Modeling and Analytical Solution of Near-Field Entrainment in Suddenly Started Turbulent Jets

摘要

The evolution of the shear-layer flowfield of a round transient turbulent jet is analytically investigated within the near-field of the jet over which the velocity potential core decays. The model results are verified and expanded by performing a large-eddy simulation of the suddenly started jet. Unlike quasi-steady approaches, the present work aims at solving the momentum conservation while preserving the time-dependent term. The governing equation is integrated with a mixing-length model to account for the turbulent mixing in the shear layer. The solution is asymptotically applicable to the shear layer of the circular jet and excludes the head vortex mixing. A reasoned calibration of the model parameters for moderate Reynolds numbers results in acceptable agreement for the streamwise entrainment both with experimental data and with large-eddy simulations. The validity limit of the present model is examined by outlining the characteristic length of the velocity potential core as well as the restricting effects of the jet dynamics on the founding assumptions of the model. The study will be instrumental in developing a hot-jet ignition model in which the rate of mass entrainment into the jet influences the prediction of ignition based on the temperature and species distribution.