pectoralfin.org - the humpback whale pectoral fin project
CFD-analysis of the flow around a humpback whale's pectoral fin - start: 2004
Digitising a humpbacks pectoral fin
How to digitise a whale's flipper
For this project, we conducted CFD-calculations (CFD - computational fluid dynamics) to simulate the flow around a humpback whale's pectoral fin. To start with, this requires a very detailed idea of a typical flipper's shape. This idea needs to be communicated to a computer in a susequent step.
The first step proved unexpectedly tricky as photographs and drawings only leave a two-dimensional impression of the flipper and we didn't have a humpback at our hands to take measurements from. But as we found out, there are publications about beached Whales, e.g. from Fish " (Leading-edge tubercles delay stall in humpback whale flippers" - Fish et al. Physics of Fluids 2004). Among other things we found quite detailed descriptions of the flipper's geometry - unfortunately these whales were dead. The gargantuan body of a perished whale applies a pressure differential of around 200-300mbar on the flippers - that is almost ten times the pressure affecting a horizontally stored human body. It seems possible, that this leads to a stronger swelling than in human corpses.
Based on these considerations we conducted an extensive foto-survey of living humpbacks for comparison. We indeed found that their flippers are significantly thinner than observed in stranded specimen. We also observed this in a crosscheck with a humpback whale skeleton in the museum in Brussels. The planform (top view) of the flipper seems unaffected from this though. Consequently we used a slimmer profile for our simulations than described by Fish.
Despite all conscientiousness the shape we used for our analysis is afflicted with many uncertainties. It ignores individual differences in length, thickness, profiling, position and shape of the tubercles, internal bone structure, etc. Also we could not reproduce the flexibility and manoeuvrability of the flipper. Its anatomy corresponds to that of a human arm with similar mechanical possibilities. Videorecordings show that humpbacks make impressive use of this mobility. In addition to that, real flippers are usually afflicted by parasites like barnacles that will inevitably influence the flow as well - maybe even improve it.
Translation in progess...... Als Zwischenergebnis unserer Untersuchungen entstanden zunächst von Hand gezeichnete Flossenumrisse (Draufsichten) mit und ohne Tuberkel. Aus diesen läßt sich - mit den benannten Einschränkungen - der zweite Schritt in Angriff nehmen: Die Aufbereitung der Geometriedaten für den Einsatz eines CFD-Programms. Die Handskizzen wurden gescant und in eine CAD-Software importiert. Mit der in unserer Fotostudie gefundenen Profilgeometrie wurde aus dem zweidimensionalen Umriss eine dreidimensionale Flosse erstellt. Wir verwendeten letztendlich ein NACA0012-Profil, statt des von Fish ermittelten NACA0020. Ein Profilgenerator für NACA-Profile findet sich hier:
digitised pectoral fin:
by Kirsten 2006