St Christopher and the vortex

Was Kármán's vortex street originally inspired by a painting of St Christopher?

A Kármán vortex in the wake of St Christopher's heels.

Theodore von Kármán, who was born in 1881, was a celebrated fluid dynamicist and pioneer of aerodynamics. In 1911, when he was studying the flow of fluid around a cylinder, he realized that the reason the flow had separated into two series of vortices which were staggered like street lights was because this asymmetrical configuration was the only stable one. His famous stability calculation, which he formulated over a weekend, gave the phenomenon its name, ‘Kármán's vortex street’.

A direct and precise motivation for this finding was clearly described in von Kármán's autobiography (The Wind and Beyond: Theodore von Kármán, Pioneer in Aviation and Pathfinder in Space, Little, Brown, 1967): “Vortices were observed and recorded many years before I came on the scene. In a museum at Bologna, Italy, I remember seeing a painting of the great Christophe (St Christopher) wading through water with the child Jesus on his shoulder. Behind his heels was a series of alternating vortices. The problem for historians may have been why Christopher was carrying Jesus through the water. For me it was why the vortices.

“I wondered about them for a long time, but the actual stimulus, which led me to study them, arose out of a curious situation in our laboratory. At the time, Prandtl was interested in measuring the pressure at different points on the surface of a circular cylinder when it was placed in a steady stream of water. He got one of his doctoral candidates, a fellow named Hiemenz, to carry out the measurement. Unfortunately Hiemenz found that the pressures he measured always fluctuated. No matter how hard he tried, he couldn't remove this unsteadiness. The motion of the water was always oscillatory.”

It was seven years after seeing the picture of St Christopher at Bologna that von Kármán calculated a stability analysis for alternative vortex arrays. We suggest that the picture that inspired him initially was a fresco picture at the museum at the Church of St Dominic in Bologna, Italy, entitled Madonna con bambino tra I Santi Dommenico, Pietro Martire e Critoforo (shown), painted by an unknown artist in the fourteenth century. Around St Christopher's heels there is a series of alternating vortices similar to those shown in the figure from von Kármán's original paper, although the pattern cited in the paper is not that from St Christopher's heels, but from other objects, not shown.

St Christopher is said to have waded across a river carrying a child on his shoulder. The child became heavier and heavier until he was as heavy as the entire Earth. This was because he was bearing the pain of the whole world — the child was Jesus. This painting adds romance to von Kármán's work and might turn out to be an artefact of significance in the history of scientific thought.