On the vortex shedding around a circular cylinder modulated by ambient density stratification

Abstract

Alternating vortex shedding could be developed when the flow past a circular cylinder. Previous laboratory studies indicate that the vortex shedding frequency, which is expressed as dimensionless Strouhal number St, depends on the Reynolds number. Numerical simulations are used to analyze the flow characteristics around cylinder in recent years, however, they cannot reproduce laboratory measurements properly and the Strouhal numbers are overestimated. This discrepancy is attributed to the fault of numerical method and more advanced turbulence models are expected to improve the results. In this study it is postulated that the overestimated Strouhal number could be attributed to the ambient density stratification, which is neglected in previous numerical simulations. Numerical methods and laboratory measurements are used to confirm this assumption. The numerical results show that the Strouhal number decreases with increasing ambient density gradient for a wide range of Reynolds numbers. An electric thermometer with two sensors is used to measure the typical density gradients for various scenarios. The experimental measurements show that the strength of measured ambient density stratification is in the range anticipated from numerical simulation, which further confirms the influence of ambient density stratification on the vortex shedding frequency.