Ph.D. (Engg):Effect of rotation on the flow organisation and instability of a slender spinning body across quiescent, axial and compressible regimes

A body of revolution spinning in a fluid drives an inherently three-dimensional boundary layer, in which a centrifugally driven secondary circulation lifts the layer off the surface and a centrifugal instability carries it into transition. Most of what is known about this flow comes from single geometries (the rotating disk, the isolated cone or cylinder) in an unbounded fluid, and from diagnostics that resolve neither the secondary meridional circulation nor the three-component velocity of the vortices. This work instead resolves the rotation-driven layer with whole-field measurements on a composite, finite cone–cylinder (half-cone angle 15°), and follows it across the settings in which such a layer arises in practice, of relevance to spin-stabilised flight and rotating machinery: a still fluid with and without confining walls, the transient establishment from rest, an imposed axial stream, and, as a bounding case, a compressible free stream.
Combining planar (two-component) and time-resolved stereoscopic (three-component) particle image velocimetry in the meridional plane with a phase-locked acquisition and a proper-orthogonal decomposition of the fluctuating field, the study shows how confinement changes the transition from a sharp two-branch switch to a smooth downstream migration coupled to a growing separation bubble; how the layer establishes advectively from rest in a single turnover, some fifty times faster than viscous diffusion; and how the base rotation ratio Sᵇ = ωR/U∞ organises transition under axial inflow, with onset at Sᵇ of order unity. As a bounding case, a slender cone spun in a Mach 2 stream leaves the shock-dominated mean flow and the surface topology essentially unchanged, adding only broadband unsteadiness with no tonal lock-in, so that the rotation-driven layer, though real, is masked at supersonic speed. Taken together, the studies trace a single rotation-driven, centrifugally unstable boundary layer, with its meridional lift-up and counter-rotating vortices, reshaped in turn by confinement, start-up, axial inflow and compressibility.

Speaker : Rajkamal Sah

Research Supervisor : Prof. Gopalan Jagadeesh