Politecnico di Milano

Researchers in the Mechanical Engineering Department of Politecnico di Milano have used the LES turbulence model in FLUENT 5 to study vortex shedding behind a cylinder for a Reynolds number of 2x104. In addition to the study of a smooth cylinder, they also used a modified cylinder on which a pair of wire helices was wrapped. Their goal was to see if the wires on the cylinder would act to reduce the amplitude of shedding-induced vibrations.

2D and 3D simulations of smooth and wired cylinders were performed using the LES model to test this hypothesis. To compare the numerical results with experimental measurements made in a water channel, a finite-length section of that channel was simulated. A small disturbance was added to every component of the velocity boundary condition at the inlet in order to reproduce the turbulence intensity for both the smooth and the wired cylinders.

Transient calculations were performed that tracked the shedding patterns behind the cylinders out to a distance of 10 cylinder diameters. Each simulation was run for over 20 seconds. One of the first conclusions drawn was that 2D simulations of the smooth cylinder did not agree well with experiment, owing to the observed variation in shedding patterns in the transverse direction.

The 3D LES simulation, on the other hand, picked up the transverse variation observed in the experimental channel, and predicted full 3D shedding patterns with frequencies close to measured values. For the wired cylinder, only a 3D analysis was performed. Results showed that the development of an organized shedding pattern did not occur, and that the amplitude of oscillations was considerably less than that observed with the smooth cylinder.

The 3D LES simulation succeeded in capturing the intrinsic three dimensionality of the phenomenon.

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