The shear stress, occurring at the outer surface of a vertical annular gap formed by a stationary outer cylinder and a rotatable inner cylinder, was measured for a wide range of conditions, using a flush mounted probe. For annular gaps of radius ratio 0.8 and 0.9, axial flows of Reynolds numbers 100, 165, 200, and 300 were imposed under adiabatic and diabatic upflow and also diabatic downflow conditions. Under these conditions, the shear stress was determined over a range of Taylor numbers approaching 107, the flow being fully developed. Diabatic conditions were achieved by the isothermal heating of the outer surface of the gap. A primary regime, in which Taylor vortices are absent or exist away from the outer surface of the gap was identified. Secondary, tertiary and quaternary regimes, in which the vortex flow is in contact with that surface ensued. The contrast between the results for diabatic upflow and diabatic downflow gives an understanding of the effect of natural convection in these regimes.

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