Direct numerical simulation of a turbulent mixing layer with a transversely oscillated inflow

抄録

Direct numerical simulation of a turbulent mixing layer with a transversely oscillated inflow is performed. The inlet flow is generated by two driver parts of turbulent boundary layers. The Reynolds number based on the freestream velocity on the low speed side, U_L, the 99% boundary layer thickness of the inflow, δ, and the kinematic viscosity, v, is set to be Re = 3000. In order to compare the results with the experimental study of Naka et al. [Naka, Tsuboi, Kametani, Fukagata, and Obi, J. Fluid Sci. Technol., Vol. 5, pp. 156-168 (2010)], the angular frequency of the oscillation was set to be Ω_c = 0:83 and 3.85 (referred to as Case A and Case B, respectively). From the three-dimensional visualization, large-scale spanwise vortical structures are clearly observed in the controlled cases. The momentum thickness and the vorticity thickness indicate that the mixing is enhanced in Case A, while it is temporarily suspended in Case B. In both cases, the Reynolds normal stresses are increased in the region right downstream of the forcing point due to the periodic forcing. Furthermore, in Case B, the Reynolds shear stress (RSS), -u´v´, is suppressed in the region downstream of the forcing point. The spatial development of the turbulent energy thickness, δ_k, and the Reynolds shear stress thickness, δ_rss, show that the Reynolds shear stress in Case B is decreased by the control despite the increase of the turbulent kinetic energy. From the spectral analysis, large-scale spanwise structures are found to be caused by the periodic forcing, while the spectra of the spanwise velocity fluctuations are nearly unchanged. Co-spectra of the Reynolds stresses show that the present forcings generally enhance the long wavelength component. In Case B, however, the long wavelength component of the Reynolds shear stress is not increased in the downstream region.