Dual PIV systems for space-time correlations in hot jets

Abstract

Digital Particle Image Velocimetry (DPIV) has developed into a production grade research tool, which is being used at NASA GRC to improve our understanding of the decay of turbulence in jet flows - a critical element for understanding the acoustic properties of the flow. Two independent DPIV systems were installed in the GRC Small Hot Jet Acoustic Rig (SHJAR), enabling correlations in time and space. The data reported here were collected at a Mach number of 0.9 and temperature ratios of 0.86 and 2.7. The DPIV systems were mounted on independent traverse systems to facilitate image acquisition over a range of time delays and spatial separations. The collected DPIV data illustrate the differences in first order flow properties of cold and hot jets. The velocity fields from the two DPIV systems were then cross-correlated to determine the degree of correlation remaining in the flow as the downstream convection distance was increased. Increasing the physical spacing and time delays between the DPIV system acquisitions provided velocity fields containing information on the decay of turbulence in the flow. The DPIV results show that there are significant differences in the mean axial velocity component and turbulent kinetic energy for cold and hot flows at the same Mach number. The data also illustrate the decay of turbulence along the jet axis as revealed by the cross-correlations of the phase delayed and spatially separated DPIV vector maps.