Nonlinear development of unsteady disturbances of a swept-wing boundary layer in the presence of intense crossflow vortices

The main goal of the present experimental study is to investigate the mechanism of weakly-nonlinear interaction in a 35-degree swept-wing boundary layer between low-amplitude nonstationary crossflow instability modes and stationary crossflow instability vortices. The crossflow instability dominates in the flow under study while the Tollmien–Schlichting instability is suppressed by a favourable pressure gradient. Stationary disturbances of large amplitude (up to 20 % at the end of the region of measurements) are excited by a surface roughness element. Controlled nonstationary disturbances of small amplitude are generated in the boundary layer by a disturbance source located upstream from the roughness element. Their amplitude in the region of main interaction does not exceed 1%. The source excites quasi-two-dimensional (spanwise-uniform) waves at low frequencies corresponding to the region of primary crossflow instability. The results of hot-wire measurements have shown that the characteristics of the base flow, as well as of the stationary and nonstationary disturbances, do not depend on the amplitude of the latter. Meanwhile, development of the nonstationary disturbances depends very much on the presence of these vortices. Excited quasi-two-dimensional instability waves decay very rapidly downstream, while the forming stationary vortices, growing in a certain spanwise-wavenumber range, transform quasi-two-dimensional waves into strongly three-dimensional ones with a spanwise-wavenumber spectrum corresponding to the most amplifying crossflow instability modes. This transformation does not occur locally in the near field of surface irregularities, but is distributed in the streamwise direction. The growth of stationary disturbance amplitudes is almost exponential. The dependence of their growth rates on the spanwise wavenumber is a characteristic of the crossflow instability modes. Meanwhile, the amplification of the nonstationary disturbances has a complicated and non-monotonous character due to their nonlinear interactions. It is found that in the plane normal to the flow and to the wall, the amplitudes of nonstationary disturbances of all frequencies are strongly localized in the regions with high values of spanwise gradients of the mean flow velocity. An essentially three-dimensional physical mechanism is proposed for the weakly-nonlinear transformation of quasi-two-dimensional waves into three-dimensional ones induced by high-amplitude stationary instability vortices. This mechanism is similar to the “lift-up” effect proposed earlier for explanation of the growth of streaky structures in two-dimensional boundary layers.