Double-diffusive gravity currents 2012/08/01 2015/12/31
A double-diffusive gravity current behaves differently than an ordinary single-diffusive gravity current. One difference is the presence of turbulent drag resulting from double-diffusive fingering. This drag is much higher than viscous drag, especially at large Reynolds numbers. The double-diffusive current might therefore be expected to propagate more slowly than a single-diffusive current, but this is not always true. Double diffusion can also affect the driving bouyancy force of the current. Assume for the sake of argument that the two components are heat and salt and that the current is a hot and salty flow over a cold and fresh ambient. Double-diffusive fingering results in adjacent upward and downward travelling fingers. Since heat diffuses more quickly, it escapes the downward moving fingers and diffuses into the upward moving fingers and is carried back into the current. In this way the current loses more salt than heat, even though it might have been originally expected that the current would lose more heat, since heat has a higher diffusivity. However, there are cases where the double diffusive fingering is not strong enough to overcome the effect of bulk diffusion. In these cases, the current loses bouyancy and has turbulent drag (although it will be smaller because the fingering is less intense), so it is slower than the single diffusive current.
- To be able to predict what factors will govern the spread on the double-diffusive current, given initial conditions
- To be able to predict the front velocity of a double diffusive-gravity current