onsdag 5 februari 2020

Skin Friction is Small

DFS with slip (zero skin friction) correctly predicts lift and drag within 2-5 percent to observations for Reynolds numbers Re beyond the drag crisis that is Re bigger than about 500.000 of relevance for flight. This shows that the contribution from skin friction to drag at high Re is at most 2-5 percent. In DFS drag is pressure/form drag with zero skin friction, in close agreement with observation.

This is in direct contradiction to a widespread belief in the fluid dynamics community that skin friction is 50 percent or more of total drag for streamlined bodies. The support comes from flat plate experiments where the flow by an attached transversal ribbon trips the flow and thus creates drag, which is translated to arbitrary streamlined body without ribbon claiming that the ribbon drag on the flat plate becomes skin friction on the body. Compare with this post.

The logic is missing and the 50% skin friction drag is an artefact from tripped experiments combined with the fact that standard CFD has been fitted to a preconceived artefact of 50% skin friction thus lacking true prediction.

To see how deeply rooted the belief in 50% skin friction is at e g Airbus, take a look at this picture:
(also note that predictive CFD is claimed to require 800.000 years of computing) as presented by Philipp Schlatter KTH:

When will Boeing and Airbus open to DFS and turn into a new era of predictive CFD?

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