Measurement of flow instability control with acoustic metamaterials

Phononic crystals and acoustic metamaterials (AMs) embedded into flat plate surfaces have previously been demonstrated theoretically to delay (or speed up) the onset of laminar-to-turbulent boundary layer transition by attenuating (or strengthening) Tollmein-Schlicting waves. Metamaterials can be advantageous for uses in flow control as they offer a passive control method that can be engineered independently without needing a priori fluid structure interactions simulations. An AM was designed to attenuate wave instabilities with a target frequency of 75 Hz. The AM was embedded into a flat plate and a circular cylinder was placed upstream in a current to generate oscillatory flow instabilities propagating in the wake of the cylinder near the flat plate. The effectiveness of the material to attenuate the artificially introduced wake instability was evaluated over a range of cylinder positions, flow speeds, and wave instability frequencies using measurements made with 2D-particle image velocimetry. It was found that the AM could attenuate instabilities at the target frequency while amplifying certain frequencies outside of the stop band, effectively increasing the vortex shedding frequency of the cylinder. This observation appears to be similar to lock-in behavior in vortex-induced vibrations, where the natural frequency of a mechanical system can alter the natural vortex shedding frequency behind a cylinder. These results are particularly significant because this is possibly the first time flow control behaviors have been observed by using an AM through physical experimentation, validating their use as passive flow control devices.