An experimental study of two flows through an axisymmetric sudden expansion

Abstract

Two turbulent separated and reattaching flows produced by a sudden expansion in a pipe have been studied. The first was produced by a simple axisymmetric sudden enlargement from a nozzle of diameter 80 mm to a pipe of diameter 150 mm. The second was the flow at the same enlargement with the addition of a centerbody 90 mm downstream of the nozzle exit. Detailed measurements of velocity and skin friction (made primarily using pulsed wires) and of wall static pressure are presented. Without the centerbody the flow structure is similar to that observed in other sudden pipe expansions and over backward-facing steps. A turbulent free shear layer, bearing some similarity to that of a round jet, grows from separation and then reattaches to the pipe wall downstream. Reattachment is a comparatively gradual process, the shear layer approaching the wall at a glancing angle. The introduction of the centerbody causes the shear layer to curve towards the wall and reattach at a much steeper angle. Reattachment is much more rapid; gradients of skin friction and pressure along the wall are many times those without the centerbody. The high curvature of the shear layer strongly influences its turbulent structure, locally suppressing turbulence levels and reducing its growth rate.

Turbulent flow of viscoelastic liquids through an axisymmetric sudden expansion

“…Escudier and Smith [12], using the same experimental set-up as in the present study but with an expansion of smaller area expansion ratio (2.37) observed a very similar value for the vorticity thickness growth. Both reattachment lengths are entirely consistent with values reported previously: Devenport and Sutton [4] list values in the range 8-11 step heights.…”

“…The reattachment length for the latter is 8-11 step heights compared to 5-8 step heights. Devenport and Sutton [4] attribute this difference to the fact that, relative to the surface area available for entrainment, the separated shear layer has to entrain a greater volume of recirculating fluid before reattaching in the axisymmetric geometry compared with the two-dimensional case. Variations in the reattachment length between studies are, as is the case for backward-facing step flow, primarily a consequence of different inlet conditions.…”

The effect of expansion ratio for creeping expansion flows of UCM fluids

“…Most of these Newtonian turbulent works are concerned with the corresponding axisymmetric geometry [2,3] given their easier construction and inherent symmetry. Limited results for viscoelastic fluids in turbulent flow have also been reported in both planar [4] and axisymmetric geometries [5][6][7].…”

Rapid multivortex mixing in an alternately formed contraction-expansion array microchannel

“…Even though the interface from two different types of fluids is perturbed by the Dean flows at the first contraction region, the perturbed interface may almost recover toward their initial interface by the Dean flows at the second contraction region that have an opposite flow direction to those of the first one. An incoming flow separated from the exit corner of the contraction region and reattached to the wall downstream, generating a recirculation flow adjacent to the wall (Devenport and Sutton 1993). However, this recovery is incomplete especially at high Re, because the strength distribution of Dean flows is not even in the whole of cross-sectional plane (Yamaguchi et al 2004a, b).…”