Transonic gas flow in a channel of variable cross-section is considered. On the lower wall of the channel there is a smooth bump, simulating an aerodynamic profile, around which a local region of supersonic flow forms. The full velocity potential equation is used as the basis for a numerical investigation of the structure of the flow field when the shape of the bump is slightly or very different from shockless form. The increase in the perturbation amplitude in the narrower part of the supersonic region due to the cumulative effect is followed, and the development of non-linear effects, leading to the formation of a shock wave, is noticed. Methods of controlling the flow by perforating the surface of the wall, thus reducing the intensity of the shock wave, are proposed.