Blubber and compliant coatings for drag reduction in fluids IV: Rotating disc drag measurements on coatings that mimic dolphin blubber

摘要

The viscoelastic properties of "live" dolphin blubber have been determined, by extrapolation, from room temperature measurements of the frequency variation from 2 to 1000 Hz of the complex shear compliance, J*=J' - iJ", shear modulus, G* = G' +iG"=1/J*, for samples, from a stranded dolphin for which rescue efforts failed, of excised blubber vs. time after excision. These properties have been matched by several polymer gel-foam composites as reported in a prior publication in this series.Rotating disc drag measurements are now presented for discs coated with several synthetic rubbers, and discs with 0.5 cm thick polymer gel-foam composite coatings. The results are compared to those for a smooth, rigid aluminum disc of the same exteriordimensions (4.5 cm thick×10 cm radius). Drag measurements on silicone rubber with shear compliance magnitudes of 5.6 to 3.4 Mpa{sup}-1 (10{sup}-7cm{sup}2/dyne), for example, with smooth coatings of 0.5 to 2.0 cm thickness all resulted in laminar toturbulent transitions at the same speed and Reynold's number as those for the rigid disc. With a rough surface and high drag, or with smooth surfaces and low drag, the discs coated with gel-foam composites showed laminar to turbulent flow transitions atrotational speeds of 255 to 260 revolutions per minute (rpm) compared to 210 rpm for the rigid disc, i.e., at a Reynold's No. of 2.7×10{sup}5 compared to 2.2×10{sup}5. The particular gel-foam composites used to coat the discs had shear compliancemagnitudes of J = 10 to 8 MPa{sup}-1 while dolphin blubber compliance magnitudes in the 2 to 1000 Hz range are J = 23 to 19 MPa{sup}-1 (10{sup}-7cm{sup}2/dyne); gel-foams with compliances closer to those of dolphin blubber may add to the delay in theonset of turbulence with increasing speed.