Evaluation of turbulence kinetic energy budget in turbulent flows by using a photobleaching molecular tagging velocimetry

Abstract

Understanding turbulence kinetic energy (TKE) budget in gas-liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. Simultaneous measurement of velocity and velocity gradients with spatial resolution smaller than the Kolmogorov scale is required to evaluate the TKE budget experimentally. We therefore proposed a molecular tagging velocimetry based on photobleaching reaction (PB-MTV) and applied it to turbulent flows in a square duct to demonstrate the possibility of evaluation of TKE budget. Although the accuracy of measured velocity and velocity gradients were examined in the previous study, the accuracy of the TKE budget has not been examined yet due to lack of information on the TKE budget. In this study, we measure TKE budget in a turbulent water flow in a square duct by using the PB-MTV at the same turbulent Reynolds number condition as DNS data provided by Iwamoto, and compare the measured data with the DNS data to validate PB-MTV for evaluation of TKE budget. The photobleaching is an irreversible photodegradation of a fluorescent dye induced by intense illumination. Uranin (fluoresceine sodium salt) is used for the fluorescent dye which is photobleached by an intense laser beam (wave length λ = 488.0 nm) to form a molecular tag. Velocity and velocity gradients are evaluated from translation and deformation of the molecular tag between two consecutive images with a time interval δt, respectively. The size of the tag is about 40 to 100 micrometers, and smaller than the Kolmogorov scale in the turbulent flow. The cross-section and length of the duct are 50×50 mm and 1.6 m, respectively. The measured result indicates that the TKE budget evaluated by PB-MTV agrees well with the DNS data. The PB-MTV is also applied to turbulent bubbly flows in the square duct.