Turbulent flow interaction with a circular cylinder

A comprehensive experimental study is carried out to investigate the unsteady pressure exerted on the surface of a round cylinder in the sub-critical Reynolds number range. Experiments are performed using a highly instrumented cylinder, with several static pressure taps and dynamic pressure transducers at dierent spanwise and peripheral locations, enabling extensive dynamic surface pressure, coherence and turbulence length-scale analysis. The effects of the free-stream turbulence are investigated by placing the smooth and turbulent-generating grids within the contraction region of the wind tunnel. For both incident flows, the surface pressure results show the emergence of the fundamental, first and second harmonics at most peripheral angles, while at the cylinder base the surface pressure spectra is dominated by the first harmonic. The amplitudes of the fundamental and second harmonic tones are observed to peak within the turbulent boundary layer region and then decrease toward the base, while that of the first harmonic is observed increases with the angle and peaks at the cylinder base. The spanwise coherence results have also shown that the vortex shedding structures have a long spanwise length, while the three-dimensional flow structures within the boundary layer have a much shorter correlation length. Furthermore, the spanwise coherence at the cylinder base is found to be purely tonal. The use of the turbulence grids is shown to lead to slight shift in the vortex shedding frequencies to lower frequencies. It has also been observed that the ow structures in turbulent incident flow have higher energy level compared to the smooth incident flow.