Quote:
Originally Posted by mjboks
This is the area you are referring to?
Attachment 33586
It appears the air pressure is lower on the roof vs the side since the upper tufts are getting pulled up and over the edge. This edge will be difficult to round. At best it will be between 1-2 inch radius. However the angle as it moves back does reduce. It is hard to see but the angle starts at about 75 degrees and ends around 45 degrees. The roof cross car dimension is consistently 48 inches. So with the edge following the truck side, as the roof comes down, the angle changes to accommodate. Here's a view of the recent CAD:
Attachment 33587
As for rear visablity, I and sticking with side mirrors. |
Yes, exactly. It's the Bournoulli Theorem at play.
Air over the truck is taking a longer path than along the side, yet arriving at the same time; requiring a higher velocity to compensate.
There's a direct trade-off between velocity and pressure, leading to the lower pressure up top, which attracts the slower, higher-pressure flow alongside.
--------------------------------------------------------------------------------------
Without radii at the intersection between the top of the new sail panel and roof, we lose the ability for some comingling of the two streams and the shearing forces they pose.
FIAT published a 'with' radii, and 'without' radii, fastback roofline comparison in an 1986 SAE Paper, and found up to a Cd 0.035 drag penalty for the roof lacking the edge softening.
Member 'bondo's' Aerolid aero shell was wind tunnel tested, courtesy of the Ford Motor Company, and returned a Cd 0.067 drag reduction on a 2012 F-150.
The FIAT data infer that, without the cab-matching radii on the aero shell, the drag reduction could be reduced to only delta-Cd 0.032, a 52% loss in performance.
------------------------------------------------------------------------------------
This vortex-drag was studied intensively by 'Kamm' at the FKFS.
There are no low-drag cars ever fabricated which did not possess this continuity between the forebody roof edges and aft-body.
--------------------------------------------------------------------------------------
Skipping ahead to adding 'fences':
* On an aircraft, like a Boeing 737, operating at 33,000-feet, and for a perfectly-streamlined wing, in two-dimensional flow, the wingtip treatment would be a must.
* On a non-streamlined, bluff-bodied, road vehicle ,operating within three-dimensional flow, and ground proximity, adding some sort of 'winglet' would be a violation of the ground rules for fluid mechanics, as spelled out by Dr. Hucho, at least in his December 1987 2nd-Edition ' Aerodynamics of Road Vehicles.'
These fences have Cd 1.15, and in the statistical 7-mph crosswind that the truck is always go to experience, you'd be installing a 'trigger' for span-wise contamination, and introduce a roll moment during wind gust.
------------------------------------------------------------------------------------
' the vehicle aerodynamicist must refer to a large amount of detail resulting from earlier development work.' Hucho, PREFACE, 2nd-Edition, 1987.
--------------------------------------------------------------------------------------
My Toyota T-100 uses a 69mm ( 2.75-inch ) radius to marry the aero shell's roof and sail panels together, mimicking the T-100's cabin.
You're not 'adding' radius, you're just providing a missing ingredient.