Let the Floodgates Open

If you’ve been following so far, we know why it’s useful to know where river water goes when it enters the ocean, why we can build a model of the situation in the lab, what the most important variables in the problem are and what the experimental setup looks like. I suppose you’re probably itching to know what actually happens when we open the floodgates and release the freshwater from the model river into the spinning saltwater tank that is our ocean. In which case, let me point you in the direction of the video below.

It probably doesn’t make much sense without some added explanation so here goes… This is a false colour image of an experiment viewed from above. The freshwater from the river is dyed red with food colouring which means that we can convert the colour intensity into a depth measurement. The more intense the red food colouring, i.e. the more of it there is, the deeper the current must be. The scale starts with black to represent no current (as is the case for the saltwater ocean), then increases with the current depth through red, yellow, green and finally blue for the deepest parts of the current.

If you look at the very beginning of the video you will see that as the river water is released from the source it travels forwards and then is immediately forced to turn to the right. This is due to the Coriolis force arising from the rotating tank (see article 3). As it turns back on itself it eventually collides with the tank wall where it then propagates as an anticlockwise boundary current. The anticlockwise direction is set by the direction of the rotation of the tank (also anticlockwise). The boundary current continues to travel around the edge of the tank, eventually filling the whole perimeter and returning back to the source by the end of the experiment.

As well as the propagating current, a second persistent feature can also be seen in the video: the outflow vortex. This is the large whirlpool-like feature that forms next to the source of freshwater. As the initial current turns to the right and back on itself to collide with the tank wall, the flow divides. One part moves anticlockwise and forms the boundary current that moves around the tank edge, whilst the other part continues in a clockwise direction and re-joins the initial jet of freshwater from the source. The result of this is to form a whirlpool next to the source which grows in size as the experiment progresses. Both features are labelled in the image below so that you can recognise them in the video.

Screen Shot 2017-11-15 at 10.45.52.png

Now that we’ve identified the two main features of the flow – the boundary current and the outflow vortex – the next step is to try to understand them in more detail. This is exactly what the first two sections of my thesis are about, beginning with the boundary current. For example, we might want to know how fast the current moves around the tank, how its depth changes as it does so and whether or not it has a constant width. For the outflow vortex, we are interested in similar properties, such as how deep the vortex is at its centre, what shape it forms at the surface and how it grows in size during an experiment. By looking at the experimental video you can begin to get a grasp on some of these questions, but to really understand them in detail you need two key ingredients: measurements and a mathematical theory.

In my thesis, I begin by discussing the mathematical models used to describe the flow and then compare this theory with the data collected from the experiments, with the hope that they will agree. The theory can only be correct if the measurements support it – which is pretty much my thesis in a nutshell. Do the predictions from the mathematical equations agree with the observations in the experiments? If we’re going to compare the two, we’d better start by forming some equations, which brings me nicely onto the next topic…

 

You can read the rest of the articles explaining my PhD thesis here.

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