Transition to turbulence in circular expansion pipe flow

Abstract : The thesis deals with numerical and experimental investigations of flow through circular pipes with smaller inlet and larger outlet diameter, also known as expansion pipes. The hydrodynamic expansion pipe flow is globally stable for high Reynolds number. In order to numerically simulate these types of flows, large computational domains that could accommodate the linearly growing symmetric recirculation region is needed. Moreover, experimental studies of expansion pipe flows indicate that the transition occurs at lower Reynolds number than predicted by the linear stability theory. The reason for early transition is due to the presence of imperfections in the experimental setup, which acts as a finite-amplitude perturbation of the flow. Three-dimensional direct numerical simulations of the Navier-Stokes equations with two different types of perturbations (i) the tilt and (ii) the vortex are investigated. First, the tilt perturbation, which applied at the inlet, creates an asymmetric recirculation region and then breaks to form localised turbulence downstream the expansion section. Second, the vortex perturbation, creates structures that looks like lower order azimuthal mode, resembles an optimally amplified perturbation. It grows due to convective instability mechanism and then breaks to form localised turbulence. Spatial correlation and the proper orthogonal decomposition reveal that this localised turbulence gains it energy from the core flow coming out of the inlet pipe.
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Kamal Selvam. Transition to turbulence in circular expansion pipe flow. Fluid Dynamics [physics.flu-dyn]. Normandie Université, 2017. English. ⟨NNT : 2017NORMLH32⟩. ⟨tel-02301768⟩

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