Trying to understand the requirement for designing an engine air scoop design. What should be the criteria for the inlet opening and plenum interior volume.
In regards to converting dynamic pressure to static pressure I have the following quote:
"Below about Mach 0.3 (or about 1/3 the speed of sound, 228 MPH at sea level), air
is considered “incompressible”. That is, even if the correct nozzle is selected, and
the air is slowed down (the official term is “stagnated”) there will be zero trade.
No kinetic energy will be traded in as work capable of compressing the air. The
reasons for this are not discussed here; the reader may consult any reputable fluid
mechanics textbook for confirmation of this fact. In plain English, a car is just too
slow for ram air to work."
Conversion of dynamic pressure to static pressure happens even ignoring effects of compressibility, in fact the commonly known definition of total pressure:
p = p0 + 0.5 rho v^2
is valid for incompressible fluid, constant density.
For compressible fluid it's:
p = p0 * (1 + (γ-1)/2 * M^2) ^ (γ / γ -1)
where γ is the ratio of specific heats, for air is commonly accepted 1.4 so it becomes:
p = p0 * (1 + 0.2 * M^2) ^ 3.5
At low speed, thus Mach, the two give roughly same result, as M grows the compressibility effects become more important and using the simplified model leads to big error.
The error you are willing to accept is what defines the M under which air can be considered incompressible, often M = 0.3 is assumed as limit but in reality it's arbitrary, one could even consider compressibility at M 0.01 if he wants, he's just wasting time using a model more complicate than needed.
Then that at the low speed (in aeronautical terms) of F1 cars the pressure gain is very small, limited to 1-2% of standard air pressure, that is undeniable, but it's still worth roughly an equal % in power, would you happily give up even just 5-10hp in an extremely competitive racing environment like F1?
Additionally you have to consider that that's the ideal gain you can get over an intake that maintains the same static pressure of freestream.
In reality a badly designed intake (too small, with edges leading to separation, badly positioned, affected by helmet's disturbance etc) can easily cause acceleration of flow and ultimately a loss of pressure; the optimization is thus aimed at having an intake that works reasonably well on the whole speed range, and in practice the effective difference between a good intake and a bad one, as average on speed range, can easily be quite a bit higher than these couple % and ultimately worth quite a bit on laptime.
After reading the paper on intake diffuser design, am I correct in assuming a low expansion ratio is better for preventing flow separation.
If so then limiting the size (2D area) of my 15 mm thick foam air filter is helpful to controlling the expansion ratio given that I have packaging (length) restrictions.
