Synthetic-Blubber Compliant Coatings for Reduction of Hydrodynamic- Drag and Flow Noise Based on the Matched Shear Impedance Hypothesis. Phase II

摘要

After some 4 years of extensive investigations, the issue of whether or not compliant surfaces can reduce hydrodynamic drag remains unresolved. Based on Kramer's initial identification of the dolphin's skin as the basic mechanism, most of the past work has concentrated on thin (approx. 0.3) compliant coatings, presumed to deflect in some preferred manner and interact with the boundary layer Tollmeir-Schlicting waves so as to delay the transition from laminar to turbulent flow. The Matched Shear Impedance Hypothesis takes a completely different approach more akin to 'acoustica' than 'hydrodynamics' Identifying the thick (approx. 3 cm) blubber as the effective compliant 'layer', the basic postulates are: (1) The Turbulent Boundary Layer (TBL) is viewed as a fluctuating Shear 'Stress Generator' coupled to the 'Compliant Layer Load' through the viscous inner boundary layer. (2) To transfer appreciable energy (power) from the TBL Shear Stress Generator to the Compliant Layer Load, the Shear Impedance of the Load, must be 'matched' to the Shear Impedance of the TBL Generator. (3) Under Matched-Load conditions, the build-up of the fluctuating energy of incipient turbulence is reduced by energy flow into the compliant layer where it is dissipated by losses in the viscoelastic compliant layer material, thus delaying the onset of turbulence. (3) Dolphin blubber represents just such a matched-load with the required high loss tangent. Preliminary Rotating Disc measurements of drag (torque) on a polymethyl siloxane gel/ polyurethane (PMS) composite sample delayed the onset of turbulence to W approx. 260 rpm as compared to W approx. 210 rpm for the rigid reference disc. The PMS sample was not fully matched to blubber and has the same surface roughness. The projection of a smooth and fully matched sample indicates a potential laminar- to-turbulent transition at W approx. 448 rpm, corresponding to peripheral velocity of approx. 10 knots and a Reynold's No. approx. 4.7 x 10(exp 5)c.