Mechanical Engineering - Theses
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ItemMechanisms of momentum transport associated with the interaction of concentrated and distributed regions of vorticity( 2018)Turbulent flows are of ubiquitous technological importance due to their wide variety of applications. The action of vortical motions plays a vital role in turbulence production, dissipation, and time-averaged turbulence statistics. Therefore, it is essential to understand the flow features responsible for the inertial mechanisms of turbulence and ultimately the mean distribution of momentum. The flow field associated with a vortex ring advecting towards a stationary/moving wall is investigated using planar PIV. This study aims to clarify the mechanisms of turbulent inertia associated with the interaction of advecting regions of concentrated vorticity and distributed vorticity. These physical simulations represent aspects of the instantaneous flow field interactions known to exist in turbulent wall-bounded flows. To allow for an explicit study of these interactions and avoid background turbulence, unsteady, laminar, vortex ring experiments are conducted under reproducible initial conditions. The experiments are conducted in a large water tank. The bottom wall of the tank is fitted with a conveyor belt driven by a servo motor to generate a time evolving shear layer. Laminar vortex rings are produced using a piston cylinder apparatus that is driven by a stepper motor and controlled using a computer. For opposite sign vorticity interactions between the vortex ring and shear layer vorticity, the passage of the vortex ring above the wall results in a lifting of the near wall fluid. This gives rise to the formation of a primary hairpin vortex with the same sign vorticity as the top core of the vortex ring. Results indicate that the generation of new coherent vortex motion introduce geometric and kinematic asymmetries that generates a contribution to turbulent inertia. This action creates local imbalances in the stress field leading to momentum inhomogeneities. To gain an in-depth knowledge of the parameters governing the formation of a primary hairpin vortex, a parametric study is conducted using four factors: the initial wall-normal location of the vortex ring, the circulation ratio between the vortex ring and shear layer, the displacement thickness, and the incidence angle of the vortex ring. This led to a precise and detailed characterisation of the primary hairpin. The development of a unique framework based upon the mean momentum equation to analyse momentum transport and exchange between the ring and the shear-layer is discussed in detail. New observations on the vortex ring/moving wall simulations are presented. In general, the time rate of change of momentum trends for the vortex ring and hairpin indicates a flux of momentum from the ring to hairpin vortex during the initial stages of interaction. Some of the momentum from the vortex ring is transported to the hairpin that is contributing towards wall-layer vorticity roll-up. There is compelling evidence that vortex rings are stable, coherent and long-lived features of the flow, and capable of transporting momentum to near-wall vorticity field without coming too close to the wall. Once instabilities are initiated in the shear-layer, the hairpin development and formation is inevitable. The resulting hairpin convection velocity is within the range of 0.6-0.8 of the wall velocity. The evolution angle of the hairpin is invariant under variations in shear-layer Reynolds number. The results answer fundamental `kernel' questions to establish an understanding of hypothetically significant events for regeneration and self-sustenance mechanisms in wall-turbulence. This information is ultimately required from flow control perspectives.