Air emission into a water shear layer through porous media: part 2 - cavitation induced pressure attenuation

Cavitation near the casing of a hydroturbine can lead to damage through both cavitation erosion and mechanical vibration of the casing and the associated piping. Cavitation erosion results from the collapse of cavitation bubbles on or near a surface such as the casing wall. Mechanical vibration is transmitted to the casing directly through the collapse of bubbles on the casing wall and indirectly through a coupling of the acoustic pressure pulse due to a nearby collapse on the turbine blade. Air emission along the casing can reduce the intensity of the tip vortex and the gap cavitation through ventilation of the cavity. Reduction in the machinery vibration is obtained by reduction of the intensity of cavitation bubble collapse and attenuation and scattering of the radiated acoustic pressure. This requires a bubble layer which may be introduced in the vicinity of the turbine blade tips. This layer remains for some distance downstream of the blades and is effective for attenuation of tip vortex induced noise and blade surface cavitation noise. For the purpose of characterizing this bubbly layer with a water pipe, we spanned a pipe with a two dimensional hydrofoil and emitted air through porous media (20 and 100 micron porosity sintered stainless steel) into the shear flow over the hydrofoil. This was mounted in the ARL Penn State 30.5 cm diameter water tunnel [This is described in detail in (Part 1): Scaling of Bubble Creation Noise.]. This paper is limited to an investigation of the attenuation of acoustic pressure propagating to the casing rather than the reduction in acoustic source level due to collapse cushioning effects. An assessment is made of the variation of insertion loss (dB reduction in acoustic pressure) with bubble size, axial velocity, and gas flow rate. Comparisons are also made to the bubbly layer created from discharge with a spanwise array of 1.2 mm circular orifices. Frequency dependent insertion losses were found to be on the order of 5-24 dB for air discharge rates of 58 to 1450 scm³/s across a 28 cm span. The insertion loss due to the bubble layer was measured by simulating broadband cavitation noise with a source hydrophone and measuring with a hydrophone on the opposite side of the layer.