The world has known about skin friction reduction technology since 1949 when a chemist from the United Kingdom, B. A. Toms, wrote that mixing small quantities of long-chain molecules (polymers) into a water based solution could reduce drag through a tube, that was in turbulent flow, by up to 70 percent. Since that time, many researchers have tried to integrate this technology on sea-faring vessels but were hindered by both the vast quantities of polymer that had to be ejected and the the vast quantities of polymer fluid that had to be carried with them. Therefore, until Cortana developed the fluidics ejector and the AHS (Additive Handling System) ships could only expect to use the technology for short bursts of speed, i.e. military vessels that would want to rapidly engage or retreat from the enemy.
Prior to Cortana’s efforts, making polymer powder down to liquid and using as-needed was considered impossible. It had been tried before and the result was an utter failure. Many believed it took one to three days for the polymer powder to go into solution and reach a state supporting skin friction reduction.
Figures 1 and 2 show the evolution of skin friction reduction technology from about the early 70's through to today.
The first generation (the far left bar which is almost invisible as its height is so short) shown in Figure 1, demonstrates that for a polymer storage volume of 38.51 m3, as can be found in an ISO storage container (20’ long x 8’ wide x 8.5’ high), could only operate for 0.047 hrs (or 2.8 minutes) before all of the polymer fluid was expended. Note also, that the total drag reduction was only 27% because this early process of ejection disrupted the boundary layer and actually increased drag at the point of ejection and it was not until some distance downstream that skin friction reduction overcame the initial increase in drag. This first generation ejector also operated at 150 Qs, where Qs is the flow rate in the viscous sublayer per unit span and is defined as the discharge per unit width of the Turbulent Boundary Layer in the region 0<y+≤11.6, and is equal to 67.3n.[1]
The second generation ejector (shown as the second bar in Figure 1) carried a 300,000 wppm concentrated slurry to sea that was post-diluted to 10,000 wppm immediately prior to ejection. Carrying the concentrated slurry enabled the ship to operate with skin friction reduction for 1.4 hours. However, post-diluting a slurry requires shearing the fluid in order to get more water into it which is detrimental to the polymer molecule. Post-diluting a slurry therefore decreases the polymer’s effectiveness for skin friction reduction versus polymer initially made from powder at the concentration necessary for ejection. Like the first generation ejector, the second generation ejector also used a 150 Qs flow rate.
The Cortana Ejector (or Fluidics Ejector) shown as the third bar in Figure 1 could theoretically operate for 52.6 hours with the slurry and achieve better drag reduction than the second generation system because it does not disturb the boundary layer upon initial ejection.
The fourth bar in Figure 1 shows how Dow Chemical’s WSR 309 PEO (Polyethylene Oxide) is twice as effective at providing skin friction reduction as the previous generation of PEO polymer due to its higher molecular weight. Therefore, one does not have to eject as much polymer (approximately half) hence 105.2 hrs of operation are theoretically possible.
Finally, for the fifth and sixth bars in Figure 1 , if one goes to sea with an ISO container filled with polymer powder, one can expect to operate for 189 to 377.9 hrs depending upon the quality of polymer powder used. Based on the initial evaluation of the Sea Flyer Tests Cortana believes it may already be operating at the 6th (far right) bar shown in Figure 1 .
Figure 1: Evolution of the AHS & Cortana Fluidics Ejector
For the fifth and sixth bars in Figure 2 , if one goes to sea with the same weight of polymer used in the liquid versions (which is perhaps a better comparison as surface ships are more weight restricted than volume restricted), one can expect to operate for 350.6 to 701.1 hrs.
Figure 2: Evolution of the AHS & Cortana Fluidics Ejector
The left side of Figure 3 shows the "stringy" behavior of the polymer fluid when a portion of it was caught in the wind. The right side of Figure 3 shows the polymer liquid in a viscometer with the center opaque spindle turning and the polymer seeking to dissipate its elastic energy build up contracting radially inward and therefore climbing the spindle.
Figure 3: Sample of Polymer for Testing
Proceed to Installation for At-Sea Test
[1] Fontaine, A.A., H.L. Petrie, and T.A. Brungart, “Velocity Profile Statistics in a Turbulent Boundary Layer with Slot-Injected Polymer” Journal of Fluid Mechanics (1992), vol. 238, pp 435-466.