Secondary flow and three-dimensional separation in curved circular ducts

Roger Edward Davis, Purdue University

Abstract

Flows in singly-curved bends and S-ducts of circular cross-section were investigated by means of flow visualization and detailed wall static pressure measurements. The emphasis was on developing flows with very short inlet tangents, although several cases were tested with fully developed pipe flow in the inlet tangent. Bends with R/a (ratio of mean radius of curvature to cross-sectional diameter) equal to 5 and 10 were studied, at Reynolds number 6,000 to 15,000 for the visualization tests and 90,000 to 150,000 for the pressure tests. The topology of the outer wall separation at entry to strongly curved pipe flow (R/a = 5) was documented by both visualization and pressure measurements. The occurrence of this separation is shown to be primarily a function of R/a. The secondary flow vortices were shown to be formed by two different mechanisms of separation on the inner wall of the bend. An "open separation" occurs at the weaker curvature (R/a = 10), but a complicated global separation occurs in the more strongly curved bend (R/a = 5). The latter case agrees qualitatively with the "two-layered" structure with singularity proposed by Stewartson & Simpson (1982). The existence of two different topologies for outer wall flow and of two different mechanisms for the formation of the secondary flow vortices are both examples of bifurcation in curved pipe flow. The parameter space is described for these bifurcations over the range of parameters studied. The pressure measurements show that the wall static pressure field is basically that of a free vortex centered at the center of curvature of the bend. The development of the secondary flow vortices is manifested as a perturbation imposed on the free vortex field. The effect of outer wall separation is shown to be very localized, with a rapid return to the free vortex field. A number of S-duct configurations were also tested. The effect of the reversal of curvature on flow in the first bend was minimal except near the plane of inflection for most configurations. The reversal of the secondary flow in the second bend was demonstrated, as was the reestablishment of a free vortex pressure field corresponding to the reversed curvature in this bend. The additional pressure loss due to the reversal of curvature was shown to be on the order of 1-2% of the dynamic pressure at the duct exit, and appeared to occur primarily near the plane of inflection.

Degree

Ph.D.

Advisors

Sullivan, Purdue University.

Subject Area

Aerospace materials|Fluid dynamics|Gases

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