Many studies have been conducted on downstream flows behind two-dimensional cylindrical sections for low Reynolds numbers. The vortex shedding phenomenon occurs in fluid flows over buildings, trailer trucks, bridge piers, heat exchangers, pipelines in the sea, etc. This phenomenon, which is due to the flow instability in the wake region results in a periodic oscillation of drag and lift forces. In experimental studies, visualization techniques such as hot wire and Laser Doppler Velocimetry (LDV) are usually employed. By performing extensive measurements and using the concept of curve fitting, correlations have been obtained for Strouhal number variation with Reynolds number. In addition to experimental works, some analytical studies on complex wake structures of forced and freely oscillating cylinders have been undertaken. Recently, numerical models have been introduced in order to simulate this phenomenon. Khalak and Williamson made several Direct Numerical Simulation (DNS) studies on freely oscillating cylinders for Re up to 350. Low Reynolds flows (up to 800) over square and circular cylinders are simulated based on a numerical method where transient 2D Navier-Stokes equations are solved. In simulations, the fluid was assumed water with properties at 25°C. The model predictions for pressure fluctuations and the variation of Strouhal number (St) with Reynolds (Re) were compared with those obtained from experiments and correlations. In this numerical model we also compared drag force (CD) against Reynolds (Re). Under sharp rising distribution and horizontal asymptotic regime which are two major parts of St-Re variations, the model results agree well with measurements. Both simulations and experiments reveal that the St-Re variations do not depend on the shape of the cylinder. The model results agreed well with measurements.

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