NIWC Drag on Inline String of Spheres

Background:

Naval Information Warfare Center (NIWC) Pacific develops seafloor hardware that support a wide range of Navy missions. Installation of seafloor equipment is often accomplished by dropping the item overboard with a string of trailing floats as shown in Figure 1. The floats reduce the in-water weight (defined as the difference of the buoyancy and force of gravity on the system) of the deployment package, enhance stability, and position the equipment in a preferred orientation. By judiciously selecting a number of floats, a desired terminal descent speed can be achieved. If the descent rate is too fast, the equipment could be damaged on impact with the seabed. If the descent is too slow, the package could be carried by currents far from the intended touch-down site. The terminal descent velocity can be accurately estimated if the in water weight and drag of the equipment are known and the buoyancy and drag of the trailing floats are known.

This project investigates drag of the trailing floats. Said floats are close to spherical, and are treated as such. While drag on a single sphere is well established, there is little literature on the drag of several attached in line. What is known is if the floats are close enough together, the wake of the leading floats will decrease the drag induced by the following floats. This effect is expected to decrease as the separation distance increases. Eventually, at a large enough distance the leading float's wake stop effecting following float's drag. It's also assumed that drag contribution of following floats will be constant by this point.