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N. and C. Orn»sc?, U c is imposed by solutal buoyancy. The right-hand side of (5.1.a) reduces to gßATN §c/v and system (5.1) admits new solutions. We check that friction and thermal buoyancy are the leading mechanisms within [ôç, § v ] as required if N and Sc verify the additional requirement N«Sc The scaling laws are given in table, Therefore, the following asymptotic cases can be distinguished : 2.1. Sc^«N«Sc which defines flows of type 2

. Sc, The scaling properties are Pr, Sc) diagram is given in figure 5. The results of the scale analysis makes it possible to predict the streamwise velocity variations for the different types of flows (figure 6) Flows of type 1 are heat-driven (N«Sc 1/3 ) : clearly, the velocity and temperature fields vary as in thermal convection (Pr«l) Only two length scales are involved. The concentration field is entirely determined by thermal effects and has no reverse action on the flow. We emphasize that strikingly flows of type 1 include flows where the amplitude of soluta! forces dominates over that of the first arising from thermal forces (~gß T ATO v ) and the second from solutal forces (~ gß T ACNo v )

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