A flexibly mounted cylinder placed in a Newtonian flow undergoes vortex-induced induced vibrations (VIV) during which the shedding frequency and the oscillation frequency are synchronized. In Newtonian fluids, VIV occurs at Reynolds numbers as low as $\mathrm{Re}=19$. Here, we show that the expected VIV response of the cylinder can be greatly affected if the viscosity of the fluid is shear thinning, by conducting VIV experiments in a series of fluids of xanthan gum solutions of increasing concentration and increasing shear-thinning intensity. While the VIV response of the weakly shear-thinning fluid closely resembles the VIV response of Newtonian fluids, we show that by increasing concentration and shear thinning, the critical Reynolds number at which VIV occurs increases, while the reduced velocity at which lock-in begins, the width of the lock-in range, and the oscillation amplitude decrease. Beyond a critical concentration, VIV is completely suppressed over the entire range of Reynolds numbers studied here. The increase in critical Reynolds number and the reduction in the width of the lock-in range and the amplitude are likely caused by the increased viscosity and increased momentum diffusion of the shed vortices in the cylinder's wake, where the shear rate is small and the viscosity is large as it has recovered back toward its zero-shear-rate limit. The reduction in the critical reduced velocity is caused by an increase in the vortex shedding frequency as the fluid becomes more shear thinning. For the most shear-thinning fluid tested where VIV is completely suppressed, the vortices are shed very far from the cylinder and do not cause any oscillations.

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• Flow-structure interactions
• Vortex dynamics