On peut en déduire que les grottes ont été colonisées par des espèces de mœurs obscuricoles. Le peuplement effectué, l’isolement est complet et peut être comparé à celui d’une île. Chaque espèce va alors varier pour son compte, influencée par les conditions du milieu, et c’est ainsi que des grottes voisines pourront abriter des formes différentes, tout comme les petites îles de la Méditerranée donnent asile à des formes différentes de Lézards.
R. H.
A. Viré, Essai sur la faune obscuricole de France (Baillière, 1899). / R. Leruth, la Biologie du domaine souterrain et la faune cavernicole de la Belgique (Le Musée, Bruxelles, 1939). / R. Jeannel, les Fossiles vivants des cavernes (Gallimard, 1943). / A. Vandel, Biospéléologie. La biologie des animaux cavernicoles (Gauthier-Villars, 1964). / P. Strinati, Faune cavernicole de la Suisse (C. N. R. S., 1966). / C. Delamare-Debouteville et L. Botosaneanu, Formes primitives vivantes (Hermann, 1970). / C. Delamare-Debouteville, la Vie dans les grottes (P. U. F., coll. « Que sais-je ? », 1971). / G. Thines et R. Tercafs, Atlas de la vie souterraine (Boubée, 1972).
cavitation
Formation, dans les liquides, de cavités remplies de vapeur ou de gaz.
Dans ce sens large, la cavitation inclut l’ébullition de l’eau et l’effervescence des boissons gazeuses. En hydrodynamique, elle peut être définie par la formation et la disparition rapides de cavités remplies de vapeur à l’intérieur de liquides en mouvement ou sur la surface de solides immergés.
Ce phénomène a une grande importance pratique par ses conséquences néfastes : abaissement du rendement des machines hydrauliques telles que pompes, turbines et hélices, production de bruit et de vibrations, et surtout érosion rapide des surfaces métalliques entraînant la destruction d’organes.
Seuil de la cavitation NPSH
L’apparition de la cavitation correspondant à une vaporisation restreinte du liquide à la température ambiante, le phénomène apparaît dans une région entourant le point M (voir fig.), où la pression absolue du liquide est théoriquement égale à sa pression de vapeur saturante ps, c’est-à-dire dans une zone où la pression est minimale. La valeur de ps dépend de la nature du liquide et de sa température : pour l’eau pure à 20 °C, ps = 1,75.10–2 bar.
Dans l’écoulement des liquides, il existe donc un seuil au-delà duquel la cavitation ne peut se produire et que nous pouvons traduire par la relation où pm est la pression minimale en un point particulier de l’écoulement.
La prise en considération, dans les calculs, non de la machine seule, mais de l’installation complète a favorisé l’emploi d’une autre relation plus pratique ; cette relation, déduite de l’équation de Bernoulli entre le niveau du réservoir amont (indice a) et l’entrée de la pompe (indice 1) — la cavitation, pour cette machine, ayant lieu dans cette région —, s’écrit ρ étant la masse volumique du liquide ; g l’accélération de la pesanteur ; c1, la vitesse du liquide dans la canalisation amont ; z1 – za la hauteur géométrique d’aspiration et Δha1 la perte de charge dans la canalisation amont.
Le premier membre de l’équation (2) représente la charge nette à l’aspiration, que l’usage désigne par l’abréviation NPSH (net positive suction head). Pour éviter la cavitation, il faut que
Causes physiques de la cavitation
Le mécanisme exact de la rupture du liquide n’est pas encore bien connu. Ainsi, certaines expériences montrent que l’eau pure, dégazée, peut résister sans rupture à des tensions de plusieurs centaines de bars et d’autres que la cavitation apparaît dans l’écoulement de l’eau ordinaire lorsque la pression est voisine de la pression de vapeur saturante. Cette contradiction n’est qu’apparente, car la présence de particules solides et de gaz dissous ainsi que l’existence de fluctuations de pression dans les écoulements turbulents rompent la continuité du liquide et abaissent sa tension superficielle. Dans les zones de basse pression, des cavités microscopiques formées à ces discontinuités, ou noyaux, se remplissent de vapeur ou de gaz tiré de la solution. Ces bulles se développent rapidement et sont entraînées dans l’écoulement vers des zones à plus forte pression, où elles disparaissent en implosant.
Effets de la cavitation
Les actions de la cavitation sur les solides immergés peuvent être mécaniques, chimiques et électriques. Quand la différence de pression entre le milieu ambiant et la bulle atteint une valeur suffisante, la vapeur se condense presque instantanément. Le liquide emplit l’espace laissé libre, et cela à une vitesse très élevée, donnant lieu à des chocs contre les parois environnantes (point N sur la figure). Les contraintes causées par ces chocs sont de l’ordre de 150 bars pour des vitesses voisines de 100 m/s. L’énergie libérée provoque soit une petite déformation locale avec ou sans fissure, soit la corrosion par fatigue. De plus, la température pouvant atteindre localement plusieurs milliers de degrés, il y a un phénomène d’ionisation favorisant l’attaque chimique des surfaces métalliques et leur oxydation.
Vibration, érosion rapide des surfaces solides, abaissement du rendement limitent les caractéristiques des turbomachines. Ce problème complexe a pourtant une solution dans le cas particulier des hélices supercavitantes installées sur les bateaux rapides : la cavitation sur les faces de l’hélice est amorcée à basse vitesse par une forme appropriée des pales, et, à vitesse de régime, chaque pale est enveloppée par un nuage de cavitation ; l’implosion des bulles de vapeur ayant lieu derrière les pales (en N′ sur la figure 1), l’érosion est supprimée.
J. G.
R. Davies, Cavitation in Real Liquids (Warren, Michigan, 1962). / G. T. Csanady, Theory of Turbomachines (New York, 1964).
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