Flapping flags in grid-induced turbulence

A fully-resolved direct-numerical-simulation approach is developed to study the flapping motion of a flexible flag forced by a turbulent incoming flow at moderate Reynolds number. Turbulence is generated by a passive grid at the inlet of the numerical domain and the turbulence level impacting the flag can be controlled by varying the distance from the grid. Our computational framework is based on the immersed boundary method and a spring-network model for dealing with deformable bodies of arbitrary geometry, and is implemented with a GPU-accelerated parallelisation increasing the computational efficiency. First, we characterize the turbulent flow comparing with well-known results for decaying turbulence and experimental measurements. Then, we revisit the flag-in-the-wind problem by exploring the effect of turbulence on the main features of self-sustained flapping. We show that, whilst the latter mechanism is still manifesting, the amplitude and frequency of the oscillation are remarkably altered. Moreover, the fingerprint of turbulent fluctuations is qualitatively detected by spectral analysis. Besides their relevance for fundamental understanding, these findings have potential impact for applications, e.g., aeroelastic energy harvesting or flow control.