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The usual hydrodynamic flow around bluff bodies (typical examples: cylinder, sphere) is an old and canonical problem of hydrodynamic research on instabilities, flow transitions, flow patters, etc. This problem is characterised by one order-parameter - the Reynolds-number Re. One of the very recent findings is that inherent 3d instabilities occur at Re~180 which are independent of the cylinder ends. In this project the flow around a cylinder is considered for an electrically conducting fluid exposed to an external uniform magnetic field. The electromagnetic forces result in a second order-parameter for the flow - the Hartmann-number Ha (which is proportional to the magnetic field strength). Compared to the usual hydrodynamic problem the flow can actively be controlled now by varying the external magnetic field. Such type of magnetic flow control is important for basic fluid dynamic research as well as applications in several Materials Processing technologies.
Theoretical/numerical work has been carried out by the groups of the co-ordinator in Dresden (Germany) and the group of ITAM Novosibirsk (Russia). Experiments on the flow around a cylinder in an aligned magnetic field have been carried out at the liquid metal loop of MAI Moscow (Russia). This loop works with the eutectic melt InGaSn, and uses a big solenoid for the longitudinal magnetic field. A special probe was developed in order to measure the temperature fluctuations in various locations downstream of the heated cylinder. In that way stability results have been obtained. In addition, experiments have been carried out at LEGI-IMG Grenoble (France). The available mercury facility with a big solenoid around it has been used for electric potential measurements downstream of a moving cylinder.
Interesting experimental results are obtained which qualitatively show the predicted behaviours. However, it turned out that the sensitivity of the instabilities and the problems connected with measurements in liquid metals (wetting, signal reproducibility, experimental noise, etc.) did not allow an exact quantitative verification of the theoretically predicted instability thresholds. The main scientific results of the project were achieved by the theoretical/numerical investigations. It has been shown that by means of the magnetic field action 3D-instabilities occur at lower Reynolds numbers than 2D-instabilities. Moreover, 3D-instabilities are found in the MHD case at Reynolds numbers where the usual hydrodynamic flow is still 3D stable. That is partly in contradiction to the textbook opinion that steady magnetic fields always show a damping influence on such flows. This represents a qualitatively new result of scientific significance for the MHD community.
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