Table of Contents

• Abstract
  
  
• Introduction
  
  
• Convection in Rotating Frames
  
  
• The Experiment
  
  
• Results
  
  
• Conclusions
  
  
• Acknowledgements
  
  
• References
  
  

This experiment was performed as a fluid flow visualization of the flow field preliminary to a liquid sodium dynamo experiment. This dynamo is presumed to be the analogue of an astrophysical α - Ω dynamo. In an α - Ω dynamo a differentially rotating conducting fluid wraps up an orthogonal field line and after many turns produces a greatly amplified toroidal field, the Ω-deformation. In astrophysical circumstances there is negligible doubt concerning the occurrence of such differentially rotating flows because of gravity and angular momentum, e.g. a Keplerian accretion disk. However, the process of conversion of a fraction of the toroidal field back into the original poloidal field, i.e. the original orthogonal field line, is far more problematic. This process is called the α-effect and requires a net helicity. This paper is concerned with the laboratory demonstration of a previously unrecognized, naturally occurring flow which produces a large scale, coherent α-effect. The large scale is important because of the magnitude of the flux affected. Coherence is important because it implies a positive α-effect for each occurrence of the flow so that averaging does not result in a null effect. This flow is the partial rotation of an off-axis, axially-aligned, expanding pulsed jet relative to its rotating frame. Although such a differential rotation may be expected on the most elementary of arguments, namely the conservation of angular momentum, a laboratory demonstration is necessary for confidence in modeling of the dynamo as well as the endeavor of attempting a dynamo experiment itself.

The structure of this paper is as follows. Section II discusses previous experiments of convection in rotating frames, and the dynamo flow field is discussed in relation to the experiment. Section III describes the experimental apparatus and flow visualization methods employed. Section IV presents the results of the experiment. The conclusions reached from the experimental results follows in Sec. V.