. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 380 THOMAS L. DANIEL 0005 j i ill 1000 5000 10,000 FIGURE 4. Local friction factor (fx) is plotted against the local Reynolds number (ReJ for the fish (solid circles) and both wax models (triangles). The solid line corresponds to the predicted friction factor for flow over a flat plate in laminar flow. The broken lines represent the regression equations which for the fish is f, = Re,"0'95 (r2 = , N = 11) and for the wax models is fx = Rex~° '3 (r2 = , N = 12). The slopes of these lines are
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 380 THOMAS L. DANIEL 0005 j i ill 1000 5000 10,000 FIGURE 4. Local friction factor (fx) is plotted against the local Reynolds number (ReJ for the fish (solid circles) and both wax models (triangles). The solid line corresponds to the predicted friction factor for flow over a flat plate in laminar flow. The broken lines represent the regression equations which for the fish is f, = Re,"0'95 (r2 = , N = 11) and for the wax models is fx = Rex~° '3 (r2 = , N = 12). The slopes of these lines are not significantly different. DISCUSSION The velocity gradient about an object moving through a fluid can be considered a measure of the transfer of momentum from fluid in the free-stream to the surface of the object, by the action of viscosity: the greater the velocity gradient, the greater the rate of momentum transfer. The velocity gradient can be modified by changes in surface texture (Goldstein, 1965) and, as this study has shown, by mucus in the boundary layer. The velocity gradient about a fish is less steep than that about a hydrodynamically smooth model of the animal. This difference reduces the local friction factor on the order of 50%, with a trend towards a greater reduction at higher Reynolds numbers. The reduction in frictional drag may reasonably be attributed to mucus. The possibility that mucus on fish is behaving as a "Toms effect" solution should be considered. In this case, high molecular weight polymers from the mucus may be decreasing the rate of momentum transfer from the fluid to the animal's surface. Several conditions must be satisfied for mucus to act as a drag reducer and for my treatment of velocity gradients to be valid: (1) Mucus must consist, in part, of polymers soluble in water and with molecular weights exceeding 50,000 (White and Hemmings, 1976). (2) There must be turbulent or pulsed laminar flow about the fish (Driels and Ayyash,
Size: 1489px × 1677px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
