discovered_01_2013

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 26 27 variations in the orbit and the tilt of Earth’s axis. However, next to buoyancy forces, precession is a potential source of energy for the geodynamo. Interestingly, you will find several typical frequencies of the Milankovitch cycle in paleomagnetic data. And this is where it gets pretty exciting in terms of climate: Because it’s not at all clear whether it was the chicken or the egg that came first. On the one hand, growing and melting ice sheets change the moment of inertia and are thus potentially able to influence the geodynamo. Or it could be that Earth’s changing magnetic field affects the climate. I didn’t want to go so far as to say that we’re planning on conducting climate research in the context of DRESDYN, even though the relationships are fascinating no doubt. _PK . To relate this complex topic to the precession dynamo: During operation, the sodium container (which weighs tons) will be spinning like the barrel of a washing machine. The goal is to get up to ten revolutions per second. The desired flow, however, depends upon the entire contraption including the turntable rotating about its perpendicular axis - up to once per second. Therefore, if we allow our dynamo container to rotate about two separate rotational axes at different angles of inclination, at some precession rate, the liquid sodium current will become turbulent and chaotic. I think we’re setting up an internationally unique facility here. At the very least, we’re looking at the mechanically most complex experiment I know. Meaning in a way, both of you are looking for chaos and turbulence. I’m guessing there are special challenges inherent in DRESDYN’s planning and construction. _PK . Definitely, just think of the mechanical impact alone. The sodium container will react to the superimposed rotations with a gigantic gyroscopic movement on the order of some five million newton meters that are conveyed to the ground through the precession experiment’s support frame. There’s really nothing you could compare it with. Typically, you have to imagine a machine sitting on a firm foundation, with only a downward pressure being exerted. However, in this case, the precession moment distorts the entire machine in such a way as if a 250-ton weight were pulling up on the cylinder, the complicated suspension, and the turntable on one side while pushing down on the other. And this all orbits about once per second. Anyone who has ever tried to use their hand to tilt their bike in the air, knows how unruly a gyroscope can be. Although, arguably, a bike is nothing like liquid sodium. In other words, we’re expecting enormous tensions resulting from the high torque within the machine. Add to that the arguably high operating temperatures. Temperature invariably means expansion: Both the sodium and the container will expand and it’ll be up to us to compensate for this expansion using suitable technological counter-measures. Similarly, we have to address the question of cooling, a particularly delicate issue given sodium’s chemical reactivity. Water and sodium just don’t get along at all. _FS . Constructing the foundation was also a real feat. Even though we haven’t yet finished construction of the dynamo, we did have to definitively determine the foundation. Of course, when we did this, we tried to be as conservative as possible but dealing with all of this uncertainty isn’t always easy for the companies involved. _PK . And just to add to that: Experimental operation requires that we thread measuring instruments and all of the necessary media like electric current and protective gas into the liquid sodium container, right into this rapidly revolving system. In other words, we have to be minimally invasive in terms of the feed-through to ensure that we don’t compromise the whole construction’s load-bearing capacity. DRESDYN ON THE COMPUTER: 3D model of the rotation container and the sodium storage tank.

discovered_01_2013

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