What is a gravity current?

Answer 1

A density current is any current set up in a fluid because some of the fluid is denser than some of the other fluid. The denser fluid will be acted on by gravity and be pulled to lower areas in the container holding the fluid. The less dense fluid will be displaced at the bottom or lower areas of the container and be forced up. The current, that density current that is created in a fluid, is driven by gravity, and a density current is also termed a gravity current. Whew! That said, let's look at some things. The "definition" supplied can be applied to broad areas, and rightly so. There are some advanced applications. But let's look at one that is often analyzed and is fairly easy to grasp. In oceanography, water of a different temperature or of a different salt content (salinity) has different densities, and you can probably see where this is going. The colder or saltier water will be more dense, and if a "quantity" of colder or saltier water is "pooled" somewhere adjacent to warmer or less salty water, the more dense water (owing to its temperature or salt content) will, by gravity, be pulled "in and underneath" the warmer or less salty water. This flow is called a density or gravity current. It really is that simple, but the hydrodynamics involved in the movement of ocean currents is only partly understood. We still have a long way to go to get a handle on it. In one more example, let's look at something you've already seen. In a container on a table is a chunk of dry ice (solid carbon dioxide). It is submerged in water and is bubbling away. The air in our room is still, and we see the white carbon dioxide gas flowing over the side of the container, across the table and down to the floor. It will even flow across the floor if left undisturbed. There is a physical current of carbon dioxide flowing here. And it is acting the way it is because the CO2 is more dense than the air in our room. A density or gravity current has been set up. You've seen this many times in films or on TV. And now you can, if you choose, put a name to the phenomenon. You know why it is happening the way it is. Note: We're not really seeing the "white carbon dioxide gas" becaue CO2 is not visible. We're seeing just a bit of water vapor in the air condensing in the presence of the cold CO2 gas, and that is what gives the "white" to the scene. Didn't want you to be confused there. If you're investigating density or gravity currents, you've just cracked the door. Push it open and see what else there is to see. Start by using the links below for more information.