Flowfield characterization of a gust generator
Fernandez, Viju John
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Windstorms such as hurricanes and tomadoes cause fetalities, injuries and have a significant societal impact. Insurance companies estimate that windstorm-related events account for up to seventy percent of their payout each year. An accurate understanding of the wind gusts that form these storms is therefore essential for damage mitigation. An active wind gust simulator was constructed which used NACA 0012 and derived airfoils to generate coherent structures in a turbulent wake, which simulated wind gusts. A constant Re = 1.9x10^^ (based on airfoil chord length) parametric study of the effects of different Strouhal numbers (4.14, 6.21 and 8.56) based on airfoil chord length, airfoils (NACA 0012, NACA 0006, NACA 0024, NACA 6412, NACA 6812) and angle of attack (10° and 20°) on three different diameter cylinders (0.2c, 0.4c, 0.6c, where c is the chord length) was conducted to study the growth, development and dissipation of coherent structures produced by the airfoil wakes and their interaction wdth those bluffbodies. Turbulent transport mechanisms and entrainment in the wakes associated with the production of a mixing layer forther downstream was also investigated. A CCD camera was used to visualize the flowfield generated. Processed flowfield images of the different conditions show a strong influence of the Strouhal nimiber, airfoil spacing and type on the flowfield produced and the nature of the wake interactions. Increasing the Strouhal number resuks in an energetic wake that does not dissipate quickly downstream in the flowfield. Decreasing the airfoil separation distance breaks down the large-scale structure of the wakes by adding additional momentum to the flow and enhancing the mixing and thereby the dissipation. The large deflection angle change does not have a significant effect of the type of structures shed from the airfoil trailing edges; it however increased the initial amplitude of the structures. The camber of the airfoils creates a fine scale turbulent flowfield by directing the flow in the direction of the airfoil tip and dissipating upon being deflected once again by the tunnel test-section wall. Any large-scale structures are quickly dissipaíed within half a lengthscale downstream of the airfoils. The flat plate wake creates structures with little entrainment and dissipation. The large-scale sinusoidal structures in the flat-plate wake maintain their size with little growth and dissipation until 3 length scales downstream. The structures caused by the NACA 0024 airfoil have a k)wer fi-equency and higher amplitude that the NACA 0012. They however behave in the same manner downstream.