Unsteady laminar flow developing in a curved duct

Measurements of the time-dependent velocity in the flow developing in a U-bend of square cross section were obtained using a laser Doppler velocimeter. Autocorrelations, energy spectra, and probability distribution functions were, in turn, derived from the time records. Numerical calculations of the flow were also performed using a 3D transient fully elliptic finite-difference scheme. The oscillations observed experimentally and numerically are linked to the formation of Goertler-type vortices on the outer-radius wall in the developing flow. At higher flow rates, the vortices, produced by the centrifugal instability of the flow on the outer-radius wall, undergo a further transition to an unsteady regime.