Two-axis Scheimpflug focusing for particle image velocimetry

The objective of this work is to demonstrate the capabilities of a two-axis, three-dimensional Particle Image Velocimetry (PIV) system for use as a largescale wind tunnel research tool. A two-axis PIV system provides the capability for camera placement virtually anywhere within a three-dimensional space relative to the laser sheet plane of measurement. The Scheimpflug focusing method is applied in two orthogonal axes to achieve off-axis focusing. The two-axis, three-dimensional PIV system uses off-axis stereoscopic viewing to determine two perspective measurements of a three-dimensional velocity field within the laser sheet. A unique calibration procedure is used to determine the optical parameters that describe the position of the camera systems and which are necessary to perform the three-dimensional reconstruction of the displacements. This paper presents this unique calibration for a two-axis, three-dimensional PIV system. This two-axis, three-dimensional PIV system was demonstrated first by measuring three-dimensional displacements of a target. Maximum instantaneous errors are estimated as 2% or less for all velocity components based upon the measured target displacements, and these error estimates were verified by a propagation of errors calculation. The error estimates are for instantaneous displacements and include all error sources. The two-axis, three-dimensional PIV system was also used to make measurements of the freestream velocity within a wind tunnel. The out-of-plane freestream component of velocity measured by the PIV system differed from the expected value by less than 1.1 %.