www.luxfamily.com/jimlux/robot/windball.htm