Spatio-temporal characteristics of turbulent flow around rectangular cylinders subject to varying oncoming flow conditions
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Date
2021
Authors
Kumahor, Sedem Kwadzo
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Abstract
This thesis presents experimental investigations on turbulent flows around wall-mounted rectangular cylinders and rectangular cylinders in a uniform flow. Time-resolved particle image velocimetry was used to perform the velocity measurements. The first set of experiments focused on the effects of upstream wall roughness on the mean and unsteady characteristics of turbulent separations induced by a wall-mounted rectangular cylinder. Examined upstream wall conditions included hydraulically smooth, transitionally rough and fully rough surfaces. The Reynolds number based on the step height and free-stream velocity was 7800 and the mean flow and Reynolds stresses were used to examine the effects of upstream wall roughness on the flow statistics. The results show that upstream wall roughness promotes early flow reattachment over the step. Distinct regions of elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step, regardless of upstream wall condition, but magnitudes of these stresses were comparatively higher for the fully rough wall. Proper orthogonal decomposition and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles.
The second set of experiments examined the effects of streamwise aspect ratio on the dynamics of flow separation induced by rectangular cylinders (a square cylinder and a longer cylinder with aspect ratio of 5) in a uniform flow. The Reynolds number based on cylinder height and oncoming flow velocity was 16200. Similarities and differences in the flow dynamics over the cylinders and in the near wake region were examined in terms of the mean flow, Reynolds stresses and triple velocity correlations. The budget of turbulent kinetic energy as well as temporal and spectral analyses were also performed. Triple velocity correlations showed low-speed ejection and high-speed sweep events transporting fluid away and toward the cylinder surface, respectively, regardless of aspect ratio. Near the leading and trailing edges of both cylinders, regions of negative turbulence production were observed and the dominant terms contributing to this occurrence were discussed. Temporal autocorrelation coefficients of the streamwise and vertical velocity fluctuations showed a periodic trend, with a periodicity that was directly linked to the Kármán shedding frequency and its second harmonic.
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Keywords
Turbulent flow, Flow separation, Uniform flow, Wall roughness, Rectangular cylinders, Particle image velocimetry