Measurements and stability calculations of the disturbance growth in an unstable three‐dimensional boundary layer

The amplification of disturbances developing under conditions of natural transition in the unstable three‐dimensional boundary‐layer flow on a swept‐back flat plate is measured with the aid of hot‐wire anemometry. A detailed analysis of the experimental data allows identification of the most amplified instability modes and determination of their growth rates. The results are compared with linear stability theory. Although the amplification process is affected by nonlinearities starting a short distance downstream of the positions where the disturbances become of measurable size, in some essential respects the applicability of linear theory can be examined. It turns out that the initial amplification rates of the stationary instability modes are fairly well predicted whereas the amplification rates of the nonstationary modes are overestimated. A remarkable feature is that the disturbances with the largest amplitudes are not, in every case, the most amplified ones in theory as well as in experiment.