N-pulse particle image velocimetry-accelerometry for unsteady flow-structure interaction

Flow-structure interaction experiments are a major area of application of instruments capable of simultaneously measuring instantaneous fields of velocity and acceleration. An N-pulse particle image velocimeter-accelerometer (N-P PIVA) employing bursts of N pulses, where N = 3 or 4, and operating in the high-image-density particle seeding mode is described and demonstrated in the context of a representative flow-structure interaction experiment. The instrument employs two double-pulsed lasers and a high-resolution, fast-framing camera to acquire successive particle images having time separations small enough to perform good interpolation or finite differencing. The interrogation procedure locates the same group of particles at each pulse time using multiple cross-correlations, and a predictor-corrector algorithm enhances the strength of the cross-correlations by centering the windows on the particle groups at each time. A flow-structure experiment was performed in liquid surrounding a horizontal cylinder suspended by two thin, flexible, vertical rods from a slider block driven horizontally and sinusoidally. The value of the Keulegan–Carpenter number is and the frequency parameter (or Stokes number) is . Data from 2-, 3- and 4-pulse systems are compared to assess their relative performance. Measurements from the 4-pulse method with interpolation have smaller mean bias errors than the 3-pulse method with interpolation or the 4-pulse method with least squares, but larger random error. To make measurements close to the surface of the cylinder, a method using near-wall transformation and correlation analysis on a transformed grid is developed. Image processing used to determine the position, velocity and acceleration of the center of the cylinder is described. These measurements, together with the N-P PIVA data allow complete evaluation of each term in the exact, stationary control surface formulation of the fluid force applied to the cylinder surface (Noca 1997 PhD Thesis California Institute of Technology, Noca et al 1997 J. Fluids Struct. 11 345–50, 1999 J. Fluids Struct. 13 551–78, Unal et al 1997 J. Fluids Struct. 11 965–71).