Leonardo di ser Piero da Vinci (1452 - 1519), was an Italian of many talents, spanning both the arts and the sciences. He is most famous as a artist, for his paintings, the Mona Lisa and The Last Supper, and his drawing of the Vitruvian Man. But he was also interested and knowledgeable in many other fields, including: invention, anatomy, mathematics, architecture, geology, cartography, music, sculpting, engineering, botany, and writing.
Turbulence, defined simply by the Oxford English Dictionary as 'an irregularly fluctuating flow of air or fluid' was something that da Vinci was fascinated by. He drew sketches of its motions, and made the following notes about it: "Observe the motion of the surface of the water, which resembles that of hair, which has two motions, of which one is caused by the weight of the hair, the other by the direction of the curls; thus the water has eddying motions, one part of which is due to the principal current, the other to random and reverse motion". Byatt is mirroring da Vinci by comparing Ash's beard hair to turbulence, just as da Vinci explained turbulence by way of hair.