Instability of viscous axial flow in annuli having a rotating inner cylinder

Hot-wire measurements are presented of the onset of instability in developed axial and tangential flow due to inner-cylinder rotation in annuli of radius ratios 0·9, 0·81 and 0·58 for axial-flow Reynolds numbers between 86 and 2000. Measurements of the minimum critical Taylor number are reported along three radii at azimuthal locations 90° apart. At the largest radius ratio these azimuthal measurements show considerable variation but the sensitivity of marginal stability to angular orientation becomes negligible at a radius ratio of 0·58, when measurements and the Galerkin predictions of Hasoon & Martin (1977) are in close accord. The greater sensitivity to orientation as the radius ratio increases appears to correlate with measured percentage circumferential variations in annulus width arising from manufacturing tolerances and non-uniform curvature of the surfaces.