An experimental investigation of Rayleigh-Blmard convection in liquid sodium has been performed in cylindrical test cells with aspect ratios between 20 and 4.6 for a range of Rayleigh numbers 1 04< Ra< 5 · 106. The Prandtl number is between 6.0·10-2+0.1·10-3. For low Rayleigh number Ra<104the dimensionless heatflux, Nu, is mainly conduction controlled and close to 1; a correlation Nu= 0.115 Ra0.25 describes the observations adequately for 2·104 < Ra < 5·106. lt is shown that the Nusselt numbers are smaller for sodium than for other fluids with larger Prandtl numbers such as mercury and helium. This contrasts with the relation Nu ~ (Ra 2 /7 • Pr· 1 n) but conforms with the theoretically predicted relation Nu -(RaPr) 1 /4 for low Prandtl number fluids. The heat transfer as weil as the statistical properties of the turbulent convection are significantly influenced by convection mode transitions in the test cell of small aspect ratio. The formation of thermal boundary layers occurs only at high Rayleigh numbers of the order Ra> 106. An instability in these thermal boundary layer triggers a new mode of large scale fluctuating motion. The analysis of the temperature time signals shows that the temperature field behaviour up to a Rayleigh number Ra -5 · 106 is essentially dissipative.Turbulente Rayleigh-Blmard Konvektion in Flüssigkeiten mit niedrigen PrandtiZahlen Zusammenfassung Eine Rayleigh-Blmard Konvektionsströmung in flüssigem Natrium wurde in zylindrischen Behältern mit einem Durchmesser-zu Höhenverhältnis zwischen 20 und 4.6 in einem Bereich der Rayleigh-Zahlen von 1 04< Ra< 5·1 06 untersucht. Die Prandti-Zahl betrug dabei 6.0·10-3±0.1·10-3. Für kleine RayleighZahlen Ra< 104 ist der Wärmefluß über die Schicht im wesentlichen durch Wärmeleitung bestimmt. Die Nusselt-Zahlliegt dann nahe bei dem Wert eins. Im Bereich der Rayleigh-Zahlen von 2·104 106 aus. Eine Instabilität dieser thermischen Grenzschicht scheint eine neue großräumige zeitlich fluktuierende Konvektionsform auszulösen. Die Analyse der zeitabhängigen Temperatursignale zeigt, daß sich die Temperatur im untersuchten Rayleigh-Zahi-Bereich wie ein passiver Skalar verhält.

The Karlsruhe Dynamo experiment is aimed at showing that an array of columnar helical vortices in liquid sodium, confined in a cylindrical container, can generate a magnetic field by self-excitation. In three test series it has been demonstrated that magnetic self-excitation occurs and a permanent magnetic saturation field develops which oscillates about a well-defined mean value for fixed flow rates. Dynamo action is observed as an imperfect bifurcation from a seed magnetic field of the environment. Two quasi-dipolar magnetic fields of opposite direction have been realized. A transition between these two states can be enforced through imposition of a sufficiently strong external magnetic perturbation on the existent dynamo field. These perturbations were induced with the aid of two Helmholtz coils. A time series analysis of the magnetic field fluctuations shows several characteristic dynamic features, which are in agreement with theoretical predictions from turbulence models available in the literature.

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A report of experimental results on dynamo action is presented, obtained at the Karlsruhe dynamo test facility. Former observations concerning the properties of the dynamo magnetic field are corroborated and complemented. The feedback of the dynamo magnetic field on the sodium velocity in the test module is analysed based on measurements employing permanent magnet potential sensors. The evaluation of time signals recorded by these probes shows that a growing dynamo magnetic field transforms hydrodynamic turbulent velocity profiles in channels into magnetohydrodynamic slug flow profiles, indicating a balance between pressure and Lorentz forces. Cross-correlations between time signals of the magnetic induction and the local velocity reveal a significant coherency between apparently random fluctuations of these relevant dynamo quantities.

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