Peter Berg wrote:
This is one of the fundamentals: the port (or gate) is 70% of the riser diameter, so according to Bernoulli's theorem, in the opening itself the speed is higher and the pressure comparatively lower. The secondary air channel is right in front of the port so this lower pressure will feed air in automatically. Behind the port both the pressure and the speed do go back to normal which in turn causing a lot of turbulence. The flames inside the riser are forming a double ram's horn pattern, coming back onto itself and spiralling up, thus lengthening the time in the riser.