Henri Claude Bénard (25 October 1874 – 29 March 1939) was a French physicist, best known for his research on convection in liquids that now carries his name, Bénard convection. In addition, the historical surveys of both Tokaty and von Kármán both acknowledge that Bénard studied the vortex shedding phenomenon later named the Kármán vortex street, prior to von Karman's own contributions. Bénard specialized in experimental fluid dynamics, and the use of optical methods to study it. He was a faculty member at the universities at Lyon, Bordeaux, and finally the Sorbonne in Paris. Bénard defended his PhD thesis at the Collège de France on 15 March 1901 entitled "Les Tourbillons cellulaires dans une nappe liquide propageant de la chaleur par convection en régime permanent". Bénard was elected President of the French Society of Physics (SFP) in 1929, following the presidency of Louis Lumière. He was succeeded as President the next year by his friend and former teacher, Jean Perrin. In 1929 Bénard received the Bordin Prize for his work on vortices from the French Academy of Sciences. After his death in 1939, his widow received the Poncelet Prize on his behalf, also from the French Academy of Sciences. A research center of the ERCOFTAC in Lyon is named after him.
Life and career
Early and student years Henri Bénard was the only son of a small investor, Felix A. Bénard (1851–1884), and his wife Hélène M. Mangeant (1837–1901). He attended elementary school in Lisieux and Caen and high school at the Lycée Louis-le-Grand. In 1894, Bénard was one of 17 students selected from 307 candidates to attend the École normale supérieure (ENS) in the sciences section. His classmates there included Henri Lebesgue and Paul Langevin, and one of his teachers was Jean Perrin. Bénard received his teaching degree in physics in 1897, and then began working as an assistant to Éleuthère Mascart and Marcel Brillouin at the Collège de France in Paris. At this time, Bénard joined the French Society of Physics (SFP). Bénard's initial scientific efforts related to the optical rotation of sugars, resulting in papers co-authored with Mascart and ENS chemistry student L.-J. Simon. The first of these was an experimental measurement of the angle of rotation of polarized light by pure sugar in solution, to determine its concentration for use in saccharimetry, undertaken at the request of the Commission on Sugars and Alcohols, of the Ministry of Finances. Bénard's results were adopted as the legal values in France by the Ministry of Finances. Meanwhile, Marcel Brillouin was teaching a course on the viscosity of liquids and gases, and asked Bénard to repeat Poiseuille's experiments on water flow rates in capillary tubes. However, Brillouin also wanted experiments done with mercury instead of water. Bénard's results (undertaken in the first 6 months of 1899) were summarized in 1907 in Brillouin's textbook based on the course. Brillouin also supervised the translation into French of Boltzmann's textbook on kinetic theory of gases, by Bénard and Alexandre Gallotti. The subject of Bénard's dissertation was cellular thermal convection, inspired by accidental observations made by Adrien Guebhard of convection in a bath of abandoned film developer. Working in Mascart's lab, Bénard carried out the first controlled, systematic scientific experiments on convection in a shallow layer of fluid heated from below. He found that the convective motions organized themselves in semi-regular, semi-permanent cellular patterns. Upflows occurred in the centers of the cells and downflows occurred at their peripheries. There was also a slight depression of the upper free surface of the fluid at each cell center, leading Bénard to speculate about the role of surface tension. He also measured the aspect ratio of the cells and discovered that there was a critical temperature below which no convection occurs. Unfortunately, he attributed this to the solidification of the fluid he was using (spermaceti, a whale oil that is solid at room temperature). Ironically, Bénard would much later become a skeptic about the very concept of the critical temperature difference, although he discovered it. In 1900–1901, Bénard presented the results of this work (and the associated optical methods) in four different journals, the Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences, the Revue Générale des Sciences Pures et Appliquées et Bulletin de l'Association Française pour l'Avancement des Sciences, the Journal de Physique Théorique et Appliquée, and the Annales de Chimie et de Physique. He also presented his findings to at least two scientific meetings, as well as in the first thesis of his dissertation. (The second part of his thesis dealt with optical rotation in sugars.) This work laid the foundation for the study of Rayleigh–Bénard convection, the buoyancy-driven flow of fluid confined between horizontal conducting surfaces, with the higher temperature at the bottom; and Bénard–Marangoni convection, the surface-tension-driven flow of a fluid with an upper free surface and a heated, conducting surface at the bottom. These problems have continued to occupy scientists beginning with Lord Rayleigh and continuing into the 21st century. Bénard spent two months as a high-school teacher in Cherbourg (Oct.-Nov. 1900) before acquiring a pension from the Thiers Foundation (Nov. 1900–April 1902). He defended his dissertation on 15 March 1901, at the age of 26, and was awarded the Docteur ès Sciences physiques, mention très honorable. His dissertation committee consisted of Gabriel Lippmann, Edmond Bouty, and Émile Duclaux. In September 1901, Bénard attended the conference of the British Association at Glasgow, where he observed a number of notable British physicists, such as Lord Kelvin, Silvanus P. Thompson, Andrew Gray, and Joseph Larmor. Unfortunately, an "excess of modesty" (Bénard's own words) prevented him from showing the results of his work to Lord Kelvin in Glasgow, as well as at the earlier Paris conference. Kelvin's late brother, James Thomson, had studied thermal convection qualitatively prior to Bénard's work. On 23 December 1901 Bénard married Clémentine Olga Malhèvre, a few months after his mother's death; they had no children.
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