Interaction of a Tollmien–Schlichting wave with a local flow inhomogeneity

The method of parabolic stability equations is used to study the laminar-turbulent transition in a boundary layer with a stationary velocity inhomogeneity concentrated in a narrow stream. The location of the transition is found as a function of the magnitude and sign of the velocity defect. It is shown that if the inhomogeneity amplitude is small, it affects only the final nonlinear stage of disturbance development. In this case, the location of the transition is independent of the sign of the velocity defect. Having a moderate amplitude, a lower-velocity inhomogeneity shifts the transition location significantly more strongly than a higher-velocity inhomogeneity of similar shape and amplitude. This is caused by amplification of unstable disturbances in the low-velocity region and, conversely, their attenuation in the high-velocity stream. The effect of disturbance amplification in the low-velocity region is shown not to be connected with inflection-type instability. Another explanation of this phenomenon is offered.