This paper studies how the turbulence intensity and macroscale generated by a passive grid located at different distances from a slender flexible cylinder (L/D = 120) affect its dynamics. The turbulence in the flow is the normalized deviation of the flow velocity, and it is important because it is related to the flow velocity variability. The macroscale length measures the size of the eddies in a flow (largest eddy size) and is related to the energy contained in the large eddies. Here we compute it using the power spectrum of the velocity at low frequencies. This helped us examine whether the size of an eddy would affect the flapping motion of the filament.

To manipulate the macroscale, a passive grid was placed at five different upstream locations with respect to the cylinder. Two high-speed cameras recorded the motion in the top and front plane while the cylinder was exposed to different wind speeds, which were increased in gradual steps.

We analyze the linear relationship of the motion in the vertical and horizontal planes. Furthermore, non-linear analysis of the data is used to determine the onset of the motion and determine whether it is a quasi-periodic or chaotic motion. The results show that the turbulence macroscale modification affects the critical velocity for the onset of the flapping motion. While the turbulence shifts the onset of the motion to the lower flow speeds, the motion exhibits less clear quasi-periodic behavior than in the case where there is no grid. The bifurcation diagrams show how the dimensionless amplitude in the vertical plane does not change for different macroscale lengths, while the motion in the horizontal plane does. Based on the analysis, turbulence does not seem to be related to the vertical plane motion, while the horizontal motion shows to be more chaotic, possibly a turbulent buffeting.

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