On the dynamics and frequency response of fully-pulsed turbulent round jets: Computations using two-time-scale/strain-sensitized eddy viscosity models

Abstract

The present paper reports simulations of ‘fully-pulsed’ (or on/off) turbulent round jets. Unsteady forcing of the jet is imposed over a range of frequencies from 2 to 10 Hz and, for a jet pulsed at 10 Hz, comparison is made with the experimental data of Bremhorst and Gehrke (2000) [Bremhorst, K., Gehrke, P.J., 2000. Measured Reynolds stress distributions and energy budgets of a fully-pulsed round air jet. Exp. Fluids 28, 519–531]. Calculations are performed using the two-time-scale, or split-spectrum, eddy viscosity turbulence model of Hanjalić et al. (1980) [Hanjalić, K., Launder, B.E., Schiestel, R., 1980. Multiple-time-scale concepts in turbulent transport modelling. In: Bradbury, L.J.S., Durst, F., Launder, B.E., Schmidt, F.W., Whitelaw, J.H. (Eds.), Turbulent Shear Flows 2. Springer-Verlag, Berlin, pp. 36–49]. The model, in addition to its split-spectrum features, places an increased emphasis on the role of irrotational straining in the energy cascade process. In seeking to distinguish between the effects of spectral partitioning and strain sensitivity in the model, a modified form is tested in which the turbulence response to shear and normal straining is treated more conventionally, while the split-spectrum approach is retained [Bremhorst, K., Craft, T.J., Launder, B.E., 2003. Two-time-scale turbulence modelling of a fully-pulsed axisymmetric air jet. In: Proc. 3rd Int. Symp. on Turbulence and Shear Flow Phenomena, Sendai, pp. 711–716]. In confirmation of the results of Bremhorst et al., it is found that, in general, the original Hanjalić et al. scheme is in better agreement with the measurements of Bremhorst and Gehrke than is the modified version. The critical role played by the starting vortex in influencing centreline flow dynamics is identified, and it is shown that it is the behaviour of the Hanjalić et al. model in a highly rotational mean velocity field that largely accounts for its generally superior performance in the computation of fully-pulsed jets.