Direct Numerical Simulations of Normal Vortex Blade Interaction



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The three-dimensional interaction between a vortex and a body-oriented normally is complex due to the interaction of inviscid and viscous mechanisms. The current thesis uses a direct numerical simulation (DNS) approach to simulate the Navier-Stokes equations in order to further understand the dynamics of the three-dimensional vortex-body interaction. The problem is modeled using a thin cylinder that travels towards and impacts a columnar vortex, cutting through it. The thesis focus on the process of boundary layer separation during the initial stages of the interaction, the evolution of secondary vorticity that is shed from the body's boundary layer and its subsequent interaction with the primary vortex, and the resulting external force affecting the body during the interaction.

The interplay between inviscid and viscous mechanisms during the interaction categorizes the interaction into different interaction regimes (the weak and strong vortex regime) governed by the impact parameter--which is the ratio of free-stream velocity to the maximum vortex swirl velocity. Additionally, although the is no explicit regime that takes distinction based on the Reynolds number, the Reynolds number is critical to the time scale associated with the entertainment of boundary layer fluid into the primary vortex during and after the interaction.

Simulations of the vortex-cylinder interaction were conducted by careful a variation of the impact parameter and Reynolds number. It was found that variation of the impact parameter revealed a gradual transition of the degree of inviscid interaction. Results from simulations done with a low impact parameter reveal an interaction dominated by ejection and inviscid interaction of secondary vorticity from the cylinder's boundary layer with the primary vortex. Simulations conducted with a high impact parameter revealed an interaction dominated by the viscous interaction of the cylinder surface and its boundary layer with the primary vortex. Flow topology around the cylinder is significantly different between impact parameter cases leading to distinct hydrodynamic force curves that contain unique peak structures. These peak structures are indicative of boundary layer fluid being pulled away from the cylinder and impingement of external flow against the cylinder's surface. Variations of the Reynolds number in both the high and low impact parameter cases identified that the Reynolds number has direct control of the unsteadiness present in the cylinder's boundary layer leading to the increase of secondary vorticity and subsequent turbulent structures that are developed during the interaction. Additionally, it is found that variations in the Reynolds number result in the formation of additional unique peak structures in the cylinder's hydrodynamic force curve.



Direct Numerical Simulation, DNS, Blade Vortex Interaction, BVI