Cd and blunt front ends

[Cd]

I just noticed that I have two typos in the name of this thread. First '.Cd' should be 'Cd' ,and second 'blunt font ends' should be 'blunt front ends'

What an idiot I am.

( Not sure what fonts have to do with aerodynamics. Helvetaca seems pretty streamlined doesn't it ? )

[Cd]

Aerohead, my original suspicions were correct - that cowl is what I thought was the base of the windshield.

[aerohead]

Originally Posted by Cd

Adding some more to this dead thread. Sorry for "beating a dead horse".

Here is an image of the car shown above as it was tested in the wind tunnel.
Of note would be how that the barn door flat front end of the car has even less of a radii than the white Camaro, no 'moonies' for the deeply inset wheels, and ( rear ) tires which stick out directly into the airflow. As shown in this image, the Cd was .292




Reading over this thread, I have to apologize for misspelling " Cd " as .Cd - as was pointed out to me, there is not period before "Cd".

I'm learning.

I also found this image interesting. The windshield is near vertical, yet there is attached flow along the roof. I guess this is a good example of how little curvature is needed to retain the airflow.

in one of my SAE papers,the researchers found that for their passenger car,a 40-mm radius was all that was necessary to achieve attached flow at the leading edges.That's not to say that it's the lowest drag radius,just that there's no separation.

[meanjoe75fan]

As alluded to earlier, the reason blunt leading edges do better than one might expect (and better than sharp ones) is that, in subsonic flow, pressure changes can propogate upstream, so that the air ahead of the vehicle "knows" what's coming, and starts to get out of the way BEFORE the car arrives. For supersonic flow, the air just sits there, fat, dumb and happy...until WHAM! it gets hit by the solid object. Thus, you NEED a sharp leading edge to "split" the air.

Also, regarding Cd increase when a sufficiently aero shape is being drafted...consider a "good" airfoil (that has laminar flow ~2/3 the way back) that suddenly has a flat plate (worst-case scenario) placed just behind it. Assuming subsonic flow, the air will "know" the barn door's coming, so it'll start to get out of the way early. I virtually certain (never ran the tests, though) this will play hell with the laminar flow, and probably even attached flow, tanking Cd and lift.

[Cd]

MeanJoe, I'm looking at that roofline on the hot rod in the post above yours. The flow certainly seems to be attached - despite the vertcal windshield.
I never would have expected this :0
I see what you mean about the pressure changes upstream . At the base of the windshield, the air is swirling and creating a smooth flow for the approaching air which slips above it ( It is " getting out of the way"as you say. )

[meanjoe75fan]

Yeah, it's all theory until you see it in motion...then it's kinda wow-ish.

Used to co-pilot Beechcraft turboprops, so I had to do the preflight. The engine nacelles hung out ahead of the wing, and the props would occasionally puke a few drops of dirty oil that would then contour to the prevailing airflow. The airflow ahead of the wing seemed about 10-15 deg upward--so the air "knew" the low-pressure upper wing was coming, and started heading that way--even though we were usually moving > 200 mph through the air.

[oldblue]

Here is a blast from the past.A 1936 scarab.

[NeilBlanchard]

Thanks for posting this picture -- another innovative aerodynamic design from the 30's! It probably needs to have a longer, and more gradual taper towards the rear, but much of it is "right".

Any idea what the Cd of the "Scarab" is?

[aerohead]

Sorry everyone,yesterday I posted a radius of 40mm to provide leading-edge attached flow.The number is correct for the 3/8-scale models tested,however for the real world and full-scale we would want at least 107mm according to that study.

[aerohead]

Originally Posted by Cd

MeanJoe, I'm looking at that roofline on the hot rod in the post above yours. The flow certainly seems to be attached - despite the vertcal windshield.
I never would have expected this :0
I see what you mean about the pressure changes upstream . At the base of the windshield, the air is swirling and creating a smooth flow for the approaching air which slips above it ( It is " getting out of the way"as you say. )

Cd,if you had a photo of the hot rod from the side,you would see that the body has a little "rake",where the nose is dropped in relation to rear.That would have the top of the roof "leaning" into the oncoming air and in a positive pressure gradient.As long as the air isn't tripped at the windshield header,it continues to be rammed onto the roof.