Page 4 - Combined_133_OCR

TECHNICAL PAPERS

built more than 30 yr ago. Much of this drag could be undersurface of the vehicle and the overall configuration Th
eliminated by closer attention to windshield and rear-end of the vehicle. The ground clearance required in modern in re;
design and by the elimination of protuberances and automobiles places them close to this critical height. With­ cosity
rough undersurfaces. The realization of drag coefficients in the allowable height variation, an underpan is more drag
lower than 0.15 is unlikely because of the presence of effective in reducing drag on a vehicle that is relatively tion,
wheels. higher than on one that is close to the ground. This crit­ the b
Little correlation of the drag reduction made possible ical height also influences the lift produced by the vehicle. body.
by clean undersurface on various types of automobiles surfac
has been found. The effect of an undersurface on the Automobile Lift descri
drag of a vehicle is related closely to the proximity of The influence of height above ground is greater upon as the
that undersurface to the ground. If a vehicle with a rough the automobile lift than upon the automobile drag. wall, t
undersurface were raised several times its height above If a circular cylinder is placed perpendicular to poten­ sition
ths ground, the disturbance to the flow caused by the tial flow in the absence of any other surface, there is no cylind<
undersurface would be much greater than if the vehicle net force produced, neither lift nor drag. If a plane par­ height
w<re so close to the ground that very little air passed be- allel to the undisturbed flow is brought near the cylinder, change
-the cy]
Feath it. But, as a body is brought near a surface, the a force will be generated that attracts the cylinder to the
velocity of the flow between the body and the surface is plane. This negative lift increases more and more rapidly relative
’ lower <
increased rapidly and the energy losses produced by as the plane approaches the cylinder and tends toward
this higher velocity flow over the rough undersurface negative infinity just as the two meet. When the plane »• bound;
vhich
of the body cause a higher drag. The resulting drag may and the cylinder are actually in contact, the direction of
mce ol
be greater when the body is at a small distance from the the force is reversed and a finite positive lift is created.
ts on
surface than when it is well-removed from the surface This positive lift diminishes as the cylinder passes into
>ositivf
or when it is very nearly in contact with the surface. This the plane and finally vanishes as the cylinder is sub­
ehicle
critical intermediate height at which the maximum drag merged completely in the plane and the flow above the
occurs is related to the boundary-layer thickness on the plane is no longer affected by the cylinder.
- U I
O C
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an affiliate of HARTFORD STEEL BALL COMPANY Dfi
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