Did someone say Yarn?

[AZPete]

I can't afford to buy yarn for air flow tests so I raided my wife's gift drawer for some ribbon. I'm saving up to buy the soon-to-be-released Zero db hood hinges. You'll see it was a breezy day by ribbons moving at 0 mph. On the video of the side vent I forgot to tape the iphone strap so that's what's flopping around. I don't do interpretations, so that's up to you.

Engine cover:

https://youtu.be/zWnan1QeWUw

Side vent:

https://www.youtube.com/watch?v=267mEVNfZjY

Here is the set up with an iphone clamped to the roll bar:
41904

[Canadian818]

Thanks for the R&D. IMO the side vent is definitely getting flow, and kurt818's scoops should be effective. The rear decklid seemed really turbulent, with only the very rear strips being consistent. After watching that, I'm still unsure if the rear top cut out is venting or sucking. Seemed to be venting initially, but then it wasn't so clear.

[apexanimal]

^agreed... long wool tufts would be easier to read... putting one color inside the engine bay around the vent, and another color on the outside spaced on a few inches apart would clear things up...

i'm not too surprised that it's a little turbulent in that area, but the bi-color, wool-tufting test at 30, 45, and 60mph would clear it up...


nice work! great start here

[Santiago]

Thanks for posting Pete!

I'm not happy with these preliminary result. At 0:20 the car has just gotten well under way and you'll note that the top ribbon (outside) is as it began - tucked into the vent. Within a short time the inside ribbon has already shown signs of reverse flow. This grows so strong that it actually pulls the outside ribbon out of the vent by 0:32 in the video. From where it's positioned, this shouldn't happen to that ribbon. As speed grows we see periods in which the flow drags the ribbon in alternating routes both into the vent and over the vent (see 0:36 for a clear shot of it).

Here's my take: The vent is either not deep enough or doesn't control the entry flow well enough. At times there's a good amount of flow that is drawn into it but it seems like it's as prone to hop over the vent as it is to stay down. A true NACA duct has a narrow throat to start the draw and sharp horizontal edges to trip the air over and into the duct. This duct may be too broad at the leading edge, it may run afoul of viscous blockage (if it's too shallow), or the edges may be too soft. I'm suspecting the broad entry, but it's going to take some experiments to confirm. You might try something simple, like slapping a layer of modelling clay on the entry and sculpt it in line with a NACA duct profile. If that fails to generate more consistent flow into the vent, either a raised scoop or a dropped floor may be in order.

Thanks again for posting the video. Hopefully we can get more like this and see what's happening on actual cars.

Best,
-j

[Harley818]

nice work Pete. Thanks for the R&D.