Vortex shedding From Single and Tandem Finned Cylinders

Abstract

<p>The effect of fins on vortex shedding and acoustic resonance in the case of single and tandem cylinders with spacing ratios of 1.5, 2 and 3 has been investigated in an open circuit wind tunnel at the Reynolds number range from 1.56 x 10⁴ to 1.13 X 10⁵. Sound pressure and velocity measurements were performed for finned cylinders with three different fin densities. Similar measurements were made on bare cylinders for the purpose of comparison. In the case of tandem bare cylinders, the first acoustic mode is excited over two different ranges of flow velocity. The first resonance range ends before the vortex shedding frequency approaches the acoustic resonance frequency and is therefore referred to as the pre-coincidence resonance. The other resonance range starts at the coincidence between the frequencies of vortex shedding and acoustic resonance and is referred to as the post-coincidence resonance. The fins in the case of single cylinders are found to reduce the strength of vortex shedding, increase the broadband turbulence level and decrease the sound pressure at the acoustic resonance. The lock-in range of acoustic resonance for the single cylinders is found to be generally smaller than that of the tandem cylinders. Before the onset of resonance, the fins cause the sound pressure to increase in the case of tandem finned cylinders with <em>S/D_e</em> = 1.5 and 2. Increasing the fin density promotes the onset of resonance, but reduces the sound pressure level at resonance for <em>S/D_e</em> = 1.5 and 2. The fins are observed to weaken the pre-coincidence resonance for <em>S/D_e</em> = 2 such that the sound pressure level is not sufficient to produce resonance. However, the fins increase the sound pressure at the pre-coincidence resonance range for <em>S/D_e</em> =3, but decrease the sound pressure at the post-coincidence resonance range. Increasing the spacing ratio between the cylinders is found to generally reduce the sound pressure level at the post- coincidence resonance, but decreases the sound pressure level at the pre-coincidence resonance range. The effect of fins is therefore rather complex and depends on the spacing ratio and the fin density.</p>