Experimental study of turbulent flow around trapezoidal cylinders in uniform flow

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Kang, Jinhao
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This thesis presents an experimental investigation on the effects of streamwise aspect ratio (AR) on asymmetric wake flows around right-angled trapezoidal cylinders with AR (upper cylinder length to height ratio) = 1, 2, 3, 4, and 5 using a particle image velocimetry (PIV) technique. The Reynolds number based on the free-stream velocity and cylinder height was 14700. The flow characteristics are examined in terms of the mean flow, Reynolds stresses, turbulent kinetic energy (TKE), turbulent transport and production term of TKE, probability density function (PDF), joint probability density function (JPDF) and two-point autocorrelations. The results show that the primary vortex in the AR1 and AR2 cases extends into the wake region but is confined to the surface of AR ≥ 3 and two asymmetric wake vortexes are only observed in these longer cases. Dual peaks of elevated streamwise Reynolds normal stress are observed in the wake region, irrespective of the aspect ratio. The magnitudes of the Reynolds stresses, TKE, and production term of TKE in the wake region are higher in the AR1, AR2 and AR3 cases compared to the AR4 and AR5 cases. The turbulent transport of TKE by streamwise and vertical fluctuating velocities show switches sign along the mean separating streamline, regardless of aspect ratios. The PDF distributions show a bimodal asymmetric shape in the shorter cases but a nearly Gaussian distribution in the AR5 case. Two-point autocorrelations of the streamwise and vertical velocity fluctuations show that the spatial coherency of the turbulent structures is highly sensitive to the streamwise aspect ratio and reference locations. Systematic comparisons between the present asymmetric results and symmetric wakes generated by rectangular cylinders with similar aspect ratios and Reynolds number show significant differences between the asymmetric and symmetric wakes, especially for the shorter cases.
Turbulence Flow, Particle Image Velocimetry, Cylinders, Vortices, Flow Dynamics, Wake turbulence