A relatively minor oval distortion in the central region of the Galaxy, turning at a representative angular pattern speed, can excite outgoing waves at the outer Lindblad resonance of that pattern speed. Associated with the density crest of these waves is fast-expanding gas flow. The physical basis of this phenomenon can be understood through a linear analysis. However, to explain the observed expanding velocity in the “3-kpc arm”, the non-linear theory must be used. In our calculations an oval distortion turning at 118 km s-1 kpc-1 with a perturbation of 10% of the mean gravitational field at the outer Lindblad resonance (located at 3 kpc in the present case) can generate an outgoing velocity of 53 km s-1 at the first density crest of the wave (located at 3.6 kpc).