Improving spatial resolution of underwater wall pressure fluctuation measurements using pinholes in a fully developed turbulent channel flow

The current measurement method of pressure fluctuations on the surface beneath a turbulent flow typically results in poor spatial resolution because no transducers simultaneously satisfy the requirements for size and sensitivity. Mounting the transducers behind a pinhole is known to be a viable solution in airborne experiments but have not yet been shown to be effective underwater. For these investigations, underwater wall pressure fluctuations were measured in a fully developed turbulent channel flow (channel half height = 3.5 mm, with 1:14 aspect ratio) with pressure transducers mounted behind a 0.5mm, 1mm-, and 2mm diameter pinholes. To prevent air-bubble blockage, the pinholes were filled with liquids with varying viscosities (water at 1 cSt, and silicone oils at 1000 and 10,000 cSt). The effect of varying the viscosities of liquids filling the pinhole on the wall pressure fluctuation frequency spectra were evaluated for flow speeds from 1.5 to 7 m/s, resulting in Karman numbers from several hundred to more than a thousand. While the results show that pinhole-mounting may be a viable method of improving spatial resolution of underwater wall pressure fluctuations, further parameter studies are warranted.