The prize was conferred to Johan Rønby from DHI’s Ports and Offshore Technology department in Denmark, for best presentation at the 2011 European Nonlinear Dynamics Conference (ENOC).
The ENOC conferences, held approximately every third year, currently constitute the largest scientific event in the broad area of nonlinear dynamics, with over 380 participants from more than 40 different countries. At the conference Johan Rønby participated with a presentation entitled “Chaos in idealized body-vortex interactions” based on his recently finished PhD project.
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Based on the simplified equations of motion, Johan Rønby’s model allowed him to discover several chaotic regimes of motion of a body interacting with a vortex. Using so-called Pointcaré sections, the animation shows how chaotic regimes arise when the body is gradually made more and more elliptic in shape. In the figure, the dots represent the system’s state. With dots aligned on a smooth curve, the system’s behaviour is not chaotic, rather it is regular. If, on the other hand, the dots are apparently randomly distributed across an area, appearing as a ‘fussy’ region in the figure, it is a sign of chaos. |
Understanding the mechanisms leading to chaotic motion when a rigid body freely moves through a fluid
In this project Johan Rønby applied dynamical system theory to study the motion of a rigid body moving freely through a fluid. It is a well-known fact that such motion can either be smooth and regular or irregular and chaotic. It is also well-known that sudden transitions between smooth and chaotic body motion may be triggered by the interaction of the body with the vortices shed in its wake. Most often in engineering science one wants to avoid the uncontrollable and unpredictable chaotic regimes of motion. But there may also be situations where such qualitative behaviour is beneficial, for instance when efficient mixing of the fluid surrounding the body is of importance.
Breaking certain symmetries in the body-vortex system leads to chaos
The detailed mechanisms of the body-vortex interplay leading to sudden transitions are not well-understood. By studying a simplified model of the body-vortex interaction, Johan Rønby and his supervisor, Professor Hassan Aref from Virginia Tech, have shed new light on these mechanisms. They have demonstrated how certain symmetries in the system lead to regular interaction, while a breaking of this symmetry leads to chaos. For instance, a circular body interacting with a single vortex cannot exhibit chaos. But if the body is slightly non-circular, the broken symmetry can lead to chaos.
Possible future application
To the question about possible future application of the knowledge gained in his phd work Johan Rønby replies: “Our main drive has been pure curiosity about the nature of Nature, you could say. As always with fundamental research, its future applications are difficult to predict. But hopefully, in the long run, our increased qualitative understanding will inspire the development of new and improved engineering tools, e.g. for the prediction of fluid forces on ships and offshore structures.”