Experimental study on passive plate pitching and coherent structures in a square channel by TR-PIV

Abstract

The interaction of plate pitching and vortex may damage the accumulator isolation passive valve in nuclear power plant. The plate pitching and turbulent flow in a square channel were studied by high-speed camera (HSC) and time-resolved particle image velocimetry (TR-PIV) at Reynolds numbers (Re) from 1474 to 58952.The plate angle captured by HSC were analyzed by edge detection algorithm. The plate is inclined in hydraulic equilibrium without oscillation when Re is less than 14738. The plate pitching start to be periodic at Re = 22107, and then the periodicity weakens with Re increasing from 22,107 to 51583. The time-averaged pitching angle increases from 0.08467 rad to 1.454 rad with Re increasing from Re = 1474 to Re = 51583. The standard deviation of pitching angle increases from zero with Re less than 14,738 to 0.1294 rad at Re = 22107, and then it decreases to 0.006536 rad at Re = 51583. The peak frequency of plate pitching is the same as that of turbulent flow extracted from the power spectral density of instantaneous velocity downstream of the pitching plate, indicating the passive plate pitching is determined by turbulence. The plate pitching is driven by lift force torque due to the leading-edge-vortex shedding from the pitching plate.The interaction between passive plate pitching and fluid generates complicated turbulence. The time-averaged flow structures are typical shear-flow due to small plate inclination angle without oscillation when Re is less than 14738. The Reynolds stresses are highly enhanced in the shear flow. At Re = 22107, the time-averaged flow structures behave as vortex bubble over the plate and shear flow surrounding the plate, and the Reynolds stresses downstream of the plate are obviously enhanced by shedding vortices from the plate. With Re increasing from 22,107 to 58952, the time-averaged vortex bubble becomes smaller and shifts downwards to the plate trailing end, while the Reynolds stresses with similar distribution patterns decrease fast. The peak frequency of vortex shedding increases with Re increasing. The large spatial and temporal length scales are induced by shear flow at low Re less than 14738. The large length scales are dominated by von Kármán street shedding from the plate at Re from 22,107 to 58952, which are identified by proper orthogonal decomposition (POD) analysis, because the first two modes are the same mode of von Kármán street with a phase difference.The experimental work improves understandings of plate-fluid-interaction of the isolation passive valve and supports validations of numerical simulations.