O!
Dear Alex. please look at the last version of my notebook https://disk.yandex.ru/d/FzwUfQGIwnDQLw.

I plotted the radial flow versus speed at a fixed angle equal to the angle of the source. It can be seen from the graph that the flux decreases linearly from the maximum at the source to zero at zero velocity. However, near zero velocity, a numerical instability appears in the form of an oscillator. The amplitude and frequency of oscillations decreased after changing the variable u -> w=u^2. They decreased even more after replacing the approximate coefficients of the equation with the exac…