Drills ‘n’ flows

So what have I been up to since my last post?

Quite a lot!

I have learned how to drill holes into thick plastic, and I’ve been writing a paper for the Research Horizons assignment. I have to write a 2000 word critique essay on a topic of my choice, I chose supercritical bedforms. The title for my paper is ‘A super critical paper’ ha… ha ha… Anyway.

Drilling

You might have laughed at the drilling part, but it’s not as simple as it sounds! To start with you need to drill a ‘pilot hole’ into the plastic, from one side to the other. If all goes well you’ll succeed in this and make it to the other side without a hitch, but if not your drill bit will get stuck and could snap in the hole which you drilled! As you can imagine, once the bit snaps it can be difficult to get back out.

My technique clearly needs some work, but it’s good to be aware of the process right? Once your pilot hole is drilled you can then drill the hole to your desired size. The reason for the pilot hole is so that the other side of the plastic doesn’t experience a ‘blow out’ at the other side where the drill bit literally blows a hole out of the other side, causing an untidy exit wound in the plastic. It also serves as a guide for when you drill the real burrow into the plastic so the drill doesn’t move around. Not exactly the most exciting thing to learn, but it didn’t strike me as obvious at the time. Many thanks to Kelvin for teaching me this.

Antidunes vs. Cyclic Steps

Now, the difference between antidunes and cyclic steps. This is something that is obvious if you observe the characteristics of each of these bedforms (bedforms are things like ripples and dunes like you would find on a beach or in the desert).

On the left we have cyclic steps and on the right we have antidunes. Now, you might be thinking “of course they’re different, they even look different!” and you’re right! Reading about these phenomena and seeing them are very different ways of learning, and the visualisation allows you to really see the difference between them. This shows that I still have much to learn throughout my Ph.D!

Antidunes are a bit like dunes – they are little (or large) bumps on the surface of sand and are formed by the erosion and deposition of sand. The difference between dunes and antidunes is that except instead of moving with the flow like dunes, antidunes move against the flow, upstream. Defiant things aren’t they?

Antidunes are formed when the pressure (or inertial) forces acting on the sandy water are greater than that of gravity (a downward force with a maximum (terminal) velocity of 9.81 meters per second) which move the sand down the slope. This is also referred to as supercritical flow, whereby the Froude number of the flow is greater than 1.25. This type of flow is thin and fast, and can have a standing wave at the surface.

If you aren’t sure if you’ve seen antidunes or supercritical flow before, if you have been to a beach before you probably have! The little ripples you see on the water when the tide has gone out and the water is draining across the surface of the sand – they are antidunes! These can appear in all sizes depending on the size of the flow.

The cyclic steps I showed above are the ones observed on Mars (crazy right?) using NASAs Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter. More details can be found here. Cyclic steps are also found here on earth where material flow undergoes a series of changes in its flow regime (whether the flow is supercritical or subcritical). They are a pretty newly discovered type of bedform, at least in an under water sense.

Under water cyclic steps  have been talked about a lot recently in the scientific community because there are some identification issues associated with them because they are under water.

Cyclic steps, are similar to antidunes because they move against the flow of water, upstream. Unlike antidunes, they are both bigger and asymmetrical. They are called ‘cyclic steps’ because they cycle between supercritical and subcritical flow, known as hydraulic jumps.

I think that’ll do for now, there’s a lot of technical information here. If you would like any further explanation feel free to contact me.