Double-diffusive convection with a nonlinear equation of state: Part II. Laboratory experiments and their interpretation |
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| Authors: | Trevor J McDougall |
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| Institution: | Research School of Earth Sciences, The Australian National University, P.O. Box 4, Canberra 2600, Australia |
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| Abstract: | The term cabbeling describes the convection that can occur when a mixture of two oceanic water masses is more dense than both of the parent water masses. When the two water masses are situated one above the other, the temperature and salinity distributions are in the correct senses for double-diffusive convection to occur and it is found that the prime effects of the nonlinearity of the equation of state are firstly to drive a greater level of double-diffusive convective activity in the lower layer than in the upper layer, and secondly, to make the lower edge of the interfacial region less gravitationally stable. Both of these effects cause the interface to migrate upwards as the lower layer grows at the expense of the upper layer. We introduce a nondimensional parameter (called the cabbeling parameter) which represents the importance of the nonlinearity of the equation of state:— is zero when the equation of state is linear and when cabbeling is normally thought to be possible, is greater than unity. Experiments set up in both the finger and diffuse sense show how the nondimensional measure of the upward interface migration (called the “entrainment” parameter ) varies with the density anomaly ration R? for various values of between zero and 2.0 and that no abrupt change in this behaviour occurs at . It is impossible to explain these observation by neglecting double-diffusive convection and considering only the convection driven by the conventional cabbeling instability. The successful interpretation of the laboratory results relies on considering the effects of a non-linear equation of state on the double-diffusive convection process. |
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