Shapes of pipes

Sammanfattning: Curved pipes are ubiquitous in several industrial applications and biological systems. The study of flows through these configurations is therefore of significant practical interest.In the present thesis, the modal stability of flows in both toroidal and spatially developing bent pipes is investigated to understand the onset of transition. In addition, turbulence characteristics and large-scale structures in flows through toroidal and helically coiled pipes are analysed.The modal stability of steady flows through toroidal pipes is studied for values of the curvature approaching zero, i.e. the straight pipe, to provide further evidence of the fact that the Hagen–Poiseuille velocity profile is stable to infinitesimal perturbations for any Reynolds number.   A novel computational technique is developed for computing the laminar pulsatile flow in a toroidal pipe. A characterisation of this flow for different values of the pulsation frequency and the pulsation amplitude is presented. Floquet theory is then applied to investigate how the stability is affected by the governing parameters, in particular the amplitude and the frequency of the pulsation.The flows through spatially developing bent pipes are also studied for understanding how the transition changes with respect to the toroidal geometry. Both a 90°- and a 180°-bend pipe with the same curvature are investigated. A transition from steady to periodic regime is observed in both cases. The stability analysis reveals that the core of the instability is located at the outer wall of the bend, approximately 15° downstream of the bend inlet, for both pipes. Similar critical Reynolds numbers are also found.Finally, the fully developed turbulent flow in toroidal and helical pipes is considered for studying the effect of torsion on the turbulence characteristics. Both curvature and torsion are found to significantly modify the anisotropy of the flow compared to the case of a straight pipe. Large-scale structures are extracted via proper orthogonal decomposition to assess the occurrence of the swirl switching.