Measurement of turbulence spectrum in pipe flow using Multiple-Eye PIV

We have investigated the turbulence spectra in pipe flow using Multiple-Eye PIV system with a mirror array and confirmed that the dynamic velocity range was increased compared with 2D2C PIV.

The mirror array consists of a flat mirror arranged to form an ellipsoid, and placed it between a camera and flow target to capture images of identical particles from N (=110 in maximum) different directions. This method is known to reduce velocity random errors in proportion to N^-0.40.[A. Maekawa et al, Meas. Sci. Technol. 29, 11(2018)] by computing ensemble average of 3D particle locations identified through 3D-PTV. In this study, m sets of n different mirrors, where nm≤N, were used to capture particle images of turbulent pipe flow, and instantaneous velocity field was estimated by averaging m sets of 3D-velocity field obtained from each n images evaluated by tomographic PIV. The mean velocity was found to match well with DNS results, and the RMS values approached the DNS results as m increased. Energy spectrum was obtained by converting the time series data from frequency to wavenumber space based on Taylor's frozen field hypothesis. The attenuation of energy at high wavenumber was better captured by increasing the m.