Transition to turbulence on the initial section of a circular pipe

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All the available data indicate that transition to turbulence in a circular pipe takes place within the initial section. This is confirmed by the conclusions of the linear theory of hydrodynamic stability, according to which the velocity profiles on the initial section of the pipe are unstable [1]. So far, however, there have been few investigations of initial-section flow at different values of the initial perturbation level e0 at the pipe inlet and different values of the length to diameter ratio of the pipe 2/d. We have now investigated the transition to turbulence in the boundary layer on the initial section of a circular pipe for various ratios of the thickness of the layer to the radius of the pipe and various levels of initial turbulence. The transition point in the boundary layer was found experimentally, since at present there are no reliable methods of calculating it. In particular, the susceptibility problem has not been solved, i.e., the problem of the initial amplitude of the Tollmien—Schlichting wave, the development of which results in transition to turbulence. It may be assumed that the initial amplitude of this wave is determined by the interaction of higher-frequency waves on the section preceding its growth zone [2]. Moreover, different views are held concerning the mechanism of transition to turbulence at e0 > 0.5%. At the same time, the results of the transition calculations for e0 > 0.5% based on the three-parameter turbulence model [3] require experimental verification.