Also note how close the motor is under the buldge under the hood. It's tight there.
Also note how close the motor is under the buldge under the hood. It's tight there.
Keith Tanner said:Best way to measure hood pressures is with a magnehelic gauge. I did a video on this recently.
I've only watched a minute of that, but I like what I hear so far. And yes, I'd prefer the use of a magnehelic gauge for finding this low pressure zone, especially since you can put the other pickup location underhood and get a pretty good sense of the differential.
re:79rex
The 'cam cover' bump you're looking at is a pretty good place. It's back a bit, but the shape of the 'cam cover bump' should itself help create a second area of 'generally' lower pressure. I'd probably bias it forward a bit from where you have it figured on. the lowest pressure for that area will happen right after the bump 'goes flat'... so to speak... which is about the forward edge of you marking place. so, if the panels can bend to match the curve, you could get more louvers into that location.
re: yarn
the answer is: "it depends"
could you use it to find the low pressure zone? yes, but it depends... and it'll be a lot harder than the aforementioned gauge.
could the yarn lift up in that low pressure zone? yes, maybe... but it depends on how low a pressure it is... and the physical property of the yarn (diameter, density, length, etc).
can yarn indicate stall? yes... although usually aerodynamicists look less at the yarn lifting off the front surface in the low pressure zone, and look more towards the rear 50% of the 'low pressure' surface and watch the propagation of tufts turning and flowing against the direction of the 'free stream flow'. All that is kind of moot... because if you're generating that high of a low pressure zone on the upper surface of a hood... cooling is probably the least of your problems.
re: pull air in from the base of the windshield?
can you do this? sure... but it depends.
one particular challenge is, I'd see it as generally encouraging the flow to go from the base of the windshield to out the bottom of the engine bay... which would generally equate to increasing front-end lift.
standard caveats:
ymmv, caveat lector, I only studied this stuff a long time ago, etc.
Back when working at major OEMs, I was involved in numerous wind tunnel tests of various types, from scale model to full size. One was even a cooling test done with a heated air and on a dynometer (at the Harrison tunnel in Buffalo). More recently my “cooling” work has been on my model airplanes (they can overheat too).
To insure good cooling, you first need to get the air going IN to the radiator – which means sealing things up around the core, having an air dam (low as possible) in front which is also sealed horizontally to the bottom of the radiator bulkhead (not to the bumper). This directs air to go up and through the core rather than under the car. Better cooling is often done with air coming in under the bumper than through the grille area. The air dam also helps by lowering the pressure underneath (behind the radiator) – as well as creating down force on the vehicle. A good underbody dam probably has side skirts on it going back to the front wheels. Incidentally, any aero work at the rear on deck spoilers, etc. can actually affect the air flow at the front. Also, just making a bigger opening in front of the radiator may not help at all.
We would always use yarn tufts and oil drops to look at boundary layer flow, as well as “smoke” sometimes (really wasn’t actual smoke but some powdered stuff which would fowl up the tunnel if used too much). The smoke was better for looking at free stream flow. Remember, the boundary layer flow is not the same as free stream flow – often it will go sideways or backward – always from high pressure to low.
The typical production car hood always has a low pressure developing once the air turns the corner from the front surfaces and is well attached flow. The windshield would always create a high pressure at the hood rear – in the center area where they put the air intakes for interior. Like Tanner said, smooth boundary layer flow will usually mean low pressure. Again, the tufts DO NOT represent the free stream flow direction.
A wind tunnel truth – you can’t force air to go where it don’t wanta. So use the natural high and low pressures to advantage. Louvers/extractor scoops can probably help on the hood, particularly when back a little from the front edge and toward the sides. That is why they are often seen there. If you want to introduce air INTO the area (for engine air), you will have to raise the (intake) scoop up above the boundary layer to get good free stream air. A good extractor scoop will probably have a little lip at the front and radius at the back. You will also see NACA ducts, which take slow moving exit air and accelerate it to higher speed by means of a narrowing duct shape. Intake NACA ducts have the wide end at the rear.
Incidentally, I was told that the most severe cooling test (at the Harrison facility) was called the Pike’s Peak Garage Door Test. The car, chained down on a dynometer, was run up to a condition simulating going up the mountain at max loads and temperatures. Then, at the end, the car was run up to a garage door (simulate by dropping an actual door right in front of the car), engine to idle speed. The airflow would then circulate in a LOOP, going through the radiator repeatedly – getting hotter and hotter. We did this on our test vehicle and it overheated instantly (implies that cooling vents in the hood might be useful to let hot air out under stopped condition after running?).
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