Pressure ratio through a duct.
(Fluiddynamics)The velocity of air through a duct is entirely dependent on the pressure ratio at the throat, and the inlet. Therefore at low vehicle speeds a duct can have 1 bar head, and 1 bar at the outlet. As the vehicle speed increases, the head pressure also increases, meanwhile, the duct pressure decreases at any and all throats, which in turn reduces the pressure at the outlet. The increased head pressure, and reduced outlet pressure together increase the pressure ratio at each throat.
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These strakes increase head pressure, as they divert air outward, the inner most strake and the body become the path of least resistance. Now these strakes also restrict air from moving into the back of the floor. Which in effect is lowering the back pressure of the throat created between the inner-most strake and the body. When the pressure ratio reaches ~2:1, you have choked flow. Once you have choked flow, the passage to the back of the floor becomes restricted, which increases the suction behind the strake which pulls the floor downward at that point. However choked flow is very lossy, unless it is carefully controlled. The shape of the outlet has to be carefully designed to exploit the fact that you can accelerate choked flow with careful expansion of the duct.
Not only that, but all subsequent ducts must have this careful sizing in mind, otherwise there are great losses, and flow instability.
https://youtu.be/45w1-lwFSzM?t=1053You can see in this experiment how critical duct geometry is to maintain low pressure, low loss flow.
