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Abstract: The secondary flow in a curved duct of square cross-section is investigated experimentally using a visualization method. Three walls of the duct (except the outer wall) rotate around the center of curvature and an azimuthal pressure gradient is imposed. Photographs of the flow in a cross-section at 180° downstream from the curved duct entrance are taken by changing the flux (Dean number) at a constant rotational speed (Taylor number) of the duct walls. Several types of secondary flow are detected. The variation of the flow patterns with change of flow parameters is compared with that of numerical calculations and is found to be in good agreement. The diagram showing types of secondary flow patterns in the Taylor–Dean number plane is obtained. [Copyright &y& Elsevier]

The Taylor–Dean flow through a curved duct of square cross section, in which walls of the duct except the outer wall rotate around the center of curvature and an azimuthal pressure gradient is imposed, is investigated numerically using the spectral method. The calculation covers a wide range of pressure gradient and duct rotational speed. The steady flow patterns of the induced secondary flow and flux through the duct are obtained. It is found that the secondary flow consists of two vortices in a cross section when the rotation of the duct is large and opposite to the negative pressure gradient. When the rotation is in the same direction as the negative pressure gradient, the secondary flow shows more complicated behavior. There appear four-vortex, eight-vortex or even non-symmetric secondary flow patterns with respect to the center line of the cross section. An analysis of linear stability is also carried out. [Copyright &y& Elsevier]

The secondary flow structure in a helical pipe with large torsion is investigated by using a numerical calculation of a fluid particle trajectory and an experiment using a smoke visualization technique. Good agreement is obtained between the experiment and the numerical calculation. The secondary flow in a cross-section is transformed from a two counter-rotating vortices structure to a one-recirculation structure with increase of the torsion of the pipe at a constant Dean number. The line dividing two vortices varies its direction from horizontal to vertical as the torsion increases. [Copyright &y& Elsevier]