FLOW VISUALIZATION AND LDV MEASUREMENTS OF LAMINAR-FLOW IN A HELICAL SQUARE DUCT WITH FINITE PITCH

Flow visualization and LDV measurements are performed on laminar flow in a helical square duct with finite pitch. The experimental observati ons are compared to results of numerical calculations employing the fi nite-volume method and assuming a fully developed flow. Good agree men t is found between measured and computed velocity profiles. This sugge sts that the physical velocity components used in the numerical calcul ations are suitable to describe the flow. It is further demonstrated t hat the contravariant velocity components employed by some authors may lead to results that are difficult to interpret. Two stable solution branches are detected in the numerical calculations. For Reynolds numb ers between the stable branches, unsteady and fully developed computat ions predict an oscillating flow between a two-vortex and a four-vorte x structure. In the experiments, the flow normally retained a stable t wo-vortex structure in the numerically predicted unstable regime. Howe ver, by disturbing the flow at the duct inlet, a four-vortex flow that showed similarities to the computed flow could occasionally be obtain ed. For Reynolds numbers above 600, unsteady flow behavior was observe d both experimentally and numerically, which might be an early sign of transition. In the experiments, Gortler-like extra vortices emerged s pontaneously from the outer wall without disturbing the flow at the in let. The same phenomenon was observed in the numerical calculations, a ssuming an unsteady and fully developed flow, but the extra vortices a ppeared with a lower frequency than in the experiments.