HUMPBACKS' BUMPY FLIPPERS

Abstract

Humpback whales' lumpy flippers don't look like anything a human engineer would design – unlike airplane wings, bumps called tubercles cover the fins' leading edges, giving them an almost serrated appearance. But humpbacks are some of the most agile of the large whales. Maybe their strange flippers actually give them an advantage in maneuvering, one that engineers haven't explored.After pondering humpback whale flippers for many years, Frank Fish decided to test this idea. Reporting in Physics of Fluids, he and his colleagues from the US Naval Academy and Duke University showed that the tubercles do increase hydrodynamic performance – they delay stall,increase lift, and decrease drag.The group built two model humpback whale flippers, one with a smooth leading edge and one with a wavy edge approximating the usual spacing of tubercles. They tested the scaled-down fins in the Naval Academy's wind tunnel by matching the Reynolds number of a swimming humpback, around 500,000, and measured the steady lift and drag forces on the fins at a variety of angles to the oncoming flow, ranging from –2° to 20°.The smooth-edged flipper behaved much like a standard airplane wing,although its shape gave it a few advantages over a normal wing. As the angle of attack increased, the lift force increased until the flipper stalled out at around 12° when the lift dropped and the drag increased substantially. This performance is very similar to a standard wing, but the lift did not drop as much as the group expected, possibly because fins are tapered towards the tip. The taper might let different parts of the fin stall later than others,leading to a more gradual drop off in the lift force.The really impressive results, however, came from the flipper with tubercles. The fin did not stall until it reached an angle of 16° and produced up to 6% higher lift and as much as 32% lower drag than the smooth fin. Over nearly the entire operating range of angles the bumpy flipper performed better.Summarizing the performance of the flipper models as an aerodynamic efficiency – the ratio of lift force to drag force – they found that tubercles increased efficiency at almost any angle, and particularly augmented the efficiency at high angles. Maximum efficiency jumped from 22.5 to 23.5.The group hypothesizes that the tubercles function like vortex generators,speeding up the flow in the gaps between bumps. The energized flow stays attached to the flipper better, helping to prevent stall at high angles and increasing the lift force. These increased forces probably contribute to humpbacks' surprising agility.While the effect of tubercles may not seem particularly huge, remember that engineers have been tinkering with the shape of airplane wings for more than 50 years. At this point, a change in force or efficiency of a few percent or so is considered large. But humpback flippers reveal a simple, passive way to increase lift by 6%, decrease drag by 32%, and delay stall by 40% –pretty astounding results in the engineering world. Simply adding tubercles– bumpy modified knuckles – to the leading edge does the trick.