Fluido wrote: ↑
Tue Apr 12, 2022 11:15 am
At 3:04 it seems there are many vanes at front part of undertray.
Any better picutre and function of it?
Why so many vanes, dont they block/slow down airflow too much?
On one side they raise pressure, on the other side they lower pressure. The vanes act like a passive air pump. The low pressure zone behind the vanes acts as a vacuum cleaner for air upstream, accelerating it.
Remember that flow velocity is dependent on the pressure difference between the inlet and throat of a venturi channel. The greater the pressure difference between the inlet and throat the greater the flow velocity. Like an air compressor, the pressure in the air lines is ~6x the pressure at ambient, so the air rushes out of the lines into ambient.
The strakes create high pressure, by the 3rd law, there's an equal but opposite reaction elsewhere.
Wings work the same way no? One side raises pressure, and the other side lowers pressure. Mass is displaced and force is generated.
How did the fan car work? It maintained low pressure underneath, which allowed the high surrounding static pressure to press down on the car. The skirts were used because air pressure functions in all directions. That fan creates high pressure on one side(the blowing side) and low pressure on the other.
The strakes at the front are basically static fans, they push air outward instead of letting it go rearward, less airmass means less air pressure. Less air pressure means more acceleration of upstream air, more momentum, means less static pressure(up to a point).
The strakes are what choke, because that channel is so small, you can see on pressure traces, there's a pressure peak where the innermost strake and the floor interact. That pressure peak indicates sonic choking. From there the only way to further accelerate the air is to lower the backpressure downstream.
Look closely, you see the pressure at the leading edge of the floor between the inner strake and the plank area have slightly high pressure, then the airflow starts accelerating despite
a diverging section. When airflow accelerates in a diverging section, it implies the flow velocity is high enough for compressibility effects to be at play, or that the flow is choked/sonic.
You notice the inner strake diverges from the floor on the RB18. With non compressible flows, diverging sections reduce airflow velocity, yet from the CFD we see that the airflow clearly accelerates.