Upstream Pressure Fluctuation Over Sonic Line
 

Hi everyone!


I am running a simulation of a transonic airfoil with buffet. However there is an event during the upstream excursion of the shock-wave which is linked to inviscid phenomena that I cannot fully understand and I was hoping that you could give me some guidance or advice.


I have attached an image with 4 figures that show the velocity divergence ( acceleration regions in red, deceleration regions in blue) with the contours of the sonic line. You can see how a pressure wave (a deceleration wave) departs from the crest of the shock-wave and travels upstream interacting with the boundaries of the sonic line. During its excursion it modifies the shape of the sonic line and I guess that for that reason a compression wave is reflected : the sonic line acts like a constant pressure boundary condition in supersonic flow since the perturbations cannot travel upstream.


My question is : why is the Mach number increased at the sonic line (the magenta line shifts to the left and therefore the supersonic region is expanded) when the pressure wave travels upstream if it is a deceleration wave? I couldn't find any reference for the effects of pressure waves in the nearby region of a sonic line and their interactions.