Controlling the instability of the air-water interface below an impacting disc

Escape of trapped air between an approaching disc and a water surface prior to the solid-liquid impact induces instability at the air-water interface. The instability occurs at the disc edges where the exit velocity of air first exceeds the critical velocity, and a dominant unstable wavelength emerges at the onset of the instability. Here we study the effect of shear flow modulation on the growth of the interfacial instability, which is achieved by imposing axisymmetric sinusoidal perturbations of different wavelengths on the impacting discs. Using a total-internal reflection visualisation technique, we observe that the instabilities on the water surface tend to grow below the crest locations of the disc cq. near the disc edges. Small wavelengths were found to attenuate the magnitude of the dominant wavelength under the disc edges. Meanwhile, larger wavelengths induce new sites of instability and could amplify the magnitude of the dominant wavelength. The amplification is significant for the imposed wavelengths that are close to the dominant wavelength. The origin of the attenuation and amplification of the instability in modulated shear flow will be discussed, accounting for the water cavity dynamics and the local air velocity maxima development under the modulated disc.