Rotating stall in a vaneless diffuser of a centrifugal compressor has been found to be guided by Rossby waves, which are composed of branches of high and low pressures (Chen, Haupt and Rautenberg, 1990a). The branch of the high pressure leads the unstalled region and that of the low pressure leads the stalled region. The phase velocity of the Rossby waves is then the pattern speed of the stall cell.

We report here an additional experimental result, according to which the flow of rotating stall is composed of a longitudinal spiral vortex pair. The vorticity and the axis of the longitudinal vortex were measured by means of two pressure transducers fixed at a distance of 2 mm to the opposite walls of the diffuser downstream of its inlet.

The analysis of the experimental result of Tsurusaki, Imaichi and Miyake (1987) about the fields of the total and fluctuating velocities of rotating stall in the vaneless diffuser reveals furthermore that the longitudinal spiral vortex is centred on the through flow. The two vortices of the pair stay side by side in touch but without mixing because of their opposite rotational sense. The longitudinal vortices make about 1 1/4 turns from the inlet to the outlet of the diffuser under the guidance of the Rossby waves. The vorticity of the longitudinal vortex is determined from the experimental result.

Furthermore, the experimental result reveals that the fronts along the high pressure ridges and the low pressure troughs of the Rossby wave pattern are themselves longitudinal vortices. Then these fronts possess a behaviour of the jet stream, which is associated with the Rossby waves of the atmosphere in the midlatitude.

Finally, the origin of the vorticity of the longitudinal vortices along the through flow and the Rossby-wave front is derived based on the experimental results obtained by Hergt and Jaberg (1988), and Hide (1958).

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