Attenuation of turbulent boundary layer induced vibration using water-repellent coatings

The performance of on board and towed sonar arrays is often limited by the self-noise generated by the structural vibration induced by the turbulent boundary layer (TBL). Several passive and some active methods based on both structural-acoustic and flow control have been designed and applied to attenuate the noise detected by the sensors. In the last decade, a number of investigations have been devoted to analyze the mechanisms of interaction between the TBL and water-repellent coatings with the specific aim to reduce frictional drag. These surfaces may also be of potential interest to increase the mission range of surface and underwater autonomous vehicles and to modify the vortex shedding from control surfaces and appendages. Little has been done to assess the efficacy of these water-repellent surfaces at modifying the TBL-induced structural response. To this aim, an experimental setup has been designed to measure, in a high speed channel, friction drag and vibration induced by turbulent boundary layer underneath flat plates at moderate Reynolds number values (Re 10⁶). The attention has been focused on assessing material performances with respect to flow velocity, surface roughness and functionalization methodology. More precisely, two biomimetic approaches for surface modification have been pursued. The first one, inspired by the lotus leaf, is based on the fabrication of a hierarchical (e.g. micro- and nano-scale) surface structure enabling the entrapment of an air plastron. The resultant superhydrophobic surfaces (SHSs) displayed water contact angle (WCA) larger than 150° and contact angle hysteresis (CAH) smaller than 10°. The second approach, inspired by Nepenthes pitcher plant, is based on the presence of a continuous film of low surface tension liquid on the surface to create a liquid-liquid interface with contacting liquids. These Slippery Liquid-Infused Porous Surfaces (SLIPSs) displayed lower WCAs compared to SHSs, but the same small CAH and excellent water-repellent behavior. For each surface the roughness values varied between 1.5 and 4 micrometers, while their performances have been tested in a velocity range between 1 and 4 m/s. Promising results in terms of both acceleration amplitude and frictional drag reduction are obtained for SLIPS with the lowest roughness value, for all tested velocities. On the contrary an increase of the panel response as well as of drag is observed for SLIPS with the higher roughness value. Some insights into capabilities and limitations of SHSs are also discussed.