Design considerations for water-flow wall-pressure fluctuation measurements using pinholes in a turbulent channel flow

Flush-mounted-transducer measurements of turbulent wall-pressure fluctuations in water flows provide poor spatial resolution because no transducers simultaneously provide the requisite size and sensitivity. However, successful air flow measurements have been made with microphones behind pinholes. This pinhole measurement method is imperfect in water flows because of the detrimental effects of trapped air bubbles, pinhole-opening flow dynamics, Helmholtz resonance(s), and residual spatial averaging by the pinhole opening. In this presentation, all four limitations are illustrated and addressed via pressure fluctuation spectra collected in a turbulent channel flow (half height = 3.5 mm, 14:1 width-to-height aspect ratio) with 5.5-mm-diameter pressure transducers mounted behind 0.5 mm, 0.75 mm, 1.0 mm, and 2.0 mm diameter pinholes at frequencies from 10 and 20 kHz at nominal average water flow speeds from less than 2 to more than 6 m/s. The effects of air bubbles are suppressed by using deaerated water (30% dissolved oxygen). The effects of pinhole-opening flow dynamics and spatial averaging are reduced by decreasing the opening size. Unfortunately, the importance of the mounting geometry's Helmholtz resonance becomes more prominent as the opening size decreases. In this study, the 0.5 mm pinhole showed the greatest potential for successful turbulent wall-bounded water-flow applications while providing more than a factor of 10 improvement is spatial resolution.